Decreased extension gap and valgus alignment after implantation of total knee prosthesis in primary varus knees

Purpose It was hypothesised that implantation of a total knee prosthesis may change the size and shape of the joint gap. To test this hypothesis, a tensor device was used which was specifically designed to reproduce the conditions before and after implantation, including attachment of the polyethylene insert trial. This study aimed to compare the joint gaps before and after implantation of a total knee prosthesis using this new tensor device. Methods A total of 259 primary varus knees were included in this study. Knees were exposed using a medial parapatellar approach, and the anterior and posterior cruciate ligaments were resected. After the trial reduction, the intraoperative joint gap kinematics was measured using the tensor device. Results Implantation of a total knee prosthesis decreased the size of the extension joint gap and made it valgus, but did not influence the size or shape of the flexion joint gap. Conclusions The present findings suggest that the classical gap technique, which creates equal and rectangular extension and flexion joint gaps in the bone cutting surface, results in an imbalance between the extension and flexion joint gaps after implantation. To achieve equal and rectangular extension and flexion joint gaps after implantation, the prepared extension joint gap should be about 2 mm larger than the flexion joint gap and slightly varus before implantation in primary varus knees. Level of evidence Therapeutic study, Level II.