Abstract
INTRODUCTION
Navigation systems have proved allowing performing measurement of the lower limb axis with a good accuracy, but the mandatory use of reference pins or screws limit their use to the operating room. The use of non-invasive navigation systems has been suggested to overcome this limitation. We conducted a prospective study to assess the validity of such a measurement system with non-invasive fixation of the reference arrays. The main goal was to compare this method with a standard, invasive navigation system requiring bony fixation of the arrays. The following hypothesis was tested: there will be a significant difference between the simultaneous measurement of the mechanical femoro-tibial angle by a standard navigation system and by the non-invasive navigation system.
MATERIAL AND 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 (fig. 1). Coronal femoro-tibial mechanical angle (CMFA) in maximal extension without 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 between non invasive and invasive measurements of the CMFA in full extension. There was a good correlation (fig. 2) and a good concordance (fig. 3) between both measurements.
DISCUSSION
The non invasive measurement technique system seems to be as accurate as conventional, invasive navigation.
CONCLUSION
This technique might be a valuable alternative to long leg x-rays, with a good accuracy but without radiation exposure.
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