The Tibial Slope–Meniscal Coupling Index (TMSI): A novel geometric MRI parameter predicting rotational stability after ACL reconstruction

PURPOSE: Residual anterolateral rotatory instability after anterior cruciate ligament reconstruction (ACLR) remains incompletely explained by graft-related parameters alone. The purpose of this study was to define and validate the Tibial Slope-Meniscal Coupling Index (TMSI), a quantitative magnetic resonance imaging (MRI) parameter integrating tibial slope and meniscal geometry, as a predictor of residual anterolateral rotatory instability after ACLR. METHODS: A retrospective cohort of 140 patients ≥12 months after primary ACLR underwent standardized 1.5- or 3-T MRI evaluation. Medial and lateral posterior tibial slope (PTS) and posterior horn displacement (PHd) of the lateral meniscus were measured to calculate the TMSI = (mean PTS × PHd)/tibial width. Clinical outcomes included pivot-shift grade, KT-1000 side-to-side difference and patient-reported outcome scores. Receiver operating characteristic (ROC) analysis and multivariable logistic regression assessed predictive performance. RESULTS: Patients with residual pivot (n = 59, 42%) demonstrated higher PTS, greater PHd and significantly higher TMSI (0.7 ± 0.1 vs. 0.4 ± 0.1; p < 0.001). A TMSI > 0.6 predicted residual pivot with an AUC of 0.8 (95% confidence interval [CI], 0.8-0.9), sensitivity 86% and specificity 82%. In multivariable analysis, TMSI was the only variable that remained independently associated with residual pivot instability (odds ratio [OR], 5.6; p < 0.001). Interobserver intraclass correlation coefficient was 0.92. CONCLUSION: Clinically, the TMSI may assist in identifying patients at increased risk of persistent rotational instability and support decision-making regarding adjunctive stabilization procedures. LEVEL OF EVIDENCE: Level III.