Bucked shins is a condition that affects the cannon bones of young racehorses in the first 6 to 8 months of their speed training. Since the horse’s bones are still developing at this time of life, the strain of training results in a cycle of stress and repair that results in weak and porous bones.

Left untreated, the bones start to develop microfractures and inflammation, which can then lead to hairline fractures and eventually catastrophic midshaft fracture.

Horses with bucked shins have painful, swollen forelegs and a short, choppy gait. Diagnosis is based on the presence of shin pain and confirmed with imaging. Treatment is aimed at reducing swelling while the horse rests.

The best way to prevent bucked shins is to design a conditioning program that includes short bursts of exercise followed by sufficient recovery time. This approach gives bone tissue time to heal properly and become strong enough to race like a champion.

Bucked Shins in Horses

Bucked shins describes a disorder in which the cannon bone undergoes cycles of damage and repair due to repetitive stress from speed training. The cannon bone, also known as the third metacarpal bone, is the main weight bearing bone in the horse’s forelimb.

Bucked shins occurs only in young horses as they begin to work at speed. [1][2][3] Other terms for this disorder include: [1][2][3][4]

  • Dorsal metacarpal disease
  • Bucked shin complex
  • Bucked shin syndrome

Bucked shins usually develop on both forelimbs at the same time. In cases where the horse trains and races in a counterclockwise direction, it may develop predominantly in the lead (left) limb. [1][2]

As a result of excessive compression during development, the cannon bones exhibit a series of conditions (the bucked shin complex).

These conditions include:

  • Periostitis
  • Stress Reactions
  • Microfractures
  • Dorsal cortical fracture

Periostitis

Periostitis is the first stage of bucked shin complex. In this stage, inflammation develops within the dense layer of tissue surrounding the cannon bones. This is the result of repeated stress and microtrauma to the front surface of the cannon bone. [1][2][3]

Stress

Bone remodeling is a normal part of healthy development in young horses. If horses have sufficient recovery time after training, remodeling leads to increased bone density, and the demands placed on the bones do not outstrip the bone’s ability to heal.

In cases where the young horse does not have sufficient recovery time, increased bone remodeling after exercise consumes bone tissue faster than new bone is laid down.

This leads to weak, porous bones and can also cause lameness in cases where remodeling progresses too quickly or excessively. This stress reaction is another contributing factor to bucked shin complex in horses. [1][2][3]

Microfractures

If bucked shins go undetected or untreated, the complex can progress to later stages, including microfractures. In this stage, tiny cracks develop in the cannon bone as the result of repetitive mechanical stress.

These are the precursor to more significant fractures if the stress continues without adequate recovery time. [1][2][3]

Dorsal cortical fracture

The latest stage of untreated bucked shins can lead to full dorsal cortical fractures.  When this occurs, a hairline stress fracture develops on the front surface of the cannon bone. These mild fractures can progress to a more significant break if not managed properly. [1][2][3][4][5]

Risk Factors

Bucked shins is a condition that is very closely associated with young horses that are exercised at speed. It typically occurs in racehorses at or around two years of age during their first six to eight months of training. Bucked shins can also develop in somewhat older horses that are entering speed training for the first time. [2][3][4]

Thoroughbreds are more at risk than other breeds with estimates suggesting that between 66 and 70% of thoroughbred horses in training develop this condition. [3][4][6] Quarter horses are also at risk. [7]

Standardbred horses, who train and race with different gaits and at different speeds, are far less likely to develop bucked shins. [2]

The prevalence of bucked shins has decreased significantly since the 2010s due to changes in training programs. Current best training practices take the impact of exercise on bone remodeling into account. Updated speed conditioning programs focus on reducing long distance galloping and increasing short distance breezing to ensure that the bone does not suffer unnecessary damage. [1]

The ground surface that horses are trained and race on also affects the risk of bucked shins. Softer surfaces are associated with lower incidence of bucked shins. Reports indicate that horses trained and raced on turf in Europe develop bucked shins when they are then brought to race on harder dirt tracks in North America. [2][4]

Causes

During the early stage of a horse’s life, the bones in its body are continuously developing. Stress placed on the bones from training, growing, and basic locomotion results in the formation of new bone tissue.

This process is called bone remodeling. Bone remodeling is greatest when horses are still growing but is an important aspect of life-long bone health and maintenance.

New bone tissue increases bone density, making bones stronger and better suited to their functions. A horse that is trained to race during this phase will have bones that are better suited to racing for the rest of its life. [1][3]

However, if bones are not given a chance to lay down sufficient new tissue before they are challenged with exercise again, the bone can become weak and porous. Bones that are weaker are more likely to develop bucked shins. [1][3]

If horses continue training with bucked shins, the condition is likely to progress to hairline fractures. [1][2][8]

In cases where a hairline fracture goes untreated and the horse continues to train, the horse could develop a catastrophic midshaft fracture, which can end the horse’s racing career. Many horses with leg fractures are euthanized. [1][2][8]

Unlike other types of fracture, bucked shins are not associated with a single trauma or event; they develop as a result of the strain put on the developing bone during speed training. [9]

Symptoms

The characteristic symptom of bucked shins is pain or tenderness of the front and inner edge of the shins. The horse is often reluctant to be touched in the area and will withdraw the limb. [1] This is typically accompanied by heat and swelling. [2]

In most cases, the onset of symptoms happens directly after intense exercise. [1][2][7]

Horse owners are usually alerted to the presence of bucked shins if the horse shows any of the following behavior: [1][2]

  • Ring sour
  • Exercise intolerance
  • Choppy gait
  • Shortened stride

In some cases, the horse exhibits lameness. [1][2]

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Severity

Bucked shins range in severity based on the extent of fracturing and inflammation in the cannon bone. [2] In many cases, bucked shins are mild to moderate and treatable with rest and anti-inflammatory medications. [2]

In chronic cases, the horse continues to experience pain after treatment. [1] In serious cases, bucked shins develop into hairline stress fractures or saucer fractures. [2]

In severe cases where a hairline fracture is left untreated, it can further develop into a catastrophic midshaft fracture where the bone breaks completely. These cases are extremely serious and treatment options are limited, often resulting in euthanasia. [2]

Diagnosis and Treatment

Bucked shins are suggested by the characteristic pain and tenderness, heat, and swelling of the shin on the front forelimbs upon physical examination. Diagnosis is confirmed with diagnostic imaging tools including: [1][1][8][10]

  • X-rays
  • Nuclear scintigraphy (the injection and tracking of a radioactive compound into the blood stream to assess bone health)
  • CT scan

Treatment for horses with bucked shins is aimed at reducing swelling and easing pain while the horse rests long enough for the bones to heal. In mild cases, recovery can take between 5 to 10 days. [1][5]

Care options during this period include: [1][8][10]

  • Hand walking
  • Ponying
  • Stall rest
  • Cold water hosing
  • Icing
  • Poultices
  • Bandaging

In cases of fracture, surgical options including the placement of screws to hold the bone together are available. [1][11][12]

Prognosis and Prevention

Once the pain has subsided and the swelling is improving, gradual reintroduction to speed and distance training are recommended with careful monitoring of the horse for signs of pain or reluctance. Gradually increasing stress on the cannon bone allows the bone’s surface to remodel according to the demands of the exercise without causing further damage. [1]

In chronic cases, the reintroduction of exercise often causes continued pain. In these cases, rest of up to 3 months is sometimes required. [1]

If hairline fractures or saucer fractures are present, the rest period may be as long as 6 months. [1]

Typically, resolution of mild, moderate, and chronic cases of bucked shins is possible. Once most cases are resolved, the bone becomes stronger and the horse never experiences the condition again. [2]

Prevention

Adopting current best training practices that allow for sufficient rest between training sessions is the most effective way to avoid bucked shins. [8]

To withstand high-impact activities like racing, horses need to be gradually trained to promote fitness and conditioning, with adequate rest periods to support recovery and prevent injury. [2][8][9]

Training programs that take rest and recovery into account have been successful in conditioning the cannon bones to withstand the demands placed on them during intense exercise.

Generally, speed training young horses involves short sessions of high-speed exercise with gradually increasing intensity, followed by sufficient recovery time. This practice helps ensure the stress on the bone does not outpace the rate of remodeling. [2][8][9]

Training without toe grabs on softer surfaces such as grass, wood fiber, or polytrack is also recommended. [1]

Surgical interventions to stimulate the growth of new bone tissue such as periosteal scraping and cortical drilling are also available to help offset the negative impact of high-impact training. [3][12]

Improved Training Practices

Previously, it was common practice to intentionally induce bucked shins in young racehorses during training. Some trainers believed that doing so was beneficial to the horse’s development and performance, and that bucking shins were an inevitable part of speed training that needed to be overcome. [2]

With recent advancements in understanding the pathophysiology of bucked shins and the associated fracture risks, most trainers have abandoned the practice of intentionally bucking shins during speed training. [2] Veterinarians strongly advise against this practice, emphasizing the importance of working with a qualified trainer to ensure the horse’s overall safety and well-being.

Summary

Bucked shins affects the forelimbs of young racehorses, particularly Thoroughbreds, when they begin speed training. It is the result of a cycle of stress and repair in the cannon bones without sufficient time for the bone to heal properly, leaving them weak and porous.

  • Periostitis, stress reaction, microfractures, and dorsal cortical fractures are all possible consequences of bucked shins in horses.
  • Symptoms develop directly after intense exercise and include pain, swelling and heat at the front and edge of the shin on the forelimb as well as a short, choppy gait.
  • If left untreated, serious fractures can occur which could end the horse’s career or even lead to euthanasia.
  • Treatment is aimed at reducing swelling and managing pain while the horse rests.
  • Training programs that incorporate gradually intensifying exercises and small bouts of high-speed activity are effective in conditioning bones and preventing bucked shins.

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References

  1. Baxter, G. M., Ed., Adams and Stashak’s Lameness in Horses. Seventh edition. Wiley-Blackwell, Hoboken, NJ. 2020.
  2. Nunamaker, D. M., On Bucked Shins. Proceedings of the Annual Convention of the AAEP. 2002.
  3. Plevin, S. and McLellan, J., Does Periosteal Scraping of the Third Metacarpal Bone Reduce the Incidence of ‘Bucked Shins’ in Young Thoroughbred Racehorses?. Equine Veterinary Journal. 2014.
  4. Katayama, Y. et al., The Influence of Exercise Intensity on Bucked Shin Complex in Horses. Journal of Equine Science. 2001.
  5. Riquelme, G. et al., Neosaxitoxin, a Paralytic Shellfish Poison Toxin, Effectively Manages Bucked Shins Pain, as a Local Long-Acting Pain Blocker in an Equine Model. Toxicon. 2018.
  6. Burr, D. B. and Milgrom, C., Musculoskeletal Fatigue and Stress Fractures. CRC Press. 2000.
  7. Ross, M. W. and Dyson, S. J., Diagnosis and Management of Lameness in the Horse. 2nd ed. Elsevier Saunders, St. Louis, Mo. 2011.
  8. Nixon, A. J. et al., Third Metacarpal Dorsal Stress Fractures. Equine Fracture Repair. 1st ed. Wiley. 2019.
  9. Nunamaker, D. M. et al., Fatigue Fractures in Thoroughbred Racehorses: Relationships with Age, Peak Bone Strain, and Training. Journal of Orthopaedic Research. 1990.
  10. Lavoie, J.-P., & Hinchcliff, K. W. Blackwell’s Five-Minute Veterinary Consult Equine. Second Edition. Wiley-Blackwell. 2009.
  11. Jalim, S. L. et al., Lag Screw Fixation of Dorsal Cortical Stress Fractures of the Third Metacarpal Bone in 116 Racehorses. Equine Veterinary Journal. 2010.
  12. Dallap, B. L. et al., Results of Screw Fixation Combined with Cortical Drilling for Treatment of Dorsal Cortical Stress Fractures of the Third Metacarpal Bone in 56 Thoroughbred Racehorses. Equine Veterinary Journal. 1999.