Abstract
Introduction: Restoring the offset after THA is positively correlated with good functional outcomes and patient satisfaction. Preoperative planning is now an essential tool for optimizing this restoration. Nevertheless, recent studies have focused mainly on the femoral offset, at the expense of the acetabular offset. Our hypothesis was that the variations in global offset after THA are mainly due to variations in acetabular offset relative to its initially planned value. Methods: This retrospective study involved 104 patients who had undergone primary anterior THA with two-dimensional preoperative planning between January 2023 and November 2024. The planned preoperative radiographs and the postoperative radiographs were analyzed independently by two attending orthopeadic surgeons who measured the global, acetabular, and femoral offsets. The intra-rater and inter-rater agreement was determined. The differences between the planned implant sizes and the size actually implanted were also determined. Results: In the end, 43 patients met all the pre and postoperative radiographic criteria and were included in the analysis. There was a correlation between the variation in acetabular offset and the variations in global offset (r = 0.74, p < 0.0001) and between the variation in femoral offset and the variations in global offset (r = 0.68, p < 0.0001). Using multiple linear regression, the coefficients of regression for the variation in the acetabular offset and femoral offset were 1.00, indicating an equal contribution (50%) to variations in global offset. Conclusion: Variations in the positioning of the acetabular cup are an important factor in the variations in global offset after THA. The postoperative variations in acetabular and femoral offsets contribute equally to changes in the global offset. Clinically, this means that practitioners should focus more on implanting the acetabular component. Registration: IRB approval N° 22727513. Level of evidence: III.
Keywords: Total hip arthroplasty, Radiographic measurement, Acetabular offset
Introduction
The aim of total hip arthroplasty (THA) is to relieve the patient’s pain while restoring the anatomy of the hip joint, namely its femoral center of rotation [1–3]. In fact, restoring the offset after THA is positively correlated with good functional outcomes and patient satisfaction.
Preoperative planning is now frequently done to determine the size and position of the implants with the aim of restoring the center of rotation during the surgical procedure [4]. It helps with planning the height of the femoral cut, the depth of acetabular reaming, and the size of the various implanted components. Two-dimensional (2D) preoperative planning using mediCAD® [5] software appears to provide good reproducibility in the positioning and choice of implants, allowing the femoral offset to be restored after THA [6]. Nevertheless, the majority of published studies focus on the femoral offset, while few studies have looked into restoring the acetabular offset [7]. We believe the acetabular offset may be more variable due to the anatomical variations observed in the acetabulum and the lack of visual landmarks during reaming of the acetabulum and positioning of the chosen acetabular cup [8].
We hypothesized that the variations in global offset after THA are mainly due to variations in acetabular offset relative to its initially planned value. The primary aim of this study was to compare the pre-planned position of the acetabular cup with its final position postoperatively. The secondary aim was to evaluate the impact of variations in acetabular offset on the global offset between the 2D preoperative plan and postoperative radiographic measurements.
Methods
Study design
This was a retrospective analysis of data collected prospectively from patients who underwent THA by a single surgeon. The inclusion criteria were patients who underwent primary THA with preoperative planning done using the mediCAD® software, no matter their age, who had postoperative control radiographs, and who provided their consent for analysis of radiographic data. The exclusion criteria were poor quality pre- or postoperative radiographs, radiographic views that do not encompass the contralateral hip, pubic symphysis, and sacrum; no calibration marker on the preoperative radiographs; THA done for femoral neck fracture; and the patient declined to participate. The study was approved by our institutional review board (IRB 22727513).
Study population
In all, 104 patients had undergone THA between January 2023 and November 2024 and had at least 3 months of follow-up. Ten patients were excluded because there was no calibration marker on their radiographs, while 51 were excluded because the radiographs were not of sufficiently high quality (Figure 1).
Figure 1.
Flowchart of the 104-patient cohort.
The surgical indication was determined during the preoperative consultation, which consisted of medical history, a full clinical exam of the hip, and a radiographic work-up with AP and lateral views of the hip. Additional imaging with a calibration marker was done on an AP view of the hip or pelvis to carry out preoperative planning with the mediCAD® 2D software (mediCAD®, Altdorf, Germany) of the THA components to be implanted.
Surgical technique
A Hueter-Gaine approach was used with the patient supine on a standard operating table. After making an 8-cm long incision, the aponeurosis of the tensor fascia lata was opened, and its muscle belly was reflected posteriorly and laterally. The ascending branches of the lateral circumflex femoral artery were identified and carefully ligated, exposing the joint capsule. The femoral neck and acetabulum were exposed with a Merle d’Aubigné retractor and Beckmann retractor. For the femoral phase, the leg was placed in adduction and external rotation, and a femoral neck elevator was applied under the greater trochanter to make it easier to insert the broaches. Throughout the procedure, the skin was protected with a commercial device (Alexis® Wound Protector-Retractor HR000, Applied Medical, USA) (Figure 2).
Figure 2.
Acetabular exposure with Alexis® wound protector-retractor.
All patients received a UTSTM short cementless femoral stem (United, Taiwan) and a U-MOTION II+TM press-fit acetabular cup (United). A ceramic-ceramic bearing was used. Its exact size was determined during the intraoperative trials, with the surgeon selecting the appropriate acetabular cup size and choosing between the available head diameters (28, 32, or 36 mm) and neck lengths (short, medium, long, extra-long). The industry partner, which also provides the two-dimensional planning software, exclusively offers prostheses with either standard or lateralized offset.
Data collection
The preoperative planning radiographs and postoperative radiographs were analyzed independently by two attending surgeons. Radiographs that met quality standards [9] were retained for the analysis. The AP views of the pelvis were classified into group 1, and the AP views of the hip that captured the pubic symphysis and sacrum were classified into group 2. The planned and postoperative offsets were measured for each patient. The acetabular offset was the distance between the femoral head center of rotation and a perpendicular line across the acetabular teardrops (trans-teardrop line). The femoral offset was the distance between the intramedullary axis of the femoral shaft and the center of rotation of the femoral head. The global offset was defined as the sum of the femoral offset, which represents the distance between the femoral axis and the femoral center of rotation, and the acetabular offset, defined as the distance between the femoral center of rotation and the acetabulum.
For the radiographs in group 1, the teardrop line was drawn directly to allow calculation of the acetabular offset [10] (Method 1) (Figure 3), while for the radiographs in group 2, it was drawn perpendicular to the line passing through the axis of the sacrum spinous processes and the center of the pubic symphysis [8, 11] (Method 2) (Figure 4).
Figure 3.
Method 1: postoperative offset measurement after total hip arthroplasty (AO: Acetabular Offset; FO: Femoral Offset; GO: Global Offset).
Figure 4.
Method 2: 2D templating to measure offsets (AO: Acetabular Offset; GO: Global Offset; FO: Femoral Offset).
Data analysis
The statistical analysis was performed using JMP software (version 7.0.1, SAS Institute). The distribution of continuous variables was evaluated with a Shapiro-Wilk test. Normally distributed data were summarized by the mean and SD values and compared using parametric tests. Comparisons between pre- and postoperative data were done using a paired Student t test. Qualitative variables were expressed as counts and percentages.
The consistency of the radiographic measurements (acetabular offset, femoral offset, and global offset) was evaluated by calculating the intraclass correlation coefficient (ICC) using a two-factor random effects model, with absolute agreement:
ICC(2,1) for intra-rater reliability (measurements 1 and 2 for each surgeon)
ICC(2,k) for the inter-rater reliability (mean of measurements by the two surgeons).
The ICC values were interpreted using the standard thresholds: <0.5 low, 0.5–0.75 moderate, 0.75–0.9 good, >0.9 excellent.
The Pearson test was used to determine the strength of a correlation between two variables. The relationship between variations in offset (Δ = postoperative value – planned value) was explored using bivariate analyses, then multiple linear regressions when several explanatory variables were included simultaneously. A partial ANOVA and Shapley values were used to estimate how much each variable contributed to the variance.
The significance threshold was set at p < 0.05.
Results
Study population
The characteristics of the study population are summarized in Table 1.
Table 1.
Patient characteristics.
| Variable | Value |
|---|---|
| Age at surgery, mean ± SD (min–max) (y) | 75.1 ± 11.7 (47–94) |
| Sex, n (%) | |
| Men | 19 (44%) |
| Women | 24 (56 %) |
| Hip right/left No. (%) | 28 (65%) / 15 (35%) |
| Method | |
| 1 | 20 |
| 2 | 23 |
n: number; SD: standard deviation.
Correlation of offsets
There was a significant correlation between the postoperative global offset and planned global offset (r = 0.62, p < 0.0001), the postoperative femoral offset and planned femoral offset (r = 0.60, p < 0.0001), and the postoperative acetabular offset and planned acetabular offset (r = 0.35, p = 0.023). The bivariate analysis found a correlation between the variation in acetabular offset and variations in global offset (r = 0.74, p < 0.0001).
There was also a correlation between the variation in femoral offset and variations in global offset (r = 0.68, p < 0.0001).
Based on the multiple linear regression, the regression coefficients for the variation in the acetabular offset and femoral offset were 1.00, showing equal contributions (50% each) to the variations in global offset. This analysis found no correlation between the variation in acetabular offset and the variation in femoral offset.
Consistency between planned and implanted THA components
The accuracy of the preoperative planning is shown in Table 2.
Table 2.
Agreement between the planned size and implanted size.
| Exact cup size | Cup ±1 size | Exact stem size | Stem ±1 size | Head size | Neck size | |
|---|---|---|---|---|---|---|
| Agreement (%) | 49 | 74 | 44 | 82 | 86 | 47 |
Validity of measurement methods
The ICC values, evidence of intra- and inter-rater reliability, were rated as good or excellent (Table 3).
Table 3.
Intraclass Correlation Coefficient for intra-rater and inter-rater reliability.
| Acetabular offset | Femoral offset | Global offset | ||||
|---|---|---|---|---|---|---|
| Planned | Post-op | Planned | Post-op | Planned | Post-op | |
| Intra-rater | ||||||
| Surgeon 1 | 0.90 | 0.84 | 0.75 | 0.85 | 0.91 | 0.84 |
| Surgeon 2 | 0.78 | 0.86 | 0.94 | 0.89 | 0.89 | 0.84 |
| Inter-rater | 0.94 | 0.90 | 0.99 | 0.94 | 0.98 | 0.88 |
Given the good intra- and inter-rater reliability, the mean of the four values (2 observations by 2 raters) was used. The values are the mean for all the patients (in mm) with SD. There was no significant difference between the mean planned offset and the mean postoperative offset.
Discussion
This study shows that the acetabular cup’s positioning has the same impact on postoperative offset as the femoral stem’s positioning. It highlights the importance of acetabular positioning and preoperative planning, which accurately predict the implant size and positioning, partially confirming our hypothesis.
We were interested in variations in the acetabular offset, as there are fewer studies on this element than on the femoral offset [11]. The acetabular cup’s positioning depends on the depth and height of reaming, which must be adjusted for each patient, combining optimal coverage, orientation, and hold of the implant [12]. It varies based on the patient’s anatomy, surgical approach, and components used [13, 14]. A hip with normal anatomy has more positioning possibilities. In cases of defective anterior or lateral coverage, the acetabular implant must be medialized; the femoral offset must compensate for this to maintain a satisfactory global offset [15]. Despite the lesser emphasis on acetabular offset in scientific literature compared to femoral offset, restoring it appears to be an important factor affecting the restoration of various offsets during THA, having clinical repercussions on the patient’s gait kinetics [15].
We did this study to understand the effect of acetabular offset on the global offset. We found that variations in global offset are explained as much by the variations in femoral offset as by the variations in acetabular offset. There was no correlation between the variation in acetabular offset and the variation in femoral offset, which means they vary independently. We believe this is a novel finding. These results can be explained by the fact that the surgeon did intraoperative trials, adjusting the length of the femoral neck to restore an optimal clinical global offset, which makes it possible to compensate for differences between the planned and final components. The variations between the neck size planned and the one implanted were actually larger than for the other components.
Despite recent published data that 3D images provide better accuracy for the reproducibility of chosen component sizes [16, 17]. We chose to use a 2D planning method because the images obtained by CT scan are not superior when measuring acetabular offset [6]. The other advantages of 2D planning are that there is less radiation, the cost is lower, and it is easier to carry out in current practice.
The accuracy of the planning for component sizes and restoring the various offsets is similar to other studies using mediCAD® software, which shows the dependability of this software package during the planning process [4]. Given this good pre- and postoperative reliability, we believe that preoperative planning is crucial as it helps to reduce surgery time [18], improves surgical precision, and reduces procedure-related costs [19]. The radiographic measurement methods used here were derived from published data and allowed us to adapt the analysis to the variable quality of the postoperative radiographs [20–22]. Since the psychometric qualities of the measurement methods have not been validated, we calculated the intra-rater and inter-rater consistency. The ICC values were good or excellent, which allowed us to be confident in the results and highlight their usefulness in current practice.
It is important to note that recent studies have evaluated the restoration of global offset using navigation or robotic assistance [23, 24]. These studies have mainly been conducted with anterior surgical approaches [25]: the use of robotics for THA by DAA provided an advantage in controlling the orientation of the cup and the restoration of its rotation center. Thanks to the 3D planning on the CT scan, it allowed us to respect the thresholds while avoiding the anterior overhangs. These findings are important to consider from the perspective of further democratizing the use of robotic assistance.
All the patients were operated on by a single surgeon, which ensures the planning technique is fully mastered and the implantation technique is highly reproducible. This study has certain limitations. Many patients were excluded because their radiographs did not meet quality standards. Consequently, two measurement methods were used to adapt to the variable quality of the radiographs. Our study focused solely on radiographic analysis of variations in offsets in patients undergoing THA; it did not determine the clinical impact of these variations, especially in the occurrence of postoperative complications (dislocation, loosening, etc.). Thus, it would be important to investigate whether restoring the global offset after THA using our planning tool produces satisfactory long-term functional outcomes.
Conclusion
The variations in positioning of the acetabular cup are an important factor in variations of the global offset after THA. The postoperative variations in acetabular and femoral offset equally contribute to the variations in global offset. This should motivate clinicians to pay close attention to the implantation of the acetabular cup. To supplement this immediate postoperative radiographic analysis, the clinical impact of these variations in global offset due to variations in acetabular offset should be determined in patients undergoing THA.
Acknowledgments
No acknowledgments were necessary for the preparation, analysis, or data gathering for this manuscript.
Cite this article as: Matray A, Cazemajou C, Vendeuvre T, Pries P, Odri GA & Severyns M (2026) Variations in Acetabular Offset Between the Preoperative Plan (mediCAD®) and Postoperative Radiographic Measurements: an overlooked source of error that affects restoration of global offset after THA. SICOT-J 12, 46. https://doi.org/10.1051/sicotj/2026050.
Funding
No funding was obtained for this study.
Conflicts of interest
The authors declare that they have no conflicts of interest.
Data availability statement
This article has no associated generated data.
Author contribution statement
Anatole Matray: Formal Analysis, Writing – Original Draft, Visualization.
Clément Cazemajou: Formal Analysis, Writing – Original Draft.
Tanguy Vendeuvre: Conceptualization, Methodology, Supervision.
Pierre Pries: Methodology, Supervision.
Guillaume Anthony Odri: Data Curation.
Mathieu Severyns: Conceptualization, Validation, Investigation, Writing- Review and Editing.
Ethics approval
The study was approved by our institutional review board (IRB 22727513).
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Data Availability Statement
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