Martijn Van Der Bom MSC, Presenter: Nothing to Disclose

M.E. Groote, Abstract Co-Author: Nothing to Disclose

Koen L. Vincken PhD, Abstract Co-Author: Nothing to Disclose

Frederik Jan Anton Beek MD, Abstract Co-Author: Nothing to Disclose

Rutger A. J. Nievelstein MD, PhD, Abstract Co-Author: Nothing to Disclose

Wilbert Bartels PhD, Abstract Co-Author: Nothing to Disclose

Diagnosis of developmental dysplasia of the hip is commonly done by measuring the acetabular angle on anterior-posterior radiographs using Hilgenreiners method. The outcome of the measurement depends on the orientation of the subject’s pelvis relative to the x-ray source. In order to obtain a reliable angle measurement the pelvis must be well positioned during the acquisition of the radiograph. This is a challenging task especially in young children. We have investigated the influence of combinations of pelvic flexion/extension and rotation on the systematic error in the acetabular angle measurement in a very reproducible way.

The study was done by using digitally reconstructed radiographs (DRRs), i.e. simulated radiographic images, of a high resolution 3D computed tomography dataset of a child. A series of 88 DRRs in different orientations was generated by varying the settings for pelvic flexion/extension and pelvic rotation. Pelvic flexion/extension was mimicked by rotation of the CT volume around the axis intersecting the volume from left to right. Pelvic rotation was mimicked by rotating the CT volume around the longitudinal axis of the volume. Acetabular angle measurements were performed on all DRRs by three experts independently.

The results show that the effects of flexion/extension and rotation accumulate and either amplify or counteract the underestimation or overestimation of the acetabular angles. For flexion/extension up to 16o and rotations up to 20o the average systematic errors in the acetabular angle varied from 17.6o underestimation to 4.6o overestimation.

We can conclude that pelvic flexion/extension and rotation led to substantial systematic errors in the acetabular angle measurement. Hence, proper alignment of the pelvis during the radiograph acquisition, for both degrees of freedom, is necessary. Additionally, we believe that using digitally reconstructed radiographs generated from a computed tomography dataset can be very useful for the investigation of systematic errors in other radiographic diagnostic measurements similar to the acetabular angle measurement.

Investigation of systematic errors in radiographic diagnostic measurements.

Van Der Bom, M, Groote, M, Vincken, K, Beek, F, Nievelstein, R, Bartels, W, The Influence of Pelvic Orientation on the Acetabular Angle.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8005561.html