Relative HU profiling on clinical CT estimates the subchondral bone plate boundary: an ex vivo cadaveric mechanical...

The subchondral bone plate (SBP) is crucial for joint biomechanics and is implicated in degenerative joint disease. Absolute CT Hounsfield units (HUs) vary with scanner settings, limiting generalisability. We evaluated whether CT-derived relative HU profiling can estimate the location and thickness of the subchondral mineralized interface by comparison with mechanical indentation-derived measurements. Eighteen distal radii from nine formalin-fixed cadaver donors underwent low-dose CT and indentation testing. Post-test scans were registered to pre-test scans so CT- and indentation-based measures were sampled at identical locations. The SBP starting point and thickness were defined from HU attenuation transitions and load-displacement curves, respectively. Association between methods was evaluated with linear mixed-effects models using donor identity as a random intercept and summarised with marginal/conditional R²; agreement was descriptively assessed using Bland-Altman analysis. Inter-observer reliability was assessed using intraclass correlation coefficients. HU-based measures were strongly associated with indentation-derived measures, with fixed-effect slopes of 0.953 for the starting point and 0.988 for thickness, and R²m/R²c values of 0.916/0.918 and 0.912/0.912, respectively. Donor-level clustering effects were minimal. Reliability was good for HU-based measurements and good-to-excellent for indentation-based measurements. Bland-Altman analysis showed smaller bias and narrower limits of agreement for thickness than for the starting point. Relative HU profiling on standard clinical CT provided reproducible estimates of subchondral mineralized boundary features in this ex vivo cadaveric model. These findings support the feasibility of using relative HU transitions as a radiologic surrogate, while further histological and in vivo validation is required before clinical application.