The Use of Navigation to Achieve Soft Tissue Balance in Total Knee Arthroplasty – A Randomised Clinical Study

All patients receive a Triathlon posterior stabilised, fixed bearing total knee replacement (Stryker Orthopedics, Mahwah, NJ) inserted via a standardised medial para-patellar approach. In the navigated group, initial gaps, at 135N of tension provided by the tensiometer, are recorded after the tibial cut at 90 degrees flexion and full extension. The Stryker version 4.0 software is able to calculate the size of the femoral component after registration of the epicondyles and registration of the bony morphology of the anterior and posterior femoral condyles. Based on the initial gap measurements, the surgeon has a number of options to equilibrate the flexion and extension gaps. These include downsizing the femoral prosthesis or modifying the position of the femoral prosthesis in the anteroposterior or proximodistal planes. The software uses algorithms that take a mean value of the registered femoral transepicondylar axis and the registered femoral anteroposterior axis (Whiteside’s line) to determine axial rotation of the femur. The surgeon aims for the femoral bony cut to have a value of 0+/-2 degrees external rotation based on the software measurement. After the femoral cuts, the gaps at 135N are measured again. The surgeon then makes further soft tissue releases as necessary to try and equalise the mediolateral gaps to within 2mm of each other. Final gap measurements (without the trial components in place) are recorded once again at 135N of tension. 135N was chosen as the tension force as it was felt that this level of tension produced sufficient strain to overcome any laxity in the soft tissues to allow reliable measurement of gaps. The duration of this procedure is approximately 80 minutes.