MRI-based normative reference values for medial and lateral meniscal extrusion in asymptomatic Korean knees

PurposeTo establish MRI-based normative reference values for medial and lateral meniscal extrusion in asymptomatic Korean adults and to assess compartmental differences using absolute and proportional indices.MethodsA retrospective study of 38 independent knees from asymptomatic adults (mean age, 30.6 ± 4.5 years; 13 male, 25 female) was performed using a single-knee-per-subject protocol. Knees with trauma, ligament injury, significant effusion, meniscal tears, meniscotibial abnormalities, cartilage defects, or bone marrow lesions were excluded. Mid-coronal 1.5-T fat-suppressed proton density MRI was used to measure absolute extrusion and calculate the Meniscal Extrusion Index (MEI). Upper reference values were determined using mean + 2 SD and the 95th percentile. Compartments were compared using paired t-tests.ResultsMean medial meniscal extrusion (MME) was 1.86 ± 0.64 mm (95% CI, 1.65-2.07), with a mean + 2 SD of 3.13 mm and a 95th percentile of 2.84 mm. Mean lateral meniscal extrusion (LME) was 1.40 ± 0.59 mm (95% CI, 1.20-1.59), with a mean + 2 SD of 2.58 mm and a 95th percentile of 2.40 mm. Medial extrusion was greater than lateral extrusion (p = 0.0029; Cohen's d = 0.52). Mean medial and lateral MEI values were 22.5% ± 8.9% and 15.4% ± 9.2%, respectively.ConclusionsMild meniscal extrusion is present in structurally intact knees. Medial values approaching 3 mm and lateral values approaching 2.6 mm may represent normal anatomical variation. Interpretation should incorporate compartment-specific reference limits and proportional indices such as the MEI.