Introduction

Proper orientation of the acetabular component is critical in total hip arthroplasty (THA) to preserve the patient’s range of motion and reduce the risk of dislocation1. Misalignment of the acetabular cup can lead to impingement2, increased wear3, and edge loading4, which can in turn result in osteolysis and aseptic loosening5.

Traditionally, cup positioning has been guided by the “safe zone” concept introduced by Lewinnek et al. in 1978, derived from an analysis of 300 THAs6. This study suggested an optimal positioning range of 30° to 50° inclination and 5° to 25° anteversion, with cups outside this zone exhibiting a fourfold increase in dislocation risk6. However, recent studies question the predictive accuracy of Lewinnek’s safe zone, particularly as it does not account for functional spine-pelvis-hip motion, which can influence postoperative outcomes7,8,9,10. Nevertheless, this standard remains widely used in preoperative planning1 and these target angles are even endorsed by implant manufacturers and integrated into surgical guides for accurate positioning11.

Patient positioning on the operating table is widely recognized as a key factor for ensuring stable pelvis orientation during surgery12,13,14. Conditions such as fixed pelvic obliquity, lumbar scoliosis, sagittal spine imbalance and change in morphology in congenital defects and postraumatic cases can alter the functional orientation of the acetabulum and should be determined prior to surgery15,16,17. Ideally, if the pelvis maintains the same tilt, obliquity, and rotation relative to the operating table as it will postoperatively, this would optimize cup placement accuracy11. However, radiographic comparisons between intraoperative and postoperative positioning have not confirmed this assumption18.

In practice, the surgeon is often unable to follow the sagittal plane of the body or the position of the patient’s pelvis due to the patient’s sterile draping. The surgeon orients the implantation of the acetabular component according to the edge of the operating table (Fig. 1A), which sets the resulting inclination of the acetabular component19. In doing so, the patient is very often positioned on the edge of the operating table for better access to the hip joint with tilted pelvis. This can lead to inaccurate implantation of the acetabular component and also to incorrect determination of the length of the lower limbs within the patient’s malposition on the operating table (Fig. 2). Although the importance of proper cup positioning and the factors influencing it are well acknowledged and extensively discussed in the literature1,20,21, the specific impact of patient positioning on the operating table during preoperative preparation on the accuracy of acetabular cup placement in non-navigated THA has not been objectively proven or quantified. Despite its presumed significance, no studies have systematically investigated this relationship and we do not know the extent to which patient positioning influences acetabular cup placement accuracy.

Fig. 1
figure 1

(A) Schematic view of patient positioning on surgical table and implantation using edge of surgical table as guide. (B) Measurement of pelvic inclination β. (C) Measurement of cup inclination Θ and leg length discrepancy (LLD).

Full size image
Fig. 2
figure 2

Schematic representation of the mechanism by which cup inclination set to external frame during surgery \(\:\alpha\:\) and pelvic inclination \(\:\beta\:\) affect the cup inclination \(\:\varTheta\:\).

Full size image

The objective of this study was to photographically record and measure pelvic inclination angles on the operating table in the supine position during preoperative preparation, comparing these to radiographic inclination angles on postoperative anteroposterior radiographs. Our study aimed to address the following questions: (1) What is the relationship between preoperative patient positioning, acetabular cup orientation, and leg length discrepancy (LLD)? (2) Is there a difference in cup placement accuracy between experienced and early-career surgeons? We hypothesized that patient positioning on the operating table would influence postoperative cup positioning, but that experienced surgeons would be better able to compensate for this effect.

Methods

This prospective, multi-surgeon, consecutive case series from a single tertiary centre had ethical approval. The study was conducted according to the Helsinki Declaration of 2008 and all patients gave informed consent.

We included a total of 135 patients indicated for total hip arthroplasty implantation (Table 1). Only patients with a diagnosis of osteoarthritis or osteoarthritis on the basis of rheumatoid arthritis were included in the study. Only patients with a maximum variation in hip length difference of less than 1 cm (LLD\(\:\le\:\)1cm) were included. The number of women in the cohort was 69 and the number of men was 66. The mean age at the time of surgery was 68.7 years (range 45–78 years). Patients were classified into 3 groups according to the experience of the surgeon. The first group of 45 THAs consisted of patients operated on by early carrier surgeon (less than 5 years of clinical experience). In the second group (45 THAs), the length of clinical experience of the surgeon was 5–15 years, while the third group (45 THAs) consists of surgeon with more than 15 years of experience.

Table 1 Patient demographics and surgical details.
Full size table

All surgeries were performed as non-navigated THA with preoperative planning based on standard supine anteroposterior radiographs. An optimal cup inclination in preoperative planning was based on reccomendation of the producer, 45° degrees to the sagital plane of the body. All cases were positioned in the supine position and as close as possible to the edge of the table with the aim of operated pelvis in neutral position.

For each patient, we identified anterior superior iliac spines (ASIS) by palpation and marked them bilaterally with a marker before surgery after the patient was positioned. After positioning the patient on the operating table, we performed preoperative photodocumentation of the patient’s position. Photodocumentation was performed each time from the mounted camera (camera Nikon D3100 Sigma DC) fixed 2 m over the center of the operating table. The focal length was adjusted to minimize image distortion, and distortion evaluation using a chess calibration image indicated an error of less than 0.3° in pelvic inclination. We have assured that in addition to the patient’s position, we may identify the edge of the operating table. From the photodocumentation, we then measured the angle between the edge of the operating table and the junction of the two ASIS using impage processing sofware ImageJ, version 1.52 (NIH, MD, USA).

An anterolateral approach in the supine position was performed in all procedures. Allofit cup and Avenir stem (Zimmer Biomet, Warsaw, IN, USA) were implanted to all patient. The acetabular component anteversion was placed taking into consideration, the orientation of the transverse acetabular ligament (TAL), the version of the femoral component and the combined anteversion value as femoral preparation proceeded that of the acetabulum. The acetabular component inclination was positioned freehand using a recommendation provided by a mechanical guide set by the manufacturer at an inclination/anteversion of 45°/15°. However, the final alignment was determined by the surgeon and not strictly dictated by the guide. For all cases, it was recomended that the intra-operative inclination was 45° (i.e. guide parallel to the edge of the surgery table).

Postoperatively, we performed a radiograph of the entire pelvis in supine position and determined the angle of inclination of the acetabular component with respect to the transverse axis of the pelvis (relative to the perpendicular to the sagittal axis of the body) (Fig. 1A, B). We further evaluated the pelvic inclination using method described in Lewinnek et al., 19786 and difference in length of legs by measuring the difference in height of the lesser trochanters on the anteroposterior postoperative radiograph between the operated and the non-operated sides as described in Hardwick-Morris, 2022 (Fig. 1C).

Data analysis

We performed a sample size calculation a priori, based upon previously published data on reported cup intraoperative inclinations11,18, resulting in a minimum cohort size of 41 patients per group (\(\:\alpha\:\) = 0.05, \(\:1=\beta\:\) = 0.95, difference of 5° in cup inclination). All data were tested for normality using the Shapiro-Wilk test. The distribution of each variable was assessed to determine the suitability of parametric tests for further analysis. Pearson’s correlation coefficient was calculated to evaluate the relationships between pelvic inclinations, cup inclination, and LLD with normal distribution, while Spearman’s rank correlation was applied for variables not meeting the assumption of normality.

Differences between groups were analyzed using independent samples t-tests for normally distributed data and the Mann-Whitney U test for data that did not meet the criteria for normality. For comparisons involving more than two groups, one-way ANOVA with post-hoc Tukey’s test was performed on normally distributed data, and the Kruskal-Wallis test was used for non-normally distributed data with Wilcoxon rank sum test. Homogeneity of variance was assessed using Levene’s test to compare the variations in cup inclination and LLD among the groups. Statistical significance was set at \(\:p\) < 0.05 for all analyses, and effect sizes were calculated to supplement the interpretation of significant findings. All analysis were performed in R, version 4.1.2 (R Foundation for Statistical Computing, Vienna, Austria).

Results

A larger pelvic inclination on the operating table (\(\:\beta\:\)) was associated with a smaller acetabular cup angle (\(\:\varTheta\:\)), with a strong negative correlation (Pearson’s R = -0.72, 95% CI [0.62, 0.79], p < 0.001). Notably, the achieved cup inclination tended to exceed the theoretical (45°-\(\:\beta\:\)) angle targeted during implantation (Figs. 2 and 3). Pelvic position on the surgical table (\(\:\beta\:\)) had a significant effect on leg length discrepancy (LLD), showing a moderate positive correlation (Spearman’s R = 0.42, 95% CI [0.27, 0.55], p < 0.001). Greater pelvic inclination during surgery was associated with leg elongation on the operated side (Fig. 4A).

Fig. 3
figure 3

(A) Pelvic inclination during surgery β and its impact on acetabular cup inclination Θ. The solid line represents the linear fit, with shaded regions indicating the 95% confidence intervals. The dashed line illustrates the theoretical dependency (45°- β), assuming an optimal acetabular cup inclination of 45° achieved during surgery as depicted on Fig. 2. (B) The influence of surgeon experience on the precision of acetabular cup inclination. While both experienced (> 15 years) and less-experienced (< 15 years) surgeons achieved a similar mean cup inclination, the experienced cohort demonstrated significantly greater precision, reflected by a lower mean absolute error from the target angle.

Full size image
Fig. 4
figure 4

(A) Relationship between postoperative leg length discrepancy (LLD), measured from radiographs, and pelvic inclination during surgery (β). The solid line represents the linear fit, with shaded regions indicating the 95% confidence intervals. (B) The influence of surgeon experience on LLD. More experience surgeons (> 15 years) achieved significantly lower discrepancies in leg length. Statistically significant differences between groups obtained from Kruskal-Wallis test are indicated as follows: p < 0.05, ** p < 0.001. The shape of individual points represents the level of surgeon experience in both graphs.

Full size image

Despite deviations in pelvic positioning during surgery, all surgeons — regardless of experience level — achieved an average postoperative cup angle close to the targeted 45°, as measured on postoperative radiographs (Table 2). Differences in average cup inclination across surgeons with varying experience levels were negligible (ANOVA, F(2, 132) = 0.37, p = 0.690, 95% CI [0.00, 1.00]). However, more experienced surgeons achieved significantly lower variance in cup placement around the target angle (Fig. 3B), as indicated by Levene’s test for homogeneity of variance (p < 0.001, F(2, 132) = 8.71). Additionally, surgeons with greater experience achieved significantly lower discrepancies in leg length (Kruskal-Wallis H(2) = 18.7, p < 0.001), as illustrated in Fig. 4B.

Table 2 Summary of measured values (mean and standard deviation) grouped by surgeon’s experience. (β) pelvic inclination measured during patient preparation, (Θ) cup inclination measured postoperatively, (LLD) leg length discrepancy measured posoperatively. Statistical significance p between measured values in different groups obtained from Kruskal-Wallis test (β and LLD) and ANOVA test (Θ).
Full size table

Discussion

The optimal position of the acetabular component in primary total hip arthroplasties is a crucial factor influencing functional outcomes and implant longevity10,22. Although it is generally understood that patient positioning on the surgical table in the supine position can influence the postoperative orientation of the acetabular component, this study provides the objective evidence quantifying its significant impact.

While earlier research has primarily addressed intraoperative changes in pelvic position15,23, this study underscores the importance of preoperative patient positioning. Initial pelvic inclination significantly affects postoperative cup positioning (Fig. 3A). Surgeons seem able to anticipate this deviation and adjust cup alignment during surgery, deviating from the strict 45° orientation relative to the operating table. This deliberate “misalignment” leads to more accurate postoperative cup positioning (Table 2). Grammatopoulos et al. (2018) reported an average cup inclination of 41° with a standard deviation of 6° in the supine position18. Our findings are comparable (Table 2), although the absolute achieved cup inclination in our study is slightly lower, likely due to different target zones (40° in Grammatopoulos et al. versus 45° in this study). Additionally, leg length discrepancy (LLD) in our findings, which ranges from − 2 to 18 mm with a mean of 4.1 mm, aligns with previous reports24. Accurate acetabular cup placement is essential not only to prevent dislocation but also to reduce wear. Daniel et al. (2016) found an increase in contact pressure when the cup inclination was above 50°, which applies across different head diameters. Higher contact pressure can increase wear, leading to an elevated risk of implant fatigue fracture3,25.

Experienced surgeons demonstrate greater consistency in both LLD and acetabular inclination (Figs. 3B and 4B), with a lower mismatch in leg length than less experienced surgeons (Table 2). Variability in less experienced surgeons may stem from concerns about restoration instability. Overall, a tendency toward elongation on the operated side may result from a higher operated joint position relative to the ipsilateral joint, potentially due to patient malpositioning, as indicated by the average positive angle \(\:\beta\:\) in Fig. 2.

Furthermore, our findings should be contextualized within the ongoing debate surrounding the traditional Lewinnek ‘safe zone.’ Recent literature has increasingly challenged this static concept, advocating for a more dynamic, patient-specific approach that considers individual spinopelvic mobility and functional anatomy7,8,9,10,26. Our results support this paradigm shift by demonstrating that patient-specific factors, such as initial positioning on the table, are a major source of variability that must be controlled to achieve any desired target, whether traditional or functional.

In this study, we chose the supine position as it was proposed that the supine position yields more accurate cup placements than the lateral position13,14. It has been shown that during THA via a posterolateral approach, the pelvis tends to roll anteriorly during cup insertion, creating a difference between the target and radiographic cup inclinations18,22,23.

Our study design employs a straightforward data collection method that requires minimal additional patient preparation before surgery. Spinopelvic relationships were not directly analyzed, and the final implant position in the standing posture may be more clinically relevant than its position in the supine position21. Additionally, different methodologies for measuring inclination and anteversion may yield varying values, as noted by Murray et al. (1993)27. However, since the study aims to compare THA outcomes between groups and the measurement of cup inclination from anteroposterior radiographs was applied consistently throughout, the assessment method may affect the absolute values of cup inclination and LLD but is unlikely to alter the study’s overall conclusions.

The measurement of pelvic inclination was performed prior to sterile draping; consequently, any subsequent patient movement may have introduced a discrepancy between the recorded value and the definitive intraoperative pelvic position. To mitigate this variable in future investigations, the utilization of real-time intraoperative sensors or advanced imaging modalities is recommended to ensure continuous positional accuracy. Our analysis was confined to static, supine radiographic assessments and did not account for the patient’s dynamic, functional spinopelvic parameters in sitting and standing positions, which are known to influence final clinical outcomes18. Furthermore, our study cohort excluded patients with significant preoperative leg length discrepancies or severe spinal deformities. This necessary exclusion introduces a selection bias, thereby limiting the generalizability of our findings to more anatomically complex primary arthroplasty cases. Achieving optimal and functional acetabular component placement in patients with such deformities presents a distinct surgical challenge. Therefore, we advocate for future research dedicated specifically to this patient population to delineate strategies for improving outcomes under functional conditions.

Our study did not explicitly account for pelvic rotation and tilt during acetabular cup implantation22. Accurately measuring pelvic tilt during preoperative positioning would require a more complex setup28. Since the cup face is oblique, pelvic tilt can influence the inclination angle. As the pelvis and cup tilt around the medial-lateral axis, the superior radiographic landmark used to measure inclination may shift medially with posterior tilt or laterally with anterior tilt, resulting in variations in the measured acetabular inclination29. Another potential source of variation is the detection of the ASIS using skin markers, which may be affected by skin movement artifacts. These factors contribute to the variability observed in patient data (Figs. 3 and 4). The study by Hardwick-Morris et al. (2022) demonstrated that functional leg-length discrepancy may not be accurately estimated from anteroposterior radiographs, as it is also influenced by hip and knee contractures as well as altered spinal mechanics20. Therefore, the LLD measured in this study does not represent true leg-length discrepancy but rather the specific contribution of cup positioning during THA to the overall discrepancy.

Computer-guided surgery presents an alternative to freehand techniques. Although CT-based navigation is highly accurate, its high cost, invasiveness, and longer operation times have limited its adoption in the clinical practice30. Other options, such as alignment guides, goniometers, and intraoperative radiography, are also available11,14,21,31,32. However, even intraoperative fluoroscopic control does not necessarily improve final positioning or limb length accuracy. Bingham et al. reported similar results in X-ray-guided (mean cup inclination of 39.4° with a 1.1 mm LLD) and control groups (mean inclination of 39.9° with a 0.8 mm LLD)33. Ogawa et al. (2018) found no significant difference in radiographic inclination between augmented reality and conventional goniometer techniques but noted more precise anteversion with augmented reality30.

The clinical implication of our findings is that the preoperative assessment of pelvic position is not merely a preliminary step but a critical determinant of surgical accuracy. To address these challenges, especially for early-career surgeons, we recommend to manually palpate the anterior superior iliac spines (ASIS) after draping to confirm the pelvis remains level. The utilization of adjustable positioning aids allows for fine-tuning to correct for any observed pelvic tilt. In the absence of such guides, knowledge of the measured pelvic tilt enables an experienced surgeon to make a cognitive adjustment to the target cup orientation. For surgeons with less tactile experience, technologies such as computer navigation or intraoperative imaging provide invaluable real-time feedback, effectively compensating for positioning inaccuracies. The integration of these strategies into standard surgical protocols and training curricula for residents and fellows could significantly mitigate the risks identified in this study.

Conclusion

Although the impact of patient positioning on cup placement accuracy is generally acknowledged, no previous prospective study has specifically examined the link between preoperative patient positioning and acetabular cup orientation. Our findings reveal that most patients are positioned with a medial rotation of the pelvis on the operated side, which significantly affects postoperative acetabular inclination and limb length discrepancy. Additionally, our study demonstrates that increased surgical experience reduces errors related to patient positioning. These results underscore the clinical importance of carefully controlling pelvic alignment prior to sterile draping. Our findings underscore that achieving a neutral and stable pelvic alignment on the operating table can minimize the risk of inaccurate cup placement and unequal limb lengths—key factors that impact implant success. Preoperative assessment of pelvic position is a straightforward procedure that can improve THA accuracy and may reduce the likelihood of revision surgeries related to improper cup positioning.