Abstract
Introduction
After total knee arthroplasty (TKA) with a PCL-retaining implant the location of the tibiofemoral contact point should be restored in order to obtain normal kinematics. The difficulty during surgery is to control this location since the position of the femur on the tibia cannot easily be measured from the back of the joint. Therefore, we developed a simple “spacer technique” to check the contact point indirectly in 90° flexion after all bone cuts are made by measuring the step-off between the distal cut of the femur and the anterior edge of the tibia with a spacer in place. The goal of this experiment was to investigate whether this new PCL balancing approach with the spacer technique created the correct contact point location.
Methods
Nine fresh-frozen full leg cadaver specimens were used. After native testing, prototype components of a new PCL-retaining implant were implanted using navigation and a bone-referenced technique. After finishing the bone cuts of tibia and femur, the spacer was inserted in flexion and positioned on the anterior edge of the bony surface to measure the step-off. If necessary, an extra cut was made to balance the PCL.
The specimen was mounted on the knee kinematics rig and a squat with constant vertical ankle force (130N) and constant medial and lateral hamstrings forces (50N) was performed between 30° and 130° of knee flexion. The trajectories of the reflective tibial and femoral markers were continuously recorded using six infrared cameras. The projections of the femoral condylar centers on the horizontal plane of the tibia were calculated and compared.
Results
Of the 9 specimens, the calculated step-off was correct in 7 after finishing the bone cuts and in 2 specimens an additional tibia cut with 2–3 degrees more slope was sufficient to achieve the correct step-off. No lift-off of the tibial tray occurred during the tests. The patterns of the kinematics of the native and replaced knee showed a considerable similarity (fig 1). The projected medial femoral condylar center of the knee implant is at the same position as the projected medial femoral condylar center of the native knee. No paradoxical roll forward is seen in the knee implants, showing that the PCL balancing apparently seems to work quite well. The projected lateral femoral condylar center of the knee has a similar kinematic pattern in flexion before and after TKA. The knee implant shows a slightly more anterior location near extension but this is only marginal.
Discussion and conclusion
The kinematics of the PCL-retaining implant are on average comparable to the kinematic pattern of the native knee. Apparently, the joint surfaces of the anatomic knee designed with a dished medial insert surface and a convex lateral insert surface and a 3 degrees varus of the joint line is guiding the motion towards that of a normal knee joint. We feel that correct balancing of the PCL during implantation is of major importance in achieving these results. The spacer technique to balance the PCL seems to work well in this experiment.
