Читать книгу Fractures in the Horse - Группа авторов - Страница 77

Far fewer studies have been conducted to investigate the risk of, or risk factors for, fracture during training compared to racing. This is due to the ready availability of data from racing in contrast to the need to design robust studies and data collection protocols and to recruit trainers who are willing to participate.

Additionally, studies of horses in training lack an obvious denominator, as with ‘per 1000 starts’ in racing which also complicates attempts to examine risk of fracture away from the racecourse. It might be optimal to quantify risk per training events at different speeds, but that level of detail is rare and it is more common to simply report fracture rates by the number of horse months. This also has the advantage that one can directly compare different trainers with different numbers of horses in training for different periods of time. An inconsistency that does arise concerns the definition of a ‘day at risk’. A horse on box rest is generally regarded as not being at risk of a training‐related fracture. But, is a horse at risk when it is walking and trotting, cantering or only when doing galloping speed exercise? Similarly, at how much greater risk is a horse that is galloping compared to when cantering? Such questions complicate markedly the investigation of training fracture risk.

The majority of training‐related work has been conducted in the UK [27–29]. The first study included details of 1178 horses providing almost 13 000 horse months in flat race training. Using total months in training as the denominator, the incidence of non‐traumatic fractures was estimated at 0.94 per 100 horse months [27]. It is important to note that this estimate excludes 22% of fractures in the same population that occurred during racing. The most common sites of fracture were Mc3 (20%), ilium (16%) and tibia (14%). The respective estimates of incidence rate were 0.22 per 100 horse months (Mc3) and 0.16 per 100 horse months (pelvis and tibia).

A similar study of horses in jump race training collected information on almost 9500 horse months (1119 horses) [29]. The incidence rate estimate for fracture in these horses was 0.6 per 100 horse months, i.e. lower than that of their flat race counterparts. However, when comparing the incidence rates for the two groups during racing (18.7 fractures per 100 horse months in flat racing and 27.6 per 100 horse months in jump racing), the reverse is true. Although exposed to a significantly greater risk of fracture during racing, horses in jump race training are at reduced risk compared with flat racehorses. This may reflect the relative infrequency of jumping during training, the reduced speed (compared to flat racehorses) or a combination of these factors.

A large study from New Zealand followed 1571 horses during 3333 training preparations over 392 290 training days [30]. A total of 55 fractures were recorded, and the authors estimated that the incidence rate for first occurrence fracture‐related lameness was 0.14 (95% confidence interval 0.1–0.18) per 1000 training days, and for second occurrence it was 0.16 (95% confidence interval 0.08–0.3) per 1000 training days. The first incidence rate equates to 0.43 fractures per 100 horse months, suggesting that there are significant differences in the rate of fracture in Thoroughbreds being trained in different parts of the world. It is important to remember that such differences could be due to different gene pools as well as differences in how horses are trained and raced.

Quantifying the number of days lost from training has been the focus of a few studies, providing valuable information about the medium‐ to longer‐term impact of fractures. In one study data from seven UK training yards reported incidence rates for two‐ and three‐year‐old horses separately [31]. Stress fracture incidence rates were similar for two‐ and three‐year olds at 1.48 and 1.43 per 100 horse months, respectively, but the incidence of fatal fracture was almost twice as high in three‐year olds compared with two‐year olds (0.3 and 0.17 per 100 horse months, respectively). The most common fracture site seen in both two‐ and three‐year olds was the pelvis with cumulative incidences of 3% and 5% in two‐ and three‐year olds, respectively. Overall, of 52 601 days available for training in two‐year olds and 29 369 days available for three‐year olds, 27% of two‐year‐old days (14 091 days) and 22% of three‐year‐old days (6324 days) were lost from training. Of the total days lost from training, fractures accounted for 18% in two‐year olds and 25% in three‐year olds. With a mean of 95 days lost per two‐year‐old fracture case and 115 days lost per three‐year‐old fracture case.

Further evidence of the impact of fracture on training is provided in a retrospective study of veterinary records from three training yards in Newmarket [32]. Over the period of study, an average of 332 horses were in training, and 50 tibial stress fractures, 35 proximal phalangeal fractures and 27 carpal fractures were recorded. Average annual injury rates (musculoskeletal injuries in general) were similar between the three yards (between 23 and 26%). However, there were significant differences in the types of fracture seen in different yards with proximal phalangeal fractures being up to three times more common in one yard compared with the other two, and tibial stress fractures being more than twice as common in one of the other yards.

These studies demonstrate how important it is to accurately record detailed information about the occurrence and impact of fractures during training as well as racing in order to clearly identify the level of risk to which horses are exposed. Without this, it is impossible to assess the impact of intervention. The fact that there were significant yard‐level differences in both studies also show how important it is, where possible, to conduct studies at the individual trainer level. There are almost certainly unique trainer characteristics that increase or decrease the risk of fracture or injury more generally. If data are collected and investigated as a whole from a number of trainers and not interrogated for individuals, subtle important differences will be lost and interventions will be less effective. That said, there also has to be a consideration of statistical power and, for some less frequent outcomes, it is often an unavoidable necessity to collect data from multiple trainers.