The interrelation of fiber bundles in the anterior cruciate ligament.

The anterior cruciate ligaments (ACL) of dogs, humans, and rabbits were studied by light and scanning electron microscopy after fixation in situ. In all species, the ACL was composed of multiple 20 microns-wide collagen fiber bundles separated by columns of cells in fibrous capsules. These bundles were in turn grouped into fascicles of varied size. The fascicles were surrounded by thin membranous sheets that ran through the ligament forming single or multiple layers between fascicles. Splaying of the ACL at insertion was created by increased volume in the cellular intervals. Bending of the fiber bundles occurred in this region--which corresponds to the fibrocartilaginous zone. We propose that the cell layers accommodate compressive forces and the membranes allow slipping among fascicles without compromising blood supply.