Bone turnover and articular cartilage differences localized to subchondral cysts in knees with advanced osteoarthritis

Objective: To investigate changes in bone structure, turnover, and articular cartilage localized in subchondral bone cyst (SBC) regions associated with knee osteoarthritis (OA). Methods: Tibial plateaus (n = 97) were collected from knee OA patients during total knee arthroplasty (TKA). SBCs were identified using micro-computed tomography, and the specimens were divided into non-cyst (n = 25) and bone cyst (n = 72) groups. Microstructure of subchondral bone was assessed using bone volume fraction (BV/TV), trabecular number (Tb.N), structure model index (SMI) and bone mineral density (BMD). In bone cyst group, the cyst subregion, which contained at least one cyst, and the peri-cyst subregion, which contained no cysts, were further selected for microstructure analysis. Articular cartilage damage was estimated using the Osteoarthritis Research Society International (OARSI) score. The numbers of TRAP⁺ osteoclasts, Osterix⁺ osteoprogenitors, Osteocalcin⁺ osteoblasts and expression of SOX9 were evaluated by immunohistochemistry. Results: Bone cyst group presented higher BV/TV, Tb.N and SMI at subchondral bone than non-cyst group. Furthermore, cyst subregion displayed increased BV/TV and Tb.N but lower BMD and SMI than peri-cyst subregion. Histology revealed a higher OARSI score in bone cyst group. SBC exhibited a weak relationship with BV/TV, etc. The numbers of TRAP⁺ osteoclasts, Osterix⁺ osteoprogenitors, Osteocalcin⁺ osteoblasts and expression of SOX9, were higher in bone cyst group. Conclusion: SBCs within knee OA are characterized by focally increased bone turnover, altered bone structure and more severe articular cartilage damage. The increased bone turnover possibly contributes to altered bone structure localized in SBC areas, and thus aggravates articular cartilage degeneration.