[Biomechanical principles of diarthroses and synarthroses. III: Mechanical aspects of the tibiofemoral joint and role of the cruciate ligaments].

The tibiofemoral joint (TFJ) is force-locked. It takes its function as it is compressively loaded. The geometrical shape of the articulating surfaces and the acting force system (given by muscles and gravity) determine the kinematics as well as the quality and extent of static stability of the knee. The mechanism of the TFJ is derived from the anatomical shape of the articulating surfaces. In antero-posterior direction the joint guidance is structurally given by a stretched and overlapped dimeric link chain in lateral and medial region, respectively. Altogether, the two chains are linked up to a four-bar-chain (link quadrangle) that solely allows the tibia to strike backwards. The extent of individual extension can be rejected to morphological data of the femoral condylus. In squat position the extent of mechanical stability of the joint can be changed and even reversed to instability by a rotation of the resulting compressive joint force around the momentary rotational axis of the gear system while the joint position remains unaltered. Thus e.g. the process of straightening up is structurally explained. The cruciate ligaments do not bear any direct mechanical guiding function. They represent a sensor system which structurally resembles a mechanical bridge circuit. Beside detecting the degree of flexion it is able to monitor the indispensable contacting of the articulating surfaces. The menisci represent an additional, similarly working sensor system. At first approximation these two sensor systems are aligned in two planes which are perpendicular. Therefore they form a spatial sensor system. The presented theory is derived from mechanical, morphological, and physiological findings and gets evidence by measurements on knee loads during seated cycling.