Functional acetabular orientation varies between supine and standing radiographs: implications for treatment of femoroacetabular impingement.

Background: Often, anteroposterior (AP) pelvic radiographs are performed with the patient positioned supine. However, this may not represent the functional position of the pelvis and the acetabulum, and so when assessing patients for conditions like femoroacetabular impingement (FAI), it is possible that standing radiographs better incorporate the dynamic influences of periarticular musculature and sagittal balance. However, this thesis remains largely untested.

Questions/purposes: The purpose of this study was to determine the effect of supine and standing pelvic orientation on (1) measurements of acetabular version and common radiographic signs of FAI as assessed on two- and three-dimensional (3-D) imaging; and (2) on terminal hip range of motion (ROM).

Methods: Preoperative pelvic CT scans of 50 patients (50 hips) who underwent arthroscopic surgery for the treatment of FAI between July 2013 and October 2014 were analyzed. The mean age of the study population was 29 ± 10 years (range, 15-50 years) and 70% were male. All patients had a standing AP pelvis radiograph, a reconstructed supine radiograph from the CT data, and a 3-D model created to allow manipulation of pelvic tilt and simulate ROM to osseous contact. Acetabular version was measured and the presence of the crossover sign, prominent ischial spine sign, and posterior wall sign were recorded on simulated plain radiographs. Measurements of ROM to bony impingement were made during (1) simulated hip flexion; (2) simulated internal rotation in 90° of flexion (IRF); and (3) simulated internal rotation in 90° of flexion and 15° adduction (FADIR), and the location of bony contact between the proximal femur and acetabular rim was defined. These measurements were calculated for supine and standing pelvic orientations. A paired Student's t-test was used for comparison of continuous variables, whereas chi square testing was used for categorical variables. A p value of < 0.05 was considered significant.

Results: When changing from supine to the standing radiographs, both mean cranial and central version increased by 2° ± 4° (95% confidence interval [CI], 1°-3°) and 2° ± 3° (95% CI, 1°-3°), respectively (both p < 0.001). However, with the numbers available, there were no changes in the proportion of positive crossover, posterior wall, and prominent ischial spine signs. Standing pelvic position tilt resulted in an increased hip flexion of 3° (95% CI, 2°-4°) as well as an increase in IRF of 2° (95% CI, 1°-3°) and FADIR of 3° (95% CI, 2°-4°) (all p < 0.001).

Conclusions: The functional orientation of the acetabulum varies between supine and standing radiographs and must be considered when diagnosing and treating patients with symptomatic FAI. Standing pelvic orientation results in posterior pelvic tilt and later occurrence of FAI in the arc of motion. Although we cannot recommend standing radiographs on the current study alone, we do recommend larger studies to determine whether any significant differences truly exist.