Medial meniscus extrusion increases with age and BMI and is depending on different loading conditions

Meniscus extrusion has always been described as an indirect sign of meniscus pathology and is associated with a loss of function of the affected meniscus. The current cut-off value of 3 mm displacement is indicated as abnormal and has been determined on magnetic resonance images (MRI) and ultrasound (US). However, it has to be considered that there is no description of the physiological meniscus extrusion in healthy knees depending on age or different weight-bearing conditions. It was hypothesized that in healthy knees there is a physiological age and BMI dependent meniscal extrusion, and meniscus extrusion depends on different loading conditions.

Healthy volunteers with non-symptomatic knee, and no history of knee injuries or operations were included in this prospective cross-sectional study. Exclusion criteria were age < 18years, subjective or objective instability, malalignment and positive medial meniscus test. Secondary exclusion criteria were osteoarthritis ICRS grade 3-4 or signs of meniscus tear on MRI. Every patient underwent standard knee examination following measurement of medial meniscus extrusion (MME) using US. In US extrusion was determined in supine position (unloaded) and in standing position with full weight-bearing and 20° of flexion (loaded). MRI was performed in a neutral knee position to compare ultrasound measurements with the current gold standard. Based on the power calculation of preliminary results a minimum of 70 volunteers was needed.

75 patients were enrolled to this study. The mean US MME was 1.1 mm ± 0.5 mm in supine position and 1.9 mm ± 0.9 mm under full weight-bearing. The mean US Δ-extrusion was 0.8 mm ± 0.6 mm. With rising age, a significant increased MME in US and MRI could be demonstrated (p < 0.001). Furthermore, elevated BMI was significantly correlated to increased US MME under full weight-bearing (p = 0.002) and to US Δ-extrusion (p = 0.003).

Based on the results of this study, medial meniscus extrusion is an age-depending phenomenon in healthy knees and depends on various load-bearing conditions. Ultrasound examination of the MME might be favorable compared to MRI due to the ability of dynamic evaluation. As a consequence, the current cut-off value of 3 mm for meniscus pathologies should be reconsidered.

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