Agreement between in vivo and ex vivo photon-counting CT measurements of subchondral trabecular bone features in patients with knee osteoarthritis.

Objectives: The aim of this study was to compare in vivo and ex vivo Photon Counting CT (PCCT) of subchondral bone features in patients with knee osteoarthritis (KOA).

Materials And Methods: Pre-surgery in vivo and post-surgery ex vivo PCCT of the tibial plateau from participants with severe KOA referred to arthroplasty surgery from January 2022 through September 2023 were compared. Linear regression and Bland-Altman plots were used to assess correlation and agreement between in vivo and ex vivo measures of bone volume fraction (BV/TV), trabecular thickness (Tb.Th.) and attenuation in healthy and sclerotic trabecular bone. Delineated areas of bone sclerosis were compared using the Dice coefficient and Hausdorff distance.

Results: 18 in vivo/ex vivo PCCT scans were included. Strong correlations were found for BV/TV, R = 0.82 and attenuation; healthy, R = 0.89, and sclerotic, R = 0.79, bone, while a moderate correlation was found for Tb.Th., R = 0.55. Bias for BV/TV and Tb.Th. was -4.1% and -0.598 mm, respectively, and -41.4 HU and -81.1 HU for healthy and sclerotic bone, respectively. A proportional bias was observed for Tb.Th. and BV/TV. There was excellent agreement between the segmentations of sclerotic areas (Dice coefficient = 0.91, Hausdorff distance = 0.11 mm).

Conclusion: In patients with severe KOA, BV/TV and attenuation can be obtained with a high correlation and small bias between in vivo and ex vivo scans, while Tb.Th. showed moderate correlation and larger bias. Longitudinal studies using in vivo PCCT are feasible, but caution may be advised when measuring Tb.Th. The key OA feature of subchondral bone sclerosis is well translated from ex vivo to in vivo PCCT.

Key Points: Question Bone changes occur with osteoarthritis development; the role of these changes is unclear, and no method for visualising bone microstructure in vivo exists. Findings Photon-counting CT showed a strong correlation between in vivo and ex vivo subchondral density measures, while a moderate correlation was found for trabecular thickness. Clinical relevance Photon-counting CT is feasible for in vivo longitudinal evaluation of bone in patients with knee osteoarthritis, allowing studies into the earlier stages of the disease.