Scrutinizing the cut-off for "pathological" meniscal body extrusion on knee MRI.

OBJECTIVES: Medial meniscal body extrusion ≥ 3 mm on MRI is often considered "pathologic." The aims of this study were to (1) assess the adequacy of 3 mm as cut-off for "pathological" extrusion and (2) find an optimal cut-off for meniscal extrusion cross-sectionally associated with radiographic knee osteoarthritis, bone marrow lesions (BMLs), and cartilage damage. METHODS: Nine hundred fifty-eight persons, aged 50-90 years from Framingham, MA, USA, had readable 1.5 T MRI scans of the right knee for meniscal body extrusion (measured in mm). BMLs and cartilage damage were read using the whole organ magnetic resonance imaging score (WORMS). Knee X-rays were read according to the Kellgren and Lawrence (KL) scale. We evaluated the performance of the 3-mm cut-off with respect to the three outcomes and estimated a new cut-off maximizing the sum of sensitivity and specificity. RESULTS: The study persons had mean age of 62.2 years, 57.0% were women and the mean body mass index was 28.5 kg/m2. Knees with radiographic osteoarthritis, BMLs, and cartilage damage had overall more meniscal extrusion than knees without. The 3-mm cut-off had moderate sensitivity and low specificity for all three outcomes (sensitivity between 0.68 [95% CI 0.63-0.73] and 0.81 [0.73-0.87], specificity between 0.49 [0.45-0.52] and 0.54 [0.49-0.58]). Using 4 mm maximized the sum of sensitivity and specificity and improved the percentage of correctly classified subjects (from between 54 and 61% to between 64 and 79%). CONCLUSIONS: The 4-mm cut-off may be used as an alternative cut-off for denoting pathological meniscal extrusion. KEY POINTS: • Medial meniscal body extrusion is strongly associated with osteoarthritis. • The 3-mm cut-off for medial meniscal body extrusion has high sensitivity but low specificity with respect to bone marrow lesions, cartilage damage, and radiographic osteoarthritis. • The 4-mm cut-off maximizes the sensitivity and specificity with respect to all three osteoarthritis features.