Purpose of the study: Compar the position of the femoral piece in relation to the transepicondylar axis (TEA) using four different techniques for regulating rotation:

cut parallel to the posterior bicondylar line (BCL),

Material and methods: One hundred patients who underwent total knee arthroplasty (TKA) had a preoperative computed tomography (CT) scan. The surgical transepicondylar axis (TEA) and the BCL were drawn on the horizontal slices. The angle measured between these two lines (1.56°–2.5°) determined the theoretical angle of external rotation for aligning the femoral piece on the TEA. During the operation, femoral valgus was set to the HKS angle, measured by goniometry. The knife of the distal femoral cut, materializing the line perpendicular to the mechanical femoral axis, came in contact with the most distal femoral condyle (generally the medial condyle but occasionally the lateral condyle for varus femurs). The distance d between the knife and the most distal point of the condyle which remained distant was then measured. The external rotation was set at 0° and 3° with the techniques 1) and 2). For the technique 3), the asymmetry of the distal cut was projected on the posterior cut leading to an automatic rotation at an angle calculated trigonometrically. For the technique 4), the rotation was calculated as a function of the distance d. The difference between the external rotation obtained for each of these techniques and the theoretical rotation was calculated for each patient.

Results: The mean error of rotation obtained for the four techniques was respectively: 2.2–1.9°; 2–1.7°; 1.8–2.2°; and 1.5–1.4° (p< 0.05). The rate of malrotations greater than 1° for the four techniques was respectively: 60%, 58%, 41% and 36%. The rate of malrotations greater than 2° was respectively: 45%, 44%, 27% and 21%. This rate varied according to the femoral morphotype. The percentage of malrotations greater than 2° by technique was as follows for femoral morphotypes normal, varus, and valgus: technique 1: 37,34,58%; technique 2: 37,53,40%; technique 3: 7.5,9,26%; technique 4:22,30,40%.

Conclusion: Interindividual variations in the TEA-BCL angle explain the high rate of malrotation after regulated rotation. An adapted regulation will enable lesser risk of error. An adaptation taking into consideration the results of the preoperative CT scan appear to provide the most reliable results.