Definition
The PTis usually defined as the angle between the anterior pelvic plane (APP) and the coronal plane[17]. However, during the course of measuring the PT on lateral radiographsobtained in the supine position, the two anterior-superior iliac spines of patients were not aligned owing to the postural change and unclearasobscured byintestinal gas, which made measurement difficult to perform. Thus, given the convenience and reproducibility of measurement, we defined the PT as the angle created by a line running from the first sacral midpoint to the symphysis pubis and a line perpendicular to the horizontal plane in the supine position(Fig. 1a).To determine the normal value, we selected six healthy volunteers (three men and three women,aged 65 to 75years) without pelvic deformities and a history of surgeries to undergo pelvic CT scan for the creation ofthree-dimensional (3D)anatomically accurate pelvic models using Geomagic Design X 2016 (Geomagic Inc., Morrisville, North Carolina, USA). The mean PT of the six 3D models measuredin accordance with our definition was 60.22° (range, 58.9°-62.3°). Accordingly, a PT of 60° was set as a natural position to eliminateall confusingfactors.
VAsmeasurement on simulated CL images
To identify the influence of the PT on the acetabular cup version measurement on CL radiography, we constructed the 3D postoperative models of the pelvis usingGeomagic Design X 2016. Initially, we used a laser equipment to scan the titanium converge acetabular cups with a diameter of 48-52mm (Trilogy® Acetabular Hip System, Zimmer, Warsaw, Indiana, USA) to establish the cup models. The pelvic models were already completed during the measurement of the normal value of the PT as previously defined. Thereafter, an inclination of 60° was set as the natural position of the pelvic models as described above. According to the safe zone described by Lewinnek et al. [1], an anteversion of 15° and an inclination of 45° were set as the initial position before the cup models were implanted. After the parameters were settled down, the six postoperative hip models were completed by implanting the prostheses into the pelves. We set the cup anteversion measurement and the PT as different units so that the actual VA could be changed. The setting range of the PT and actual VA was 20°-80°and 0°-40° at intervals of 5°.
The procedure forCL radiographyhas been described in detail in a previous study[18]. To simulate this, we placedthe completed3D model in thesupine position, rotated it clockwise to 45°, and centered it on the top of the acetabular cup. Thereafter, the model was projected onto the coronal plane, and a two-dimensional image was captured by screen as a CL film simulation.The measurementof the VA on CL radiography was performed using the method introduced by Woo and Morrey(Fig.1b)[9].
After different groups of the PT and actual VA were set up, the VAswere measured on the CL simulated radiographs from thesix 3D models. From these data, a mathematical equationrelated tothe PT, actual VA, and measured VA was derived. Through the formula, the VA measurement could be converted into the actual VA of the 3D model under the PT of 60°, which indicates that using a mathematical solution can transfer values of cup version from the individual pelvic position to the natural pelvic position.
VAs measurement on real CL films
A total of 50 patients who underwent primary THAfor hip degeneration diseases were recruited in this study between July 2019 and December 2019, including four men and nine women with an average age of 56 years (range, 32-78 years) andbody mass index of 22.74 kg/m2(range, 17.60-24.84 kg/m2). None of the patients had spinal or pelvic surgeries or deformities. The same team of surgeons performed allcementless THAs using a posterolateral approach or a direct anterior approach. All acetabular cups were selected from the Trilogy® Acetabular Hip Systemand R3®Acetabular Hip System (Smith&Nephew, Inc., Memphis, Tennessee, USA). This study was approved by the ethics committee of our institution. All patients provided written informed consent.
To measure the VA and PT, we performed CL radiography and additional lateral radiography1 day after surgery for each patient. For CL radiography, the patients were placed in the supine position with the contralateral hip flexed at 90° and the contralateral leg placed on a small stand to maintain the position. The radiation beam was positioned parallel to the examination table at 45° to the long axis of the body, and the radiographic film was positioned perpendicular to the examination table[18]. The method used to measure the anteversion of the acetabular cup on the CL radiographs was in accordance with the method used in the 3D simulation (Fig.1c).For additional lateral radiography,the patients’ posture was maintained at the level of the CL film to ensure that the PT was invariable. However, the difference was thatthe radiographic incidence was adjusted from 45° to 90°, enabling the measuring markers to be displayed completely and clearly. The method used to measure the PT on this lateral radiograph was based on the PT definition(Fig.1d).
Consistencyverification by CT
The accuracy and reliability of the measured VA and corrected VA were verified by comparing them with the CT values.The patientsunderwenta CT scan for the pelvis 7 days after surgery and were placed in the supine position with the bilateral hip joints in a neutral position. To ensure that the VA measured on CT and CL radiographywas compared at a consistent PT, we used Geomagic Wrap 2017 (Geomagic Inc.) to create the 3D pelvic model based on CT data so that the PT could be set to a certain value.The VAwas measured under the PT set to the measured value onthe radiographs and 60° to compare the CL radiography-measuredVA and the formula-corrected VA.The method of measuring the VAon CT was referenced to a previous study(Fig.1e)[10].
Statistical analysis
Three qualified orthopedic surgeons performedall measurements. The intra-class correlation coefficient (ICC) and 95% confidence interval were calculated to assess the intra-and inter-observer reliabilities. We used the two-way randomeffects intra-class correlation model and absolute agreement to calculate the ICC: an ICC of 1 indicated perfect reliability, while an ICC of 0 indicated the opposite. Linear regression was used to assess the correlation between thePT and acetabular cup anteversion. The ICC was used to determine the validity of the measured VA and corrected VA by comparing them with the corresponding CT values.Statistical analyses were performed using SPSS ver. 22.0 (SPSS Inc., Chicago, Illinois, USA). The significance level was set at p-values of <0.05.