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 S1 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 (CEAs) 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.