Accurate placement of unicompartmental knee arthroplasty components is thought to be essential for the long-term survival and efficacy of the prosthesis. Computer navigation is being explored as a means of improving the accuracy of component position. There are few published studies comparing conventional and computer-navigated techniques using the same prosthesis. Twenty-two Allegretto [Zimmer] medial unicompartmental knee prostheses were placed in 18 patients using the AxiEM [Medtronic] computer-navigated system. The immediate post-operative AP and lateral radiographs were analysed and compared with an equivalent cohort of 30 prostheses in 29 patients with medial unicompartmental arthritis in whom the Allegretto was placed without the aid of computer navigation. All operations were performed by the senior author in a rural Queensland hospital. No cases were lost to follow-up. The data was not normally distributed. The mean, SD and variance of the data sets was calculated and significance tested with a 2-tailed Mann-Whitney U-test. Computer navigated tibial components were implanted with a mean of 2 degrees of varus compared with 1 degree of valgus with conventional navigation [p = 0.027]. Our target was 0–4 degrees of varus. Eighteen of the 20 computer-navigated cases, 90% fell within the recommended range [0–4 degrees of varus] compared with only 40%, 12 of the 30 conventionally-implanted cases. This is demonstrated by the greater range and variance of the conventional navigation data set. Posterior slope for the computer navigated components was 1 degree compared with 3 degrees for conventional navigation [0.010]; only 1 computed navigated component [5%] was implanted with anterior slope compared with 4 cases for conventional navigation [13%]. Measurements of femoral component flexion and position with respect to the tibial component were not significantly different but demonstrated greater variance for the conventionally navigated data set. Accurate component positioning improves efficacy and prosthesis survival for patients who meet the indications for unicompartmental surgery. However proponents acknowledge the weaknesses of conventional jigs for unicompartmental prostheses. In this study computer navigation has been shown to improve the accuracy of component placement.
