NEURAL SHAPE MODEL QUANTIFIES EARLY AND PROGRESSIVE BONE SHAPE CHANGES AFTER ACLR

Abstract

INTRODUCTION

Femoral bone shape scores (B-Score) derived from shape models quantify 3D structural features associated with OA^1,2. A higher B-Score is indicative of more OA-like bone shape. B-Scores have high sensitivity to quantify OA progression and stratify patients for interventions¹. Neural Shape Models (NSM) capture non-linear bone shape features and outperform traditional Statistical Shape Models (SSMs) in encoding OA-related shapes³. Prior work that used a SSM-based B-Score showed that anterior cruciate ligament reconstructed (ACLR) knees exhibit higher B-Scores than their contralateral knees 2 years post-surgery, reflecting OA-like bone shape features⁴. However, little is known about how femoral bone shape changes immediately following ACLR and how it progresses during the early post-surgical period—a critical window when post-traumatic osteoarthritis (PTOA) may still be most responsive to intervention.

OBJECTIVE

To use a Neural Shape Model-based B-Score to quantify femoral shape differences between ACLR and contralateral knees immediately post-surgery (3-weeks) and to detect early PTOA bone shape changes over 30 months.

METHODS

ACLR and contralateral knees of 17 subjects (11M/6F, age=38±10 yrs, BMI=24±2 kg/m²) were scanned at 3 weeks (baseline), 3, 9, 18, and 30 months post-ACLR in a 3T MRI scanner (GE Healthcare, USA) using a qDESS sequence (TE/TR=6/22 ms_, flip angle=25°, FOV=160 × 160 mm, bandwidth=31.25 kHz, pixel spacing=0.42 × 0.50 mm, slice thickness=1.5 mm). The femur was automatically segmented, and the B-Score was computed for each subject at all visits using a NSM that was trained on 9,376 femoral segmentations from the baseline DESS images in the OAI dataset¹. To assess bone shape differences immediately after surgery, we compared B-Scores between the ACLR and contralateral knees at the baseline visit using a linear mixed effects model. To capture longitudinal bone shape changes after surgery, we calculated change in B-Score at each follow-up visit with respect to the baseline visit. We used a linear mixed effects model to assess the effect of knee-type and time post-surgery on B-Scores. Effect sizes [η_p² is small (0.01), medium (0.06), or large (0.14)] were computed for significant effects (p<0.05).

RESULTS AND DISCUSSION

At baseline, the ACLR knee B-Score was significantly lower than the contralateral knee (η_p²=0.40, p=0.005; Fig. 1A). Longitudinally, ACLR knees showed a significantly greater increase in B-Score than contralateral knees (η_p²=0.19, p<0.001; Fig 2A). The lower B-Scores in ACLR knees at baseline indicate that the surgical knee had a healthier, less OA-like bone shape than the contralateral knee. Visualization revealed that ACLR knees had a wider intercondylar notch compared to their contralateral knee resulting from notchplasty that were confirmed on surgical notes (Fig. 1B). Since idiopathic OA-like features typically include notch narrowing², the surgically altered geometry, particularly the widened intercondylar notch yields a shape less characteristic of OA, resulting in a lower B-Score. Longitudinally, however, we observe early osteophyte lipping, particularly in the trochlea, intercondylar notch, and medial-posterior condyle—bone shape changes that align with idiopathic OA and likely explain the steep increase in B-Score for ACLR knees over time (Fig. 2B and C).

CONCLUSION

Neural shape modeling characterizes femoral shape changes due to ACLR surgery. Accounting for surgically induced shape changes enables detection of OA-like features as early as 3 months post-ACLR and enhances sensitivity to track these changes longitudinally, potentially serving as a sensitive biomarker for early detection and monitoring of PTOA.