The immediate effect of navigation on implant accuracy in primary mini-invasive unicompartmental knee arthroplasty.

The success of unicompartmental knee arthroplasty (UKA) is highly dependent on the accuracy of the component alignment. Objective of the present study was to evaluate the immediate effect of image-free computer navigation technology on implant accuracy in primary mini-invasive UKA. This study reviews 40 patients with primary isolated arthritis of the medial compartment of the knee that underwent unicompartmental knee arthroplasty through a minimally invasive approach. A cohort of the 20 most recent consecutive UKA's implanted with standard instrumentation was followed by a cohort of the very first 20 consecutive cases after conversion to the navigated technique. There was no variability regarding implant (Oxford meniscal unicompartmental knee system--Biomet Orthopedics, Inc., Warsaw, Indiana 46580, USA), surgeons and surgical technique, except for the use of the navigation system (Treon plus--Medtronic Inc., Minnesota, MI, USA). The axis alignment and accuracy of implant positioning was measured on postoperative long-leg standing radiographs and standard lateral X-rays with regard to the valgus angle and the coronal and sagittal component angle. In addition, preoperative deformities of the mechanical leg axis, tourniquet time, age, gender, and body mass index were correlated. Statistical analyses were performed using the SPSS 14.0 (SPSS Inc., Chicago, IL, USA) software package. Optimal implant alignment including all measurements in the desired angular range was significantly (P=0.041) higher in the navigated cohort. Navigation eliminated outliers in the frontal mechanical alignment and coronal orientation of the femoral component totally and significantly (P<0.02). Furthermore, navigation narrowed the range of outliers in all other planes of component orientation. There were no statistically significant differences in the mean numerical values between the cohorts, except for the frontal mechanical alignment (P<0.009) and coronal tibial alignment (P<0.037). The average tourniquet time was increased by 10.95 min in the navigated cohort. Our results indicate that navigation immediately improves accuracy of bone cuts and reduces the number of outliers with implementation in UKA.