Is anatomic acetabular orientation related to pelvic morphology? CT analysis of 150 healthy pelvises.

Background: Links between sagittal spinal alignment and acetabular orientation attract considerable research attention with the goal of optimising prosthetic cup position. However, whether pelvic incidence (PI) is related to anatomic acetabular orientation remains unknown. We therefore conducted a radiological study with the following objectives: to look for correlations between PI and anatomic acetabular parameters; to describe the sacro-pubic angle (SPA), defined by fixed bony pelvic landmarks, and its relations with acetabular anteversion; and to determine whether anatomical parameters (PI and SPA) correlate with demographic characteristics.

Hypothesis: PI correlates with anatomical acetabular parameters.

Materials And Methods: We conducted a computed tomography (CT) study of the pelvises of 150 patients free of degenerative disease. Three parameters were measured: anatomic acetabular orientation in the Lewinnek reference plane, PI, and the SPA subtended by the line connecting the midpoint of the sacral endplate to the pubic symphysis and the anterior pelvic plane. Statistical tests were performed to look for correlations among these parameters.

Results: Intra-observer and inter-observer reproducibility was considered highly satisfactory (inter-class correlation coefficient, >86% and >82%, respectively). Mean PI was 58.6°±10.2° (range, 32.8°-97.6°), with no significant differences between genders or across age groups. Mean SPA was 34.7°±5.5° (range, 18.3°-49.8°). Mean anatomic acetabular anteversion (AAA) was greater in females (23.4°; range, 11.5°-34.5°) than in males (20°; range, 7.5°-34.5°) (P<0.001). PI did not correlate with any of the acetabular parameters (PI/AAA, r=0.8 and P=0.33; PI/acetabular inclination on the horizontal, r=-0.96 and P=0.24). SPA correlated significantly with both PI (r=0.33 and P<0.001) and AAA (r=0.33 and P<0.001).

Discussion: This CT study of normal pelvises showed that AAA was significantly greater in females and that SPA correlated significantly with both PI and acetabular anteversion. SPA could serve to define the "theoretical" AAA of each individual patient and could thus be incorporated into surgical planning protocols or intra-operative guidance methods for hip replacement surgery.

Level Of Evidence: IV, retrospective study with no control group.