3D Bone Shape from CT-Scans Provides an Objective Measure of Osteoarthritis Severity: Data from the IMI-APPROACH Study

Decisions regarding total knee arthroplasty are usually made using a patient’s own assessment of pain and the structural disposition of the joint as seen on plain film radiographs. Pain severity can fluctuate, and radiographs may be misleading, with apparent joint status affected by anatomical orientation. An important component of the surgical management of knee osteoarthritis (OA) is the timing of surgical intervention; knee arthroplasty performed too early in the course of the disease may result in increased need for revision surgery. Femoral 3D bone shape (B-score) from MR images is an objective measure of OA severity and has been correlated with current and future risk of pain. CT images are used in planning robot-assisted knee arthroplasty. We aimed to derive the B-score from CT images. We used baseline and 24-month image data from the IMI-APPROACH 2-year prospective cohort study. The femur was automatically segmented using an active appearance model, a machine-learning method, to measure B-score. Linear regression was used to test for correlation between measures. Limits of agreement and bias were tested using Bland-Altman analysis. CT-MR pairs of the same knee were available from 424 participants (78% female). B-scores from CT and MR were strongly correlated (Lin’s Concordance Correlation Coefficient, CCC = 0.980) with negligible bias of 0.0106 (95% CI: −0.0281, +0.0493). The strong correlation and small B-score bias suggests that B-score may be measured reliably using CT images. B-score derived from CT surgical planning images may provide a useful objective input to deciding the appropriateness and timing of knee arthroplasty.