[Rotational alignment of femoral component with computed-assisted surgery (CAS) during total knee arthroplasty]

Accurate implantation of the prosthesis components is a prognostic factor for long-term total knee arthroplasty survival as it reduces wear and loosening failure. Computer-assisted navigation systems have proved to produce accurate bone cuts orthogonal to the mechanical axis. Proper rotational alignment of the femoral component is one of the requirements for optimal positioning of the femoral prosthesis. The posterior bicondylar axis of the femoral prosthesis should therefore be parallel to the transepicondylar axis. The purpose of the present study was to determine whether computer-assisted navigation provides an accurate rotational alignment of the femoral implant, when preoperatively defined with CT scan.

This prospective study, carried out between December 2003 and June 2005, included 70 patients of average age 74 years old (range 57-85) who underwent primary total knee arthroplasty, with a SAL prosthesis (Zimmer). Preoperative investigations with computed tomography scanning produced accurate measurements of distal epiphyseal femoral torsion (DEFT). The posterior bicondylar axis was found to be a reliable landmark for the rotational orientation of the femoral cutting-guide during bone-cuts. The rotational orientation of the cutting-guide was based on the preoperative CT data. A three-month follow-up CT scan was carried out to evaluate final rotational position of the femoral component.

The mean DEFT evaluated on the preoperative CT scan was 6.9+/-2.9 degrees . The mean rotational orientation of the cutting guide was 4.8+/-2 degrees . The mean postoperative measurement of DEFT was 1.56+/-2.7 degrees . The mean adjustment of DEFT was 5.34 degrees . Adopting a +/- 2 degrees cutoff, 77 % of patients achieved acceptable alignment within +/- 2 degrees compared with our objectives. These findings were compared to a previous series of 34 cases using an arbitrary 3 degrees standardized rotation of the femoral component and following an identical radiological protocol. Among the knees, 44% reported alignment within +/- 2 degrees .

When femoral and tibial bone cuts are performed independently, conventional instrumentation techniques seem insufficient to adapt patient's specific anatomy and prove inadequate to provide precise rotational alignment of the femoral component. Computed tomography scan is a reliable mean to produce precise preoperative measurements for proper DEFT. Moreover, it allows accurate postoperative control of the implant positioning. Other studies have documented a higher degree of precision in the rotational alignment of the femoral component with computed navigation systems in comparison to conventional instrumentation. However, in such studies, rotational alignment was always determined by computer navigation, and based on a controversial intraoperative identification (epicondyles and Whiteside's line referencing). We believe that preoperative CT scanning is a more favourable method. Actually, 77% of the cases reported satisfactory rotational alignment of the femoral component using this technique.