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

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. A retrospective cohort of 140 patients &#x2265;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&#x2009;=&#x2009;(mean PTS&#x2009;&#xd7;&#x2009;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. Patients with residual pivot (n&#x2009;=&#x2009;59, 42%) demonstrated higher PTS, greater PHd and significantly higher TMSI (0.7&#x2009;&#xb1;&#x2009;0.1 vs. 0.4&#x2009;&#xb1;&#x2009;0.1; p&#x2009;<&#x2009;0.001). A TMSI&#x2009;>&#x2009;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&#x2009;<&#x2009;0.001). Interobserver intraclass correlation coefficient was 0.92. Clinically, the TMSI may assist in identifying patients at increased risk of persistent rotational instability and support decision-making regarding adjunctive stabilization procedures. Level III.