Abstract
Introduction
Reliability of a gap control technique with the tensor/balancer during PS-TKA was assessed by means of fluoroscopic images after TKA.
Methods
Thirty-one subjects were selected for assessment. The mean age of the subjects was 73.0 years old. During PS-TKA, a parapatellar approach was used. Cruciate ligaments were excised, and distal femoral and proximal tibial cuts were made. After all osteophytes were removed, the joint gap angle and distance were measured in full extension and at 90° flexion using a tensor/balancer. Medial soft tissue releases were performed and soft tissue balancing was obtained in full extension so that the joint gap angle was 3° or less than 3°. The joint gap angle and distance between femoral and tibial cut surfaces in full extension, and between a tangent to the posterior femoral condyles and tibial cut surface at 90° flexion were measured. The external rotation angle of the anterior and posterior cuts of the femur was decided based on the joint gap angle at 90° flexion. The size of the femoral component was decided based on the joint gap distance in full extension and at 90° flexion. Then only the trial femoral component was inserted. The joint gap angle and distance between the tangent to the condyles of the trial femoral component and tibial cut surface in full extension and at 90° flexion were measured.
More than one month after TKA, the fluoroscopic images of the prostheses were taken during knee extension/flexion. Then, a torque of about 5 Nm was applied to the lower leg in order to assess the varus/valgus flexibility during flexion. The pattern matching method was used to measure the 3D movements of the prostheses from the fluoroscopic images. The joint gap angle was calculated in full extension and at 90° flexion. The varus/valgus flexibility at each flexion angle was also assessed.
Results
During TKA, the mean joint gap angle was 0.9° varus in full extension, and was 0.3° valgus at 90° flexion. The mean difference of the gap distance between extension and flexion was 2.3 mm. The results from fluoroscopic images showed that the mean joint gap angle was 0.1° valgus in extension, and was 0.6° varus at 90° flexion. The mean joint gap in full extension and at 90° flexion was less than 1° both during TKA and after TKA. The mean varus/valgus flexibility in the implanted knees was 1.6° in full extension, and was 3.9° at 90° flexion.
Discussion
The results showed that the joint gap was almost rectangular both in extension and flexion both during TKA and after TKA. The tensor/balancer, with a load of 30 inch-pounds, was reliable during PS-TKA. Muscles function had recovered and the implanted knees might be stable. However, the results of this study clearly showed the theoretical ground for the reliability of the tensor/balancer during TKA.
Conclusion
During PS-TKA by means of the gap control technique, the tensor/balancer with 30 inch-pounds can provide reliable joint gap angle and distance.
