In osteoarthritis of the knee, degenerative changes occur in the articular cartilage and underlying subchondral bone, particularly of the medial tibial condyle. Cancellous bone sclerosis that accompanies osteoarthritis is not only the result of an increase in bone volume fraction but also a change in trabecular structure. In a comparison with age-matched controls (n = 4), osteoarthritis (n = 11) demonstrated a significant (P < or = 0.05) increase in bone volume fraction and trabecular thickness. Overall trabecular orientation in the osteoarthritic group was more vertical or perpendicular to the articular surface than the control group (P < or = 0.05) especially in the trabeculae of the cancellous bone layer closest to the articular surface. These alterations in trabecular bone structure could have significant consequences for the mechanical properties of osteoarthritic bone.