DEVELOPMENT OF SUBCHONDRAL CYSTIC LESIONS AFTER ARTICULAR-CARTILAGE AND SUBCHONDRAL BONE DAMAGE IN YOUNG HORSES

The objective of this study was to determine if damage to the articula r cartilage alone or articular cartilage plus subchondral bone of the distal medial femoral condyle of young, exercised horses resulted in t he formation of subchondral cystic lesions. Twelve Quarter Horses (age 1-2 years), free of clinical and radiographic signs of osteochondrosi s and lameness were used. In 6 horses (Group 1), a 15 x 1 mm linear fu ll thickness defect in the articular cartilage was made arthroscopical ly on the weightbearing surface of the distal aspect of the medial fem oral condyle. In the other 6 horses (Group 2), a 15 x 3 mm full thickn ess elliptical cartilage defect was made, followed by burring a 5 mm d iameter, 4 mm deep hole into the subchondral bone. Three weeks after s urgery, all horses were hand walked and trotted for 2 weeks and then e xercised for 6 min daily, 5 days a week for the next 14 weeks on a tre admill. They were then turned onto a small paddock for 6 weeks (6 mont hs total). The development of subchondral cystic lesions was determine d using radiography. Bone activity in the femoral condyle was monitore d with nuclear scintigraphy. All horses that developed subchondral cys tic lesions radiographically were subjected to euthanasia for gross an d histological examination of the lesions. No subchondral cystic lesio ns and no clinical abnormalities were detected in the horses in Group 1, Subchondral cystic lesions developed radiographically in 5 of 6 hor ses in (Group 2). Scintigraphic findings of horses with subchondral cy stic lesions were inconsistent. Histological examination of lesions re vealed variable quantities of fibrous connective tissue, fibrocartilag e and bone, with evidence of bone remodelling adjacent to the subchond ral cystic lesion. The results suggest that damage to articular cartil age plus subchondral bone, but not articular cartilage alone, of the d istal medial femoral condyle may lead to the development of subchondra l cystic lesions. These findings indicate that all subchondral cystic lesions in horses may not be osteochondrosis-type lesions and that tra uma to weightbearing articular surfaces of young horses may be a predi sposing factor.