3D simulation to investigate the relationship between incidence and cup version
SL radiography could be regarded as a variation of CL radiography, with the patients positioned from supine to standing, and the X-ray incidence adjusted from 45° to 90° (Fig.1). The main difference observed in the image was the projected shape of the cup rim caused by the change in the incident angle of the beam. Cup version on CL radiograph was measured between the vertical line of the film and the tangent line of the opening face of the acetabular cup . The same method may be applied to SL radiograph, but it was necessary to identify whether the change in X-ray incidence significantly affects cup version measurement. However, it was impractical to perform multiple lateral radiographs with different X-ray incidences in patients considering the radiation exposure and ethical issues. In a previous study, a THA postoperative 3D model was built, in which some parameters, including the cup version, inclination, pelvic tilt, and incidence, were independently adjusted . Independent variables and confounding factors were controlled using the standard physical model. Thus, in this study, standard 3D models were used to investigate the effect of incidence on cup version measurements.
This study recruited four healthy volunteers (two men and two women) without pelvic deformity who underwent surgery to perform a pelvic CT scan. The 3D pelvic models were built based on the pelvic CT data. Then, laser equipment was used to scan the titanium converge acetabular cups with a diameter of 48–52 mm (R3◊ Acetabular System, Smith & Nephew, Inc., Memphis, Tennessee, USA) to establish the cup models. Finally, all postoperative models were created by integrating the cup model with the pelvic model. All 3D models were constructed using Geomagic Design X 2016.
In these 3D postoperative models, the actual versions and incidences can be independently set. Five groups of actual versions were set from 10° to 30°, at intervals of 5°. The X-ray incident angles were adjusted at intervals of 5° in the range of 45–90°. Measurements of five groups of actual versions using the CL radiograph method described by Woo et al.  were performed on the 3D models under different groups of incidences (Fig. 2). The above measurements were repeated for the four postoperative 3D models.
Clinical validation of the SL radiograph method
Patients undergoing primary cementless THA between April 2020 and December 2020 were included in this study. Patients with a pelvic surgical history or spinal or pelvic deformity were excluded. In total, 87 consecutive patients were evaluated, including 43 men and 44 women with a mean age of 58.6 years (range, 19–84 years) and body mass index of 23.3 kg/m2 (range, 15.8–33.3 kg/m2) at the time of the operation. The indications for THA were femoral head osteonecrosis in 37 hips (42.5%), osteoarthritis in 25 hips (28.7%), femoral neck fracture in 15 hips (17.2%), and others in 10 hips (11.5%). Four experienced orthopedic surgeons performed all operations using a posterolateral approach. All prostheses were selected from the R3◊ Acetabular System and POLARSTEM◊ Cementless Stem System (Smith & Nephew, Inc., Memphis, Tennessee, USA). The patient information was summarized in Table 1. This study was approved by the ethics committee of our institution, and informed consent was obtained from all participating patients.
One week after THA, images including SL radiograph and CT scan were obtained to measure the acetabular component version. For a SL radiography, patients naturally stood with their feet together and were instructed to stand as still as possible. The radiation beam was centered over the greater trochanter and intersected the longitudinal axis of the body at a right angle. Imaging ranged from the upper edge of the sacrum to the lower edge of the stem (Fig. 1b). During the CT scan, the patients were in a supine position with the bilateral hip joints in a neutral position. All standardized radiographs and CT scans were performed by the same group of radiology technicians.
Measurement of version
Unlike the CL radiograph, the edge of the cup opening face was not clearly observed on the SL film to take the tangential line. We used the image processing system in our hospital to establish a circle through the outer edge of the cup. This circle intersected the elliptical arc of the cup opening face. The line of the two intersecting points was the long axis of the elliptical open face of the acetabular cup, which was similar to the tangential line of the cup opening face on the CL radiograph. The line is defined as the matching tangent line. The cup version on the standing radiograph was measured between the vertical line of the longitudinal axis of the body and the matching tangent line (Fig. 3a). All measurements were performed using an image-processing system in our hospital.
Assessment of reliability and accuracy
All measurements were performed by two observers who were blinded to the patients’ information and the other observers’ values. All images were randomly assigned to each observer by a research assistant who did not participate in the reliability assessment. Reliability refers to the consistency of the measurements. The intra-observer reliability of each method was assessed using the values measured by one examiner who performed the reassessment 4 weeks later. The inter-observer reliability of each method was assessed using the same two examiners.
Accuracy was defined as the proximity of the reference standard. CT scans was known as an accurate method to learn the true cup version . However, variations of position had been proved to affect cup version by changing the PT [19, 22] and the supine position of CT scan was different from the standing lateral radiograph. Thus, we did not compare the radiographic measurements directly with the CT measurements. When measuring cup version in CT images, we adjusted the PT of CT images (Fig. 3b) matched with that measured on the SL radiograph (Fig. 3a). We then captured the sagittal plane through the center of the femoral head to show the actual version. Cup version was the angle between the line through the cup anterior and posterior edge and the horizonal line (Fig. 3c). Pelvic tilt measured in radiographs and CT images was based on the reported method: pelvic tilt was defined as the angle between a horizontal line and a line connecting the upper border of the symphysis with the sacral promontory (Fig. 3a, 3b) .
In the 3D simulation analysis, the measurements of each group of versions at different incidences were evaluated using the two-way classification ANOVA. The intra- and inter-observer reliabilities of all measurements were calculated using the intraclass correlation coefficient (ICC) and 95% confidence interval (CI). The two-way random effects intraclass correlation model and absolute agreement were used to calculate the ICC; an ICC of 1 indicated perfect reliability, while an ICC of 0 indicated the opposite. To determine the convergent validity, version measurements of the radiographs and CT scans were compared using the paired t-test. The correlation between mean radiological and CT measurements was evaluated by Pearson’s correlation coefficient (r). Correlation was characterized as poor (0.00 to 0.20), fair (0.21 to 0.40), moderate (0.41 to 0.60), good (0.61 to 0.80) or excellent (0.81 to 1.00) . Bland–Altman plots were presented to illustrate the difference between the methods. If the differences were within the 95% limits of agreement (95% LoA), the differences were clinically acceptable and the measurements of the two methods had good agreement. Statistical analyses were conducted using SPSS for Windows (version 25.0; SPSS Inc., Chicago, Illinois, USA), and statistical significance was set at p < 0.05.