Aspects of arthrology in the lower leg of the opossum.

Abstract

Locomotion of the opossum Didelphis marsupialis is characterized by several rotatory movements of the bones in the lower leg. Some of these movements may be analyzed by manipulation of anatomical specimens simulating the motional sequences of the leg during the step cycle. During the stance phase of the step cycle, from touchdown onwards, external rotation of the femur induces external rotation of the tibia by tautening of the cruciate ligaments of the knee joint. Consequently, this rotational movement is transmitted to the talus by tightening of the intercrossing tibiotalar ligaments. The talar external rotation exerts pulling forces on the meniscoid cruciate ligaments within the fibulotalar joint, which eventually will cause an exorotation of the fibula. As a result of the simultaneous external rotation of the tibia and fibula, the head of the fibula shifts backward with respect to the femur. In the ankle, the dorsolateral shift of the talus imposes inversion of the calcaneus, the tuber calcanei shifting medially. The inversion is, however, not transmitted to the cuboid, since the calcaneus may rotate around the pin-like processus calcaneus of the cuboid. Medially, extension of the talonavicular joint causes progressive elevation of the heel, leading to the late stance phase which is characterised by increasing inversion of the calcaneus and by elevation of the sole of the foot until tiptoe. In the swing phase of the step cycle, after toe-off, the lower legg will internally rotate, during which process the movements in knee- and ankle joint are reversed.