RANKL-mediated osteoclastic subchondral bone loss at a very early stage precedes subsequent cartilage degeneration and uncoupled bone remodeling in a mouse knee osteoarthritis model.

introductionUncoupled bone remodeling in the subchondral bone (SB) has recently been considered as an important process in the progression of knee osteoarthritis (KOA). In this study, we aimed to investigate changes in SB and articular cartilage using a mouse model of destabilization of the medial meniscus (DMM) and determine the effects of bone metabolism on KOA progression.

methodsDMM or sham surgery was performed on the left knees of 40-week-old male wild-type (WT) mice and Tsukuba hypertensive mice (THM), which exhibit high-turnover bone metabolism. Bone volume/tissue volume (BV/TV) and bone mineral density (BMD) in the medial tibial SB were measured longitudinally in vivo using μCT at 0 (immediately after surgery), 1, 2, 4, 8, and 12 weeks postoperatively. Concurrently, histological evaluations of the articular cartilage in the medial tibial plateau were conducted. Furthermore, the number of endo-periosteal tartrate-resistant acid phosphatase-positive osteoclasts, trabecular RANKL-positive osteocytes, and osteocytes in the trabeculae were measured at 0, 1, 2, and 4 weeks.

resultsIn the WT + DMM group, BV/TV and BMD in the SB significantly decreased with time, whereas cartilage degeneration significantly increased. In the THM + DMM group, these changes in BMD and cartilage degeneration were significantly pronounced. Interestingly, in the THM + DMM group, BV/TV significantly decreased up to 4 weeks but then began to increase, although BMD continued to decrease until the 12-week mark. The number of osteoclasts and the percentage of RANKL-positive osteocytes per total number of osteocytes within the total trabecular bone area (%) in the WT + DMM group significantly increased with time, with a significant difference between the WT + DMM and WT + sham groups at 4 weeks. The number of osteocytes in the WT + DMM group significantly decreased with time, and the difference between the WT + DMM and WT + sham groups was significant at 4 weeks postoperatively. These histological changes were significantly enhanced in the THM + DMM group.

conclusionsThe results indicate that early-stage osteocyte death in the SB and RANKL-mediated osteoclastic SB loss precede histological cartilage degeneration and contribute to uncoupled bone remodeling at the later stage. Acceleration of disease processes in the THM + DMM group suggests that high-turnover bone metabolism is a potential risk factor for KOA. Maintaining SB integrity and avoiding continuous SB overload may be key strategies for mitigating disease progression.