The restraining function of the cruciate ligaments on hyperextension and hyperflexion of the human knee joint.

Each of the cruciate ligaments contains functionally different fiber groups; one fiber bundle is always taut; numerous others are taut in intermediate or extreme positions. The bulk of the fibers of the anterior cruciate ligament (ACL) is taut in maximal extension, while that of the posterior cruciate ligament (PCL) is taut in the intermediate positions and in maximal flexion. Fibers taut in extreme positions serve as restraints: during hyperextension, the ACL restrains forward migration of its tibial attachment, while the PCL interacts with other structures to prevent posterior opening of the joint. The inverse situation occurs in hyperflexion. Cruciate fibers are dissimilar in length and angular arrangement so that, when movements are restrained, they lengthen to different extents. To define this phenomenon in quantitative terms, the term isokolyons was coined for lines from which fibers showing identical elongation in percentage on exposure to a force take their origin.