New insight on the subchondral bone and cartilage functional unit: Bone mineral density and cartilage thickness are spatially correlated in non-osteoarthritic femoral condyles

OBJECTIVE: This study aimed to improve our understanding of the relationship between bone and cartilage by characterizing the morphological coupling between these mechanosensitive tissues exposed to the same mechanical environment within each knee. Specifically, it reanalyzed a prior dataset to test the hypothesis that the locations of thickest cartilage and densest subchondral bone are correlated in non-osteoarthritic femoral condyles. METHOD: Anatomically standardized maps of cartilage thickness (CTh) and subchondral bone mineral density (sBMD) were calculated for 50 non-osteoarthritic distal femurs based on computed tomography arthrography examinations. The locations of thickest CTh and densest sBMD were identified in the load-bearing region of the medial and lateral compartments, and correlation analyses were performed to quantify the associations between these locations, with inclusion of age, gender, femoral bone size and femorotibial angle as cofounding variables. Paired Student's t-tests were also performed to compare CTh and sBMD locations. RESULTS: Locations of thickest CTh and densest sBMD were positively correlated along the anteroposterior direction in both compartments (r ≥ 0.45, p ≤ 0.001). Furthermore, thickest CTh was more posterior than densest sBMD in the medial (p = 0.014) and lateral (p < 0.001) compartments, and more lateral than densest sBMD in the lateral compartment (p < 0.001). On average, these location differences were of 1.3, 5.3 and 2.1% of the subchondral bone size. CONCLUSION: The positive spatial relationship between the locations of thickest CTh and densest sBMD supports the idea of a functional cartilage/subchondral bone unit with morphological coupling conditioned by the individual loading pattern.