Background: To our knowledge, no previous studies have compared periprosthetic bone remodeling around monoblock versus modular tapered fluted titanium stems with different stem length and thickness. Methods: A retrospective comparative study was performed on 139 consecutive total hip arthroplasties (THAs) revised with a tapered fluted modular titanium stem and 114 consecutive THAs revised with a tapered fluted monoblock titanium stem. The latest follow-up radiographs were compared with immediate postoperative radiographs to assess bone restoration in residual osteolytic areas, femoral stress-shielding, spot-welds, and radiolucent lines. Diameter and medullary canal filling of the 2 stems were measured. Results: More patients in the monoblock group demonstrated osseous restoration than in the modular group (p = 0.009), and the modular stem exerted more severe stress-shielding on the femur (p < 0.001). Stem tip spot-welds developed in 88.5% of modular stems compared with 47.4% of the monoblock stems (p < 0.001). Spot-welds developed in 38.8% of modular stems at the modular junction. Partial or circumferential radiolucent lines were observed at the proximal segment of 30.9% of modular stems, compared with 14.0% of monoblock stems (p = 0.002). Conclusions: Compared with the longer and thinner monoblock stems, modular stems had less proximal osseous restoration in residual osteolytic areas and more severe femoral stress-shielding, stem tip spot-welds, and radiolucent lines around the stems, which were stiffer and had a shorter distal section. The stem diameter and stiffness (which were influenced by stem length, curvature, and modularity) determined bone remodeling patterns. Level of Evidence: Therapeutic Level III. See Instructions for Authors for a complete description of levels of evidence.

Aims: The aim of this study was to evaluate the reliability and validity of a patient-specific algorithm which we developed for predicting changes in sagittal pelvic tilt after total hip arthroplasty (THA).Methods: This retrospective study included 143 patients who underwent 171 THAs between April 2019 and October 2020 and had full-body lateral radiographs preoperatively and at one year postoperatively. We measured the pelvic incidence (PI), the sagittal vertical axis (SVA), pelvic tilt, sacral slope (SS), lumbar lordosis (LL), and thoracic kyphosis to classify patients into types A, B1, B2, B3, and C. The change of pelvic tilt was predicted according to the normal range of SVA (0 mm to 50 mm) for types A, B1, B2, and B3, and based on the absolute value of one-third of the PI-LL mismatch for type C patients. The reliability of the classification of the patients and the prediction of the change of pelvic tilt were assessed using kappa values and intraclass correlation coefficients (ICCs), respectively. Validity was assessed using the overall mean error and mean absolute error (MAE) for the prediction of the change of pelvic tilt.Results: The kappa values were 0.927 (95% confidence interval (CI) 0.861 to 0.992) and 0.945 (95% CI 0.903 to 0.988) for the inter- and intraobserver reliabilities, respectively, and the ICCs ranged from 0.919 to 0.997. The overall mean error and MAE for the prediction of the change of pelvic tilt were -0.3° (SD 3.6°) and 2.8° (SD 2.4°), respectively. The overall absolute change of pelvic tilt was 5.0° (SD 4.1°). Pre- and postoperative values and changes in pelvic tilt, SVA, SS, and LL varied significantly among the five types of patient.Conclusion: We found that the proposed algorithm was reliable and valid for predicting the standing pelvic tilt after THA.Cite this article: Bone Joint J 2024;106-B(1):19–27.

Aims: This study aimed to report the surgical techniques and early clinical results of robotic arm-assisted acetabular reconstruction in revision total hip arthroplasty (THA).Methods: Between October 2019 and May 2021, we used the Mako robotic system to perform 62 revision THAs at our hospital. This study included 54 patients who underwent robotic arm-assisted revision THA for acetabular reconstruction. Surgical techniques using the robotic system to reconstruct the acetabulum, including preoperative planning, intraoperative registration, and the accuracy of different registration methods, are reported. The accuracy between the target acetabular component orientation and final orientation was evaluated. The latest follow-up Harris Hip Score (HHS) and radiographs were analyzed.Results: Among the 54 hips included, four types of intraoperative registration methods with different surfaces were developed and registered 65 times. The overall success rate of the registration process was 98.5%. The mean accuracy of successful registration was 0.38 mm (0.2 to 0.5). The median difference between the target and final acetabular component orientations assessed by Mako was 1.0° (-2.0° to 0.0°) for inclination and 0.0° (-1.0° to 1.0°) for anteversion. Four hips were classified as outliers for acetabular component orientation. The reconstructed centres of rotation (CORs) were slightly lower than the anatomical CORs by a mean 4.72 mm (SD 4.71), and shifted laterally by 3.92 mm (SD 4.62) on postoperative radiographs. The median HHS improved significantly from 46.0 (IQR 33.0 to 58.3) preoperatively to 89.0 (IQR 78.3 to 93.0) postoperatively (p < 0.001). The overall satisfaction rate was 86.0% (n = 50). There were no radiological failures at the latest follow-up.Conclusion: Robotic arm-assisted revision THA provides valuable information for the analysis of bone defects to guide reconstructive strategies. Robotic arm-assisted reaming and acetabular component or augment positioning facilitates accurate component position and orientation. The radiological and preliminary clinical results of this cohort were satisfactory.Cite this article: Bone Joint J 2025;107-B(4):404–412.

Background: Three-dimensional computed tomography (3D CT) reconstruction is the reference standard for measuring component orientation. However, functional cup orientation in standing position is preferable compared with supine position. The low-dose bi-planar radiographs can be used to analyze standing cup component orientation. We aimed to assess the validity and reliability of the component orientation using the low-dose bi-planar radiographs compared with the 3D CT reconstruction, and explore the differences between the functional cup orientation in standing radiographs and supine CT scans.Methods: A retrospective study, including 44 patients (50 hips) with total hip arthroplasty (THA), was conducted. CT scans were taken 1 week after surgery and the low-dose bi-planar radiographs were taken in the follow-up 6 weeks later. Component orientation measurement was performed using the anterior pelvic plane and the radiographic coronal plane as reference, respectively.Results: The study showed no significant difference in cup anteversion (p = 0.160), cup inclination (p = 0.486), and stem anteversion (p = 0.219) measured by the low-dose bi-planar radiographs and 3D reconstruction. The differences calculated by the Bland–Altman analysis ranged from − 0.4° to 0.6° for the three measured angles. However, the mean absolute error was 4.76 ± 1.07° for functional anteversion (p = 0.035) and 4.02 ± 1.08° for functional inclination (p = 0.030) measured by the bi-planar radiographs and supine CT scans.Conclusions: The low-dose bi-planar radiographs are the same reliable and accurate as 3D CT reconstruction to assess post-THA patients’ component orientation, while providing more valuable functional component orientation than supine CT scans.