Predicting subchondral bone stiffness using a depth-specific CT topographic mapping technique in normal and osteoarthritic proximal tibiae

Background: Subchondral bone stiffness is thought to be involved in osteoarthritis pathogenesis. Our objective was to determine if a CT imaging technique, which measures density in relation to depth from the subchondral surface, could predict the stiffness of proximal tibial subchondral bone. A second objective was to determine whether cartilage degeneration (an indicator of osteoarthritis) affected predictions. Methods: Thirteen proximal tibial compartments (4 medial, 9 lateral) from 10 male donors (age: mean 73.2, SD 10.6 years) were scanned using quantitative CT. We assessed average subchondral bone mineral density across different depths (0-2.5, 0-5, 0-10 mm) and layers (2.5-5, 5-10 mm) measured relative to the subchondral surface. We classified cartilage status as normal or degenerated using the International Cartilage Repair Society system. We performed macro indentation testing directly at the subchondral surface, and related stiffness to density measures using power-law regression models adjusted for side, age and cartilage status. We tested the coincidence of normal and degenerated regression models using F test statistics. Findings: Density measures nearest the subchondral bone surface (0-2.5 mm) were most effective at predicting subchondral bone stiffness (r(2) = 0.67, p<0.001). The predictive ability of depth-specific density measures decreased when density was averaged across larger depths or layers deep to the subchondral surface. Cartilage status did not affect model predictions. Interpretation: Depth-specific density measures have potential use as in vivo imaging tools for characterizing subchondral bone density and estimating stiffness. This information could help explain the role of subchondral bone in osteoarthritis pathogenesis. (C) 2011 Elsevier Ltd. All rights reserved.