Microstructure of the ligament-to-bone attachment complex in the human knee joint

Clinical and experimental studies have shown that injuries in the human knee ligaments occur in the ligament midsubstance, at the transition between bone and ligament, and in the bone in the vicinity of the ligament-to-bone attachment site. Whereas ligament and bone have been thoroughly described, the way they connect to each other remains unclear. The goal of this study is to provide a description of the microstructure of the ligament-to-bone insertion, with the view of providing a mechanical model capable of predicting the injuries that occur at this insertion. The preparatory literature review showed that there was no description of the insertion microstructure for the human ligaments. The results found for human tendons and animal tendons/ligaments were used to lead the histological and electron - scanning and transmission - microscopy analysis. The posterior cruciate ligament (PCL), and the lateral collateral ligament (LCL) were sampled from one post mortem human subject. Slices were cut along the longitudinal direction of the ligaments, following the fibers direction. The histology analysis showed that the insertion has the same structure as reported in the literature: it is made of a mineralization front between calcified and uncalcified fibrocartilage, which is not crossed by the ligament fibers. The transmission electron microscopy analysis of the calcified fibrocartilage revealed a collagenous structure which has a direction drastically different from the direction of the ligament fibers. The mechanical function of the insertion was discussed and combined with the histological findings to hypothesize the microstructure of the insertion.