• View in gallery
    Figure 1—

    Causal web to illustrate proposed associations (derived from multivariable conditional logistic regression models) between explanatory variables and retirement from racing attributable to tendon injuries among 175 case and 525 control racehorses in Hong Kong. Explanatory variables are considered most likely to be causal risk factors (yellow) or an effect of tendon injury (blue); variables highlighted in green are also most likely to be an effect of previous tendon injury (given the direction of the association [ie, OR < 1). Earnings per year* may be regarded as either an effect of tendon injury or potentially a proxy measure of the quality of the horse, which itself may be associated with the likelihood of retirement attributable to tendon injury.

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Evaluation of detailed training data to identify risk factors for retirement because of tendon injuries in Thoroughbred racehorses

Kenneth K. H. LamDepartment of Veterinary Regulation and International Liaison, Hong Kong Jockey Club, Sha Tin Racecourse, Sha Tin, NT, Hong Kong
Epidemiology Group, Faculty of Veterinary Science, University of Liverpool, Leahurst, Neston, CH64 7TE, UK

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Tim D. H. ParkinCentre for Preventive Medicine, Animal Health Trust, Lanwades Park, Newmarket, Suffolk, CB8 7UU, UK

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Christopher M. RiggsDepartment of Veterinary Clinical Services, Hong Kong Jockey Club, Sha Tin Racecourse, Sha Tin, NT, Hong Kong

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Kenton L. MorganEpidemiology Group, Faculty of Veterinary Science, University of Liverpool, Leahurst, Neston, CH64 7TE, UK

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Abstract

Objective—To identify the risk factors for premature retirement because of tendon injury in a Thoroughbred racehorse population.

Animals—175 Thoroughbred racehorses (cases) at the Hong Kong Jockey Club that were retired from racing because of tendon injury between 1997 and 2004 and for which the last preretirement exercise was at a fast pace were each matched with 3 control horses that were randomly selected from all uninjured horses that had galloped on the same date as that last exercise episode.

Procedures—Training data for all horses were examined. Conditional logistic regression analyses were performed to identify risk factors for retirement from racing attributable to tendon injury. Two multivariable conditional logistic regression models were created; each contained 8 explanatory variables.

Results—Compared with control horses, case horses were older at the time of import, accumulated more race distance soon after import, were more likely to have had previous official veterinary or ultrasonographic examinations, raced fewer times during their career, and were in training for a longer period and had exercised at a reduced intensity during the 180-day period preceding the last fast-paced work date.

Conclusions and Clinical Relevance—In addition to identification of risk factors for tendon injury among racing Thoroughbreds, results have suggested that resources focused on obtaining accurate training data may be misdirected in the absence of internationally agreed criteria for incident tendon injury among racehorses. Nevertheless, changes in training intensity and findings of previous clinical examinations could be used to identify horses at risk of tendon injury–associated retirement.

Abstract

Objective—To identify the risk factors for premature retirement because of tendon injury in a Thoroughbred racehorse population.

Animals—175 Thoroughbred racehorses (cases) at the Hong Kong Jockey Club that were retired from racing because of tendon injury between 1997 and 2004 and for which the last preretirement exercise was at a fast pace were each matched with 3 control horses that were randomly selected from all uninjured horses that had galloped on the same date as that last exercise episode.

Procedures—Training data for all horses were examined. Conditional logistic regression analyses were performed to identify risk factors for retirement from racing attributable to tendon injury. Two multivariable conditional logistic regression models were created; each contained 8 explanatory variables.

Results—Compared with control horses, case horses were older at the time of import, accumulated more race distance soon after import, were more likely to have had previous official veterinary or ultrasonographic examinations, raced fewer times during their career, and were in training for a longer period and had exercised at a reduced intensity during the 180-day period preceding the last fast-paced work date.

Conclusions and Clinical Relevance—In addition to identification of risk factors for tendon injury among racing Thoroughbreds, results have suggested that resources focused on obtaining accurate training data may be misdirected in the absence of internationally agreed criteria for incident tendon injury among racehorses. Nevertheless, changes in training intensity and findings of previous clinical examinations could be used to identify horses at risk of tendon injury–associated retirement.

Injuries of the SDF tendon are common debilitating problems among Thoroughbred racehorses worldwide.1-8 In Hong Kong, such injuries are the single most important veterinary medical reason for retirement of racing Thoroughbreds.9 The annual incidence of SDF tendon injury–related retirement increased from 2.3% to 4.2% between 1992 and 2004.10 Among the racehorse population during that period, premature retirement associated with tendon injury reduced the duration of racing career, number of race starts, and earnings by 25.6%, 41.2%, and 53.3%, respectively. Thirteen percent of horses that retired as a result of tendon injury never raced in Hong Kong.10

Several risk factors for tendon injury in horses during racing have been identified in previous studies.3,4,11-13 These include horse-related factors such as age, sex, weight, previous steeplechase experience, and SDF tendon abnormalities detected during prerace examination and course-related factors such as racecourse, surface, and distance.3,4,11-13

It has been suggested that tendon injury is associated with exercise intensity and duration; however, in most studies, accurate data on exercise carried out during training are not available. In Hong Kong, daily exercise data for racehorses have been recorded since 1996. The purpose of the study reported here was to identify the risk factors for premature retirement because of tendon injury in a Thoroughbred racehorse population through analysis of detailed training data.

Materials and Methods

Study population—Details of the horse population, training facilities, and procedures of veterinary inspections to evaluate the suitability of horses to race in Hong Kong have been previously described.14,15 Briefly, horse racing in Hong Kong consists entirely of flat racing. The season runs from July 1 to June 30 the following year, with races taking place from late August or early September through to the end of June. All race-horses are stabled at Sha Tin racecourse where a maximum of 1,200 horses can be accommodated at any 1 time. All clinical care is provided by a team of veterinarians employed by the HKJC.

Horses in which an injury has been diagnosed must pass an official veterinary examination before they are allowed to race again. Horses can potentially remain in training and racing until compulsory retirement at 10 years of age. Premature compulsory retirement may also occur following 2 officially recorded episodes of either epistaxis or heart rhythm irregularity. Horses can be retired voluntarily at any time during their racing career.

Data collection—For each horse, data of epidemiologic interest including details of the horse, race career, earnings, and racing and medical histories were obtained from the official computerized Jockey Club information system (Appendix). Training work distances and times were obtained from a local commercial track-work database group.a

HKJC database—The HKJC began computerized data collection in 1972. In 1992, an official racing information system was established; a separate veterinary system within the racing information system (designed to store all veterinary clinical records) was established in 1996. At the time of the study, the racing information system database consisted of > 3,700 fields in > 400 database tables, in which clinical records and other related information (eg, racing history and horse and owner details) were collected. Officially published data and clinical record entries were routinely validated by internal audit processes.

Trackwork database and limitations—A local commercial trackwork database groupa recorded daily training data since July 1, 1996, for all racehorses in Hong Kong. A team of 8 experienced observers recorded the daily trackwork activities of horses with the aid of binoculars from different observation points at Sha Tin racecourse. Each horse was exercised with a numbered and color-coded (associated with year of import) saddlecloth. According to the Rules of Racing, trainers are required to have each of their horses display the correct saddlecloth during trackwork. The Duty Stipendiary Steward performs random inspections during trackwork to ensure that the saddlecloth matches the freeze brand number on the left shoulder of the horse. A penalty fine may be imposed for use of incorrect saddlecloths, and in the last 10 years, there have been no more than 10 discrepancies reported. For any given horse, the estimated distance covered at trotting or canter pace on the all-weather tracks was calculated by multiplying the number of circuits observed by the mean circumference of the specified track.

Case definition and selection—Between July 1, 1997, and June 30, 2004, 337 of 3,585 (9.4%) horses were retired as a result of tendon injury. Three horses with traumatic tendon injuries (eg, lacerations and wounds), 1 with hind limb tendon injury, and 1 with extensor tendon injury were excluded from the potential case population. The remaining 332 horses that were retired because of strain injuries to forelimb tendons were eligible for inclusion as case horses in the study. Only horses that performed work at a fast pace (eg, training gallop, barrier trial [a form of race practice], or race) as their final exercise event prior to retirement attributable to tendon injury were included in the study. This definition was used in an attempt to identify the date of tendon injury, as it was assumed that horses performing work at a fast pace were free from serious tendon injury at that point in time.

Control horse selection—For each case horse, 3 matched control horses were selected at random from all other horses that raced, performed a barrier trial, or galloped during training on the same day as that defined as the last date on which the case horse undertook exercise at a fast pace. Exact exercise periods prior to the final fast work dates could then be examined for both case and control horses without the need to account for seasonal differences in exercise intensity. Exercise intensity was measured in terms of number of episodes and total distance covered for each type of exercise, including trotting in ring (warm-up), trotting or canter exercise, galloping, barrier trial, and racing.

Statistical analysis—Univariate conditional logistic regression was performed with the outcome being retirement attributable to tendon injury. All variables with a value of P ≤ 0.25 identified in the univariate screening process were available for inclusion in the final multivariable model. Multivariable conditional logistic regression models that accounted for confounding of explanatory variables were developed by use of a forward selection procedure. Variables with strong a priori biological reasons for inclusion were also considered in the final model if they significantly reduced the residual deviance of the model (likelihood ratio; P < 0.05). Collinearity of continuous explanatory variables in the final models was assessed by examination of the VIFs with computer software.16,b

Two multivariable models were developed. The first included all eligible variables. In the second, risk factors that were not considered to be part of the causal web were removed (Figure 1). For both models, the deviance residuals versus the individual observations were plotted to assess goodness of fit. Observations with large deviance residuals were excluded from the dataset, and the models were refitted to evaluate the influence of the removed observations on the estimated ORs and variables retained in the models.17 A statistical packagec was used for the conditional logistic regression analysis.

Figure 1—
Figure 1—

Causal web to illustrate proposed associations (derived from multivariable conditional logistic regression models) between explanatory variables and retirement from racing attributable to tendon injuries among 175 case and 525 control racehorses in Hong Kong. Explanatory variables are considered most likely to be causal risk factors (yellow) or an effect of tendon injury (blue); variables highlighted in green are also most likely to be an effect of previous tendon injury (given the direction of the association [ie, OR < 1). Earnings per year* may be regarded as either an effect of tendon injury or potentially a proxy measure of the quality of the horse, which itself may be associated with the likelihood of retirement attributable to tendon injury.

Citation: American Journal of Veterinary Research 68, 11; 10.2460/ajvr.68.11.1188

Results

One hundred seventy-five of the 332 horses that were retired from racing because of tendon injuries met the case definition (ie, were doing fast-paced work as their final exercise prior to retirement), and 525 horses were selected as control horses.

Univariate analysis—The univariate relationships between independent variables and retirement from tendon injuries were evaluated (Tables 1 and 2). With regard to horse details, the likelihood of retirement attributable to tendon injuries was strongly associated with age at the time of the final fast-paced work. This was identified when age was entered as a continuous (OR, 1.19; 95% CI, 1.07 to 1.33; P = 0.002) or categoric variable, with the odds being highest for horses in the 5-year-old group (OR, 2.34; 95% CI, 1.44 to 3.81; P < 0.001).

Table 1—

Results of univariate conditional logistic regression analysis of horse details, training and racing variables, and medical history in association with retirement from racing attributable to tendon injuries among 175 case and 525 control racehorses in Hong Kong.

Table 1—
Table 2—

Results of matched univariate analysis of training and racing variables assessed in 90-day periods preceding the last fast-paced work date in association with retirement from racing attributable to tendon injuries among 175 case and 525 control racehorses in Hong Kong. For each variable, values represent (in descending order) the OR, P value, and 95% CI. Significance set at P < 0.005.

Table 2—

Retirement attributable to tendon injury among racehorses was also associated with the number of years in training in Hong Kong (OR, 1.16; 95% CI, 1.04 to 1.29; P = 0.008) and being a sexually intact male (OR, 2.32; 95% CI, 1.12 to 4.78; P = 0.03); the association with a change of trainer was less strong (OR, 1.29; 95% CI, 0.9 to 1.9; P = 0.17). Although age at time of import into Hong Kong (OR, 1.17; 95% CI, 0.86 to 1.58; P = 0.33) was weakly associated with retirement because of tendon injuries, it was included in the multivariable analysis because of a priori evidence for age as a risk factor in a previous study.13

Retirement and race career—Increased odds of retirement attributable to tendon injury were associated with the age at first race in Hong Kong (OR, 1.42; 95% CI, 1.05 to 1.91; P = 0.02) but not with the number of race starts (Table 1). This included the total number of race starts (comprised of overseas race starts [OR, 0.99; 95% CI, 0.98 to 1.01; P = 0.23] and the number of races in Hong Kong [OR, 0.99; 95% CI, 0.98 to 1.0; P = 0.16]). The number of races per year in Hong Kong was strongly associated with a reduced likelihood of retirement attributable to tendon injury (OR, 0.88; 95% CI, 0.84 to 0.92; P < 0.001), as were cumulative distances raced during the entire career (OR, 0.99; 95% CI, 0.98 to 1.01; P = 0.02) and cumulative distances raced per year (OR, 0.91; 95% CI, 0.88 to 0.94; P = 0.08).

Career earnings of racehorses in Hong Kong were examined as a proxy measure of performance of the horses. As a result of the skewed distribution and the extreme range of the continuous variable of earnings per year ($0 to $6.6 million/y in HK$), the variable was log transformed to normalize the distribution. The natural logarithm forms of earnings per year were associated with the likelihood of retirement attributable to tendon injury (OR, 0.94; 95% CI, 0.88 to 1.0; P = 0.05). Earnings per year, in the form of a piecewise linear relationship derived from the logtransformed variable, were also associated with the likelihood of tendon injury–related retirement. There was a constant likelihood of retirement attributable to tendon injury in the lower earnings group (earnings ≤ HK$ 1.1 million/y; OR, 0.97; 95% CI, 0.9 to 1.03; P = 0.29) followed by a linear reduction in the odds of being a case horse as earnings increased from HK$ 1.1 million/y to a maximum of HK$ 6.6 million/y (OR, 0.78; 95% CI, 0.5 to 1.23; P = 0.05).

Retirement and racing intensity after import to Hong Kong—Both the number of races and cumulative distance raced during a number of time periods after import to Hong Kong were strongly associated with a reduced likelihood of retirement attributable to tendon injury (Table 1). The ORs, 95% CIs, and P values for number of races during specified time periods were calculated for 0 to 180 days after import (OR, 0.87; 95% CI, 0.76 to 0.99; P = 0.03), 181 to 360 days after import (OR, 0.87; 95% CI, 0.79 to 0.96; P = 0.004), and 0 to 360 days after import (OR, 0.88; 95% CI, 0.82 to 0.95; P < 0.001). During the first 90 days after import, there was a weak association between the likelihood of retirement attributable to tendon injury and both the number of races and cumulative distance raced (OR, 0.78; 95% CI, 0.49 to 1.22; P = 0.26).

Retirement and training and racing intensity (assessed for periods preceding the final fast-paced work event)—The number and cumulative distance (in kilometers) of races were strongly associated with reduced odds of retirement attributable to tendon injury for 90day periods up to 360 days before the last fast-paced work date (P < 0.001; Table 2). The number and cumulative distance of all fast-paced work events during 90-day periods up to 540 days before the last fast-paced work date were strongly associated with a reduced likelihood of retirement attributable to tendon injury (P ≤ 0.001). The number and cumulative distance of training gallops during 90-day periods up to 540 days before the last fast-paced work date were also associated with the likelihood of tendon injury–related retirement (P ≤ 0.01). The number of episodes (exercise events on the training or race track) and distance covered during trotting and canter exercise were strongly associated with reduced odds of retirement attributable to tendon injury in the 90-day periods from 0 to 360 days (P < 0.001) and 91 to 360 days (P ≤ 0.015) before the last fast-paced work date, respectively. However, the number and cumulative distance of all fast-paced work events and trotting or cantering exercise in the periods of 30, 60, and 360 days before the last fast-paced work date were not associated with tendon injury–related retirement among racehorses.

When compared with gallop-pace trackwork, the likelihood of tendon injury retirement was strongly associated with the type of final fast-paced work, namely barrier trial (OR, 7.3; 95% CI, 3.1 to 17.2; P < 0.001) and racing (OR, 3.2; 95% CI, 1.74 to 5.93; P < 0.001).

Retirement and medical history—History of a previous ultrasonographic examination for tendon injuries (OR, 11.5; 95% CI, 7.28 to 18.07; P < 0.001), official veterinary examination for tendon injuries (OR, 35; 95% CI, 12.62 to 97.21; P < 0.001), and previous anti-inflammatory drug treatment (OR, 1.63; 95% CI, 1.07 to 2.5; P = 0.02) were all strongly associated with an increased likelihood of retirement attributable to tendon injuries (Table 1).

Multivariable analysis—Two multivariate models were developed, and a causal web was constructed to identify the potential relationships between factors that were considered likely to be causally associated with retirement attributable to tendon injuries (Figure 1).

Model 1 (all variables)—Horses that were older at the time of import into Hong Kong were more likely to be retired from racing because of tendon injury (Table 3). For every additional year of age at import, horses were at least 2.5 times as likely to be retired as a result of tendon injury (OR, 2.51; 95% CI, 1.3 to 4.87; P = 0.006).

Table 3—

Multivariable conditional logistic regression model of retirement from racing attributable to tendon injuries among 175 racehorses in Hong Kong: analysis 1 (all potential explanatory variables included; likelihood ratio = 289.4 [9 degrees of freedom]; P < 0.001).

Table 3—

Distance raced in the first 6 months after import to Hong Kong was also associated with increased odds of retirement attributable to tendon injury. For every additional kilometer raced in this period, the odds of tendon injury–related retirement increased 1.24 times (95% CI, 1.04 to 1.48; P = 0.018). In contrast, an increased number of fast-paced work events in 90-day periods up to 180 days prior to the last fast-paced work date was associated with a reduced likelihood of a horse becoming retired because of tendon injury (ie, likelihood of becoming a case; OR, 0.86 [95% CI, 0.81 to 0.91; P < 0.001] for the 0- to 90-day period and OR, 0.92 [95% CI, 0.87 to 0.98; P = 0.005] for the 91- to 180-day period). Barrier trial and racing as the last fast-paced work event were also associated with an increased likelihood of retirement attributable to tendon injury with an OR of 10.96 (95% CI, 2.33 to 51.49; P = 0.002) and 10.12 (95% CI, 3.09 to 33.17; P < 0.001), respectively.

Previous evidence revealed that tendon injury was an important risk factor. Case horses were more likely to have had a previous tendon ultrasonographic examination (OR, 10.91; 95% CI, 4.84 to 24.59; P < 0.001) or official veterinary examination because of tendon injury (OR, 19.39; 95% CI, 4.01 to 93.79; P < 0.001) than control horses. There was a reduction in the likelihood of retirement attributable to tendon injury as natural log earnings per year increased (OR, 0.63; 95% CI, 0.53 to 0.75; P < 0.001).

Model 2 (removal of variables unlikely to be part of the causal web)—A second model was built without ultrasonographic and official veterinary examination variables. The risk factors identified in the first model remained, but 2 additional factors were identified; horses that remained in training and racing in Hong Kong longer had increased odds of tendon injury–related retirement (Table 4). The OR was 3.13 (95% CI, 1.89 to 5.17; P < 0.001) for every additional year in training. An increased number of races in Hong Kong was associated with a reduced likelihood of a horse being retired because of tendon injury (ie, likelihood of becoming a case horse; OR, 0.89; 95% CI, 0.84 to 0.94; P < 0.001). Estimates for the ORs related to the type of last fast-paced work were significantly reduced in the second model (barrier trial OR decreased from 11 to 6.08 [95% CI, 1.68 to 21.99; P = 0.006]; racing OR decreased from 10.1 to 5.94 [95% CI, 2.41 to 14.68; P < 0.001]). In this model, earnings per year in a piecewise linear form provided the best fit for the model. There was a gradual reduction in the likelihood of retirement attributable to tendon injury as earnings per year increased from HK$ 0 to 1.1 million (OR, 0.81; 95% CI, 0.71 to 0.94; P = 0.004), followed by a much greater reduction in the odds of tendon injury–related retirement for horses earning < HK$ 1.1 million/y up to a maximum of HK$ 6.6 million/y (OR, 0.001; 95% CI, 0 to 0.31; P = 0.017). Odds ratios for other risk factors in model 2 were not significantly altered.

Table 4—

Multivariable conditional logistic regression model of retirement from racing attributable to tendon injuries among 175 racehorses in Hong Kong: analysis 2 (excluding risk factors considered likely to be intermediate steps on the causal pathway [previous ultrasonographic and official veterinary examination for tendon injury]; likelihood ratio = 233.7 [10 degrees of freedom]; P < 0.001).

Table 4—

Colinearity diagnostic evaluations—Examination of VIFs for each of the independent variables revealed that the largest individual VIF scores were associated with the number of episodes of fast-paced work in the 91- to 180-day period prior to the final fast-paced work date and the history of previous ultrasonographic examination (1.33 from model 1) or the number of years of training in Hong Kong (6.09 from model 2). The mean VIF values were 1.2 and 2.3 for models 1 and 2, respectively.

Goodness of fit of the multivariable models—For model 1, removal of data sets containing observations with the largest deviance residuals had no effect on the variables retained within the final model. However, 4- to 5-fold increases in the estimates for the ORs related to the type of last fast-paced work before retirement and history of previous ultrasonographic or official veterinary examination for tendon injury were identified.

For model 2, removal of data sets containing observations with the largest deviance residuals had no effect on the variables retained within the final model. There was an approximately 2.5-fold increase in the estimate for the OR for barrier trial as the last fast-paced work undertaken. The OR for racing as the last fast-paced work undertaken increased by approximately 30%. The OR for the number of years of racing and training in Hong Kong increased by approximately 65%.

Discussion

The aim and challenge of the present study were to assess the unique training data collected by the HKJC for relationships between a selection of variables and premature retirement of racehorses as a result of tendon injury. Analysis of this data set identified events that occurred at or near the time of importation, performance of ultrasonographic and official veterinary examinations, and reduced exercise frequency in the days preceding retirement as being associated with the likelihood of retirement attributable to tendon injury. Although some of these factors, such as previous ultrasonographic or official veterinary examination, are of value in the management of risk in the racehorse population, they are examples of associations that are unlikely to be causative. Two multivariable models were therefore established to examine the effects of these variables on the likelihood of retirement of racehorses as a result of tendon injuries.

Within the population evaluated, horses that were older at the time of import into Hong Kong and spent longer in training and racing were at greater odds of retirement attributable to tendon injury. This is consistent with findings of a study by Perkins et al,13 which indicated that older horses were more likely to sustain tendon injuries, compared with 2-year-old horses. It has been suggested that tendon strength gradually diminishes after maturation at a rate that is dependent on the amount of training undertaken.18 There is evidence that the SDF tendon in adult horses operates close to its physiologic limits during maximal exercise or racing.19-23 It has been proposed that a combination of exercise- and age-associated microdamage and the limited adaptive ability of the SDF tendon after maturation at 2 to 3 years of age contributes to increased risk of tendon fatigue failure in horses.19,21,24-27

In the present study, horses that raced a greater distance during their first 6 months after import were also at greater risk of retirement attributable to tendon injury. It is possible that such horses were already in race training prior to import into Hong Kong, which enabled them to race more often early in their career at the HKJC. Our finding may therefore reflect more extensive exercise-related tendon degeneration in those horses prior to racing in Hong Kong. It was not possible to quantify the size of this effect because exercise history prior to import into Hong Kong was not available. Results of a biomechanical study19 suggest that excessive exercise at a young age may predispose horses to SDF tendon injury later in life. It is also possible that the case horses in the present study might have had tendon injuries before import. The unknown history of previous tendon injury and unmeasured exercise intensity prior to import may be contributing factors to the increased odds of tendon injury–related retirement among racehorses in Hong Kong. Nevertheless, a limit on the distance raced immediately after import into Hong Kong may reduce the likelihood of horses being retired from racing because of tendon injury later in their careers.

Case horses were more likely to be performing a barrier trial (race practice) or racing rather than galloping in normal training on their last day of fast-paced work prior to retirement. This finding may suggest that these types of exercise are more likely to result in tendon injury. However, it is also possible that some case horses were already injured prior to the defined last fast-paced work date. Further information on the motivation behind trainer decisions to enter horses into barrier trials or races after reduced levels of fast-paced work in training may help to explain the apparent difference in risk associated with different types of fast-paced work. For example, horses that performed a barrier trial as the last type of fast-paced work undertaken may be more likely to be retired because this type of exercise may be used to evaluate horses that have recently recovered from previous tendon injury. The following data obtained in the present study supported this hypothesis: 46% (28/61) of horses that had barrier trial as the last fast-paced work had undergone a previous veterinary or ultrasonographic examination for tendon injury. However, only 25% (113/445) and 38% (73/194) of horses that had training gallop or racing as their final exercise, respectively, had undergone a previous veterinary or ultrasonographic examination.

Both previous ultrasonographic and official veterinary examinations for tendon injury were strongly associated with the likelihood of retirement attributable to tendon injury among the racehorses included in the present study. Although it was not possible to introduce interventions that would reduce the risk of tendon injury, it would be possible to monitor the number of examinations for individual horses at the HKJC in the future. On the basis of those data, it may then be possible to identify those horses with greater potential susceptibility to tendon injury and subsequent retirement from racing. In addition, the reduced amount of fast-paced work undertaken by case horses in the 1- to 90-day and 91- to 180-day periods prior to the last fastpaced work date suggested that these horses most likely already had some form of injury. Thus, identification of changes in training patterns could also be used to enable closer examination of horses at risk, thereby potentially preventing career-ending tendon injury. Specific preventive measures that have been implemented at the HKJC since completion of our study include reviewing training regimens and closer monitoring of horses with a history of ultrasonographic or veterinary examination for tendon injury.

Exclusion of the 2 variables that were considered most likely to be intermediate steps on the causal pathway (ie, previous veterinary or ultrasonographic examination for tendon injury) resulted in some significant changes to the final multivariable model (model 2) in the present study. Most notably, 2 new risk factors were identified, a different best-fitting form of the association with earnings per year was included, and the estimated ORs for both barrier trial and racing as the type of final fast-paced work prior to retirement were reduced by approximately 50%. Although not dramatically changing the overall findings of our study, these changes underscore the value of causal webs and the importance of hypothesis-driven analysis. Without the second model, we would have failed to identify 2 additional risk factors because of the overwhelming presence of 2 other variables that were strongly associated with the outcome but unlikely to play any part in increasing the likelihood of tendon injury–related retirement from racing.

The impact of tendon injury–related retirement of racehorses in Hong Kong has previously been quantified in terms of a 26% reduction in the duration of racing career, a 41% reduction in the number of race starts, and a 53% decrease in career earnings.10 These observations were consistent with findings in the present study. Case horses had a reduced number of lifetime race starts, compared with control horses, and horses that earned more were less likely to be retired from racing because of tendon injury. It is probable that these factors reflect the effect of chronic or repeated tendon injury, which ultimately results in retirement, rather than being causal risk factors for retirement.

In the present study, the VIFs calculated for both models indicated little evidence of multicollinearity. The mean VIFs were not considerably larger than 1, and the largest VIFs did not exceed a value of 10.16 The analysis of goodness of fit for both models indicated that there were some influential observations within the original data set. When case-control sets that included these observations were removed, the final multivariable models retained the same risk factors but there were some important changes in the estimated ORs for some variables. On all occasions, the ORs were more removed from 1, indicating that the original model that was based on the full data set provided conservative estimates of the effects of most risk factors.17

The unique clinical and trackwork training records available at the HKJC revealed that 47% (156/332) of the horses that were initially eligible for inclusion in the study trotted or cantered (often for many days after their last fast-paced work) before they were retired. One additional noneligible horse did no fast-paced work in Hong Kong before retirement attributable to tendon injury. This may represent an attempt to rehabilitate a horse that had previous tendon injuries or may indicate that serious tendon injuries can also occur during lower intensity exercise. Arguably, there is little incentive for trainers to have horses with mild tendon injuries examined by veterinarians because this is unlikely to change the management of these horses (the most common conservative treatment being rest and rehabilitation). 28,29 An assumption in our study was that a tendon injury that was associated with retirement from racing would occur during exercise at gallop pace. In an attempt to identify the date of injury, we included only those horses for which the last exercise was at a fast pace. This enabled us to compare exercise intensity for case and control horses over the same time periods, thereby removing the effect of seasonal differences in training and racing. However, results of the present study highlight a major problem in the design of this type of study—identification of the time of first tendon injury. There is evidence that we identified risk factors associated with chronic tendon injury in the study of this report, and this is likely applicable to other epidemiologic studies.

Information regarding the distance covered at different velocities by horses during training and racing and regarding the surface characteristics of training and racing tracks was not available for investigation in the present study. Collecting information about these variables remains a challenge. It has been suggested that the development of miniaturized electronic equipment that incorporates global positioning system capabilities may facilitate this process in future studies.13 However, with respect to tendon injuries in horses, definition of the incident case is the limiting factor in epidemiologic studies rather than the availability of more detailed or accurate training data. Further investigation of the data available at the HKJC by use of survival analysis techniques should remove assumptions about the exact date of injury through analysis of all cases of tendon injury–related retirement and would identify incident cases by selecting those cases with official records of veterinary examinations associated with tendon injury.

ABBREVIATIONS

SDF

Superficial digital flexor

HKJC

Hong Kong Jockey Club

VIF

Variance inflation factor

OR

Odds ratio

CI

Confidence interval

HK$

Hong Kong dollars

a.

Turf Timers Co, Jockey Club Trackwork Service Provider, Hong Kong.

b.

Stata, version SE 9.2, StataCorp LP, College Station, Tex.

c.

Egret, version 2.0.3, Cytel Software Corp, Cambridge, Mass.

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Appendix

Racing history, horse details, and training variables investigated in a study to assess risk factors for tendon injury–related retirement from racing among a population of Thoroughbreds in Hong Kong.

table5

Contributor Notes

Supported by The Hong Kong Jockey Club.

The authors thank Iris Yu, Anthony Leung, Brian Stewart, and Leo Cheung for technical assistance.

Address correspondence to Dr. Lam.