Game of bones

New insights into how bone forms, adapts and heals are revolutionising fracture prevention and repair, explains Patrick Pollock FRCVS

- Patrick Pollock

BONE is a substance that changes throughout life, in a process called remodelling. This allows the bones of foals and young horses to grow and change shape, and in adult horses enables the repair of damage and prevents bones from becoming brittle.

The amount of remodelling is affected by the horse’s age, the type of exercise he does and the surface on which he works. Research in thoroughbreds discovered that if horses undergo controlled exercise early in life, the bone “learns” to model in a way that protects it from injury in later life.

Interval training with short bursts of high-intensity exercise is more likely to result in healthy, strong bone, than low-intensity, endurance-type work. Exercise causes tiny areas of damage, leading to “stress remodelling”, where the bone adapts to the forces placed on it. This process, called an adaptive response, is critical to bone health.

WHY BONES FRACTURE

WE now know that the die is cast for a fracture long before the bone actually fails. Inappropriate training before the adaptive bone response is complete leads to too much strain on the bone. This results in “fatigue injury”, microscopic damage to the bone.

If training continues, the osteoblasts (see box, right) lay down too much new bone, causing the bone in those parts of the skeleton placed under excessive strain to become more dense than normal. In horses, extra new bone tends to be laid down at the joints, such as around the condyles (rounded ends) of the fetlock and in the front part of the limb bones. This dense bone, called sclerotic bone, is actually quite brittle.