Why bucked shins are so prevalent in the racehorse
/Article by Adam Jackson MRCVS
One of the most common causes of lost days to training and racing in racehorses is dorsal metacarpal disease (DMD), often referred to as “bucked shins” or “sore shins.”
Often a frustration to trainers and owners, this problem rears its ugly head at the time of highest expectations, such as arising the last day of work before a horse’s first race, right after a horse’s first victory or after a horse was purchased at a two-year-old sale.
This disease presents with heat, pain with or without inflammation (swelling) on the dorsal (front) surface or the dorsomedial (front inside) surface of the third metacarpal bone (cannon), which is referred to as acute periostitis. With rest and reduced exercise, the condition can improve, but catastrophic fractures of the cannon may occur at the site of previous DMD episodes. A good understanding of this disease and strategies of prevention are vital in order to improve the welfare of the horse and reduce the potential expenses to all shareholders.
Introduction
The cannon bone is an important structure in the weight bearing and absorbing shock. As the horse moves, the bone bends a little and then returns to its original shape like an elastic band, which is often referred to as elastic deformation. In addition, it has been observed that horses that work slowly have tension on the front of the cannon bone; in other words, the bone is stressed by a stretching force rather than a compressing force. However, at higher speeds, these forces change from stretching to compressing forces.
Repeated bending forces (stress cycle) on the cannon bone causes dorsal metatarsal disease. When the horse is young, it has a thin bone cortex. As the horse grows and is repeatedly subjected to these forces, the bones remodel and the cortex thickens, making it stronger. However, if the bending forces exceed the bone’s ability to remodel, then this leads to stress fatigue and bone damage.
The occurrence of bucked shins is most common when horses are developing, typically at two to three years old as training becomes more intensive. But it must be noted that if the horse is not bone fit, any aged racehorse is susceptible to these diseases when they begin training. Roughly at the age of five years, when a horse is fit, they are at a low risk of this disease. Within the first six months of training, DMD may present in one or both front limbs. If the condition does occur in both front limbs and the horse is being trained on a circular track, then it is likely the inside leg is where it will occur first. In other words, if the training tends to be in a counterclockwise training circuit, then there are greater forces on the left limb than the right, thus the left is more likely to develop the disease before the right limb.
Risk Factors of DMD
Age: DMD occurs most commonly in two to three-year-olds often within their first six months of training. It is rarely seen in horses with a mature skeleton (age four and over). However, this disease has been seen in five-year-olds especially if they have been stalled for a long amount of time after weaning and not racing until that age.
Gender: It is believed that the gender of the horse does not alter its risk to DMD.
Breed: Most common in Thoroughbreds but may be seen in both Standardbreds and Quarter horses.
Genetics: The risk of DMD is influenced by genetics as variation in limb bone geometry (inherited) behaves differently to force/strains on the bone. In addition, the longer the cannon bone, the greater the load is at flexion of the dorsal cortex of the bone, making it more susceptible to DMD.
Training and racing surfaces: The different types of training and racing surface alter the risk to DMD because there are variations in the force applied to limbs as well as the acceleration rates of hoof impact. Furthermore, the impact of these forces is increased with greater speed. Dirt tracks tend to be the hardest surface, whereas synthetic tracks reduce hoof and limb impact and loading force. However, it is important to remember that the hardness all of these surfaces can be altered by a number of other factors such as:
Training and racing surfaces
Different surface materials
Changes in weather, temperature and humidity
Surface maintenance (i.e., soaking, harrowing)
Changes in horse body weight
Age of surface, wear and tear of surface
Human opinion of condition of track
6. Training: The length of time for bones to respond to different training practices is unknown. Although further research is required, it is suggested that fast work should be avoided in the early stages of training as it is thought that high-speed exercise introduced too quickly (within one month) was detrimental to bone health.
7. Direction of training: Track direction varies globally. Thoroughbreds tend to lead with the inside forelimb around turns then switch to the outer forelimb on the straight. It has been suggested that due to greater forces on the leading limb on the turn, that limb is more at risk of bucked shins. However, more research is required to make accurate conclusions.
8. Speed: Current research is contradictory. Some research indicates a reduction in the risk of DMD if the horse is trained at high speeds with every extra mile worked, and canter work increases the risk. However, other research suggests that short periods of work (< 1 month) at high speed increases the risk of DMD.
9. Camber: In the U.S., tracks are usually flat in contrast with European tracks, which tend to vary in their design and often include slopes, twists, turns, uphill sections, and cambers, with turf being the prevalent surface. In addition, races may be run straight, clockwise or counterclockwise. Although it is known that this variation in track characteristics alters the horse’s gait, thus altering forces on the forelimbs, further research is needed to understand if these variations increase the risk of DMD.
How does DMD develop?
Bucked shin is the formation of tiny stress fractures on the front or inside of the cannon bone of the horse’s front legs. DMD occurs when the stress on the legs with high-speed training exceeds the bone’s ability to adapt to those stresses.
Bone is a dynamic tissue that is constantly adapting its structure. Once the bone is formed in immature animals, the bone grows and changes shape by a process called modeling. Bone remodeling is different to modeling in that its function is to renew the skeleton and involves both bone resorption and formation to occur at the same location in a sequential manner.
With high-speed training, there is high-strain fatigue, which causes excessive compression of the bone. During this compression, there is an insufficient amount of bone remodeling at the point of stress. At this site, this new bone is much weaker; thus, it is susceptible to inflammation, and pain and may lead to fractures.
Treatment of dorsal metacarpal disease
Treatment of DMD is designed to alleviate pain and inflammation while allowing the remodeling process of the bone to catch up with the damage that has been caused from stress cycling.
The core of the treatment is rest and providing pain relief, followed by a slow and gradual increase in exercise levels.
Fractures of the bone cortex can be treated with surgery using lag screw fixation and osteostixis. Osteostixis is the drilling of many holes around the site of fracture in order to promote bone healing. Lag screw fixation is the drilling of a screw across the fracture line to compress and stabilize the bone. However, fracture recurrence is common with both techniques and requires five to six months out of training.
There are additional treatments that may be used to complement core treatments. Extracorporeal shock wave therapy (ESWT) is commonly used for treatment and involves a highly concentrated, powerful acoustic (sound) energy source being applied to the site of injury. The rationale is that ESWT increases blood flow, increases growth of new blood vessels and increases the production of natural healing factors in the treated area. The research findings are limited on its effectiveness but anecdotally among the veterinary profession, it seems to work on bucked shins and stress fractures.
In North America, horses are not permitted to race or breeze for 30 days following treatment as per the Horseracing Integrity and Safety Authority’s (HISA) rulings. In Europe, horses must not have had shock wave therapy on the day of racing, or on any of the five days before the race day in which the horse is declared to run.
With all treatment options, there must be a careful and considered discussion with the veterinarian and all stakeholders on the desired outcome while bearing in mind the important factor of the horse’s welfare and wellbeing.
What about bisphosphonates?
Some clinicians are using a combination of shockwave and bisphosphonates (TildrenTM, OsPhosTM) to treat DMD. Bisphosphonates were first seen in human medicine and used for osteoporosis. Bones are constantly remodeling in a process that removes old bone cells and deposits new ones. Bisphosphonates help prevent bones from losing calcium and other minerals by slowing or stopping that natural process that dissolves bone tissue, thus, helping bones remain strong and intact. Veterinary surgeons report mixed results with these therapies, and long-term use of bisphosphonates is expensive and has serious consequences. Bisphosphonates are toxic to the gastrointestinal and renal systems, thus, potentially causing colic and kidney disease. Their safety has not been evaluated for the use in horses younger than four years old nor in pregnant and lactating mares.
RULES ARE CHANGING - Bisphosphonates
HISA’s Anti-Doping and Medication Control (ADMC) Program came into effect on March 27 and with it, new regulations regarding the presence and use of bisphosphonates.
The Horseracing Integrity & Welfare Unit (HIWU) states “The ADMC Program regulations categorize bisphosphonates as a banned substance, meaning that they are prohibited from being administered to, or present in, covered horses at any time. Covered horses that test positive for bisphosphonates under the ADMC Program are subject to lifetime ineligibility, and associated covered persons may incur an Anti-Doping Rule Violation.”
“HIWU will not pursue disciplinary action against Covered Horses or their associated covered person(s) for the presence of bisphosphonates if the covered person(s) can provide documentation (e.g., medical records or a positive test result) to HIWU of the administration or presence of bisphosphonates prior to the implementation date of the ADMC Program.”
In Europe, bisphosphonates are not to be administered to a racehorse under the age of three years and six months as determined by its recorded date of birth, on the day of the race or on any of the 30 days before the day of the race in which the horse is declared to run as per The International Federation of Horseracing Authorities rulings.
Training Regimens
With DMD, it must be remembered that it is an appropriate response for new bone formation when the cannon endures cyclic stress and injury. This injury cannot be ignored but addressed to reduce the risk of serious consequences. Exercise is the root of the problem; therefore, the solution is to alter the patterns of exercise.
Dr. David Nunamaker, DVM, of the University of Pennsylvania, has developed a training program, which is believed to reduce the risk of DMD. The rationale when developing this modified training program is that horses are not born with the right bone structure for racing. The bones are to develop and adapt to racing. By providing training programs that mimic racing, the bones can adapt to the forces that are applied during racing, thus, reducing the risk of developing bucked shins.
When initiating this training regimen, it is assumed that young horses are broken to ride in autumn and able to gallop a mile by January so that training can start.
Stage 1 (5-week duration) – Horses finish the gallops two times a week with the last 1/8th of the mile (last 200 meters of 1600 meters) completed in an open gallop in 15 seconds.
Stage 2 (5-week duration) – Twice a week, open gallops for ¼ of a mile (400 meters of 1600 meters) in 30 seconds including a one-mile (1600 meters) gallop.
Stage 3 (7-week duration) – The addition of speed work once per week. Breezing (moderate speed) for ¼ mile (400 meters) and daily gallops lengthened to 1¼ miles twice per week for four weeks. In the following three weeks, the ¼ mile breeze is continued with a strong gallop out for another furlong (roughly 40 seconds total for a breeze).
Conclusion
The findings of exercise research are often varied and contradictory due to many research variables, making comparisons and conclusions difficult. In addition, most of the research of musculoskeletal issues in racehorses uses racing data, but most injuries occur during training.
Because more research is needed, there remain conflicting views of the effects of racing on horses before skeletal maturity and the most effective and safe way to introduce speed exercise. At present, the data suggests that distance and speed should be implemented gradually and should include high-speed work at full racing speed.
The racing industry must continue to work cooperatively to address the welfare concerns associated with horses experiencing DMD.
