Effect of exercise on bone density in distal regions of the equine third metacarpal bone in 2-year-old thoroughbreds.

The present study addressed the question of whether training induces changes in bone density in the younger racehorse in regions relevant to the genesis of the common sagittal condylar fracture pattern and, if so, the nature of such alteration. Third metacarpal bones (McIII) were obtained from 12 experimental Thoroughbred, unbroken fillies, age 18.1-21.0 months at entry to the study. Six (Group E) undertook 19 weeks of progressive high intensity exercise on a high-speed treadmill at 3% incline. Six (Group C) walked 40 min each day. Clinical bone density was studied by Computed Tomography (CT) of the right McIII. 1:1 aspects of bone density were studied by digital radiography of slices and microscopic aspects by backscattered electron (BSE) imaging. There was a distinct pattern of regional variation in bone density throughout the distal condyles of McIII. Bone density was greater in specimens from Group E than those from Group C by CT, by radiographic density and by bone volume fraction as determined from backscattered electron images: the latter also showed that the most important change was in the increased amount of bone deposited within antecedent marrow space in cancellous tissue in Group E compared with Group C, with more extensive regional involvement in the highly loaded (distopalmar) aspects of the medial and lateral condyles. However, this space-filling bone fraction had a lower degree of mineralisation than the prior bone tissue, showing that CT and radiography underestimate the resultant increased bone volume fraction. Stress fractures of the distal condyles of the McIII are common in Thoroughbred racehorses. We propose that excess stiffening within the condyles due to extensive new bone formation may lead to concentration of strain at the condylar grooves and incipient cracking through calcified cartilage and into the subchondral bone.