What Is the Functional Spinopelvic Relationship in Three Dimensions? A CT and EOS Study

Understanding the spinopelvic relationship is essential in THA planning, especially given the elevated hip dislocation risk in patients exhibiting abnormal spinopelvic movements. Rotations of the spinopelvic unit affect the functional orientation of the acetabulum and, in turn, the placement of the acetabular cup. Currently, however, the kinematic behavior of the pelvis is not considered preoperatively. Standard CT scans and radiographs only guide component positioning based on the supine position, which may result in suboptimal acetabular cup alignment in more functionally relevant positions. Therefore, the ideal imaging for 3-dimensional (3D) planning of hip surgery is full-length standing CT, which is yet to be implemented given the technical and logistical challenges and high radiation doses involved.

(1) What is the pelvic tilt in the functional positions of supine, standing, and seated? (2) How does the pelvic orientation change when transitioning between these positions?

Between 2020 and 2023, we treated 36 patients for osteoarthritis (OA) who underwent preoperative CT and EOS imaging for their primary THA at our tertiary center. We considered all of those with a satisfactory CT and EOS scan as potentially eligible for this study. Based on that, 86% (31) were confirmed as eligible for analysis here; 14% (5) were excluded because of an incomplete scan field of view, presence of a spinal implant, or because of difficulty in identifying the anterior pelvic plane (APP). The final analysis included 31 patients with OA (23 women and 8 men, mean ± SD age 63 ± 13 years). We proposed a comparison method that uses 3D models of the supine CT-generated bony anatomy and standing and seated full-body biplanar radiography (EOS scans) to obtain the absolute and relative values of pelvic orientation in this patient cohort. To answer our first research question, we performed computational measurements of the patients' pelvic tilt in all three planes (sagittal, coronal, and axial) and in three functional positions: supine, standing, and seated. To answer our second question, we compared and studied each patient's pelvic rotation as they transitioned between these positions. The outcome measures were the angular measurements of (1) pelvic tilt from CT (supine) and EOS (standing and seated) and (2) the change in pelvic tilt when transitioning from supine-standing, supine-seated, and standing-seated.

The mean ± SD sagittal pelvic tilt was greatest in the seated position, least in the standing position, and intermediate in the supine position (-26° ± 12° versus -2° ± 9° versus 6° ± 7°, respectively; p < 0.001). A positive pelvic tilt value denoted an anterior tilt of the APP with respect to the coronal plane and a negative value denoted a posterior tilt. The mean difference in the sagittal pelvic tilt between the supine and standing position was 8° ± 6° (95% confidence interval [CI] 6° to 10°; p < 0.001). The mean difference in the sagittal pelvic tilt between the supine and seated position was 32° ± 13° (95% CI 27° to 38°; p < 0.001). The mean difference in the sagittal pelvic tilt between the standing and seated position was 26° ± 15° (95% CI 20° to 31°; p < 0.001). When comparing the supine to standing transition to the standing to seated transition, the mean difference in the sagittal pelvic rotation was -19° ± 18° (95% CI -26° to -12°; p < 0.001). When comparing the supine to standing transition to the supine to seated transition, the mean difference in the sagittal pelvic rotation was -26° ± 15° (95% CI -31° to -20°; p < 0.001). When comparing the supine to seated transition to the standing to seated transition, the mean difference in the sagittal pelvic rotation was 7° ± 5° (95% CI 5° to 9°; p < 0.001).

The findings from this study show that patients with hip arthritis exhibit pelvic rotations between their daily movements and postures; this rotation may be excessive for some individuals given the large sagittal pelvic tilts reported. This puts some patients at risk of dislocation, edge loading, and limited ROM given that cup orientation in THA is widely known to be affected by the sagittal pelvic tilt. Our work therefore emphasizes the importance of measuring patient-specific pelvic positions and rotations before hip replacement procedures and offers a method to do so.

The functional orientation of the pelvis and the anatomic-functional relationship between the pelvis and spine are seldom considered by surgeons when positioning prosthetic acetabular components. Performing EOS scans in addition to pelvic CT scans can help the surgeon better understand the spinopelvic relationship and the 3D pelvic orientation in the functional supine, standing, and seated positions. This can aid the planning of THA as patients with extreme pelvic parameters and an increased risk of instability can be identified prior to the surgery, and the orientation of the acetabular cup may be adjusted accordingly.