How Does Radiographic Acetabular Morphology Change Between the Supine and Standing Positions in Asymptomatic Volunteers?
Authors: Vorimore Camille , Verhaegen Jeroen C. F. , Kashanian Koorosh , Horton Isabel , Beaule Paul , Grammatopoulos George 
Published: April 2024

Background
The radiographic appearance of the acetabulum differs between the supine and standing positions in patients with hip conditions. The pelvis undergoes a change in tilt when transitioning between positions, resulting in variations in version and acetabular coverage. However, the extent of these variations in well-functioning volunteers without compensatory patterns caused by pain is unknown.

Questions/purposes
We performed this study to (1) quantify differences in radiographic acetabular measurements when transitioning between supine and standing among asymptomatic, well-functioning volunteers; (2) assess differences in pelvic tilt between positions; and (3) test whether individual anatomic parameters are associated with the change in tilt.

Methods
This was a prospective, single-center study performed at an academic referral center. One hundred volunteers (students, staff, and patients with upper limb injuries) with well-functioning hips (Oxford hip score ≥ 45) were invited to participate. A total of 45% (45) of them were female, their mean age was 37 ± 14 years, and their mean BMI was 25 ± 2 kg/m2. Supine and standing AP pelvic radiographs were analyzed to determine numerous acetabular parameters including the lateral center-edge angle (LCEA), acetabular index (AI), anterior wall index (AWI), posterior wall index (PWI), crossover sign (COS), crossover ratio (COR), posterior wall sign (PWS), ischial spine sign (ISS), and femoroepiphyseal acetabular roof index (FEAR), as well as pelvic parameters including the sacrofemoral‐pubic angle (SFP). Spinopelvic parameters were measured from lateral standing spinopelvic radiographs. Radiographic measurements were performed by one hip preservation research fellow and a fellowship-trained staff surgeon. Differences in parameters were determined, and correlations between postural differences and morphological parameters were tested. Clinically important differences were defined as a difference greater than 3° for acetabular angle measurements and 0.03 for acetabular ratio measurements, based on previous studies.

Results
Lateral coverage angles did not show a clinically important difference between positions. AWI decreased when standing (0.47 ± 0.13 versus 0.41 ± 0.14; p < 0.001), whereas acetabular retroversion signs were more pronounced when supine (COS: 34% [34 of 100], PWS: 68% [68 of 100], and ISS: 34% [34 of 100] versus COS: 19% [19 of 100], PWS: 38% [38 of 100], and ISS: 14% [14 of 100]; all p values < 0.05). Pelvic tilt increased by a mean of 4° ± 4° when standing, but the range of change was from -15° to 7°. The change in AWI (ρ = 0.47; p < 0.001), PWI (ρ = -0.45; p < 0.001), and COR (ρ = 0.52; p < 0.001) between positions correlated with ΔSFP. Volunteers with spinal imbalance (pelvic incidence lumbar lordosis > 10°) demonstrated greater change in pelvic tilt (ΔSFP) (-7° ± 3° versus -4° ± 4°; p = 0.02) and a greater reduction in AWI (by 10%). These volunteers demonstrated reduced standing lumbar lordosis angles (45° ± 11° versus 61° ± 10°; p = 0.001).

Conclusion
Acetabular version increases from supine to standing because of an increase in pelvic tilt. The change in pelvic tilt between positions exhibited substantial variability. Individuals with reduced lumbar lordosis for a given pelvic incidence value demonstrated greater pelvic mobility. No features on supine radiographs were associated with the change in tilt.

Clinical Relevance
Performing standing radiographs in addition to supine views can help identify aberrant physiologic patterns in patients with diagnostic dilemmas and might thus help with management. Normative data of pelvic tilt change can help clinicians identify patients who demonstrate excessive change in tilt that contributes to abnormal hip pathomechanics.