There seems to be little doubt that musculoskeletal injury–including injury to bones, joints, tendons, and ligaments–is a major problem for Thoroughbred racehorses. This impression has been borne out by studies of “wastage” in the racing industry, where wastage describes the losses that occur during the training and racing of a horse. In some studies, more than 50% of these racehorses experienced a period of lameness and, in about 20% of affected horses, lameness was severe enough to prevent racing during the period of investigation.

Although the economic impact of this wastage due to injury has not been quantified, the losses are probably substantial. Some have estimated that in the United States alone, the annual economic loss from these injuries approaches $1 billion. These losses can be measured in terms of days out of training, decreased athletic performance, veterinary care, reduced income from race winnings, loss of training fees, the cost of replacement horses, etc.

Perhaps even more important than the economic losses are the animal welfare implications. Those involved in the equine racing industry realize they must make it a priority to better understand the factors that contribute to the development of musculoskeletal injuries during racing and training, and to develop means for preventing these injuries.

This article gives an overview of musculoskeletal injury rates in Thoroughbreds and discusses current research that is focused on developing strategies for preventing these injuries. Note that this discussion focuses on Thoroughbred racehorses in large part because most research has focused on this breed rather than other racehorses (Standardbreds, Quarter Horses, or Arabians).

Injury Rates

Epidemiological studies (dealing with the incidence, distribution, and control of disease or injury in a population) over the past decade have shed much light on the type and frequency of musculoskeletal injuries in Thoroughbred racehorses. There are several ways to classify these musculoskeletal injuries. One way is to record the number of injuries that occur during races, and to classify those injuries as catastrophic or non-catastrophic. Catastrophic means the injury is fatal–the horse dies as a result of the injury or is euthanized because of the severity of the injury. The most common catastrophic injuries include fractures of the humerus (the bone located between the shoulder and elbow joints), cannon or third metacarpal bone, and sesamoid bones, with 90% of these injuries occurring in the forelimbs.

All other non-fatal injuries are in the non-catastrophic category. However, these non-catastrophic injuries are no less important, because there is increasing evidence that seemingly minor injuries can eventually lead to more serious problems. For example, a recent study at the University of California, Davis, looked at the effects of mild suspensory apparatus injury in 219 Thoroughbred racehorses.¹ (The suspensory apparatus includes the suspensory ligament and the distal sesamoidean ligaments; it is crucial for support of the fetlock joint.) Horses which had a mild suspensory injury were at increased risk for a more severe musculoskeletal injury, including complete failure of the suspensory apparatus–a condition that is often fatal. The moral of the story is that minor injuries can quickly turn into big, potentially serious problems.

One study in Kentucky over an 18-month period involving 35,484 racing starts among 7,649 horses identified 132 musculoskeletal racing injuries among 117 horses.² Of these injuries, 51 were classified as catastrophic and 66 as non-catastrophic. This means that about one in 700 racing starts is associated with a catastrophic musculoskeletal injury (most commonly, fractures of the lower limbs). Recent studies in Britain have revealed a similar overall death rate in flat racing Thoroughbreds (0.9 per 1,000 starts). Although some might argue that this number of catastrophic injuries is small in relation to the number of racing starts, the opposing view is that any number other than zero is unacceptable.

It is also important to realize that many (if not the majority of) racehorse injuries actually occur during training. Therefore, the statistics on race day injuries can be viewed as the tip of the iceberg in relation to the overall injury rate in racehorses in training. The injuries occurring during training might occasionally be fatal, but more commonly result in wastage–days lost in training, missed racing opportunities, premature retirement from racing, etc.–and can lead to more serious injuries later on.

However, an Australian study of 169 young Thoroughbreds (distributed among 24 trainers) during their 2- and 3-year-old racing seasons found that catastrophic injury was uncommon during training.³ However, during the 2-year-old season, there was a high incidence of low-grade injuries that had a marked impact on training and racing activities. The most common injury was bucked shins (also termed shin soreness), which affected 42% of the horses, followed by fetlock problems (16%) and a mixed bag of other leg injuries. Many of these horses developed shin soreness a second or third time in the same year (2-year-old) or subsequent (3-year-old) racing campaign.

In 3-year-olds, knee (carpus) problems–such as carpitis (knee inflammation) and chip fractures–were the most common causes of lameness, although fetlock injuries were also important in this age group. These lameness problems were the most common reason for alterations in training; in particular, bucked shins accounted for about 35% of reduced training days (i.e., training was modified in some fashion) and 4% of lost training days. Overall, days lost to training because of injury or other illnesses (e.g., respiratory disease) were higher in 2-year-olds (3.1%) than in 3-year-olds (2.2%), probably reflecting the greater impact of shin soreness in the younger horses.

Tendon and suspensory ligament injuries were uncommon in this small group of young Thoroughbreds. However, injuries of these structures are much more common in older racehorses. In a survey of 222,000 racing starts in Britain, involving both flat and National Hunt (steeplechase) races, there were a total of 2,358 clinical problems (about 10 per 1,000 starts). Of this total, 1,790 were injuries of the musculoskeletal system–81% affected the forelimbs and 46% involved the flexor tendons and/or suspensory ligament.⁴

Without getting too bogged down in the numbers, we can conclude that:

• Catastrophic musculoskeletal injuries (mainly long bone fractures and “breakdown” of the fetlock and pastern area) occur at the rate of about one per 1,000 race starts in flat racing Thoroughbreds;
  
• Injury occurs during both training and racing;
  
• Forelimb fractures, including chip fractures that involve the knee and fetlock joints, are common in all age groups;
  
• The frequency of tendon and ligament injuries increases dramatically with age; and
  
• Bucked shins are very common problems in young racehorses and contribute to the apparently higher rate of injury in this age group compared to older horses.

The 2-Year-Old Issue

This last point raises the question of 2-year-old racing. A view long held by many in the horse racing community is that 2-year-old training and racing contribute to the unacceptably high rate of musculoskeletal injuries in racehorses. It is also argued that 2-year-old racing shortens racing careers. Buried in this argument is the assumption that a delay in the commencement of training and racing until a horse’s 3-year-old year will minimize injury problems, largely because the skeleton and supporting structures are more mature at the start of training and therefore better able to withstand the rigors of repetitive exercise.

This debate has recently resurfaced because of the popularity of “2-year-olds in training sales” and reports of breakdown injuries in horses being prepared for the solitary sprint (each sales horse performs alone against the clock) that is part of the sales process. Because these sales are held in early spring, horses enter training at around 16 to 18 months of age. The level of fitness required for what is in effect “race readiness” necessitates an intense training program with these young horses.

As yet, there is no easy answer to this perplexing issue. True, there is some evidence that injury rates are higher in 2-year-olds than other age groups. However, bucked shins are a big part of this difference, and it is important to realize that this injury will also occur in horses which commence serious training later in life. As well, some horses can suffer recurrences of shin soreness following a period of stall or paddock rest. Therefore, development of bucked shins is not so much due to the age of the horse, but rather is the consequence of horses entering intensive training, particularly on the first occasion.

Recent studies have also demonstrated that Thoroughbreds which begin racing as 2-year-olds have a longer career compared to those which begin training and racing at an older age. There is growing evidence that in order to maximize the increases in bone and tendon strength that result from training, horses should begin training at an early age. Although this debate is sure to continue, so far there is no convincing evidence that age per se is the biggest factor in musculoskeletal injuries.

Bone Fatigue

As discussed in a recent article⁵, bone is a dynamic tissue that is constantly remodeling. In the case of physical conditioning (training), this remodeling process serves to increase bone strength so that the bone is better able to withstand the stresses placed upon it during exercise. However, this beneficial effect might not always occur and these areas of extensive remodeling could actually be prone to fatigue failure or “stress fracture” under the continued rigors of high-speed exercise.

For example, it is now thought that chip fractures involving the knee bones in racehorses are not an acute traumatic event, but rather the end result of a negative remodeling balance that leads to a loss of bone mass and strength (termed osteoporosis). The weakened area of bone is then predisposed to fracture. Unfortunately, just why this osteoporosis occurs is not yet understood.

Bucked shins can also be regarded as the bone modeling process gone wrong. With the onset of galloping exercise, additional bone is deposited on the front portion of the cannon bone, which should ultimately result in improved bone strength. However, early on this new bone appears to be prone to microfractures similar to the stress fractures that can occur in human athletes during training.

Stress fracture of the bottom portion of the cannon (distal condyle) is another common injury in racing Thoroughbreds that might be the result of unbalanced bone modeling. Recent research has found that training results in an uneven increase in the density of the condyles. The resultant variation in bone density might then predispose the bone to stress fractures in the weaker areas under the strain of normal training and racing.

Progress Toward Prevention

Veterinarians have become highly skilled in the diagnosis and treatment of many musculoskeletal injuries that afflict racehorses. As an example, the advent of arthroscopic surgery was a huge step forward in the treatment of joint problems and, compared to older therapies, has greatly improved the prognosis for a return to successful racing. Still, the ultimate goal must be prevention of this and other types of injuries, or at least a marked reduction in overall injury rate and a decrease in the number of catastrophic or career-limiting injuries.

There is much to be done before we can achieve this ideal. However, researchers around the world are working hard to develop better training methods and means for preventing musculoskeletal injury. This research effort has many facets. Some groups are working on the early diagnosis of mild injuries in racehorses–that is, diagnosis before these injuries result in lameness or other obvious signs such as swelling. For example, Tracy Turner, DVM, MS, Dipl. ACVS, of the University of Minnesota, has been using thermography (a pictorial representation of skin temperature) as a means to identify areas of heat associated with underlying bone or tendon problems (e.g., stress fractures or tendon strains).⁶ Few will dispute that the early diagnosis and treatment of mild problems are important in preventing more severe, even catastrophic injuries. Perhaps in the near future, it will be routine to use these highly sensitive scanning tools on a weekly basis during training.

Other research teams are investigating the effects of training methods and age at commencement of training on the development of bone, joints, and tendons. Others continue to focus on factors that could contribute to the development of musculoskeletal injuries. These factors include conformation, shoeing techniques, track design and surface, and the volume and intensity of training. These epidemiological studies, which will identify common risk factors, are a vital first step in the development of effective preventive strategies.

Recent findings from some of these studies are challenging conventional Thoroughbred training methods. For example, recent English studies reported at a conference on racehorse injuries in November of 2001 have shown that galloping at least one furlong for every seven furlongs at lower speed is associated with a decreased risk of fracture compared with no galloping work.⁷ Surprising as it might seem, this finding is actually in line with the concept that a certain amount of very hard exercise is necessary for maximum increases in bone strength and resistance to fracture.

Consistent with this idea, studies of young racehorses in the United States have shown that the allocation of more training effort to regular short-distance breezing (one to two furlongs of full gallop) and less to longer-distance cantering helps to minimize the incidence and severity of bucked shins.⁷ The implication is that beyond a period of base training, the emphasis in conditioning programs should be on quality rather than quantity of work efforts.

Clinical experience and epidemiological studies have also identified that certain stress fractures are more likely to occur when horses race during the two- to three-week period following a time of inactivity (such as a lay-up because of injury). One theory is that the period of rest results in a decrease in bone strength that makes the bone more susceptible to injury when fast exercise is resumed. Perhaps horses returning from lay-up periods need a longer period of reconditioning before they are raced again, with a particular emphasis on short bursts of hard galloping to rebuild bone strength.

As with all research, patience is a must. At this point, there are more questions than there are answers. Still, the evidence so far points to changes in training methods (speed, distance, and gait) as important in the prevention of musculoskeletal injuries. As always, the “proof is in the pudding.” The challenge now is to compare specific training protocols and figure out which method(s) work best in terms of injury prevention.

REFERENCES

¹ Hill, A.E.; Stover, S.M.; Gardner, I.A.; et al. Risk factors for and outcomes of non-catastrophic suspensory apparatus injury in Thoroughbred racehorses. Journal of the American Veterinary Medical Association, 218, 1136-1144, 2001.

² Peloso, J.G.; Munday, G.D.; Cohen, N.D. Prevalence of, and factors associated with, musculoskeletal injuries of Thoroughbreds. Journal of the American Veterinary Medical Association, 204, 620-626, 1994.

³ Bailey, C.J.; Reid, S.W.; Hodgson, D.R.; Rose, R.J. Impact of injuries and disease on a cohort of 2- and 3-year-old Thoroughbreds in training. Veterinary Record, 145, 487-493, 1999.

⁴ The Epidemiology of Death and Injury in Racehorses. Suffolk, England. Animal Health Trust. www.aht.org.uk.

⁵ Boston, R.C.; Nunamaker, D.M. Gait and speed as exercise components of risk factors associated with onset of fatigue injury of the third metacarpal bone in 2-year-old Thoroughbred racehorses. American Journal of Veterinary Research, 61, 602-608, 2000.

⁶ The Horse staff. Lameness in Horses. The Horse, AAEP Wrap-Up, February 2002, 19-22. Article Quick Find #3324 at www.TheHorse.com.

⁷ Conference on Racehorse Injuries. Cheltenham, England. November 15, 2001.

⁸ Riggs, C.M. Fractures–A preventable hazard of racing Thoroughbreds. The Veterinary Journal, 163, 19-29, 2002.

FURTHER READING

Geor, R.J. Are Your Horse’s Bones Tough Enough?, The Horse, July 2002, 79-86. Article Quick Find #3673 at www.TheHorse.com.

5 TIPS: Racehorse Injuries

1. Studies have shown that 50% of racehorses suffer from musculoskeletal injuries that caused lameness.
   
2. The most common catastrophic injuries (those which result in death) in racehorses include fractures of the humerus (the bone located between the shoulder and elbow joints), cannon (third metacarpal bone), and sesamoid bones, with 90% of these injuries occurring in the forelimbs.
   
3. About one in 700 racing starts is associated with a catastrophic musculoskeletal injury (most commonly, fractures of the lower limbs).
   
4. Injury occurs during training and racing.
   
5. Training methods might contribute to or prevent injuries

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