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The Deep Digital Flexor Tendon
Chapter
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The Deep Digital Flexor Tendon Sue J. Dyson ANATOMY
References on page 1315
In the forelimb the deep digital flexor tendon (DDFT) has three heads: the humeral head, the largest, and the smaller radial and ulnar heads.1 The tendon of the humeral head
develops 8 to 10 cm proximal to the antebrachiocarpal joint, but muscular tissue persists to the level of the antebrachiocarpal joint, where the ulnar and radial heads join. The merged tendon is triangular in cross-section within the carpal canal but becomes more rounded in the metacarpal region. The accessory ligament of the DDFT (ALDDFT) merges with the DDFT in the middle third of the metacarpal region. In the hindlimb the DDFT is formed by a large lateral digital flexor tendon and a smaller medial digital flexor tendon. The lateral digital flexor tendon incorporates the caudal tibialis tendon and passes over the sustentaculum tali within the tarsal sheath. The medial digital flexor tendon passes over the proximal tubercle of the talus, on the medial aspect of the talus, in its own synovial sheath. These two tendons fuse in the proximal metatarsal region.
Chapter 70 The Deep Digital Flexor Tendon
In the proximal metatarsal region the DDFT is a large oval structure that becomes smaller farther distally. The ALDDFT in the hindlimb varies in size and is generally comparatively smaller than in the forelimb and may be absent; in some horses it is a bifid structure. At the fetlock region the DDFT becomes wider, elliptical, and fibrocartilaginous and is enclosed within the digital flexor tendon sheath (DFTS). In the pastern region the tendon becomes bilobed. At the level of the proximal part of the middle phalanx the dorsal part of the tendon becomes a fibrocartilaginous pad. Distally the DDFT is molded to the palmar or plantar aspect of the navicular bone. The DDFT is broad, has a terminal fanlike expansion containing cartilage, and inserts on the facies flexoria of the distal phalanx, delineated dorsally by the semilunar line and the adjacent surface of the cartilage of the foot. The DDFT has a high modulus of elasticity (1585 MPa) and a considerable strength to rupture (approximately 1700 daN).1 The ALDDFT has a low modulus of elasticity (490 MPa) and a moderate strength to rupture (approximately 490 daN). The DDFT limits carpal and fetlock extension under high loads. In the fetlock region the DDFT is under tension and compression. It is therefore fibrocartilaginous in this region and in the pastern, where the tendon is under pressure from the tuberositas flexoria, a transverse prominence on the proximal palmar aspect of the middle phalanx. In the digit the DDFT facilitates flexion of the proximal interphalangeal joint during weight bearing and stabilizes the distal interphalangeal joint. The position of the DDFT and the navicular bone varies considerably during the stance phase. In the full weight-bearing position the DDFT is in close contact with only the distal border of the navicular bone, but during propulsion it comes into full contact with the palmar aspect of the bone. The tendon is stretched maximally as active contraction of the muscle bellies and the elasticity in the tendon result in elevation of the fetlock and extension of the distal interphalangeal joint. During the swing phase of the stride the DDFT relaxes. The DFTS facilitates displacement of the digital flexor tendons during flexion and extension. DDFT injuries occur most commonly in the fetlock or pastern regions within the DFTS or within the hoof capsule. Injuries in the metacarpal or metatarsal regions and the carpus or tarsus are less common.2-22
DEEP DIGITAL FLEXOR TENDONITIS ASSOCIATED WITH RECURRENT DESMITIS OF THE ACCESSORY LIGAMENT OF THE DEEP DIGITAL FLEXOR TENDON Injuries of the DDFT in the carpal or metacarpal region, proximal to the DFTS, are rare except in association with chronic desmitis of the ALDDFT (see Chapter 71). Recurrent desmitis may be accompanied by pathological lesions of the DDFT.2 Because of the close proximity of the DDFT and its accessory ligament, it is difficult to assess each structure accurately by palpation, especially with chronic enlargement of the ALDDFT, which may wrap around the borders of the DDFT. Ultrasonographic examination may reveal slight enlargement of the DDFT. The dorsal
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border may be less well defined, and diffuse hypoechogenic regions may occur within the DDFT, extending a variable distance proximodistally. These injuries usually result in recurrent lameness. It has also been noted that in association with substantial enlargement of the superficial digital flexor tendon (SDFT) because of chronic tendonitis, the DDFT becomes smaller in cross-sectional area.3 With chronic enlargement of the ALDDFT, the DDFT also may reduce in size. Primary deep digital flexor tendonitis in the proximal metacarpal region is rare. A single case was recorded by Genovese and Rantanen4 in an 8-year-old Quarter Horse used for English pleasure riding. Lesions have been identified using magnetic resonance imaging in a small number of horses.5,6 Occasionally, traumatic injuries of the DDFT have been seen within the carpal sheath (see Chapter 75).
DEEP DIGITAL FLEXOR TENDONITIS IN THE CARPAL SHEATH SECONDARY TO SOLITARY OSTEOCHONDROMA OR A DISTAL RADIAL PHYSEAL EXOSTOSIS Lesions of the DDFT within the carpal sheath are an unusual cause of lameness except secondary to irritation by a solitary osteochondroma or a distal radial physeal exostosis.7 An osteochondroma is an exostosis continuous with the cortex of the bone and is covered by cartilage. The osteochondroma develops immediately proximal to the distal radial physis, often medial to the midline. Lameness is sudden in onset and usually is accentuated by carpal flexion. There is usually distention of the carpal sheath, but some horses with physeal exostoses have severe, sporadic lameness with no localizing clinical signs. An osteochondroma is readily identifiable radiologically, but some physeal exostoses are more readily identified using ultrasonography. Ultrasonographic examination from the distal medial aspect of the antebrachium also reveals the abnormal bone contour, an abnormal amount of fluid within the carpal sheath, and an irregular dorsal contour of the DDFT. Treatment is by tenoscopic surgical removal of the osteochondroma or exostosis and debridement of any torn fibers of the DDFT. The prognosis for return to athletic function is excellent.
DEEP DIGITAL FLEXOR TENDONITIS WITHIN THE DIGITAL FLEXOR TENDON SHEATH IN THE FETLOCK REGION Some enlargement of the DFTS is common in hindlimbs, often unassociated with lameness, but occurs less frequently in forelimbs. Sudden-onset lameness associated with distention of a DFTS in a forelimb or a hindlimb may be caused by a variety of different lesions, but deep digital flexor tendonitis always should be considered5,6 (see Chapter 74). It is rare to identify lesions of the DDFT within a DFTS that is not distended. Some horses develop deep digital flexor tendonitis after long-term chronic enlargement of the DFTS. Lameness associated with DDFT lesions within the DFTS occurs more frequently in hindlimbs than in forelimbs and in horses from a variety of disciplines. The
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A
B
Fig. 70-1 • Transverse (A) and longitudinal (B) ultrasonographic images of the left hindlimb of a horse with sporadic left hindlimb lameness. There was chronic moderate distention of the digital flexor tendon sheath of both hindlimbs. There is diffuse reduction in echogenicity in the dorsal half of the deep digital flexor tendon (A, arrows) and loss of fiber pattern (B, arrows).
condition usually occurs unilaterally, although it has been seen bilaterally in the hindlimbs of several Warmblood dressage horses3 and in show jumpers. Lameness varies from mild to moderately severe. Distention and thickening of the DFTS may make accurate palpation of the DDFT difficult. In some horses pain can be elicited by palpation of the margins of the tendon or by firm pressure applied to its palmar (plantar) aspect. The tendon should be assessed throughout its length, proximal and distal to the fetlock. In the acute stage there may be localized heat. Passive flexion of the lower limb may induce pain. If forelimb lameness is only mild in straight lines, it may be exaggerated on the lunge on a soft surface, especially in medium and extended trot. Distal limb flexion often accentuates the lameness. Occasionally in hindlimbs lesions of the DDFT within the DFTS have been the cause of sporadic lameness. The intermittent nature of the lameness makes definitive identification of the cause a diagnostic challenge. Such a history seen in conjunction with distention of the DFTS should prompt ultrasonographic examination (Figure 70-1).16 Intrathecal analgesia of the DFTS usually results in substantial improvement but rarely alleviates lameness. Better improvement is seen after perineural analgesia of the palmar or plantar nerves and palmar metacarpal (plantar metatarsal) nerves (a so-called low 4-point block) proximal to the distended DFTS. In horses in which the metacarpophalangeal (metatarsophalangeal) joint capsule also is distended, performing intraarticular analgesia may be necessary to be sure that distention is not contributing to pain. Definitive diagnosis requires ultrasonographic examination. Four types of lesions involving the DDFT have been identified: enlargement and change in shape of the tendon, focal hypoechoic lesions within the tendon or on its border, mineralization within the DDFT, and marginal tears.8-14 The first three are readily diagnosed using
diagnostic ultrasonography, but the marginal tears are much more difficult to identify. Surgical exploration may be required for definitive diagnosis.8 Acute-onset focal hypoechogenic areas generally are not seen with preexisting adhesion formation, although any of the other lesions may be. The normal DDFT changes in its shape and crosssectional area from proximally to distally, but it is usually bilaterally symmetrical. A normal DDFT is uniform in its echogenicity, and its margins are clearly defined. At the site at which the ALDDFT merges with the DDFT there may be a relatively hypoechogenic region, especially in the hindlimbs. This is a normal variant. Hypoechoic artifacts are induced readily in the distal fetlock and pastern regions if the ultrasound transducer is not perpendicular to the tendon, and in these regions evaluating the SDFT and DDFT simultaneously is difficult. Echogenic synovial plicae (mesotendon) extend medially and laterally from the DDFT to the DFTS wall in the proximal recess of the DFTS (Figure 70-2, A). These are seen much more obviously when the tendon sheath is distended and should not be mistaken for marginal tears or adhesions. With chronic tenosynovitis, these plicae may become thickened. Distal to the fetlock is an echogenic palmar (plantar) synovial fold that should not be confused with an adhesion (see Figure 70-2, B). The ergot on the palmar (plantar) distal aspect of the fetlock prohibits ultrasonographic evaluation at this level, and lesions of the DDFT may be missed. The size of the palmar (plantar) annular ligament (PAL) and the presence of subcutaneous fibrosis should also be evaluated because this may influence prognosis.13
Enlargement or Change in Shape of the Deep Digital Flexor Tendon
In horses with low-grade injuries the only detectable ultrasonographic abnormality is a change in shape or size of the
Chapter 70 The Deep Digital Flexor Tendon
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B
Fig. 70-2 • Transverse ultrasonographic images of the digital flexor tendon sheath (DFTS) in (A) the distal metacarpal region and (B) the pastern. Medial is to the left. A, An abnormal amount of fluid is within the DFTS, but the deep digital flexor tendon (DDFT) appears normal. Note the prominent synovial plica extending from the lateral aspect of the DDFT. This plica also is thickened slightly, and other echogenic material is within the anechogenic synovial fluid, but the horse had no associated lameness. B, Note the abnormal amount of fluid within the DFTS, and the synovial plica on the palmar aspect of the DDFT. These are normal anatomical structures that are visible most readily when the DFTS is distended.
DDFT. The echogenicity and fiber pattern may appear normal; therefore these injuries are easily overlooked. The DDFT may look rounder in cross-section, rather than elliptical. Careful comparison with the contralateral limb at the same distance distal to the accessory carpal bone is important for accurate diagnosis. Comparison of the relative sizes of the SDFT and DDFT can also be helpful. Cross-sectional area measurements of the SDFT are prone to error at this level, and therefore measurements of the dorsopalmar thickness of the SDFT and DDFT are more accurate. If the horse fails to respond satisfactorily to conservative management, then consideration should be given to the presence of a marginal tear that may not be detectable with ultrasonography.3 Constriction of the DFTS and its contents by an enlarged PAL may result in secondary compression of the DDFT.
Focal Hypoechoic Lesions
The cross-sectional area of the DDFT may be enlarged. Focal hypoechoic lesions vary in size and position within the cross-section of the tendon and in length. Some are small, occupying less than one tenth of the cross-sectional area of the tendon and extending less than 1 cm proximodistally, whereas others are considerably more extensive (Figure 70-3).5 Generally, larger lesions are associated with more severe lameness. These lesions generally occur immediately proximal to, or at the level of, the fetlock joint. An abnormal amount of synovial fluid is found within the DFTS, and sometimes adhesions are identified. In some horses, inspection of the medial or lateral border of the DDFT reveals loss of definition of the margin
of the tendon and an area of reduced echogenicity, indicating major fiber disruption. These lesions are easy to diagnose. The marginal tears described subsequently (see page 730) are more difficult to detect. Small lesions have resolved with rest, and horses have returned to full athletic function. However, large lesions tend to persist, and ultrasonographic examination may reveal no change, even if the horse has been rested for more than 1 year. Large lesions have been associated with recurrent lameness.8 Intrathecal medication with hyalu ronan, triamcinolone acetonide, or methylprednisolone acetate has resulted in only temporary relief. Desmotomy of the PAL has resulted in only temporary remission of clinical signs. The prognosis for return to full athletic function is guarded. A few sports horses have been successfully managed by intralesional injection of cultured mesenchymal stem cells, with subsequent ultrasonographic resolution of lesions and return to full athletic function, without recurrent injury for up to 3 years after treatment.23
Fibrosis and Mineralization within the Deep Digital Flexor Tendon
Some horses develop widespread hyperechogenic foci within the DDFT in the fetlock region. Some of these create shadowing artifacts and therefore represent mineralization (Figure 70-4). The foci generally but not invariably reflect a chronic injury.14 If extensive, they make evaluation of the remaining tendon architecture difficult because of acoustic shadowing. The cross-sectional area of the tendon usually is enlarged. Ill-defined hypoechogenic regions
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A
B
Fig. 70-3 • Transverse ultrasonographic images of the distal metatarsal region of a horse with acute-onset right hindlimb lameness associated with distention of the digital flexor tendon sheath (DFTS). Medial is to the left. An abnormal amount of fluid is within the DFTS. A, There is a well-defined anechogenic lesion within the deep digital flexor tendon, close to the plantar border. Note also the rather poorly defined dorsal border. B, Slightly farther distally the lesion can be seen to involve the dorsal and plantar borders of the tendon.
injections of corticosteroids into the DFTS. Most of the affected horses are middle-aged Warmblood-type horses used for show jumping or dressage. These lesions tend to result in persistent long-term lameness.
Marginal Tears of the Deep Digital Flexor Tendon
Fig. 70-4 • Transverse ultrasonographic image of the distal metatarsal region of an 8-year-old event horse with chronic right hindlimb lameness associated with distention of the digital flexor tendon sheath. Medial is to the left. The deep digital flexor tendon (DDFT) is enlarged. Many hyper echogenic foci are within the DDFT, resulting in shadowing artifacts that make accurate evaluation of the structure of the more dorsal aspects of the tendon impossible.
often extend proximally or distally from hyperechogenic foci or are remote from them. Fibrous adhesions may extend from the DDFT to the DFTS wall. The cause of these hyperechogenic lesions is unknown, although some affected horses have a history of previous
Marginal tears of the DDFT have been identified on the medial, lateral, and dorsal borders.3,8 Lesions involving the lateral margin have been identified most commonly, varying in length (Figure 70-5). Frequently the proximal extent of the tear is at the level of the manica flexoria. Some lesions extend as far distally as the proximal digital annular ligament. In some horses the lesion is a frontal (dorsal) plane split in the margin of the tendon; in other horses a sagittal tear of fibers occurs. Frontal plane splits in the DDFT, which may be the result of compression of the tendon with the fetlock in extension, may not be detectable by ultrasonography, although often a detectable enlargement of the cross-sectional area of the DDFT exists. The affected margin may be slightly less well defined compared with normal findings. Occasionally a torn echogenic strand is seen partially detached from the DDFT. Care must be taken not to confuse the synovial plicae on the medial and lateral borders of the tendon with a marginal tear. It is important to recognize that lesions may occur at more than one location; therefore evaluation of all accessible regions of the DDFT is recommended. Diagnosis is based on surgical exploration of the DFTS, which is indicated if a horse has pain associated with the DFTS along with abnormalities of the DDFT detected by ultrasonography, or no detectable changes, but a failure to
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skin surface and so is vulnerable to the effects of direct trauma and puncture wounds. In the pastern region the DDFT is a bilobed structure, each lobe being similar in size and shape at any level. Care must be taken when evaluating the tendon by ultrasonography because off-incidence artifacts are created readily. These may be seen as central, round hypoechogenic areas and should not be mistaken for lesions.
Deep Digital Flexor Tendonitis
Fig. 70-5 • The lateral margin of the deep digital flexor tendon (DDFT) within the digital flexor tendon sheath in the distal metacarpal region, viewed tenoscopically. The DDFT has a marginal tear. Ultrasonographic examination revealed slight enlargement of the DDFT, but otherwise the tendon appeared structurally normal. (Courtesy I.M. Wright, Newmarket, England.)
respond to conservative management (see Chapter 74). Tenoscopic evaluation via a portal on the proximopalmar (proximoplantar) aspect of the pastern enables the most comprehensive view of the flexor tendons.17 However, proper evaluation of the DDFT underneath the manica flexoria may not be possible unless the manica flexoria is transected. Treatment can be by debridement of the lesion, which can be done tenoscopically, or by debridement and suturing. The latter may be achieved most effectively by transection of the palmar PAL and opening of the DFTS, followed by primary closure of the PAL; however, this technique is potentially associated with some morbidity. Some horses develop thickening of the PAL and/or the proximal digital annular ligament postoperatively, together with subcutaneous fibrosis, which results in persistent lameness. Five of six horses used for dressage, show jumping, or horse trials treated by open surgery to repair defects in the margin of the DDFT had persistent lameness associated with this type of reaction.3 Therefore tenoscopic debridement is the preferred method of management, although the prognosis remains guarded. Prognosis for horses treated by tenoscopic debridement depends in part on the length of the lesion, with 40% to 59% of horses returning to full athletic function.10,12 Generally horses with hindlimb injuries have had a better prognosis than those with forelimb injuries.
DEEP DIGITAL FLEXOR INJURIES IN THE PASTERN REGION The DDFT within the pastern remains within the DFTS, over which lie the thin proximal and distal digital annular ligaments. The tendon is therefore relatively close to the
Deep digital flexor tendonitis restricted to the pastern region is not a common cause of lameness and occurs less often than sprain of the oblique sesamoidean ligaments or strain of one of the branches of the SDFT (see Chapter 32). Many horses with lesions of the DDFT have lesions that extend into the hoof capsule and are discussed in Chapter 32. Tendonitis of the DDFT in the pastern region is seen most often in skeletally mature horses and occurs more often in forelimbs than in hindlimbs. Deep digital flexor tendonitis occurs in a variety of sports horses, but the incidence in racehorses is lower than in other performance horses. Deep digital flexor tendonitis in the pastern region usually results in acute-onset, unilateral, moderate-tosevere lameness that is persistent. Less commonly lameness is detectable only after maximal exertion and has resolved with rest but has progressively worsened over 1 to 2 years.3 Slight, firm soft tissue swelling may occur on the palmar midline of the pastern region. Often, slight distention of the DFTS occurs in the pastern region, but an obvious windgall may not be apparent. Firm palpation on, or just to one side of, the palmar midline of the pastern region may cause pain. Lameness is often worse on a soft surface compared with a hard surface and generally is improved substantially by perineural analgesia of the palmar nerves at the level of the proximal sesamoid bones. Diagnosis is based on ultrasonographic evaluation. Careful comparison of the size and shape of the contralateral DDFT is useful. The ease with which the tendon can be evaluated distally in the pastern depends on the conformation of the pastern and foot. Evaluation is more difficult in horses with a narrow heel and a deep cleft between each bulb. A lesion may involve just one lobe of the tendon or both and is usually characterized by enlargement and alteration in shape of the tendon, with or without hypoechogenic regions within the tendon. Off-incidence imaging may be necessary to identify areas of fibrosis. The medial and lateral margins of the tendon should be carefully evaluated. In horses with chronic injuries focal hyperechogenic foci consistent with dystrophic mineralization may be seen. Occasionally a complex of injuries is identified involving not only the DDFT, but also a distal sesamoidean ligament or the distal digital annular ligament. All structures should be examined carefully and systematically. Horses with lesions restricted to the DDFT that have been recognized early usually respond reasonably to prolonged rest (6 to 12 months), with progressive resolution of hypoechoic defects as assessed by ultrasonography. Some enlargement of the tendon may persist long term. Horses with more chronic injuries or complex injuries including adhesion formation respond less favorably, with a high incidence of recurrent lameness, even with surgical intervention. There are anecdotal reports of successful
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PART VIII The Soft Tissues
treatment by intralesional injection of platelet-rich plasma or cultured mesenchymal stem cells. However, if the periphery of the DDFT is not intact, leakage of mesenchymal stem cells may result in echogenic tissue being laid down within the DFTS, with associated chronic lameness. Lesions of the DDFT have been described in association with primary injuries of the distal digital annular ligament diagnosed using magnetic resonance imaging.24 Such injuries have been characterized by focal thickening of the distal digital annular ligament and increased signal intensity in at least T1- and T2-weighted images. It was suggested that lameness could be successfully treated by transection of the distal digital annular ligament, but given the integral relationship between the DDFT and the distal digital annular ligament it is difficult to see how this can be achieved. In my clinical experience injuries of the DDFT and the ipsilateral aspect of the distal digital annular ligament are usually identified in association with other soft tissue injuries on the ipsilateral aspect of the foot.
Injury Caused by Blunt Trauma in the Pastern Region
Direct blunt trauma caused by, for example, a severe overreach can result in severe inflammation of the subcutaneous tissue, the digital annular ligaments, and sometimes also the DDFT. If the proximal or distal digital annular ligament becomes thickened and fibrosed, this can create undue pressure on the DDFT and chronic pain, without any structural abnormality of the DDFT. In the acute stage, generalized swelling occurs in the pastern region, and palpation elicits pain. Ultrasonographic examination is required to determine which structures have been damaged. This may need to be repeated as fibrotic reactions develop in the ensuing weeks. The prognosis is generally favorable unless the digital annular ligaments become substantially thickened or the palmar aspect of the DDFT is torn. An overreach sometimes results in laceration of the skin and the palmar aspect of the DDFT. These lacerations may not coincide because of the mobile nature of the skin, and damage to the DDFT may be overlooked unless the tendon is examined by ultrasonography. One or both branches of the SDFT may also be damaged, so all structures should be evaluated carefully and systematically. Such lesions often heal with extensive fibrotic reactions and adhesion formation to the DFTS, which may result in chronic lameness.
was embedded within the DDFT that might seriously compromise the outcome. Alternatively, blunt trauma to the pastern region may result in an innocuous wound followed by the subsequent development of severe lameness in association with a core lesion of the DDFT. It is postulated that this may be the result of avascular necrosis of the tendon.18 The prognosis is poor.
Rupture of the Deep Digital Flexor Tendon
Rupture of the DDFT in the pastern region or within the hoof capsule is usually a sequela to a previous neurectomy of the palmar digital nerves. Neurectomy is usually performed because of suspected navicular disease or to relieve chronic foot pain of unknown cause. Preexisting lesions of the DDFT are likely to predispose to tendon rupture. I have never examined a horse with spontaneous rupture of the DDFT that did not have a history of neurectomy. Lameness may be insidious and progressive or sudden in onset, associated with extensive swelling in the pastern region. Thickening results from enlargement of the DDFT, peritendonous fibrosis, and, in horses with long-term injuries, secondary superficial digital flexor tendonitis. Complete rupture results in the toe of the limb flipping up when the limb is bearing weight (Figure 70-6). These clinical signs are pathognomonic. In such horses the proximal end of the DDFT may retract proximally, resulting in the SDFT becoming opposed to the fibrocartilaginous scutum on the palmar aspect of the pastern region. Radiologically a variable degree of subluxation of the distal interphalangeal joint exists, depending on the degree of integrity of the DDFT. The prognosis is hopeless for athletic function. Extensive peritendonous fibrosis ultimately develops, providing adequate
Puncture Wounds of the Deep Digital Flexor Tendon and Blunt Trauma
Puncture wounds in the palmar (plantar) aspect of the pastern region, caused by sharp objects such as a flint, can cause sudden-onset, moderate-to-severe lameness and rapid development of effusion within the DFTS. Ultrasonographic examination can be used to determine whether the DDFT was penetrated. An anechoic defect may be seen in its palmar border. With prompt treatment by vigorous lavage of the DFTS combined with systemic broad-spectrum antimicrobial therapy, the outcome may be favorable. Ideally the DFTS and its contents should be inspected using an arthroscope to determine if any foreign material
Fig. 70-6 • The distal aspect of the right forelimb of a horse with rupture of the deep digital flexor tendon 5 months after palmar digital neurectomy. The toe is flipped upward, associated with subluxation of the distal interphalangeal joint. There is considerable thickening in the pastern region caused by peritendonous fibrosis. Note the surgical scar.
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support to preserve life for breeding, assuming that the contralateral limb can withstand the strain in the interim. Arthrodesis of the distal interphalangeal joint remains a management option for salvage of a horse for breeding purposes or pasture soundness.
LESIONS OF THE DEEP DIGITAL FLEXOR TENDON WITHIN THE HOOF CAPSULE Lesions of the DDFT within the hoof capsule are discussed in Chapter 32.
LESIONS OF THE DEEP DIGITAL FLEXOR TENDON IN THE TARSAL AND PROXIMAL METATARSAL REGIONS Primary deep digital flexor tendonitis in the tarsal or proximal metatarsal regions is an unusual cause of hindlimb lameness (see Chapter 76). Lesions in the proximal metatarsal region have been identified in young Thoroughbreds in race training, with lameness associated with mild swelling in the proximal plantar metatarsal region. Lameness was improved transiently by intraarticular analgesia of the tarsometatarsal joint in two horses, presumably because of local diffusion of local anesthetic solution.3 Subtarsal analgesia of the plantar and plantar metatarsal nerves alleviated lameness in one horse. Ultrasonographic examination revealed enlargement of the DDFT in the proximal metatarsal region, poor definition of the margins, and diffuse hypoechogenic areas within the tendon. Rest for 3 months resulted in resolution of lameness, and the prognosis is favorable for return to racing. Primary lesions of the DDFT in the tarsal region have been identified only in skeletally mature horses and are comparatively rare. Lameness is sudden in onset and moderate in degree. Mild distention of the tarsal sheath may occur, but localizing signs may be absent. Lameness is improved by perineural analgesia of the tibial and fibular nerves, but it is unaffected by intraarticular analgesia of the hock joints. Diagnosis relies on ultrasonographic examination. The DDFT may be examined from the plantaromedial aspect of the hock within the tarsal sheath. The DDFT is a large oval structure with well-defined margins. The dorsolateral aspect of the tendon may appear slightly less echogenic than the remainder of the tendon due to structural adaptation, and care should be taken not to misinterpret this as a lesion (Figure 70-7). Careful comparison with the contralateral limb is essential. Abnormalities include enlargement of the tendon, poor demarcation of the borders, and hypoechogenic areas. Lesions may also be identified by tenoscopic examination of the tarsal sheath (see Chapters 24 and 76). Lesions of the DDFT also have been identified in association with bony lesions of the sustentaculum tali of the
Fig. 70-7 • Transverse ultrasonographic image of the plantaromedial aspect of the distal tarsal region of a normal horse. Medial is to the left. Note the relatively hypoechogenic area dorsolaterally (arrows), a normal finding.
fibular tarsal bone (calcaneus) and ectopic mineralization within the tarsal sheath.19 These horses may have fibrillation of the DDFT and adhesion formation. These lesions generally are seen with obvious distention of the tarsal sheath and long-term lameness and probably occur secondary to primary pathological conditions of the bone. The sustentaculum tali should be evaluated radiologically using plantarolateral-dorsomedial oblique and flexed dorsoplantar images. The prognosis for athletic performance is poor with conservative or surgical management. Dorsomedial luxation of the DDFT secondary to congenital malformation of the sustentaculum tali has been recognized in young Saddlebred horses and occasionally in Thoroughbreds.3,20 Clinically the plantar aspect of the tarsal region appears broader than normal, with tarsus valgus. Lameness may not be obvious. Diagnosis is based on radiological examination: the sustentaculum tali appears flattened in a dorsoplantar (flexed) image.21 This may result in a mechanical lameness. Surgical treatment has been attempted, but the horse did not race.22 Congenital medial displacement of both the DDFT and SDFT in a hindlimb has also been seen, with a secondary valgus deformation of the metatarsal region and distal physeal trauma of the third metatarsal bone.23
INFECTION OF THE DEEP DIGITAL FLEXOR TENDON Infection of the DDFT is discussed in Chapter 37 (see page 423).
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The Deep Digital Flexor Tendon Sue J. Dyson ANATOMY
References on page 1315
In the forelimb the deep digital flexor tendon (DDFT) has three heads: the humeral head, the largest, and the smaller radial and ulnar heads.1 The tendon of the humeral head
develops 8 to 10 cm proximal to the antebrachiocarpal joint, but muscular tissue persists to the level of the antebrachiocarpal joint, where the ulnar and radial heads join. The merged tendon is triangular in cross-section within the carpal canal but becomes more rounded in the metacarpal region. The accessory ligament of the DDFT (ALDDFT) merges with the DDFT in the middle third of the metacarpal region. In the hindlimb the DDFT is formed by a large lateral digital flexor tendon and a smaller medial digital flexor tendon. The lateral digital flexor tendon incorporates the caudal tibialis tendon and passes over the sustentaculum tali within the tarsal sheath. The medial digital flexor tendon passes over the proximal tubercle of the talus, on the medial aspect of the talus, in its own synovial sheath. These two tendons fuse in the proximal metatarsal region.
Chapter 70 The Deep Digital Flexor Tendon
In the proximal metatarsal region the DDFT is a large oval structure that becomes smaller farther distally. The ALDDFT in the hindlimb varies in size and is generally comparatively smaller than in the forelimb and may be absent; in some horses it is a bifid structure. At the fetlock region the DDFT becomes wider, elliptical, and fibrocartilaginous and is enclosed within the digital flexor tendon sheath (DFTS). In the pastern region the tendon becomes bilobed. At the level of the proximal part of the middle phalanx the dorsal part of the tendon becomes a fibrocartilaginous pad. Distally the DDFT is molded to the palmar or plantar aspect of the navicular bone. The DDFT is broad, has a terminal fanlike expansion containing cartilage, and inserts on the facies flexoria of the distal phalanx, delineated dorsally by the semilunar line and the adjacent surface of the cartilage of the foot. The DDFT has a high modulus of elasticity (1585 MPa) and a considerable strength to rupture (approximately 1700 daN).1 The ALDDFT has a low modulus of elasticity (490 MPa) and a moderate strength to rupture (approximately 490 daN). The DDFT limits carpal and fetlock extension under high loads. In the fetlock region the DDFT is under tension and compression. It is therefore fibrocartilaginous in this region and in the pastern, where the tendon is under pressure from the tuberositas flexoria, a transverse prominence on the proximal palmar aspect of the middle phalanx. In the digit the DDFT facilitates flexion of the proximal interphalangeal joint during weight bearing and stabilizes the distal interphalangeal joint. The position of the DDFT and the navicular bone varies considerably during the stance phase. In the full weight-bearing position the DDFT is in close contact with only the distal border of the navicular bone, but during propulsion it comes into full contact with the palmar aspect of the bone. The tendon is stretched maximally as active contraction of the muscle bellies and the elasticity in the tendon result in elevation of the fetlock and extension of the distal interphalangeal joint. During the swing phase of the stride the DDFT relaxes. The DFTS facilitates displacement of the digital flexor tendons during flexion and extension. DDFT injuries occur most commonly in the fetlock or pastern regions within the DFTS or within the hoof capsule. Injuries in the metacarpal or metatarsal regions and the carpus or tarsus are less common.2-22
DEEP DIGITAL FLEXOR TENDONITIS ASSOCIATED WITH RECURRENT DESMITIS OF THE ACCESSORY LIGAMENT OF THE DEEP DIGITAL FLEXOR TENDON Injuries of the DDFT in the carpal or metacarpal region, proximal to the DFTS, are rare except in association with chronic desmitis of the ALDDFT (see Chapter 71). Recurrent desmitis may be accompanied by pathological lesions of the DDFT.2 Because of the close proximity of the DDFT and its accessory ligament, it is difficult to assess each structure accurately by palpation, especially with chronic enlargement of the ALDDFT, which may wrap around the borders of the DDFT. Ultrasonographic examination may reveal slight enlargement of the DDFT. The dorsal
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border may be less well defined, and diffuse hypoechogenic regions may occur within the DDFT, extending a variable distance proximodistally. These injuries usually result in recurrent lameness. It has also been noted that in association with substantial enlargement of the superficial digital flexor tendon (SDFT) because of chronic tendonitis, the DDFT becomes smaller in cross-sectional area.3 With chronic enlargement of the ALDDFT, the DDFT also may reduce in size. Primary deep digital flexor tendonitis in the proximal metacarpal region is rare. A single case was recorded by Genovese and Rantanen4 in an 8-year-old Quarter Horse used for English pleasure riding. Lesions have been identified using magnetic resonance imaging in a small number of horses.5,6 Occasionally, traumatic injuries of the DDFT have been seen within the carpal sheath (see Chapter 75).
DEEP DIGITAL FLEXOR TENDONITIS IN THE CARPAL SHEATH SECONDARY TO SOLITARY OSTEOCHONDROMA OR A DISTAL RADIAL PHYSEAL EXOSTOSIS Lesions of the DDFT within the carpal sheath are an unusual cause of lameness except secondary to irritation by a solitary osteochondroma or a distal radial physeal exostosis.7 An osteochondroma is an exostosis continuous with the cortex of the bone and is covered by cartilage. The osteochondroma develops immediately proximal to the distal radial physis, often medial to the midline. Lameness is sudden in onset and usually is accentuated by carpal flexion. There is usually distention of the carpal sheath, but some horses with physeal exostoses have severe, sporadic lameness with no localizing clinical signs. An osteochondroma is readily identifiable radiologically, but some physeal exostoses are more readily identified using ultrasonography. Ultrasonographic examination from the distal medial aspect of the antebrachium also reveals the abnormal bone contour, an abnormal amount of fluid within the carpal sheath, and an irregular dorsal contour of the DDFT. Treatment is by tenoscopic surgical removal of the osteochondroma or exostosis and debridement of any torn fibers of the DDFT. The prognosis for return to athletic function is excellent.
DEEP DIGITAL FLEXOR TENDONITIS WITHIN THE DIGITAL FLEXOR TENDON SHEATH IN THE FETLOCK REGION Some enlargement of the DFTS is common in hindlimbs, often unassociated with lameness, but occurs less frequently in forelimbs. Sudden-onset lameness associated with distention of a DFTS in a forelimb or a hindlimb may be caused by a variety of different lesions, but deep digital flexor tendonitis always should be considered5,6 (see Chapter 74). It is rare to identify lesions of the DDFT within a DFTS that is not distended. Some horses develop deep digital flexor tendonitis after long-term chronic enlargement of the DFTS. Lameness associated with DDFT lesions within the DFTS occurs more frequently in hindlimbs than in forelimbs and in horses from a variety of disciplines. The
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A
B
Fig. 70-1 • Transverse (A) and longitudinal (B) ultrasonographic images of the left hindlimb of a horse with sporadic left hindlimb lameness. There was chronic moderate distention of the digital flexor tendon sheath of both hindlimbs. There is diffuse reduction in echogenicity in the dorsal half of the deep digital flexor tendon (A, arrows) and loss of fiber pattern (B, arrows).
condition usually occurs unilaterally, although it has been seen bilaterally in the hindlimbs of several Warmblood dressage horses3 and in show jumpers. Lameness varies from mild to moderately severe. Distention and thickening of the DFTS may make accurate palpation of the DDFT difficult. In some horses pain can be elicited by palpation of the margins of the tendon or by firm pressure applied to its palmar (plantar) aspect. The tendon should be assessed throughout its length, proximal and distal to the fetlock. In the acute stage there may be localized heat. Passive flexion of the lower limb may induce pain. If forelimb lameness is only mild in straight lines, it may be exaggerated on the lunge on a soft surface, especially in medium and extended trot. Distal limb flexion often accentuates the lameness. Occasionally in hindlimbs lesions of the DDFT within the DFTS have been the cause of sporadic lameness. The intermittent nature of the lameness makes definitive identification of the cause a diagnostic challenge. Such a history seen in conjunction with distention of the DFTS should prompt ultrasonographic examination (Figure 70-1).16 Intrathecal analgesia of the DFTS usually results in substantial improvement but rarely alleviates lameness. Better improvement is seen after perineural analgesia of the palmar or plantar nerves and palmar metacarpal (plantar metatarsal) nerves (a so-called low 4-point block) proximal to the distended DFTS. In horses in which the metacarpophalangeal (metatarsophalangeal) joint capsule also is distended, performing intraarticular analgesia may be necessary to be sure that distention is not contributing to pain. Definitive diagnosis requires ultrasonographic examination. Four types of lesions involving the DDFT have been identified: enlargement and change in shape of the tendon, focal hypoechoic lesions within the tendon or on its border, mineralization within the DDFT, and marginal tears.8-14 The first three are readily diagnosed using
diagnostic ultrasonography, but the marginal tears are much more difficult to identify. Surgical exploration may be required for definitive diagnosis.8 Acute-onset focal hypoechogenic areas generally are not seen with preexisting adhesion formation, although any of the other lesions may be. The normal DDFT changes in its shape and crosssectional area from proximally to distally, but it is usually bilaterally symmetrical. A normal DDFT is uniform in its echogenicity, and its margins are clearly defined. At the site at which the ALDDFT merges with the DDFT there may be a relatively hypoechogenic region, especially in the hindlimbs. This is a normal variant. Hypoechoic artifacts are induced readily in the distal fetlock and pastern regions if the ultrasound transducer is not perpendicular to the tendon, and in these regions evaluating the SDFT and DDFT simultaneously is difficult. Echogenic synovial plicae (mesotendon) extend medially and laterally from the DDFT to the DFTS wall in the proximal recess of the DFTS (Figure 70-2, A). These are seen much more obviously when the tendon sheath is distended and should not be mistaken for marginal tears or adhesions. With chronic tenosynovitis, these plicae may become thickened. Distal to the fetlock is an echogenic palmar (plantar) synovial fold that should not be confused with an adhesion (see Figure 70-2, B). The ergot on the palmar (plantar) distal aspect of the fetlock prohibits ultrasonographic evaluation at this level, and lesions of the DDFT may be missed. The size of the palmar (plantar) annular ligament (PAL) and the presence of subcutaneous fibrosis should also be evaluated because this may influence prognosis.13
Enlargement or Change in Shape of the Deep Digital Flexor Tendon
In horses with low-grade injuries the only detectable ultrasonographic abnormality is a change in shape or size of the
Chapter 70 The Deep Digital Flexor Tendon
A
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B
Fig. 70-2 • Transverse ultrasonographic images of the digital flexor tendon sheath (DFTS) in (A) the distal metacarpal region and (B) the pastern. Medial is to the left. A, An abnormal amount of fluid is within the DFTS, but the deep digital flexor tendon (DDFT) appears normal. Note the prominent synovial plica extending from the lateral aspect of the DDFT. This plica also is thickened slightly, and other echogenic material is within the anechogenic synovial fluid, but the horse had no associated lameness. B, Note the abnormal amount of fluid within the DFTS, and the synovial plica on the palmar aspect of the DDFT. These are normal anatomical structures that are visible most readily when the DFTS is distended.
DDFT. The echogenicity and fiber pattern may appear normal; therefore these injuries are easily overlooked. The DDFT may look rounder in cross-section, rather than elliptical. Careful comparison with the contralateral limb at the same distance distal to the accessory carpal bone is important for accurate diagnosis. Comparison of the relative sizes of the SDFT and DDFT can also be helpful. Cross-sectional area measurements of the SDFT are prone to error at this level, and therefore measurements of the dorsopalmar thickness of the SDFT and DDFT are more accurate. If the horse fails to respond satisfactorily to conservative management, then consideration should be given to the presence of a marginal tear that may not be detectable with ultrasonography.3 Constriction of the DFTS and its contents by an enlarged PAL may result in secondary compression of the DDFT.
Focal Hypoechoic Lesions
The cross-sectional area of the DDFT may be enlarged. Focal hypoechoic lesions vary in size and position within the cross-section of the tendon and in length. Some are small, occupying less than one tenth of the cross-sectional area of the tendon and extending less than 1 cm proximodistally, whereas others are considerably more extensive (Figure 70-3).5 Generally, larger lesions are associated with more severe lameness. These lesions generally occur immediately proximal to, or at the level of, the fetlock joint. An abnormal amount of synovial fluid is found within the DFTS, and sometimes adhesions are identified. In some horses, inspection of the medial or lateral border of the DDFT reveals loss of definition of the margin
of the tendon and an area of reduced echogenicity, indicating major fiber disruption. These lesions are easy to diagnose. The marginal tears described subsequently (see page 730) are more difficult to detect. Small lesions have resolved with rest, and horses have returned to full athletic function. However, large lesions tend to persist, and ultrasonographic examination may reveal no change, even if the horse has been rested for more than 1 year. Large lesions have been associated with recurrent lameness.8 Intrathecal medication with hyalu ronan, triamcinolone acetonide, or methylprednisolone acetate has resulted in only temporary relief. Desmotomy of the PAL has resulted in only temporary remission of clinical signs. The prognosis for return to full athletic function is guarded. A few sports horses have been successfully managed by intralesional injection of cultured mesenchymal stem cells, with subsequent ultrasonographic resolution of lesions and return to full athletic function, without recurrent injury for up to 3 years after treatment.23
Fibrosis and Mineralization within the Deep Digital Flexor Tendon
Some horses develop widespread hyperechogenic foci within the DDFT in the fetlock region. Some of these create shadowing artifacts and therefore represent mineralization (Figure 70-4). The foci generally but not invariably reflect a chronic injury.14 If extensive, they make evaluation of the remaining tendon architecture difficult because of acoustic shadowing. The cross-sectional area of the tendon usually is enlarged. Ill-defined hypoechogenic regions
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A
B
Fig. 70-3 • Transverse ultrasonographic images of the distal metatarsal region of a horse with acute-onset right hindlimb lameness associated with distention of the digital flexor tendon sheath (DFTS). Medial is to the left. An abnormal amount of fluid is within the DFTS. A, There is a well-defined anechogenic lesion within the deep digital flexor tendon, close to the plantar border. Note also the rather poorly defined dorsal border. B, Slightly farther distally the lesion can be seen to involve the dorsal and plantar borders of the tendon.
injections of corticosteroids into the DFTS. Most of the affected horses are middle-aged Warmblood-type horses used for show jumping or dressage. These lesions tend to result in persistent long-term lameness.
Marginal Tears of the Deep Digital Flexor Tendon
Fig. 70-4 • Transverse ultrasonographic image of the distal metatarsal region of an 8-year-old event horse with chronic right hindlimb lameness associated with distention of the digital flexor tendon sheath. Medial is to the left. The deep digital flexor tendon (DDFT) is enlarged. Many hyper echogenic foci are within the DDFT, resulting in shadowing artifacts that make accurate evaluation of the structure of the more dorsal aspects of the tendon impossible.
often extend proximally or distally from hyperechogenic foci or are remote from them. Fibrous adhesions may extend from the DDFT to the DFTS wall. The cause of these hyperechogenic lesions is unknown, although some affected horses have a history of previous
Marginal tears of the DDFT have been identified on the medial, lateral, and dorsal borders.3,8 Lesions involving the lateral margin have been identified most commonly, varying in length (Figure 70-5). Frequently the proximal extent of the tear is at the level of the manica flexoria. Some lesions extend as far distally as the proximal digital annular ligament. In some horses the lesion is a frontal (dorsal) plane split in the margin of the tendon; in other horses a sagittal tear of fibers occurs. Frontal plane splits in the DDFT, which may be the result of compression of the tendon with the fetlock in extension, may not be detectable by ultrasonography, although often a detectable enlargement of the cross-sectional area of the DDFT exists. The affected margin may be slightly less well defined compared with normal findings. Occasionally a torn echogenic strand is seen partially detached from the DDFT. Care must be taken not to confuse the synovial plicae on the medial and lateral borders of the tendon with a marginal tear. It is important to recognize that lesions may occur at more than one location; therefore evaluation of all accessible regions of the DDFT is recommended. Diagnosis is based on surgical exploration of the DFTS, which is indicated if a horse has pain associated with the DFTS along with abnormalities of the DDFT detected by ultrasonography, or no detectable changes, but a failure to
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skin surface and so is vulnerable to the effects of direct trauma and puncture wounds. In the pastern region the DDFT is a bilobed structure, each lobe being similar in size and shape at any level. Care must be taken when evaluating the tendon by ultrasonography because off-incidence artifacts are created readily. These may be seen as central, round hypoechogenic areas and should not be mistaken for lesions.
Deep Digital Flexor Tendonitis
Fig. 70-5 • The lateral margin of the deep digital flexor tendon (DDFT) within the digital flexor tendon sheath in the distal metacarpal region, viewed tenoscopically. The DDFT has a marginal tear. Ultrasonographic examination revealed slight enlargement of the DDFT, but otherwise the tendon appeared structurally normal. (Courtesy I.M. Wright, Newmarket, England.)
respond to conservative management (see Chapter 74). Tenoscopic evaluation via a portal on the proximopalmar (proximoplantar) aspect of the pastern enables the most comprehensive view of the flexor tendons.17 However, proper evaluation of the DDFT underneath the manica flexoria may not be possible unless the manica flexoria is transected. Treatment can be by debridement of the lesion, which can be done tenoscopically, or by debridement and suturing. The latter may be achieved most effectively by transection of the palmar PAL and opening of the DFTS, followed by primary closure of the PAL; however, this technique is potentially associated with some morbidity. Some horses develop thickening of the PAL and/or the proximal digital annular ligament postoperatively, together with subcutaneous fibrosis, which results in persistent lameness. Five of six horses used for dressage, show jumping, or horse trials treated by open surgery to repair defects in the margin of the DDFT had persistent lameness associated with this type of reaction.3 Therefore tenoscopic debridement is the preferred method of management, although the prognosis remains guarded. Prognosis for horses treated by tenoscopic debridement depends in part on the length of the lesion, with 40% to 59% of horses returning to full athletic function.10,12 Generally horses with hindlimb injuries have had a better prognosis than those with forelimb injuries.
DEEP DIGITAL FLEXOR INJURIES IN THE PASTERN REGION The DDFT within the pastern remains within the DFTS, over which lie the thin proximal and distal digital annular ligaments. The tendon is therefore relatively close to the
Deep digital flexor tendonitis restricted to the pastern region is not a common cause of lameness and occurs less often than sprain of the oblique sesamoidean ligaments or strain of one of the branches of the SDFT (see Chapter 32). Many horses with lesions of the DDFT have lesions that extend into the hoof capsule and are discussed in Chapter 32. Tendonitis of the DDFT in the pastern region is seen most often in skeletally mature horses and occurs more often in forelimbs than in hindlimbs. Deep digital flexor tendonitis occurs in a variety of sports horses, but the incidence in racehorses is lower than in other performance horses. Deep digital flexor tendonitis in the pastern region usually results in acute-onset, unilateral, moderate-tosevere lameness that is persistent. Less commonly lameness is detectable only after maximal exertion and has resolved with rest but has progressively worsened over 1 to 2 years.3 Slight, firm soft tissue swelling may occur on the palmar midline of the pastern region. Often, slight distention of the DFTS occurs in the pastern region, but an obvious windgall may not be apparent. Firm palpation on, or just to one side of, the palmar midline of the pastern region may cause pain. Lameness is often worse on a soft surface compared with a hard surface and generally is improved substantially by perineural analgesia of the palmar nerves at the level of the proximal sesamoid bones. Diagnosis is based on ultrasonographic evaluation. Careful comparison of the size and shape of the contralateral DDFT is useful. The ease with which the tendon can be evaluated distally in the pastern depends on the conformation of the pastern and foot. Evaluation is more difficult in horses with a narrow heel and a deep cleft between each bulb. A lesion may involve just one lobe of the tendon or both and is usually characterized by enlargement and alteration in shape of the tendon, with or without hypoechogenic regions within the tendon. Off-incidence imaging may be necessary to identify areas of fibrosis. The medial and lateral margins of the tendon should be carefully evaluated. In horses with chronic injuries focal hyperechogenic foci consistent with dystrophic mineralization may be seen. Occasionally a complex of injuries is identified involving not only the DDFT, but also a distal sesamoidean ligament or the distal digital annular ligament. All structures should be examined carefully and systematically. Horses with lesions restricted to the DDFT that have been recognized early usually respond reasonably to prolonged rest (6 to 12 months), with progressive resolution of hypoechoic defects as assessed by ultrasonography. Some enlargement of the tendon may persist long term. Horses with more chronic injuries or complex injuries including adhesion formation respond less favorably, with a high incidence of recurrent lameness, even with surgical intervention. There are anecdotal reports of successful
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treatment by intralesional injection of platelet-rich plasma or cultured mesenchymal stem cells. However, if the periphery of the DDFT is not intact, leakage of mesenchymal stem cells may result in echogenic tissue being laid down within the DFTS, with associated chronic lameness. Lesions of the DDFT have been described in association with primary injuries of the distal digital annular ligament diagnosed using magnetic resonance imaging.24 Such injuries have been characterized by focal thickening of the distal digital annular ligament and increased signal intensity in at least T1- and T2-weighted images. It was suggested that lameness could be successfully treated by transection of the distal digital annular ligament, but given the integral relationship between the DDFT and the distal digital annular ligament it is difficult to see how this can be achieved. In my clinical experience injuries of the DDFT and the ipsilateral aspect of the distal digital annular ligament are usually identified in association with other soft tissue injuries on the ipsilateral aspect of the foot.
Injury Caused by Blunt Trauma in the Pastern Region
Direct blunt trauma caused by, for example, a severe overreach can result in severe inflammation of the subcutaneous tissue, the digital annular ligaments, and sometimes also the DDFT. If the proximal or distal digital annular ligament becomes thickened and fibrosed, this can create undue pressure on the DDFT and chronic pain, without any structural abnormality of the DDFT. In the acute stage, generalized swelling occurs in the pastern region, and palpation elicits pain. Ultrasonographic examination is required to determine which structures have been damaged. This may need to be repeated as fibrotic reactions develop in the ensuing weeks. The prognosis is generally favorable unless the digital annular ligaments become substantially thickened or the palmar aspect of the DDFT is torn. An overreach sometimes results in laceration of the skin and the palmar aspect of the DDFT. These lacerations may not coincide because of the mobile nature of the skin, and damage to the DDFT may be overlooked unless the tendon is examined by ultrasonography. One or both branches of the SDFT may also be damaged, so all structures should be evaluated carefully and systematically. Such lesions often heal with extensive fibrotic reactions and adhesion formation to the DFTS, which may result in chronic lameness.
was embedded within the DDFT that might seriously compromise the outcome. Alternatively, blunt trauma to the pastern region may result in an innocuous wound followed by the subsequent development of severe lameness in association with a core lesion of the DDFT. It is postulated that this may be the result of avascular necrosis of the tendon.18 The prognosis is poor.
Rupture of the Deep Digital Flexor Tendon
Rupture of the DDFT in the pastern region or within the hoof capsule is usually a sequela to a previous neurectomy of the palmar digital nerves. Neurectomy is usually performed because of suspected navicular disease or to relieve chronic foot pain of unknown cause. Preexisting lesions of the DDFT are likely to predispose to tendon rupture. I have never examined a horse with spontaneous rupture of the DDFT that did not have a history of neurectomy. Lameness may be insidious and progressive or sudden in onset, associated with extensive swelling in the pastern region. Thickening results from enlargement of the DDFT, peritendonous fibrosis, and, in horses with long-term injuries, secondary superficial digital flexor tendonitis. Complete rupture results in the toe of the limb flipping up when the limb is bearing weight (Figure 70-6). These clinical signs are pathognomonic. In such horses the proximal end of the DDFT may retract proximally, resulting in the SDFT becoming opposed to the fibrocartilaginous scutum on the palmar aspect of the pastern region. Radiologically a variable degree of subluxation of the distal interphalangeal joint exists, depending on the degree of integrity of the DDFT. The prognosis is hopeless for athletic function. Extensive peritendonous fibrosis ultimately develops, providing adequate
Puncture Wounds of the Deep Digital Flexor Tendon and Blunt Trauma
Puncture wounds in the palmar (plantar) aspect of the pastern region, caused by sharp objects such as a flint, can cause sudden-onset, moderate-to-severe lameness and rapid development of effusion within the DFTS. Ultrasonographic examination can be used to determine whether the DDFT was penetrated. An anechoic defect may be seen in its palmar border. With prompt treatment by vigorous lavage of the DFTS combined with systemic broad-spectrum antimicrobial therapy, the outcome may be favorable. Ideally the DFTS and its contents should be inspected using an arthroscope to determine if any foreign material
Fig. 70-6 • The distal aspect of the right forelimb of a horse with rupture of the deep digital flexor tendon 5 months after palmar digital neurectomy. The toe is flipped upward, associated with subluxation of the distal interphalangeal joint. There is considerable thickening in the pastern region caused by peritendonous fibrosis. Note the surgical scar.
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support to preserve life for breeding, assuming that the contralateral limb can withstand the strain in the interim. Arthrodesis of the distal interphalangeal joint remains a management option for salvage of a horse for breeding purposes or pasture soundness.
LESIONS OF THE DEEP DIGITAL FLEXOR TENDON WITHIN THE HOOF CAPSULE Lesions of the DDFT within the hoof capsule are discussed in Chapter 32.
LESIONS OF THE DEEP DIGITAL FLEXOR TENDON IN THE TARSAL AND PROXIMAL METATARSAL REGIONS Primary deep digital flexor tendonitis in the tarsal or proximal metatarsal regions is an unusual cause of hindlimb lameness (see Chapter 76). Lesions in the proximal metatarsal region have been identified in young Thoroughbreds in race training, with lameness associated with mild swelling in the proximal plantar metatarsal region. Lameness was improved transiently by intraarticular analgesia of the tarsometatarsal joint in two horses, presumably because of local diffusion of local anesthetic solution.3 Subtarsal analgesia of the plantar and plantar metatarsal nerves alleviated lameness in one horse. Ultrasonographic examination revealed enlargement of the DDFT in the proximal metatarsal region, poor definition of the margins, and diffuse hypoechogenic areas within the tendon. Rest for 3 months resulted in resolution of lameness, and the prognosis is favorable for return to racing. Primary lesions of the DDFT in the tarsal region have been identified only in skeletally mature horses and are comparatively rare. Lameness is sudden in onset and moderate in degree. Mild distention of the tarsal sheath may occur, but localizing signs may be absent. Lameness is improved by perineural analgesia of the tibial and fibular nerves, but it is unaffected by intraarticular analgesia of the hock joints. Diagnosis relies on ultrasonographic examination. The DDFT may be examined from the plantaromedial aspect of the hock within the tarsal sheath. The DDFT is a large oval structure with well-defined margins. The dorsolateral aspect of the tendon may appear slightly less echogenic than the remainder of the tendon due to structural adaptation, and care should be taken not to misinterpret this as a lesion (Figure 70-7). Careful comparison with the contralateral limb is essential. Abnormalities include enlargement of the tendon, poor demarcation of the borders, and hypoechogenic areas. Lesions may also be identified by tenoscopic examination of the tarsal sheath (see Chapters 24 and 76). Lesions of the DDFT also have been identified in association with bony lesions of the sustentaculum tali of the
Fig. 70-7 • Transverse ultrasonographic image of the plantaromedial aspect of the distal tarsal region of a normal horse. Medial is to the left. Note the relatively hypoechogenic area dorsolaterally (arrows), a normal finding.
fibular tarsal bone (calcaneus) and ectopic mineralization within the tarsal sheath.19 These horses may have fibrillation of the DDFT and adhesion formation. These lesions generally are seen with obvious distention of the tarsal sheath and long-term lameness and probably occur secondary to primary pathological conditions of the bone. The sustentaculum tali should be evaluated radiologically using plantarolateral-dorsomedial oblique and flexed dorsoplantar images. The prognosis for athletic performance is poor with conservative or surgical management. Dorsomedial luxation of the DDFT secondary to congenital malformation of the sustentaculum tali has been recognized in young Saddlebred horses and occasionally in Thoroughbreds.3,20 Clinically the plantar aspect of the tarsal region appears broader than normal, with tarsus valgus. Lameness may not be obvious. Diagnosis is based on radiological examination: the sustentaculum tali appears flattened in a dorsoplantar (flexed) image.21 This may result in a mechanical lameness. Surgical treatment has been attempted, but the horse did not race.22 Congenital medial displacement of both the DDFT and SDFT in a hindlimb has also been seen, with a secondary valgus deformation of the metatarsal region and distal physeal trauma of the third metatarsal bone.23
INFECTION OF THE DEEP DIGITAL FLEXOR TENDON Infection of the DDFT is discussed in Chapter 37 (see page 423).