BIOMECHANICAL PRINCIPLES IN DIARTHROSIS A ND SYNARTHROSIS .3. MECHANICS OF THE TIBIOFEMORAL JOINT AND THE 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) d etermine the kinematics as well as the quality and extent of static st ability of the knee. The mechanism of the TFJ is derived from the anat omical shape of the articulating surfaces. In antero-posterior directi on the joint guidance is structurally given by a stretched and overlap ped dimeric link chain in lateral and medial region, respectively. Alt ogether, the two chains are linked up to a four-bar-chain (link quadra ngle) that solely allows the tibia to strike backwards. The extent of individual extension can be rejected to morphological data of the femo ral condylus. In squat position the extent of mechanical stability of the joint can be changed and even reversed to instability by a rotatio n of the resulting compressive joint force around the momentary rotati onal axis of the gear system while the joint position remains unaltere d. Thus e.g. the process of straightening up is structurally explained . The cruciate ligaments do not bear any direct mechanical guiding fun ction. They represent a sensor system which structurally resembles a m echanical bridge circuit. Beside detecting the degree of flexion it is able to monitor the indispensable contacting of the articulating surf aces. The menisci represent an additional, similarily working sensor s ystem. At first approximation these two sensor systems are aligned in two planes which are perpendicular. Therefore they form a spatial sens or system. The presented theory is derived from mechanical, morphologi cal, and physiological findings and gets evidence by measurements on k nee loads during seated cycling.