Multiscale Imaging Reveals Compartment-Specific Mechanobiology of the Osteochondral Unit in Knee Osteoarthritis.

Abstract

Mechanical overloading is a key risk factor for knee osteoarthritis (OA), yet its compartment-specific effects on the mechanobiology of the osteochondral unit and its interplay with synovial inflammation remain insufficiently defined. Here, multiscale imaging of clinical specimens is applied to uncover how mechanical and inflammatory cues shape the osteochondral unit in knee OA. Medial and lateral femoral condyles from patients with primary OA and secondary OA due to rheumatoid arthritis (RA), spanning varus to valgus alignment, were analyzed. A comprehensive multiscale imaging pipeline - including micro-computed tomography, cartilage- and bone-specific histomorphometry, quantitative backscattered electron imaging, and osteocyte lacunocanalicular network analysis - is integrated with targeted proteomic profiling of synovial fluid. Mechanical alignment is strongly associated with compartment-specific cartilage degeneration and subchondral bone adaptation in OA, as reflected by robust correlations between loading and OARSI score, subchondral bone thickness, bone formation, and osteocyte lacunocanalicular network parameters. Notably, the medial compartment exhibits substantially greater susceptibility to mechanically induced osteochondral remodeling than the lateral compartment. In contrast, these structure-function relationships are attenuated in RA. Collectively, these findings indicate that osteochondral remodeling in knee OA is predominantly associated with mechanical loading and reveal disease- and compartment-specific patterns in the balance between mechanical and inflammatory contributions, supporting a more tailored approach to treating knee arthropathies by prioritizing biomechanical or anti-inflammatory strategies according to disease context and compartmental involvement. STATEMENT OF SIGNIFICANCE: There remains ongoing debate regarding the extent to which osteoarthritis (OA) is a mechanical disease. The investigation of leg axis deviations (i.e., mechanical alignment) provides an ideal model to address this question. Here, we present a comprehensive multiscale characterization of the osteochondral unit in knee OA, integrating clinical imaging, micro-CT, histomorphometry, quantitative backscattered electron imaging, and targeted synovial proteomics. Mechanical alignment emerged as the predominant factor associated with cartilage degeneration, pathological subchondral bone formation, and osteocyte lacunocanalicular network alterations. In contrast, synovial TNF-α independently predicted cartilage degeneration in an inflammatory OA subtype secondary to rheumatoid arthritis. These findings reveal disease- and compartment-specific patterns and support a tailored approach in which biomechanical and biological strategies are prioritized according to context.