EFFECT OF LATERAL MENISCUS POSTERIOR ROOT TEARS ON CARTILAGE AND MENISCAL MECHANICS

INTRODUCTION

Measuring cartilage and meniscal mechanics in loaded knees is essential to understanding the effects of lateral meniscus posterior root tears (LMPRTs) and the effectiveness of meniscal repair procedures that seek to protect the joint from degeneration. Studies have assessed mechanics with thin-film pressure sensors or finite element models, but their conclusions are limited by the invasiveness or inherent assumptions of the techniques employed. Ultra-high field MRI provides sufficient resolution to measure cartilage and meniscal mechanics during loading in a compatible loading device, without requiring disruption or simulation of the articulating joint surfaces. However, no studies have evaluated the impact of LMPRTs on the cartilage and meniscal mechanics in a human cadaveric knee using such a method.

OBJECTIVE

Test the hypothesis that LMPRTs increase femoral and tibial cartilage strain and meniscal extrusion.

METHODS

Six human knee lateral compartments (mean age 70 yrs) were tested. Anatomical alignment in full extension was maintained during preparation. The lateral meniscus and its roots, meniscotibial ligament, and attachment to the popliteus, as well as the ACL, were preserved. Specimens were placed in a novel pneumatic compression apparatus customized for use a 9.4T MRI scanner. Morphologic scans with a resolution of 0.06 × 0.12 × 0.4 mm were acquired before loading and after 2 hours of loading (Figure 1). The load applied was constant and equivalent to 48% body weight to simulate two-legged standing. An artificial LMPRT was then created, and specimens were left unloaded until testing the next day with the same protocol. Joint tissues were manually segmented for both intact and LMPRT conditions, in both loaded and unloaded states. Flattened cartilage profiles were generated to calculate cartilage strain in the axial direction, with negative strain indicating compression. The mean and maximum strains in the tibiofemoral contact area were determined in both the femoral and tibial cartilage. Meniscal extrusion was measured as the perpendicular distance between the external edge of the meniscus and the line bisecting the external edge of the tibial plateau and femoral condyle in the most anterior slice of the popliteus’ insertion. All measures were compared between conditions with paired Student’s t-tests with significance set to 0.05.

RESULTS

Maximum compressive strain in the tibiofemoral contact region of the femoral (p = 0.013) and tibial (p = 0.010) cartilage increased significantly after the LMPRT (Figure 2). The increase in mean compressive strain in the tibiofemoral contact region after the LMPRT was not significantly different for the femoral (p = 0.103) or tibial (p = 0.065) cartilage. Likewise, the increase in meniscal extrusion after the LMPRT was not significantly different (p = 0.143). Specimens with a greater increase in meniscal extrusion after the LMPRT tended to have a greater increase in maximum cartilage strain after the LMPRT.

CONCLUSION

Increases in maximum cartilage strain after LMPRT reflect higher cartilage stress, which is associated with cartilage degeneration. Our finding of more meniscal extrusion in specimens with greater increases in cartilage strain highlights a potential relationship between cartilage and meniscal mechanics, as well as the importance of restoring normal meniscal mechanics through a LMPRT repair. A key advantage of this approach to studying knee mechanics is the ability to simultaneously assess meniscal and cartilage mechanics with minimal disruption to the alignment and critical soft tissue. The approach has potential for assessment of the effectiveness of meniscal repair techniques.