FUNCTION OF THE MEDIAL MENISCUS & IMPLICATIONS FOR REPLACEMENT

Introduction The major function of the medial meniscus has been shown to be distribution of the load with reduction of cartilage stresses, while its role in AP stability has been found to be secondary. However several recent studies have shown that cartilage loss in OA occurs in the central region of the tibia while the meniscus is displaced medially. In a lab study (Arno, Hadley 2013) it was confirmed that the AP laxity was greatly reduced with a compressive force across the knee, while the femur shifted posteriorly and the AP laxity was increased after a partial meniscetomy of the posterior horn. It is therefore possible that under load, the compression of the meniscus and the cartilage, 2–3mm in total, allows load transmission on the central tibial plateau, and causes radial expansion and tension of the meniscus providing restraint to femoral displacements. This leads to our hypotheses that the highest loading on the medial meniscus would be at the extremes of motion, rather than in the mid-range, and that the meniscus would provide the majority of the restraint to anterior-posterior femoral displacements throughout flexion when compressive loads were acting. Methods & Materials MRI scans were taken of ten knee specimens to verify the absence of pathology and produce computer models. The knees were loaded in combinations of compressive and shear loading over a full flexion range. Tekscan sensors were used to measure the pressure distribution across the joint as the knee was flexed continuously. A digital camera was used to track the motion, from which femoral-tibial contacts were determined by computer modelling. Load transmission was determined from the Tekscan for the anterior horn, central body, posterior horn, and the uncovered cartilage in the center of the meniscus. An analysis was carried out (Fig 2) to determine the net anterior or posterior shear force carried by the meniscus. Results For the three types of loading (Fig 1); compression only, compression and anterior shear, compression and posterior shear; between 40–80% of the total load was transmitted through the meniscus, the overall average being 58%. The remaining 42% was transmitted directly through the uncovered cartilage. The anterior horn was loaded only up to 30 degrees flexion, and played a role in controlling anterior femoral displacement. The central body was loaded 10–20% and would provide some restraint to medial femoral subluxation. Overall the posterior horn carried the highest percentage of the shear load (Figure 3), especially after 30 degrees flexion when a posterior shear force was applied, for which the meniscus was estimated to carry 50% of the shear force. Discussion The hypotheses were largely supported. There was high anterior horn loading in early flexion, but in the remaining range, the posterior horn was the highest loaded especially under posterior shear. Supporting the posterior shear force under load bearing conditions is evidently an important role of the meniscus. Hence in any attempts at repair or replacement, these dual functions of load-sharing and stability need to be incorporated.