Summary

This study investigates the utility of ultrasound in analyzing meniscal extrusion in weight-bearing states rather than the traditional supine position. Meniscal extrusion measurements made under weight-bearing conditions using ultrasound may provide more useful information about meniscal function compared to a supine MRI.

Abstract

Introduction

The current standard for assessing meniscal injuries is MRI. However, meniscal extrusion measured in a supine, unloaded position may not accurately reflect normal or pathologic meniscal behavior. Ultrasonography (US), can relate meniscal pathology to altered joint mechanics and allows for evaluation of meniscal extrusion during loading and flexion. The aim of this study was to examine physiologic medial meniscal extrusion (MME) in both unloaded and loaded states as well as between different knee flexion angles in asymptomatic volunteers of different age groups.

Methods

This prospective study was approved by an IRB and included twenty volunteers with asymptomatic knees and no history of knee injury or surgery (33 ± 12 years, BMI 23.0 ± 2.3 kg/m2, 9 females). Ultrasound imaging of the medial meniscus of both knees was performed by an orthopedic surgeon with 6 years of experience with musculoskeletal ultrasonography using an Aplio i800 system (Canon Medical Systems, USA) with an 18 MHz linear array transducer. The transducer was placed in the coronal plane parallel to the medial collateral ligament to measure medial meniscal body extrusion in supine and weight-bearing positions using the medial collateral ligament as a landmark. Ten retroreflective markers were placed for real-time assessment of joint angles using a 16-camera video-motion capture system. Three ultrasound images were taken in each position and joint angles were captured simultaneously (supine position at individual neutral (0-5°), 20°, and 45° flexion, standing on two legs with balanced weight in a neutral position (0-10° flexion) and squatted to 20° and 45° flexion). This procedure was repeated with participants standing on a single leg at a time, alternating between left and right legs. MME was measured on each image using a custom MATLAB script and averaged for each position.

Results

There were significant relationships between MME and age (p < 0.001), stance (p < 0.001), and knee flexion angle (p < 0.001) but not between right and left legs (p = 0.257). Increasing knee flexion angle was associated with significantly lower MME, with the greatest extrusion in a neutral angle compared to 20 and 45? of flexion. Further, MME was significantly lower in the supine position compared to MME in single- and double-leg stances but there was no difference between the two weight-bearing stances.

Discussion

Weight-bearing on 1 or 2 legs increased meniscal extrusion compared to the supine, unloaded position. Meniscal extrusion measurements are highly correlated between MRI and ultrasound in a supine, unloaded position; however, this position may not allow for the best assessment of meniscal function. These findings suggest that MME may be most evident in an upright weight-bearing position with the knees in a neutral position. While the 1-leg stance trended toward higher MME in each position, both a 1- and 2-leg stance may provide similar information about meniscal behavior under load. Standing on both legs allows for even weight distribution and provides an alternative position for patients with difficulties in weightbearing or painful knees, such as in cases with meniscal tears.