The Risk of Injury on Different Racing Surfaces

Musculoskeletal injuries are the most common reason for time lost in training, retirement from racing, and euthanasia in racehorses worldwide.^3-7 In the majority of locations, nonturf surfaces appear to be associated with an approximately twofold greater risk of injury. From 1999 to 2003 in the United Kingdom, the overall incidence of catastrophic distal limb fractures (including fractures of the carpus and tarsus) in all-weather flat races was 0.74 per 1000 starts compared with 0.37 per 1000 starts in turf flat races.⁸ The risk of catastrophic musculoskeletal injury on dirt has been reported to be 1.12 per 1000 starts compared with 0.58 per 1000 starts on turf on two racecourses in Ontario, Canada,⁹ The risk of fracture injury for horses racing on three New York Racing Association tracks from February 1, 1983 to mid October 1985 was 2.1 per 1000 starts on dirt compared with 1.1 per 1000 starts on turf.¹⁰

Results in a study from Florida did not concur.¹¹ In this study the incidence of injury was significantly higher in turf races (2.3 per 1000 starts) than in dirt races (0.9 per 1000 starts).¹¹ This may be a particular characteristic of racing in Florida because the turf races were more likely to involve larger fields and be longer races than those reported in other studies. Both of these factors were associated with the risk of injury in previous studies,^12-17 indicating that turf racing may be a proxy marker for these other risk factors.

Fractures predominate as a major type of injury, and the bones distal to the carpus or tarsus are most often affected.^3,4 Postmortem investigations conducted in the United Kingdom have enabled estimates of the risk of different types of fracture on different racing surfaces (Figure 105-1).⁸ The overall incidence of catastrophic distal limb fractures in all-weather flat races was 0.74 per 1000 starts compared with 0.37 per 1000 starts in turf flat races. In other words, the relative risk (RR) of catastrophic distal limb fracture on all-weather surfaces compared with turf was 2.0 (95% confidence interval [95% CI] 1.3 to 3.1). There were also significant differences in the risk of certain types of fracture on these surfaces. In particular, fractures of the lateral condyle of the third metacarpal bone (McIII) were 3.2 times (95% CI 1.6 to 6.6) more likely to occur on all-weather surfaces than turf racecourses, and biaxial proximal sesamoid bone (PSB) fractures were 4.6 times (95% CI 1.8 to 11.6) more likely to occur on all-weather surfaces than turf racecourses.⁸ Some fracture types were more common on the turf. For example, catastrophic fractures of the proximal phalanx were twice as common (RR 2.0; 95% CI 0.6 to 6.7) on turf than on all-weather surfaces.⁸

Fig. 105-1 The distribution of catastrophic forelimb distal limb fractures on UK racecourses, by flat race surface type, from January 1999 to December 2003 (showing standard error bars). MCIII, Third metacarpal bone.

Similar postmortem studies have been conducted in other racing jurisdictions. In California^3,18 and Kentucky^14,19 the PSBs followed by the McIII were the most common sites of fracture. These were catastrophic fractures included in the California Horse Racing Board postmortem program from dirt tracks^3,18 and all injuries recorded at four tracks in Kentucky.^14,19 In contrast, injuries on three New York Racing Association dirt and turf tracks from 1986 to 1988 were more similar to those in the United Kingdom, with fractures of the McIII being most common.²⁰ Information on fatal musculoskeletal injuries from a number of racing jurisdictions within the United States (33 racetracks in 15 different states) revealed that McIII fractures were slightly more common than those of the PSBs.²¹ However, an unknown number of “ankle breakdowns” in these data would have been caused by a PSB fracture and suspensory apparatus disruption. PSB fractures were also reported as the most common “serious accident” on eight racecourses in Japan.²² In Australia, the situation is more similar to that in the United Kingdom, with fractures of the McIII and then of the PSBs predominating.²³ Clarification of differences in the risk of individual fracture types on different surfaces and further international comparisons will aid in the understanding of the pathogenesis of different fractures and ultimately may result in novel preventative measures being developed.

Simple comparisons of injury rates are useful in establishing priorities for future work at the local or regional levels. However, when establishing causal relationships, raw comparisons should be interpreted with caution. To state that the differences in injury rates on different surfaces in different racehorse populations are directly caused by surface characteristics is dangerous. There are many differences in the Thoroughbred populations racing in different locations. For example, at the time of the following study, different shoe types such as toe grabs were permitted in the United States.²⁴ There were strong associations between the use of toe grabs and the risk of both suspensory apparatus failure and condylar fracture of the McIII. In particular, the risk of suspensory apparatus failure was substantially increased in horses using regular-height toe grabs compared with those using no toe grabs.²⁴ In many other racing jurisdictions, toe grabs are not permitted, so meaningful international comparisons need to account for differences in shoe types as well as many other important contributory factors.

Even within a racing jurisdiction, direct comparisons between turf and all-weather tracks should not be made without consideration of all potential explanatory factors. For example, in the United Kingdom the prize money available and class of flat race held on the all-weather racecourses are generally lower than for flat races held on turf. This is obviously potentially an important confounding factor, as one of the reasons for poor performance may be subclinical or previous injury. It is therefore possible that horses in all-weather flat races are inherently more likely to sustain an injury, regardless of the surface on which they are racing. The type of surface may be important, but it is imperative to consider all potential factors in the same analysis. In order to deal with confounding variables, epidemiologists build multivariable or multiple regression models. These models adjust the size of effect and significance of risk factors by accounting for the effect of other confounding variables that are associated with both the outcome and other explanatory variables (Figure 105-2).

Fig. 105-2 An example of the relationship among explanatory, confounding, and outcome variables that should be accounted for when determining the true size and significance of associations between important risk factors and the outcome of interest.

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