ACCURACY OF ROTATION OF FEMORAL COMPONENT FOR NAVIGATION IMPLANTED TOTAL KNEE ARTHROPLASTY: PRE- AND POSTOPERATIVE COMPUTED TOMOGRAPHY STUDY

Abstract

Purpose of the study: Recent studies have demonstrated that navigation systems provide highly accurate cuts for orthogonal alignment of the lower limb. The accuracy has not to our knowledge been assessed for rotation. Rotation of the femoral piece, which results from a strategy independent of the bone cut, is designed to «correct» for epiphyseal torsion of the distal femur and thus obtain a biepicondylar axis parallel to the «surgical» posterior bicondylar line described by Berger (line drawn between the medial sulcus and the lateral epicondyle), i.e. forming un angle of 2° with the anatomic biepicondylar line described by Yoshioka (line from the medial to lateral condyles). The purpose of this study was to access the precision of navigation rotation.

Material and methods: This prospective consecutive study included 40 osteoarthritic knees undergoing total knee arthroplasty (TKA). The anatomic angle of distal femoral torsion (Yoshioka angle: angle formed by the posterior bicondylar line and the biepicondylar line) was measured on the pre- and post(3 months)-operative scans. Navigation (Navitrack, Zimmer) used the rotation given by the preoperative scan to guide the femoral cut with the objective of achieving a residual Yoshioka angle of 2°, i.e. parallel to Berger’s surgical biepicondylar line. The postoperative HKA measured on the pangonogram in the standing position was 179.6±2° with 85% of patients between −2° and +2°, confirming the reliability of the navigation system.

Results: The mean preoperative epiphyseal rotation of the distal femur was 6.4±1.8°. The mean postoperative measurement was 1.1±2.4°. Eighty percent of patients were within ±2° of the objective.

Discussion: We demonstrated in previous work that navigation-based rotation using intraoperative data is satisfactory as long as the degree of rotation is based on the preoperative scan (and thus takes into account the wide rang of distal femur torsion). Navigation-based rotation is a progress compared with standardized rotation. The few errors observed were related to insufficient identification of the posterior bicondylar line during navigation or to difficulties in interpreting the postoperative scan.