The figure-of-four axis as a reference to determine stem rotation in hip arthroplasty. What does it really measure? A cadaver study

Background and purpose Implantation of the femoral component at 10–15° of anteversion is recommended in THA. Surgical guidelines suggest that the lower leg be positioned horizontally or vertically with the knee flexed to 90° (figure of four). By constructing a perpendicular axis (a “figure-of-four” axis) to the lower leg, anteversion of the stem is approximated. We assessed whether the figure-of-four axis is a reliable intraoperative tool to approximate the retrocondylar line as a reference for stem version.

Methods Cadavers (21 in total) were placed supine on an operating table and the lower legs were aligned to the horizontal plane. Cannulated titanium screws were inserted perpendicular to the lower leg into the medial epicondyle, representing the figure-of-four axis. The femoral neck axes, retrocondylar lines, and the figure-of-four axes were determined using CT images of the specimen.

Results The anteversion of the femoral neck was median 9.8 (4.5–15.1) degrees (interquartile range). The figure-of-four axis deviated by 0.5 (-2.1–2) degrees, whereas the median difference in the axis in relation to the femoral neck axis was 9.5 (-13.6 to -2) degrees.

Interpretation The figure-of-four axis, being nearly parallel to the retrocondylar line, is a valid indirect method for determination of stem version intraoperatively in patients without varus/valgus deviations of the knee.