Effect of combined flexion and external rotation on measurements of the proximal femur from anteroposterior pelvic radiographs

Introduction: Fixed flexion and external rotation contractures are common in patients with hip osteoarthritis and, in particular, before total hip replacement (THR). We aimed to answer the following question: how does combined flexion and external rotation of the femur influence the radiographic assessment of (1) femoral offset (FO) (2) neck-shaft angle (NSA) and (3) distance (parallel to the femoral axis) from greater trochanter to femoral head center (GT-FHC)? Hypothesis: Combined flexion and external rotation impact the accuracy of two-dimensional (2D) proximal femur measurements. Materials and methods: Three-dimensional (3D) CT segmentations of the right femur from 30 male and 42 female subjects were acquired and used to build a statistical shape model. A cohort (n = 100; M:F = 50:50) of shapes was generated using the model. Each 3D femur was subjected to external rotation (0°–50°) followed by flexion (0°–50°) in 10° increments. Simulated radiographs of each femur in these orientations were produced. Measurements of FO, NSA and GT-FHC were automatically taken on the 2D images. Results: Combined rotations influenced the measurement of FO (p < 0.05), NSA (p < 0.001), and GT-FHC (p < 0.001). Femoral offset was affected predominantly by external rotation (19.8 ± 2.6 mm [12.2 to 26.1 mm] underestimated at 50°); added flexion in combined rotations only slightly impacted measurement error (20.7 ± 3.1 mm [13.2 to 28.8 mm] underestimated at 50° combined). Neck-shaft angle was reduced with flexion when external rotation was low (9.5 ± 2.1° [4.4 to 14.2°] underestimated at 0° external and 50° flexion) and increased with flexion when external rotation was high (24.4 ± 3.9° [15.7 to 31.9°] overestimated at 50° external and 50° flexion). Femoral head center was above GT by 17.0 ± 3.4 mm [3.9 to 22.1 mm] at 50° external and 50° flexion. In contrast, in neutral rotation, FHC was 12.2 ± 3.4 mm [3.9 to 22.1 mm] below GT. Discussion: This investigation adds to current understanding of the effect of femoral orientation on preoperative planning measurements through the study of combined rotations (as opposed to single-axis). Planning measurements are shown to be significantly affected by flexion, external rotation, and their interaction. Level of evidence: IV Biomechanical study.