Objectives: The goal of the study was to compare a new method of non-invasive knee navigation with a standard, invasive navigation system requiring bony fixation of the arrays.

Methods: 20 patients scheduled for total or partial knee arthroplasty were included after giving their informed consent. There were 7 men and 13 women with a median age of 65 years (range, 55 to 90). The median coronal deformation measured by X-rays was 8° of varus (range, 5° valgus to 22 ° varus). The same navigation system was used for both invasive and non-invasive measurements, but the basic algorithms were adapted for the non-invasive technique. For the non-invasive technique, metallic plates were strapped on the thigh and the calf to allow arrays fixation. Coronal femoro-tibial mechanical angle (CMFA) in maximal extension and at 30° of knee flexion without stress and with manual varus/valgus stress was recorded by the non invasive system. This non-invasive analysis was immediately followed by surgery, and the same angle was measured intra-operatively with the invasive system. Comparisons between non-invasive and invasive measurements were performed using a Wilcoxon test, after checking that their distribution followed a normal distribution, and an equivalence testing with limits of ±3°. The correlation between the two sets of measurements was analyzed using a correlation test Spearman rank. The analysis of the concordance of the two sets of measurements was performed using Bland and Altman tests. The significance level p was set at 0.05.

Results: There was no significant difference for all measurements in full extension. There was a significant difference for all measurements at 30° of flexion. There was a good correlation between non-invasive and invasive measurements of the CMFA in full extension, but not at 30° of flexion. The same results were observed for stress measurements. There was a good concordance between all measurements.

Conclusions: The non-invasive navigation system seems a good alternative or a good supplement to conventional, invasive navigation. Non-invasive navigation is a reliable technique of measurement of the mechanical axis of the lower limb and of the varus/valgus knee laxity in extension, but the measurements in flexion are less reliable. The non-invasive navigation may be more accurate than radiographic measurements without radiation exposure. This system may be used for pre-operative and post-operative evaluation of the knee anatomy and function before and after knee arthroplasty.