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

J. D. O'Connor, M. Rutherford, J. C. Hill, D. E. Beverland, N. J. Dunne, A. B. Lennon*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)
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Abstract

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.

Original languageEnglish
Number of pages6
JournalOrthopaedics and Traumatology: Surgery and Research
Early online date10 Apr 2018
DOIs
Publication statusEarly online date - 10 Apr 2018

Keywords

  • Femoral offset
  • Femoral orientation
  • Neck-shaft angle
  • Preoperative planning

ASJC Scopus subject areas

  • Surgery
  • Orthopedics and Sports Medicine

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