Physiologic Medial Meniscal Extrusion With Loading and Flexion Detected by Ultrasound

Medial meniscal extrusion (MME) is a risk factor for osteoarthritis of the knee and is usually assessed using a supine, unloaded magnetic resonance imaging. However, protocols for examining meniscal extrusion with ultrasound in the literature vary regarding specific flexion angles and the extent of weightbearing during the examination, complicating interpretations of the influence of loading and position on meniscal extrusion.

The hypothesis was that loading and increased flexion angles would increase physiologic MME relative to supine and neutral positions.

Descriptive laboratory study.

Asymptomatic volunteers with no history of knee pain, injury, or surgery were included. Real-time knee joint angles were obtained by a motion-capture system using retroreflective markers placed on the lower limbs. Ultrasound images of the medial meniscus of both knees were acquired in the coronal plane for supine, single-leg, and double-leg standing positions and with the knee in neutral (0°-5°), 20°, and 45° of flexion. MME was compared across legs, stances, and angles using a repeated-measures mixed-effects linear model, and pairwise comparisons were made using a post hoc Tukey test.

A total of 21 volunteers (age, 33 ± 12 years; body mass index, 23 ± 2.3 kg/m²; 9 women) participated in the study. MME was significantly lower in the supine position compared with single- and double-leg stances (P < .0001, estimated marginal mean MME: supine = 1.1, 1-leg = 1.3, 2-leg = 1.3), with no significant difference between weightbearing stances. Increasing knee flexion angles were associated with significantly lower extrusion (P < .0001, estimated marginal mean MME: neutral = 1.4; 20° = 1.3; 45° = 1), with extrusion greatest at neutral flexion and lowest at 45° of flexion. There was no significant difference in MME between right and left legs (estimated marginal mean MME = 1.2 for both left and right legs; P = .7255).

Our study demonstrated that unloaded, supine examinations of MME may be of limited value. Also, including ultrasound assessment under loaded conditions may better reflect the biomechanical function of the meniscus during activities of daily life.

Examining physiologic meniscal behavior is important to provide context for altered meniscal function in the presence of pathology, which may help inform clinical decisions regarding the management of meniscal degeneration or tears.