Several studies have shown that the rotation of the femoral component is an essential factor in total knee replacement. Consequently, different intra-operative landmarks for femoral implantation were established but most of them are either hard to define or have a high variance, so reducing their utility value. The aim of this randomised, prospective study was to prove that a preoperative CT scan is a usable help for femoral orientation.
In 2006, 57 consecutive patients designated for implantation of a knee arthroplasty were split up in two groups (a and b) corresponding to a randomisation scheme. The implantation of the femoral component in group a) was done with a posterior condylar angle of three degrees of external rotation as invariable determined by the resection guide. In addition the surgeon could correct the rotation following any other landmarks (surgeon's own method). In group b) the posterior condylar angle was measured preoperatively with the help of a CT scan and transferred on a resection guide allowing variation of the posterior condylar angle in single degree steps. In this case variation of femoral rotation (surgeon's own method) was not possible. The rotation of all knee replacements (groups a and b) was measured postoperatively with a CT scan following the technique of Berger et al. The implantation with the femoral component lying parallel to the transepicondylar axis was regarded as correct. Differences were measured in degrees.
Regarding the degree of femoral malrotation without consideration of the direction (external/internal rotation) the difference between group a) and group b) was highly statistically significant (p < 0.000001). The highest range of malrotation in group b) was two degrees versus six degrees in group a). In group b) 19 of 30 (= 63.3 %) femoral components showed no malrotation at all, in group a) only 5 of 27 (= 18.5 %) components were implanted completely correctly.
Preoperative measurement of the posterior condylar angle was proven to be a simple, reproducible examination method which increases the rotational accuracy of total knee replacement.