Biomechanical implications of transverse centers of rotations of normal and pathologic knees (joints, biomechanics, axis)
Description
The concept of instant centers and axes of rotation were used to elucidate the biomechanical aspects of transferring part or all of the biceps femoris tendon. The centers were determined in two ways. The longitudinal center of rotation (for internal and external knee rotations) was determined by digitizing sequential CAT scans of the four in vitro knees at 0, 30, 60 and 90 degrees of flexion with the joint positioned at full internal, neutral and full external rotation. These views were obtained before and after anterior cruciate ligament (ACL) sacrifice. Results from a computer algorithm which determined the centers indicated that they were dispersed about the tibial spine approximately between the insertions of the cruciate ligaments. Following ACL sacrifice, the dispersion doubled in the coronal direction while staying fairly constant in the sagittal direction The sagittal (flexion-extension) and longitudinal (internal-external) axes were determined for in vivo and in vitro knees utilizing a mechanical axis finder. Each axis was demonstrated by minimizing the precession of a freely locatable axle. Subsequent to axes demonstration, all knees were photographed and radiographed. The average in vitro longitudinal axis was located in the mid sagittal plane, passing near to the tibial origin of the ACL. The sagittal in vitro axis passed through the femoral origins of the collateral ligaments. The axes position of the in vivo knees supported these average locations The biomechanical aspects of the biceps transfers were determined for four fresh, unembalmed extremities. The rotational torques and flexural forces were measured before and after the transfer with the knees positioned at 10, 20, 30 and 45 degrees of flexion, in neutral and 5(DEGREES) internal rotation. The applied load to the biceps was a ramp function (66.75 N in 15 seconds). The partial transfer resulted in an average decrease of 2% and 15% in the rotational and flexural capacity of the biceps respectively. Hence the functional lever arm of the transfer tendon about the longitudinal axis is essentially unchanged, while the functional lever arm for sagittal motion is decreased. With the total transfer the rotational capacity increased 28% while the flexural capability was decreased an average of 75%. The total transfer resulted in the biceps becoming an extensor at 10 and 20 degrees of flexion. Thus, the functional lever arm about the sagittal axis is markedly reduced while the function lever arm about the longitudinal axis is augmented as compared to before the transfer. The total and partial biceps femoris transfers were not found to be effective ancillaries in the treatment of anterolateral rotary instability