FP1.9 Changes in Acetabular Coverage in Supine and Simulated Standing Position: A 3-Dimension Modeling Study

Background: Accurate acetabular coverage analysis is essential in surgical planning for periacetabular osteotomies (PAO) to determine adequate rotational correction. A 2D analysis may fail to characterize the 3D morphology of the hip joint. Coverage assessment in the weight-bearing position may be more clinically relevant for the treating surgeon.

Purpose: To evaluate the utility of dynamic CT-derived 3D models to assess changes in acetabular coverage patterns from native supine position to a simulated standing position.

Methods: On standing 2D radiographs, the distance from the pubic symphysis to the sacrococcygeal (PS-SC) joint was used as standing sagittal pelvic tilt. The 3D pelvis and sacrum were rotated around the center of the femoral head to match the PS-SC distance to that of the standing radiograph. The acetabular rim was superimposed onto the superior surface of the femoral head using previously described techniques. The femoral head was divided into quadrants and percent coverage was collected. The anterior pelvic plane (APP) was established at the anterior most point of the anterior superior iliac spine and the pubic symphysis. The angle between the APP and a vertical z-line was used to find the APP angle. A Pearson correlation analysis was performed to determine what factors were most associated with pelvic tilt as measured by the APP angle.

Results: Twenty hips undergoing PAO were selected for analysis (age: 23.9±8.0 years, % female: 95%, body mass index: 26.9±5.4, acetabular version: 23.0±5.4, femoral version: 16.8±9.9). From supine to standing, there was increased posterior pelvic tilt for 70.0% of hips, minimal change in tilt for 20.0% of hips, and increased anterior pelvic tilt for 10.0% of hips. Coverage in supine to standing position trended towards decreasing anterior coverage and increasing posterior coverage 3D models. This was most prominent for the anteromedial (AM) quadrant which decreased from 80.1±10.3% to 77.6±8.8% and the posterolateral (PL) quadrant which increased from 30.2±13.0% to 31.6±12.9%. On correlation analysis, decreased AM coverage was associated with increased pelvic tilt (r=-0.360, p=0.031) and increased posteromedial coverage was associated with increased pelvic tilt (r=0.376, p=0.024). Further, decreased pelvic incidence was associated with increased pelvic tilt (r=-0.458, p=0.006).

Conclusions: Supine to standing position is associated with increasing posterior pelvic tilt for most hips. Increased pelvic tilt was correlated to decreased anterior coverage and increased posterior coverage. Dynamic 3D models can be used to simulate pelvic tilt in weight-bearing position which may provide utility for preoperative PAO planning.