study design
From a database of 439 patients who underwent primary unilateral THA at Geneva University Hospital between 2016 and 2019 and had biplane radiographs 1 week to 1 month before and 2 months later, 28 retrospectively. patients were randomly selected. surgery. Patients with end-stage primary osteoarthritis were included, but patients with other medical conditions such as severe hip dysplasia, post-traumatic arthritis, or post-infective sequelae were excluded. Three reconstructions were performed in each session for each patient by two operators using a research version of the dedicated sterEOS software (EOS Imaging, Paris, France).Details of the protocol and statistical methods have been outlined in previous studies3 Data is shared in the online repository YaretaFourThis study was approved by the local ethics committee (CCER Geneva, Switzerland). Informed patient consent was obtained and protected by the Geneva Ahtroplasty Registry, and all experiments were performed in accordance with relevant guidelines and regulations.
Definition of acetabular offset
Two definitions of acetabular offset are used in the literature. The first used in CT scans is defined as the distance between the center of the femoral head and the true floor of the acetabulumFive Or, for standard X-rays, by the horizontal distance to the pelvic teardrop6The second definition concerns Pauwels balance.7 measured by the distance between the center of the acetabulum and the sagittal plane of the pelvis8Since the true floor is not visible on sagittal X-rays and the second definition is directly related to hip biomechanics, we follow this definition and generalize it in 3D by defining additional anteroposterior and vertical offsets. will be
Rationale for the method
Several pelvic anatomical landmarks are available when processing biplane X-rays with sterEOS software: acetabulum, center of sacral slope, symphysis pubis, and anterior superior iliac spine. To compare pre- and post-operative offsets, the anatomical plane of the pelvis should be identical before and after surgery. For biplane x-rays before and after THA, the operated acetabulum is the only affected landmark, so other landmarks can be used.However, the location of the anterior superior iliac spine is less reliable3Therefore, the anterior pelvic plane is not used as a reference plane. Reliable and available landmarks therefore include the center of the sacral slope, the pubic symphysis, and the contralateral acetabulum. These points do not define the anatomical plane of the pelvis, but to link preoperative and postoperative measurements while the anatomical plane is defined only once on his preoperative X-ray. can be used for A 3D Cartesian coordinate system is used for this purpose.
principle of the method
Acquiring similar anatomical coordinate systems in pre- and postoperative X-rays requires five steps (Figure 1). First, an anatomical coordinate system is defined in preoperative X-rays. This coordinate system defines the anatomical plane of the pelvis and the scale of the acetabulum offset. Second, the technical coordinate system is identified on the same preoperative X-ray. Third, the orientation of the technical coordinate system relative to the anatomical coordinate system is identified. A technical coordinate system is then identified on the postoperative X-ray. Finally, a geometric transformation between the technical and anatomical coordinate systems is applied postoperatively to obtain the same anatomical coordinate system orientation on postoperative X-rays as preoperatively. The origin of this new coordinate system is set as the anatomical landmark specified by definition and identified on the post-operative X-ray.
method workflow. Tech technical abbreviation, Anat anatomically, CS For the coordinate system, APs for front and back, ML For mediolateral Ⅴ for vertical, and measure. for measurement. \({{\varvec{R}}}_{T\to A}\) Represents the orientation of the technical coordinate system relative to the anatomical coordinate system.
Since the pre- and post-operative anatomical planes of the pelvis are defined in the same way, it is possible to calculate the acetabular offset and assess post-operative changes with the same definition of the anatomical plane. Acetabular offset describes the distance of the center of the acetabulum to the origin of the anatomical coordinate system expressed along the medial-lateral, anterior-posterior, and vertical axes of the coordinate system. These parameters are lengths, so the values are positive.
Coordinate system definition
Multiple definitions of technical and anatomical coordinate systems are possible based on available anatomical landmarks. We first present the definitions of multiple coordinate systems (Fig. 2) and then show how to assess the reliability of the measurements.
Definitions of the technical (Tech) and anatomical (Anat) coordinate system (CS) and two medial-lateral (ML) axes. Images of the pelvis were obtained from www.biodigital.com.
technical coordinate system
The coordinate system has three axes and one origin. Since three points are available, only one set of three axes can be defined. Multiple origins are possible, but the origin of the post-operative anatomical coordinate system is an anatomical landmark measured post-operatively, so the origin of the technical frame does not affect the value of the offset. The origin of the technical coordinate system was arbitrarily chosen as the center of the acetabulum. Calculation details are provided in Supplementary File 1.
anatomical coordinate system
Two definitions are possible for the medial-lateral axis. The first shows the line from one acetabulum to the other acetabulum (ML1). The second is a vector orthogonal to the plane defined by the midpoint of the acetabulum, the pubic symphysis, and the center of the plane tilt (ML2).
Two definitions are also possible for the front-back axis and the vertical axis. First, the anterior-posterior axis is defined by the midpoint of the acetabulum and the pubic symphysis. The vertical axis is defined as the cross product of the anterior-posterior axis and the medial-lateral axis. In the second definition, the vertical axis is defined by the midpoint of the acetabulum and the center of the sacral tilt. Anteroposterior axis is defined as the cross product of the medial lateral axis and the vertical axis. Calculation details are provided in Supplementary File 1.
To avoid zero offset values, the origin of the anatomical coordinate system should be outside the plane of the three points used to construct the three axes. Therefore, for the first anatomical coordinate system, the origin is only the center of the sacral slope, and for the second anatomical coordinate system, the origin is only the symphysis pubis.
Combining different axis definitions, there are four possible anatomical coordinate systems.
Method evaluation
As acetabular offset in the contralateral acetabulum is not affected by surgery, this parameter can be used to assess intra-operator, inter-operator and test-retest reliability. According to COSMIN classification and checklist9, the reliability domain includes reliability assessed by the intraclass correlation coefficient (ICC) and measurement error assessed by the smallest detectable change (SDC). The ICC was classified as: Bad (< 0.5)、中程度 (0.5–0.75)、良い (0.75–0.9)、優れた (> 0.9)TenSDC was classified as follows: poor (> 10 mm/°), moderate (5 to 10 mm/°), good (3 to 5 mm/°), excellent (< 3 mm/° ).3.
The reliability of the calculated offsets in 12 combinations of technical and anatomical coordinate systems was evaluated. The most reliable combination of acetabulum and global offset was chosen as the best definition. The goal of acceptable measurements was to achieve an ICC and SDC equal to or better than the femoral offset approved for clinical use.These reliability results of femoral offset were evaluated in a previous study using the same database3 The results yielded good to excellent ICC (0.853 to 0.916) and average SDC of 4.8 mm (4.3 to 5.7 mm).
Average offset values are shown in Table 1. All ICCs were rated good to excellent for acetabular offset (Table 2) and global offset (Table 3). The anatomical coordinate system defined by the center of the acetabulum and sacral slope (A2) and the medial-lateral axis (ML1) defined by the acetabulum have a lower SDC than other acetabular offsets, 3 It showed an average SDC of 6.3 mm across the axis. (4.3 mm to 8.7 mm) (Table 2). With this definition, the mean SDC was equal to 13% of the offset value and was 13.5%, 5.3%, and 26.1% in the anteroposterior, mediolateral, and vertical directions, respectively. The mean SDCs for global offsets were all moderate (6.3–8.8 mm, Table 3) and 5.2% (4.7–6.7%) of mean offset values.
