Abstract
Objectives:
Medial meniscus posterior root (MMPR) tears can result in rapidly progressing osteoarthritis if left untreated. Prior research has demonstrated that MMPR tears are frequently preceded by medial meniscotibial ligament (MTL) disruption and medial meniscus extrusion. Although meniscus extrusion is a physiologic phenomenon observed after sustained activity, the impact of excessive medial meniscus extrusion resulting from MTL disruption on forces experienced at the MMPR remains uncertain. Moreover, repairing MMPR tears does not always correct medial meniscus extrusion, suggesting that MTL repair or tenodesis may provide some benefit as a concomitant procedure to MMPR repair in terms of restoring native forces. The aim of this study was to assess the relationship between medial meniscus extrusion and MMPR forces in the setting of MTL insufficiency and following MTL tenodesis while subjecting the joint to progressive cyclic loading. We hypothesized that 1) medial meniscus extrusion resulting from MTL insufficiency would increase MMPR forces; 2) MMPR forces and extrusion would both increase as the number of loading cycles increased; and 3) MTL tenodesis would decrease medial meniscus extrusion and restore MMPR forces to native levels.
Methods:
Fifteen pairs of fresh-frozen, cadaveric human knees were organized in a balanced incomplete block design (BIBD) consisting of three testing groups: a native group, a group in which the MTL was disrupted (MTL Cut), and a group in which the MTL was disrupted and then repaired via a peripheral centralization procedure using two knotless all-suture anchors (MTL Tenodesis). Specimens were dissected down to the knee capsule, and the femur/tibia bones were potted. A three-axis force sensor was installed inferior to the MMPR such that forces acting on the MMPR could be measured across three axes. Specimens were then mounted in full extension to a dynamic materials testing machine (Fig. 1), and baseline meniscus extrusion and three-dimensional (3D) force measurements were captured. 3D MMPR forces were measured from the force sensor, and ultrasound (US) was used to image the medial meniscus. Each specimen was subjected to compressive cyclic loading over 10,000 cycles at a 1 Hz frequency ranging from 500 N to 20 N. After the 0th cycle, 100th cycle, 1,000th cycle, and 10,000th cycle, a 500 N load was applied, the medial meniscus was imaged, and 3D forces were measured. Medial meniscus extrusion was measured from the resulting US images. To date, means and standard deviations have been calculated for 11 cadaveric pairs, and Pearson correlations were calculated between outcomes. The full data set will be analyzed with an analysis of variance method corresponding to the BIBD. Pairwise group comparisons will be made among the estimated marginal means (Tukey’s method).
Results:
Eleven pairs of specimens have been tested as of this submission: 6 native, 8 MTL Cut, and 8 MTL Tenodesis. For all states, medial meniscus extrusion increased as the number of loading cycles increased. The MTL Cut state demonstrated increased meniscus extrusion relative to the native state, and MTL tenodesis restored extrusion back to the native level (Fig. 2). In all states, MMPR medial shear force and compression decreased as the number of cycles increased. MMPR medial shear force and compression increased to a greater extent in the MTL Cut state compared to the native state, and MTL tenodesis did not restore shear nor compression forces to native levels (Fig. 3 and Fig. 4). Meniscus extrusion was weakly correlated to MMPR medial shear at lower numbers of cycles (0th: R = 0.2, 100th: R = 0.5) and strongly correlated to MMPR medial shear at higher numbers of cycles (1,000th: R = 0.93, p < 0.001; 10,000th: R = 0.82, p = 0.002).
Conclusions:
As hypothesized, medial meniscus extrusion, MMPR medial shear force, and MMPR compression were higher in the MTL Cut state compared to the native state. While the MTL tenodesis reduced medial meniscus extrusion to the native level, MMPR forces were not restored to native levels. Also in line with our hypothesis, meniscus extrusion increased with progressive loading. Interestingly, MMPR medial shear force decreased with progressive loading cycles, and the Pearson correlation between meniscus extrusion and MMPR medial shear force was stronger at higher levels of cyclic loading. Based on these findings, MTL tenodesis should be considered as an effective treatment option for reducing medial meniscus extrusion both in patients with isolated MTL insufficiency and in the setting of MMPR tears with medial meniscus extrusion. Further research is necessary to determine how the various MTL repair and tenodesis techniques affect MMPR forces. The observed correlation between meniscus extrusion and MMPR medial shear force provides an interesting insight into native meniscal function. As the knee joint undergoes repetitive loading, the medial meniscus undergoes physiologic extrusion, which decreases MMPR medial shear force and may thereby protect the MMPR. Conversely, the MMPR may be at increased risk for injury when the knee undergoes acute loading, and the medial meniscus does not have the opportunity to undergo physiologic extrusion or in the setting of excessive medial meniscus extrusion resulting from MTL insufficiency. This finding also experimentally supports the theoretical assertion that the meniscus, anchored by the roots, transmits femoral compressive loads into circumferential loads.
