Characterization of the mechanical properties and mineral distribution of the anterior cruciate ligament-to-bone insertion site.

The anterior cruciate ligament (ACL) connects the femur to the tibia through direct insertion sites and functions as the primary restraint to anterior tibial translation. The ACL-to-bone insertion sites exhibit a complex structure consisting of four zones of varied cellular and matrix components, consisting of ligament, non-mineralized fibrocartilage, mineralized fibrocartilage and bone, which allow for the effective load transfer from ligament to bone, thereby minimizing stress concentrations and preventing failure. The mineral content and distribution within the fibrocartilage region may be an important structural component of the insertion site which may influence the mechanical properties. The goals of this study are to characterize the compressive mechanical properties of the fibrocartilage region of the ACL-to-bone insertion site and evaluate how the mineral distribution at the interface relates to these compressive properties. In order to determine the compressive mechanical properties we have utilized a novel microscopic mechanical testing method combined with digital image correlation and employed energy dispersive X-ray analysis (EDAX) in order to evaluate the mineral content and distribution across the femoral and tibial insertion sites. The results reveal that a regional mineral gradient is observed across the fibrocartilage which corresponds to depth-dependent variations in compressive mechanical properties. This depth- dependent mechanical inhomogeneity strongly correlates to the increase in mineral content of the mineralized fibrocartilage (MFC) region compared to the non-mineralized fibrocartilage (NFC). Additionally, the tibial NFC and MFC mechanical properties are greater than those of the femoral NFC and MFC which corresponds to a greater mineral content in the NFC and MFC regions of the tibial insertion. The findings of this study suggest that a structure-function relationship exists at the ACL-to-bone interface.