OBJECTIVES: To determine the relationship between the severity of the tear gap resulting from medial meniscus posterior root (MMPR) tear and medial meniscal extrusion, cartilage, bone, and ligament lesions on MRI. METHODS: A total of 133 patients with MMPR tear were retrospectively evaluated. The patients were divided into two groups according to the tear gap width as minor (≤ 4 mm) and widely (4 mm<) displaced. Medial meniscal extrusion and medial compartmental chondromalacia, bone and ligament lesions were analyzed. RESULTS: There were 61 patients (56 women and 5 men) in the minor displaced group (mean age: 56.3 years, range: 29-82 years) and 72 patients (59 women, 13 men) in the widely displaced group (mean age: 53.2 years, range: 20-86 years). There was no significant difference in terms of age and sex (p = 0.31 and p = 0.09, respectively). The mean absolute extrusion was 3.51 mm (range: 1.5-5 mm) in the minor displaced group and 4.52 mm (range: 2.4-7.2 mm) in the widely displaced group (p < 0.001). High-grade medial femoral condylar chondromalacia was more common in the widely displaced group (p = 0.002). Osteophyte, bone marrow edema, and subchondral cyst in the medial compartment, and ligament injury were higher in the widely displaced group, but were not statistically significant (p > 0.05). CONCLUSION: The amount of medial meniscal extrusion and the prevalence of high-grade medial femoral condylar chondromalacia were found to be significantly more in patients with wider tear gap. Determining the amount of tear gap in the evaluation of root ligament tears on MRI is important to predict knee joint internal derangement.