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.

Purpose: Tibial osteotomy for valgisation is a difficult procedure with a risk of over-correction or under-correction which can have significant aesthetic, functional and medicolegal consequences. In order to improve the precision, we adapted a navigation system to tibial osteotomy for valgisation. The purpose of the present work was to evaluate the feasibility and results of this technique.

Material and methods: From March to November 2001, we used the Orthopilot system for 19 patients with genu varum. Mean patient age was 50.8±11.7 years (range 18–71). The indication for valgisation was genu varum, associated with joint degeneration (grade 1, 2, or 3 in the modified Ahlback classification) in 18. The aesthetic effect of genu varum was the only indication in one patient. Preoperatively, the mean HKA was 173.73±3.24° (range 169–178). The mechanical axis was determined with Orthopilot before performing an open medial osteotomy for valgisation which was fixed temporarily with a metal wedge to control as needed lower limb alignment. Orthopilot enabled verification of the desired axis. When obtained, the metal wedge was removed and replaced by a wedge of tricalcium phosphate (Biosorb®, B-Pharm) of the same size. The osteotomy was stabilised with a screw plate. The objective was to obtain a femorotibial axis between 182° and 186°, i.e. 2° to 6° valgus.

Results: HKA measured peroperatively with Orthopilot was 174.05±3.06°, exactly the same as on the preoperative x-rays. After oseotomy, HKA was 183.47±1.07° (180°–184°) with Orthopilot, and 183.47±1.44° (179°–186°) on the x-rays. Eighteen knees were within the objective of 182°–186° giving a success rate of 94.7%.

Discussion: Computer-assisted tibial osteotomy for val-gisation using Orthopilot is quite feasible. To have a valid assessment of this new method, results would have to be compared with a series performed without computer assistance.