Subchondral bone changes have been proposed as an early event in the pathogenesis of osteoarthritis. In this study, microscopic computed axial tomography was used to evaluate the subchondral bone structure in femoral heads from a guinea pig model of osteoarthritis. Examination of trabecular bone within the femoral head showed a highly significant increase in bone fraction in the experimental animals. This was due to the development of trabeculae that were thicker and closer together. We conclude that trabecular remodeling may be an early event in this model of osteoarthritis. Submitted for publication November 2, 1987; accepted in revised form June 9, 1988. changes in subchondral bone is unclear (1). It is also unclear whether the primary lesion occurs in cartilage ( 2 4 , synovium (9, or the subchondral bone (6,7).Subchondral bone absorbs most of the mechanical force applied across diarthrodial joints (8-10). Experimental studies of subchondral bone changes induced by repetitive impact-loading indicate that subsequent trabecular remodeling results in less compliant trabecular bone and thereby transfers excessive mechanical stress to the overlying articular cartilage (1 1-14).We investigated subchondral bone changes in an animal model of early osteoarthritis. In the animal model we chose, the resection of gluteal musculature was used to alter the biomechanical forces acting across the hip joint. Microscopic computed tomography (pCT) was used to assess the architecture of the subchondral bone in an effort to evaluate subchondral bone changes in a model of early osteoarthritis. MATERIALS AND METHODSAnimal preparation. Ten male Hartley strain guinea pigs (Charles River Breeding Laboratory, Wilmington, MA) averaging 700 gm in weight and 8 weeks in age were divided into 5 pairs. One member of each pair underwent a right gluteal myectomy and infrapatellar tendotomy according to a standardized procedure (15). Briefly, a 0.5-cm wide X 2.5-cm long segment consisting of the right gluteus superficialis, biceps femoris, and gluteus medius was removed at the sacral origin. Extraarticular resection of the right infrapatellar ligament was also performed. The hip joint was not invaded at any time during this procedure. Postoperatively, the animals limped for approximately 2 weeks, after which time, no difference in gait or activity level was visually distinguishable between the animals that had undergone operations and the control animals. The second member of each animal pair was used as an unoperated control. The