Soft tissue balancing: the knee.

During flexion of the normal knee, the tibia stays within a plane that is aligned anterior-to-posterior and passes near the center of the hip, knee, and ankle. To align the knee during total knee arthroplasty, the distal femoral cuts are aligned in 5 degrees to 7 degrees valgus to the long axis of the femur, and the tibial surface is cut perpendicular to the long axis of the tibia. To align the knee in the flexed position, the femoral surfaces are resected perpendicular to the anteroposterior axis of the femur. After alignment, sizing, and implant positioning are done, only tight ligaments are released. This technique generally results in a knee that is balanced to varus and valgus stresses in flexion and extension, but it often leaves anteroposterior and rotational instability, which may require a more highly conforming tibial component or posterior stabilized knee.