Does Spinal Alignment Influence Acetabular Orientation: a Study of Spinopelvic Variables and Sagittal Acetabular Version

Objective Although hip alignment and spinal alignment have been studied individually, there is little information concerning the relationship between them. The questions addressed in this study are: (i) Is there a reproducible measure of sagittal plane acetabular orientation; (ii) Is sagittal plane acetabular orientation determined more by pelvic incidence ( PI ) or acetabular wall coverage? Methods Data on patients who had undergone screening by CT scan of the pelvis for non‐orthopedic indications from 2005–2010 were retrospectively studied. There were 164 patients of mean age 59 years (range, 27–87). Patients with pelvic trauma, hip arthroplasty or other hip pathology were excluded. Measurements of relevant acetabular and spinopelvic variables were made in the sagittal plane. The sacro‐acetabular angle ( SA ) was defined as the angle between a tangent line to the anterior and posterior walls of the acetabulum and the S 1 endplate. Multiple regression analysis was used to determine which factors contribute to SA angle. Results The mean sacro‐acetabular angle was 72° ( SD = 14.8°). Center edge angles ( CEA s) were measured at the anterior and posterior walls of the acetabulum. Mean anterior CEA was 69° ( SD = 8.3°) and posterior CEA 107° ( SD = 16.4°). Regression analysis revealed the largest significant predictors of SA angle were PI and A ‐ CEA . Conclusions Sagittal acetabular orientation is related to spino‐pelvic balance and morphological characteristics. Increased PI or posterior wall coverage corresponds to a more vertical acetabular orientation. Sagittal plane acetabular alignment may be an important variable in achieving favorable results after reconstruction.