Participants
The study was approved by the institutional review board of our hospital (approval number 1912) and performed in line with the principles of the Declaration of Helsinki (1964) and its subsequent amendments. All patients provided written informed consent for their participation in the study and the publication of their data. Between January 2015 and December 2018, 285 primary THAs were performed at our institution. Of these, 246 patients (270 hips), who were Asian, completed a minimum follow-up of 2 years and were enrolled into this study. From this group, we excluded 34 patients (58 hips) who had undergone a staged bilateral THA history (46 hips) or had a history of spinal surgery (five hips), new vertebral compression fracture (three hips) [18], THA with subsequent lumbar spine fusion (two hips), or simultaneous THA (two hips) during the follow-up period. For a few patients, the femoral head was not visible on radiographs and, thus, the pelvic incidence (PI) could not be evaluated (four hips) [19]. Ultimately, 208 patients (208 hips) were included in our study (Fig. 1). Of these, secondary hip OA was the most common (165 hips, 79.3%), followed by primary OA and osteonecrosis of the femoral head (18 hips for each, 8.7%) [20].
Surgical procedure and post-operative protocol
All THAs were performed by six experienced arthroplasty surgeons using a direct lateral approach with the patient in the lateral decubitus position [21, 22]. Of these, 130 required acetabular structural bone grafting for the dysplastic acetabulum [22]. The highly cross-linked polyethylene flanged socket (K-MAX CLHO flanged cup, Kyocera Medical, Osaka, Japan) and a cobalt-chromium head with a polished stem (SC stem, Kyocera Medical, Osaka, Japan) were fixed using bone cement (CMW Endurance, DePuy, Blackpool, UK). All patients were allowed full weight-bearing post-operatively, with the use of crutches encouraged, as needed, for the first 3 months. This was according to a standardised fast-track post-THA protocol, which included standardised physical therapy with mobilisation after drain removal.
Clinical evaluations
Before and at 2 years after THA, we used the modified Harris Hip Score (HHS) and the Trendelenburg sign as measures of hip function [23, 24]. The incidence of complications was investigated. Data were analysed in a blinded fashion.
Patient-reported outcome measures
We evaluated the patient-reported outcomes pre-operatively and at 2 years post-operatively. The Hip disability and Osteoarthritis Outcome Score, Joint Replacement (HOOS-JR) is a short PROM developed to efficiently evaluate end-stage hip OA in patients undergoing THA. The HOOS-JR is a six-question survey derived from the original 40-question HOOS. Each item on the HOOS-JR is scored from 0 to 28 and then converted into an interval score from 0 (total joint related disability) to 100 (perfect joint health) [24, 25]. A 100-mm visual analogue scale (VAS) was used to evaluate hip pain and patient satisfaction. The 100-mm VAS-pain and satisfaction score was categorised for analysis from a range of “0” mm (no pain and very satisfied) to “100” mm (worst pain imaginable and completely dissatisfied) [15, 24]. The EuroQol 5-Dimension 5-Level (EQ-5D) scale was used as a measure of patient-reported quality of life [24, 26].
Radiological evaluations
Spinopelvic alignment was assessed before and at 2 years after THA, with the patients in the standing position [27]. Radiographs obtained within 1 month pre-operatively were reviewed for vertebral fractures by an independent arthroplasty surgeon with 10 years of experience. Vertebral fractures were identified using a semiquantitative method, namely a decrease in the height of the vertebral body > 20% [18]. Radiological measures of the sagittal spinopelvic alignment were obtained using a protractor with 1° increments as follows: C7 sagittal vertical axis (SVA), lumbar lordosis (LL), PI, pelvic tilt (PT), and T1 pelvic angle (T1PA) [4, 6, 19, 23, 28] (Fig. 2). The T1PA, a measure of the global malalignment and/or compensation through pelvic retroversion, was defined as the angle between the line from the femoral head axis to the centre of the T1 vertebral body and the line from the femoral head to the centre of the S1 superior end plate. A T1PA divided by PI (T1PA/PI) > 0.2, which provides an angular measure of global sagittal spinal deformity, was associated with lower health-related quality of life in patients undergoing treatment for adult spinal deformity [6]. Osseous complications at the reattached fragment were evaluated on anterior-posterior radiographs obtained 2 years after THA [21, 22].
To calculate the reliability of the spinopelvic alignment, three experienced arthroplasty surgeons independently evaluated the radiographic parameters, with each observer completing three randomly selected measurements at a mean interval of 4.1 (range, 3.6 to 4.4) weeks for 15 patients each. All observers were orthopaedic surgery specialists, with > 6 years of experience. Additionally, they had at least completed a 1-year fellowship in hip surgery under a mentor. Intra- and inter-rater reliability was calculated with a tolerance error of < 2° [29].
Statistical analysis
Statistical analyses were performed using JMP 14 software (SAS Institute Inc., Cary, NC, USA), with p-values < 0.05 considered statistically significant. We defined a HOOS-JR of 70 as a clinically significant cut-off value and divided patients into the following two groups for comparison: the disability group, who had a post-operative HOOS-JR < 70, indicating hip disability, and the control group, who had a HOOS-JR ≥70, indicating no disability [30].
Differences in the measured variables between the two groups were evaluated using the Mann-Whitney U test for continuous variables. Categorical variables were compared using Fisher’s exact or chi-squared tests as per the data distribution. The Steel-Dwass test was used to reveal the relationship between the grade of OA according to the Kellgren-Lawrence classification and spinopelvic parameters. To identify independent risk factors for the residual disability group, logistic regression analyses were performed. Factors, such as age, sex, body height, body mass index, spinopelvic parameters, and surgeon experience, were analysed using an exploratory univariate analysis followed by a multivariate analysis [1, 4, 7, 13, 15,16,17, 22]. Short stature was typically defined as a height < 147.23 cm by legal standards [13]. Surgeons were classified into the following groups: orthopaedic specialists < 8 years’, 8–15 years’, and ≥ 15 years’ experience after certification [15].
A multicollinearity test was performed with the variance inflation factor set at < 10. Age was included as a confounding factor. To identify the cut-off value of the parameters for predicting disability, we used the receiver-operating characteristic (ROC) curve.