Sagittal Pelvic Orientation A Comparison of Two Methods of Measurement.

Pelvic tilt is an essential parameter in spinal deformity surgery and in acetabular positioning for total hip arthroplasty. However, the measurement of tilt varies between the hip and spine literature. Hip surgeons measure the anterior pelvic plane tilt, whereas spine surgeons measure the spinopelvic tilt. This study uses stereoradiography (EOS imaging SA, Paris, France) to assess the relationship and the inter-observer and intra-observer reliability of measuring these two common references for pelvic tilt. Retrospective analysis of full-body, standing stereoradiographic studies of 100 patients with varying degrees of spinal deformity was performed at a single institution. Assessment of anterior pelvic plane and spinopelvic tilt were undertaken by two orthopaedic surgeons and two orthopaedic residents using validated software. The pelvic incidence and sacral slope were also measured. The mean difference between anterior pelvic plane and spinopelvic tilt was 13.98° ± 7.04°, and the values were linearly inversely related. Both measures of tilt were strongly correlated with each other. Spinopelvic tilt has greater inter- and intra-user reliability and was a more precise measurement than anterior pelvic plane. Spinopelvic tilt is a more precise and reliable measurement than the anterior pelvic plane tilt; however, both measurements are strongly correlated. The clinical implications of this are not completely understood; however, it may be important for hip surgeons when placing acetabular components with precision. Further investigation is needed to assess which is a more accurate reference for the placement of acetabular components in hip arthroplasty.