Pre- and postoperative 4D CT analysis of thumb CMC joint kinematics after first metacarpal extension osteotomy

INTRODUCTION: First metacarpal extension (Wilson) osteotomy combined with modified Brunelli ligamentoplasty is performed for early thumb carpometacarpal (CMC1) osteoarthritis to improve pain and function. The purpose of this study was to quantify in vivo changes in CMC1 joint contact distribution and kinematics after this combined procedure and to determine whether postoperative patterns resemble those of healthy volunteers. We hypothesized that the procedure would reduce cumulative palmar contact during opposition-retroposition. MATERIALS AND METHODS: In a prospective single-center observational cohort study, six adults with early CMC1 osteoarthritis (Eaton-Littler I-II) underwent standardized preoperative and 6-month postoperative four-dimensional computed tomography (4D CT) of the CMC1 joint during active opposition-retroposition. Five healthy volunteers served as reference with a single 4D CT scan. First metacarpal-trapezium motion was registered across frames. Joint contact was estimated using a 2.0 mm proximity surrogate on the trapezial surface. The primary outcome was change in cumulative palmar contact area. Secondary outcomes included change in dorsoradial contact, temporal variability of contact curves, and contact-centroid path length and mean position. Within-subject pre- versus postoperative comparisons were performed; healthy scans provided reference patterns. RESULTS: Healthy volunteers showed reproducible patterns with increasing palmar contact toward terminal opposition and near-mirror symmetry on retroposition. Preoperatively, patients demonstrated elevated palmar contact persisting into retroposition and loss of symmetry. Postoperatively, the procedure did not consistently reduce cumulative palmar contact. Instead, most patients exhibited increased palmar contact, flattening of contact-versus-time curves, and a shorter contact-centroid path with a palmar-ulnar shift in mean position. These changes suggest a stabilizing effect more consistent with functional partial arthrodesis than selective palmar unloading. CONCLUSION: Wilson osteotomy with modified Brunelli ligamentoplasty did not produce the anticipated palmar unloading during functional thumb motion. The dominant postoperative effect was palmar loading with reduced dynamic variability. These findings refine the biomechanical rationale for this joint-preserving procedure and support future studies isolating the osteotomy from ligamentoplasty using load-standardized tasks. LEVEL OF EVIDENCE: Therapeutic, IV.