Mechanical axis-derived femoral component rotation in extramedullary total knee arthroplasty: a comparison between femoral transverse axis and transepicondylar axis

Abstract

Correct rotational alignment of the femoral component is paramount to the success of total knee arthroplasty, but debate continues as to which method is the most reliable. The purpose of this study was to evaluate mechanical axis-derived rotational axis of the femoral component using an extramedullary femoral alignment guide system. Between 2009 and 2010, 60 patients (120 knees) underwent simultaneous bilateral total knee arthroplasty. Postoperative CT scans were performed on all 120 knees. The mechanical axis-derived rotational axis of the femoral component was compared with the surgical transepicondylar axis on CT scans. The femoral component was externally rotated compared with the surgical transepicondylar axis (mean = 1.6°, SD = 2.2°, range: from 4.8° of internal rotation to 7.9° of external rotation). One hundred and nine of the 120 knees (90.8%) were rotated by less than 5° from the surgical transepicondylar axis. All inter-observer observations on CT scans were within a range of ±2.1° and showed no significant differences. Mechanical axis-derived rotational axis of the femoral component was found to be closely related to the surgical transepicondylar axis, to an extent which suggests that the mechanical axis-derived rotational axis could be a new alternative option in determining optimal rotational alignment of the femoral component during total knee arthroplasty. II.