Morphological and histological features of thicker cartilage at the posterior medial femoral condyle in advanced knee osteoarthritis.

Objective: To assess morphological and histological features of cartilage at the posterior medial condyle in advanced pre-prosthetic osteoarthritis (OA), which is notably thicker compared to non-OA knees.

Design: Cartilage thickness was measured pre-operatively using MRI in 10 subjects with medial femorotibial OA (mean age: 70.2 years). Posterior condyles were obtained during arthroplasty and cartilage thickness, relative collagen content and subchondral bone volume fraction (BV/TV) were determined using phosphotungstic acid (PTA)-enhanced micro-CT. Regions of interest (ROI) around the maximum cartilage thickness were further analyzed through histomorphometry (Mankin score) and immunohistochemistry (cell density and apoptosis rates).

Results: Maximum cartilage thickness was 2.63 ​± ​0.51 ​mm and 3.04 ​± ​0.55 ​mm and both measurements were strongly correlated ( ​= ​0.84,  ​= ​0.003). Cartilaginous collagen content measured by PTA-enhanced micro-CT was negatively correlated with maximum cartilage thickness ( ​= ​-0.70,  ​= ​0.02). Average subchondral BV/TV was 31.6 ​± ​3.4% and did not correlate with cartilage thickness. Extensive loss of proteoglycan staining and tidemark multiplication were common histomorphological features around the maximum cartilage thickness. Chondrocyte densities were 315 ​± ​67 and 194 ​± ​36 ​cells/mm at the superficial and transitional cartilage zones, respectively. Chondrocyte apoptosis rates were approximately 70% in both zones. Maximum cartilage thickness correlated with superficial chondrocyte densities ( ​= ​0.79,  ​= ​0.01).

Conclusions: Thicker cartilage at the posterior medial condyle in OA knees displayed degenerative changes both in cartilage tissue and at the osteochondral junction. Cartilage thickening may be influenced by alterations in the superficial zone, necessitating further investigation through molecular studies.