Neural Shape Modeling Reveals Early and Progressive Femoral Bone Shape and Cartilage Thickness Changes After Anterior Cruciate Ligament Reconstruction.

Early and longitudinal shape changes in femoral bone and cartilage after anterior cruciate ligament reconstruction (ACLR) remains poorly understood. It is also unknown whether post traumatic shape changes follow similar trajectory as progressive alterations seen in idiopathic osteoarthritis (OA). Two neural shape models (NSMs)-B-NSM (bone-only) and BC-NSM (bone + cartilage) encoded femoral bone and cartilage shapes from 17 ACLR, contralateral, and healthy control knees at 3 weeks and 3, 9, 18, and 30 months post-ACLR. Knee OA severity shape scores (B-Score, BC-Score), bone surface area, and cartilage thickness were computed. Linear mixed-effect models assessed cross-sectional and longitudinal changes in these metrics; ηp 2 evaluated sensitivity to change. Cosine similarity quantified post-ACLR shape change trajectories compared to idiopathic OA progression. At 3 weeks, ACLR knee B-Scores were significantly lower (p = 0.010) than contralateral knees from surgical notchplasty. Over 30 months, ACLR knee B- and BC-Scores significantly increased compared to baseline (p < 0.001 for both). Bone surface area (ηp 2 = 0.01) increased from periarticular osteophyte lipping with concomitant region-specific cartilage thickening and thinning (ηp 2 = 0.02). B-Score (ηp 2 = 0.45) was more sensitive to longitudinal change than BC-Score (ηp 2 = 0.30). ACLR bone shape changes aligned with idiopathic-OA progression (cosine similarity = 0.22), while cartilage did not (cosine similarity = 0.08). Early and progressive joint remodeling were observed post-ACLR. Both B- and BC-Score were more sensitive than bone surface area and cartilage thickness measures in quantifying longitudinal changes in ACLR knees. While ACLR bone shape features exhibited a linear progressive pattern similar to idiopathic OA, cartilage thickness changes were nonlinear. These findings support the use of NSM-based metrics for tracking post-surgical joint remodeling in future clinical trials.