Diagnostic Performance of MRI for Posterolateral Meniscocapsular Detachment in ACL-Injured Knees: A Retrospective...

Background/Objectives: Posterolateral meniscocapsular detachment represents a potentially underrecognized component of knee trauma in patients with anterior cruciate ligament (ACL) injury. Altered rotational kinematics in ACL-deficient knees may increase stress on posterolateral corner structures. Although an association between ACL tears and posterolateral abnormalities has been reported, the true diagnostic performance of magnetic resonance imaging (MRI) for these lesions remains incompletely defined. This study aimed to evaluate the diagnostic accuracy of MRI for posterolateral meniscocapsular detachment in patients with confirmed ACL injury, to describe associated meniscal tears, and to assess the association between lateral tibial plateau bone marrow edema, age, and sex with the presence of detachment. Methods: In this retrospective study, 117 knee MRI examinations (1.5 T and 3 T) of patients with native ACL rupture or ACL graft failure were reviewed. Two readers independently assessed posterolateral meniscocapsular detachment. Ramp lesions, other meniscal tears and lateral compartment bone marrow edema were evaluated in consensus. Arthroscopy served as the reference standard. Diagnostic performance and inter-observer agreement were calculated. Detachment was considered present only when both readers independently identified the lesion. Results: MRI demonstrated high specificity (90.8%; 95% CI: 83.7-95.5%) for posterolateral meniscocapsular detachment, and promising sensitivity (87.5%; 95% CI: 47.3-99.7%) based on a small number of confirmed cases (n = 8); this estimate should be interpreted with caution given the wide confidence interval. One arthroscopically confirmed lesion was missed, and false-positive findings were limited. MRI also showed good diagnostic performance for ramp lesions and other associated meniscal tears, assessed as secondary endpoints. Lateral tibial plateau bone marrow edema, age, and sex were not significantly associated with detachment. Conclusions: Routine MRI shows high specificity and promising, although preliminary, sensitivity for detecting posterolateral meniscocapsular detachment in ACL-injured knees; given the small number of confirmed cases, this estimate warrants confirmation in larger cohorts. Careful multiplanar assessment and detailed anatomical knowledge are essential to optimize detection. Accurate preoperative MRI characterization may help guide surgical planning and reduce the risk of missed posterolateral meniscocapsular pathology at arthroscopy.