Mechanical axis of the lower limb. Comparison between invasive and non invasive navigated measurement.
Jenny J. (France)
Jenny J. (France)
Dillmann G.
Dillmann G.
ESSKA Academy. Jenny J. May 9, 2018; 209561; P09-1017 Topic: Knee
Assoc. Prof. Jean Yves Jenny
Assoc. Prof. Jean Yves Jenny
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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.

Knee, mechanical axis, navigation, non invasive
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