Alterations to the subchondral bone architecture during osteoarthritis: bone adaptation vs endochondral bone formation

Objective: Osteoarthritis (OA) is characterized by loss of cartilage and alterations in subchondral bone architecture. Changes in cartilage and bone tissue occur simultaneously and are spatially correlated, indicating that they are probably related. We investigated two hypotheses regarding this relationship. According to the first hypothesis, both wear and tear changes in cartilage, and remodeling changes in bone are a result of abnormal loading conditions. According to the second hypothesis, loss of cartilage and changes in bone architecture result from endochondral ossification. Design: With an established bone adaptation model, we simulated adaptation to high load and endochondral ossification, and investigated whether alterations in bone architecture between these conditions were different. In addition, we analyzed bone structure differences between human bone samples with increasing degrees of OA, and compared these data to the simulation results. Results: The simulation of endochondral ossification led to a more refined structure, with a higher number of trabeculae in agreement with the finding of a higher trabecular number in osteochondral plugs with severe OA. Furthermore, endochondral ossification could explain the presence of a "double subchondral plate" which we found in some human bone samples. However, endochondral ossification could not explain the increase in bone volume fraction that we observed, whereas adaptation to high loading could. Conclusion: Based on the simulation and experimental data, we postulate that both endochondral ossification and adaptation to high load may contribute to OA bone structural changes, while both wear and tear and the replacement of mineralized cartilage with bone tissue may contribute cartilage thinning.