INTRODUCTION
Lateral lumbar interbody fusion (LLIF) has become an established minimally invasive option for treating a range of spinal pathologies, including degenerative disc disease, spondylolisthesis, spinal stenosis, and deformity correction. Compared with traditional anterior or posterior approaches, LLIF offers advantages such as reduced blood loss, shorter operative time, and faster recovery [1-3]. However, performing LLIF in a single lateral position presents unique challenges in maintaining or restoring physiological lumbar lordosis (LL) and optimal spinopelvic parameters intraoperatively. Achieving appropriate alignment is critical for postoperative balance, fusion success, and prevention of complications such as adjacent-segment disease and mechanical failure [4,5]. The relationship between hip position and lumbar alignment has been demonstrated in supine and prone postures, where varying degrees of hip flexion significantly influence pelvic tilt (PT), sacral slope (SS), and overall LL [6-8].
A comprehensive understanding of how varying hip positions influence LL and pelvic alignment in the right lateral decubitus position (RLDP) is essential for optimizing surgical technique and outcomes during single-position LLIF [7]. Although prior studies have examined how posture affects spinopelvic parameters in standing, sitting, or prone positions, the influence of hip configuration in the lateral decubitus position remains insufficiently characterized. Existing evidence suggests that hip flexion alters pelvic orientation and spinal curvature; however, most data are derived from non-lateral models that do not accurately reflect intraoperative positioning [9-13].
Clarifying how hip position modulates LL and pelvic parameters in RLDP has direct clinical relevance, as optimal alignment facilitates cage placement, minimizes mechanical stress, and contributes to harmonious spinopelvic balance. Therefore, this study aimed to quantify the effects of different hip positions on LL, PT, SS, and pelvic incidence (PI) in RLDP among healthy volunteers, and to identify the configuration that best reproduces physiological standing alignment. We hypothesized that moderate flexion of the lower hip with the contralateral hip neutral would best preserve lumbar alignment during RLDP.
MATERIALS AND METHODS
1. Study Design and Participants
This cross-sectional descriptive study was approved by the Institutional Review Board of the Faculty of Medicine of Chulalongkorn University (IRB No. 0867/65). All participants provided written informed consent. The study was conducted at our university hospital, between August 2022 and December 2023, in accordance with the Declaration of Helsinki.
Thirty healthy volunteers (15 males, 15 females; mean age, 27.8±8.6 years; range, 20–48 years) without spinal deformity, hip pathology, or prior spinal surgery were prospectively recruited. Exclusion criteria included radiographic evidence of spinal or pelvic asymmetry, previous hip or spinal operations, and neurological or systemic conditions affecting spinal posture.
2. Sample Size and Power Consideration
A minimum of 24 subjects was estimated to achieve 80% power (α=0.05) for detecting a medium within-subject effect size (Cohen f=0.25) across 6 repeated measures, assuming moderate intermeasure correlation (ρ≈0.5). To ensure adequate power and precision for both comparative and reliability analyses, 30 participants were enrolled. This sample size aligns with prior radiographic posture studies evaluating spinopelvic parameters in healthy adults, which typically include 25–30 participants [9,10,12].
3. Radiographic Protocol and Positioning
Each participant underwent true lateral lumbar spine radiography in the standing position and in 5 distinct RLDP configurations (Fig. 1): (1) neutral position of both hips (NN); (2) 30° flexion of both hips (30FF); (3) 30° flexion of the right (dependent) hip with the left (upper) hip in neutral (30FN); (4) 60° flexion of both hips (60FF); and (5) 60° flexion of the right (dependent) hip with the left (upper) hip in neutral (60FN).
Participants lay on their right side with a pillow supporting the head. The right leg was flexed according to the target angle, confirmed using a goniometer. A small cushion was placed under the left thigh to maintain the upper (left) hip in a neutral position and avoid adduction, thereby standardizing pelvic alignment across positions. Lateral radiographs were obtained using a digital radiography system (Konica Minolta AeroDR, Konica Minolta Healthcare Americas, Inc., USA).
4. Radiographic Measurements
Images were analyzed using picture archiving and communication system (PACS) software (Infinitt PACS; Infinitt Healthcare Co., Ltd., Korea). Two orthopedic spine surgeons independently measured:
• LL: angle between the superior endplate of L1 and the superior endplate of S1.
• PT: angle between a vertical line and a line connecting the midpoint of the sacral plate to the bicoxofemoral axis.
• SS: angle between a horizontal reference and the superior endplate of S1.
• PI: angle between a perpendicular to the sacral plate midpoint and a line joining this point to the bicoxofemoral axis (PI=PT+SS).
5. Reliability Analysis
Each observer performed 2 independent measurements of all radiographs at least 2 weeks apart. Intra- and interobserver reliability were evaluated using intraclass correlation coefficients (ICCs) with 95% confidence intervals. ICC values >0.90 were interpreted as excellent reliability.
6. Statistical Analysis
Statistical analysis was performed using IBM SPSS Statistics ver. 23.0 (IBM Co., USA). Continuous variables are presented as mean±standard deviation. Differences in LL, PT, SS, and PI between the standing reference and each RLDP configuration were assessed using paired t-tests.
The primary objective was to quantify the biomechanical deviation of each RLDP configuration relative to the physiological reference (standing alignment). Our statistical analysis was strictly limited to 5 predefined comparisons, each position against the standing control.
Direct statistical comparisons (pairwise testing) between the 5 different RLDP configurations were not performed. This choice aligns with our a priori objective. We did not aim to establish the statistical superiority of one position over the others, which would have required a repeated measures analysis of variance and complex post hoc corrections (such as Bonferroni) to control the Family-Wise Error Rate. Limiting the analysis to paired t-tests was deemed the most statistically sound and efficient method for addressing our specific research question.
Consequently, post hoc corrections were not applied. Statistical significance was set at p<0.05.
RESULTS
Thirty volunteers (15 males, 15 females; mean age, 27.8±8.6 years; range, 20–48 years) completed all radiographic evaluations. Mean height, weight, and body mass index were 159.8±7.2 cm, 56.8±9.2 kg, and 22.4±5.8 kg/m², respectively (Table 1).
Intraobserver ICCs for LL, PT, SS, and PI ranged from 0.92 to 0.99, and interobserver ICCs from 0.90 to 0.96, confirming excellent measurement reliability. Detailed intra- and interobserver ICC values for each vertebral level and parameter are provided in Supplementary Tables 1-3.
The mean LL in standing was 51.1°±3.8°. All 5 RLDP configurations showed a significant reduction in LL compared with standing (p<0.001). When comparing the magnitude of change, the 30FN position (right hip flexed 30°, left hip neutral) exhibited the smallest mean deviation from standing alignment (ΔLL=-4.9°) (Fig. 2). LL decreased progressively with greater or bilateral hip flexion, reaching the largest deviation in the 60FF position (ΔLL=-15.0°) (Table 2). However, direct statistical comparisons between these positions were not conducted, as the study did not seek to test hypotheses regarding the statistical superiority of one lateral position over another.
At the segmental level, L5–S1 lordosis in the 30FN position (11.43°±2.89°) was not significantly different from standing (11.98°±3.45°, p=0.156), confirming near-physiologic preservation of curvature at the lumbosacral junction.
Analysis of pelvic parameters revealed consistent changes in pelvic orientation (Table 2). PI demonstrated only minimal variation across positions (Fig. 3). Although small but statistically significant increases were noted during bilateral hip flexion (30FF and 60FF), the absolute changes were less than 1°, indicating no meaningful alteration in pelvic geometry.
In the 30FN position, PT decreased from 18.7°±4.5° to 16.4°±5.9° (p=0.013), while SS increased from 32.3°±4.0° to 35.2°±5.1° (p=0.003), reflecting mild anterior pelvic rotation (Figs. 4 and 5). Conversely, the 60FF position demonstrated increased PT and reduced SS, consistent with posterior PT and flattening of lumbar curvature.
When ranked by deviation from standing, LL changed progressively according to hip configuration in the following order: 30FN<NN<30FF<60FN<60FF (Fig. 2). This pattern demonstrates that increasing bilateral hip flexion produced progressively greater lordotic loss. The 30FN configuration most closely replicated physiologic standing alignment, whereas 60FF produced the largest sagittal flattening.
For sex dimorphism, sex-stratified subgroup analysis demonstrated that both males and females exhibited a similar directional response to hip positioning in RLDP. Although female participants showed slightly larger absolute decreases in LL in several positions, the overall pattern of change across NN, 30FN, 30FF, 60FN, and 60FF was consistent between sexes. Mixed-model analysis revealed no significant position×sex interaction for LL, PT, SS, or PI (all p>0.05), indicating that sex did not significantly modify the effect of hip configuration on spinopelvic alignment. Numerical values are provided in Supplementary Tables 4 and 5.
DISCUSSION
Single-position LLIF has gained increasing popularity owing to its efficiency and reduced operative time, yet achieving optimal sagittal alignment remains challenging when the procedure is performed entirely in the lateral decubitus position. The present study demonstrates that hip configuration significantly influences LL and pelvic orientation during RLDP. All tested hip positions produced measurable reductions in LL compared with standing, confirming that lateral positioning inherently decreases lumbar curvature. Among the configurations tested, the 30FN position demonstrated the smallest numerical reduction in LL relative to standing. While we did not statistically compare the positions to one another, the data descriptively suggests that the 30FN configuration minimized the ‘sagittal flattening’ effect more effectively than the bilateral hip flexion (60FF) positions in this cohort.
Our findings support and expand upon previous studies showing posture-dependent variations in spinopelvic parameters. Prior radiographic analyses in standing, sitting, and prone positions have consistently demonstrated that hip flexion influences PT, SS, and lumbar curvature [9-13]. However, few studies have examined these relationships in the lateral decubitus position, which is now widely used for single-position LLIF. The present results clarify that moderate unilateral hip flexion can help maintain physiologic spinal alignment during RLDP, providing a practical intraoperative reference for patient positioning.
Biomechanically, the observed pattern reflects the interdependence of LL, PT, and SS. In the 30FN configuration, PT decreased and SS increased relative to standing, signifying mild anterior pelvic rotation caused by flexion of the dependent (right) hip. Despite a modest reduction in overall LL (ΔLL=-4.9°), this anterior rotation helps counteract gravitational flattening of the spine in the lateral posture. In contrast, deeper or bilateral hip flexion, as in 60FF, induces posterior pelvic rotation, increasing PT and reducing SS, which together produce greater loss of lordosis (ΔLL=-15°) and a mechanically less favorable alignment for cage placement and sagittal correction.
Although the 30FN position exhibited reduced PT and increased SS compared with standing, this does not contradict the overall decrease in LL. The reduction reflects the absence of axial loading and paraspinal muscle tone present in standing, while the mild anterior rotation in 30FN compensates for this effect, maintaining near-physiologic curvature. This interplay between PT and SS highlights that preserving balanced spinopelvic alignment, rather than replicating the absolute magnitude of standing lordosis, should be the primary intraoperative goal. The 30FN position achieved this balance most effectively, offering a stable, reproducible setup that minimizes fluoroscopic adjustments, shortens operative time, and supports sagittal harmony during single-position LLIF.
The interaction between hip position and psoas morphology is clinically important during LLIF. Hip flexion shortens the psoas, reducing passive tension and shifting the muscle belly posteriorly, whereas a neutral hip position elongates and tensions the psoas, decreasing its anterior cross-sectional area and enlarging the retroperitoneal oblique corridor (ROC) [14-16]. These opposing effects underscore the need for a balanced positioning strategy rather than maximal flexion or full neutrality. Our findings suggest that the 30FN configuration achieves this balance: flexing the right hip (lower) to 30° provides adequate lordosis and does not adversely affect the morphology of the upper psoas, while keeping the left (upper/surgical-side) hip neutral preserves ROC size and reduces psoas bulk. This interpretation aligns with prior work by Kotheeranurak et al. [7], that a neutral upper hip increases the ROC by approximately 20% and reduces psoas bulk. Because traction-related neuropraxia is more commonly associated with excessive or prolonged retraction than with positioning alone, appropriate hip orientation combined with careful retraction technique and neuromonitoring may further enhance procedural safety [17,18].
Although this study focused on how hip configuration influences lumbar alignment, other anatomic factors, particularly iliac crest morphology, also affect LLIF feasibility, especially at L4–5. Lower crest height and a steeper crest slope have been associated with a wider safe working zone and a more posterior lumbar plexus position, improving transpsoas access [19]. Beyond crest height, the intrinsic characteristics of the oblique or transpsoas surgical corridor also influence approach selection. Recent evidence shows that the OLIF corridor can be expanded safely with careful psoas retraction and that even relatively narrow corridors do not negatively impact clinical or radiographic outcomes at L4–5 [20,21]. While hip positioning optimizes sagittal alignment, corridor anatomy, governed by iliac crest height, vascular location, and psoas morphology, ultimately determines access. In cases where a high crest restricts L4–5 LLIF, OLIF provides a reliable anterior-to-psoas alternative that is unaffected by crest height. This corridor-related constraint exists independently of RLDP hip configuration and remains an important determinant of approach planning.
From a clinical standpoint, these findings suggest that the 30FN configuration may serve as a practical starting point during right lateral LLIF because it provides a biomechanically favorable alignment close to physiologic standing posture. This descriptive finding should, however, be individualized based on patient anatomy and intraoperative imaging. By approximating physiologic alignment and optimizing pelvic orientation, surgeons can achieve more accurate cage placement, minimize compensatory segmental strain, and reduce the risk of postoperative imbalance or adjacent-segment degeneration. Standardizing this position can streamline the preoperative setup, decrease the need for repeated intraoperative adjustments, and reduce radiation exposure, while individualized radiographic assessment remains essential. Future studies incorporating real-time fluoroscopic evaluation under anesthesia and studies in patients with degenerative spinal pathology are warranted to further validate the clinical applicability of this positioning strategy.
Given the known sex dimorphism in pelvic shape and spinopelvic morphology, we performed a post hoc sex-stratified analysis. Female participants demonstrated slightly wider variability in ΔLL, PT, and SS, consistent with anatomical differences such as greater anterior PT and wider pelvic morphology. However, the position×sex interaction was not significant for any parameter, indicating that the biomechanical response to hip positioning is similar in males and females. These findings suggest that, although absolute pelvic orientation varies by sex, the relative effect of hip configuration on sagittal alignment in RLDP is largely sex-independent.
This study has several limitations. First, the cohort consisted of healthy young adults without degenerative spinal pathology, which allowed isolation of the biomechanical effect of hip positioning but limits generalizability to older LLIF candidates who may have fixed pelvic retroversion, segmental stiffness, or reduced hip mobility. Prior sagittal alignment studies [22,23] have shown that age-related degeneration alters pelvic compensation and may prevent patients from achieving the postures observed in our volunteers. Our aim was therefore to define the physiologic baseline response to hip positioning, establishing a biomechanical target rather than a universally achievable clinical position. In practice, the applicability of this target may be limited by patient-specific pathology, including fixed pelvic retroversion, hip stiffness, or spinal ankylosis.
Second, imaging was performed in awake individuals with preserved paraspinal muscle tone. Under general anesthesia, paraspinal relaxation is known to cause additional reductions in LL compared with standing or awake positioning [24], suggesting that the lordotic flattening observed in our study may underestimate the intraoperative effect. Accordingly, the clinical importance of using a lordosis-preserving configuration such as 30FN may be even greater during surgery.
Third, although all RLDP positions measurements demonstrated excellent inter- and intraobserver reliability, subtle measurement variability is inherent to manual angle determination on radiographs.
Fourth, this study utilized a reference-based statistical design, focusing solely on the deviation from the standing physiological alignment. Direct statistical comparisons (pairwise testing) among the 5 different RLDP positions were considered outside the defined, a priori scope of our investigation. While the 30FN position was descriptively identified as having the smallest numerical loss of lordosis (closest to standing), conclusions regarding its statistical superiority over other configurations (such as NN or 30FF) cannot be drawn from this data. Our final positioning recommendation is based on this descriptive finding that the 30FN configuration resulted in the most favorable maintenance of the sagittal profile in our cohort. Also, although no significant sex-based interaction was detected, the study was not powered specifically to detect subtle sex-related effects, and larger cohorts may better characterize sex-dependent alignment responses.
Finally, this study assessed static postures only; dynamic changes in alignment under surgical load, retraction, and instrumentation were not evaluated. Future research incorporating intraoperative fluoroscopic or navigation-based measurements in anesthetized patients with spinal pathology is warranted to validate and extend these findings to clinical practice.
CONCLUSION
Hip positioning has a significant effect on lumbar and pelvic alignment in the RLDP, with all tested configurations resulting in a statistically significant reduction in LL compared to the standing reference. Among the configurations evaluated, the 30FN position, with the right (lower) hip flexed 30° and the left (upper) hip in neutral, descriptively demonstrated the smallest numerical deviation from the physiologic standing alignment, effectively preserving both LL and spinopelvic balance.
This configuration is biomechanically advantageous as it simultaneously achieves the most favorable sagittal alignment while maintaining a neutral position for the left (upper/surgical access) hip. This neutral left hip is a critical, proven factor for minimizing psoas muscle tension, reducing the risk of neurological complications, and optimizing the surgical retroperitoneal corridor.
Adopting the 30FN configuration provides a reproducible and biomechanically favorable starting posture for single-position LLIF, enhancing surgical efficiency, supporting optimal interbody cage placement, and aiding in the maintenance of postoperative sagittal balance.
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