Superficial Digital Flexor Tendon Healing: Ultrasonographic Evaluation of Therapies

MODERN DIAGNOSTIC IMAGING

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SUPERFICIAL DIGITAL FLEXOR TENDON HEALING: ULTRASONOGRAPHIC EVALUATION OF THERAPIES Virginia B. Reef, DVM

Until recently, it was difficult to critically evaluate tendon healing in vivo. Superficial digital flexor tendon injuries were considered healed when the injured tendon was cold and nonpainful, adequate time had passed for tendon healing to occur, and no recurrence of the injury was detected when the horse returned to athletic work. Ultrasonography has revolutionized the diagnosis, treatment, and management of tendon injuries. The presence, type, and severity of the tendon injury can be determined ultrasonographically.14, 26, 38, 47, 48, 50, 53, 54 The ultrasonographic findings associated with tendon injury can be differentiated from the sonographic findings in young horses in race training?' 10, 27, 28, 38, 39, 42, 52, 61 The sonographic evaluation of tendon healing can be used to assess the response of injured tendons to treatment in a controlled fashion, enabling more critical assessment of the available treatments. A rehabilitation program for a horse with an injured tendon can now be tailored to the individual horse, the injury sustained, the response of the injured tendon to the treatment instituted, and the exercise program recommended. 1, 13, 17-19, 21, 26, 34, 38, 50, 51, 54, 58 Essential to the critical evaluation of treatments for injured tendons is the accurate assessment of injury severity. This enables the veterinary clinician to compare treatment results in horses with superficial digital flexor tendon injury of similar severity. It

From the Department of Clinical Studies, Section of Sports Medicine and Imaging, New Bolton Center, University of Pennsylvania, Kennett Square, Pennsylvania

VETERINARY CLINICS OF NORTH AMERICA: EQUINE PRACTICE VOLUME 17 • NUMBER 1 • APRIL 2001

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is well documented that the outcome of horses sustaining an injury to the superficial digital flexor tendon is related to injury severity.17, 26, 38, 54 SONOGRAPHIC ASSESSMENT OF INJURY SEVERITY

The normal superficial digital flexor tendon in the metacarpal region has a uniform echogenicity, parallel fiber pattern, and cross-sectional area that ranges from 0.6 to 1.2 cm2 depending on the breed of horse. 24, 25,29,50 Width and thickness measurements of the superficial digital flexor tendon have been described in normal horses, but measurements of cross-sectional area have largely supplanted these assessments of tendon size. 8, 20, 25, 50 The cross-sectional area of the superficial digital flexor tendon should be similar at the same level in both front or hind limbs. 2o, 24,29,30,51,63,67,68 Tendon enlargement, with a decrease in echogenicity and preservation of fiber alignment, is an early indication of injury.50 This often precedes the development of an area of fiber disruption, which appears sonographically as an anechoic or hypoechoic region lacking parallel fibers. The area of fiber damage most frequently occurs in the center of an affected tendon and appears as an anechoic or hypoechoic "core" (Fig. 1).7, 10, 14, 17-22,38,39,42,44,47,50--56,58,61,62 Diffuse areas of fiber disruption also occur but are less common. Injury severity is a combination of the length of the lesion, tendon cross-sectional area, lesion cross-sectional area, lesion echogenicity, and fiber alignment within the area of injury.13, 17,21,22,26,51,53--55 Several methods

Figure 1. Transverse (left) and sagittal (right) sonograms of the right fore superficial digital flexor tendon (arrows) obtained from a 6-year-old Arabian gelding with a severe acute tendon injury extending from 10-30 cm distal to the accessory carpal bone. The total crosssectional area of the superficial digital flexor tendon for the entire tendon (8 zones) was 13.01 cm 2 with a percent injury of 46.7%, an average echogenicity score of 2.1 and an average fiber score of 1.9. These sonograms were obtained in the worst injury zone (see Fig. 38) at 26 cm distal to the point of the accessory carpal bone. At this level, the crosssectional area of the tendon was 1.92 cm 2 with 50.5% damage. The echo score at this level is 3, as is the fiber score.

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Table 1. SONOGRAPHIC GRADING SCALE OF LESIONS IN HORSES WITH SUPERFICIAL DIGITAL FLEXOR TENDINITIS Echogenicity Grading

0: 1: 2: 3:

Normal to near-normal echogenicity Mostly echogenic 50% anechoic and 50% echogenic Mostly anechoic

Fiber Alignment Grading

0: 1: 2: 3:

75%-100% parallel fiber alignment 50%-75% parallel fiber alignment 25%-50% parallel fiber alignment 0%-25% parallel fiber alignment

for assessing injury severity have been described. 13, 17-19, 21, 23, 26, 38, 50, 53--55 Measurements of tendon and lesion cross-sectional area are superior to width and thickness measurements of the tendon and lesion. Measurements of the tendon and lesion at the area of maximal injury only partially describe injury severity. The most complete assessment to date incorporates the tendon and lesion cross-sectional area in each zone, lesion echogenicity, and fiber alignment. Tendon cross-sectional area (TCA), lesion cross-sectional area (LCA), echogenicity, and fiber alignment are evaluated at the most severe area of injury in each zone and are summed to obtain a total measurement or score for the entire tendon (see Fig. 1). The lesion echogenicity and fiber alignment are graded using a scale from 0 to 3 (Table 1). For tendon injury severity (metacarpus): ILCA in zones lA + 18 + 2A + 2B + 3A + 3B + 3C ITCA in zones lA + 18 + 2A + 2B + 3A + 3B + 3C For tendon injury severity (metatarsus): ILCA in zones lA + 18 + 2A + 2B + 3A + 3B + 4A + 4B + 4C -:- ITCA in zones lA + 18 + 2A + 2B + 3A + 3B + 4A + 4B + 4C For tendon injury severity (pastern): ILCA in zones PIA + P18 (M) + PIB (L) + PIC (M) + PIC (L) ITCA in zones PIA + P18 (M) + PIB (L) + PIC (M) + PIC (L) where ILCA is the lesion cross-sectional area and ITCA is the tendon cross-sectional area. Most normal Thoroughbred or Standardbred racehorses have a total tendon cross-sectional area measurement for seven zones summed that ranges between 6 and 7 cm2 and for eight zones that ranges between 7 and 8 cm2, with a total echogenicity score of 0 and a total fiber score of O. Eight zones are included if the injury extends into the carpal canal (zone 0). Mild injuries involve 15% or less of the total tendon crosssectional area, moderate injuries involve greater than 15% but less than or equal to 25% of the total tendon cross-sectional area, and severe injuries involve greater than 25% of the total tendon cross-sectional area.17, 21, 50, 54

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PROGNOSIS FOR HORSES WITH SUPERFICIAL DIGITAL FLEXOR TENDINITIS

The ultimate evaluation of a therapy for an injured tendon is the ability of the horse to return to its previous type of athletic work successfully. The prognosis for horses with superficial digital flexor tendinitis varies with the success criteria used and the type of athletic work performed. Success rates of 20% to 90% have been reported for horses with superficial digital flexor tendinitis. 1, 3--5,12,13,17-19,21,26,30,36,38, 40,43,53,54,56,66,69 Racehorses usually have a poorer prognosis than other types of performance horses because they are performing high-intensity exercise. Most reports of horses treated for superficial digital flexor tendinitis do not have a control group, or the control group was not selected prospectively. Few studies discuss injury severity, which is a critical factor in determining the success of horses returning to athletic competition. Tailoring the Rehabilitation Program to the Severity of the Lesion

A gradually increasing controlled exercise program should be part of the treatment of a horse with superficial digital flexor tendinitis, regardless of the treatment selected. Table 2 presents an example of a rehabilitation program tailored to sonographic lesion severity.I-5, 9, 12, 13, 17-19, 21-23, 26, 30-32, 35, 36, 38, 43, 44, 48, 50, 51, 54-56, 62 The controlled exercise program optimizes the healing process in the injured tendon, promotes parallel fiber alignment, and minimizes the restriction of any adhesions that form. In horses with fiber tearing detected ultrasonographically, controlled walking exercise should be performed for the first few months after injury before beginning any jogging exercise. The length of time that the horse should be stall rested with only walking exercise varies with the severity of the tendon injury. Turnout exercise should be postponed for as long as possible and should initially occur in a small paddock with the horse tranquilized. Most horses with injuries to the superficial digital flexor tendon are not able to continue training and racing without an intervening period of rest. In one study, however, 50% of Standardbred racehorses with significant superficial digital flexor tendinitis were able to race twice while remaining in training, although only 12% of Thoroughbred racehorses were able to do SO.22 In general, only Thoroughbred racehorses with extremely mild superficial digital flexor tendinitis are able to remain in race trainingY Resting horses with significant superficial digital flexor tendinitis before returning them to race training yields a better prognosis than does keeping the horses in training. 17, 22 Most rested Thoroughbred (64%) and Standardbred (81%) racehorses with superficial digital flexor tendinitis were able to complete two races. 22 In another study evaluating horses with superficial digital flexor tendinitis of varying severity treated conservatively, 55% of Thoroughbred racehorses and 75% of Standard-

Table 2. CONTROLLED EXERCISE PROGRAM' FOR HORSES WITH SUPERFICIAL DIGITAL FLEXOR TENDINITIS BASED ON SONOGRAPHIC LESION SEVERITY Exercise Level

Exercise Level 1 (Ell)

Exercise Level 2 (EL2)

Exercise Level 3 (EL3)

Exercise Level 4 (EL4)

Exercise Level 5 (EL5)

Exercise Level 6 (EL6)

Exercise Level 7 (EL7)

Exercise Program

Walking: 30--45 minutes. 30 minutes for weeks 1--4, increased to 45 minutes for weeks 5-8. In hand, on walker or treadmill, with pony, or under saddle, under whichever condition the horse is most manageable. Jogging: 5-10 minutes. 5 minutes for first 4 weeks of trotting exercise (weeks 9-12 after treatment, at the earliest, and only with improved sonographic findings) , 10 minutes for the second 4 weeks of trotting exericse (weeks 13-16 after treatment). On treadmill, in free walker, with pony, or under saddle, under whichever condition the horse is most manageable. Jogging: > 10 minutes. 15 minutes for the third 4 weeks of jogging exercise (weeks 17-20 after treatment, at the earliest, and only with improved sonographic findings), 20 minutes for the fourth 4 weeks of jogging exercise (weeks 21-24 after treatment). With moderate and severe tendon injuries, 25 minutes for the fifth 4 weeks of jogging exercise (weeks 25-28 after treatment, at the earliest, and only with sonographic improvement), 30 minutes for the sixth 4 weeks of jogging exercise (weeks 29-32 after treatment). On treadmill, in free walker, with pony, or under saddle, under whichever condition the horse is most manageable. Tum out in small paddock if quiet or tranquilized initially. Jogging exercise should continue if rapid return to racing desired during this time. For mild tendon injuries, no earlier than 4 months after treatment, for moderate or severe tendon injuries, no earlier than 6 months after treatment. Canter or slow gallop. 1 mile daily for first 4 weeks of galloping with sonographic improvement only, 2 miles daily for second 4 weeks with sonographic improvement only. For mild tendon injuries, begin no earlier than week 25 after treatment and only with sonographic improvement. For moderate tendon injuries, begin no earlier than week 33 after treatment and only with sonographic improvement. For severe tendon injuries, begin no earlier than 41 weeks after treatment and only with sonographic improvement. Breeze. For mild tendon injuries, begin no earlier than 33 weeks after treatment and only with sonographic improvement. For moderate tendon injuries, begin no earlier than 41 weeks after treatment and only with sonographic improvement. For severe injuries, begin no earlier than 49 weeks after treatment and only with sonographic improvement. Race. For mild tendon injuries, begin no earlier than 37 weeks after treatment and only with sonographic improvement. For moderate tendon injuries, begin no earlier than 45 weeks after treatment and only with sonographic improvement. For severe tendon injuries, begin no earlier than 53 weeks after treatment and only with sonographic improvement.

'Controlled exercise program from the intralesional BAPTEN trials."

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bred racehorses were able to complete three starts. 51 ,53 Severe superficial digital flexor tendon injuries (anechoic lesions with almost total fiber tearing involving over 25% of the tendon's cross-sectional area at the maximal injury zone) have been associated with a poor prognosis in Thoroughbred and Standardbred racehorses. The successful Thoroughbred racehorses in this study usually had less than 25% injury in the maximal injury zone. 22 In another group of horses treated conservatively for superficial digital flexor tendinitis, an increasing severity rating was associated with a decreased likelihood of returning to racing, shortened racing career, and drop in racing class. 19 Injury severity was also related to success in returning to racing in the double-blind placebo-controlled intralesional beta-aminopropionitrile fumarate (BAPTEN [PR Pharmaceuticals, Fort Collins, CO]) trials. The successful placebo-treated horses (raced five or more times) had a total percentage of superficial digital flexor tendon injury of less than 12%. The placebo-treated horses that failed to race with 2.5 years of treatment had a total percentage of injury to their superficial digital flexor tendon of up to 41.2%. No severely injured placebo-treated horse was able to return to racing and complete five or more starts, and only 33% of the severely injured placebo-treated horses were able to complete one to five starts. 54 The presence of bilateral superficial digital flexor tendinitis also affects the prognosis for return to racing in Thoroughbreds, with only 25% of Thoroughbreds able to complete three starts in one study compared with 75% of Standardbred racehorses. 51 ,53 TREATMENT OF SUPERFICIAL DIGITAL FLEXOR TENDINITIS: SONOGRAPHIC ASSESSMENT AND OUTCOMES COMPARISON Anti-Inflammatory Treatment of Superficial Digital Flexor Tendinitis

Aggressive "cooling out" of the acutely injured superficial digital flexor tendon is recommended with cold water hydrotherapy or icing of the affected limb and anti-inflammatory therapy, regardless of the severity of the lesion detected sonographically.32,50 The limb should then be bandaged with a support wrap, and the horse should be stall rested. A poultice may also be applied under the bandage for its local effects. The foot of the affected limb should be trimmed so that it is balanced and leve1. 50 Initial anti-inflammatory therapy should include steroidal and nonsteroidal anti-inflammatory drugs if the tendon is severely injured. Nonsteroidal anti-inflammatory drugs such as phenylbutazone are indicated after injury to reduce the severity of the initial inflammatory response for the first week or so. Initial treatment with systemic corticosteroids to reduce swelling and inflammation is often indicated but should not be continued long-term or used intralesionally.31,46 Dimethyl sulfoxide is a potent anti-inflammatory agent and free radical scavenger and may benefit an acutely injured superficial digital flexor tendon. 31

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Horses should be stall rested and hand walked at this time, unless the injury results in loss of support in the limb. Cold water hydrotherapy, icing, and the application of a topical poultice can be discontinued once clinical improvement is no longer detected. Surgical Treatment for Superficial Digital Flexor Tendinitis

Several different surgical procedures for the treatment of superficial digital flexor tendinitis are available and may be used alone or in combination depending on the type of injury sustained by the horse. It is difficult to separate the prognosis for horses treated surgically for superficial digital flexor tendinitis, because multiple surgical procedures are frequently performed. Placebo-controlled double-blind trials to demonstrate the efficacy of the surgical treatments for superficial digital flexor tendinitis are lacking. Tendon Splitting Surgery

Tendon splitting surgery has some beneficial sonographic and histopathologic effects in horses with acute superficial digital flexor tendinitis; however, no beneficial effect has been demonstrated for treating horses with chronic superficial digital flexor tendinitis.57 A significant decrease in lesion size and an improvement in lesion echogenicity have been reported in horses that had ultrasound-guided tendon puncture/ tendon splitting surgery in conjunction with superior check desmotomy, and in horses with collagenase-induced tendon injuries. 31,34 Figure 2 shows an example of a horse with a large central core lesion treated with tendon splitting surgery. Most (81 %) horses (primarily 3-day event horses) treated with ultrasound-guided tendon puncture were able to return to performance, with 68% being able to return to their previous level of athletic function or better.1 There was no control population of similarly affected horses in this study with which to compare these results, however. There was no difference in the successful return to racing of National Hunt and English flat Thoroughbred racehorses treated with superior check desmotomy plus tendon splitting surgery (61.4%) compared with superior check desmotomy alone (68.6%).43 Although tendon splitting surgery may be of benefit in treating the acute anechoic core lesion in the injured superficial digital flexor tendon, minimizing the adverse effects of the central hematoma, there are no reports indicating a significant change in the return to racing performance of the horse or recurrence rate of superficial digital flexor tendinitis. Desmotomy of the Fetlock Annular Ligament

If an injury to the superficial digital flexor tendon involves the portion of the tendon contained within or immediately proximal to the fetlock annular ligament, surgical transection of the annular ligament

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Figure 2. Sonograms of the left foreleg obtained from a 4-year-old Thoroughbred stallion with a central core lesion in the superficial digital flexor tendon. The initial sonograms (A) were obtained 2-3 weeks after the injury occurred. The follow·up sonograms (B) and (C) were obtained 6 weeks and 8 months, respectively, after tendon splitting surgery was performed. A, Transverse (left) and sagittal (right) sonograms of the left fore superficial digital flexor tendon (arrows) obtained in zone 2A at 16 cm distal to the accessory carpal bone (the worst injury zone) 2-3 weeks after the injury occurred. Note the central to slightly medial hypoechoic to anechoic core lesion (echo score 2) with near complete fiber disruption within the lesion (fiber score 3). The total tendon cross·sectional area of the tendon was 9.55 cm2 with a total percent injury of 18.1 %. The total echo score was 2 and the total fiber score was 2. The cross-sectional area of the tendon at this level was 1.64 cm2 with 17% injury. The tendon injury extended from 4-27 cm distal to the accessory carpal bone. B, Transverse (left) and sagittal (right) images of the left fore superficial digital flexor tendon (arrows) obtained in zone 2A at 16 cm distal to the accessory carpal bone (the worst injury zone) 6 weeks after tendon splitting surgery was performed (same location as in A). Note the central to slightly medial core lesion has filled in with hypoechoic to echoic random echoes (echo score 1), creating a random fiber pattern within the lesion (fiber score 2). The borders of the core lesion are more difficult to distinguish in the transverse image but still readily detectable in the sagittal image. The total tendon cross-sectional area of the tendon was 9.71 cm 2. The total echo score for the entire tendon was 0.85 and the total fiber score was 1.14. C, Transverse (left) and sagittal (right) images of the left fore superficial digital flexor tendon (arrows) obtained in zone 2A at 16 cm distal to the accessory carpal bone (the worst injury zone) 8.5 months after tendon splitting surgery was performed (same location as in A and 8) . Most of the core lesion has a normal to near-normal echogenicity (echo score 1). The core lesion is difficult to discern in the transverse image but is still visible in the sagittal image, although the fiber pattern has improved and short linear echoes are evident within the lesion (fiber score 1). The total tendon cross-sectional area of the tendon was 8.95 cm 2. The total echo score for the entire tendon was 0.71 and the total fiber score was 0.86.

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may be indicatedY This surgery is recommended by some surgeons for any horse with "low bows" in the distal metacarpal or metatarsal region to restore the gliding function of the tendon in the annular ligament region. 56 There are no reports of the long-term results (return to racing) of horses treated with an annular ligament desmotomy. Desmotomy of the Superior Check Ligament

This procedure involves the transection of the superior check ligament near its origin along the distomedial aspect of the radius, effectively lengthening the bone-ligament-tendon-bone axis and effectively increasing the elastic limit of the healed tendon. This surgery has been advocated as a treatment for mild to severe tendon injuries in Standardbred racehorses and has been most successful in Thoroughbred racehorses with mild to moderate tendon injuries. 30, 36, 56 The success rate for horses returning to racing after superior check desmotomy surgery has been variable, with reports of success ranging from 40% to 80% depending on the type of racehorse and the criteria for success imposed. 3-s, 21, 23, 30, 36, 43 Seventy-one percent of Thoroughbred racehorses treated with superior check desmotomy returned to racing, and 51% made more than five starts after surgery.4 In another study, only 40% of horses returned to racing after superior check ligament desmotomy, with a recurrence rate of 32.5%.21 In the United Kingdom, 53% of flat Thoroughbred racehorses, 58% of steeplechasers, and 73% of hurdlers competed in five or more starts after surgery.43 In the only prospective controlled study comparing horses with superficial digital flexor tendon injuries treated with a controlled exercise program with horses treated surgically with superior check desmotomy, no benefit to performing this surgery was found in Thoroughbred racehorses.23 There was no difference in injury severity between these two groups of horses at the start of this study. The horses treated surgically were 1.3 times as likely compared with the conservatively treated horses to race five or more times after surgery, 1.2 times as likely to have a recurrence of the superficial digital flexor tendinitis, and 5.5 times as likely to sustain an injury to the suspensory ligamenU3 An increased risk of suspensory desmitis has also been reported in Standardbred racehorses after superior check desmotomy.30 Return to racing results after superior check desmotomy are better in Standardbred racehorses, with 87%30, 56 and 69%,36 respectively, starting more than five times. The time for Standardbred racehorses to return to racing was much shorter than in the treated Thoroughbred racehorses in these studies, with a mean time to first start of 237 days30 compared with 353 days.4 Medical Treatment for Superficial Digital Flexor Tendinitis

A variety of medical treatments for superficial digital flexor tendinitis are available, including BAPTEN, sodium hyaluronate, and polysulfated glycosaminoglycans.

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Intralesional 8eta-Aminopropionitrile Fumarate

Intralesional BAPTEN trials in horses with superficial digital flexor tendinitis were one of the few well-controlled prospective clinical trials demonstrating treatment efficacy.6, IS, 50, 54, 55 Intralesional BAPTEN coupled with a controlled exercise program resulted in a better sonographic quality of tendon repair, with improved fiber alignment and better return to racing in horses with moderately to severely injured tendons.ls, 50, 54, 55 In one study evaluating the long-term success of horses with superficial digital flexor tendinitis, 76% of horses treated with intralesional BAPTEN were able to return to racing. Although 41% of these horses ultimately rebowed tendons when their entire racing career was evaluated/I the prognosis for returning to racing for all other treatments was significantly lower, and the treatments were not different from one another in terms of outcome; 43% of non-BAPTEN-treated racehorses returned to racing, and 53% ultimately rebowed tendonsY For racehorses treated with intralesional BAPTEN in the double-blind placebocontrolled trials, the prognosis for returning to racing was similar (approximately 70%), regardless of the severity of the initial injury. 54 More than 48% of racehorses treated with intralesional BAPTEN were able to complete five or more starts, although only 33% of placebo-treated horses were able to do SO.54 The severity of tendon injury in successful placebo-treated horses was mild «12%) compared with the injury severity in the successful BAPTEN-treated horses (33%).54

Figure 3. See legend on opposite page

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BAPTEN is the only treatment so far that has been shown to improve fiber alignment. 50, 54, 55 BAPTEN has been shown to significantly improve fiber pattern by 16 weeks after treatment when compared with placebo-treated horses (Figs, 3 and 4).54,55 A better quality of fiber repair has been associated in several studies with improved chances of horses returning successfully to racing and competing without reinjury for at least five starts (see Figs. 3 and 4),13, 26, 54, 55 The probability of a horse with a bowed tendon being successful can be predicted from the fiber score at 16 weeks after treatment and was significantly increased for BAPTEN-treated tendons compared with control tendons. 54 Horses with an average fiber score of less than 0.5 16 weeks after treatment with either intralesional BAPTEN or placebo had an 83% chance of being

Figure 3. Sonograms of the left and right forelimbs obtained at the worst injury zone from an unraced 3-year-old Thoroughbred colt before (A and B) and 16 weeks after (C and D) intralesional BAPTEN injections of both forelegs. The colt had bowed in the right foreleg as a 2-year-old. The tendon was treated conservatively and was reinjured late in his 2year-old year, 2-3 months before the sonograms in A. The left foreleg was also injured this time. Both superficial digital flexor tendons were worse in zone 1A and 1B and had core lesions. This colt was placed in a controlled exercise program based upon periodic sonographic monitoring . Four and a half months later when sonograms C and 0 (left and right foreleg, respectively) were obtained, the colt had been jogging for 5 minutes daily for 4 weeks. Although the colt progressed more rapidly than advised, he has returned to raCing successfully (> 5 starts). A, Transverse (left) and sagittal (right) sonograms of the left fore superficial digital flexor tendon (arrows) obtained in zone 1A at 8 cm distal to the accessory carpal bone (one of the worst injury zones) . Note the enlarged superficial digital flexor tendon (1.68 cm2 at this level), hypoechoic areas (echo score 2) visible in the dorsal medial and in the palmar aspect of the left fore superficial digital flexor tendon with an amorphous and random fiber pattern (fiber score 2). The total cross-sectional area for 8 zones of the left fore superficial digital flexor tendon (including the carpal canal) was 9.08 cm2 with an average echogenicity score for all 8 zones of 1.6 and an average fiber score of 1.4. The tendon injury extended from 4-17 cm distal to the accessory carpal bone. 8 , Transverse (left) and sagittal (right) sonograms of the right fore superficial digital flexor tendon (arrows) obtained in zone 1A at 8 cm distal to the accessory carpal bone (one of the worst injury zones). Note the enlarged superficial digital flexor tendon (1.54 cm2 at this level) with the dorsal hypoechoic area (echo score 2) still visible in the medial aspect of the left fore superficial digital flexor tendon, with an amorphous and random fiber pattern (fiber score 2). The total cross-sectional area of the right fore superficial digital flexor tendon for 8 zones (including the carpal canal) was 9.44 cm2 with an average echogenicity score for all 8 zones of 1.3 and an average fiber score of 0.9. The tendon injury extended from 3-22 cm distal to the accessory carpal bone. C, Transverse (left) and sagittal (right) sonograms of the left fore superficial digital flexor tendon (arrows) obtained in zone 1A at 8 cm distal to the accessory carpal bone (one of the worst injury zones) 4.5 months after A. Note the small hypoechoic area (echo score 1) still visible in the medial aspect of the left fore superficial digital flexor tendon; however, the fiber pattern is fairly long and parallel (fiber score 1). The overall fiber score for the superficial digital flexor tendon at this time was 0.5. 0, Transverse (left) and sagittal (right) sonograms of the right fore superficial digital flexor tendon (arrows) obtained in zone 1A at 8 cm distal to the accessory carpal bone (one of the worst injury zones) 4.5 months after A. Note the uniform echogenicity and fiber pattern in the right fore superficial digital flexor tendon (arrows) 4.5 months after 8. The tendon is only slightly hypoechoic (echogenicity score 1) with a good-quality fiber pattern (fiber score 1). The overall fiber score for the superficial digital flexor tendon at this time was 0.625.

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Figure 4. See legend on opposite page

successful (racing at least five times without recurrence of the injury), regardless of the treatment administered. 54 Horses with an average fiber score of greater than 0.5 but less than 1 had a 58% chance of successfully returning to racing, although those with an average fiber score greater than 1 had only a 34% chance of racing successfully.54 The recurrence of superficial digital flexor tendinitis was lower in BAPTEN-treated horses «50%), although the placebo-treated control group's recurrence rate was 87.5%. The mean percentage of injury for the placebo-treated horses that failed to return to racing was 41.2%, although 30% of the successful BAPTEN-treated tendons had a mean percentage of initial injury that exceeded 41.2%.54 BAPTEN treatment thus enabled a significant percentage of horses with severely damaged superficial digital flexor tendons to return successfully to racing compared with similarly managed placebotreated horses. Intralesional Sodium Hyaluronate

Sonographic and histopathologic evidence of tendon healing has been reported after intrathecal injections of sodium hyaluronate into the digital sheath of horses with collagenase-created and surgically created defects in the deep digital flexor tendon. 16 There have been conflicting results on the response of the injured tendon to peritendinous sodium hyaluronate. 15, 16, 31, 44, 60 In a blind placebo-controlled study of horses with naturally occurring superficial digital flexor tendinitis treated with

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intralesional high-molecular-weight sodium hyaluronate, significant improvement was seen in the sonographic resolution of the lesions 6 months after treatment compared with results in placebo controls. 62 In several other studies, improvement in the sonographic appearance of the superficial digital flexor tendon and return to performance were reported, although there was no matched control group.2, 35 Some benefit has also been reported in the treatment of horses with chronic superficial digital flexor tendinitis. 9 Return to performance for horses treated with intralesional sodium hyaluronate in a controlled study with horses of matched injury severity was no different than in horses treated with a

Figure 4. Transverse (left) and sagittal (right) sonograms of the right fore superficial digital flexor tendon obtained in zone 3B at 24 cm distal to the accessory carpal bone (the worst injury zone) from a 13·year·old Thoroughbred gelding, This horse originally had an acute core lesion in the left fore superficial digital flexor tendon 5 years ago that involved 11 % of the total tendon cross-sectional area. This injury was treated conservatively and did not heal well. Eighteen months later (without competing) the horse experienced an acute injury to the proximal and midportion of the right fore superficial digital flexor tendon. At this time, the distal portion of the right superficial digital flexor tendon in zones 3B-3C was normal (A). The tendon was treated with intralesional BAPTEN and 16 weeks later had an average fiber score of 0.5. The horse then competed successfully at the advanced level in combined training for 18 months, before re-injuring the right fore superficial digital flexor tendon distal to the original injury (B). The tendon was treated conservatively and at 24 weeks still had an overall fiber score of only 1.5. The tendon at 36 weeks (C) has a poor-quality repair with an echogenicity of 2 and a fiber score of 2 at the worst injury zone. A central hypoechoic area remains at the site of the reinjury with a random pattem of fiber alignment. This does not bode well for his being able to return successfully at the advanced level of combined training, A, Transverse (left) and sagittal (right) sonograms obtained in zone 3A at 24 cm distal to the accessory carpal bone at the time of the first injury to the right fore superficial digital flexor tendon. Note the normal size (1.14 cm2 at this level) and shape of the superficial digital flexor tendon (arrows) at this location and the normally echoic tendon (echo score 0) and fiber alignment (fiber score 0). The tendon injury was confined to the proximal and midportion of the tendon (8-22 cm distal to the accessory carpal bone) with a normal to near-normal distal tendon. The tendon at this time had a total cross-sectional area for 8 zones of 9.76 cm2 with an overall percentage injury to the entire superficial digital flexor tendon of 24.5%, an average echo score of 2.4, and an average fiber score of 2.4. B, Transverse (left) and sagittal (right) sonograms of the right fore superficial digital flexor tendon at the time of its reinjury 18 months later, obtained from the same location as in A (24 cm distal to the accessory carpal bone). At the time of the reinjury the tendon had a total cross-sectional area for 8 zones of 12.24 cm2 with an overall percentage injury to the entire superficial digital flexor tendon of 9.6%, an average echo score of 2.1, and an average fiber score of 1.7. Note the hypoechoic central core lesion (echo score 2) in the previously uninjured portion of the superficial digital flexor tendon and the fiber disruption present (arrows). The tendon lesion is filled with amorphous echoes and short linear echoes at this time (fiber score 3). This recent injury extended from 19-30 cm distal to the accessory carpal bone. C, Transverse (left) and sagittal (right) sonograms obtained at the same level as in Band C (24 cm distal to the accessory carpal bone) 9 months after the reinjury in B. The tendon was treated conservatively during this time with a controlled exercise program based on periodic sonographic monitoring. Note the enlargement of the superficial digital flexor tendon (1 .72 cm2 at this level) with a persistent dorsal and central hypoechoic region (echo score 2), poor-quality fiber pattern (fiber score 2), and peritendinous thickening (arrows). The total cross-sectional area of this tendon for 8 zones is 11.6 cm2 with an average echo score of 1.1 and an average fiber score of 1.0.

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controlled exercise program, and the risk for recurrent tendinitis was similarY Intralesional sodium hyaluronate also did not increase the success of horses with superficial digital flexor tendinitis treated with superior check desmotomy when these horses were compared with those treated with surgery alone. 43 Polysulfated Glycosaminoglycans

An improved sonographic appearance and significantly decreased external circumference of the injured superficial digital flexor tendon have been reported with the use of intralesional polysulfated glycosaminoglycans, but neither of these studies included a control populationp,58 Intramuscular or intralesional polysulfated glycosaminoglycans have been used for treating horses with tendinitis; however, no difference in return to racing or recurrence of tendinitis has been demonstratedp,13, 32,38,58 A recent study in rabbits demonstrated a significant distribution of tritium-labeled polysulfated glycosaminoglycans to the superficial digital flexor tendon in rabbits. A similar distribution may occur in the horse after intramuscular injection. Similar populations of horses treated with intramuscular or intralesional polysulfated glycosaminoglycans should be evaluated long-term to determine the efficacy of this treatment for horses with tendon injuries.

Alternative Treatment for Superficial Digital Flexor Tendinitis

Noninvasive treatment of tendon injuries has long been popular, and some beneficial effects on tendon healing have been demonstrated with alternative treatments in humans and laboratory animals. Further controlled studies of these noninvasive treatment modalities (laser, therapeutic ultrasound, electromagnetic therapy, and extracorporeal shock wave therapy) need to be performed in horses to determine their usefulness in the treatment of superficial digital flexor tendinitis. 37, 40 Low-Power Laser

Sixty-six percent of Standardbred racehorses with superficial digital flexor tendinitis were able to return to racing after low-power laser treatment of the tendinitis. 40 The rate of return to racing for these Standardbred racehorses is no better than that reported for other Standardbred racehorses with superficial digital flexor tendinitis treated conservatively, however. Similarly, in National Hunt horses with superficial digital flexor tendinitis treated with the low-power laser, there was no difference in return to racing compared with that in conservatively managed horses or those receiving polysulfated glycosaminoglycans. 39

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Therapeutic Ultrasound, Electromagnetic Therapy, and Extracorporeal Shock Wave Therapy

There are no long-term studies evaluating the sonographic changes or return to performance of horses with superficial digital flexor tendinitis treated with therapeutic ultrasound, electromagnetic therapy, or extracorporeal shock wave therapy.

Counterirritation for Superficial Digital Flexor Tendinitis

The use of counterirritation such as pin-firing and blistering in the treatment of tendinitis in horses results in severe local inflammation and secondary fibrosis in and around the tendon that is not beneficial to tendon healing. 32 The prognosis for horses treated with counterirritation to return to racing was not improved when compared with all other forms of therapy in one study.21

SONOGRAPHIC MONITORING OF TENDON HEALING

The ultrasonographic findings associated with a healing tendon lesion have been well described. 50 As tendon healing progresses, the cross-sectional area of the lesion and tendon usually decreases. The lesion repairs with hypoechoic amorphous echoes that usually represent granulation tissue and immature fibrous tissue. The echogenicity of the injured area gradually increases, and short linear echoes appear with fibrous tissue maturation. The fiber pattern of these new linear echoes is usually random. Fibers become more parallel as healing progresses; however, a conservatively treated core lesion rarely returns to a completely normal sonographic appearance (Fig. 5). Sonographic evidence of tendon scarring (e.g., random fiber pattern, short linear echoes, demarcation of the original core lesion from the previously uninjured tendon) usually remains.

Recommended Follow-Up Sonographic Monitoring

The primary cause of failure to return to racing is tendon injury in the same (most frequently) or contralateral forelimb. Ultrasonographic assessment of tendon healing is an important aspect in the rehabilitation of the horse with an injured superficial digital flexor tendon. Several studies have demonstrated the importance of imaging a good fiber pattern throughout the entire superficial digital flexor tendon before the onset of cantering or galloping exercise.13, 26, 54 In all these studies, the success of horses returning to racing or competition was significantly

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Figure 5. Sonograms of the right forelimbs obtained at the worst injury zone from a 7-yearold Warm blood stallion with superficial digital flexor tendinitis that was treated conservatively. The stallion had developed acute swelling in the right foreleg with local heat and sensitivity 4 weeks prior to the initial sonogram. The stallion was placed in a controlled exercise program based on periodic sonographic monitoring. A, Transverse (left) and sagittal (right) images of the right fore superficial digital flexor tendon obtained in zone 3A at 24 cm distal to the accessory carpal bone. Note the large medial hypoechoic to anechoic core lesion (echo score 3) on the periphery of the superficial digital flexor tendon (arrows), with no linear fibers imaged in the lesion (echo score 3). The total cross-sectional area of the tendon was 6.16 cm2 with a total lesion cross-sectional area of 0.78 cm 2, yielding a total percent damage of 12.7% for the right fore superficial digital flexor tendon. The total echo score for the right fore superficial digital flexor tendon was 1.6, with a total fiber score of 1.3. The tendon injury extended from 17-26 cm distal to the accessory carpal bone. The cross-sectional area of the tendon at this level was 1.12 cm2 with a total lesion crosssectional area of 0.3 cm 2, for a percent injury of 26.8% at this level. B, Transverse (left) and sagittal (right) images of the right fore superficial digital flexor tendon obtained in zone 3A at 24 cm distal to the accessory carpal bone 12 months after the initial injury (11 months after the initial sonograms) . This tendon has a relatively poor quality of repair in spite of the horse being in a carefully controlled exercise program based on sonographic monitoring for the last 11 months. Note the persistent visualization of the large medial hypoechoic core lesion (echo score 1) on the periphery of the superficial digital flexor tendon (arrows) with short and long random to linear fibers imaged in the lesion (fiber score 1). The total cross-sectional area of the tendon was 6.15 cm 2, with a total lesion cross-sectional area of 0.51 cm 2, yielding a total percent damage of 8.3% for the right fore superficial digital flexor tendon . The total echo score for the right fore superficial digital flexor tendon was 0.57 with a total fiber score of 0.71. The cross-sectional area of the tendon at this level was 1.04 cm2 with a total lesion cross-sectional area of 0.18 cm 2, for a percent injury of 17.3% at this level.

associated with the quality of tendon repair determined ultrasonographically before beginning cantering or galloping exercise. A follow-up sonographic examination should be performed 8 weeks after injury or after treatment has been instituted. Additional follow-up sonographic examinations should be performed every 8 weeks or at pivotal points before any increase in the horse's exercise level. Each exercise increase should occur only when sonographic improvement is detected, including a decrease in the total cross-sectional area of the tendon, a decrease in the total cross-sectional area of the lesion, an increase in tendon echogenicity, and an improvement in the parallel fiber alignment. The exercise level should stay the same if total tendon cross-sectional area is increased by as much as 10% or if the cross-

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sectional area at one injury zone is increased by as much as 20% compared with the previous examination.50,55 Increases in tendon crosssectional area of greater than 10% for the entire superficial digital flexor tendon or greater than 20% at anyone injury zone are possibly indications of excessive loading for the current state of healing and may indicate possible reinjury.50,55 These horses should have their exercise program reduced until improvement in the sonographic appearance of the superficial digital flexor tendon occurs. The horse should not begin cantering or galloping before 6 months after treatment, or reinjury is likely. In more severely injured tendons, the time before beginning cantering or galloping should be longer, up to 9 to 12 months in severely injured horses (see Table 2). The fiber alignment before cantering or galloping exercise is a critical factor in determining success during the rehabilitation of horses with tendon injuries. FUTURE DIRECTIONS FOR TREATMENT OF SUPERFICIAL DIGITAL FLEXOR TENDINITIS

Additional placebo-controlled double-blind trials and prospective controlled trials need to be performed to critically evaluate the results in horses treated for superficial digital flexor tendinitis with the treatments available and with therapy being developed. New treatments such as insulin-like growth factor I therapy that have been shown to have some beneficial effects in vitro41 should be studied in vivo. Double-blind placebo-controlled trials with matched populations of horses with tendon injuries of similar severity are needed to critically evaluate the efficacy of "tried and true" and new and upcoming treatments for tendon injuries. Diagnostic ultrasound has provided the veterinarian with an objective method of categorizing injury severity so that matched populations of horses with superficial digital flexor tendinitis can be evaluated with different treatments. References 1. Allen KA: Experience with ultrasound-guided tendon puncture or splitting. Proc Am Assoc Equine Pract 38:273-277, 1992

2. Blobel K: Ein beitrag zur sehnenbehandlung bei reit-und trabrennpferden. Der praktische Tierarzt 69:10--14, 1988 3. Bramlage LR: Superior check ligament desmotomy as a treatment for superficial digital flexor tendinitis: Initial report. Proc Am Assoc Equine Pract 32:365-370, 1986 4. Bramlage LR, Hogan P: Career results of 137 Thoroughbred racehorses that have undergone superior check ligament desmotomy for the treatment of tendinitis. Proc Am Assoc Equine Pract 42:162-163, 1996 5. Bramlage LR, Rantanen NW, Genovese RL, et al: Long-term effects of surgical treatment of superficial digital flexor tendinitis by superior check ligament desmotomy. Proc Am Assoc Equine Pract 34:655-656, 1988 6. Chvapil M: Present status of the pharmacology of fibrosis and scar contractures. Proceedings of the Dubai International Equine Symposium 1:395-406, 1996

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7. Crass JR, Genovese RL, Render JA, et al: Magnetic resonance, ultrasound and histopathologic correlation of acute and healing equine tendon injuries. Vet Radiol Ultrasound 33:206-216, 1992 8. Cuesta I, Riber C, Pinedo M, et al: Ultrasonographic measurement of palmar metacarpal tendon and ligament structures in the horse. Vet Radiol Ultrasound 36:131-136, 1995

9. Damsch S, Drommer W, Schmidt W, et al: Intratendinose injektion von hochmolekularem Natrium hyaluronat bei pferden mit chronischer tendonitis-klinische, licht und elektronrnikrokopische befunde. Pferdeheilkunde 8:333--343, 1992 10. Denoix J, Mialot M, Levy I, et al: Etude anatomo-pathologique des lesions associees aux images echographiques anormales des tendons et ligaments chez Ie cheval. Recueil de Medecine Veterinaire 166:45--55, 1990 11. Dik KJ: Ultrasonographic evaluation of fetlock annular ligament constriction in the horse. Equine Vet J 23:285-288, 1991 12. Dow SM, Wilson AM, Goodship AE: Treatment of acute superficial digital flexor tendon injury in horses with polysulphated glycosaminoglycan. Vet Rec 139:413--416, 1996

13. Dyson S: Treatment of superficial digital flexor tendinitis: A comparison of conservative management, sodium hyaluronate, and glycosaminoglycan polysulfate. Proc Am Assoc Equine Pract 43:297-300, 1997 14. Dyson SJ: Ultrasonographic examination of the metacarpal and metatarsal regions in the horse. Equine Vet Educ 4:139-144, 1992 15. Foland JW, Trotter GW, Powers BE, et al: Effect of sodium hyaluronate in collagenaseinduced superficial digital flexor tendinitis in horses. Am J Vet Res 53:2371-2376, 1992 16. Gaughan EM, Nixon AJ, Krook LP, et al: Effects of sodium hyaluronate on tendon healing and adhesion formation in horses. Am J Vet Res 52:764-773,1991 17. Genovese R, Longo K, Berthold B, et al: Quantitative sonographic assessment in the clinical management of superficial digital flexor injuries in Thoroughbred race horses. Proc Am Assoc Equine Pract 43:285-290, 1997 18. Genovese RL: Sonographic response to intralesional therapy with beta-aminopropionitrile fumarate for clinical tendon injuries in horses. Proc Am Assoc Equine Pract 38:265--272, 1992 19. Genovese RL, Rantanen NW, Simpson BS, et al: Clinical experience with quantitative

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29. Gillis CL, Meagher DM, Cloninger A, et al: Ultrasound cross-sectional area and mean echogenicity of the superficial and deep digital flexor tendon. Proc Am Assoc Equine Pract 39:273-274, 1993 30. Hawkins JF, Ross MW: Transection of the accessory ligament of the superficial digital flexor muscle for the treatment of superficial digital flexor tendinitis in Standardbreds: 40 cases (1988--1992). J Am Vet Med Assoc 206:674--678, 1995 31. Henninger R: Treatment of superficial digital flexor tendonitis in the horse. Equine Vet Educ 4:287-291, 1992 32. Henninger R: Treatment of superficial digital flexor tendinitis. Vet Clin North Am Equine Pract 10:409-424, 1994 33. Henninger RW, Bramlage LR, Bailey M, et al: Effects of tendon splitting on experimentally-induced acute equine tendinitis. Vet Comp Orthop Traumatol 5:1-9, 1992 34. Henninger R, Bramlage L, Schneider R: Short-term effects of superior check ligament desmotomy and percutaneous tendon splitting as a treatment for acute tendinitis. Proc Am Assoc Equine Pract 36:539-540, 1990 _ 35. Hertsch B, Schmidt H, Tilkom P, et al: Ergebnisse der behandlung von tendopathien des pferdes mit hochmolekularem Na-Hyaluronat. Pferdeheilkunde 5:235-243, 1989 36. Hogan PA, Bramlage LR: Transection of the accessory ligament of the superficial digital flexor tendon for treatment of tendinitis: Longterm results in 61 Standardbred race horses (1985-1992). Equine Vet J 27:221-226, 1995 37. Marcos MB, Aswad A: Histologic studies of the effects of ultrasonic therapy on surgically split flexor tendons. Equine Vet J 10:267-269, 1978 38. Marr CM, Love S, Boyd JS, et al: Factors affecting the clinical outcome of injuries to the superficial digital flexor tendon in National Hunt and point-to-point horses. Vet Rec 132:476-479, 1993 39. Marr CM, McMillan I, Boyd JS, et al: Ultrasonographic and histopathological findings in equine superficial digital flexor tendon injury. Equine Vet J 25:23-29, 1993 40. McKibbon LS, Paraschak DM: A study of the effects of lasering on chronic bowed tendons at Wheatley Hall Farm Limited, Canada. Lasers Surg Med 3:55-59, 1983 41. Murphy DI, Nixon AJ: Biochemical and site-specific effects of insulin-like growth factor Ion intrinsic tenocyte activity in equine flexor tendons. Am J Vet Res 58:103-109, 1997 42. Nicoll RG, Wood AKW, Rothwell TLW: Ultrasonographical and pathological studies of equine superficial digital flexor tendons: Initial observations, including tissue characterisation by analysis of image grey scale, in a Thoroughbred gelding. Equine Vet J 24:318-320, 1992 43. Ordidge RM: Comparison of three methods of treating superficial digital flexor tendinitis in the racing Thoroughbred by transection of its accessory ligament alone (proximal check ligament desmotomy) or in combination with either intralesional injections of hyaluronidase or tendon splitting. Proc Am Assoc Equine Pract 42:164-167, 1996 44. Palmer SE, Genovese R, Longo KL, et al: Practical management of superficial digital flexor tendinitis in the performance horse. Vet Clin North Am Equine Pract 10:425481, 1994 45. Pharr JW, Nyland TG: Sonography of the equine palmar metacarpal soft tissues. Vet Radiol 25:265-273, 1984 46. Pool RR, Wheat JD, Ferraro GL: Corticosteroid therapy in common joint and tendon injuries of the horse: Part II. Effects on tendons. Proc Am Assoc Equine Pract 26:407410,1980 47. Rantanen NW, Hauser ML, Genovese RL: Superficial digital flexor tendonitis: Diagnosis using real-time ultrasound imaging. J Equine Vet Sci 5:115-119, 1985 48. Rapp HI, Becker M. Heisse K, et al: Diagnose und therapie von tendinitiden am beispiel des athleten pferd. Sportverl-Sportschad 6:77-88, 1992 49. Reef VB: Managing superficial digital flexor tendinitis in horses. Vet Med (Praha) (Suppl 3: The Veterinary CE Advisor):3-20, 1998 50. Reef VB Musculoskeletal ultrasonography. In Equine Diagnostic Ultrasound. Philadelphia, WB Saunders, 1998, pp 39-186 51. Reef VB: Ultrasonic diagnosis of tendon and ligament disease. In White NA, Moore IN (eds): Current Practice in Equine Surgery. Philadelphia, JB Lippincott, 1990, pp 425-435 52. Reef VB, Martin BB, Stebbins K: Comparison of ultrasonographic, gross and histologic

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Address reprint requests to Virginia B. Reef, DVM Section of Sports Medicine and Imaging Department of Clinical Studies New Bolton Center University of Pennsylvania 382 West Street Road Kennett Square, PA 19348 e-mail: [email protected]