Racing performance of Thoroughbreds with superficial digital flexor tendonitis treated with desmotomy of the accessory ligament of the superficial digital flexor tendon: 332 cases (1989–2003)

• Autores: Alaine J. Hu
• Localización: JAVMA: Journal of the American Veterinary Medical Association, ISSN-e 0003-1488, Vol. 244, Nº. 12, 2014, págs. 1441-1448
• Idioma: inglés
• Texto completo no disponible (Saber más ...)
• Resumen

  • Objective—To assess postoperative probability of racing, career longevity, and convalescent time in Thoroughbred racehorses with moderate to severe superficial digital flexor tendonitis (SDFT) in the forelimbs treated by desmotomy of the accessory ligament of the superficial digital flexor tendon (ie, superior check ligament desmotomy [SCLD]).

    Design—Retrospective case series.

    Animals—332 Thoroughbred racehorses with SDFT consecutively treated by means of SCLD.

    Procedures—Medical records and racing records were reviewed to assess return to racing, number of races completed, time to first race, and lifetime performance. The horses were categorized as raced or unraced prior to and after surgery. Descriptive statistics including age and treated limb were also recorded.

    Results—Of 332 horses, 228 (69%) returned to racing following injury and treatment. Seventy-eight of 118 (66%) horses that had not raced prior to injury and 150 of 214 (70%) horses that had raced prior to injury raced after treatment. Seventeen of 39 (44%) horses ≥ 5 years old raced following injury and treatment and 211 of 293 (72%) horses ≤ 4 years old returned to racing. There was no difference in the percentages of horses returning to racing for 2-, 3-, or 4-year olds. Postoperative infections occurred in 6 of the 332 (2%) horses. Median time to first race for horses that raced after surgery was 302 days (range, 48 to 1,120 days; mean ± SD, 341 ± 153 days), with a median of 8 starts/horse after surgery (range, 1 to 109 starts; mean ± SD, 14 ± 15.8 starts). Of 228 horses that returned to racing, 159 (70%) raced ≥ 5 times after surgery. Sex and treated limb did not have a significant effect on return to racing. However, horses ≥ 5 years old were significantly less likely to return to racing, compared with younger horses. In horses with unilateral SDFT and < 5 starts, the affected and contralateral limbs were both treated, but return to racing was not significantly different between horses treated bilaterally versus unilaterally.

    Conclusions and Clinical Relevance—228 of 332 (69%) horses with SDFT of the forelimb treated with SCLD successfully returned to racing. Convalescent times were shorter, compared with previous recommendations, and treated horses had a longer racing career after surgery than has been described for other treatment modalities. The results of the present study support consideration of SCLD as part of a treatment plan for SDFT in Thoroughbred racehorses.

    Superficial digital flexor tendonitis is a common career-threatening injury in Thoroughbred and Standardbred racehorses,1 with a prevalence of approximately 11% of racing Thoroughbreds in 1 study.2 Superficial digital flexor tendonitis is an injury that requires prolonged rehabilitation, carries an unfavorable prognosis for return to racing in flat-racing Thoroughbreds, and reduces the economic value of a racehorse.1,3,4 Of Thoroughbreds and Standardbreds with mild, moderate, and severe tendonitis, 63% to 68% raced but only 15% completed 5 races without reinjury.1 Of 4 Thoroughbred racehorses in a group of horses with medically treated tendonitis and ultrasonographic monitoring, 2 were reinjured, with 1 horse lost to follow-up.4 Horses in nonracing disciplines treated similarly had a 23% to 43% reinjury rate, and National Hunt racehorses had a 56% reinjury rate. The treating clinician did the follow-up examinations in that study.4 No data on longevity of performance were described.

    Genovese et al5 found that return to racing in a mixed group of horses depended on the severity of SDFT lesion; approximately 60% of Thoroughbred and Standardbred racehorses with mild selected lesions (severity rating < 3.8 on a 10-point scale) returned to race ≥ 5 times with a variety of medical or surgical treatment modalities, rest, and a controlled exercise program. Lesions with a severity rating ≥ 3.8 of 10 significantly decreased the chance for return to racing. Severity ratings were calculated by the grade of the lesion (grade I to IV), the percentage of the length of tendon the lesion encompasses, and a scaling factor.5 A case series of 131 National Hunt horses and 10 flat-racing Thoroughbreds treated with intralesional injection of bone-marrow–derived mesenchymal stem cells for SDFT has been reported.6 The article describes a 98% rate of return to racing, but 28 of 141 (20%) horses were removed from the analysis because they were lost to follow-up or because of career changes, and the follow-up was by telephone conversation.6 Desmotomy of the accessory ligament of the superficial digital flexor tendon (ie, SCLD) has been proposed as a treatment based on the premise that transection and subsequent healing increase the check ligament–tendon length and reduce the likelihood of the tendon exceeding the maximum check ligament–tendon unit excursion, reducing the likelihood of reinjury.7 In vitro cadaver studies8,9 have shown increased strain in both the superficial digital flexor tendon and the suspensory ligament, under load, after acute SCLD. Those investigators considered this a detriment to horses. We believe that in vivo, with a disabled check ligament, the acute increased strain on the tendon is gradually neutralized by stretching of the muscle proximal to the check ligament after transection of the ligament, resulting in an increase in the check ligament–tendon length after healing (Figure 1). This ability to lengthen the check ligament–tendon unit may be an advantage to horses because the load is partially transferred from the injured tendon to the suspensory ligament that cosupports the metacarpophalangeal (fetlock) joint.9 In an initial study7 of SCLD, we found a 79% rate of success, defined as starting at least 2 races, after SCLD. A subsequent investigation found reoccurrence of SDFT in 4 of 36 (11%) horses on resumption of racing.10 A study11 in Standardbreds found a 25% reoccurrence in SDFT and a decrease in earnings after SCLD; some horses with bilateral SCLD developed suspensory ligament desmitis, which affected their ability to race. Another study12 suggested that horses that underwent SCLD were 1.3 times as likely to return to race ≥ 5 times than were nonsurgically treated horses and were 5.5 times as likely to have a reinjury or develop suspensory ligament desmitis while racing. Horses with SCLD were more likely to injure the suspensory ligament, and nonsurgically treated horses were more likely to reinjure the tendon in that study.12 No significant difference was found between surgically and nonsurgically treated horses sustaining new or recurrent injuries in that study12; however, 37% of the nonsurgically treated horses were removed from analysis because they were lost to follow-up or retired from their racing careers before resumption of racing, and only 12% of the surgically treated horses were removed from analysis for similar reasons.

    An Australian study13 demonstrated an improvement in rate of return to racing (≥ 2 starts) in horses that underwent SCLD, compared with nonsurgical treatment. Standardbred racehorses with SDFT treated with SCLD have also been reported to have a more favorable prognosis than Thoroughbreds for return to racing and performance.1,13–15 It has been suggested14 that this improved prognosis for successful (≥ 5 races) return to racing after surgery in Standardbreds is because of decreased stress on the superficial digital flexor tendon at the trot or pace, compared with the gallop.

    View larger version(87K) Figure 1— Representative postmortem photograph showing the axial aspect of the accessory ligament of the superficial digital flexor tendon (superior check ligament)–tendon unit of 1 of 6 horses that had iatrogenic bilateral tendonitis created 30 days prior to SCLD in the left forelimb. The right forelimb was untreated. Sixty days after SCLD of the left forelimb, the tendons of both limbs were harvested, and the superior check ligaments were measured with calipers at the site indicated (arrows). The SCLD site healed in all 6 horses. Note the axial aspect of the elongated left superior check ligament, which has been transected and has healed, compared with the untreated right superior check ligament. The superior check ligament measured at the site indicated (arrows) in 6 horses was a mean ± SD of 2.04 ± 0.34 cm longer in the treated limb, compared with the untreated limb.

    Objective, complete data on the rate of return to performance and career longevity following injury and treatment of a large number of racing Thoroughbreds after SDFT and SCLD would be useful for evaluation of SCLD as a treatment and for comparison with other treatment options. Currently, these data are not available in the literature because so many evaluation systems have been used and the populations of horses have been used for a variety of purposes. Most studies4,10,12 have evaluated the reoccurrence rate of tendonitis, sometimes evaluated by the treating clinician, sometimes by the attending veterinarian, and sometimes by phone interview. Reoccurrence is a subjective judgment that can be difficult to clearly define,10 in our experience. In addition, horses that may not have clearly definable SDFT reoccurrence may still be unproductive after injury and treatment because it has been demonstrated that treatments of SDFT can create morbidity in the ipsilateral suspensory ligament and in the contralateral SDFT.4,6,12 Surgeons can more accurately use data from evaluation of the number of races competed after injury, as determined by an objective third party, to advise owners as to the probability of performance after injury and treatment.

    In some studies,6,12 the tendency to remove horses from analysis because of loss to follow-up and decisions to retire may have skewed the interpretation. The racing industry has a highly accurate system of documentation of racing performance,a so it can be certain that any horse lost to follow-up did not race. Frequently, retirement is not a totally independent decision. Poor progress in a horse's rehabilitation often leads to a decision for retirement from racing or change to a breeding career. Therefore, we believe that all horses undergoing treatment are best maintained as part of the study for a complete, objective evaluation.5,7,10 The purpose of the study reported here was to evaluate rate of return to racing, number of races completed, time to first race, and lifetime performance in a large group of flat-racing Thoroughbreds with SDFT treated by means of SCLD. No data on control horses treated without SCLD and evaluated in this manner were available in the literature for comparison, so a hypothesis of improved or decreased performance could not be formulated. No data on convalescence time were present in the literature.