Abstract
Purpose: Over recent years, several authors have estimated that the distal femur presents an epiphyseal torsion which can be measured intraoperatively or on the preoperative scan. This measurement does not however take into account the dynamic mechanical axis, particularly the mechanical axis at 90° flexion when walking. We used a computer-assisted navigation system (Ortho-pilot®) to attempt to measure femoral rotation by dynamic gonometry in extension and 90° flexion before and after implantation of a total knee arthroplasty.
Material and methods: We recorded the preoperative (Rx) and intraoperative (Orthopilot) HKA in extension and in 90° flexion before and after implantation of 50 total knee prostheses (Search®, Aesculup, AG, Tuttlingen) and again postoperatively (Rx). The series included 19 knees with genu valgum (mean HKA 187.36±5.4°, range 181°–203°), 30 knees with genu varum (HKA 169.2±4.11, range 160°–176°), and one normal axis knee.
Results: The radiographic values obtained preoperatively were confirmed by Orthopilot, respectively 186.68±5.25° and 169.76±3.84° in extension. At 90° flexion, HKA was 178.63±5.7° before implantation for genu valgum giving a significant varus due to lateral opening during flexion,and 171.6±4.15° for genu varum, showing persistence of varus. After implantation of the total knee prosthesis, the values were as follows. For the genu varum: HKA in extension 180.57±0.82° and HKA in 90° flexion 176.86±2.55° giving a mean residual varus of 3.16±2.86° (from 4–8° varus) without external rotation of the femoral implant. For genu valgum, HKA in extension was 179.60±0.92° and HKA in 90° flexion was 176.1±3.23°, giving a mean residual varus of 3.26±2.86° (0–10° varus), recalling that in the event of genu valgum we impose external rotation due to the frequent hypoplasia of the lateral condyle.
Discussion: Orthopilot-assisted implantation of total knee prostheses provides new information concerning dynamic gonometry, particularly the varus or valgus in flexion, which corresponds to measuring natural external or internal rotation. Measuring epiphyseal torsion of the distal femur with classical methods does not take into account the global rotation of the femur which is often an external rotation (up to 8° for genu varum). Systematic implantation of the femoral component in external rotation raises the risk of increasing considerably the varus forced to the implant during 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.
