REPRODUCIBILITY OF PEROPERATIVE DIGITAL ACQUISITION OF THE FEMORAL BIEPICONDYLAR AXIS

Abstract

Purpose: The biepicondylar axis of the femur is considered by many authors as a reliable reference axis for flexion-extension of the knee and to establish desirable orientation of the femoral component of a total knee arthroplasty. We studied the reproducibility of axis measurments made using an automatic digital acquisition system (OrthoPilot®, Aesculap, Chaumont, France). The system localises anatomic points in space from information obtained with a palpation probe carrying an infrared diode.

Material and methods: A consecutive series of 20 total knee arthroplasties (Search®, Aesculap, Chaumont, France) implanted by two senior surgeons on the same surgical team were studied. The mechanical axis of the femur was calculated prior to the study using kinematic acquisition of the position of the centres of rotation of the hip and the knee. The frontal reference plane was then defined from the most posterior point on the femoral condyles palpated with the probe as the plane containing the mechanical axis of the femur and parallel to the posterior bicondylar line. The apex of the two femoral epicondyles was obtained by direct palpation with the probe. A second plane passing through the apex of the epicondyles and parallel to the mechanical axis of the femur was thus defined. Three acquisitions were made for the same patient by each of the two surgeons without changing the posterior bicondylar reference plane. The angle between the frontal plane of reference and the biepicondylar plane was calculated directly by the software for each acquisition. The variability of the three measurements taken by each operator and between the two operators was studied with the Wilcoxon test for paired series and with Spearman’s coefficient of correlation.

Results: Mean intraobserver variability for the orientation of the biepicondylar axis was 4° for the two operators, with a maximum of 11° for the first operator and 9° for the second, the directions being random. The mean interobserver variability for this orientation was 4° with a maximum of 14°, again at random. All differences were statistically significant.

Discussion, conclusion: Measurements of the biepicondylar axis exhibit high intra- and interobserver variability, probably due to the anatomic conditions; the apex of the epicondyles is a blunt surface difficult to identify with precision. Use of this axis to determine the rotation of the femoral component of a total knee arthroplasty is thus an element of wide variability with measurement inaccuracy of a mean ± 5° but with a maximum that can reach 10°. The question remains to determine whether this uncertainty is tolerable or whether more precision is required.