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Effect of combined flexion and external rotation on measurements of the proximal femur from anteroposterior pelvic radiographs - 11/06/18

Doi : 10.1016/j.otsr.2018.03.004 
J.D. O’Connor a, M. Rutherford a, J.C. Hill b, D.E. Beverland b, N.J. Dunne c, d, e, f, A.B. Lennon a,
a School of Mechanical and Aerospace Engineering, Queen's University Belfast, 125, Stranmillis road, BT9 5AH, Belfast, United Kingdom 
b Primary Joint Unit, Musgrave Park Hospital, Stockman's Lane, Belfast, BT9 7JB, United Kingdom 
c School of Mechanical & Manufacturing Engineering, Dublin City University, Glasnevin, Dublin 9, Ireland 
d Centre for Medical Engineering Research, Stokes Building, School of Mechanical & Manufacturing Engineering, Dublin City University, Collins avenue, Dublin 9, Ireland 
e Trinity Centre for Bioengineering, Trinity College Dublin, 152-160, Pearse street, Dublin 2, Ireland 
f School of Pharmacy, Queen's University Belfast, 97, Lisburn road, BT9 7BL, Belfast, United Kingdom 

Corresponding author.

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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±2.6mm [12.2 to 26.1mm] underestimated at 50°); added flexion in combined rotations only slightly impacted measurement error (20.7±3.1mm [13.2 to 28.8mm] 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±3.4mm [3.9 to 22.1mm] at 50° external and 50° flexion. In contrast, in neutral rotation, FHC was 12.2±3.4mm [3.9 to 22.1mm] 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.

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Keywords : Femoral orientation, Preoperative planning, Femoral offset, Neck-shaft angle


Plan


 Presented to Closed Meeting International Hip Society, London September 2017.


© 2018  Elsevier Masson SAS. Tous droits réservés.
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Vol 104 - N° 4

P. 449-454 - juin 2018 Retour au numéro
Article précédent Article précédent
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