Two large consecutive epidemiological studies were performed at the University of Liverpool between 1998–2003 investigating risk factors for fatal distal limb fractures during racing. The methodologies of these studies involved identifying all fatal distal limb fractures that occurred distal to the tarsus and carpus on UK racecourses, collecting and performing a post mortem on the limb (and the contralateral [opposite] limb) and then using classical epidemiological (case–control methods) identifying the differences between either the horses that sustained fractures and other horses not affected by fractures (horse-level risk factors), as well as defining differences in races and racecourses at which fractures occurred in comparison to those which did not have fractures (race- and course-level risk factors).
This study identified that the risk of fracture varied depending on the type of racing. Flat turf racing was the safest (0.4 fatal fractures/1000 starts) whilst National Hunt flat racing was associated with the highest risk (2.2 fatal fractures/1000 starts). The type of fracture sustained did vary with the type of racing. Lateral condylar fractures of the third metacarpus/metatarsus (Mc/MtIII) (cannon bone) (Figs 1, 2) were the most common fracture overall and were the most common type in all forms of National Hunt racing. In flat turf racing proximal phalanx fractures were the most common diagnosis in fatal fractures (Fig 3) (Parkin et al. 2004a). However, in all-weather flat racing, biaxial sesamoid fractures predominated. In the case of proximal sesamoid bone fractures (Figs 4, 5), the overall incidence of such fractures in all racing was 0.63/10,000 starts, which increased to 1.63/10,000 starts in all-weather flat racing. This equated to a risk ratio of 4.4 for these fractures occurring in all-weather flat racing compared with flat turf racing (Kristoffersen et al. 2010). Overall, it was concluded that the incidence of fatal fractures in racing in the UK had not changed since a previous survey a decade earlier (McKee 1995). Furthermore, the risk of a particular fracture occurring varied with the type of racing indicating that the reasons why particular fractures occur are different (Parkin et al. 2004a).
When the reasons why horses are at altered risk of fracture during racing were assessed, it was discovered that horses that had either done no galloping work during training and those in their first year of racing were at an increased risk of sustaining a fatal distal limb fracture when they raced. Furthermore, horses trained on a sand gallop were at an increased risk of fracture. When the analysis was extended to just include reasons why horses sustain the most common fracture, lateral condylar fracture of Mc/MtIII, it was discovered that affected horses again were horses that had either done no galloping work during training and those in their first year of racing were at an increased risk of this particular fracture. Also, horses that sustained a lateral condylar fracture of Mc/MtIII were more likely to have commenced their racing career as 3- or 4-year-olds rather than as 2-year-olds (Parkin et al. 2004b,d).
When the reasons why particular races may be at risk were examined, it was determined that longer races that contained more runners were at increased risk. Both firmer going and fewer days since the last race on the same course, were also associated with an increased risk of a horse sustaining a fatal distal limb fracture. When the analysis was extended to just include reasons why races may cause an altered risk of sustaining the most common fracture, lateral condylar fracture of Mc/MtIII, it was found that such fractures were more common in races with a larger number of runners, races in which professional jockeys were not permitted to ride and races in which the going was described as good or firm (Parkin et al. 2004c,d).
Using videos to analyse risks and occurrences of such fatal distal limb fractures, it was found that approximately 75% of all fractures occurred spontaneously and were not associated with a fall or interference from another horse. In National Hunt racing, most fractures (74%) occurred in the second half of the race, whilst in flat racing fractures occurred equally at any time through the race. The majority of horses (66%) that sustained a forelimb fracture did so when the affected leg was the lead leg at the time of fracture (Parkin et al. 2006a).
The pathological configurations of lateral condylar fracture of Mc/MtIII were examined and level of pre-existing pathology determined in the limbs. This study concluded that there was a much higher degree of bone comminution (both affecting the joint and further up the bone) in comparison with previous reports of such fractures, as well as a greater presence of other concurrent fractures. Many limbs had evidence of pre-existing pathology (Fig 6), particularly in the medial and lateral parasagittal grooves of the distal articular surface of Mc/MtIII, although the presence of such pathology was not associated with horse age, length of career and number of race starts during their career (Parkin et al. 2006b; Barr et al. 2009).
Fractures during training
There have been a number of studies performed to describe the risk factors for severe musculosketelal injury in horses during training and, whilst such studies inevitably involve other injuries, by virtue of their frequency fractures are often the most common injury described.
An epidemiological study recruited a cohort of 13 flat racehorse trainers and followed 1178 horses over a 2 year period to give a total of 12,893 months at risk during training. This produced a figure of nontraumatic fracture incidence of 1.15/100 horse months, with 78% of the fractures occurring during training. This would translate to a trainer with 100 horses in their year having just over one horse sustain a fracture each month in their yard. They found a wide variety of bones were involved in fracture, with tibial and pelvic stress fractures making up 28% of the total number of fractures (Verheyen and Wood 2004).
A subsequent publication from this study analysed in detail the reasons why horses sustain tibial and pelvic stress fractures, the 2 commonest causes of fracture in this population. Results from this study identified some influences in the training of the horse on the risk of sustaining such fractures. Looking at the 30 day period prior to the horse sustaining a fracture, the risk of sustaining a tibial and pelvic stress fracture increased with increasing distance cantered by the horse, reaching a peak at 50 km, before declining. However, if the period of 60 days prior to fracture was examined, no effect on distance cantered and the risk of fracture was identified. There was some association with horses sustaining such fractures and the use of a particular sand-based all-weather training surface, although this finding was recommended to be treated with caution as there may be undetermined influences from the construction and maintenance of the surface. There was no effect of gender or age of horse on the risk of fracture, but there was an identifiable trainer effect. Whilst the authors concluded that there was evidence that such injuries could be potentially modified by scientifically based training regimes, it was premature to make any firm specific recommendations on training regimes (Verheyen et al. 2006a).
A further study using this same cohort of flat racehorses in training was performed to identify specific training regimes that could influence the risk of bone injury. Results from this study showed a strong interactive effect of exercise distances at different speeds on fracture risk. Horses that exceeded 44 km at canter (< or = 14 m/s) and 6 km at gallop (>14 m/s) in a 30 day period were at a particularly increased risk of fracture. These distances equate to approximately 7700 bone loading cycles at canter and 880 loading cycles at gallop. Fifty-six fractures occurred in the subset of study horses that were followed since entering training as yearlings, when skeletally immature (n = 335). Cohort analysis of this data set showed that, in previously untrained bones, accumulation of canter exercise increased the risk of fracture whereas accumulation of high-speed gallop exercise had a protective effect (P<0.01). However, increasing distances at canter and gallop in short time periods (up to one month) were associated with an increasing fracture risk. Results from this study provide further data on the effects of physical exercise on bone adaptation and injury risk (Verheyen et al. 2006b).
The same investigators using the same cohort of trainers and horses described epidemiological factors associated with dorsal metacarpal disease ([DMD] sore shins). They determined that in their cohort of horses in training DMD had an incidence of 1.87/100 horse months. Thus a trainer with 100 horses in training would expect to have just less than 2 cases each month. The DMD was associated with horses that had increasing exercise distances at canter and increased high speed exercise over a short period of time (<1 month). However, the incidence was decreased in horses that had greater cumulative exercise distances since entering training. Overall, it was concluded that the risk of DMD in young Thoroughbreds decreases with accumulation of distances exercised at canter and high speed, probably reflecting adaptive response of the third metacarpal bone to the loads placed upon it. However, increasing exercise distances in short periods (up to 1 month) increases the risk of DMD, probably as a consequence of microdamage and associated remodelling response and should therefore be avoided. For the first time, investigators gave specific recommendations on training and recommended that early but gradual introduction of small amounts of high-speed exercise may be beneficial. Canter exercise should be kept to a minimum on high-speed work days and large amounts of both cantering and high-speed work per week, 2 weeks, or per month should be avoided, particularly during the early stages of training, in order to minimise the risk of DMD (Jackson et al. 2005).
A further study was performed to estimate the incidence of both fractures and tendon and suspensory ligament injuries in National Hunt racehorses in training and determine particular risk factors for these injuries. This study used the same methodology as the earlier training studies with data collected on horses from 14 UK National Hunt training yards for 2 racing seasons. Data was available from 1223 horses that spent 9466 months at risk of injury. The fracture incidence rate was 1.1/100 horse months (very similar to that seen in flat horses) and varied significantly by trainer (P<0.001) but not by gender, age or background (whether a National Hunt store or previous flat-trained horse). The fracture incidence rate in racing was 9.2 per 1000 starts. The pelvis and third metacarpal bone (McIII) were the most commonly fractured bone, with McIII fractures occurring more commonly during racing whilst pelvic fractures occurring predominantly during training (Ely et al. 2009).
The Verheyen group determined the effects of dam age and parity on the rate of fracture in offspring of Thoroughbreds in training for flat racing. Using data from 8 trainers on 335 horses, monitored since the start of their training as yearlings, it was shown that first foals had a significantly (approximately one-third the risk) lower fracture rate than subsequent ones and rate of fracture decreased with increasing dam age (Risk ratio = 0.91 per year increase in dam age). This study identified that the rate of equine injury may be influenced by factors that affect skeletal development and, whilst no mechanism or reason for the phenomenon was proposed, it was stated that further research on intra-uterine and perinatal determinants of injury risk in later life in horses was needed (Verheyen et al. 2007).