Influence of Tibial Translation on Estimates of Patellar Tendon Force During Knee Flexion

The mechanics of the knee was analyzed in the sagittal plane to study effects on patellar tendon force as influenced by placement of external flexing loads on the tibia and by translation of the tibial bone 7 mm anterior to the femoral bone during the flexion range 0-120°. Anatomical parameters and measurements for orientations and moment arms of the patellar tendon during the flexion range were estimated from experiments on cadaver knees available in the literature. The analysis suggests that the force in the patellar tendon varies directly with the distance of external flexing load placed distal to the tibial surface. This effect is uniform throughout the joint flexion range. Further, 7 mm anterior translation of the tibia relative to the femur resulted in significantly reduced anterior component of net shear force for all flexion positions and all placements of loads distal to the joint line. This has relevance to ACL-deficient knees where large tibial translations may be necessary to compensate for the deficiency (ACL is anterior cruciate ligament). Two critical factors that may require special attention to protect the ACL during rehabilitation exercises are the flexion angle and the position of flexing load below the tibial surface.