Characterizing the Rotational Profile of the Distal Femur: A Roadmap for Distal Femoral Replacement Surgery.

Introduction: Standard references for determining rotational alignment are often unavailable when performing distal femoral replacement (DFR) surgery. This study aimed to describe the rotational profile of the distal femoral osteology at common resection levels.

Methods: Adults with CT scans of the bilateral legs were included. Exclusion criteria included fractures, congenital deformity, prior arthroplasty, or inadequate imaging. Referencing the transepicondylar axis, angles were measured in the axial plane using the following reference lines: anterior condylar axis or anterior femoral cortex axis (AFCA), posterior condylar axis or posterior femoral cortex axis (PFCA), lateral projection of the linea aspera (LAA), and Whiteside line (WL). These rotational measurements were assessed at 3, 5, 7, and 9 cm proximal of the joint line. External and internal rotation were denoted as positive and negative, respectively.

Results: Fifty-one patients (102 femora) were included (mean age: 62.6 ± 13.4 years; mean body mass index: 26.0 ± 6.7 kg/m 2 ). Proximally, the anterior condylar axis/anterior femoral cortex axis became increasingly internally rotated (3-cm: -13.1 ± 3.5°, 9-cm: -21.2 ± 6.6°), whereas the posterior condylar axis/PFCA became increasingly externally rotated (3-cm: -6.7 ± 2.3°, 9-cm: 9.6 ± 6.1°). WL remained nearly perpendicular (3-cm: 88.8 ± 2.3°, 5-cm: 89.8 ± 3.7°) but was not reliably measured beyond 5 cm. The LAA was measurable proximal to 5 cm from the joint line and was internally rotated (5-cm: -58.5 ± 14.3°, 9-cm: -45.6 ± 13.4°).

Conclusion: Distally, WL is a reliable anatomic landmark for femoral implant rotation. At more proximal resection levels, the posterior cortex may be a valuable landmark as referencing the linea aspera may lead to gross internal rotation of the femoral implant.