Abstract
Purpose: The position of the femoral implant in external rotation remains a controversial issue. It can be determined using bone landmarks (Whiteside line, parallel to the biepicondylar axis, 3° external rotation from the posterior condylar plane). For the last seven years, we have related femoral rotation to the orientation of the tibial cut in order to ensure good femorotibial stability in flexion using specific instruments (Cores®). This prospective study was conducted to examine the position of the femoral implant determined with this method and to measure the position from bone landmarks.
Material and methods: Twenty consecutive patients were included in this study. Bilateral computed tomographic measurements were made before and after surgery. Joining 8mm/8 slices were obtained for the femoral necks and 5mm/3 slices for the knees. The angle of femoral torsion was defined in two ways: the first by the angle formed between the axis of the femoral neck (on two superimposed slices) and the tangent to the most posterior part of the femoral condyles; the second by the angle formed between the epidondylar line and the posterior condylar line.
Results: The preoperative scans demonstrated that the angle between the biepicondylar line and the posterior condylar line was 5.8±1.5°. Using Cores®, led to an external rotation of the femoral implant to 2.7±0.6°. The postoperative scans demonstrated that the angle between the biepicondylar line and the posterior condylar prosthetic play was a mean 3.3°. The measurements using the femoral neck were less precise, with, in one case, an external rotation of 5°. The patella was well balanced postoperatively (irrespective of the external rotation position of the femoral implant).
Discussion: The angle of about 6° between the biepicondylar line and the posterior plane of the condyles has also been reported by others (Beaufils, Matsuda). To obtain a rectangular space in flexion, the posterior condyle cuts are more important medially than laterally. We found a correlation between the correction provided by the specific instrument set and the difference in the posterior condyle cuts, demonstrating the intraoperative precision of Cores®. It is difficult to orient the femoral piece parallel to the biepicondylar axis. This study demonstrates that there always remains 2 to 3° of inclination of the biepi-condylar axis from the posterior condylar plane.
Conclusion: The positioning the femoral implant parallel to the biepicondylar line leads to inducing an important external rotation. While using 3° rotation systematically would reduce the risk of internal malrotation, we feel it better to adapt the rotation to each individual knee depending on the anatomic presentation. Cores® enables positioning the femoral implant in external rotation as a function of the ligament balance obtained in flexion after peripheral tension is applied. This enables avoiding medial femorotibial laxity in flexion.
The abstracts were prepared by Docteur Jean Barthas. Correspondence should be addressed to him at Secrétariat de la Société S.O.F.C.O.T., 56 rue Boissonade, 75014 Paris.
