Abstract
Introduction
To evaluate whether lateral decubitus positioning increases the pleura–plexus distance at the supraclavicular level and to characterize its role as a modifiable anatomical factor relevant to ultrasound-guided interventions.
Methods
This prospective observational ultrasound study included 90 adults comprising individuals with complex regional pain syndrome, cervical spine stenosis, and healthy controls. Pleura–plexus distance was measured at end expiration in three standardized positions: semi-reclined supine, 45° lateral tilt, and lateral decubitus. Positional effects were assessed using mixed repeated-measures analysis of variance, and associations with demographic variables were examined using correlation and multivariable linear regression.
Results
Pleura–plexus distance increased progressively across positions in all groups. Compared with the semi-reclined position (0.37–0.46 cm), lateral decubitus positioning more than doubled the distance (0.82–1.04 cm; p < 0.001), with a significant group interaction. Body mass index independently predicted pleura–plexus distance in the lateral position (+0.028 cm per kg/m2, p < 0.001), whereas age was not significantly associated.
Conclusions
Lateral decubitus positioning was consistently associated with increased pleura–plexus distance at the supraclavicular level across all study groups. These findings suggest that patient positioning represents a measurable and potentially modifiable anatomical factor that may influence anatomical conditions relevant to ultrasound-guided supraclavicular procedures.
Supplementary Information
The online version contains supplementary material available at https://doi.org/10.1007/s40122-026-00856-5.
Keywords: Supraclavicular brachial plexus, Ultrasound-guided procedures, Patient positioning, Lateral decubitus, Pleura-plexus distance, Anatomical configuration, Body mass index
Plain Language Summary
Ultrasound-guided procedures near the brachial plexus can be technically challenging because the plexus lies close to the pleura, creating limited anatomical separation between neural and pleural structures. This study showed that changing patient position from semi-reclined to lateral decubitus significantly increased the distance between the brachial plexus and pleura. The effect was consistent across clinical populations, and higher body mass index was associated with greater positional widening. These findings suggest that patient positioning may represent a simple and noninvasive strategy to optimize anatomical conditions relevant to ultrasound-guided supraclavicular procedures without requiring additional equipment or procedural modification.
Supplementary Information
The online version contains supplementary material available at https://doi.org/10.1007/s40122-026-00856-5.
Key Summary Points
| Why carry out this study? |
| Ultrasound-guided supraclavicular brachial plexus procedures are widely used in regional anesthesia and interventional pain medicine; however, the close proximity of the brachial plexus to the pleura remains an important anatomical consideration. |
| Optimizing anatomical conditions through simple, noninvasive strategies may improve procedural planning and facilitate ultrasound-guided targeting. |
| This study investigated whether lateral decubitus positioning increases pleura–plexus distance compared with conventional semi-reclined and lateral tilt positions. |
| What was learned from the study? |
| Lateral decubitus positioning significantly increased pleura–plexus distance across all study groups and more than doubled the distance compared with the semi-reclined position (p < 0.001). |
| Body mass index was an independent predictor of pleura–plexus distance in the lateral decubitus position, whereas age was not significantly associated. |
| These findings demonstrate that patient positioning is a measurable and modifiable anatomical factor influencing supraclavicular anatomy. |
| Position optimization may represent a simple, noninvasive strategy to improve anatomical conditions relevant to ultrasound-guided supraclavicular brachial plexus targeting and warrants further clinical investigation. |
Introduction
Ultrasound-guided supraclavicular brachial plexus procedures are widely used in regional anesthesia and interventional pain practice owing to their effectiveness in providing dense and rapid blockade of the upper limb [1–4]. However, the close anatomical relationship between the brachial plexus and the pleura at the supraclavicular level results in a limited neuropleural anatomical separation, which may increase procedural complexity and necessitate careful needle trajectory planning [3, 4].
Although pleural complications are uncommon with modern ultrasound guidance, they remain clinically relevant. Pneumothorax has been reported even under ultrasound guidance, often related to suboptimal needle visualization or trajectory misalignment [5]. The estimated incidence remains low (approximately 0.4 per 1000 procedures), yet this risk underscores the importance of maintaining adequate anatomical separation between neural and pleural structures during supraclavicular interventions [6]. Accordingly, current safety strategies emphasize meticulous needle control, continuous in-plane visualization, and the use of the first rib as a mechanical backstop [3, 7–9].
The supraclavicular region is anatomically and procedurally dynamic. Cervical alignment, thoracic orientation, scapulothoracic positioning, and surrounding soft tissue structures may influence the spatial relationship between the brachial plexus and pleura [7–10]. These observations suggest that body positioning may represent a modifiable anatomical factor with potential procedural relevance. Recent anatomy-informed frameworks further support the concept that spatial relationships within neural compartments may be influenced by anatomical targeting and tissue mechanical properties [11].
Building on this conceptual framework, preliminary observations from independent studies have suggested that lateral decubitus positioning may increase the pleura–plexus distance at the supraclavicular level [12, 13]. However, these findings were derived from relatively small cohorts and heterogeneous methodologies, without systematic evaluation across different clinical populations or quantitative modeling of anatomical determinants. As such, the generalizability and consistency of position-related anatomical changes remain incompletely defined.
In addition, supraclavicular anatomy varies across individuals. Sonographic studies have shown that body habitus, tissue depth, and local anatomical characteristics influence needle trajectory and the depth of key anatomical targets during ultrasound-guided procedures [14]. These factors may also influence baseline pleura–plexus distance and its positional variability.
Clinical conditions may further influence anatomical behavior. Complex regional pain syndrome is associated with autonomic dysregulation and altered soft tissue properties [15–18], whereas cervical spine disorders are characterized by structural and biomechanical constraints that may affect neural mobility [19–22]. Whether such factors influence position-related anatomical changes at the supraclavicular level remains incompletely understood.
Accordingly, this study was designed to evaluate the effect of body positioning on pleura–plexus distance during ultrasound-guided supraclavicular brachial plexus targeting. Using a standardized high-resolution ultrasound protocol, pleura–plexus distance was measured across three clinically relevant positions: semi-reclined supine, 45° lateral tilt, and full lateral decubitus. The primary objective was to determine the magnitude and consistency of position-related changes across different clinical populations and healthy controls. Secondary objectives included identifying anatomical and demographic determinants of positional variation using correlation and multivariable regression analyses.
Methods
Study Design and Participants
This prospective observational ultrasound imaging study was conducted at a tertiary regional anesthesia and pain intervention center between 2024 and 2025. Ethical approval for this study was obtained from the Institutional Review Board of Chang Gung Medical Foundation, Taiwan (IRB No. 202401968A3). All participants provided written informed consent prior to participation. The study was conducted in accordance with the principles of the Declaration of Helsinki and applicable institutional regulations.
The study was designed as an exploratory observational imaging study intended to characterize positional anatomical variation across clinical groups. Sample size selection was based on feasibility considerations, anticipated anatomical variability, and the ability to evaluate repeated-measures positional effects across multiple participant cohorts.
A total of 90 adults were enrolled, comprising individuals with complex regional pain syndrome (CRPS) [15–18], cervical spine stenosis (CSS) [19–22], and healthy controls. Exclusion criteria included prior cervical or thoracic surgery; known pleural or pulmonary disease; previous supraclavicular brachial plexus block or intervention on the examined side; or inadequate sonographic visualization of the supraclavicular region. Ultrasound assessments were performed on the symptomatic side in participants with CRPS and CSS, and on a randomly assigned side in healthy controls. CRPS diagnosis was established according to the Budapest clinical diagnostic criteria. Cervical spine stenosis diagnosis was based on compatible clinical symptoms and confirmatory imaging findings.
Ultrasound Examination and Measurement Protocol
Ultrasound examinations were performed using a high-frequency linear array transducer with standardized depth, focal zone, and gain settings consistent with ultrasound-guided supraclavicular brachial plexus targeting technique. The supraclavicular brachial plexus, subclavian artery, scalene muscles, first rib, and pleural line were identified in a reproducible short-axis view at the level of the first rib. Probe orientation and imaging planes followed conventional anatomical landmarks used in clinical practice.
The pleura–plexus distance was defined as the shortest perpendicular distance from the pleural line to the most superficial margin of the inferior trunk of the brachial plexus. Measurements were obtained using electronic calipers aligned orthogonally to the pleural interface to minimize angular error and ensure consistency across examinations.
To reduce respiratory-related variability, all measurements were obtained at end expiration, when diaphragmatic excursion and pleural motion are minimal. For each body position, three consecutive measurements were acquired and averaged to improve measurement precision. All ultrasound examinations were performed by two experienced operators with expertise in musculoskeletal and regional anesthesia ultrasound, following a harmonized acquisition protocol. Interobserver agreement was assessed during pilot testing and confirmed to be acceptable prior to study initiation.
Image Management and Quality Control
All sonographic images and cine loops were digitally stored in a secure, time-stamped archive and anonymized prior to analysis. Each image set was independently reviewed by both operators using identical measurement presets. When measurements differed by more than 10%, images were jointly reevaluated and a consensus value was determined. This adjudication process was used to minimize interobserver variability and ensure uniform methodological standards across all evaluations.
Body Positions
Pleura–plexus distance was assessed in three standardized body positions. In the semi-reclined supine position (D1), participants were positioned at approximately 30°, with the head supported and gently rotated away from the examined side. In the 45° lateral tilt position (D2), pillows were used to maintain a stable tilt, with the ipsilateral upper limb resting alongside the body. In the full lateral decubitus position (D3), participants lay fully on the side, with the ipsilateral shoulder adducted and the humerus internally rotated. The arm was supported anteriorly on a pillow to minimize brachial plexus tension and facilitate gravitational redistribution of soft tissues.
Outcomes
Primary Outcome
The primary anatomical outcome was pleura–plexus distance, defined as the shortest perpendicular distance from the inferior trunk of the brachial plexus to the pleural line in each body position. This measurement represents the anatomical separation relevant to ultrasound-guided supraclavicular brachial plexus targeting. For each position, three measurements were obtained using standardized digital calipers, and the mean value was used for analysis.
Secondary Outcomes
Secondary analyses examined demographic and anatomical factors potentially associated with pleura–plexus distance across positions, including body mass index (BMI), age, ultrasound depth to the brachial plexus, scalene muscle thickness, cervical range-of-motion limitation, and pain-related clinical characteristics. Correlation analyses were performed to assess associations between participant characteristics and pleura–plexus distance in each position.
A multivariable linear regression model was constructed to identify independent predictors of pleura–plexus distance in the lateral decubitus position (D3). Candidate variables included body mass index (BMI), age, diagnostic category, and side of examination. Additional exploratory adjustment analyses considering baseline semi-reclined pleura–plexus distance (D1) and ultrasound depth were performed to further evaluate positional anatomical variation.
Statistical Analysis
Statistical analyses were performed using SPSS version 29.0 (IBM Corp., Armonk, NY, USA) and R version 4.3.2. Data distribution was assessed using the Shapiro–Wilk test and visual inspection of Q–Q plots. Homogeneity of variance was evaluated using Levene’s test.
Between-group comparisons of continuous variables were conducted using one-way analysis of variance (ANOVA), with Bonferroni-adjusted post hoc comparisons when appropriate. Two-group comparisons (CRPS versus CSS) were analyzed using independent-samples t tests. Categorical variables were evaluated using chi-square or Monte Carlo exact tests as appropriate.
To assess positional effects on pleura–plexus distance, a mixed (split-plot) repeated-measures ANOVA was performed, with body position (semi-reclined supine, 45° lateral tilt, lateral decubitus) as the within-subject factor and diagnostic group (CRPS, CSS, healthy control) as the between-subject factor. Mauchly’s test of sphericity was conducted; when sphericity assumptions were violated, Greenhouse–Geisser corrections were applied. Effect sizes are reported as partial eta squared (η2). Bonferroni-adjusted pairwise comparisons were performed for post hoc positional contrasts.
Pearson correlation coefficients were calculated to examine associations between demographic variables and pleura–plexus distance in each position.
A multivariable linear regression model was constructed to identify independent predictors of pleura–plexus distance in the lateral decubitus position (D3). Predictor variables included body mass index (BMI), age, and diagnostic category. Standardized β coefficients were calculated to estimate relative effect magnitude. Model assumptions were evaluated using residual plots, and multicollinearity was assessed using variance inflation factors (VIF). Given the high adjusted R2, additional diagnostics, including Cook’s distance, leverage statistics, and examination of studentized residuals were performed, confirming model robustness without evidence of influential outliers. Statistical significance was defined as a two-sided p value < 0.05.
Results
Participant Flow and Data Completeness
A total of 112 individuals were screened for eligibility. Approximately 22 were excluded due to prior cervical or thoracic surgery, known pleural pathology, inadequate sonographic visualization, or intolerance of positional testing. A total of 90 participants completed all assessments, including 30 individuals with complex regional pain syndrome (CRPS), 30 with cervical spine stenosis (CSS), and 30 healthy controls. Participant flow is illustrated in the STROBE diagram (Fig. 1). Complete raw measurements for all three body positions are provided in Supplementary Tables S1 and S2, confirming full data acquisition with no missing values.
Fig. 1.
STROBE flow diagram. Flow diagram showing screening, exclusions, enrollment of 90 participants into three diagnostic groups, and completion of ultrasound measurements in three body positions with no missing data
Baseline Characteristics
Baseline characteristics are summarized in Table 1. Participants with CSS were significantly older than those with CRPS and healthy controls (73.4 ± 3.9 versus 55.4 ± 5.5 and 54.8 ± 5.1 years, respectively; F(2,87) = 214.6, p < 0.001). Body mass index (BMI) also differed significantly across groups (F(2,87) = 33.4, p < 0.001), with the highest values observed in CSS (26.1 ± 1.3 kg/m2), followed by CRPS (24.6 ± 1.3 kg/m2) and healthy controls (23.7 ± 1.0 kg/m2).
Table 1.
Baseline demographic, clinical, and ultrasound characteristics of the study population
| Variable | CRPS (n = 30) | CSS (n = 30) | Healthy controls (n = 30) | Test statistic | p value |
|---|---|---|---|---|---|
| Age, years | 55.4 ± 5.5 | 73.4 ± 3.9 | 54.8 ± 5.1 | F(2,87) = 214.6 | < 0.001 |
| Female sex, n (%) | 19 (63.3) | 17 (56.7) | 20 (66.7) | χ2(2) = 0.67 | 0.72 |
| Examined side, right/left, n | 16/14 | 15/15 | 14/16 | χ2(2) = 0.27 | 0.87 |
| BMI, kg/m2 | 24.6 ± 1.3 | 26.1 ± 1.3 | 23.7 ± 1.0 | F(2,87) = 33.4 | < 0.001 |
| Symptom duration, yearsa | 3.5 ± 1.2 | 6.8 ± 1.7 | – | t(58) = 8.79 | < 0.001 |
| Pain severity (VAS 0–10)a | 7.7 ± 0.9 | 5.9 ± 0.7 | – | t(58) = 9.16 | < 0.001 |
| Functional disability score (0–100)a | 53.3 ± 4.2 | 45.5 ± 2.9 | – | t(58) = 8.63 | < 0.001 |
| Neck ROM limitation, n (%) | 11 (36.7) | 23 (76.7) | 4 (13.3) | χ2(2) = 23.1 | < 0.001 |
| Ultrasound depth to plexus, cm | 3.05 ± 0.10 | 3.23 ± 0.10 | 2.89 ± 0.07 | F(2,87) = 118.2 | < 0.001 |
| Scalene muscle thickness, mm | 7.3 ± 0.4 | 7.8 ± 0.4 | 7.0 ± 0.2 | F(2,87) = 47.5 | < 0.001 |
Values are presented as mean ± SD unless otherwise indicated
Continuous variables were compared using one-way ANOVA for three-group comparisons. Two-group comparisons were performed using independent samples t-test. Categorical variables were analyzed using chi-square testing. All tests were two-sided. Statistical significance was defined as p < 0.05
Assumptions of normality and homogeneity of variance were verified using Shapiro–Wilk and Levene’s tests prior to ANOVA
Examined side distribution did not differ significantly across groups
aCompared between CRPS and CSS only (healthy controls not applicable)
Pain severity and functional disability were significantly greater in the CRPS group compared with CSS (both p < 0.001). Cervical range-of-motion limitation was most prevalent in CSS (76.7%), intermediate in CRPS (36.7%), and lowest among controls (13.3%; χ2(2) = 23.1, p < 0.001). Ultrasound depth to the brachial plexus differed significantly across groups (F(2,87) = 118.2, p < 0.001), whereas scalene muscle thickness demonstrated smaller but statistically significant differences (F(2,87) = 47.5, p < 0.001). The distribution of examined sides was balanced across groups.
Positional Effects on Pleura–Plexus Distance
Pleura–plexus distance increased progressively across the three standardized body positions in all diagnostic groups (Table 2; Fig. 2).
Table 2.
Positional pleura–plexus distance (cm) across body positions
| Group | D1: semi-reclined supine | D2: 45° side-tilt | D3: lateral decubitus | Within-group p value† |
|---|---|---|---|---|
| CRPS (n = 30) | 0.41 ± 0.03 | 0.46 ± 0.03 | 0.89 ± 0.07 | < 0.001 |
| CSS (n = 30) | 0.46 ± 0.03 | 0.52 ± 0.03 | 1.04 ± 0.08 | < 0.001 |
| Healthy controls (n = 30) | 0.37 ± 0.02 | 0.42 ± 0.02 | 0.82 ± 0.06 | < 0.001 |
| All participants (n = 90) | 0.41 ± 0.04 | 0.47 ± 0.05 | 0.92 ± 0.12 | < 0.001 |
Values are presented as mean ± SD
A mixed (split-plot) repeated-measures ANOVA demonstrated: Main effect of position: F(2,174) = 12,648.33, p < 0.001, partial η2 = 0.993. Main effect of group: F(2,87) = 81.57, p < 0.001, partial η2 = 0.652. Group × Position interaction: F(4,174) = 72.72, p < 0.001, partial η2 = 0.626
Bonferroni-adjusted pairwise comparisons confirmed significant increases from D1 to D2, D2 to D3, and D1 to D3 across all groups (all p < 0.001)
†Within-group positional differences were analyzed using repeated-measures ANOVA (D1–D3)
Fig. 2.
Positional changes in pleura–plexus distance across diagnostic groups. Line plot showing mean pleura–plexus distance measured in three body positions for each diagnostic group; distances increased progressively from semi-reclined supine to lateral decubitus
Mixed (split-plot) repeated-measures ANOVA demonstrated a highly significant main effect of position (F(2,174) = 12,648.33, p < 0.001, partial η2 = 0.993), indicating that body posture accounted for the vast majority of within-subject variance in pleura–plexus distance. A significant main effect of diagnostic group was also observed (F(2,87) = 81.57, p < 0.001, partial η2 = 0.652), along with a significant group × position interaction (F(4,174) = 72.72, p < 0.001, partial η2 = 0.626), demonstrating that the magnitude of positional widening differed among cohorts.
In the semi-reclined supine position (D1), pleura–plexus distance was uniformly narrow across groups (CRPS 0.41 ± 0.03 cm; CSS 0.46 ± 0.03 cm; controls 0.37 ± 0.02 cm). Transition to a 45° lateral tilt (D2) produced a modest but statistically significant increase in all groups (all p < 0.001). The greatest increase occurred in the full lateral decubitus position (D3), where pleura–plexus distance more than doubled relative to D1. In D3, mean distances were 0.89 ± 0.07 cm in CRPS, 1.04 ± 0.08 cm in CSS, and 0.82 ± 0.06 cm in healthy controls.
Bonferroni-adjusted pairwise comparisons confirmed significant increases for D1 → D2, D2 → D3, and D1 → D3 transitions in all groups (all p < 0.001). Effect sizes for the supine-to-lateral transition were large (Cohen’s d range 1.4–1.9), indicating a substantial positional effect on anatomical separation at the supraclavicular level.
Correlation Analysis
Associations between participant characteristics and pleura–plexus distance are presented in Supplementary Table S3. BMI demonstrated strong correlations with pleura–plexus distance in the lateral decubitus position (D3), with correlation coefficients of 0.72 in CRPS, 0.78 in CSS, and 0.68 in healthy controls (all p < 0.001). Across the entire cohort, the association remained robust (r = 0.74, p < 0.001).
In contrast, age showed no significant association with pleura–plexus distance in D3 (p > 0.05). In the semi-reclined position (D1), BMI demonstrated only weak correlations with pleura–plexus distance, supporting the interpretation that the increased pleura–plexus distance observed in D3 reflects position-related anatomical modification beyond baseline anatomical depth alone. The linear association between BMI and D3 distance was demonstrated by the correlation analysis (Supplementary Table S3).
Multivariable Regression Analysis
Multivariable linear regression identified BMI as the dominant independent predictor of pleura–plexus distance in the lateral decubitus position (Table 3). The overall model was highly significant (F(4,85) = 533.4, p < 0.001), explaining 96% of the variance in D3 distance (adjusted R2 = 0.96).
Table 3.
Multivariable linear regression model for predictors of pleura–plexus distance in lateral decubitus (D3)
| Predictor | β (cm) | Standard error | 95% CI (cm) | Standardized β | p value | VIF |
|---|---|---|---|---|---|---|
| Body mass index (kg/m2) | 0.028 | 0.003 | 0.022–0.034 | 0.82 | < 0.001 | 1.54 |
| Age (years) | 0.002 | 0.002 | −0.002–0.006 | 0.04 | 0.28 | 1.61 |
| CRPS (versus control) | 0.045 | 0.021 | 0.003–0.087 | 0.11 | 0.04 | 1.32 |
| CSS (versus control) | 0.092 | 0.019 | 0.055–0.129 | 0.24 | < 0.001 | 1.48 |
Dependent variable: pleura–plexus distance in lateral decubitus (D3), cm
Reference category: healthy controls
Overall model: F(4, 85) = 533.4, p < 0.001; adjusted R2 = 0.96
All variance inflation factors were < 2.0, indicating no evidence of multicollinearity
Additional exploratory adjustment analyses considering baseline D1 pleura–plexus distance and ultrasound depth demonstrated that the observed association between body mass index and D3 pleura–plexus distance remained directionally consistent after accounting for baseline anatomical variation. These findings support the interpretation that positional widening reflects position-related anatomical modification rather than baseline anatomy alone.
Each one-unit increase in BMI was associated with an adjusted increase of 0.028 cm in pleura–plexus distance (95% CI 0.022–0.034, p < 0.001; standardized β = 0.82). Diagnostic category independently influenced D3 distance: compared with healthy controls, CRPS was associated with an adjusted increase of 0.045 cm (p = 0.04), while CSS demonstrated a larger adjusted increase of 0.092 cm (p < 0.001). Age was not independently associated with D3 distance after adjustment (p = 0.28).
Variance inflation factors were all below 2.0, indicating no evidence of multicollinearity. Residual analysis demonstrated homoscedasticity, approximate normal distribution of residuals, and no influential outliers (Cook’s distance < 1.0 for all observations).
Exploratory Analyses
Ultrasound depth demonstrated a moderate association with the magnitude of positional change, suggesting that deeper thoracic inlet anatomy may permit greater gravitational displacement during lateral positioning. Scalene muscle thickness was not significantly associated with pleura–plexus distance in any posture (all r < 0.20, p > 0.05).
Feasibility and Safety
Ultrasound imaging was successfully completed in all 90 participants across all body positions. No missing measurements or adverse events occurred. A small number of participants with CSS reported transient discomfort during transition to the lateral decubitus position; however, all examinations were completed without protocol modification, and overall tolerability was high.
Discussion
The present study demonstrates that lateral decubitus positioning is associated with a consistent increase in pleura–plexus distance at the supraclavicular level. These findings support the concept that patient positioning represents a modifiable anatomical factor that may influence anatomical conditions relevant to ultrasound-guided supraclavicular procedures.
From a biomechanical perspective, the supraclavicular region is not a static anatomical space but rather a dynamically responsive compartment influenced by cervical alignment, thoracic orientation, scapulothoracic mechanics, and surrounding fascial tension [24–28]. Changes in body position may alter gravitational loading and soft tissue distribution, resulting in displacement of neural and adjacent structures and modification of their spatial relationships. These considerations provide a plausible mechanistic explanation for the increased pleura–plexus distance observed in the lateral decubitus position.
The present findings are in agreement with previous reports suggesting that lateral positioning may increase the pleura–plexus distance [12, 13]. These earlier studies provided important initial insights into position-related anatomical variation. Building upon this foundation, the current study further explores this phenomenon using a larger cohort, multigroup comparisons, and quantitative modeling of anatomical determinants, thereby offering a more comprehensive characterization of positional anatomical variability.
The clinical relevance of pleura–plexus distance lies in its relationship to procedural conditions and safety. Although pneumothorax is uncommon in ultrasound-guided supraclavicular procedures, it remains a recognized complication, often associated with suboptimal needle visualization or unintended trajectory deviation [6, 7]. In this context, increased anatomical separation between the pleura and brachial plexus may reduce the likelihood of inadvertent pleural proximity during needle advancement. Nevertheless, as clinical outcomes were not directly assessed, these findings should be interpreted as reflecting anatomical conditions that may support procedural safety rather than direct evidence of complication reduction.
Beyond neuropleural separation, an increased pleura–plexus distance may also facilitate procedural performance. A wider anatomical window may enable improved needle trajectory control, enhance real-time visualization of the needle tip, and reduce the need for steep or deep needle advancement. These factors are particularly relevant in ultrasound-guided procedures, where visualization quality is a key determinant of procedural safety [3].
From a practical perspective, lateral decubitus positioning may also influence procedural ergonomics. Altered body orientation may affect probe stability, needle handling, real-time visualization, and patient comfort during ultrasound-guided targeting. In particular, patients with cervical spine disorders may experience positional discomfort or restricted mobility during lateral positioning. Although feasibility was generally favorable in the present cohort, these procedural considerations should be evaluated further in future clinical studies.
The present findings are also aligned with emerging anatomy-informed approaches that emphasize the importance of spatial relationships and tissue mechanics in guiding interventional procedures [11]. Within this conceptual framework, optimization of anatomical context such as through patient positioning may influence procedural feasibility and safety, complementing traditional technique-focused strategies.
Interindividual variability in positional response was observed, suggesting that anatomical and demographic factors may influence the degree of positional modification. Previous studies have demonstrated that body habitus, tissue depth, and regional anatomy affect supraclavicular needle pathways [18, 19]. The current findings extend this understanding by indicating that these factors may also influence responsiveness to positional change, highlighting the potential value of individualized positioning strategies.
Importantly, the pleura–plexus distance measured in the present study represents a standardized perpendicular sonographic measurement rather than the exact oblique needle trajectory encountered during clinical procedures. In practice, needle advancement angles may vary according to operator technique, patient anatomy, and procedural objectives. Accordingly, the measured anatomical separation should be interpreted as a reproducible imaging-derived anatomical parameter rather than a direct procedural trajectory distance.
Clinical conditions may further contribute to this variability. Complex regional pain syndrome is associated with altered autonomic function and tissue characteristics [20–23], whereas cervical spine disorders, including degenerative conditions, may impose structural and biomechanical constraints on neural mobility [24–28]. Although causal relationships cannot be established from the present data, these findings suggest that disease-related factors may influence anatomical responsiveness to positioning.
Several limitations should be acknowledged. First, this study evaluated anatomical measurements rather than direct procedural or clinical outcomes; therefore, conclusions regarding complication reduction or procedural safety cannot be directly established. Second, ultrasound-based measurements are inherently operator-dependent. Although standardized acquisition protocols and consensus adjudication procedures were implemented, image acquisition and measurements were not independently blinded, which may introduce expectation bias. Third, the observational design limits causal inference, and residual confounding cannot be excluded. Fourth, body positions were assessed sequentially rather than randomly assigned, introducing potential order-related effects. Finally, subgroup analyses may have been limited by sample size and should be interpreted cautiously. In addition, the study was not prospectively registered, which should be acknowledged as a methodological limitation.
Despite these limitations, the present study provides evidence that patient positioning represents a simple and potentially effective strategy to modify supraclavicular anatomy. By integrating biomechanical reasoning with quantitative imaging, these findings support a mechanism-informed approach to procedural optimization. Future studies are warranted to determine whether position-related anatomical changes translate into measurable improvements in procedural performance and clinical outcomes, and to explore the incorporation of positioning strategies into routine clinical practice.
Conclusions
This study provides ultrasound-based evidence that lateral decubitus positioning is associated with a consistent increase in pleura–plexus distance at the supraclavicular level across multiple clinical populations. Body mass index independently influenced the magnitude of positional widening, supporting the contribution of soft tissue redistribution and biomechanical factors to supraclavicular anatomical variation. These findings suggest that patient positioning may represent a simple and potentially modifiable anatomical factor relevant to procedural planning during ultrasound-guided supraclavicular brachial plexus targeting. Further studies are warranted to determine whether these anatomical changes translate into measurable improvements in procedural performance and clinical outcomes.
Supplementary Information
Below is the link to the electronic supplementary material.
Acknowledgements
I would like to acknowledge the late Professor Carl P. C. Chen, MD, PhD, for his substantial intellectual contributions to this work and for our many years of scientific collaboration. Together, we shared a common vision that the true value of research lies in transforming innovative ideas into rigorous scientific evidence and translating that evidence into patient-centered care that improves lives around the world. Through open scientific dialogue, mutual respect, scientific rigor, research integrity, and a shared commitment to excellence, we developed innovative concepts, critically refined them using the best available evidence, and contributed to advances in rehabilitation medicine, pain medicine, ultrasound-guided interventions, and translational research. Professor Chen’s scientific insight, intellectual generosity, integrity, and unwavering dedication to improving patients’ lives profoundly influenced this work and many of our collaborative contributions. It is my sincere hope that this work will continue to support clinicians, inspire researchers, and ultimately be translated into better patient-centered care for patients worldwide, carrying forward Professor Carl P. C. Chen’s enduring scientific legacy.
Author Contributions
Carl P. C. Chen and Areerat Suputtitada contributed to the conception and design of the study, participant recruitment, and oversight of all methodological procedures. Both authors performed the ultrasound examinations and standardized measurements, including image acquisition and end-expiration measurement protocols. They jointly conducted the statistical analyses and interpretation of the data, drafted and critically revised the manuscript, and approved the final version for submission. Both authors meet the ICMJE criteria for authorship and agree to be accountable for all aspects of the work.
Funding
This work and the journal’s Rapid Service Fee were supported by the National Science and Technology Council of Taiwan, R.O.C. (Grant No. NSTC114-2314-B182-021-MY2; NMRPD1Q0921-2). The funding source had no role in the design of the study; collection, analysis, or interpretation of data;preparation of the manuscript; or the decision to submit the manuscript for publication. No funding or sponsorship was received for the publication of this article.
Data Availability
Deidentified data supporting the findings of this study, including Supplementary Tables S1 and S2, are available from the corresponding author upon reasonable request and in accordance with institutional and ethical regulations.
Declarations
Conflicts of Interest
Carl P. C. Chen and Areerat Suputtitada declare no conflicts of interest relevant to this work.
Ethical Approval
Ethical approval for this study was obtained from the Institutional Review Board of Chang Gung Medical Foundation, Taiwan (IRB No. 202401968A3). All participants provided written informed consent prior to participation. The study was conducted in accordance with the principles of the Declaration of Helsinki and applicable institutional regulations.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
Deidentified data supporting the findings of this study, including Supplementary Tables S1 and S2, are available from the corresponding author upon reasonable request and in accordance with institutional and ethical regulations.