Focal concavity of posterior superior acetabulum and its relation with acetabular dysplasia and retroversion in adults without advanced hip osteoarthritis

In the present study, we included adults less than 80 years old from consecutive patients with hip pain who visited our hospital and had three-dimensional CT images of the pelvis from January 2010 to August 2012. We excluded patients without standing pelvic radiographs of the anteroposterior and false profile views or with radiographic hip osteoarthritis Tönnis grades 2 and 3; mild hip osteoarthritis (Tönnis grade 1) was judged to be acceptable for the analysis of original morphology. Patients were also excluded if they had a history of hip fracture or surgery and diseases that affect the morphology of the hip including osteonecrosis of the femoral head and rheumatoid arthritis, or if CT images limited to measure angles precisely because of poor positioning and there were no raw data available to recreate reconstructed images. In addition to the analysis of all subjects, we performed a subgroup analysis that was limited to only younger adult patients up to 50 years to further focus on the original morphology after growth. The institutional review board of the Saitama Medical University Hospital approved the present study (approval No. 13-047-1); informed consent was waived because of the retrospective design.

Standing anteroposterior radiographs of the hip were made with the limbs parallel and with the feet internally rotated approximately 20°. The central beam was directed to the midpoint between the superior border of the pubic symphysis and the center of a line connecting both anterior superior iliac spines, at a distance of 120 cm from the film. False-profile radiographs of the hip were obtained in a standing position. Affected hip was positioned against the film cassette, with the ipsilateral foot parallel to the cassette stand. The pelvis was rotated 65° relative to the cassette. The x-ray beam was directed toward the center of the femoral head at a tube-to-film distance of 120 cm.

All CT images were acquired with a 16-slice or 128-slice multidetector CT scanner system (Somatom Emotion 16 or Somatom Difinition Flash; Siemens Healthcare, Forchheim, Germany). The scan parameters for the 16-slice CT scanner were tube voltage 130 kV, reference mAs 140 mAs, collimation 1×16×0.6 mm, gantry rotation time 0.6 s, pitch 0.9, pixel matrix size 512×512, and those for the 128-slice CT were tube voltage 120 kV, reference mAs 185 mAs, collimation 2×64×0.6 mm, gantry rotation time 1.0 s, pitch 0.8, pixel matrix size 512×512. Automatic exposure control (CARE Dose 4D, Siemens Healthcare, Forchheim, Germany) was activated in all scans. For a given reference mAs, this technique can adjust the tube current in real-time to optimize radiation dose utilization. The radiation doses of all patients were recorded; the average CT dose index volume (CTDI _vol) on 16-slice and 128-slice CT was approximately 12 mGy and 8 mGy, respectively, while the corresponding dose-length product (DLP) was approximately 375 mGy*cm and 238 mGy*cm. Patients were placed spine with the limbs parallel and with enough internal rotation for the feet to touch each other. Images were obtained from anterior superior iliac spines to the proximal portion of the femurs. Axial and coronal images were reconstructed at 3-mm slice thickness using filtered back projection. Three-dimensional volume-rendered images were acquired with a 0.75-mm reconstructed slice thickness and a 0.5-mm reconstruction increment, on Aquarius iNtuition 3D workstation (TeraRecon, Foster City, CA, USA).

Focal concavity of posterior superior acetabulum (Fig.  1 and Additional file 1: Figure S1) was evaluated by three-dimensional CT image of the pelvis and the selection was performed under the agreement of all authors. Acetabular dysplasia was determined by not only lateral center edge (LCE) angle <25° on standing anteroposterior radiographs, but also Tönnis angle >10° and anterior center edge (ACE) angle <25° on standing radiographs of the anteroposterior and false-profile views [ 8], respectively. LCE angle was formed by a vertical line through the center of the femoral head and a second line through the lateral edge of the acetabulum to the center of the femoral head. Tönnis angle was created by a horizontal line and a line connecting the lateral and inferior aspects of the acetabular sourcil. ACE angle was composed of a vertical line through the center of the femoral head and a second line through the most anterior point of the acetabulum to the center of the femoral head. Acetabular retroversion was judged by version angle <0° at the one-fourth cranial level of the acetabulum in an axial CT image according to a recent validation study [ 9]; we did not use cross-over sign because recent studies suggest that it might not provide the accurate diagnosis of acetabular retroversion [ 10, 11]. This angle was formed by a reference line which is perpendicular to a horizontal line connecting the posterior margins of both acetabuli, and a line connecting the anterior and posterior margins of the acetabulum. Two authors (HT and KW) with more than 10 years of experience in this field performed all measurements independently and their mean values were used for the analyses after confirming the inter-rater reliability shown in Additional files 2: Table S1, 3: Figures S2 and 4: Figure S3.

Comparisons of continuous variables for two groups and associations between categorical variables were analyzed by Mann–Whitney U test and Fisher’s exact test, respectively, using StatMate v4.01 (ATMS Co., Ltd., Tokyo, Japan). A p-value of <0.05 was considered statistically significant.