The infra-meniscal fibers of the anterolateral ligament are stronger and stiffer than the supra-meniscal fibers despite similar histological characteristics.

PURPOSE: The purpose of the current investigation was to characterize biomechanical differences between the supra- and infra-meniscal sections of the anterolateral ligament (ALL). We hypothesized that the supra-meniscal fibers of the ALL would be stronger and stiffer than the infra-meniscal fiber.

METHODS: Nine cadaveric knee specimens [mean (SD) age = 79 (14.6) years] were dissected to identify the borders of the ALL while maintaining the anatomy of the lateral meniscus. The specimens were randomly assigned to either a supra-meniscal (the ALL below the meniscus was sectioned leaving only the supra-meniscal ALL intact) or an infra-meniscal (the ALL above the meniscus was sectioned leaving only the infra-meniscal attachment intact) group. The specimens were potted into dental cement such that the ALL was pulling laterally on the meniscus when the specimens were secured within an Instron materials testing machine. The specimens were subjected to a tensile failure test at 1 mm/s. The load at failure and stiffness were calculated from the force-displacement curves, while peak stress was calculated by normalizing the peak force to the cross-sectional area of the ALL. Furthermore, one intact knee specimen was used to perform a histological analysis on the two ALL sections using Masson's Trichome staining.

RESULTS: The infra-meniscal ALL had a significantly (p = 0.03) higher load to failure (195.0 vs. 132.1 N) and was significantly (p = 0.03) stiffer than the supra-meniscal fibers (24.8 vs. 12.3 N/mm). The relatively similar cross-section areas also resulted in the infra-meniscal sections having a greater peak stress (p = 0.04) (11.1 vs. 5.4 MPa). Histological analysis showed relatively consistent fiber orientation with similar organization noted throughout the fibers.

CONCLUSIONS: The ALL-meniscal construct that includes the infra-meniscal fibers was significantly stronger and stiffer than the construct that includes the supra-meniscal fibers. The infra-meniscal ALL is another important component of the anterolateral complex of the knee, and should be considered when presented with an ACL and/or meniscal injury.