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Keywords:

  • horse;
  • superficial digital flexor tendonitis;
  • performance;
  • racehorse;
  • Thoroughbred

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Reasons for performing study: There is limited information regarding the number of races and the period for evaluation of outcome which is critical for assessment of SDF tendonitis treatments.

Objective: To evaluate the re-injury rate and racing performance of Thoroughbred racehorses that sustain SDF tendonitis in relation to matched controls in terms of number of races post treatment and maximum racing performance ratings before and after injury.

Study design: Clinical records and racing histories of 401 racehorses with a first occurrence of SDF tendonitis diagnosed by ultrasonography. Controls were of the same age, sex and were horses training in the same establishment at the time of injury as the case horses and where the trainer reported that the horse had not had a previous SDF tendon injury or treatment.

Results: Eighty percent of both case and control horses returned to racing after the date of injury, and the re-injury proportion within 3 years of treatment was 53%. The difference in Racing Post Rating(max) (RPR(max)) and the Racing Post Rating in the race immediately before the treatment date was significantly smaller in case horses (mean = 9.6 lbs; range = 0–75) compared to control horses (mean = 17.0 lbs; range = 0–79). No significant decrease in RPR(max) was noted post injury. No difference between case and control horses was found for return to racing and racing 3 times, but control horses were significantly more likely to compete 5 races post treatment date than case horses.

Conclusion and clinical relevance: Injury was associated with an individual's pre-injury maximum performance level and return to racing and completion of 3 races are not useful indicator of the outcome of horses with SDF tendonitis. The assessment of the outcome of horses with an SDF injury in a population of racehorses using the number of races post injury requires a minimum of 5 races post injury to be a useful indicator. Further, a re-injury proportion in a population of horses in training for 3 years post treatment.


Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Superficial digital flexor (SDF) tendonitis of the forelimb is among the most common musculoskeletal injuries in racehorses. A prevalence of 11–30% (over a 1–10 year period) of SDF tendonitis have been reported in Thoroughbred racehorses (Marr et al. 1993; Williams et al. 2001; Takahashi et al. 2004; Pinchbeck et al. 2004a; Lam et al. 2007a; Avella et al. 2009); and Ely et al. (2009) reported an incidence of 1.7/100 horse months in training. Epidemiological studies have identified a range of risk factors for SDF tendon injuries, such as older age, male gender, longer race distance, frequent high-speed work, heavier mean bodyweight at race time, race track surface and longer training careers (Mohammed et al. 1992; Estberg et al. 1995; Kasashima et al. 2004; Perkins et al. 2005b; Lam et al. 2007b). Other potential risk factors are fatigue and lack of fitness outlined in an experimental study (Butcher et al. 2007) and less clearly defined risk factors, such as conformation and incoordinate action (Jorgensen and Genovese 2003; Weller et al. 2006).

It is now generally accepted that clinical injury is often preceded by age- and exercise-related degenerative changes to the matrix of the SDF tendon (Goodship et al. 1994; Smith et al. 2002). However, high loads are required for a partial rupture of the SDF tendon to occur. During maximal exercise the SDF tendon operates close to its physiological limits with a narrow biomechanical safety margin (Dowling and Dart 2005). The likelihood of a clinical lesion of the SDF tendon would therefore be highest when horses are competing at their maximum performance level. The first objective of this paper was therefore to investigate the hypothesis that horses were at or near their maximum pre-injury performance level in the race before presentation for treatment of a SDF tendonitis than matched control horses.

Most horses with a strain-induced SDF tendonitis require at least 9 months of rehabilitation before resuming full performance. In severe cases, the rehabilitation period is prolonged up to 18 months until the maturation phase of tendon healing is completed (Davis and Smith 2006). It is well recognised that ‘healed’ tendon tissue does not regain its original mechanical properties, because persistent scar tissue both within and between fascicles is likely to interfere withnormal function and as a consequence a remodelled tendon has a decreased elastic modulus (Crevier-Denoix et al. 2005). In normal tendons, the muscular work required during galloping is halved by storing and returning elastic strain energy in spring-like muscle-tendon units (Wilson et al. 2001). This would suggest that horses with a ‘healed’ SDF tendonitis require an increased amount of muscular work to compensate for the reduced tendon elasticity. A resultant decrease in performance post injury may therefore be expected. The second objective was to investigate the hypothesis that racehorses that have recovered from an episode of SDF tendonitis injury have a reduced maximum performance level.

Return to racing and the number of races post treatment after an incident of SDF tendonitis have been used as a marker of the outcome of a tendon treatment (Marr et al. 1993; Fulton et al. 1994; Hawkins and Ross 1995; Hogan and Bramlage 1995; Dyson 1996; Gibson et al. 1997; Pacini et al. 2007). Jorgensen and Genovese (2003) suggested classification of tendon treatment outcomes into one of 3 categories: 1) successful, when treated horses competed 5 or more races; 2) partially successful, meaning completion of 1–4 races; and 3) failure, because re-injury occurred before the first race was competed. The third objective in the current investigation was to investigate the return to racing, 3 or 5 races post treatment groups in relation to matched controls. We hypothesised that case horses were less likely to return to racing and to compete 3 or 5 races post treatment date than control horses.

The number of races competed does not reflect the number of horses that have been re-injured, retired for other reasons or have been lost to follow-up; potentially, the number of races competed post treatment may not be sufficiently sensitive indicators to allow assessment of individual treatments. There is age- and exercise-related degeneration of the tendon fibres present in both tendons and therefore a subsequent injury may occur in the affected or in the contralateral limb (Davis and Smith 2006). Dyson (2004) reported the success of various treatments for SDF tendonitis based on the proportion of horses that suffered a further incidence of tendonitis to either the affected or the other forelimb, during 2 years of athletic activity after the convalescent period. A further objective of this study was to calculate the re-injury proportion in the study population as a potential alternative to the number of races competed post treatment as a measure of the outcome of horses with SDF tendonitis.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Study population

Thoroughbred racehorses with a first occurrence of a SDF tendonitis were identified from clinical records at Donnington Grove Veterinary Surgery, Newbury, UK between 1997 and 2004. The inclusion criteria were racehorses that had been diagnosed by ultrasonography as having a first occurrence of an SDF tendon core lesion of a severity warranting treatment and convalescence, comparable to Grade III–VI as outlined by Jorgensen and Genovese (2003). Only horses treated with intralesional insulin-like growth factor-type 1, traditional bar firing or by a superior check ligament desmotomy were included.

Data acquisition

Clinical, racing and sales records were examined and trainers questioned about the histories of case and control horses on 2 occasions in 2003 and 2008. The name, age, sex, date of first treatment and the owners/trainers details were gathered from clinical records and the Racing Post website (http://www.racingpost.com). Telephone interviews were conducted to ascertain follow-up information and to clarify that it was the case horse's first occurrence of a tendon injury and the absence of a tendon injury in the control horses.

Furthermore the horse's veterinary history was examined and the details of the horses racing history were examined and, if there had been a prolonged absence of racing, for example a year, then the trainer was asked specifically why it had not raced over that period. If the history could not be verified then the horse was excluded from the study.

Controls

One control was selected at random (using a random number generator) for each case horse from ‘Horses in Training’ (Anon 1997–2004). Controls were horses training in the same establishment at the time of injury and were of the same age and sex as the case horse. The treatment date of the case horse was used as the date for each matched control horse to define pre- and post treatment date periods. The racing performance information was calculated from this date. All controls were reported by the trainer/owner as not to have had a previous SDF tendon injury or treatment.

Racing data and performance ratings

Racing data were gathered for both case and control horses from the Racing Post website. The Racing Post Rating (RPR) was used as a measure of performance. The RPR is published by the Racing Post newspaper, as an aid to gambling; it is calculated for every race in the UK at the end of every day by a small team of observers. The figure is calculated according to the distance won/beaten in lengths in relation to the other horses (collateral form) and their ratings and the amount of weight the horse carried. However, unlike the official handicap rating RPR reflects the horse's performance in a single race and not the potential of a horse. In the present study the RPR(max) was used as a measurement of the best racing performance within a certain period (i.e. pre-injury, post injury) to gauge the best performance effort of a horse in that time period. If a case horse or matched control horse did not have the required RPR figure (i.e. never raced or was never attributed a rating because the horse had not had a meaningful racing effort as determined by the RPR specialists) or if all the ratings required were not of the same type (i.e. flat or national hunt (NH)) then that case horse and matched control were excluded from the analyses using RPR.

The maximum RPR (RPR(max)) before each case horse's treatment date and the RPR in the race immediately before treatment were collected for case horses and matched controls. The difference between the RPR(max) before treatment and the rating in the race immediately before treatment was calculated for case horses and matched controls. It was assumed that the smaller the difference, the nearer the horse was to its maximum performance in the race before injury.

The (RPR(max)) pre- and post treatment date was recorded for case and matched controls. The difference between the pre-injury/treatment RPR(max) and post injury/treatment RPR(max) was calculated for case horses and matched controls.

The number of races competed post injury was gathered from the Racing Post website for case horses and matched controls. Each horse was categorised as having competed a minimum of one (i.e. returned to racing), 3 or at least 3 races post injury.

Horses that were sold or the owner/trainer was not contactable were categorised as ‘lost to follow-up’. Horses that were retired for a reason unrelated to a SDF tendon re-injury were categorised as ‘retired for other reasons’. A 3-year period was chosen to allow for one year of convalescence and 2 years of full athletic function (training and racing). ‘Successfully treated’ horses were those horses that remained in race training and did not re-injure within 3 years of treatment. ‘Re-injured’ horses were horses that had a subsequent SDF tendon injury in one or both forelimbs of a severity that excluded it from race participation within 3 years of treatment. The re-injury proportion was defined as the number of horses that ‘re-injured’ as a percentage of all case horses, excluding those lost to follow-up or retired for other reasons.

Data analysis

Shapiro-Wilks tests were used to identify normally distributed data. Wilcoxon signed rank tests and paired t tests were used to identify differences between matched cases and controls as regards the association of tendon injury and maximum performance and as regards a decrease in performance post treatment, respectively. McNemar tests was used to identify significant associations between case status and a return to racing or racing at least 3 and 5 times after treatment date.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The study population included a total of 802 racehorses (401 case horses consisting of 371 male, 30 female). Of these, 106 horses were lost to follow-up and 103 retired for other reasons. Ninety-one horses were treated successfully and 101 horses were regarded as having been treated unsuccessfully. Mean age at time of the first injury was 6.6 years (range 2.2–12.4 years). The racing history of the case horses prior to the SDF tendon injury was as follows: 29 horses (11 flat trained, 18 NH trained) had not raced; 42 case horses had only raced on the flat; 48 had raced only in NH flat races; 41 had raced only in NH (hurdle and chase) races and 241 had raced in both flat and NH (hurdle and chase) races. Mean number of races per case and control horse was 11.2 and 11.7 before treatment date and 6.5 and 10.3 after treatment date, respectively. The total number of races before and after treatment are reported (Table 1).

Table 1. The total number of races and race types before and after superficial digital flexor tendonitis treatment date for case and matched controls
 Before treatment dateAfter treatment date
Case horses (n = 401)Control horses (n = 401)Case horses (n = 401)Control horses (n = 401)
National Hunt flat races5013103636
Hurdle172516759291290
Chase64666611201507
Flat160120395121285
Total races4473469025974118

On average, case horses (n = 181) were 7.4 lbs nearer their maximum pre-injury performance level in the race before presentation for treatment of a SDF tendonitis than matched control horses. The difference in RPR(max) and the Racing Post Rating in the race immediately before the treatment date was significantly smaller for case horses (mean = 9.6 lbs; range = 0–75) compared to control horses (mean = 17.0 lbs; range = 0–79) (P<0.001).

The mean reduction in RPR(max) was 6.6 lbs (range = -58 lbs to 59 lbs) and 4.5 lbs (range = -76 lbs to 63 lbs) for case and control horses, respectively (n = 146). There was no significant difference in the change in RPR(max) pre- and post treatment date between case and control horses (P = 0.35).

There was no statistical significant difference in the likelihood that case or control horses would return to racing (P = 0.94) or race at least 3 times (P = 0.23) post treatment date (Table 2). Control horses were significantly more likely to race at least 5 times post treatment date compared to case horses (P = 0.04).

Table 2. Re-injury proportion and the percentage of horses that returned to racing, and completed three or five races after superficial digital flexor tendonitis treatment date for case and matched controls
 Case horses (n = 401)Control horses (n = 401)P valueRe-injury proportion
Return to racing (≥one race competed)80%80%0.9442% (68/161)
Three races competed63%68%0.2334% (46/137)
Five races competed46%56%0.0422% (25/115)

The overall re-injury proportion was 53% (101/192). The re-injury proportion for the ‘Return to racing’, ‘Competed in 3 races’ and ‘Competed in 5 races’ groups was increasingly lower in each group being 42, 34 and 22%, respectively (Table 2).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

Racehorse performance is difficult to quantify and there is no established method of measuring the change of performance of racehorses before and after treatment of a medical or surgical condition. As a consequence, a number of markers of performance have been used in clinical studies including earnings, finishing position in a race (performance index = PI), racing post ratings (RPR), top speed rating (TPS), official rating (OR) or a combination of these (Woodie et al. 2005; Weller et al. 2006; Cheetham et al. 2008; Parente et al. 2008; Reardon et al. 2008). RPR accounts for the allotted weight carried in a race and, furthermore, it varies according to the horse's performance in each race and therefore allows for a comparison with other races in the career of a horse (Racing Post personal communication).

Horses in this study were significantly nearer to their pre-injury maximum performance level in the race immediately before injury when compared to matched controls suggesting that they were competing nearer their individual maximum performance level when the SDF tendon injury occurred. Ultimate failure of the SDF tendon may be explained by the combination of exercise-induced progressive degenerative changes and high load. During maximal exercise the SDF tendon operates close to its physiological limits with a narrow biomechanical safety margin (Dowling and Dart 2005). Once the peak load of the tendon overcomes its structural strength, there is physical disruption of the tendon (Davis and Smith 2006). Fatigue of the synergistic, faster contracting deep digital flexor muscle causing overloading of the SDF tendon (Butcher et al. 2007) may be more likely to occur during a maximal performance effort. The concept that SDF tendonitis is related to elite performance in competition has been suggested by others (Murray et al. 2006; Singer et al. 2008).

The decrease in RPR(max) after treatment date was greater in the case horses compared to the control horses; however, this was not statistically significant. With the small numbers available for this part of the analysis, the statistical power may have been limited. Although the authors believe that RPR is the most sensitive marker of a horses performance in a race, it may have not have been sufficiently so to uphold the second hypothesis. Alternatively, as the results in this study suggest SDF tendonitis injury may not affect performance substantially. Crevier-Denoix et al. (2005) has demonstrated a substantial reduction in the elastic modulus of a diseased tendon and so a decrease in performance would be expected; however, our results suggest that it is unlikely to be clinically relevant.

The high incidence of re-injury in racehorses with SDF tendonitis is again highlighted in this study and in others (Fulton et al. 1994; Hawkins and Ross 1995; Dyson 1996; Gibson et al. 1997; Southwood et al. 1999). New treatments aim at regenerating the tendon tissue (Richardson et al. 2007). The hope is that these new biotechnologies will promote new tendon tissue that has the compositional and functional properties of non-degenerated tendons. Re-injury rate remains an important marker to evaluate these new tissue engineering techniques. Previous studies have used the completion of a number of races to measure the outcome of different treatments (Marr et al. 1993; Fulton et al. 1994; Hawkins and Ross 1995; Hogan and Bramlage 1995; Dyson 1996; Gibson et al. 1997; Pacini et al. 2007). The percentage of horses that returned to racing in this study was similar to the percentage previously reported in racehorses (Gillis et al. 1995) and in point-to-point and NH horses (Marr et al. 1993; Ely et al. 2009). In the current study of 401 horses a return to racing and the ability to compete in 3 races post injury did not differ from matched control horses suggesting that these are not useful measurements of the outcome of horses with tendon injury. However, case horses were significantly less likely to compete in 5 races post treatment date than control horses. This suggests that to compete at least 5 races post injury should be regarded as a useful indicator of the outcome of horses with SDF tendonitis. A significant difference between case and controls was determined only in the horses that competed a minimum of 5 races, indicating that the effects of SDF tendon re-injury outweighed the other reasons for the control horses not to compete in further races However, the re-injury proportion in the group of horses that competed 5 races was 22%. This figure emphasises the fact that the number of races competed post injury does not take into account the number of horses that have re-injured.

This study has the inherent limitations of a retrospective design. Furthermore the study was conducted over a 10 year period and there is a potential for recall bias and inaccuracy as regards determining the true absence of previous tendon injury. The case horses were horses presented for treatment that had a SDF injury of a severity warranting treatment and convalescence; therefore, there are many potential reasons why horses would not have been treated and hence would not have been included in the study. Re-injury in horses was confirmed by telephone interview and used to calculate the re-injury proportion. If the history was unclear then these horses were categorised as lost to follow-up; work by Lam et al. (2007b) and Avella et al. (2009) suggests that trainers are aware of acute tendon injury before an ultrasonographic examination is carried out. However, clearly, there is still a potential for inaccuracy.

The over-representation of males (371/401) to females (30/401) may be explained in part by gender bias within the racing thoroughbred population (Anon 2008) and, in part, by the possibility that a larger proportion of females retire to stud, following tendon injury, rather than seek treatment aimed at a return to racing. Furthermore, some previous studies have highlighted that male horses may be at an increased risk of SDF tendon injury (Takahashi et al. 2004; Perkins et al. 2005a). In the study population, NH racehorses were over-represented compared to the practice population (approximately 40% NH trained/60% flat trained). This difference concurs with the studies by Williams et al. (2001), Takahashi et al. (2004) and Pinchbeck et al. (2004b) who reported that steeple chasing is a risk factor for SDF tendonitis. Meershoek et al. (2001) have shown that the relative loading of the SDF tendon is very high while jumping; therefore, providing an explanation for the over-representation of jumping horses in the present population. Furthermore, NH horses are generally older than the flat horse population in this study, and age has been shown to be a risk factor for SDF tendonitis injury (Perkins et al. 2005b; Lam et al. 2007b).

This study has established a relationship between maximum performance levels and the occurrence of SDF tendonitis in the Thoroughbred racehorse. The study has identified that a return to racing and the completion of 3 races are not useful indicators of the outcome of horses with SDF tendonitis. Perhaps the completion of 5 races may be a useful measure of outcome in a study of this size. However, calculation of a re-injury proportion over a period of 3 years after injury appears the more appropriate measure of outcome in horses with SDF tendonitis.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References

The authors would like to acknowledge the efforts of Karen Brewer and Sam Oats for their help in gathering data; the clinicians at O'Gorman, Slater, Main & Partners, the trainers, stable staff, owners and horses of the Newbury and Lambourn area for their participation in this study.

Bryan O'Meara is in receipt of a scholarship from the Horse Trust.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Acknowledgements
  8. References
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Author contributions All authors contributed to the initiation, conception, planning, writing and statistics for this study. Its execution was by B.B., B.F., B.O'M. and C.J.L.