Measurements of pelvic flexion angle using three-dimensional computed tomography.

Area of Science:

• Orthopedic Surgery
• Biomedical Engineering
• Radiology

Background:

• Acetabular component positioning is critical for total hip arthroplasty (THA) success.
• Pelvic tilt significantly influences acetabular orientation.
• Understanding positional variations is key to optimizing component placement.

Purpose of the Study:

• To evaluate if safe acetabular component positioning is affected by pelvic location differences.
• To determine the impact of supine, standing, and sitting positions on pelvic tilt.
• To establish a reliable reference frame for acetabular component orientation in THA.

Main Methods:

• 101 THA patients underwent pre-operative and 1-year post-operative pelvic radiographs (AP) in supine, standing, and sitting positions.
• Pre-operative CT scans were used to create 3D pelvic models.
• Image matching between CT models and radiographs calculated pelvic flexion angles for each position.

Main Results:

• Significant inter-subject variability in pelvic flexion angles was observed across positions.
• The mean pre-operative pelvic flexion angles were: supine (5° ± 9°), standing (3° ± 12°), and sitting (-29° ± 12°).
• Pelvic position pre-operatively was largely maintained 1 year post-THA, with significant pelvic extension (≥20°) from supine to sitting.

Conclusions:

• Anatomic landmarks are insufficient for determining acetabular component orientation due to pelvic tilt variability.
• The supine pelvic position can be considered a functional reference frame for acetabular component orientation in approximately 90% of cases.
• Adjustments may be necessary for the remaining cases, but supine positioning offers a reliable baseline for THA component placement.