Preoperative magnetic resonance imaging of lateral meniscus extrusion likely indicates posterior root tear in anterior cruciate ligament-injured knees.

Background: A lateral meniscal posterior root tear (LMPRT) is a known complication of anterior cruciate ligament (ACL) injury. Complete repair of meniscal posterior root tears is essential for preventing degenerative changes in the knee that may result from meniscal extrusion and rotational instability. This study aimed to assess the relationship between LMPRT morphology and preoperative lateral meniscal extrusion (LME) and to determine the cutoff value for complete LMPRT based on preoperative LME measurements.

Methods: We retrospectively analyzed 403 patients who underwent ACL reconstruction at our hospital between 2011 and 2021. The study finally included 35 patients with LMPRT and 20 with an intact lateral meniscus (LM) as controls. Based on the arthroscopic findings during ACL reconstruction, the 35 patients were divided into two groups: partial (type 1) and complete (types 2-5) LMPRT, in accordance with the LaPrade Classification. The degree of LME was measured using magnetic resonance imaging (MRI).

Results: The partial LMPRT group comprised 17 knees (10 male and 7 female patients), and the complete LMPRT group comprised 18 knees (9 male and 9 female patients). MRI revealed notably greater preoperative LME measurements in cases of complete LMPRT than in cases of both partial LMPRT and intact LM. Receiver operating curve analysis established the optimal cutoff threshold of 2.2 mm for a preoperative LME, with a sensitivity of 78 % and specificity of 71 % for predicting complete LMPRT.

Conclusions: The findings of this study indicate a higher degree of LME in complete LMPRT than in partial LMPRT in knees with ACL injury. Additionally, in patients with ACL injuries, a preoperative MRI measurement of LME ≥2.2 mm suggests a greater likelihood of complete LMPRT.