Treating 'bucked shins' in the thoroughbred 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), which is 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) 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, 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 two–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 old 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 2–3 year olds, often within their first 6 months of training. It is rarely seen in horses with a mature skeleton (age 4 and over). However, this disease has been seen in 5 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 of all of these surfaces can be altered by a number of other factors such as:

1. Different surface materials

2. Changes in weather, temperature and humidity

3. Surface maintenance (i.e., soaking, harrowing)

4. Changes in horse body weight

5. Age of surface – wear and tear of surface

6. Human opinion of track’s condition

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 1 month) was detrimental to bone health.

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.

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.

Camber:  European tracks, with turf being the prevalent surface, tend to vary in their design, often including slopes, twists, turns, uphill sections, and cambers. In addition, races may be run straight, clockwise or counterclockwise. This is in contrast to the USA where the tracks are usually flat. 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?

Buck 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 modelling. Bone remodelling is different from modelling 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 insufficient amount of bone remodelling 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 remodelling 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 stabilise the bone. However, fracture recurrence is common with both techniques and requires 5–6 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 amongst the veterinary profession, it seems to work on bucked shins and stress fractures. 

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. 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. 

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 (Tildren TM, OsPhos, TM) to treat DMD. Bisphosphonates were first seen in human medicine and used for osteoporosis. Bones are constantly remodelling 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

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 for Europe. 

In America, 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 categorise 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.” 

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 programme, which is believed to reduce the risk of DMD. The rationale when developing this modified training programme 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 programmes 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 metres of 1600 metres) completed in an open gallop in 15 seconds.

Stage 2 (5 week duration) – Twice a week open gallops for ¼ of a mile (400 metres of 1600 metres) in 30 seconds, including a 1 mile (1600 metres) gallop.

Stage 3 (7 week duration) – The addition of speed work once per week.  Breezing (moderate speed) for ¼ mile (400 metres) and daily gallops lengthened to 1 ¼ miles twice per week for 4 weeks. The following 3 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 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.

Racehorse Bone Health: From a Nutritional Perspective

By Louise Jones

Strong, healthy bones are the foundation for racehorse soundness, but unfortunately skeletal injuries are an issue that every trainer will face. There are many factors involved in the production of strong bones; however, two key factors that we can influence are training and nutrition. 

Every trainer knows how important exercise is to ‘condition’ the bones, and we are constantly striving to improve training programmes so that sufficient strain is applied to signal an increase in bone development, whilst not straining the bones to the point of fracture; this is a difficult balancing. Perhaps more fundamental to this is the role of diet in supporting bone density, strength and repair.  Even minor nutrient deficiencies or imbalances can mean that the horse doesn’t receive the nutrients it requires for healthy bones and thus increases the risk of potential problems down the track.

Understanding how bone is formed and adapts in response to training, alongside the critical role optimal nutrition plays in these processes, can help to ensure skeletal soundness and minimise the risk of bone-related injuries.

Bone formation & remodelling

Bone formation occurs by a process of endochondral ossification; this is where soft cartilage cells are transformed into hard bone cells. Bone consists of three types of cells and an extracellular matrix. This extracellular matrix is made mainly from the protein collagen, which makes up to 30% of mature bone and is a key element in connective tissue and cartilage. The three types of cells in bone are:

• Osteoblasts: These are the cells that lay down the extracellular matrix and are responsible for the growth and mineralisation/hardening of bone.

• Osteoclasts: These cells are involved in the breakdown of bone, so that it can be replaced by new stronger bone. 

• Osteocytes: These cells work to maintain and strengthen when a bone requires modelling or remodelling.

Bone mineral content (BMC) is a measure of the amount of mineral in bone and is an accurate way of measuring the strength of a bone. Interestingly, about 70% of bone strength is due to its mineral content; calcium being the most notable and accounting for 35% of bone structure. A horse’s bones do not fully mature until they are about 5-6 years old. So, whilst a horse will have reached 94% of their mature height when they are a yearling, they will have only reached 76% of their total BMC. 

Although it may seem like mature bone is inert, it is in fact a highly dynamic tissue, and BMC is constantly adapting in response to exercise and rest by a process called remodelling.zBone remodelling is a complex process involving several hormones and nutrients. Essentially when mature bone ages or is placed under stress, such as exercise, small amounts of damage occur. This results in the osteoclast cells removing the old or damaged bone tissue. In turn, this triggers osteoblasts and osteocytes to repair the bone by laying down collagen and minerals over the area, thus strengthening the bones. It’s estimated that 5% of the horse’s total bone mass is replaced (remodelled) each year. It should be noted that during the remodelling process, bone is in a weakened state. Therefore, if during this period, the load applied to the bones exceeds the rate at which they can adapt, injuries such as sore shins can occur.

Bone strength & exercise

When galloping, a horse places up to three times its body weight in force on the lower limbs. The more load or pressure put on a bone, the greater the bone remodelling that will need to take place. Ultimately, this will result in new, stronger bones being formed. 

Studies have shown that correct exercise can increase bone density in the cannon bone, the knee and sesamoid bones; and this can help reduce the likelihood of skeletal injury. However, the intensity of training is key; low intensity exercise (trotting), whilst essential for muscle development, has been shown to only result in small change in cannon bone density. Whereas training at high speeds for a short amount of time (sprinting), rather than repetitive slow galloping, has shown to result in a significant increase in bone density. This is highlighted in a study using a treadmill where short periods of galloping at speeds over 27mph (43 km/hour) were associated with a 4-5% increase in the density of the cannon bone.

Whilst exercise clearly plays a pivotal role in bone density, doing too much too soon can be disastrous and result in issues such as:

• Sore/buck shins: This is a common injury in young racehorses. It is caused by excessive pressure on the bones resulting in tiny fractures on the cannon bone, which may not have fully mineralised (strengthened and hardened). This results in the periosteum (a fibrous membrane of connective tissue covering the cannon bone) becoming inflamed. 

• Bone chips: Another common skeletal injury in racehorses, mostly seen in joints, particularly in the knee. This is when a tiny fracture occurs in the joint, weakening the bone and ultimately resulting in a ‘chip’ of the bone becoming separated. 

When trying to maximise skeletal strength, periods of lower intensity exercise or rest are just as important as gallop work, as they give the bone a chance to remodel. However, prolonged rest will have a negative effect on skeletal health.  Research has looked at the loss of BMC in the cannon bone when horses were placed on box-rest (with 30 minutes on the walker) and found overall BMC was reduced. Therefore, even horses returning to work after a short period of 1-2 weeks of box-rest could potentially have a significant decline in bone density and thus be at increased risk of skeletal injury once exercise recommences. 

It’s also important to bear in mind that when a young horse starts training, it is normally coming from a 12–24-hour turnout. This is where the horse has the ability to gallop and play. However, once training begins, they are typically stabled from long hours with short intervals of low intensity training. Consequently, bone demineralisation can occur. In addition, during this early stage of training, bone will undergo a significant degree of remodelling in response to exercise. Initially this process makes the bone more porous and fragile before it regains its strength. As a result, research has shown that horses can have reduced bone density during the first few months of training, with bones being at their weakest and the horse more prone to issues such as sore shins between day 45–75 of training. 

It should be noted that even when training is carried out slowly, conditions such as sore shins can still happen as bone remodelling occurs at different rates in every horse and is influenced by factors such as track surface and design. While there is some information on exercise and bone development from which to make inferences, a definitive answer as to the perfect amount of exercise to support optimal bone development has not yet been found.

Nutrition & bone health

Exercise is essential to bone health, but nutrition plays an equally important role. Bone is continuously being strengthened, repaired and replaced. And if we can aid bone remodelling with good nutrition, we can decrease the likelihood of skeletal injury. The essential nutrients for bone health are protein, minerals and vitamins, including calcium (Ca), phosphorus (P), zinc (Zn) copper (Cu), vitamins A, D and K. 

Protein: Collagen is a protein and forms the bony matrix on which minerals are deposited. Feeding sufficient high-quality protein, rich in essential amino acids such as lysine and methionine, is therefore a key factor in the development of strong healthy bones. When selecting an appropriate feed for horses in training, both the level and quality of the protein it provides should be carefully considered; not all protein is equal.  

Calcium & Phosphorus: It is well documented that these essential minerals are the foundation of strong and healthy bones, making up 70% of the BMC. The ratio of calcium and phosphorus in the diet is also very important for bone mineralisation. This is because imbalances in the Ca:P ratio can result in the removal of calcium from the skeleton and may lead to bone demineralisation. The minimum Ca:P ratio in the diet should be 1.5:1, with the ideal ratio being at least 2:1 for young horses. It is important to note that adding other feedstuffs such as chaffs or cereals to the horse’s feed can alter the Ca:P ratio in the overall diet. For example, adding oats, which are high in phosphorus, will reduce the calcium to phosphorus ratio and this may adversely affect calcium absorption. On the other hand, including some alfalfa, which is high in calcium, can help to increase the Ca:P ratio if required. 

Copper & Zinc: Copper is an important mineral for bone, joint and connective tissue development. Lysyl oxidase is an enzyme that requires copper. It is responsible for cross-linking of collagen, and therefore copper plays an important role in the formation of new bone which requires a collagen matrix. Similarly, zinc is integrally involved in cartilage turnover; and research has shown that horses supplemented with zinc, as part of a complete mineral package, have increased bone mineral density compared to horses fed an unsupplemented diet. Copper and zinc are frequently found to be low in forage and therefore must be provided in the form of a hard feed or supplement. 

Vitamins: A number of vitamins play essential roles in skeletal health. For example, vitamin A is involved in the development of osteoblasts—the cells responsible for laying down new bone—whilst vitamin D is needed for calcium absorption. More recent research has also shown that feeding vitamin K improves the production of osteocalcin, the hormone responsible for facilitating bone metabolism and mineralisation. Furthermore, research in two-year-old thoroughbreds suggests that feeding vitamin K may help increase bone mineral density and thus potentially be beneficial for decreasing the incidence of sore shins. Although standard feed manufactures include vitamin A and D in their feeds, a few also now include vitamin K.

Supplementation for bone health

Young horses in training, those recovering from injury or returning to work following a rest will benefit from additional nutritional support targeted at maintaining improving bone health. In these situations, supplementing with elevated levels of calcium and phosphorus will help improve bone health. Look for a supplement containing collagen, which is rich in type I and II collagen, proteoglycans and glycosaminoglycans—all of which aid the bone remodelling process and help to maintain bone health. Choosing a supplement that also contains chelated copper and zinc, as well as vitamins A and D, will also help support bone mineralisation. 

In summary, skeletal injuries have a huge adverse effect on the racing industry and are a common cause of lost training days. Undoubtedly, adapting our training regimes, modifying our gallops and improving our management practices will all help to reduce the risk of bone-related injuries. Equally, the role of nutrition in bone health should not be overlooked. A balanced diet, rich in nutrients, minerals and vitamins, can contribute significantly to bone density and strength. Proper nutrition is an essential parameter of skeletal health, participating in both the prevention and treatment of bone diseases.  To achieve a strong, sound skeleton, you must feed the bones.

The Importance of forage testing

Forage (hay/haylage) is an important source of nutrients for horses in training. However, the levels of minerals such as calcium, phosphorus and copper present can vary enormously and depend on factors such as the species of grass and the land on which it was grown. It is recommended that you regularly test the nutritional value of your forage. This will highlight any mineral excess/deficiencies and allow for the ratios of certain minerals such as calcium and phosphorus to be assessed. In most cases, any issues identified can be corrected through using an appropriate hard feed and/or supplement.