From April 2018 to April 2019, we investigated 6 patients (6 hips) with symptomatic DDH. All patients had bilateral dysplasia of the hip and had pain in one hip joint. The mean age at the time of the first medical examination was 30.2 years (18 to 48). All patients were females. The mean body mass index was 21.6 kg/m2 (18.9 to 23.0). DDH was classified using the Tönnis et al. system. All hips in this study had Grade 0 DDH, and none had Grade 1, 2, or 3 [6]. None of the patients had undergone surgery during infancy to reduce congenital dislocation of the hip.
At the time of the first medical examination, patients began a 6-month trial of nonsurgical management consisting of patient education, activity modification, physical therapy, and/or anti-inflammatory medications [7].
Surgical management, specifically PAO [8] [9] [10] [11], was performed in 3 patients (3 hips) in whom pain was still present after 6 months of nonsurgical management for DDH (PAO group). The remaining 3 patients (3 hips) reported reduced pain after conservative treatment and did not undergo PAO (non-PAO group). The mean follow-up duration after the first medical examination was 23 months (8 to 41), and the mean follow-up duration after surgery was 11.3 months (8 to 18).
The Japanese Orthopaedic Association (JOA) scoring system was used to evaluate hip joint function [12]. The JOA system consists of a 100-point scale comprising the following subcategories: pain (0 to 40 points), ability to walk (0 to 20 points), range of motion (0 to 20 points), and ability to complete tasks of daily living (0 to 20 points). Higher scores indicate better function. Scores at the final follow-up were compared to those obtained preoperatively.
Radiographic examination was performed both at the first and final medical examinations to calculate the center-edge (CE) angle [13] [14], Sharp angle [15], acetabular-head index (AHI), and vertical-center-anterior (VCA) angle in the false profile view [16].
Image acquisition was performed using a computed tomography (CT) scanner (Philips Brilliance® 64 scanner; Marconi Medical Systems, Best, Netherlands) and an X-ray flat panel detector system (FPD, Zexira®; Toshiba, Tokyo, Japan). CT scans were taken of the hip area from the bilateral anterior superior iliac spines to the distal ends of the femurs. DICOM-compliant CT images were taken under the following conditions: resolution, 512×512 pixels; slice thickness, 0.67 mm; and pixel size, 0.391 mm × 0.391 mm. The CT data were then converted to voxels to construct a 3D gray-scale digital image. The 3D gray-scale model was located in a virtual 3D space, and computer simulation of the radiographic process was carried out to generate virtual radiographic images in which the light source and projection plane parameters were set identical to the actual FPD imaging conditions. The relative geometric relationship between the X-ray light source and the projection plane (flat panel sensors) of the FPD system was determined using a coordinate building frame. The simulated value A of a voxel at a point (x, y) on the project plane was defined by:
A (x,y) =\({\sum }_{i}^{n}{a}_{i}{L}_{i},\) where ai is the value of a property of interest (e.g., bone mineral density) per unit length of the ith voxel through which a virtual X-ray beam passes, Li is the length of the ith voxel, and n is the number of voxels through which a virtual X-ray beam travels (Fig. 1).
Virtual 2D images generated from the 3D gray-scale model were then compared with the serial X-ray images acquired using the FPD. Correlations of the pixel values between the virtual and real images were used to fine-tune the 3D model (Fig. 2). Multiple small image windows that spanned the bone edge were defined for the image-matching analysis [5].
Using the FPD, DICOM-compliant X-ray images of the hip joint were obtained, each measuring 2048 × 2048 pixels with a 0.148-mm pixel pitch. The hip joint was positioned near the flat panel sensors during motion, and images were taken from the anteroposterior side. The frame rate was set at 3 frames/sec to acquire high-resolution images. The pelvic and femoral coordinate systems were determined based on the study by Cappozzo et al. [17] [18].
Dynamic instability of the hip joint was defined as the mean 3D translation between the maximum and minimum values of the femoral head center for the acetabular center at hip abduction angles from 0° to 30° (Fig. 5).
Clinical assessments and radiographic measurements were completed twice by 2 orthopedic surgeons, each with more than 15 years of experience in assessing hip function. Both surgeons were blinded to the radiographic results at the time of the evaluation. The time between measurements was at least 2 weeks. Intra- and interobserver variances were calculated.
Statistical analysis
The normality of continuous data was assessed with Levene’s test. Since the data were normally distributed, the unpaired Student’s t-test was used. Intraobserver variances in the JOA hip score were determined by comparing separate radiographic assessments of the same patient, performed by the same observer with at least a 2-week interval between assessments. Intra- and interobserver variances in the JOA hip score were determined by comparing radiographic measurements and are expressed using interclass correlation coefficients (ICCs), with ICC < 0.20 indicating slight agreement; 0.21 to 0.40, fair agreement; 0.41 to 0.60, moderate agreement; 0.61 to 0.80, substantial agreement; and > 0.80 almost perfect agreement [19]. JMP® for Windows version 15.1 (SAS Institute Japan) was used for all statistical analyses. A p value of < 0.05 indicated statistical significance.
Ethics
This study was approved by our institution’s Ethics Committee and was conducted in accordance with the World Medical Association Declaration of Helsinki Standard of 1964, as revised in 1983 and 2000. All patients were informed about the study in detail before providing written informed consent for enrollment, including consent for postoperative CT imaging.