Chapter
69
Superficial Digital Flexor Tendonitis Mike W. Ross, Ronald L. Genovese, Sue J. Dyson, and Joan S. Jorgensen
The first two sections of this chapter consider the general clinical manifestations of tendonitis and then the specific surgical management of tendonitis in racehorses. The third section discusses some of the variable clinical presentations in other competition and pleasure horses and factors influencing treatment and prognosis. In this chapter we have chosen to retain the term tendonitis to refer to the clinical syndrome of injury of the superficial digital flexor tendon, because clinical signs of inflammation are hallmarks and tendonitis is pertinent. We recognize that others prefer the term tendonopathy (see Chapter 68).
SUPERFICIAL DIGITAL FLEXOR TENDONITIS IN RACEHORSES Joan S. Jorgensen, Ronald L. Genovese, and Mike W. Ross Superficial digital flexor tendon (SDFT) injuries substantially compromise athletic performance and may culminate in a career-ending injury. The incidence of SDFT injuries in Thoroughbred (TB) racehorses ranges from 7%
to 43%,1,2 and such horses are most at risk because of high racing speeds or high speeds associated with jumping (steeplechase racing, see Chapter 112).3 In National Hunt horses the prevalence of tendonitis of the SDFT as detected using ultrasonographic examination was found to be 24%.4 In that study a reference range of cross-sectional area (CSA) measurement was obtained (77 to 139 mm2), but ultrasonographic examination could not predict injury, and variation in prevalence among yards suggested that training methods may influence injury rate.4 Tendonitis is quite common in the Standardbred (STB) racehorse, more so in pacers in North America than in trotters (see Chapter 108). Other performance horses, including upper-level event horses (see Chapter 117), have an increased risk of SDFT injury (see page 721). Horses used for dressage (see page 725), high-level show jumpers (see Chapter 115 and page 724), racing Arabians (see Chapter 111) and Quarter Horses (see Chapter 110), polo ponies (see Chapter 119), and fox hunters incur SDFT injuries less frequently.3,5,6 SDFT injury from athletic use in racehorses commonly is seen because of repetitive speed cycles over distance and possibly genetic predisposition to SDFT injury.7 We are aware of several TB racehorse mares and at least one TB stallion and one STB stallion that are known to have progeny with an increased susceptibility to SDFT injury compared with the normal racehorse population. Additional factors that may predispose a horse to SDFT injury include conformation (see Chapters 4 and 26), working surfaces, shoeing, training methodology, and the relationship between the level of physical fitness and the current exercise. SDFT injury also occurs spontaneously in sedentary or lightly used horses older than 15 years of age. These tendon injuries often are severe and generally involve the proximal metacarpal region and the carpus and extend to the musculotendonous junction in the antebrachium. Many of
References on page 1314
Chapter 69 Superficial Digital Flexor Tendonitis
these injuries result in overt lameness, tendon thickening, and carpal sheath effusion (see Figure 6-18). Sometimes, however, the only clinical sign is lameness, which is often pronounced to severe, with little palpable thickening of the SDFT. Subtle swelling in the proximal aspect of the SDFT is easily missed, particularly when the limb is held in flexion. These injuries occasionally can be difficult to diagnose, requiring local analgesia and ultrasonography. Although older nonracehorses appear prone to the development of superficial digital flexor (SDF) tendonitis in the proximal metacarpal and palmar carpal regions, the condition is sometimes seen in older STB racehorses, usually pacers. In a recent study, only two of 12 nonracehorses with tendonitis of the proximal aspect of the SDFT returned to previous use with medical management; horses were significantly older (mean age of 18 years, median 17 years, and range 11 to 23 years) than a comparison group with tendonitis of the SDFT in the midmetacarpal region, and prognosis was significantly worse than in the same comparison group (10 of 22 horses with midmetacarpal SDF tendonitis returned to previous use after injury).8 In an unpublished study of 29 horses with tendonitis of the proximal aspect of the SDFT, mean age was 13 years (median 15, range 2 to 30 years), there were six racehorses and 23 nonracehorses of mixed use, lameness was generally pronounced, and ultrasonographic evaluation revealed injury that often involved the palmar carpal region from near the musculotendonous junction to the proximal metacarpal region.9 Overall, 14 of 24 horses (58%) became sound and returned to work, and five of seven horses managed surgically (desmotomy of the accessory ligament of the SDFT [ALSDFT],otherwise known as superior check desmotomy) in combination with carpal retinaculotomy and proximal metacarpal fasciotomy, or transection of the ALSDFT in combination with intralesional injections of fresh bone marrow) returned to work.9 SDFT injuries from athletic use occur in the forelimb far more frequently than in the hindlimb. In one U.S. study of 143 TB racehorses, 58% of SDFT injuries occurred in the left forelimb and 42% in the right forelimb.10 Bilateral injury is common and has been recognized more frequently since veterinarians have been examining both limbs routinely by ultrasonography. Most injuries in the SDFT caused by athletic use occur in the midmetacarpal region (zones 2B to 3B), but injuries also occur at the musculotendonous junction of the antebrachium, in the carpal canal and subcarpal region, and in the pastern (see Chapter 82). The plantar hock region is the most common site of SDFT injury in hindlimbs, especially in the STB racehorse (see Chapters 78 and 108). SDF tendonitis in the hindlimb in the plantar tarsal region is referred to as curb and is one of the collection of soft tissue injuries comprising this injury (see Figure 4-30). Occasionally this injury extends into the midmetatarsal region. Infrequently, a subtle SDFT injury is associated with tenosynovitis of the digital flexor tendon sheath (DFTS) in hunters, jumpers, and dressage horses. In the STB racehorse, tendonitis often extends to the distal metacarpal region, involving the DFTS and palmar annular ligament, and there is generalized soft tissue thickening in the palmar aspect of the fetlock region. Racehorses with SDFT injuries traditionally have been regarded as having a guarded-to-poor prognosis for return
707
to racing, although the prognosis for other athletic disciplines is more optimistic. Before ultrasonography was used routinely, documentation of SDFT injury was limited. Diagnosis was based on gait evaluation and palpation of a swollen or thickened tendon. The injury was referred to as a bowed tendon, and morphological abnormality and severity of injury were little appreciated. Substantial progress has been made in understanding the nature of tendon injury and the mechanisms of healing (see Chapter 68). Historically, tendon injuries have healed by the process of scar tissue formation and maturation, known as reparative healing. During repair, injured elastic tendon fibers are replaced with modified fibrous scar tissue, resulting in a tendon repair that is never totally normal. The quality of repair can vary greatly. Some tendon injuries repair and resolve with enough mature collagen so that they return to nearly normal size, with sufficient remodeling that approximately parallel alignment of the repair tissue results. Other injuries develop a scar, with an overall increase in tendon size, poor or random fibrous tissue alignment, and peritendonous fibrosis. Many of the proposed therapeutic approaches are directed at maximizing the chances for a more physiologically functioning tendon. Therapy requires a multifaceted approach that reduces the acute inflammatory response and hemorrhage in the acute phase and improves fiber alignment during the long rehabilitation phase. The ultimate goal of any treatment and management program is to maximize the chances for a tendon to repair with adequate strength and elasticity for a return to a similar level of performance with the lowest risk of reinjury. Recently attempts have been made to heal equine soft tissue injuries using principles of regenerative healing. Regenerative healing occurs in fetal tissues and involves restoration of tissue without scar formation. Regenerative techniques involve the use of freshly harvested or cultured mesenchymal stem cells (MSCs) derived from bone marrow, adipose tissue, or other sources in an attempt to heal adult tissues in a manner similar to healing seen in a fetus (see Chapter 73). Improvement was reported in nine of 12 horses with SDF tendonitis injected with undifferentiated autologous MSCs, and the technique was found to be safe and efficacious.11 Reimplantation of autologous bone marrow-derived MSCs in 168 TB racehorses (National Hunt horses) with SDF tendonitis resulted in a substantially smaller reinjury rate (18%)12 than that previously reported (56%13). It is important to recognize the variables that may affect the ultimate prognosis when SDFT injuries are managed. Not all tendon injuries are the same, and case management depends on the specific injury, medical factors, and other nonmedical factors.
CLINICAL SIGNS Clinical signs of SDFT injury in racehorses vary considerably depending on the location of the primary injury, type of injury, severity, and timing of the examination. Occasionally, clinical signs may be delayed by days or weeks. Furthermore, a lack of correlation may exist between the severity of the injury and the severity of tendonitis in any given individual, especially in the more common core tendon injuries experienced by TB racehorses. In contrast, STB racehorses more often experience lateral and medial
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PART VIII The Soft Tissues
border tendon injuries that result in substantial swelling but less severely injured tendon fascicles. Identification of subtle, yet important, reinjury by clinical evaluation may be difficult because of previous tendon thickening. Thus ultrasonographic imaging is essential to confirm clinical signs and to evaluate the extent of injury to the SDFT.
Lameness
Pain on direct compression of the SDFT and mild swelling are the earliest signs of SDF tendonitis. Lameness is often not present at first, a characteristic that often delays veterinary intervention. Because horses are not often overtly lame in the early stages of tendonitis, and subtle pain and swelling either are missed or respond to symptomatic therapy, lesions can progress in severity. The degree of lameness associated with a tendon lesion in the midmetacarpal region is usually correlated with the severity of the injury. Slight to high-slight (category II to IV) injuries generally are not associated with any appreciable lameness, whereas mid-moderate to high-moderate (category V) injuries cause only transient lameness. Most severely injured tendons (category VI), or a total rupture of the SDFT, result in at least transient lameness, which may be severe. In contrast, lesions in the carpal canal or proximal metacarpal region (zones 0 and lA) consistently are associated with pronounced lameness.
Swelling
For assessing tendon injuries, swelling is defined as subcutaneous or peritendonous fluid accumulation. Digital palpation reveals a soft or semifirm, diffuse or focal fluid accumulation that may prohibit exact palpation of the SDFT. Subcutaneous swelling can be associated with tendon injury, especially in the acute stage of injury. Careful digital palpation of the limb held in a semiflexed position may reveal slight crepitus in an acutely injured tendon. However, subcutaneous inflammation or hemorrhage is not associated invariably with tendon injury. Examples of focal edema or hemorrhage without substantial SDFT injury include swelling associated with cording of the midmetacarpal region secondary to a malpositioned bandage or subcutaneous swelling in the proximal or distal metacarpal region caused by malpositioned tendon boots or stable (stall) bandages. An example of diffuse swelling is pitting edema (see Chapter 14), occasionally caused by external blistering. Diffuse filling also may reflect a subsolar abscess or cellulitis.
Thickening
Thickening or enlargement specifically indicates SDFT swelling secondary to injury or a thickened end-stage repair from a previous injury. In this context, subcutaneous swelling is not appreciable, but a palpable enlargement of the SDFT occurs. In many slightly and diffusely injured tendons, thickening may be difficult to appreciate, and careful comparison with the normal contralateral limb may be required to identify SDFT enlargement. In more severely injured tendons examined in the subacute phase, enlargement generally can be felt. Two clinical situations in which assessing SDFT thickening is difficult are a focal SDFT injury in the subcarpal region, where the tendon is enveloped by the retinaculum, especially in colder climates when limb hair is long, and instances of SDFT injury within
the DFTS that also is associated with tenosynovitis. Tenosynovitis makes distinguishing between tendon thickening and tendon sheath fibrosis difficult. When palpating the SDFT, one should always determine if the medial and lateral borders of the SDFT can be separated clearly from the accessory ligament of the deep digital flexor tendon (DDFT) and the tendon itself. If both digital flexor tendons are slightly enlarged, detection of abnormality is more difficult; however, frequently the margins of both tendons are more rounded than normal. Assessing the flexibility of the SDFT also is useful, because abnormal stiffness usually reflects previous injury.
Heat
Increase in surface temperature often can be the earliest and most subtle clinical sign of SDFT injury or reinjury. Digital appreciation of an increase in skin temperature, or thermography, often can indicate tendon inflammation. Because extensive use of liniments and daily bandaging in racehorses also increases skin temperature, one must be careful when making this assessment.
Sensitivity to Direct Digital Palpation
A painful response to direct digital palpation is often a reliable clinical test for tendon injury and may be the earliest clinical sign detectable (see Figure 6-15). Examination is best performed by holding the limb in a semiflexed position and palpating with the thumb and forefinger systematically from proximal to distal in the metacarpal region in an effort to elicit a painful response. When a sensitive area is palpated, the horse generally flinches. The examination has many caveats. If a sensitive response is elicited bilaterally, the horse may merely be hyperresponding to increased pressure and possibly has no injury. Not all horses with tendon injury have a painful response. Horses with blistering of the skin, adverse local drug reaction, infection, or cording are also hyperresponsive and more reactive than those with a tendon injury. In addition, extreme sensitivity to direct palpation coupled with focal or diffuse swelling may indicate a problem not related to the tendon.
Tendon Profile
Evaluation of the tendon profile with the limb in a full weight-bearing position can provide valuable information. In a normal limb the metacarpal region has a straight palmar profile. A normal SDFT should be superficial and parallel to the DDFT. It is important to examine the profile from all possible angles. With a slight injury, the tendon often has a normal profile when viewed from the lateral aspect and a convex or bowed profile from the medial aspect, or vice versa. In fact, slight changes on tendon profile are often most obvious when examining the horse visually from the opposite side (Figure 69-1). It takes considerable damage to the SDFT to change the visually detected profile of the tendon, and ultrasonographic evidence of tendon injury is often much more pronounced than expected. In a horse with an acute total rupture, little swelling and thickening may be present if the leg is examined within 2 hours of the injury. However, with the limb in full weight-bearing position, one may note hyperextension of the metacarpophalangeal joint. In this case, digital palpation along the palmar aspect of the tendon reveals a
Chapter 69 Superficial Digital Flexor Tendonitis
A
709
B Fig. 69-1 • A, Photograph of the right front (RF) metacarpal region taken from the left side showing swelling of the superficial digital flexor tendon (SDFT) in the midmetacarpal region (arrow). Often the change in profile of the SDFT is best seen from this perspective (the left forelimb can be seen in the foreground). B, Transverse (left) and longitudinal ultrasonographic images of the RF SDFT in this 3-year-old Thoroughbred gelding racehorse with substantial injury of the SDFT. A central core lesion (double-headed arrow) and enlargement of the entire SDFT can be seen. Ultrasonographic evidence of tendonitis is often much more extensive than is realized during visual inspection of the limb, a fact that may delay veterinary intervention. This horse underwent successful surgical management using desmotomy of the accessory ligament of the SDFT (superior check desmotomy) and desmoplasty (tendon splitting).
1- to 2-cm defect in the SDFT. Digital palpation with the limb in a semiflexed position also reveals laxity and excessive mobility of the tendon.
Swelling in the Distal Metacarpal Region
In horses with chronic tendonitis of the SDFT or in those with only tendonitis of the SDFT in the distal metacarpal region, there can be involvement of the DFTS and the palmar annular ligament (PAL). Chronic, distal metacarpal tendonitis of the SDFT in the region of the PAL is common in STB racehorses, polo ponies, and older TB racehorses (Figure 69-2). It is important to differentiate clinical syndromes in this region. In horses with chronic tendonitis of the SDFT, the primary lesion is the tendon injury with subsequent restriction of movement through the “fetlock canal” (reduced gliding function) by the PAL. Thus the PAL is not primarily involved but is merely a “passenger” in the clinical syndrome. In these horses palmar annular desmotomy is critical to restore gliding function and to decompress the swollen SDFT, but there is no actual palmar annular desmitis. Primary palmar annular desmitis, tenosynovitis of the DFTS, and deep digital flexor (DDF) tendonitis are other clinical syndromes that cause swelling in the distal, palmar metacarpal region and should be differentiated from distal SDFT lesions and compression by the PAL (see later discussion and Chapters 70 and 74).
Tenosynovitis of the Carpal Sheath or Digital Flexor Tendon Sheath
Tenosynovitis may be associated with a tendon injury or may be a clinical entity without tendon injury (see
Fig. 69-2 • Photograph of the left metacarpal region of an aged Standardbred gelding racehorse with chronic, severe tendonitis of the left front superficial digital flexor tendon (SDFT). Notice that the thickened SDFT is compressed by the palmar annular ligament (PAL) (arrows) in the palmar fetlock region. The PAL is a secondary problem causing restriction of gliding function of the injured SDFT. Palmar annular desmotomy in combination with desmotomy of the accessory ligament of the SDFT was performed successfully. Previous cryotherapy for suspensory desmitis was performed (white hairs).
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PART VIII The Soft Tissues
Chapters 74 and 75). Ultrasonographic evaluation is required to appreciate tendon injury in the presence of tenosynovitis.
Tendon Injury Limited to the Pastern
Injury to one or both of the SDFT branches of the pastern is generally but not always associated with branch thickening and a painful response to direct digital pressure (see also Chapter 82). This is best appreciated with the limb held in a semiflexed position and direct pressure placed on the branch with the clinician’s thumb. Injury to the SDFT branch(es) may be associated with tenosynovitis. Injury to the branches of the SDFT should be carefully differentiated from injuries of the distal sesamoidean ligaments.
MANAGEMENT OF THE ACUTE PHASE OF TENDON INJURY IN RACEHORSES In most horses with subtotal SDFT injuries in the acute phase, antiinflammatory and supportive management is instituted. A variety of treatment regimens are available. For the most part, systemic nonsteroidal antiinflammatory drugs (NSAIDs) such as phenylbutazone (4.4 mg/kg/day) for 7 to 10 days and a single dose of systemic corticosteroids such as dexamethasone (0.04 mg/kg) are included in the initial therapy. Perilesionally administered corticosteroids are considered contraindicated in horses with tendon injuries, especially long-term use, because these drugs are thought to delay collagen formation. However, some clinicians use a single perilesional dose of triamcinolone aceto nide (6 to 9 mg) or methylprednisolone acetate (40 mg) (dystrophic mineralization occasionally has been associated with methylprednisolone therapy) in horses with slight, peripheral tendon injuries in STB racehorses, especially when associated with curb (see Chapter 78). Practitioners often administer a course of polysulfated glycosaminoglycans (PSGAGs; 1 vial per week for 4 weeks) in the acute stage. Physical therapy is indicated, and we recommend icing for 1 to 2 hours once or twice a day, with application of a poultice, or simple support bandaging for horses with subtotal injuries. Casting may be indicated for horses with rupture of the SDFT. Exercise generally is restricted to stall (box) rest or limited hand walking. In horses with subtotal injuries, we prefer to keep the injured limb shod in a fully grooved bar shoe, with a straight hoof-pastern axis, so that the metacarpophalangeal joint position is normal in a standing position. We do not advocate raising or lowering the heel to an exaggerated position. We perform an ultrasonographic examination within the first few days and then again 3 to 4 weeks later. Sometimes tendon splitting is advised for horses with small core lesions.
INJURY ASSESSMENT AND GOALS FOR AN ATHLETIC OUTCOME Qualitative assessment combines the physical findings and a subjective ultrasonographic appraisal. This gives an accurate diagnosis, but we also strongly encourage the use of quantitative ultrasonographic evaluation. This includes data such as CSA and echogenicity and fiber alignment scores (see Chapter 16). Optimal healing of SDFT injuries depends on managing a number of variables, including the personality of the
horse; its age, sex, athletic use, conformation, injury episode number, and maximal level of exercise attained; and the severity of the injury. For instance, a 4-year-old TB gelding racehorse that has an upright conformation, has never raced, and experiences a severe (category VI) SDFT injury after one gallop has a poor prognosis with any treatment. If this horse were to sustain a small core lesion of the SDFT, its prognosis would be more guarded because of its conformation and the exercise level at which injury occurred compared with a similar injury in a wellconformed, seasoned racehorse. Athletic outcome of a racehorse may be divided into three categories: successful, meaning completion of five or more races (I); partially successful, completion of one to four races (II); and failure, meaning reinjury occurred before the first race was completed (III). Horses with successful outcomes can be further subdivided into those with reinjury and those with no reinjury. Horses with partially successful outcomes can be further subdivided into those sustaining reinjury and those injuring the contralateral SDFT or a forelimb suspensory ligament.
ULTRASONOGRAPHIC EVALUATION AND CATEGORIZATION OF INJURIES If clinical evaluation indicates a possible SDFT injury, ultrasonography should be used to confirm the diagnosis and objectively assess the severity of injury. Sequential examinations provide a guide to controlled exercise management and are used to assess progress of repair and attempt to establish an optimal time to return to full work. The acquisition and assessment of accurate ultrasonographic data require high-quality images, and it is important to develop a rigid, standardized technique (see Chapter 16). The ultrasonographer must take primary responsibility for image interpretation. For a second person to give an opinion on images previously obtained by someone else is often difficult. Quantitative ultrasonographic data include CSA, percentage of CSA occupied by a lesion, grade of echogenicity of a lesion (type or echo score [TS]), and assessment of fiber bundle alignment in longitudinal images (fiber alignment score [FAS]). Each of these data points is assessed at every defined level of the limb (zone), and they are then summed to provide total scores. These scores then can be used to categorize an injury as minimal (category III), slight (category IV), moderate (category V), or severe (category VI) (see Chapter 16). The following comments apply to forelimb and hindlimb injuries, although reference is made to only the metacarpal region.
Initial Evaluation
Early examination of a suspected new injury of the SDFT may not reveal any anechoic or hypoechoic lesions. However, if the CSA of a single zone is more than 39% larger than in the contralateral limb, or if the total of six of seven zones is more than 14% larger than in the contralateral limb, then tendonitis should be suspected (category II). If tendonitis cannot be substantiated by ultrasonography, despite soft tissue swelling, then conservative management is indicated. The horse should be restricted to walking exercise for at least 72 hours and then reevaluated clinically
Chapter 69 Superficial Digital Flexor Tendonitis
and ultrasonographically. Symptomatic therapy includes systemic NSAIDs, daily icing, and mild leg liniments or sweats with limb bandaging. One must recognize that sometimes a lesion(s) cannot be identified ultrasonographically for at least several days or longer. Within 7 days of injury a hypoechoic or anechoic lesion may represent tendon fascicle damage, hemorrhage, or inflammatory exudates and is most likely a combination of all three. Distinguishing the relative contributions of each or determining accurately the severity of injury, which may be underestimated or overestimated, is not possible. Initially, damaged collagen fibers may be grossly intact but nonfunctional, resulting in reflecting echoes. Ongoing enzymatic degradation and further injury caused by pressure necrosis may result in a lesion deteriorating over 3 to 4 weeks. In contrast, infrequently the hemorrhage and inflammatory exudates resolve over the following 3 to 4 weeks and result in a great improvement of the lesion, and the injury to fiber bundles may not be as serious as initially indicated.
Baseline Evaluation
If an initial examination is done within 1 to 7 days of a new injury or a reinjury, we strongly advise that a second ultrasonographic evaluation be performed 2 to 4 weeks (preferably 4 weeks) later to obtain baseline data about the severity of injury (see Chapter 16).
SUBACUTE PHASE TREATMENT AND LONG-TERM REHABILITATION History of Treatment in Racehorses
A wide variety of short-term and long-term treatment programs for SDF tendonitis are used, sometimes implemented regardless of the severity of the injury or reinjury, and with or without prolonged rest or controlled exercise. No comprehensive reports comparing various treatments of similarly injured tendons are available, and therefore proposing specific recommendations that will give the best prognosis for return to racing is difficult. A comprehensive retrospective study of TB and STB racehorses currently is being performed to compare the rate of return to racing among a variety of therapeutic regimens for minimal (category III), slight (category IV), moderate (category V), and severe (category VI) tendon injuries documented by ultrasonography. Therapies include pasture turnout, external blistering, internal blistering, intralesional therapies (excluding β-aminopropionitrile fumarate [Bapten]), or a combination of these. In addition, the amount of layup time is being considered for each category of injury. If all therapeutic regimens are combined (Figure 69-3), preliminary data indicate that few racehorses successfully return to racing without reinjury (athletic outcome I, completed five or more races), especially with severe injuries.14 In addition, these data demonstrate that few racehorses are able to return to racing and not experience reinjury of the SDFT, sustain injury to the contralateral SDFT, or injure the suspensory apparatus (subcategories IB and IC). Ultimately, we hope that this research helps to determine optimum therapy for specific lesions and better equip the veterinarian to provide accurate prognostic information.
711 Injury Category
80 Percentage of tendons in each injury category
70
IV (n = 213)
60 50
V (n = 97) VI (n = 141)
40 30 20 10 0
IA
II IB+C One race Athletic outcome category
III
Fig. 69-3 • A successful return to racing becomes increasingly difficult as the severity of tendon injury increases. Treatments for slight (category IV), moderate (category V), and severe (category VI) superficial digital flexor tendon injuries including pasture turnout, external or internal blistering, intralesional injections (excluding β-aminopropionitrile fumarate), or a combination of these were assessed for their success in return to racing. Athletic outcome categories (AOCs) were separated into five groups. AOC IA refers to those horses that have successfully raced at least five times with no reinjury. AOC IB + C includes those that have raced at least five times but have reinjured the same tendon (group B) or the contralateral tendon (group C). AOC II includes those that have raced successfully one to four times and includes tendons that have been reinjured, whereas horses with tendon injuries in AOC III have never raced successfully. An additional group of all those that completed at least one race was included. Data are represented as the percentage of tendons in each injury category that resulted in the ultimate AOC.
Symptomatic Treatment with Continued Exercise If the decision is made to treat a horse with an injured tendon symptomatically and continue exercise (racing), serial ultrasonographic examinations should be performed, because changes in size or echogenicity may be detected before clinical signs are obvious. If the ultrasonographic examination demonstrates progressive injury or instability and the horse is not athletically (economically) productive, stopping racing and considering long-term rehabilitation would be wise. Naturally, any racehorse racing with an injured tendon is risking more serious injury. A retrospective study of 209 tendons from 207 TB racehorses was performed to determine if quantitative ultrasonographic assessment could aid in accurately defining an SDFT injury and provide evidence for determining a prognosis for racing in horses that underwent symptomatic treatment with continued exercise. Eighty-eight percent of horses that had no or minimal injury (categories I and III) started more than three races, whereas 12% failed. Thirtyfive percent of horses with slight (IV), moderate (V), or severe (VI) injuries started more than three races, but 82% had recurrent injury.10 Ultrasonography is therefore helpful to determine the prognosis for returning to racing. Symptomatic therapy while continuing to race is a viable therapeutic option for minimal tendon injuries. Consider a 3-year-old TB gelding racehorse with a swollen left front SDFT after a race (Figure 69-4). Quantitative ultrasonographic analysis revealed a total lesion area of 13%, TS of 7, and total FAS of 7, indicating a mildly injured (category IV) tendon. The horse was treated symptomatically with antiinflammatory medication and continued to race. After 4 1 2 months and six races the horse was racing well, having earned more than $34,000. After two additional races, the total lesional area increased and the
PART VIII The Soft Tissues
712
Zone
Structure Size (mm 2 )
Lesion Size (mm 2 )
Fiber Alignment Score
Type/Echo Score
1A
87
—
—
—
Lameness: 0/5
1B
82
11 (14.25%)
—
1
Swelling: 2/5
2A
88
5 (6.18%)
—
1
Thickening: 1/5
2B
108
16 (14.84%)
1
1
Sensitivity: 2/5
3A
139
14 (10.47%)
3
2
Heat: 2/5
3B
123
36 (29.19%)
3
2
Tendon sheath involvement: 1/5
Clinical Findings
Fetlock sinking: 0/5 Totals
627
82 (13%)
7
7
Fetlock flexion: Negligible
Bold type indicates maximal injury zone. Fig. 69-4 • Data derived from an initial computerized scan of a 3-year-old Thoroughbred racehorse actively racing with a category IV injury (<15% tendon lesion). The type (or echo) score and fiber alignment score are similar, indicating a new lesion. Zone 3B was determined to be the maximal injury zone, with a 29% increase in size compared with the contralateral limb.
Zone
Structure Size (mm 2 )
Lesion Size (mm 2 )
Fiber Alignment Score
Type/Echo Score
1A
109
—
—
—
Lameness: 0/5
1B
95
—
—
—
Swelling: 1/5 (zone 3C)
2A
122
—
—
—
Thickening: 2/5
2B
157
11 (7.38%)
1
2
Sensitivity: 0/5
3A
136
14 (9.59%)
1
1
Heat: 1/5
3B
178
28 (15.68%)*
2
1
Tendon sheath involvement: 0/5
3C
164
23 (13.90%)*
3
3
Fetlock sinking: 0/5
Totals
961
76 (8%)
7
7
Fetlock flexion: Negative or no abnormality detected
Clinical Findings
Bold type indicates maximal injury zone. *Evidence of focal instability of echogenicity near the metacarpophalangeal joint. Fig. 69-5 • Data derived from a computerized scan of a right forelimb superficial digital flexor tendon injury 11 12 months after the baseline scan and 6 weeks after galloping was resumed. Before galloping, the total cross-sectional area was 975 mm2, which decreased to 961 mm2. However, new hypoechogenic lesions were documented in zones 3B and 3C, indicating an unstable healing process.
horse’s performance decreased. Long-term therapy with time off was instituted. STB racehorses are generally more successful than TBs in continued performance with a tendon injury.14 A retrospective study of pretraining (exercise level 5) ultrasonographic data from 106 injured racehorses provided four criteria that we use as a guideline for an optimum return to racing15: 1. At least a 60% decrease in category IV total lesional area, or fewer than 12% total hypoechogenic fiber bundles for all categories of severity of injury 2. At least a 10% to 15% decrease in total CSA from baseline for all categories of injury (a relatively greater decrease in more seriously injured tendons) 3. At least a 70% decrease in the total TS (ideally <4; the closer to zero, the better)
4. At least a 75% decrease in the total FAS (ideally <4; the closer to zero, the better) before advancement to exercise level 5 The horse should meet at least three of the four criteria, with only minimal failure of the other criterion. Horses that met all criteria or failed only one had a 50% chance to be successful as defined by completing five races. If a horse failed more than one criterion, the chance of reinjury before racing one race was 85%. All horses (100%) that failed all criteria had reinjury. Ultrasonographic evaluation is used to monitor tendon stability during training. For example, ultrasonographic examination of a right front SDFT 111 2 months after the baseline scan and after 6 weeks of galloping indicated stable total CSA values but increased hypoechogenic tendon fascicles in zones 3B and 3C (Figure 69-5). Clinically, increased
Chapter 69 Superficial Digital Flexor Tendonitis
Zone
Structure Size (mm 2 )
Lesion Size (mm 2 )
Fiber Alignment Score
Type/Echo Score
1A
88
—
—
—
Lameness: 0/5
1B
116
—
—
—
Swelling: 0/5
2A
114
9 (7.71%)
1
1
Thickening: 2/5
2B
151
17 (11.27%)
1
1
Sensitivity: 0/5
3A
160
16 (10.40%)
1
1
Heat: 0/5
3B
202
53 (26.37%)*
3
3
Tendon sheath involvement: 0/5
3C
191
27 (14.06%)*
3
3
Fetlock sinking: 0/5
Totals
1022
122 (12%)
9
9
Fetlock flexion: Negative or no abnormality detected
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Clinical Findings
Bold type indicates maximal injury zone. *Evidence of focal instability of echogenicity near the metacarpophalangeal joint. Fig. 69-6 • Data derived from a computerized scan of the same limb as in Figure 69-5. After 16 days of continued exercise, clinically significant reinjury is documented by an increase in total cross-sectional area to 1022 mm2, worsening type score and fiber alignment score in zones 3B and 3C, and an increase in the total percentage hypoechogenic volume of the injury. These findings illustrate the importance of quantitative analysis in monitoring tendon healing and predicting responses to an aggressive treatment program.
heat and swelling in the distal metacarpal region were found, which indicated tendon instability at the current exercise level and a high risk of reinjury with continued training. The trainer was unwilling for economic reasons to pursue another long-term treatment program and decided on an intermediate program of 30 days of ponying (leading the horse from another horse) and swimming. Six weeks later substantial reinjury of the SDFT occurred in the distal metacarpal region (Figure 69-6). Use of serial ultrasonographic monitoring is discussed in detail elsewhere (see Chapter 16).
Common Long-Term Treatment Programs Controlled Exercise and Time Out of Training
A graded exercise program based on severity of the injury and the ultrasonographic progress during rehabilitation has been used.16 Turnout in a large paddock was not permitted. The time out of training ranged from 9 to 12 months, depending on the initial severity of the injury and ultrasonographic progress. Twenty-eight TB racehorses were managed in this manner, and 20 (71%) of the 28 returned to racing. Only two (25%) of eight horses treated with time off and large pasture exercise had a successful outcome. The difference between horses managed in an uncontrolled fashion, using unlimited exercise while being turned out, and horses managed in a vigorously controlled exercise program is hugely important.16 One of us (MWR) feels strongly that “turnout is the antithesis of healing,” and the results of the study by Gillis emphasize the point. Currently no consensus exists on the treatment of choice for SDF tendonitis in racehorses, but most treatment regimens are combined with a controlled exercise program and serial ultrasonographic assessment. One of us (MWR) feels strongly that surgical management is important. Most horses with category II (tendonitis without lesions perceptible by ultrasonography) or III (total echogenicity score less than 3) lesions have a reasonably favorable prognosis
for successful return to racing (defined as five or more races [athletic outcome I]) if they are confined to controlled exercise for sufficient time.
External and Internal Blistering, Pin Firing, and Time Off for Long-Term Rehabilitation
One of the most common therapeutic regimens in the United States for rehabilitating racehorses with SDFT injuries includes some sort of counterirritation (see Chapter 88), plus controlled exercise or turnout into a large pasture. Many counterirritation options exist, including external blistering agents such as a variety of iodine-based liniments, internal peritendonous injection of 2% iodine in almond oil (internal blister), and pin firing. Several reasons exist for the persistent use of these treatment regimens, in spite of evidence suggesting that turnout exercise is contraindicated and the limited research suggesting that external blistering and especially pin firing have no beneficial effect on tendon injury repair.17 No alternative therapeutic regimens result in a consistent return to racing without reinjury or injury to another soft tissue structure. Tradition is strong, and most newer surgical and medical treatment regimens are expensive, even though the prognosis remains guarded. Counterirritation promotes angiogenesis, and, when used, exercise is restricted. In a retrospective study of 54 TB and STB racehorses treated with pin firing or external blistering and given more than 6 months out of training, 23 (43%) returned to race at least once.18 Ten (19%) of these horses returned to racing without experiencing reinjury to the tendon. These data indicate that racehorses treated by counterirritation can return successfully to racing, and so counterirritation will continue to be used until alternative methods prove to have significant and consistent improved athletic outcomes. If counterirritation is the treatment option selected by the trainer or owner, athletic outcomes may be improved if treatment is combined with a controlled exercise program and serial
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ultrasonographic examinations. We recommend reexamination every 2 months to evaluate the quality of repair and to determine if the current exercise program is excessive for any given stage of healing.
Intralesional β-Aminopropionitrile Fumarate and Controlled Exercise
The following information regarding β-aminopropionitrile fumarate is included as it appeared in the first edition of this textbook, because it is historically important and lessons learned from the long-term study of the effects of the chemical are important. During study of the effects of the chemical, work by Genovese, Reef, and others demonstrated the value of serial ultrasonographic examinations and fiber score assessment in horses being managed in the various treatment protocols. They and others, including Dyson (see discussion of SDF tendonitis in nonracehorses, later) believed strongly in the value of the chemical in combination with strict rehabilitation. Currently there is no commercially available source to acquire the chemical, and use of it has largely ceased in North America (MWR). β-Aminopropionitrile fumarate is a toxic substance found in the seeds of the plant Lathyrus odoratus (sweet pea). If β-aminopropionitrile fumarate is injected into an injured tendon 30 to 90 days after injury, it binds to the enzyme lysyl oxidase and inhibits the deamination of lysine. This temporarily blocks cross-linking between collagen fibers and improves the quality of repair, if combined with controlled exercise, to generate piezoelectric forces and encourage axial alignment of the repairing collagen fibers. In a naturally healing tendon the collagen fibers may be aligned randomly, whereas treatment with β-aminopropionitrile fumarate may encourage parallel alignment of the fibers, resulting in a stronger, more physiological repair. Treatment with β-aminopropionitrile fumarate combined with a controlled exercise program has resulted in superior healing, with more type I collagen fibers and improved longitudinal fiber alignment than in horses treated without exercise.19 β-Aminopropionitrile fumarate does not increase tenocyte formation and only affects the orientation of the scar and does not hasten repair. Theoretically intralesionally administered β-aminopropionitrile fumarate is indicated for horses with moderate to severe injuries, when a large volume of the tendon will be repaired by scarring. β-Aminopropionitrile fumarate is administered 30 to 90 days after injury and is used at a rate of 1 mL (0.7 mg) per 3% total lesional area, up to a maximum of 10 mL (7 mg). Pretreatment quantitative ultrasonographic evaluation is important for establishing the anatomical extent of the lesion to be treated and the dose required. Several clinical trials with β-aminopropionitrile fumarate have been performed. Genovese reported an improvement in quantitative ultrasonographic morphology.20 In a study in the United Kingdom, six of seven TB flat racehorses failed to complete one race,3 but a trial in the United States determined that 50% of treated horses returned to racing and completed at least five races.21 Clinical trials performed in Ohio compared the results of β-aminopropionitrile fumarate treatment (75 horses) with use of a placebo (10 horses). All horses followed a similar exercise program. In phase 1, horses were out of training for less than 6 months, but the rate of reinjury was high. In phase 2, convalescence
Categories IV, V, and VI Phase 1 Bapten (21)
Phase 2 Bapten (55)
Placebo (1)
Placebo (9)
Categories V and VI Phase 1 Bapten (16)
Phase 2 Bapten (34)
Placebo (1)
Placebo (6)
Success Partial success Failure (No. of tendons in group)
Fig. 69-7 • Superficial digital flexor tendon injuries treated with β-aminopropionitrile fumarate were compared with placebo groups in two phases of the study. Phase 1 required less than 6 months of layup time, whereas phase 2 studies increased the convalescent time to at least 6 months for category IV injuries, 8 months for category V, and 10 months for category VI injuries. The percent of the total number of tendons was divided into three groups: success (raced at least five times after injury), partial success (raced one to four times), and failure (never raced). In phase 1, injuries including mild (category IV), moderate (category V), and severe (category VI) were grouped together to determine outcome; in phase 2 only moderate and severe categories were grouped.
was increased, based on the ultrasonographic grading of the severity of the injury. Horses with a category IV lesion had at least 6 months out of training; this was extended to 8 months for horses with category V lesions and 10 months for those with category VI injuries. In phase 2 the success rate increased (Figure 69-7). Thirty-seven percent of treated horses with moderate (category V) or severe (category VI) injuries returned to racing without recurrence of injury; 57% failed to race once. None of the placebo group returned to racing without recurrent injury, and 71% failed to race once. Although slight (category IV) and moderate (category V) tendon lesions may appear healed clinically and by ultrasonography (with qualitative and quantitative assessment) 4 to 5 months after treatment, the strength of repair is inadequate to permit return to full training, and further time is essential for collagen maturation and remodeling. Improvements in management have resulted in increased success with the use of β-aminopropionitrile fumarate. Racehorses with career-ending category VI injuries also have been rehabilitated for low-level dressage or eventing, or as show hunters or show jumpers.
Intralesional or Perilesional Administration of Hyaluronan
Hyaluronan is a component of the tendon matrix, directly influences collagen fibril formation and aggregation, and stimulates fibrillogenesis of type 1 collagen. Hyaluronan may decrease adhesion formation during tendon repair. In a clinical field trial, treatment of horses with acute anechoic tendon lesions with a single intralesional injection of highmolecular-weight hyaluronan (Hylartin-V) was compared
Chapter 69 Superficial Digital Flexor Tendonitis
with use of a saline placebo. Ultrasonography showed that 60% of lesions resolved in the hyaluronan group compared with 24% resolved in the placebo-treated group, but athletic outcome results were not reported.22 A separate longterm study comparing reinjury rate at a minimum follow-up of 2 years after treatment with intralesional high-molecularweight hyaluronan combined with a controlled exercise program or controlled exercise alone found no significant difference in recurrence of injury. Neither study reported any adverse reaction to intralesionally administered highmolecular-weight hyaluronan.22,23
Corticosteroids
Repeated treatments with corticosteroids may impair tendon healing, but many veterinarians have anecdotally reported that low doses of corticosteroids combined with hyaluronan can be used successfully to manage category III or IV peripheral SDFT lesions in STB racehorses. This practice seldom is used in TB racehorses because of the concern of acute breakdown, an uncommon occurrence in the STB racehorse. A single perilesional injection of 6 to 9 mg triamcinolone acetonide with 10 to 20 mg hyaluronan is suggested. Methylprednisolone acetate is not recommended, because it may be associated with development of dystrophic mineralization. It is important to note that continued exercise during this treatment may lead to more serious injury. Therefore close clinical and ultrasonographic monitoring, modification of training schedules (downscaled), and spaced, selected races for optimal results are recommended.
Intralesional and Systemic Administration of Polysulfated Glycosaminoglycans
PSGAGs are reported to inhibit macrophage activation and collagenase and metalloproteinase activity and therefore may be useful in the acute stage of tendon injury or reinjury.2 The suggestion has been made that PSGAG may stimulate tenocyte repair. However, in a long-term clinical study, no significant difference was found in the recurrent injury rate of horses with tendons treated with systemic or intralesionally administered PSGAGs compared with controlled exercise alone.23,24 However, because tendon injuries are potentially career threatening, one author (RLG) often recommends weekly systemic administration of PSGAGs for 4 to 6 weeks for horses with acute injuries of less than 8 weeks’ duration. This is combined with other long-term rehabilitation procedures.
Physical Therapies
Physical therapeutic approaches for long-term rehabilitation of a tendon injury, such as therapeutic ultrasound, low-frequency laser, extracorporeal shock wave therapy, cryotherapy, and electromagnetic field therapy frequently are used by owners, but no clinical studies document their therapeutic value over conservative, controlled exercise management.
Other Proposed Long-Term Treatments
See Chapter 73 for a complete discussion of alternative treatments including MSCs, growth factors, and other methods to stimulate regenerative or reparative healing. See the following discussion regarding the surgical management of SDF tendonitis.
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SURGICAL MANAGEMENT OF SUPERFICIAL DIGITAL FLEXOR TENDONITIS Mike W. Ross Surgical management remains a most useful and viable alternative to all other methods currently employed to augment reparative and regenerative healing of an injured SDFT. Time-honored principles such as decompression, improved gliding function, reduced load, and protection of inelastic scar formation are achieved with surgical management. Postoperative exercise and rehabilitation must be vigorously controlled and are optimized if a philosophy of restricted, controlled exercise without uncontrolled, unlimited turnout exercise is adopted.
TRANSECTION OF THE ACCESSORY LIGAMENT OF THE SUPERFICIAL DIGITAL FLEXOR TENDON Since Bramlage first described transection of the ALSDFT, also known as superior or proximal check desmotomy, as a novel surgical treatment for tendonitis of the SDFT, there has been controversy regarding efficacy of the procedure.1-6 Early, optimistic results were reported in TB racehorses and included 32 of 36 horses (89%) that returned to racing and 25 horses that competed at a level equal to or above the preinjury level.3 The results of that study were criticized because criteria for success, defined as completing two races and starting a third, were lenient, and horses developing contralateral limb lameness, including tendonitis, were excluded. Results from a larger group of TB racehorses, using a more strict definition of success, revealed that 97 of 137 horses raced (71%) and 70 horses (51%) made more than five starts after surgery, but average earnings decreased in 58% of horses. In that study the mean time from surgery to first start was 353 days.4 The proposal was that the ALSDFT heals after transection but in an elongated fashion, causing an increased length of the bone-ligament-tendonbone (radius-ALSDFT-SDFT–proximal and middle phalanges) construct.5 In a smaller, separate study evaluating the long-term effects of transection of the ALSDFT and other treatments, 53% of flat racehorses, 58% of steeplechasers, and 73% of hurdlers competed in five or more races after surgery.6 Clearly, with more strict definitions of success, earlier results have been downgraded but do appear superior to results from unmonitored conservative therapy alone. For instance, one estimate claims that only 20% of TB racehorses will complete three or more races after injury.7 In a separate study of TB horses treated without surgery, 52% returned to racing, but 48% had recurrent tendonitis.8 More recently, 20 of 28 (71%) TB racehorses were managed successfully nonsurgically with careful rehabilitation (controlled exercise, no turnout), serial ultrasonographic examinations, and a minimum of 8 to 9 months of rest.9 Clearly, controlled exercise and careful clinical and ultrasonographic examinations are important components of any rehabilitation program. In contrast to other reported results, another study found no difference in prognosis in TB racehorses treated with transection of the ALSDFT compared with conservative management and in fact found horses that underwent surgery to be at risk to develop suspensory desmitis.10
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In the STB racehorse, results after transection of the ALSDFT are clearly superior to those achieved in the TB racehorse and published results in horses receiving only conservative management. In our study, 35 of 38 horses raced after surgery (92%), and 33 horses (87%) started more than five races, but tendonitis recurred in six horses.11 Using a strict definition of success, 71% of horses started five or more times after surgery without recurrence of tendonitis, median earnings per start decreased significantly, and mean time from surgery to first start was 237 days. Suspensory desmitis developed in five horses, all of which had bilateral transection of the ALSDFT.11 In a similar study, 82% of STB horses raced after transection of the ALSDFT, and 69% competed in five or more starts. In that study, horses that raced before injury had a better prognosis.5 Published results of conservative management in STB racehorses are scant, but in one retrospective study, 31 (76%) of 41 STB racehorses completed two races and started a third, but tendonitis recurred in 43% of these horses.12 In summary, transection of the ALSDFT is clearly beneficial in the STB racehorse. Results of transection of the ALSDFT in other types of sports horses have not been published, but I suspect hunters, jumpers, event, and dressage horses have a prognosis somewhere between those published for TB and STB racehorses. Unpublished results in the Three Day Event horse suggest that transection of the ALSDFT may be of limited value, although reasons for this are not known.13 In a study of 33 horses in which 22 were nonracehorses, tendon splitting under ultrasonographic guidance, combined in some horses with transection of the ALSDFT, resulted in a 68% return to previous level of competition.14 In the TB racehorse, results of transection of the ALSDFT may be superior to those achieved with intratendonous injections and uncontrolled rehabilitation but are not as favorable as conservative management with controlled exercise.
Surgical Procedure
Currently I prefer conventional surgery using a transthecal approach through the flexor carpi radialis tendon sheath.3 Compared with the original description of transection of the ALSDFT, this modified approach allows the surgeon to close the incision in three layers, providing a much more secure closure. Transection of the ALSDFT is performed in a caudal location where the ligament is well defined. The surgical procedure is performed using a medial approach, with the horse in lateral recumbency (repositioned if bilateral transection of the ALSDFT is performed) without tourniquet application. The initial incision is made directly over or just cranial to the cephalic vein. The vein is carefully dissected free from underlying antebrachial fascia and retracted caudally. The cranial approach to the vein is less vascular than is the caudal approach, and in most horses a vein penetrating the antebrachial fascia is clamped and ligated. It is important to sever the ALSDFT completely, because incomplete division does not allow immediate transfer of load to the muscle. Intuitively, with incomplete division, it is likely that the ALSDFT would heal faster after surgery and there is less likelihood that the ligament would heal in an elongated fashion. To sever the ligament completely, careful dissection of the proximal fibers from the nutrient artery and vein often is necessary, and in some horses these structures are cut inadvertently or division is
necessary. Division of the vessels causes no known clinical problem, but the vessel ends should be ligated to prevent edema and seroma formation. Often the proximal aspect of the carpal canal is penetrated, because the ALSDFT is attached to this structure distally. In horses undergoing transection of the ALSDFT an occasional complication is inadvertent transection of the nutrient artery. Ligation of the vessel ends has no adverse clinical consequence. Complications with the surgical procedure are unusual, but the clinician should be aware of what normal healing of the surgical site entails. Nearly all sites develop diffuse swelling within 1 to 3 days after surgery, and in some horses hematoma or seroma formation occurs. The medial antebrachial region is highly vascular, and swelling is common and expected. If necessary, fluid can be evacuated, but this is seldom necessary. Firm, fibrous tissue forms at the surgical site by 4 to 6 weeks after surgery and may persist for several months. However, in most horses it is difficult to see residual swelling by 6 to 8 months after surgery, and cosmetic appearance is acceptable, if not normal, thereafter. I do not believe the postoperative morbidity associated with transection of the ALSDFT is sufficient to warrant a change to the tenoscopic approach. Transection of the ALSDFT can be accomplished using an endoscopic (tenoscopic) approach through the carpal sheath. Both tenoscopic portals are made on the lateral aspect of the limb. Positioning a horse in lateral recumbency is preferred, but the surgical procedure can be done with the horse in dorsal recumbency if bilateral transection of the ALSDFT is necessary.15 The ALSDFT can be seen through the sheath and severed, but problems with hemorrhage from transection of the nutrient vessel can be encountered, and whether the entire ligament has been transected is sometimes unclear. Some surgeons prefer the endoscopic approach to avoid swelling that occurs with conventional surgery, but I feel this concern is overrated. Although this approach may be considered more elegant than conventional surgery, I prefer the conventional approach described previously.
How Transection Works
Originally, transection of the ALSDFT was thought to reduce tendon strain, thereby reducing the risk of recurrence of tendonitis. If a gap remained in the ALSDFT after transection, one could reasonably assume load was transferred to the SDF muscle. In this case the muscle rather than the inelastic scar within the tendon could stretch, thereby protecting the healed portion of tendon. Experimental evidence in cadaver specimens suggests that after transection of the ALSDFT, load is transferred immediately to the muscle, but tendon strain increases because of a decrease in the metacarpophalangeal joint angle (hyperextension).16 Significantly increased strains in both the SDFT and the suspensory ligament were measured in equine cadaver limbs after transection of the ALSDFT in another study, and significant changes in metacarpophalangeal and carpal joint angles had occurred.17 Both of these studies were performed using cadaver limbs and in vivo results are unknown. Increased tendon strain after transection of the ALSDFT may promote optimal tendon remodeling and collagen cross-linking. It has been proposed that the ALSDFT likely heals after transection, but in an elongated fashion, allowing increased length of the bone-ligament-tendon-bone
Chapter 69 Superficial Digital Flexor Tendonitis
axis, rather than replacing this load-bearing axis with muscle. This in theory would increase elastic limit of the damaged tendon and negate the intrinsic loss of elasticity found in healed but scarred tendon.5 I was initially skeptical of the mechanical effects of transection of the ALSDFT, but immediately after surgery horses may exhibit back-at-theknee conformation (calf knee). Circumstantially the fact that suspensory desmitis and changes in carpal angle occur in horses after transection of the ALSDFT implicates a shift in distribution of load from the SDFT to the suspensory ligament.10,11 These clinical observations were reinforced with data from the experimental study using cadaver limbs in which increase strain in the suspensory ligament and changes in joint angles were confirmed.17 Although suspensory desmitis is a serious soft tissue injury and possibility of suspensory injury has altered recommendations for transection of the ALSDFT, I do not consider the risk of suspensory desmitis after surgery a reason to abandon the procedure either in racehorses or in other, nonracehorse sports horses. Based mostly on clinical impression I believe transection of the ALSDFT applies the time-honored principles of decompression, reduced load (increased strain), and protection of the inelastic scar achieved during reparative healing.
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A
When Transection Should Be Performed
I used to perform transection of the ALSDFT with the assumption that the procedure was useful in preventing recurrence of tendonitis, and no attempt was made to perform surgery immediately after discovery of tendonitis. However, my current recommendation is that the procedure be performed as early as possible after injury. Obvious, visible reduction in tendon size occurs in the first 5 to 10 days after surgery, and although I cannot substantiate this claim, the procedure may be important in limiting or reducing inflammation and thus may improve the quality of early healing. Immediate reduction of load on the damaged tendon may explain decreased swelling, but possibly transection of the ALSDFT may function to decompress the injured tendon or may alter blood flow. Reduction in tendon swelling is most marked in horses undergoing transection of the ALSDFT and palmar annular desmotomy but occurs in those undergoing ALSDFT desmotomy alone. Ultrasonographic examination as early as 3 to 4 weeks after surgery often reveals marked improvement of anechoic or hypo echoic regions and reduced CSA measurements that compare favorably with those shown by others recommending intralesional injection of various products aimed at stimulating reparative or regenerative healing of the SDFT (Figure 69-8).
Horse Selection for Transection
Racehorses with mild, diffuse tendonitis or those with core lesions involving 10% or less of the CSA of the tendon likely will heal with conservative management and generally are not considered surgical candidates unless injury is recurrent. Racehorses with recurrent diffuse tendonitis, severe diffuse tendonitis, or core lesions involving 10% to 15% or more of the CSA of the tendon are surgical candidates. In other sports horses, transection of the ALSDFT is recommended in the affected limb in upper-level performance horses using similar guidelines as racehorses, but in lower-level horses conservative management of more severe injuries is often successful.
B Fig. 69-8 • Transverse (left) and longitudinal ultrasonographic images at 21 cm distal to the accessory carpal bone of the right front superficial digital flexor tendon (SDFT) of a 4-year-old Standardbred mare, pacer, with severe superficial digital flexor tendonitis before (A) and 28 days after (B) transection of the accessory ligament of the SDFT and palmar annular desmotomy. There is marked improvement in the echogenicity of the SDFT soon after surgery, confirming the observation that the clinical appearance of the tendon improves quickly after surgical management.
Bilateral Transection
Bramlage’s early results using transection of the ALSDFT in TB racehorses were complicated by the development of SDF tendonitis in the contralateral limb.2,3 In any racehorse, bilateral tendonitis is not uncommon. Ultrasonographic evaluation of young racehorses with obvious tendonitis in one limb often reveals mild tendonitis in the contralateral limb, and I suggest that both limbs be examined carefully before surgery. These facts led me to consider performing transection of the ALSDFT in racehorses bilaterally, even when the procedure was done prophylactically. However, the horse’s age, existence of concomitant suspensory desmitis in the contralateral limb, gait (in STB racehorses), type of sports horse, and cause of tendonitis play a role in decision making. Bilateral transection of the ALSDFT is recommended in horses with bilateral tendonitis; in young, unproven (unraced or early in training) 2- or 3-year-old TB or STB racehorses with unilateral tendonitis in which the contralateral limb is operated on prophylactically; and in horses with subtle ultrasonographic evidence of contralateral tendonitis. Unilateral transection of the ALSDFT (surgical procedure performed in affected limb only) is recommended in show horses; in young racehorses,
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particularly STB racehorses with suspensory desmitis in the contralateral limb; and in horses with tendonitis caused by direct trauma (tendonitis is unlikely to occur in the contralateral limb), such as a bandage bow. In older TB and STB racehorses I most often recommend transection of the ALSDFT in the affected limb only. In STBs, pacers are more likely than trotters to develop contralateral tendonitis, so in young, unproven pacers I will likely recommend bilateral transection more often than in trotters. In older, proven pacers, I recommend transection only in the affected limb.
Aftercare
Horses are given 2 weeks of absolute stall rest, followed by 6 weeks of stall rest with an increasing handwalking program, beginning with 10 to 15 minutes twice daily. After 8 weeks, an additional handwalking program, walking in the jog cart (STB racehorses), or swimming physiotherapy is recommended for an additional 8 weeks. Turnout exercise is not permitted. Horses then are placed back into early training by trotting and light galloping (TBs) or by walking and light jogging (STBs). Serial ultrasonographic examinations should be performed each time an incremental increase in exercise is planned or when clinical evidence of mild inflammation exists. Time to first start varies considerably but in TB and STB racehorses is 11 to 12 months and 8 to 9 months, respectively. Large, full-limb support bandages are considered important in limiting motion and swelling at the surgical site(s) and are maintained and changed as needed for a minimum of 14 days. Once bandages have been removed, swelling inevitably occurs at the site(s) of transection of the ALSDFT. An inner, lighter bandage is covered by a heavy cotton, full-limb dressing. Concomitant systemic use of PSGAGs or hyaluronan is controversial, and clear benefits have not been demonstrated, but the use of these medications makes sense to me theoretically. Eight weekly injections of PSGAGs are recommended. Dramatic clinical improvement in tendon size occurs when transection of the ALSDFT is combined with other surgical procedures (see Figure 69-8) or in horses in which peritendonous injection of corticosteroids or a combination of corticosteroids and other antiinflammatory products is used adjunctively with surgery. Phenylbutazone (4.4 mg/kg, intravenously or orally [PO], twice daily for 10 to 14 days) administration appears useful in reducing swelling and improving comfort after surgery.
PALMAR ANNULAR DESMOTOMY Much attention has been given to transection of the ALSDFT and to tendon splitting, but little to no recent mention has been made of palmar annular desmotomy. This procedure remains a most useful surgical treatment, usually combined with transection of the ALSDFT, to manage tendonitis of the distal aspect of the SDFT. Two distinct clinical situations occur in which palmar annular desmotomy is recommended. The most successful is adjunct use of palmar annular desmotomy in horses with SDF tendonitis. The PAL is a passenger in the disease process, which results in annular constriction of the SDFT. As the SDFT enlarges, the PAL impedes gliding function of the tendon. Further tendon enlargement and inflammation may cause thickening of the PAL, but the primary
disease process involves the SDFT, not the PAL or DFTS. The PAL often becomes thickened and adheres to underlying DFTS and SDFT. Once the PAL impedes SDFT function, lameness, continued swelling, and inflammation occur. Palmar annular desmotomy usually is combined with transection of the ALSDFT but can be a career-saving procedure when done alone. Palmar annular desmotomy provides immediate decompression and improved gliding function of the SDFT, which are important, time-honored surgical principles to follow. The decision to transect the PAL is based on clinical observations. Typically a notch appears along the palmar aspect of the limb when viewed from the side, at the location of the PAL. Even if impingement is not severe, I recommend palmar annular desmotomy. Viewed with ultrasonography the PAL may be normal to moderately thickened and adhesions may exist, but actual evidence of PAL desmitis is lacking. The SDFT is abnormal. After palmar annular desmotomy a reduction in tendon size is seen within 5 to 10 days, and clinically initial swelling appears to redistribute proximal to the PAL and distal to the level of the surgical site. The PAL likely reforms in an elongated fashion after surgery but well after tendon size has decreased. Adhesion formation between the PAL, DFTS, and SDFT may occur after surgery, and inadvertent damage to the edge of the SDFT is possible, but the benefits appear to outweigh the risks of the procedure. The second and less successful clinical situation in which palmar annular desmotomy is used is in horses with chronic tenosynovitis without tendonitis of the SDFT. These horses may have primary tendonitis of the DDFT, injury of the manica flexoria, or rarely primary palmar annular desmitis (see Chapter 74). I prefer a minimally invasive conventional surgical approach. A 1.5-cm stab incision is made medially (when used with transection of the ALSDFT) just proximal to the PAL through skin, subcutaneous tissues, and DFTS, but care should be taken to avoid incision of the medial (lateral) aspect of the SDFT. In horses with chronic, severe tendonitis the tissue can be quite thick and care must be taken not to incise the underlying SDFT. A curved bistoury (Sontec Instruments, Englewood, Colorado, United States) or one blade of a pair of straight Mayo scissors is inserted deep to the PAL, superficial to the SDFT. The PAL is incised in its entirety, and care must be taken to transect the distal aspect completely. Reversing the bistoury or scissors blade and transecting the remaining fibers of the PAL and thickened DFTS proximal to the incision may be necessary. The small incision then is closed using subcutaneous and skin sutures. Occasionally a larger incision is necessary in horses with severe and chronic tendonitis, because adhesions preclude accurate insertion of the bistoury or scissors. Care after surgery is the same as described for transection of the ALSDFT when the procedures are done simultaneously, but if palmar annular desmotomy is performed alone, an accelerated exercise program often is advised. Long stall rest periods after palmar annular desmotomy may promote earlier healing of the severed PAL and adhesion formation. After an initial 2-week period of stall rest, handwalking is undertaken for 2 weeks and the tendon is reevaluated. Acceleration of the exercise program at this point may be recommended depending on factors such as the horse’s value, class of racing, time of season, and owner’s and trainer’s proposed schedule.
Chapter 69 Superficial Digital Flexor Tendonitis
Other surgical approaches, including an extended open approach or an open approach without entering the DFTS, have been described, but I do not recommend them. The approach described previously can be done with horses in the standing position or while they are under general anesthesia. Endoscopic examination of the DFTS and transection of the PAL have been described elegantly (see Chapters 24 and 74).18 This is a novel visually pleasing surgical procedure, but in my hospital it is expensive and time-consuming to perform. Complications with any approach include incisional dehiscence, delayed healing, infection, and damage to the ipsilateral edge of the SDFT. Horses with severe tendonitis in which the SDFT is severely enlarged and those with extensive adhesions and thickening of the PAL and DFTS are at greatest risk for complications.
TENOSCOPY OF THE DIGITAL FLEXOR TENDON SHEATH AND CARPAL SHEATH See Chapter 24 for additional discussion of tenoscopy of the DFTS and carpal sheath. I perform tenoscopy of the DFTS for management of tenosynovitis primarily in nonracehorses and most commonly in the hindlimb. Tenoscopy is valuable for validation of ultrasonographic findings, to evaluate the surfaces and debride defects in the flexor tendons, to debride tears in the manica flexoria, to remove the manica flexoria in horses with complete tears of the structure, to remove soft tissue masses and perform adhesiolysis, and to perform palmar or plantar annular desmotomy under endoscopic guidance. Carpal tenoscopy is valuable to perform ostectomy of the distal caudal aspect of the radius in horses with the unusual condition of exostosis of the distal caudal aspect of the radius and accompanying tenosynovitis, to remove supracarpal exostoses, and to inspect and debride lesions associated with the flexor tendons and the ALSDFT. Occasionally a trotter develops acute tenosynovitis and lameness as a result of hemorrhage in the carpal sheath, and careful ultra sonographic examination reveals injury to the ALSDFT. Desmitis of the ALSDFT has been reported in trotters and nonracehorses and was managed conservatively.19 However, tenoscopic examination provides additional information about the soft tissue structures and extent of injury, and, importantly the torn edges of the ALSDFT can be debrided.
PROXIMAL METACARPAL FASCIOTOMY AND CARPAL RETINACULAR RELEASE For horses with proximally located SDF tendonitis an effective method to decompress the damaged, enlarged SDFT and improve gliding function is to transect the proximal metacarpal fascia and carpal retinaculum (desmotomy, retinaculotomy). Clinical diagnosis in horses with only proximal SDF tendonitis can be challenging because swelling and pain can be easily missed during palpation. Lameness is often inappropriately severe based on the minimal clinical signs present; horses often stand with an over-atthe-knee conformation (bucked knee), a positive response to carpal flexion is manifested, and horses may have mild
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carpal tenosynovitis. Lameness may worsen after palmar digital analgesia, a clinical observation most commonly seen in horses with proximal palmar metacarpal and palmar carpal region pain. Horses with proximal SDF tendonitis have been reported to have a poor prognosis (two of 12 horses returned to full work).20 My experience with conservative management appears to be somewhat better than in this publication, but I believe surgical management offers the best chance for full recovery.21 Horses with severe mid-to-distal SDF tendonitis may have proximal involvement, and fasciotomy or carpal retinacular release appears beneficial. Old horses with inexplicably severe SDF tendonitis often have extension of tendonitis to the region of the carpal canal and may have moderate to severe carpal tenosynovitis (see discussion on page 707). Carpal retinacular release is described for use in horses with carpal tenosynovitis, but it can be useful in horses with SDF tendonitis in which transection of the metacarpal fascia is done concomitantly. This procedure most often is performed in combination with transection of the ALSDFT but can be combined with palmar annular desmotomy also. Prognosis in horses with SDF tendonitis in which transection of the ALSDFT and fasciotomy or carpal retinacular release are performed is guarded to good. Prognosis in old horses requiring transection of the ALSDFT, fasciotomy or carpal retinacular release, or, in some, palmar annular desmotomy is better than expected, because swelling and lameness are often severe in these horses. Substantial reduction in tendon size and lameness score after surgery can be expected, and some horses have returned to light riding or field hunting. Fasciotomy or carpal retinacular release is done using conventional surgical techniques and usually via a medial approach, because the procedure often is combined with transection of the ALSDFT (Figure 69-9). To perform fasciotomy alone a 2- to 3-cm incision is made through skin and subcutaneous tissues just dorsal to the cephalic vein at the level of the proximal aspect of the second metacarpal bone. Because underlying tendonitis is present, bleeding can be excessive. A 1-cm incision is made in the dense, underlying metacarpal fascia, and straight Mayo scissors are used to extend the fasciotomy proximally through distal aspect of the carpal retinaculum to the level of the distal aspect of the accessory carpal bone and distally to the midmetacarpal region. No distinction is perceivable between fascia and retinaculum because the tissues blend together. Complete transection of the carpal retinaculum is often necessary and requires tedious dissection and careful ligation of vessels. The radial artery and vein run between the inner and outer lamina of the carpal retinaculum and care must be taken to avoid severing these vascular structures. It is mandatory to sever both the inner and outer lamina of the carpal retinaculum to provide decompression and improved gliding function of the SDFT (see Figure 69-9). I prefer using conventional surgical techniques but tenoscopic approaches exist. Conventional surgical incisions often are quite long since it is necessary to sever the carpal retinaculum to the level of the proximal aspect of the accessory carpal bone. Subcutaneous tissues and skin are closed routinely. Bandaging and care after surgery are the same as for transection of the ALSDFT because the procedures usually are performed in combination.
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PART VIII The Soft Tissues Dorsal
A
Distal
Distal
Dorsal
B Fig. 69-9 • A, Intraoperative photograph (taken from a palmar perspective) of a 5-year-old Standardbred stallion, pacer, with pronounced lameness from tendonitis of the proximal aspect of the left front superficial digital flexor tendon (SDFT; distal is to the right and dorsal is to the top). The horse is positioned in left lateral recumbency under general anesthesia, and desmotomy of the medial metacarpal fascia (fasciotomy) and carpal retinaculum is underway. The thick, outer lamina of the carpal retinaculum (black arrows) is an extension of the metacarpal fascia that has been cut, but it is important to completely sever the inner lamina (white arrowheads on top of Metzenbaum scissors) that encircles and constricts the swollen SDFT. B, Intraoperative photograph (taken from a palmar perspective) of the same horse after metacarpal fasciotomy and desmotomy of the carpal retinaculum showing that the enlarged SDFT is now decompressed, expanding the cut edges of the severed fibrous tissue (double-headed arrow). This horse raced successfully after surgery.
TENOPLASTY—TENDON SPLITTING Interest in tendon splitting as an adjunct method to manage SDF tendonitis has been renewed. Originally the procedure was developed to promote vascularization of the tendon in horses with chronic tendonitis, and early results showed promise.22 Experimental studies questioned the value of tendon splitting, and the conclusion was that splitting induced excessive granulation tissue and slow healing of areas of tendon necrosis.23,24 Clinical use of the technique then fell out of favor, but renewed interest was sparked by reports of combined use of splitting with transection of the ALSDFT in a clinical study and improved healing and revascularization of acute collagenase-induced tendon injuries in an experimental study.25,26 The collagenase model produces severe tendonitis with extensive necrosis, and I question the value of this model in mimicking the naturally occurring disease. Various authors have reported clinical experiences with tendon splitting used with other procedures, but clinical studies using splitting alone are lacking.4,5,11,14 Tendon splitting currently is used in horses
with core lesions early in the disease process rather than in horses with chronic tendonitis, the group for which the procedure was designed originally. Tendon splitting, done to decompress areas of hemorrhage, or to provide vascular access channels early after injury, makes theoretical sense for horses with anechoic lesions, but it is of questionable value if done once granulation tissue has formed or mature collagen fibers exist. The rationale given for use in early lesions is to decompress the area of hemorrhage and to provide vascular access channels to improve vascularization by vessel ingrowth. Soon after injury areas of hemorrhage become granulation tissue in which the value of decompression would be minimal. The surgical procedure would have to be performed before granulation tissue develops (3 to 5 days, but certainly before 2 weeks) and may be best reserved for horses with anechoic lesions. Damage caused to peritendonous tissues and surrounding intact tendon fibers must be considered and may outweigh any benefit. However, the procedure has been done apparently successfully or at least without outward harmful effects. In clinical practice I use the procedure when requested by referring veterinarians or in TB racehorses with anechoic or hypoechoic core lesions. In fact, my best results in TB racehorses have been achieved using tenoplasty in combination with transection of the ALSDFT (see Figure 69-1). The percutaneous technique is preferred, and using a double-edged tenotome is recommended. The procedure seldom is used in the STB racehorse, and in our report detailing results of transection of the ALSDFT in this racing breed, tendon splitting was done in only one horse.11 In STB racehorses managed with transection of the ALSDFT, I often recommend palmar annular desmotomy if tendonitis extends to the distal metacarpal region, but seldom recommend tenoplasty. In the STB racehorse, I will consider tenoplasty in horses with lesions involving the lateral aspect of the SDFT. Currently I recommend that tenoplasty be done in TB racehorses or other non-STB sports horses with transection of the ALSDFT. Needle decompression of anechoic lesions early after injury may make theoretical sense to provide early decompression, but creation of vascular access channels using this method appears implausible and I do not use the technique.
COMBINED SURGICAL PROCEDURES Because SDF tendonitis is career-limiting in many sports horses, early and aggressive surgical management should be considered. Potentially, injection of growth factors, MSCs, fresh liquid bone marrow or bone marrow concentrate, or other substances used with surgery and strict rehabilitation may offer the best hope for a successful outcome. Combined surgical management using transection of the ALSDFT and palmar annular desmotomy; transection of the ALSDFT, annular desmotomy, and fasciotomy or carpal retinacular release; or in some horses various combinations with tenoplasty has been successful even in horses with severe tendonitis. In my experience, using transection of the ALSDFT without palmar annular desmotomy in horses with distal SDF tendonitis often results in failure, as does using transection of the ALSDFT without fasciotomy or carpal retinacular release in those with lesions involving the proximal aspect of the SDFT.
Chapter 69 Superficial Digital Flexor Tendonitis
The ideal management program for horses with SDF tendonitis has yet to be discovered but should rely on the principles of tendon healing, including minimizing peritendonous scar tissue formation, minimizing the effect of hemorrhage and subsequent granulation tissue formation and disorganized fiber alignment, maximizing gliding function within and external to the SDFT, reducing load and protecting inelastic scar tissue by increasing the length of the bone-ligament-tendon-bone construct, and using an exercise regimen that allows healing and maturation of collagen fibers without deleterious effects of uncontrolled exercise. Surgical management preserves many of these time-honored principles and has proved useful in many types of sports horses. Treatment of tendon injuries in racehorses is a challenge to clinicians and researchers. Return to racing is associated with a high rate of recurrent injury. However, much progress has been made in the past two decades in diagnostic identification and classification of injury, and the biochemical and biomechanical aspects of tendon injury. Treatment and management programs by necessity depend heavily on economics and trainer decisions relative to continued exercise or long-term rehabilitation. Of vital importance are accurate injury severity categorization, controlled exercise in long-term rehabilitation programs, and ultrasonographic monitoring at all levels of treatment. Improved athletic outcome results gradually evolve as continued basic science research and clinical investigations of past and proposed therapeutic regimens unfold.
SUPERFICIAL DIGITAL FLEXOR TENDONITIS IN EVENT HORSES, SHOW JUMPERS, DRESSAGE HORSES, AND PLEASURE HORSES Sue J. Dyson SDF tendonitis is a potentially career-limiting injury in event horses and Grand Prix–level show jumpers. Although the incidence of SDF tendonitis is much lower in dressage horses, it is also an important injury. The clinical manifestations vary considerably.
SUPERFICIAL DIGITAL FLEXOR TENDONITIS IN EVENT HORSES Clinical Signs
Although all event horses are at risk for injury of the SDFT, the incidence of injury is highest in horses competing at advanced or international level, especially those competing in Three Day Events.1,2 This is probably because of the combination of galloping long distances and jumping on very variable terrain and footing. The introduction of short format Three Day Events does not appear to have reduced the risk of injury. After training or competing, riders commonly apply a proprietary clay to the forelimbs and bandage the limbs at least overnight. This practice may result in early warning signs of impending tendon damage being missed. Because lesions frequently occur bilaterally, subtle changes in limb temperature may be missed except by the most astute and vigilant riders or grooms.
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The clinical signs associated with tendon damage vary markedly. A horse may pull up lame after the cross-country phase of an event and rapidly develop peritendonous soft tissue swelling and pain on palpation of the tendon. In some horses the degree of pain is severe and the horse may be distressed and reluctant to bear weight on the limb. If lesions occur bilaterally, the horse may be reluctant to move and its behavior may mimic that of a horse that is tying up. These horses may require both analgesia and sedation to relieve the distress. Clinicians should be aware that a horse may appear very lame, but it may not be possible to elicit pain by palpation of the tendon, although the tendon may feel abnormally soft. In contrast, a horse may complete the cross-country phase and appear sound, with lameness developing several hours later. Although soft tissue swelling may develop, in some horses there may be absolutely no clues as to the cause of lameness (i.e., no heat, pain, or swelling). This may persist for many days, and then suddenly enlargement, subtle or obvious, of the SDFT may be seen despite resolution of lameness. If swelling is subtle, it may be overlooked and only recognized after localization of the pain causing lameness by local analgesic techniques and clipping of the limb in preparation for ultrasonographic evaluation. In other horses, lameness is never present. The horse initially may exhibit poor performance, and clinical examination may reveal that the SDFTs are slightly enlarged. Alternatively, the horse may have localized heat and pain on pressure applied to the SDFT, with no history of lameness. Some horses have obvious clinical signs of SDF tendonitis after the first training gallop, cross-country schooling, first event, or a period of reduced work for some months after a Three Day Event. It seems highly likely that these horses sustained damage to the SDFT(s) at the Three Day Event without associated detectable clinical signs. Clinical signs and the severity of the tendon injury are not necessarily correlated. The clinical signs of SDF tendonitis can easily be masked. I have examined a number of horses at events when the horses have finished lame and had clinical signs compatible with SDF tendonitis. The horses were treated by application of a modified Robert Jones bandage and systemic NSAIDs. Ultrasonographic examination after 5 to 7 days was recommended. The horses were then examined by the owners’ regular veterinary surgeons, who were unable to detect any palpable abnormality after removal of the bandage and therefore elected not to perform an ultrasonographic examination. Return to work was recommended by the second veterinary surgeons with catastrophic consequences. Some horses that have completed the speed and endurance phase of a Three Day Event and sustained a tendon injury can be managed to pass the final horse inspection and complete the show jumping without marked deterioration of the tendon injury. In older horses (usually >12 years of age) lesions of the most proximal aspect of the SDFT may develop, extending into the carpal sheath.3 These can be quite difficult to palpate because heat is difficult to detect and the tight flexor retinaculum may conceal swelling, but they usually cause lameness. The lameness is variable in severity and may be inapparent following rest for several days, but recurs after fast work. Lameness is frequently accentuated by carpal flexion. Lameness may be improved by palmar and
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PART VIII The Soft Tissues
palmar metacarpal nerve blocks, but median and ulnar nerve blocks are sometimes required to abolish lameness. SDFT injury can also arise after an overreach injury; there may also be damage to other structures. The effects of local trauma can vary from localized peritendonous edema with no evidence of tendon damage, to localized hypoechoic or anechoic lesions on the palmar aspect of the SDFT (the result of blunt trauma), to partial or complete laceration (see Chapter 81). Local traumatic injuries do not extend far proximodistally, but partial lacerations can be associated with the development of longitudinal splits extending proximally or distally, which are the result of altered shear stresses. Careful ultrasonographic examination of all adjacent structures is important to establish the severity of injury, bearing in mind that the site of a skin abrasion may not coincide with the site of tendon injury. The subsequent development of peritendonous fibrosis may influence the prognosis.
the constraints of ultrasonographic examinations from both the skill of the veterinarian and the limitations of the resolution of ultrasonographic images. If an SDFT appears normal ultrasonographically but the clinical signs raise suspicion of injury, the horse should be treated as if it has a tendon injury, with repeat examination after a further 7 to 14 days. In horses with early subtle tendonitis the only abnormality detectable may be slight, localized enlargement in CSA of the tendon; therefore area measurements and comparisons with the contralateral limb can be extremely valuable. If the injury is bilateral, subtle enlargement in both SDFTs is easily missed. It is important to examine the relative sizes of the SDFT and DDFT and to be aware of the way in which a normal SDFT changes shape from proximally to distally in the metacarpal region (see Chapter 16). It is also important to look carefully at the echogenicity of the SDFT and compare it with the DDFT and more proximal and distal sites within the SDFT. Subtle lesions often result in a slight diffuse reduction in echogenicity of part or all of the CSA of the tendon, but only in a localized region. Such lesions are easily missed, especially if the gain controls of the ultrasound machine are set too high or if the limb is examined without fine clipping of the hair coat. Detection of these early injuries is very important, because a horse can make a relatively rapid complete recovery at this stage, whereas continued work may result in a much more severe, career-limiting injury. In these early injuries, longitudinal images of the tendon often appear normal. Both forelimbs should be examined routinely. Because lesions may be very localized, it is important to carefully and systematically examine the tendon from proximally to distally. Although gross core lesions of the
Diagnostic Ultrasonography
In my opinion, whenever unexplained lameness occurs in an event horse, the SDFTs should be examined ultrasonographically. I also recommend routine ultrasonographic examination 10 to 14 days after the horse has completed a Three Day Event. Baseline ultrasonographic images should be available for comparison, with measurements of CSA at 4-cm intervals distal to the accessory carpal bone throughout the metacarpal region (Figure 69-10). Mild asymptomatic lesions may be identified and should be monitored serially. Ultrasonographic examination should be mandatory for any horse with a history of slight heat or filling in the metacarpal region unless there is an obvious cause (see Figure 69-6). However, it is equally important to recognize
A
B
Fig. 69-10 • A, Transverse ultrasonographic image of the palmar aspect of the right forelimb of a 9-year-old advanced event horse; the scan was obtained at 17 cm distal to the accessory carpal bone. Medial is to the left. The horse initially had slight heat in the metacarpal region but no lameness. The superficial digital flexor tendon is enlarged (cross-sectional area 1.2 cm2), and there are focal anechogenic regions on the lateral border. The lesion extended from 14 to 20 cm distal to the accessory carpal bone. B, Lateral parasagittal ultrasonographic image of zone 2B; proximal is to the left. There is some loss of long linear echoes, with an anechoic lesion on the palmar aspect.
Chapter 69 Superficial Digital Flexor Tendonitis
SDFT are relatively easily identified without fine clipping of the hair coat, image resolution is inferior. Application of mineral oil helps to improve resolution, but this can destroy the ultrasound standoff pad. Subtle lesions will be missed unless the hair is clipped, although shaving is unnecessary. Owners may be reluctant to allow the limbs to be clipped, suggesting that this may jeopardize the horse’s evaluation at an inspection at a forthcoming Three Day Event if the limbs actually appear normal and the horse is able to compete. Owners must be persuaded that it may be essential to clip the limbs and that only a narrow strip is necessary; the clinician should suggest that if the horse is fit to compete, the entire horse should then be clipped.
Treatment
Management of SDF tendonitis in the event horse is difficult. Individual horses’ capacities to recover from tendon injuries vary considerably. In some horses a marked improvement is seen in the clinical and ultrasonographic appearance of the tendon within 3 months of first injury, whereas little progress is seen in other horses with a similar injury. In almost all horses a convalescent period of 1 year after injury is required. In some horses, normal echogenicity is never restored, and a fairly obvious central hypoechoic area persists ultrasonographically. These horses may be able to withstand one day events but frequently sustain reinjury at a Three Day Event. Lesions in the distal one fourth of the metacarpal region (zones 3B and 3C) are particularly at risk for reinjury. Even in those horses in which relatively normal echogenicity is restored, it is relatively unusual for them to be able to complete more than two Three Day Events without reinjury unless the original injury was mild. Most published studies of the treatment of SDF tendonitis relate to the racing TB and STB. It is generally accepted that the rate of reinjury is high, especially for horses competing in Three Day Events. Although a horse may be managed successfully to complete one Three Day Event after injury, the risk of subsequent injury is very high. Of 23 event horses treated with a controlled exercise program alone, 57% were able to return to full athletic function without recurrent injury for a minimum of 2 years after resumption of full work,4 compared with 56% of 25 horses that received intralesional hyaluronan and the same controlled exercise program and 58% of 31 horses that received intralesional PSGAGs administered intralesionally, systemically, or by both routes. Almost all horses with a unilateral injury reinjured the same tendon. No horses completed more than two Three Day Events without recurrent injury. The results of intralesional treatment with βaminopropionitrile fumarate are similar if both the treated limb and the noninjured limb are considered.5 However, the reinjury rate in the treated limb was considerably better (16%). Of 22 advanced-level event horses, 14 have completed two to six Three Day Events, including nine that have completed three or more Three Day Events at championship level four-star FEI events without recurrent injury. An additional five horses completed two to five Three Day Events before injuring the contralateral limb; three of these horses have completed two additional four-star Three Day Events. Overall, in my experience this treatment seems to
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offer the best long-term prognosis for event horses with moderate-to-severe injuries of the SDFT, although a licensed product is no longer available. After treatment, horses are walked for 30 to 60 minutes daily for 4 months and are then reexamined ultrasonographically. Successfully treated horses had a remarkably good strength of fiber pattern in longitudinal ultrasonographic images 4 months after treatment. This feature appears to be a good prognostic indicator. Work intensity is slowly and progressively increased thereafter, with horses generally reaching full work by 12 months after treatment. It is premature to judge the efficacy of stem cell therapy, but early results from 109 horses from all racehorse and sports horse disciplines showed a reinjury rate of 13% 1 year after resumption of work.6 My clinical experience with event horses has been less favorable, although some horses have responded extremely well with sustained return to full athletic function. More recent data from 83 flat racehorses and eight National Hunt racehorses (which excludes horses lost to follow-up) indicate a reinjury rate of 31% (flat racehorses 50%; National Hunt racehorses 23%) 2 years after resumption of full work.6 A problem remains with medical management, by whatever means, of a horse with a unilateral injury, with no detectable ultrasonographic lesion in the contralateral limb. There is no doubt that despite successful management of the injured limb, the noninjured limb is at risk of subsequent injury. Desmotomy of the accessory ligament of the SDFT has been less successful in the management of SDFT lesions in event horses than in racehorses, and the disappointing results achieved in my clinic and elsewhere in the United Kingdom have led to this technique being abandoned. It has also fallen out of favor in the United States. Tendon splitting has been used for anechoic central core lesions, combined with a controlled exercise program, with successful results, but the number of treated horses does not compare with the studies described previously. Regardless of the method of management, serial ultrasonographic examinations during the convalescent period seem to be the most accurate predictors of the final outcome. If the echogenicity and fiber pattern have improved markedly by 4 months after injury, then the longer-term prognosis is much better than in horses in which improvement is only slight. The strength of the fiber pattern when full work is resumed is a good predictor of whether the horse will sustain reinjury.4,5 It does appear that some horses have an innately better ability to repair tendon lesions than others. Use of Doppler ultrasonography can be helpful for monitoring repair and for identification of reinjury. Normal SDFTs usually have minimal discernible blood flow. After injury a vascular pattern is usually obvious and subsides 3 to 6 months after injury. Reappearance of blood flow indicates reinjury. Horses with proximal lesions that extend into the carpal canal have generally responded poorly to conservative management, with a high rate of recurrent injury. Surgical release of the flexor retinaculum has enabled several horses to return to full athletic function. The results of treatment with intralesional MSCs have been disappointing.
Pastern Lesions
Injury to the medial or the lateral branch of the SDFT in the pastern region occurs most commonly in forelimbs but
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PART VIII The Soft Tissues
also occasionally in hindlimbs (see Chapter 82). Such injury can occur as an isolated acute lesion or as a sequela to previous tendonitis in the metacarpal region. There is usually acute-onset, moderate lameness. In some horses, soft tissue swelling is immediately apparent on the palmar aspect of the pastern, but in others, obvious swelling may take several days to develop. In these horses, perineural analgesia may be required in the acute stage to localize the source of pain. Lameness is alleviated by perineural analgesia of the palmar nerves at the level of the proximal sesamoid bones. Diagnosis is confirmed ultrasonographically. The affected branch is usually enlarged, with poor demarcation of its margins and diffuse or focal areas of reduced echogenicity. There may be some peritendonous hyper echogenic tissue representing fibrosis. Horses with acute lesions with no preceding tendonitis have a fair prognosis for complete recovery after adequate rest for at least 6 months. However, horses with lesions secondary to tendonitis in the metacarpal region have a more guarded prognosis.
SUPERFICIAL DIGITAL FLEXOR TENDONITIS IN SHOW JUMPERS Injury to the SDFT is comparatively unusual in show jump ing horses, except either those competing at international
A
level,1,7 or horses approximately 15 years of age or older. Injury to the SDFT is most commonly unilateral. There are six typical manifestations: 1. Insidious-onset peritendonous swelling and enlargement of the SDFT, but no associated lameness (Figure 69-11, A). Such horses can generally be managed, but if maintained in full work it is likely that the lesion will deteriorate. 2. Sudden onset of severe lameness while jumping, with rapid development of heat, pain, or swelling. In some horses this is a sequela to previous lowgrade injury, but in others there were no previous warning signs. Injuries occur most frequently during competition and may result in an acute, severe lameness with the horse abruptly stopping during a round. Alternatively, the horse may finish lame. 3. Sudden-onset lameness in an older horse, with a lesion extending into the carpal sheath. 4. Sudden-onset lameness associated with distention of the DFTS. There may be no palpable abnormality of the digital flexor tendons, but ultrasonography will reveal a marginal lesion of the SDFT. 5. Progressive enlargement of the SDFT after a previous injury of the accessory ligament of the DDFT (ALDDFT) and lameness. There is usually adhesion formation between the SDFT and the ALDDFT.
B
Fig. 69-11 • A, Transverse ultrasonographic image of the palmar aspect of the left forelimb of a 13-year-old Grand Prix show jumper; the image was obtained at 11 cm distal to the accessory carpal bone. The horse initially had localized heat and swelling but no lameness. A focal hypoechoic lesion is seen on the palmar margin of the superficial digital flexor tendon (SDFT). The lesion extended less than 1 cm proximodistally. The horse’s condition was managed symptomatically, and the horse continued to compete successfully for 6 months with no change in the lesion; the horse then developed acute lameness with exacerbation of the injury. B, Transverse ultrasonographic image of the palmar aspect of the distal metacarpal region of a 14-year-old Grand Prix show jumper with acute-onset, severe lameness associated with distention of the digital flexor tendon sheath. Medial is to the left. The medial aspect of the SDFT is enlarged, its margin is irregular, and there are focal anechoic lesions.
Chapter 69 Superficial Digital Flexor Tendonitis
6. Occasionally, traumatic injuries occur to the SDFT within the carpal sheath after the horse has sustained a fall (see Chapter 75). Injuries restricted to the proximal one third of the metacarpal region are common, and there appears to be a tendency for the development of peritendonous adhesions and recurrent lameness. Horses with these lesions have been difficult to manage successfully, as are those with lesions that extend proximally into the carpal canal. However, horses with lesions restricted to the metacarpal region, without peritendonous adhesions, can be rehabilitated and returned to competition with judicious, symptomatic management relatively quickly compared with event horses with a comparable injury. Some horses can withstand return to full work despite persistence of quite obvious ultrasonographic abnormalities of the SDFT. However, the tendon must be monitored carefully, because some horses continue to sustain recurrent low-grade injuries, which result in the tendon becoming progressively larger and wrapping around the DDFT and becoming contiguous with the ALDDFT. Severe injuries predispose to the development of secondary desmitis of the ALDDFT (see Chapter 71). Desmitis of the ALDDFT may occur as a primary lesion. Severe lesions predispose to the development of secondary SDF tendonitis. Horses with these injuries are difficult to manage successfully. Marginal tears of the SDFT within the DFTS occur occasionally, manifesting as an acute-onset lameness associated with distention of the DFTS (see Figure 69-11, B).
A
725
Fourteen (93%) of 15 show jumpers with unilateral SDF tendonitis that were treated conservatively after a controlled exercise program were able to return to full athletic function for at least 2 years without recurrent injury, compared with four (80%) of five horses treated with intralesional hyaluronan and three (75%) of four horses treated with PSGAG.4 Five horses with severe lesions that had not responded adequately to conservative management were subsequently treated with β-aminopropionitrile fumarate, and all have withstood return to international competition without recurrent injury.5 Horses with lesions extending into the carpal canal have been treated by carpal retinacular release with variable results. Older horses (>15 years of age) with SDT tendonitis generally have a more guarded prognosis. Some of these injuries appear to be progressive degenerative lesions, which deteriorate despite rest.3 Injury of the SDFT in ponies is unusual, but I have experience of several teenage event and show jumping ponies that have sustained injuries of the SDFT in the proximal metacarpal region extending into the carpal canal. Although these ponies had unilateral lameness, lesions were identified ultrasonographically bilaterally. Bilateral carpal retinacular release resulted in a favorable outcome.
SUPERFICIAL DIGITAL FLEXOR TENDONITIS IN DRESSAGE HORSES SDF tendonitis is not a common injury in dressage horses, but it does occur occasionally (Figure 69-12). The
B
Fig. 69-12 • A, Transverse ultrasonographic image of the palmar aspect of the left forelimb of a 12-year-old Grand Prix dressage horse. The image was obtained at 22 cm distal to the accessory carpal bone; medial is to the left. The horse had exceptionally extravagant forelimb paces and acute-onset, moderate lameness. There is a focal hypoechoic lesion on the lateral aspect of the superficial digital flexor tendon (SDFT). The lesion extended from 16 to 26 cm distal to the accessory carpal bone. At this level the tendon is slightly enlarged (cross-sectional area 1.2 cm2), but at all other levels it was of normal size. B, Longitudinal ultrasonographic image of zone 3A obtained from the palmar midline; proximal is to the left. The SDFT appears normal. Continued
PART VIII The Soft Tissues
726
with lesions in the proximal one third of the metacarpal region are much more difficult to manage, and clinical signs resolve very slowly regardless of the method of management. Hindlimb lesions have been restricted to the DFTS and are sometimes bilateral. They often are associated with adhesion formation within the DFTS and tend to result in chronic lameness. Some marginal tears of the SDFT are recognized only during exploratory tenoscopy of a distended DFTS. Concurrent tears of the manica flexoria have also been identified during tenoscopic evaluation. The results of debridement of marginal tears of the SDFT within the DFTS in dressage horses have been rather disappointing. Although lameness has often improved, minor career-limiting gait abnormalities have persisted.
C Fig. 69-12, cont’d. • C, Longitudinal ultrasonographic image of zone 3A obtained from the palmarolateral aspect of the metacarpal region shows reduced echogenicity of the SDFT.
at-risk horses appear to be young, very extravagantly moving horses and older Grand Prix horses. Lesions are often quite focal, may be marginal, and are usually associated with low-level lameness. Most lesions occur in the forelimb, but lesions also occur in the distal metatarsal and pastern regions of hindlimbs. Lesions also occur in some relatively young horses disproportionate to the quality of their paces and work history, and these are often difficult to manage and may reflect primary degenerative lesions. Young, big-moving horses with low-grade lesions are managed by restriction to stall rest and controlled exercise, and horses usually resume work after resolution of localized heat, pain, and peritendonous swelling. Horses
RUPTURE OF THE SUPERFICIAL DIGITAL FLEXOR TENDON IN PLEASURE HORSES SDF tendonitis is an uncommon injury in general-purpose riding horses, but spontaneous rupture occurs occasionally in old horses in the midmetacarpal region.8 This results in acute-onset, severe lameness often identified in horses turned out in a field. There is diffuse soft tissue swelling in the metacarpal region, and hyperextension of the fetlock if the horse will bear weight on the limb. If the limb is picked up, a palpable defect can be identified in the SDFT, with the tendon flaccid both proximal and distal to the site of rupture. Ultrasonographic examination reveals complete loss of fiber pattern and an anechogenic region at the site of rupture. Treatment consists of application of a Robert Jones bandage, NSAIDs, and box rest for at least 2 months, with a progressive increase in walking exercise thereafter. Nine of nine horses aged 18 to 22 years made a complete functional recovery, returning to light work within 6 months of injury.8 One horse subsequently ruptured the contralateral SDFT. Rupture of the SDFT at the musculotendonous junction is discussed in Chapter 75.