Bucked shins seem is a disease of young horses being trained at speed. This would include Thoroughbreds, Standardbreds, and Quarter Horses and it is seen in some surveys that almost 3/4 of TB's develop bucked shins. The classical explanation for bucked shins is related to micro-fracture of the front cortex of the cannon bone from the trauma of running. If enough time is allowed between exercises, these fractures heal and the bone remodels and becomes stronger. However in typical training regimens the exercises are too frequent to allow complete healing and the result is the damage accumulates and the bone becomes inflammed. Despite an increasing understanding of bucked shins this problem stills plagues us.

A complication of past bucked shins and with a very similar clinical appearance are fissure and saucer fractures of the cannon bone. This article presents some of the new thoughts about bucked shins and cannon fissure/saucer fractures. It covers some of the new thoughts on these related problems, diagnosis, treatment, and ways to greatly reduce the incidence.

The details of this model have been challenged. In the past bucked shins were thought to be small fractures in the cannon bone. However recent research by Dr. Nunmaker in conjunction with New Bolton Equine Center, indicates this is not true of most cases. We think of bones has hard unchangeable structures and nothing could be further from the truth. From a structural standpoint there is a fair amount of flex in a bone, particularly young bone. From a metabolic standpoint, bone is constantly being torn down and rebuilt in response to stress.

With work, the bones natural remodeling process is accelerated: bone is torn down and then rebuilt. The rebuilding process takes time to complete, at least two days, following a workout and three is better. When exercise is repeated without adequate rest for rebuilding or when very stressful work is done that is substantially different from that the horse has been prepared for the bone begins to remodel with fibrous tissue and inadequate calcium deposition.

When the fibrous bone is laid down at the level of the surface of the bone it lifts and stretches the periosteum off of the calcified bone. This stretching is painful and also results in inadequate blood supply further increasing pain. This is a bucked shin.

Instead of inadequate time for rest and rebuilding between gallops, it has been suggested that the type of exercise may be responsible and that classical training does not provide the type exercise needed to strengthen the bone for racing. After 20years of investigation, Dr Nunamaker in a landmark presentation to the AAEP reached the following conclusions about bucked shins:

• It seems that horses are not born with the right bone structure for racing. They must develop it. Bone can only develop based on its own experience (Wolff's Law). Training adapts bone to training and training that mimics racing adapts bone to racing.
• It is important to change the inertial properties of MCIII to resist bending in the dorsopalmar direction.
• Different exercise programs can change the shape and substance of bone (modeling and remodeling) with hard surfaces giving faster results.
• High-speed exercise in small doses seems highly protective against bucked shins, whereas long galloping exercise increases the risk for bucked shins.
• Exercise programs exist to decrease the incidence of bucked shins.
• Horses that do not develop bucked shins do not develop stress or saucer fractures in their dorsolateral cortex.
• Horses that do not develop stress or saucer fractures do not develop midshaft Mc3 fractures while training and racing.
• Institution of an effective training program for young Thoroughbreds to significantly decrease the incidence of bucked shins will therefore significantly decrease the incidence of fatal musculoskeletal mid-cannon bone fractures during racing and training.

As Dr. Nunamaker points out recent research has shown modifications in training regimens can reduce the incidence of bucked shins to less than 15%, see Prevention below.