Bone Remodeling is an Early Sign of Biomechanically Induced Pre-Osteoarthritis

Purpose: Despite the degenerative nature of osteoarthritis (OA), early diagnosis of a so-called pre-osteoarthritis - prior to the onset of irreversible degradations – may enable to either treat or modify its progression. Therefore, detection of early stage OA has become very important nowadays. Among various risk factors, excessive mechanical loading has been recognized to be involved in OA. Response of cartilage and bone to loading reflects mechanosensitivity of both tissues as a reaction to deformation and whether the applied mechanical forces can induce pre-OA conditions. In this study, we aimed to design different running models proceeding to mild damage or pre-OA in rats. For this purpose, we applied treadmill-running exercise and performed several assays to elucidate the diminutive initial changes in the involved tissues. Methods: Sixty male rats (Wistar; Charles River, 8 weeks old) were used in this study and divided into three groups of 10 animals each: an adaptive running group that runs for 8 weeks in daily sessions of gradually increasing running on a treatmill up to 1120 m per hour (adaptive running regime) followed by 6 weeks of one hour per day at 1120 m/h (constant running regime); a non-adaptive (constant) running group that has rest for 8 weeks followed by 6 weeks of constant running regime; and a control group without running (Fig. 1). At baseline and after 14 and 20 weeks (additional 6 weeks of rest), different joint analyses were performed: micro computed tomography (micro-CT) - in epiphysis, metaphysis, and diaphysis -, histology, HSP70 immunohistochemistry, and micro-indentation. Results: MicroCT data showed that, after 6 weeks (at 14 weeks) bone parameters including subchondral bone tibia plateau thickness and trabecular bone thickness for both running groups decreased in the load bearing region of the medial knee joint (epiphysis) as compared to the control (non-running) group (P ≤ 0.005). This is opposite of the cortical bone further away from the joint that increased. At 20 weeks, (after the 6 weeks rest) for the running groups the differences between running animals and control disappeared for the most part (Fig. 2). Safranin-O stained histological sections revealed a significant variation after 20 weeks between adaptive running and control groups (P <0.05). The two running groups showed chondrocyte proliferation, colony formation and hypertrophy (Fig. 3A), more HSP70 expression (Fig. 3B) and a lower stiffness, in particular at the lateral side and more in the adaptive running group (Fig. 3C). Conclusions: In summary, we investigated how bone morphological changes and cartilage responses within the joint region contribute to a premature stage of OA. As a result of loading that could be considered excessive for these rats, bone adaptation occurs indicating a situation of pre-OA. This seems to lead to abnormal stress stimuli in the chondrocytes, kicking them to proliferation and hypertrophy. Furthermore, bone and cartilage behave differently in biomechanically induced pre-OA. While changes in articular cartilage are mainly exposed at the end, at week 20 (after the 6 weeks rest), bone turnover started earlier during the running period at week 14. In addition, whereas cartilage tissue changes appeared not to recover from adaptive running exercise, the subchondral bone normalized after the sedentary period of 6 weeks (Fig. 2). In conclusion, overloading by running exercise can induce a mild or pre-OA status in the knee joint of the rats.