Quantitative magnetic resonance imaging during knee hyperextension for the evaluation of screw-home mechanism abnormalities in patients with anterior cruciate ligament injury: a clinical study.

Background: Anterior cruciate ligament (ACL) injury, a common sports-related knee ligament injury, often leads to screw-home mechanism (SHM) dysfunction and joint instability. Clinical testing and conventional magnetic resonance imaging (MRI) do not provide satisfactory quantitative assessment of functional instability post-acute ACL injury. To improve ACL injury diagnosis and treatment, this study examined the ability of MRI during knee hyperextension to clinically evaluate postinjury SHM abnormalities. Methods: This retrospective study enrolled 56 participants who underwent positional MRI of the knee in hyperextension from June 2020 to June 2025, including 21 controls, 15 patients with acute partial tear of the ACL, and 20 with acute complete rupture of the ACL (acute injury was defined as an injury-to-MRI interval ≤14 days). None of the enrolled patients had knee extension deficit, and standard hyperextension position was maintained during scanning to simulate terminal knee extension. The standardized tibiofemoral translation distances and tibiofemoral rotation angle were measured. One-way analysis of variance was used for intergroup comparison, and receiver operating characteristic (ROC) curve analysis was applied to determine diagnostic efficacy, with P<0.05 indicating statistical significance. Results: In the control group, the tibiofemoral rotation angle and standardized lateral tibiofemoral translation during hyperextension were 6.27±2.35° and 0.016±0.035, respectively, which is consistent with the physiological function of a normal SHM; these two parameters were -4.51±5.88° and 0.073±0.059 in the partial tear group and -6.18±4.21° and 0.103±0.051 in the complete rupture group, respectively. Compared with controls, patients showed significantly elevated standardized lateral tibiofemoral translation and reduced tibiofemoral rotation angle (F=17.218 and F=52.013, respectively; P<0.001). No significant differences in these kinematic parameters were observed between the partial and complete tear groups or between patients with isolated ACL injury and those with concomitant injuries to other knee-joint-stabilizing structures (P>0.05). Standardized medial tibiofemoral translation showed no significant intergroup difference (P=0.269) and had limited diagnostic value. In the ROC curve analysis, the tibiofemoral rotation angle had the highest single-index efficacy in diagnosing ACL injury-related SHM abnormalities [area under the curve (AUC) =0.985; sensitivity =100%; specificity =97.1%; P<0.001]. When this rotation angle was combined with standardized lateral tibiofemoral translation, diagnostic performance further improved (AUC =0.996; P<0.001). The intraclass correlation coefficients for interobserver reliability of all measurements were >0.95 (P<0.001), indicating excellent measurement consistency. Conclusions: By simulating the physiological characteristics of the knee joint in terminal extension, MRI in the hyperextended position can quantify the abnormal kinematic parameters of the tibiofemoral joint after acute ACL injury and accurately evaluate the rotational and translational stability of the joint, with an extremely high sensitivity (100%) and specificity (97.1%) demonstrated by the tibiofemoral rotation angle. This technique is simple, noninvasive, and equal in duration to conventional MRI and is thus a practical, reliable, and objective evaluation method for clinical practice.