We investigated the importance of the posterior cruciate ligament (PCL) and the medial and lateral compartmental structures for translatory and simultaneous axial rotatory instability in 25 osteoligamentous knee preparations. Instability was registered continuously from 0 degree to 90 degrees of flexion with application of a constant force to the tibia. Isolated transection of the PCL increased the posterior tibial displacement with flexion to a maximum of 10 mm at 90 degrees of flexion; when combined lesions to the lateral structures were included, the popliteal tendon (PT) in particular turned out to have a major secondary stabilizing function. The posterior tibial displacement in flexion was doubled when all lateral structures were included in the lesions. Transection of the PCL and all the medial structures led to a notable increment in posterior displacement increasing with flexion. Major increments in simultaneous tibial rotation were recorded only after combined lesions to either medial or lateral structures. A reverse pivot shift was provoked after combined lateral lesions when the PT was included. Even an anteromedial subluxation was released after lesions to the medial structures. Regardless of the type of lesion, the specimens remained stable concerning anterior-posterior displacement in extension. No changes in the anterior tibial displacement were observed.