In vivo dynamic intrusion and extrusion of the menisci in varus and valgus load within a healthy population

Purpose: Axial loading, varus and valgus stress lead to meniscal motion towards the joint periphery, defined as meniscal extrusion. Direction and amount of extrusion is unknown as this is a dynamic process within a 3D environment dependent on joint loading as well as individual anatomy. We propose that there is motion in all compartments of the medial and lateral meniscus during valgus and varus stress.
Method: MRI scans of 31 healthy subjects in varus or valgus stress positions were acquired with the help of a pneumatic loading device. Semiautomatic segmentation of the menisci, the femur and the tibia (with corresponding cartilages) was carried out. An individual 3D model of the joint was generated. The meniscal movement was calculated within a tibia-based coordinate system and broken down into total and partial meniscal movement (anterior/posterior horn, intermediate part).
Results: Under valgus load the medial meniscus (MM) showed average movement of 1.5 (±0.5) mm in lateral-posterior direction with most lateral motion of 1.4 (±0.7) mm in the intermediate part. The lateral meniscus averaged 1.6 (±1.0) mm in lateral-anterior motion, exhibiting maximal lateral motion in the anterior horn (AH) 0.7 (±0.8) mm and posterior horn 0.6 (±0.6) mm. In response to the varus load, average MM motion was 0.9 (±0.5) mm in medial-anterior direction with the largest medial movement in the AH 0.9 (±1.1) mm. The lateral meniscus moved in average 1.6 (±0.8) mm into lateral-posterior direction with the intermediate part showing the largest medial motion of 0.6 (±0.4) mm.
Conclusion: In a healthy population, the menisci extrude up to 1.5 mm during varus and valgus loading. The anterior and posterior horn show greater dynamic extrusion than the intermediate part. However, an in vivo dynamic intrusion mechanism of meniscus when discharged (medial 1.45 mm, lateral 1.56 mm) could be demonstrated. Quantification and reconstruction of this phenomenon might be of crucial importance during meniscal root or meniscal transplantation surgery. Level of Evidence: Level II, descriptive laboratory study.