Chapter 105Track Surfaces and Lameness
Epidemiological Aspects of Racehorse Injury
The racing surface is often regarded as the most important factor likely to influence the risk of injury. The effect that different course designs and surface types have on injury has been discussed for many years,1,2 and the surface and its management are obvious targets for immediate scrutiny after a racing injury. Certainly there are significant differences in the risk of injury among races held on turf, all-weather, and dirt tracks. On turf tracks there is also a significant effect of the firmness of the ground on injury risk. However, there are many other potentially less obvious factors that may also be related to the type and quality of the racing surface that influence the risk of injury during racing. In this chapter, the strength of evidence provided by a range of epidemiological studies is examined. Associations between injury and surface characteristics during both racing and training are described. The risk of injury on different surfaces and evidence from multivariable epidemiological studies that include surface characteristics are summarized. Comparisons of the same racecourse where changes have been made are discussed, and priorities for future research are identified.
Racing Surfaces
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 California3,18 and Kentucky14,19 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 tracks3,18 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.
Further examples of potential confounding variables include track configuration,25 training regimens of the local trainers who most often use the tracks,26-28 and weather conditions and experience of the jockeys.29 All of these variables and many more should be considered and accounted for when interpreting differences in injury risk on different racing surfaces. A causal web can be built to account for all potential confounders. A simplified version of the causal web designed for investigations of fatality during racing in Victoria, Australia30,31 is shown in Figure 105-3. It is particularly useful with such complex diagrams to divide variables into the different horse, race, track, and jockey factors and then to identify which of these factors are likely to influence others as well as affecting the likelihood of the outcome of interest.
Evidence from Multivariable Epidemiological Studies
There are relatively few multivariable epidemiological studies that have included surface type in models while also accounting for the effect of many other factors. The likely reason for this is that when performing case-control studies it is common to select control (unaffected) horses from the same race in which the case horse was running. This obviously means that race-matched case-control sets are also matched on surface type, making comparison of this factor impossible. The rationale behind this study design is that in order to identify horse-level factors it is easiest to exclude (by matching) the potentially strong influence that race-level factors may have on the outcome. Even among studies that have specifically investigated race-level factors, control races, in which no horses were injured, are often matched on race and surface type.17 This is done to account for the strong effect of race type and surface on injury risk so that other, more subtle race-level risk factors can be identified. Given the fact that until recently the majority of courses were not in a position to change their track type, the priority was to identify modifiable risk factors to reduce the likelihood of injury on the surfaces already in use.
Results from the few multivariable studies that did include surface type have not been consistent. In New York, racetrack location (one particular track) and dirt surfaces were significantly associated with an increased risk of injury.20,32 However, in Florida, horses racing on turf tracks were more likely to be injured than those racing on dirt.11 This association remained significant even when field size and race distance (both associated with surface type) were accounted for.
Therefore, to date complex multivariable models have not fully addressed the effect of surface type on injury risk. Ideally, large-scale collaborations among racing jurisdictions with different racing surfaces, using multivariable analysis to account for other potential risk factors, are needed if firm conclusions are to be drawn. Although expensive and time-consuming, such an initiative would also help to avoid repetition of studies and duplication of investment in different jurisdictions investigating the same problems. Large-scale collaborations can dramatically improve statistical power, ensuring that small effects of different risk factors can be identified.
Studies within the Same Racecourse
The goal of most multivariable epidemiological studies is to identify modifiable risk factors to enable the design and implementation of appropriate and effective interventions. In the field of racetrack epidemiology, monitoring the effect of a change is as close as the industry will get to a randomized controlled trial. If all other factors remain the same and sufficient time is given for interventions to have a statistically significant effect, a change in one variable (e.g., surface type) may be interpreted as causing a change in the outcome variable (e.g., risk of injury). Although there has been a change from dirt to synthetic surfaces at a number of tracks around the world, to date few have reported how these new surfaces have affected injury rates. It is likely too early to see statistically significant differences in changes of surfaces on racetracks in the United States. However, in Japan, changes were made to Hanshin Racetrack after it was found to have a higher rate of racing injuries than other racetracks.25 Changes included slowing the pace of races, widening the third and fourth corners, and including an upgrade slope in the finishing stretch. A better cushion was installed in both the turf and dirt tracks. In the following year there were significant reductions in the incidence of severe racing injuries (both fracture and nonfracture).25
In 2000 the British Horseracing Authority established an Equine Welfare database that provides all race, horse, course, trainer, jockey, and racecourse details for every race on all 60 racecourses in the United Kingdom. Comparison of the risk of fatality and long-term injury on the different courses before and after the introduction of new surfaces provides some of the most convincing evidence available to assess the effect of surface type on injury risk. These data were provided by members of the Equine Science and Welfare department at the British Horseracing Authority.33 The synthetic surfaces at three all-weather racecourses were replaced or relayed from 2001 to 2007. At two of these tracks the surface was changed to Polytrack from either Equitrack or Fibresand. At a third track an old Fibresand surface was replaced by a new Fibresand surface that was subsequently relayed 2 years later.
The change from Fibresand to Polytrack on one course resulted in a statistically significant reduction (50%) in the risk of fatality (RR 0.48 [95% CI 0.3 to 0.78]; P = 0.002). However, there was no significant reduction in the risk of long-term injury on this track (P = .026). In contrast, a change from Equitrack to Polytrack on a second course resulted in a significant drop in the risk of long-term injury (RR 0.37 [95% CI 0.2 to 0.7]; P = 0.002) but no significant change in the risk of fatality (P = 0.73).
These studies demonstrate that all aspects of course design must be considered to enable the provision of the safest racing conditions. On some courses with certain climatic conditions, turf may be the only appropriate surface. However, in other situations, synthetic surfaces (especially the more recent designs) may present a similar level of risk as turf. All racing jurisdictions should endeavor to provide the best racing conditions and seek the most robust current evidence when deciding which surface to introduce. In addition, comparisons and subsequent judgments on success or failure should be made only once sufficient time has passed for statistically significant differences in risk to be identified.
Quality of the Racing Surface
Turf Tracks
The firmness of the turf racing surface was reported as a risk factor for musculoskeletal injury.6,17,34 In all studies there was an approximately linear increase in risk of injury from heavy and soft going (footing) through to firm or hard. The firmer the surface, the greater the risk of injury. Racing on firm or hard ground increased the risk of catastrophic lateral condylar fracture of the McIII fivefold, compared with racing on anything between good-firm and heavy.29 Harder tracks, with less moisture content, may have less cushion, causing greater forces to be exerted and increasing the risk of injury.35-37 This may be the case for most fracture types, but when lateral condylar fractures of the McIII were investigated, there was a threshold of good-firm going, above which the risk was greatly increased. Observations such as these could help elucidate the true pathogenesis underlying what is believed to be a stress-related fracture.38 Further studies to identify methods to both effectively measure and modify the track firmness, specifically in jump racing, in which lateral condylar fractures of the McIII are most common, are warranted.
All-Weather Tracks
Recently there has been considerable interest in the replacement of dirt surfaces with new synthetic surfaces, particularly in the United States. Anecdotally the effect on injury rates has been variable. There remains much to learn about track maintenance with newer surfaces, and a particular type of synthetic surface may be more suitable than others in certain locations with particular climatic conditions. It may be years before statistical analysis of injury rates can be performed; therefore it is essential that accurate records of the number of starts and injuries be maintained for all tracks to enable effective future comparison.
Synthetic surfaces may lose the properties that make them safer than dirt surfaces, and inappropriate maintenance may well result in significant deterioration of the surface. Based on race data from the United Kingdom from 1987 to 1993, the overall fatality rate in all-weather flat races was determined to be 0.6 per 1000 starts compared with 0.8 per 1000 starts in turf flat races.4 At the time there were only two all-weather racecourses in use, both of which were still relatively new, having started racing toward the end of 1989. In contrast, today starts at all-weather tracks in the United Kingdom are significantly more likely to result in injury or fatality than starts at turf tracks.6,8 All-weather surfaces must be installed to customize maintenance regimens and surface characteristics with prevailing local weather conditions.
Training Surfaces
It is understandable that the focus of epidemiological studies be race day fatalities and injuries, because they attract much media attention and have a substantial impact on the public perception of racing. In addition, epidemiological race day data are much more readily available than training data, simplifying study design and logistics. Training injuries are extremely important, and given the amount of time at risk there are many more nonfatal injuries during training than racing. One study in the United Kingdom followed a population of approximately 1200 horses for 2 years and reported that 78% of nontraumatic fractures occurred during training.39 It is important to consider the effect of the training surface on lameness incurred during training and also during racing. The quality, type, and maintenance of training surfaces are at least as important as the same aspects of the racing surface. However, to date there are relatively few studies that have investigated the effect of different training surfaces on injury or lameness.
In some racing jurisdictions where the majority of training occurs at the same location as racing and sometimes on the main racetrack itself, it is difficult to compare the effects of different surfaces. However, in other jurisdictions, such as the United Kingdom, the vast majority of training occurs on private or public gallops located away from racecourses, and in this scenario there are many other factors that may contribute to differences in injury rates on different surfaces (e.g., training regimen, topography, and population demographics). Therefore identifying definitive associations with surface type has proved difficult. Nevertheless, there are differences. There is a higher incidence of dorsal metacarpal disease on dirt than on wood fiber training surfaces in the United States.40 Horses in the United Kingdom that spent a majority of time training on Equitrack were less likely to develop a fracture than those training on other surfaces.41 During a study investigating racecourse catastrophic fracture a marginal association with the use of sand gallops during training was identified.26
Recent work from Ontario, Canada indicated that catastrophic musculoskeletal injuries were more likely to occur on turf (0.64 per 1000 workouts) than on dirt (0.39 per 1000 workouts).9 Although not statistically significant, these differences are in contrast to the findings of previous studies and defy the common perception that turf is the safer surface. There is limited detailed discussion in the report, and results may be an example of an unavoidable limitation of observational epidemiological studies. Such studies identify associations, but often it is impossible to determine cause or effect. Horses may have been trained on turf, a more forgiving surface, because they were not as sound as those training on dirt. Turf-trained horses may have been inherently more likely to sustain a catastrophic musculoskeletal injury, regardless of the surface on which they were being trained. A further example of this potential effect was seen in a study in the United Kingdom,26,29 in which horses doing no fast work during training were found to be significantly more likely to sustain a catastrophic distal limb fracture during racing. Bones of these horses may not have been adapted and may have been at risk to fail when racing, or the horses may have had subclinical injury or preexisting pathology that prevented them from training at full speed but increased the likelihood that they would sustain a fracture during racing.
Horses training in the United Kingdom on an unnamed all-weather surface were eight times more likely to have sustained a pelvic or tibial stress fracture.28 Horses were included if they had been trained on this surface 70% of the time during the exposure period (30 or 60 days before fracture date). Horses that had trained less than 70% of the time on one surface were defined as the reference group (mixed surfaces), with which other surfaces were compared. The 70% cutoff was arbitrary. Incline and maintenance of the training surface were not taken into consideration, and there was a significant degree of unexplained trainer-level variability.28 Further investigations measuring other trainer- or training yard–level variables are required before definitive statements about the effect of training surfaces on injury and lameness can be made.
Future Directions
Physical properties of racing surfaces and injury rates were studied. As early as the 1970s a dynamic testing machine was developed to simulate the peak impact force of a horse’s hoof on a racing surface.35 There was a decreased risk of lameness on surfaces with a loose cushion and soft base in a survey of racecourses in California conducted at the same time as the testing machine was being used.35 Significant differences in surface moisture content at six different locations around Canterbury Downs racecourse, Shakopee, Minnesota, were identified, even though the subbase moisture did not change significantly.13 Several areas of abnormal compaction were identified at starting chutes and areas of the course exposed to high traffic density.13,42 During the study six horses broke down in the compacted areas. More recently, moisture content of the track cushion was shown to alter the energy return and impact resistance.43 It is interesting to note that these authors also suggest that these properties may be optimal for horses traveling at relatively narrow ranges of speeds. Given the potential differences in the way individual horses interact with the racing surface43 and the fact that tracks are not uniform,13,42 there is a requirement for large-scale population level studies using accelerometers (in conjunction with Global Positioning Systems [GPSs] and heart rate monitors) to monitor hoof-surface interactions during normal training.
Accelerometers placed on different anatomical locations are now being used more frequently. In the early 1980s, accelerometers placed on the dorsal aspect of the hoof showed typical decelerations on a sand track of 85 g (g = acceleration of gravity, 9.8 meters/sec2) on impact.36 This author also cited Fujisawa (personal communication), who reported that the typical deceleration on impact on turf was 175 g compared with 84 g to 125 g on dirt. Promising recent developments in wireless data acquisition systems (WDASs) suggest that population-wide studies are not far off. A WDAS was used to identify significant differences in impact accelerations in horses using toe grabs compared with flat racing plates44 and on different surfaces.45
The potential to use the horse as the device to measure hoof-surface interactions is being pursued by a number of groups around the world. Technological advances have reduced the cost and weight of GPSs and accelerometers. Assuming jockey and horse safety are not compromised, WDASs will enable population studies to be conducted during normal training. The volume of data acquired will require specialist analytical techniques so that these data can be combined with routinely collected training, medical, and racing data to enable new risk factors for injury and lameness to be identified. These risk factors will be at the level of hoof-surface interaction and will provide novel insights into the pathogenesis of many different injuries. Only then will the true extent of the role the surface plays in the development of injury in the Thoroughbred be realized.