The knee joint of the flying fox (Pteropus rufus).

As the flying fox (Pteropus rufus) knee does not exhibit any sign of allowing rotation, the aim of this study was to evaluate the factors preventing rotation. Menisci are not present. Yet, the femoral and tibial articular surfaces are not anatomically congruent as the kinematic mechanism of the knee is not based on a pure hinge but on a "four-bar linkage" construction, which consists of femur, tibia and the guiding bundles of the cruciates. Therefore, the cruciate ligaments have the same function as in man, i.e., they contain extension and flexion restricting fibers, fibers taut in intermediate positions and a guiding bundle which is constantly taut. Also in the collateral ligaments these fiber groups can be identified and hence parts of them are taut throughout the entire range of motion (5 degrees hyperextension, 170 degrees flexion). These parts prevent rotation together with the femoral and tibial articular surfaces, the latter of which are concave.