Acetabular anteversion with CT in supine, simulated standing, and sitting positions in a THA patient population

Abstract

Background: Appraisal of the orientation of implants in THA dislocations currently is based on imaging done with the patient in the supine position. However, dislocation occurs in standing or sitting positions. Whether measured anteversion differs in images projected in the position of dislocation is unclear.

Questions/purposes: We compared measured acetabular cup orientations on axial CT scans taken with the patient in a supine position with those from CT sections at angles to the sacral slope reflecting standing and sitting positions.

Methods: We retrospectively reviewed the radiographs of 328 asymptomatic patients who had THAs. Anatomic acetabular anteversion (AAA) was measured from the plain CT scan (supine position, axial CT sections). The AAA also was measured on reformatted CT scans in which the orientation was adjusted individually to the sacral slope on lateral radiographs with patients in the standing and sitting positions.

Results: The mean/(SD) AAA changed from 24.2° (6.9°) in the supine position to 31.7° (5.6°) and 38.8° (5.4°) in simulated standing and sitting positions, respectively. The supine AAA correlated with the standing AAA (r = 0.857) but not with the sitting AAA (r = 0.484).

Conclusions: These data suggest measurement of the AAA on a plain CT scan used in current practice is biased. In patients with recurrent posterior dislocation from a sitting position, accounting for the functional variations in measurement of the position of the acetabular cup provides more relevant information regarding component positioning.

Fig. 1A–D

(A) AP and (B) lateral radiographs of a patient in a standing position, and (C) AP and (D) lateral radiographs of the same patient in the sitting position are shown. Measurement of the abduction angle (AA) on the AP radiograph and the acetabular sagittal inclination (ASI) and sacral slope (SS) is shown. Variations in orientation of the acetabular cup in the standing versus the sitting position can be seen.

Fig. 2A–B

Lateral schematics of the pelvis in (A) standing and (B) sitting positions show variations of the sacral slope (SS) and the acetabular orientation. The SS increases in the standing position, whereas the acetabular sagittal inclination (ASI) angle decreases. The SS decreases in the sitting position and the ASI angle increases. The pelvic incidence (PI) represents the offset between the lumbosacral junction and the hips. The PI is a constant anatomic parameter. It does not vary whatever the pelvic tilt is.

Fig. 3A–C

(A) The standard CT scan protocol is performed with the patient in a supine position. It provides routine measurement of acetabular anteversion (AAA). (B) The proposed CT scan protocol includes an adjustment to the sacral slope, which replicates the standing position. First, the sacral slope (SS) is measured on the lateral radiograph in a standing position. Second, the sacral slope is transferred to the CT scan scout view (1 = sacral end plate; 2 = horizontal plane in a standing position). (C) Adjustment to the SS when measured on a lateral radiograph in a sitting position is shown. Changes of the measured AAA according to the plane of section can be seen.