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Bone fractures are a serious cause of injury for the equine athlete. Small cracks, known as microcracks, may develop in bone as a result of exercise but these can be repaired and the on-going mechanism of bone growth and removal during the lifetime of the animal can even increase the overall strength of the bone.
Problems occur when new bone growth cannot keep up with the rate of bone damage, for example if exercise levels are excessive. In this case, individual microcracks may join up with each other to form much larger cracks and eventually lead to fracture.
Although the processes that repair microcracks might actually help to protect the joint in the short term through adaptive change, their effect in the longer term may be much less beneficial. An important aspect of microcrack repair includes changes to the appearance of the bone that resemble osteoarthritis (OA). OA is a joint disease that commonly affects racehorses from as early as two years of age.
This study will carry out mechanical testing of direct and shearing loads on cartilage and bone at the end of the cannon bone in the fetlock joint - the joint most likely to sustain OA or fracture in racehorses - to investigate the relationship between microcracking, fracture and OA in healthy horses and those that have been diagnosed with early-onset OA.
This study has focused on comparing the articular cartilage at two sites on the distal end of the cannon bone: the palmar region, which is subject to high in vivo loading and frequently affected by degenerative changes; and the dorsal region, which experiences low loads. By comparing these regions that are subject to different mechanical demands, a better understanding of the effects of loading history on the structure and mechanical response of the tissue may be obtained.
The study has identified significant differences in the morphology, mechanical properties, and response to impact loading between the two sites of interest. These differences, measured in healthy tissue, indicate that the palmar region may be inherently more vulnerable to mechanical damage and disruption, which may in part account for the high incidence of degenerative lesions that occur at this site.
We are currently investigating the mechanical properties and morphology of these same palmar and dorsal sites in Thoroughbred racehorses of various ages. A highlight of this work so far is the acquisition of high resolution images that capture dramatic changes in the articular cartilage and subchondral bone of the palmar region. Of particular note are images that show varying degrees of microcrack accumulation in the subchondral bone, up to and including collapse of regions of bone directly subjacent to the articular cartilage. Interestingly, it has been noted that in some cases extreme subchondral bone damage can occur with minimal disruption to the overlying articular cartilage. When completed, this work may reveal the progression of structural changes involved in the osteochondral degeneration of the palmar aspect of the cannon bone.
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Last updated on Tuesday 16 August 2016
Distal (bottom) end of the cannon bone. Thinning of the cartilage is clearly visible at the back of the joint (arrows).
This work is funded by the Equine Trust.