Effects of combined knee loadings on posterior cruciate ligament force generation.

Resultant forces in the posterior cruciate ligament were measured under paired combinations of posterior tibial force, internal and external tibial torque, and varus and valgus moment. The force generated in the ligament from a straight 100 N posterior tibial force was highly sensitive to the angle of knee flexion. For example, at 90 degrees of flexion the mean resultant force in the posterior cruciate ligament was 112% of the applied posterior tibial force, whereas at 0 degree, only 16% of the applied posterior force was measured in the ligament. When the tibia was preloaded by 10 Nm of external torque, only 9-13% of the 100 N posterior tibial force was transmitted to the posterior cruciate ligament at flexion angles less than 60 degrees; at 90 degrees of flexion, 61% was carried by the ligament. This "off-loading" of the posterior cruciate ligament also occurred when the tibia was preloaded by 10 Nm of internal torque, but only at knee flexion angles between 20 and 40 degrees. The addition of 10 Nm of valgus moment to a knee loaded by a 100 N posterior tibial force increased the mean force in the posterior cruciate ligament at all flexion angles except hyperextension; this represents a common and potentially dangerous loading combination. The addition of 10 Nm of varus moment to a knee loaded by a 100 N posterior tibial force decreased the mean force in the ligament between 10 and 70 degrees of flexion. External tibial torque (alone or combined with varus or valgus moment) was not an important loading mechanism in the posterior cruciate ligament. The application of internal torque plus varus moment at 90 degrees of flexion produced the greatest posterior cruciate ligament forces in our study and represented the only potential injury mechanism that did not involve posterior tibial force.