Lameness in cutting horses is a major cause of loss of use and can lead to career-limiting injuries. Most cutting horses are treated for hind limb lameness as opposed to forelimb lameness.1 Tendonitis of the SDF in the forelimbs is rarely reported as a cause of lameness in cutting horses, and to our knowledge, there are no reports on the development of SDF tendonitis in this particular class of equine athletes.
In racehorses, tendonitis of the SDF tendon in the forelimb is among the most common musculoskeletal injuries2 and has been reported to account for 6% to 13% of racing-related injuries.3 Most of the published data regarding SDF tendonitis in horses are associated with sport and racing disciplines,4 with the lesion most commonly located in the center of the tendon within the mid-metacarpal region5,6 (generating a classic core lesion appearance in ultrasonographic images). It has been postulated that core lesions account for most of the tendon injuries in racehorses because of the repeated high-strain cycling in the center of the tendon, which results in microtrauma.7 Tendonitis of the SDF in horses other than cutting horses is commonly considered to have a poor prognosis with a high recurrence rate and is often a career-ending injury.4,8–11
After tendon injury, the chances of affected racehorses returning to racing are < 50%, and of those that do return to racing, nearly half will reinjure the tendon.12 Clinical signs of SDF tendonitis in racehorses include acute swelling on the palmar aspect of the metacarpus (resulting in a bowed tendon appearance), mild to severe lameness, and detectable heat and signs of pain on palpation of the limb. Diagnosis of tendon lesions is commonly based on history, clinical findings, and results of an ultrasonographic examination. In ultrasonographic images, the affected region of the tendon appears enlarged and hypoechogenic with decreased linear fiber pattern and changes in shape, margin, or position of the tendon, compared with the unaffected regions of the tendon.
Tendon failure occurs as a single overstrain incident or cumulative fatigue failure as a result of cyclic loading.9,13 Exercise-induced degenerative change within the SDF tendon has been proposed as a possible mechanism of injury in racehorses and sport horses,2 and results of a study8 suggest that with increasing age, the risk of SDF tendon injury increases. There are also reports10,11 that SDF tendons may be at increased risk for injury with fatigue of the deep digital flexor muscle. The purpose of the study reported here was to characterize SDF tendon lesions in cutting horses and determine the recurrence rate and prognosis for this condition. Additionally, we obtained information regarding treatment and rehabilitation protocols commonly used to treat these injuries in a clinical setting.
Materials and Methods
Case selection—All cutting horses currently in training or being shown that were initially examined from January 1, 2007, to December 31, 2011, for which an ultrasonographic examination of the affected SDF tendon was performed and subsequently a diagnosis of tendonitis of the SDF tendon was made were included in the study.
Medical records review—Data obtained from the medical record of each horse included the following: age, sex, affected limb, lameness grade, and treatment. Diagnosis of SDF tendonitis was made in all cases on the basis of results of clinical and ultrasonographic examination. All ultrasonographic images for each horse were reviewed. Information obtained from images included location of the lesion, severity of the lesion, tendon CSA, lesion CSA, and CSA of the lesion as a percentage of total tendon area.
For each horse, a lameness examination was performed on an asphalt surface and graded on a scale of 1 to 5 by means of the recommended American Association of Equine Practitioners’ grading scheme for lameness examination.14 Ultrasonography was performed with a digital ultrasound machine and a linear tendon probe (7.5 to 12 mHz), and images were stored on a server. All images were reviewed and graded by the same investigator (TET).
From the ultrasonographic images, CSA of the lesion as a percentage of the total tendon area was used to classify lesion severity; severity was graded on a scale of 1 to 3 by use of a grading system described by Gibson et al12 as follows: < 25% = mild, 25% to 50% = moderate, and > 50% = severe. The area of tendon damage was also recorded in terms of the zone of the tendon damaged (zones 1A, 1B, 2A, 2B, or 3A) as described by Avella et al.15 As with all retrospective case analyses, incomplete medical records were a concern. Longitudinal images were not available for review in all cases. However, the CSA of the tendon itself, CSA of the lesion, and CSA of the lesion as a percentage of the total tendon area were available for all horses; thus, those values were used to determine lesion severity. Although longitudinal sections of the tendon would have been very beneficial in the present study, Genovese et al11 stated that tendon and lesion CSAs were the most reliable indicators of tendon damage obtained by means of ultrasonographic examination.
Treatment received by each horse was recorded. Successful treatment was defined as the return of the horse to cutting training and showing at or better than the level of competition prior to tendon injury.
Follow-up information was obtained by telephone interview with the owner or trainer to determine whether the horse was able to return to its previous level of performance and whether the horse had recurrence of lameness associated with the previous SDF tendon injury. Other information obtained from owners and trainers included the following: duration of the convalescent period, level of competition prior to and after treatment of the tendon lesion, whether there was evidence of the previous injury, and overall opinion of the efficacy of the administered treatment. The rehabilitation protocol was also recorded for each horse. Rehabilitation programs were tailored for individual horses depending on the severity of the lesion and the clinical judgment of the attending clinician. These protocols included aquatic treadmill exercise, hand walking, and light riding under a saddle with a structured increase in exercise intensity. The time to follow-up interview was at least 6 months (range, 6 months to 3 years) after the horse's return to athletic activity.
Results
Medical records of 19 horses that met the inclusion criteria for the study were reviewed. All cutting horses with SDF tendonitis that were initially examined during the study period were included in the analysis. All horses were Quarter Horses that were 3 to 6 years old (mean age, 3.9 years). There were 9 females, 4 stallions, and 6 geldings. Among the 19 horses, 7 had an SDF lesion located in the left forelimb and 11 had an SDF lesion located in the right forelimb; 1 horse was affected in both forelimbs. In all horses, the lesions were located on the lateral aspect of the SDF tendon in the mid-metacarpal region. Ultrasonographic examination revealed an area of hypoechogenicity in the lateral aspect of the affected SDF tendons in all horses (Figure 1); moreover, the most intense hypoechoic area was in the lateral aspect of the tendon. There was no ultrasonographically identifiable damage to the medial aspect of the tendon in any of the ultrasonographic images.
Representative cross-sectional ultrasonographic view of an SDF tendon in the right forelimb of a cutting horse with SDF tendonitis. Notice a discrete lesion at the lateral aspect of the tendon with no observable damage to the central region of the tendon or the medial aspect of the tendon. Marks at the bottom of the image represent distance in 0.5-cm increments.
Citation: Journal of the American Veterinary Medical Association 243, 8; 10.2460/javma.243.8.1162
Among the 19 horses, mean lameness grade was 1.26 (range, 0 to 3). Mean CSA of the tendon was 99 mm2, and the mean CSA of the lesion as a percentage of CSA of tendon was 30%. Eight horses had lesions that were classified as mild, and 11 horses had lesions that were classified as moderate. Most horses had lesions in zone 1B (Table 1).
Information obtained from medical records of 19 cutting horses with forelimb SDF tendonitis (during 2007 through 2011), including age of affected horses, severity of lameness, location and extent of lesion, duration of convalescence, and rate of recurrence.
| Region of SDF tendon affected | No. of horses | Mean age (y) | Mean lameness score* | Mean CSA of the lesion (% of the total tendon area) | Mean duration of convalescence period (mo)† | No. of horses with recurrence of SDF tendonitis† |
|---|---|---|---|---|---|---|
| Zone 1A | 3 | 5 | 0.33 | 23 | 5 | 0 |
| Zone 1B | 7 | 3.14 | 1.14 | 34.9 | 4 | 1 |
| Zone 2A | 6 | 4.16 | 1.83 | 30.3 | 3.6 | 2 |
| Zone 2B | 3 | 4.3 | 1.33 | 26.7 | 3.6 | 0 |
| Group mean | — | 3.94 | 1.26 | 30 | 4 | — |
For each horse, a lameness examination was performed on an asphalt surface and graded on a scale of 1 to 5 by means of the recommended American Association of Equine Practitioners' grading scheme for lameness examination.14
Follow-up data were available for 17 horses.
— = Not applicable.
Administration of platelet-rich plasma, tendon splitting, extracorporeal shock wave therapy (involving application of a focused shock wave), and rest and rehabilitation were all used as treatments for SDF tendonitis in this group of horses. Mean duration of convalescence for horses treated with platelet-rich plasma (n = 5), tendon splitting (4), and shock wave therapy (3) was 4 months for each method. The mean convalescent period for horses treated with rest and rehabilitation alone (n = 10) was also 4 months, with the exception of 1 horse that never returned to performance because of chronic lameness associated with the SDF lesion. One horse was treated with both platelet-rich plasma and tendon splitting, and 1 horse was treated with platelet-rich plasma, tendon splitting, and shock wave therapy. Among the 18 horses that returned to athletic function, duration of convalescence ranged from 3 to 9 months.
Two horses were lost to follow-up because they were sold, but they were not lame at the time of sale. As reported by owners and trainers, 3 of the 17 horses for which follow-up information was available had recurrence of tendon lesions, and 1 horse was examined because of development of a lesion in the SDF tendon of the contralateral forelimb. Sixteen of the 17 horses returned to their previous level of activity, and 1 horse had chronic lameness associated with the SDF lesion. Two horses that were able to return to their previous level of activity were subsequently retired as breeding stock with no signs of lameness because of tendon reinjury. Overall, 16 of the 17 owners and trainers available for follow-up interviews were satisfied with the treatment rendered and the outcome of the patient.
Mean duration of convalescence was similar on the basis of the zone of tendon injury in these cases; however, a larger population of horses may be needed to determine whether the zone of tendon injury has an effect on healing time. Also, when assessing the duration of convalescence and recurrence rate, there did not appear to be a relationship between the time it took to heal the lesion and recurrence of the lesion.
The rehabilitation program for each horse differed depending on the lesion severity and the clinical judgment of the attending clinician. Each horse was prescribed an individualized rehabilitation protocol that the attending clinician considered appropriate for the patient. The protocols all included a period of rest prior to beginning rehabilitation; once initiated, the rehabilitation program included aquatic treadmill exercise, hand walking, and riding under a saddle with a gradual increase in exercise amount and intensity. Because of the varied nature of the rehabilitation programs and the variation in the rest period, the duration of rehabilitation was not assessed. We believed that total convalescence was a more clinically relevant time period, so that period was analyzed.
Discussion
It was our hypothesis that the pathogenesis of SDF tendonitis in the forelimbs of cutting horses involves acute overstrain of the SDF tendon with fatigue of the palmar support structures of the limb as a possible risk factor. The location of the tendon lesion in cutting horses appeared to differ from that in racehorses (lateral aspect of the tendon vs central core of the tendon, respectively). In racehorses, thermal injury in the SDF tendon has been described,16,17 with peak temperatures occurring in the central core region of the tendon. It is plausible that thermal injury is less of a risk factor in cutting horses; consequently, lesion location is dissimilar. Overall, the difference in lesion location suggested that other pathological processes are the cause of the lesions seen in cutting horses.
It is also likely that age is less of a risk factor for SDF tendonitis in cutting horses versus racehorses. In the present study, the mean age of affected cutting horses was 3.9 years, whereas in another report,18 horses < 5 years old were less likely to sustain SDF tendon injury. The risk of tendon injury in racehorses increases with age.3,10 The early age at which SDF tendonitis develops in cutting horses is likely attributable to the different intensity and type of training for cutting horses.
Training for a cutting horse usually begins at 18 to 24 months of age in preparation for the first major shows as a 3 year old. This is the time at which a cutting horse undergoes the most strenuous training. The training requirements for a cutting horse are much different than those for a racehorse. In general, cutting horses are ridden and worked 5 to 6 d/wk and at times are worked twice per day. It is plausible that microtruama, changes in crimp pattern, and other exercise-induced pathological changes occur in the SDF tendons but in a different area of the tendon, compared with damage that occurs in SDF tendons of racehorses that are performing at high speeds. However, no data were collected in the present study to prove or disprove any of these theories with regard to cutting horses. Further research to determine whether these proposed changes occur in the periphery of the SDF tendon in cutting horses is warranted.
The area of injury in the SDF tendon of cutting horses in the present study was consistent; all 19 horses, which included all cutting horses that were evaluated for SDF tendonitis at our facility during 2007 through 2011, had lesions in the lateral aspect of the SDF tendon within the mid-metacarpal region of 1 or both forelimbs. Although injuries in this region of the SDF tendon are not exclusive to cutting horses, all cutting horses examined for SDF tendonitis in the present study had lesions localized to this area. The postulated reason that central core lesions were not detected in the study horses is that cutting horses do not perform at top speeds and often perform athletic turn backs and stops in deep sandy arenas, which could potentially place excessive strain on the lateral aspect of the forelimb. It has been shown that at high speeds, hard surfaces are a predisposing factor for SDF tendon injury in racehorses, but it is also known that very deep surfaces have been implicated in induction of tendon lesions.19 The stress of deep footing coupled with the quick side-to-side and turning-around movements required of cutting horses could possibly produce selective tensional forces on the lateral aspect of the tendon, leading to damage of the tendon fibers.
Fatigue may also have an important role in development of SDF tendonitis in cutting horses. Most horses used for the sport of cutting perform a warmup or are loped for 1.5 to 3 hours prior to competition, which consists of a 2.5-minute run. This amount of preper-formance exercise could potentially result in fatigue of many structures affecting the locomotion of a horse. Butcher et al20 proposed that SDF tendon injury could be a result of fatigue of the deep digital flexor muscle, resulting in overstrain on the SDF tendon.
The rehabilitation period for most SDF tendon injuries is at least 8 to 9 months and may last for as long as 18 months.2,4,6,19 It has been recommended that horses with SDF lesions should not return to cantering exercise for 12 to 16 months or until ultrasonographic evidence of healing of the tendon is found.4 Horses of the study reported here required a mean convalescent period of 4 months (range, 3 to 12 months) before returning to full exercise. The convalescent period for the horses was determined by a follow-up telephone interview with the owner or trainer. As with all retrospective case analysis, there may be unintended bias in these results owing to the reliance on the memory of the owners.
Treatment of SDF tendonitis in horses has commonly involved cold therapy, bandaging, controlled exercise, intralesional therapy, extracorporeal shock wave therapy, tendon splitting, and systemic administration of medications, such as NSAIDs. Horses of the present study were all treated on an individual basis depending on the severity of the tendon lesion and preference of the attending clinician. Therefore, no comparison of treatment was performed for this group of horses.
Rehabilitation typically consisted of a period of stall rest, followed by stall rest with hand walking. After rest with hand walking, the horses either were put on an underwater treadmill system or underwent short periods of riding at a walk, then trotting exercise, followed by the introduction of loping exercise. If the horse became lame at any point during the rehabilitation protocol, the level of activity was decreased until ultrasonography revealed notable healing of the tendon, at which time the intensity of exercise would again be increased.
Although not a common problem in cutting horses, SDF tendonitis is an important cause of lameness and loss of use. On the basis of the results of the present retrospective study, the prognosis for cutting horses with SDF tendonitis is much better than that for affected racehorses. The percentage of racehorses with SDF tendonitis that return to racing ranges from 20% to 60%, with recurrence of tendon lesions in up to 80% of affected racehorses.8,10 In comparison, the data obtained in the present study indicated that 14 (82%) of the cutting horses with SDF tendonitis returned to training and only 3 (18%) of them had recurrence of tendon lesions.
ABBREVIATIONS
| CSA | Cross-sectional area |
| SDF | Superficial digital flexor |
References
1. Smith BA. Cutting, in Proceedings. 43rd Annu Meet Am Assoc Equine Pract 1993;31–34.
2. O'Meara B, Bladon B, Parkin TDH, et al. An investigation of the relationship between race performance and superficial digital flexor tendonitis in the Thoroughbred racehorse. Equine Vet J 2010; 42: 322–326.
3. Takahashi T, Kasashima Y, Ueno Y. Association between race history and risk of superficial digital flexor tendon injury in Thoroughbred racehorses. J Am Vet Med Assoc 2004; 225: 90–93.
4. Dyson SJ. Medical management of superficial digital flexor tendonitis: a comparative study in 219 horses (1992–2000). Equine Vet J 2004; 36: 415–419.
5. Hoscka YZ, Uratsuji T, Ueda H, et al. Comparative study of the properties of tendinocytes derived from three different sites in the equine superficial digital flexor tendon. Biomed Res 2010; 31: 35–44.
6. Avella CS, Smith RKW. Diagnosis and management of tendon and ligament disorders. In: Auer J, Stick J, eds. Equine surgery. 4th ed. St Louis: Saunders, 2012; 1157–1177.
7. Patterson-Kane JC, Wilson AM, Firth EC, et al. Exercise-related alterations in crimp morphology in the central core regions of superficial digital flexor tendons form young Thoroughbreds: a controlled study Equine Vet J 1998; 30:1;61–64.
8. Chesen AB, Dabareiner RM, Chaffin MK, et al. Tendinitis of the proximal aspect of the superficial digital flexor tendon in horses: 12 cases. J Am Vet Med Assoc 2009; 234: 1432–1436.
9. Dowling BA, Dart AJ, Hodgson DR, et al. Superficial digital flexor tendonitis in the horse. Equine Vet J 2000; 32: 369–378.
10. Thorpe CT, Clegg PD, Birch HL. A review of tendon injury: why is the equine superficial digital flexor tendon most at risk? Equine Vet J 2010; 42: 174–180.
11. Genovese RL, Reef VB, Longo KL, et al. Superficial digital flexor tendonitis long term sonographic and clinical study of racehorses, in Proceedings. Dubai Equine Int Symp 1996;187–205.
12. Gibson KT, Burbidge HM, Pfeiffer DU. Superficial digital flexor tendonitis in Thoroughbred race horses: outcome following non-surgical treatment and superior check desmotomy Aust Vet J 1997; 75: 631–635.
13. Dowling BA, Dart AJ. Mechanical and functional properties of the equine superficial digital flexor tendon. Vet J 2005; 170: 184–192.
14. Keegan KG. Evidence based lameness detection and quantification. Vet Clin North Am Equine Pract 2007; 23: 403–423.
15. Avella CS, Ely ER, Verheyen KLP, et al. Ultrasonographic assessment of the superficial digital flexor tendons of National Hunt racehorses in training over two racing seasons. Equine Vet J 2009; 41: 449–454.
16. Brich HL, Wilson AM, Goodship AE. The effect of exercise-induced localized hyperthermia on tendon cell survival J Exp Biol 1997; 200: 1703–1708.
17. Dahlgren LA, van der Meulen MC, Bertram JE, et al. Insulin like growth factor-1 improves cellular and molecular aspects of healing in a collagenase-induced model of flexor tendinitis J Orthop Res 2002; 20: 910–919.
18. Ely ER, Avella CS, Price JS, et al. Descriptive epidemiology of fracture, tendon and suspensory ligament injuries in National Hunt racehorses in training. Equine Vet J 2009; 41: 372–378.
19. Crevier-Denoix N, Robin D, Pourcelot P, et al. Ground reaction force and kinematic analysis of limb loading on two different beach sand tracks in harness trotters. Equine Vet J Suppl 2010;(38):544–551.
20. Butcher MT, Hermanson JW, Ducharme NG, et al. Superficial digital flexor tendon lesions in racehorses as a sequella to muscle fatigue: a preliminary study. Equine Vet J 2007; 39: 540–54.
