To determine the MRI-based threshold of lateral meniscal body extrusion (LMBE) that are associated with meniscal damage, cartilage damage and radiological knee osteoarthritis (OA).

A total of 142 patients (59 men and 83 women) with a mean age of 57.2±7.9 (SD) years (range: 41–77 years) with symptomatic knee OA were included. Radiological assessment was performed using the Kellgren-Lawrence scoring system. Meniscus and cartilage damage were assessed using the whole-organ magnetic resonance imaging score. Meniscal extrusion was quantified on coronal sections of intermediate-weighted MRI sequences. Differences between medial and lateral compartments in meniscal extrusion and incidence of tibiofemoral OA-related structural changes were assessed using Wilcoxon signed rank test and Bowker test. Receiver operating characteristic curves and Youden index were used for determining thresholds for meniscal extrusion.

A total of 142 knees were assessed. Meniscal body extrusion distances between medial and lateral compartments were significantly different in the entire sample, and in subjects with and without radiological knee OA (P<0.05 for all). The incidence of structural changes between medial and lateral compartments were significantly different (P=0.003 for meniscal damage; P=0.001 for femoral cartilage damage). Three mm and 2mm were determined to be the optimal thresholds for medial and lateral meniscal body extrusion, respectively.

Medial and lateral meniscal body extrusion were associated with the incidence of OA-related knee structural changes in symptomatic patients with knee OA. A threshold of 2mm for LMBE with respect to meniscal damage, cartilage damage and radiological knee OA was determined.