Abstract
Background
The interscalene brachial plexus block (ISB) is widely regarded as the most commonly used regional anesthesia technique for perioperative analgesia in arthroscopic shoulder surgery; however, it is associated with a high incidence of phrenic nerve paresis and profound, often complete, motor blockade of the upper extremity. In contrast, the superior trunk block (STB) represents an anatomically targeted alternative specifically designed to preserve distal motor function while maintaining effective surgical anesthesia. This randomized comparative study evaluated the motor-sparing profile of an identical liposomal bupivacaine–based admixture administered via either STB or ISB, as well as their comparative analgesic effects.
Patients and Methods
In this prospective, randomized, participant- and outcome-assessor-blinded, active-controlled trial, 68 patients undergoing elective arthroscopic shoulder surgery were randomly assigned to receive either ultrasound-guided ISB (n = 35) or STB (n = 33). Both groups received a 20 mL admixture containing 10 mL of liposomal bupivacaine (133 mg) and 10 mL of 0.9% normal saline. The primary outcomes were postoperative pain intensity measured by the visual analogue scale (VAS; range 0–10 cm) at rest and during activity at 30 min, 6 h, 12 h, and 24 h. Secondary outcomes included the modified Bromage scale (MBS; range 0–4) for upper extremity motor function (assessed by shoulder abduction, elbow flexion, and elbow extension) and the University of California at Los Angeles (UCLA) shoulder score (range 2–35) for early functional assessment at 24 h. No block-related complications were observed in either group.
Results
No statistically significant differences were observed in either resting or activity VAS scores between the two groups at any postoperative time points (all P > 0.0125 after Bonferroni correction). Crucially, the STB group achieved significantly higher MBS scores compared to the ISB group (median difference 0.00, 95% CI: 0.00 to 1.00; Z = −2.706, P = 0.007), demonstrating a profound motor-sparing effect on the upper extremity. Early shoulder function at 24 h was comparable between the two groups, with an identical median UCLA score of 30.00 in both cohorts (median difference −1.00, 95% CI: −3.00 to 1.00; Z = 0.858, P = 0.391).
Conclusion
Ultrasound-guided superior trunk block with a liposomal bupivacaine admixture provides postoperative analgesia comparable to the traditional interscalene approach, with no statistically significant differences in VAS scores observed between the two groups. Crucially, the superior trunk approach offers a significant clinical advantage by preserving upper extremity motor function, making it an optimized regional anesthetic technique aligned with modern enhanced recovery after surgery (ERAS) principles.
Keywords: liposomal bupivacaine, superior trunk block, interscalene brachial plexus block, shoulder arthroscopy, motor-sparing anesthesia, enhanced recovery after surgery (ERAS)
Introduction
Arthroscopic shoulder surgery has emerged as a widely adopted, minimally invasive intervention for managing various shoulder pathologies, including rotator cuff tears, impingement syndrome, and shoulder instability.1 Despite its minimally invasive nature, shoulder arthroscopy is consistently associated with moderate-to-severe postoperative pain, predominantly attributable to localized surgical trauma and secondary tissue edema.2,3 Inadequate pain control not only delays early functional rehabilitation but also significantly compromises patients’ psychological well-being and health-related quality of life.4 Therefore, developing a safe, effective, and evidence-based postoperative analgesic strategy is a critical clinical priority in shoulder arthroscopy.
From an anatomical perspective, effective analgesia for shoulder arthroscopy requires blockade of the joint’s primary sensory nerves-the suprascapular nerve (C5–C6) and the axillary nerve (C5–C6).5 Although ultrasound-guided interscalene brachial plexus block (ISB) has historically been regarded as the reference standard for perioperative pain control in shoulder surgery, it achieves analgesia by targeting the C5 and C6 nerve roots at the level of the interscalene groove.6,7 This anatomical proximity to critical adjacent structures, however, confers substantial procedural risks. Conventional ISB is associated with a consistently high incidence (>90%) of ipsilateral phrenic nerve blockade-often resulting in diaphragmatic dysfunction-and carries a non-negligible risk of stellate ganglion spread, potentially precipitating Horner’s syndrome.8–10 These adverse effects substantially limit its clinical utility and undermine perioperative safety, particularly in patients with preexisting pulmonary compromise.
To address these limitations, regional anesthesia research has increasingly prioritized the development of anatomically refined techniques. The superior trunk block (STB) has recently emerged as a promising alternative to interscalene block. By targeting the superior trunk-the anatomical site where the C5 and C6 nerve roots converge-the STB provides reliable sensory blockade of both the suprascapular and axillary nerves, which originate from this structure.11,12 Critically, by relocating the injection site to a more distal and lateral position relative to the phrenic nerve course, the STB reduces the incidence of hemidiaphragmatic paresis to 14%-25%,13,14 offering a potential safety advantage without sacrificing analgesic quality.
Concurrently, advances in pharmacology have yielded novel extended-duration local anesthetic formulations. Liposomal bupivacaine, a sustained-release agent formulated using multivesicular liposome technology, delivers prolonged analgesia lasting up to 72 h.15 Although ISB is a primary FDA-approved indication for liposomal bupivacaine, with robust evidence from multiple randomized controlled trials supporting its efficacy in providing extended-duration analgesia, reduced total local anesthetic exposure, and lower systemic toxicity,16,17 the safety and analgesic performance of liposomal bupivacaine administered via the STB approach remain insufficiently characterized.
Recent studies have evaluated the analgesic efficacy and effects on upper extremity motor function associated with varying concentrations of liposomal bupivacaine administered via upper trunk brachial plexus block in patients undergoing arthroscopic shoulder surgery—providing a theoretical foundation for identifying the optimal concentration.18 Moreover, comparative evidence indicates that, in this patient population, a combined suprascapular and axillary nerve block using liposomal bupivacaine yields analgesia non-inferior to that of the traditional interscalene brachial plexus block, while significantly reducing the incidence of hemidiaphragmatic paralysis.19 However, data on the combination of STB with liposomal bupivacaine—particularly regarding its motor-sparing profile and its impact on early functional recovery—remain limited. Therefore, this prospective, randomized, participant- and outcome-assessor-blinded, active-controlled trial was designed to compare the analgesic efficacy and motor-sparing properties of a standardized dose and volume of liposomal bupivacaine administered via either the STB or the ISB in patients undergoing elective arthroscopic shoulder surgery. The primary endpoint was postoperative pain intensity within 24 h after surgery; secondary endpoints included preservation of upper extremity motor function and shoulder-specific functional recovery, both assessed at 24 h postoperatively. It is important to note that although the phrenic-sparing anatomical rationale underpins this study, diaphragmatic function was not evaluated in the present trial.
Materials and Methods
Participants
This prospective, randomized, parallel-group, active-controlled trial was conducted at Zhongnan Hospital of Wuhan University from January to April 2026. The study protocol was approved by the Medical Ethics Committee of Zhongnan Hospital of Wuhan University (Approval No. 2025027) and prospectively registered in the Chinese Clinical Trial Registry (Registration No. ChiCTR2600115932). Written informed consent was obtained from all participants before enrollment. The study was conducted in full accordance with the Declaration of Helsinki and reported following the CONSORT statement.
Adult patients scheduled for elective arthroscopic shoulder surgery, including rotator cuff repair, subacromial decompression, and Bankart repair, were screened for eligibility. Inclusion criteria were: (1) age 18–70 years; (2) American Society of Anesthesiologists (ASA) physical status I, II, or III; (3) Body Mass Index (BMI) between 18.0 and 28 kg/m2; (4) The operation is expected to last between 1 h and 3 h.
Patients were excluded if they met any of the following criteria: (1) Pre-existing neurological deficits or peripheral neuropathy in the surgical upper extremity; (2) History of severe chronic pain or opioid dependence; (3) Known allergy to local anesthetics or liposomal bupivacaine; (4) Severe cardiopulmonary, hepatic, or renal dysfunction; (5) Inability to comprehend the visual analogue scale (VAS) or cooperate with postoperative assessments.
Study Design
Eligible patients were randomly assigned to either the interscalene brachial plexus block group (the ISB group, n = 35) or the superior trunk block group (the STB group, n = 33). Randomization was performed using a computer-generated 1:1 randomization sequence with random permuted blocks of size four. Allocation assignments were concealed in consecutively numbered, opaque, sealed envelopes, which remained unopened until immediately before the nerve block procedure—when they were opened solely by the investigator performing the block, after the patient had entered the operating room. The randomization sequence was generated by an independent statistician not involved in trial conduct or participant management. Patient enrollment was performed by designated researchers according to prespecified inclusion criteria. To preserve outcome assessor blinding, the investigator responsible for administering the nerve block did not participate in any subsequent assessment of study outcomes.
This was a participant- and outcome-assessor-blinded study. Due to the distinct anatomical landmarks and needle trajectories inherent to the two block techniques, the senior anesthesiologist performing the procedures could not be blinded to the allocations. Patients remained blinded to their assigned intervention through a standardized preoperative briefing: all participants were informed they would receive an ultrasound-guided peripheral nerve block for postoperative analgesia, without disclosure of the specific technique (eg, selective vs non-selective), the targeted nerve(s), or anticipated motor-sparing or motor-blocking effects. The outcome assessor remained blinded to group allocation throughout the study period. Specifically, the investigator conducting all postoperative follow-up assessments was neither present during the nerve block procedure nor the surgical intervention, had no access to the randomization sequence, allocation envelopes, or intraoperative records, and had no communication with the anesthesiology or surgical teams regarding the assigned intervention.
To mitigate the risk of assessor unblinding arising from observable intergroup differences in motor function—a secondary outcome explicitly measured in this trial—the outcome assessor was instructed to strictly adhere to a standardized assessment script, to avoid all discussion with patients regarding the nerve block procedure or sensorimotor symptoms, and to base evaluations solely on prespecified, objective scoring criteria. Nevertheless, we acknowledge that complete blinding of the assessor to motor-function differences between groups may not have been fully attainable; this limitation is inherent to the design and is explicitly addressed in the Discussion section.
Ultrasound-Guided Nerve Block Interventions
Upon arrival in the operation room, standard clinical monitoring (electrocardiogram, non-invasive blood pressure, and pulse oximetry) was established. All regional block procedures were performed under strict aseptic conditions using a high-frequency linear ultrasound transducer:
ISB Group (n = 35): The ultrasound transducer was positioned transversely over the interscalene groove at the cricoid level. After clear identification of the C5 and C6 nerve roots between the anterior and middle scalene muscles, a 22-gauge block needle was advanced in-plane from lateral to medial. A total volume of 20 mL admixture, consisting of 10 mL of liposomal bupivacaine (133 mg) and 10 mL of 0.9% normal saline, was injected into the interscalene space.
STB Group (n = 33): The transducer was traced laterally and caudally from the interscalene groove to identify the superior trunk, which is situated deep to the posterior belly of the omohyoid muscle where the C5 and C6 roots merge. The block needle was advanced using an in-plane approach. The identical 20 mL local anesthetic admixture, consisting of 10 mL of liposomal bupivacaine (133 mg) and 10 mL of 0.9% normal saline, was deposited circumferentially around the superior trunk.
Intraoperative Management
Following completion of the nerve block, general anesthesia was induced and maintained according to a standardized protocol. Induction consisted of midazolam (0.03–0.05 mg/kg), propofol (1.5–2.5 mg/kg), sufentanil (0.3–0.5 μg/kg), and rocuronium (0.6 mg/kg), followed by endotracheal intubation. Intraoperative ventilation was delivered with an inspired oxygen fraction (FiO2) of 50% and a tidal volume of 6 mL/kg based on ideal body weight. Anesthesia maintenance employed a balanced regimen comprising continuous intravenous propofol (4–8 mg·kg−1·h−1), inhaled sevoflurane (1–1.5% end-tidal concentration), and intravenous remifentanil (0.1–0.3 μg·kg−1·min−1); neuromuscular blockade was titrated to maintain a train-of-four (TOF) count of 0 −1, and the bispectral index (BIS) was targeted at 40–60. Upon surgical completion, all patients were transferred to the post-anesthesia care unit (PACU) for standardized recovery. No intra-articular or portal-site local anesthetic infiltration was administered by the surgical team during any procedure, to prevent confounding of the primary outcome—block-related analgesia—as assessed by standardized postoperative pain scores.
Outcome Measurements
Postoperative outcomes were systematically assessed at prespecified timepoints: resting and movement-evoked (ie, active) VAS scores were recorded at 30 min post-extubation, and at 6 h, 12 h, and 24 h postoperatively. No standardized postoperative analgesia regimen was implemented; rescue analgesia was administered exclusively when the patient’s VAS score exceeded 4 at rest or during active movement. The rescue analgesic of choice was intravenous flurbiprofen axetil 100 mg, which could be repeated once after a 12 h interval if pain persisted. If pain remained inadequately controlled (VAS ≥ 4) despite two doses of flurbiprofen, intravenous tramadol 100 mg was administered as a second-line rescue, with a maximum of two doses at 6-hour intervals. All analgesic administrations and VAS assessments were recorded in the electronic medical record.
Additionally, the University of California at Los Angeles (UCLA) Shoulder Rating Scale was administered to evaluate affected-side shoulder function at 24 h postoperatively, and the modified Bromage score (MBS) was assessed concurrently to quantify the motor activity of the upper extremity on the surgical side.
The UCLA Shoulder Rating Scale was administered 24 h postoperatively to capture an early, shoulder-specific functional assessment during the immediate recovery phase. The UCLA scale evaluates five domains: pain (scored 1–10), function (scored 1–10), active forward flexion (scored 0–5), strength of forward flexion (scored 0–5), and patient satisfaction (scored 0–5), with a total possible score ranging from 2 to 35. In this study, the scale was administered via structured face-to-face interview by the outcome assessor, who was blinded to group allocation and had no involvement in intraoperative or PACU care. The 24 h timepoint was selected to reflect functional status after emergence from anesthesia and stabilization in the ward setting, serving as an early reference point for within-subject comparison across subsequent follow-up visits, not as a measure of long-term functional recovery.
Motor function was assessed using three standardized, gravity-resisted movements corresponding to key myotomes: shoulder abduction (C5), elbow flexion (C6), and elbow extension (C7) - a protocol consistently applied in contemporary shoulder arthroscopy trials. For each movement, the MBS was assigned based on the highest level of voluntary motor performance achievable against gravity, using the following five-point ordinal scale: 4 = full active movement against gravity with normal strength; 3 = active movement against gravity with reduced strength; 2 = active movement only with gravity eliminated (ie, performed in horizontal plane); 1 = visible or palpable muscle contraction without joint motion; 0 = no detectable contraction.20 All follow-up assessments—including VAS scores at 30 min, 6 h, 12 h, and 24 h postoperatively, modified Bromage scores, and UCLA Shoulder Rating Scale scores at 24 h—were conducted by a dedicated, blinded researcher who had no involvement in intraoperative management or PACU care of the enrolled participants.
Statistical Analysis
The sample size for this study was determined a priori based on the primary endpoint-postoperative pain intensity measured using the 100-mm VAS. According to the methodologically rigorous and widely cited study by Myles et al, the minimally clinically important difference (MCID) for acute postoperative pain on this scale is 9.9 mm (equivalent to 1.0 point on a 0–10 metric).21 Drawing on pooled data from recent randomized trials evaluating long-acting liposomal bupivacaine formulations, the anticipated common standard deviation was conservatively estimated at 1.4 points. Using SPSS version 27.0 (IBM Corp., Armonk, NY, USA), a two-sample independent t-test indicated that 31 participants per group would provide 80% power (1−β) to detect the prespecified MCID of 1.0 point between the intervention groups at a two-sided significance level of α = 0.05. To accommodate an anticipated 10% rate of protocol deviations—including clinical withdrawal, loss to follow-up, or incomplete outcome assessment—the target enrollment was increased to 68 participants (34 per group), ensuring sufficient statistical precision in the primary analysis. The sample size was calculated a priori based on a two-sample t-test, using effect size estimates derived from published literature. However, post hoc assessment of the VAS data—using both the Shapiro–Wilk test and visual inspection of histograms—revealed marked deviations from normality and pronounced floor effects at all scheduled timepoints. Consequently, the pre-specified primary analysis was conducted using the Mann–Whitney U-test, a non-parametric procedure robust to distributional violations and appropriate for ordinal pain scores. This analytical adaptation was prespecified in the statistical analysis plan and implemented prior to unblinding or outcome adjudication.
All statistical analysis were performed using IBM SPSS version 27.0 (IBM Corp., Armonk, NY, USA). Normality of continuous variables was assessed rigorously using the Shapiro–Wilk test and corroborated by visual inspection of histograms. Parametric continuous variables that followed a normal distribution were expressed as mean ± standard deviation (Mean ± SD) and compared between the two groups using the independent-samples t-test. Non-normally distributed continuous outcomes—including the UCLA shoulder score—were reported as median and interquartile range (median [IQR]), and analyzed using the Mann–Whitney U-test. Categorical and ordinal variables—including MBS scores for upper extremity motor blockade—were analyzed using either the Mann–Whitney U-test (for ordinal data) or the chi-square test (for categorical data with expected cell counts ≥5); results were presented as frequencies and percentages (n, %). Given pronounced non-normality and floor effects in serial VAS measurements (at 30 min, 6 h, 12 h, and 24 h postoperatively), intergroup comparisons at each timepoint were conducted using independent Mann–Whitney U-tests. To account for multiple testing across the four repeated VAS assessments, a Bonferroni-adjusted significance threshold was applied: α’ = 0.05 / 4 = 0.0125; thus, P < 0.0125 was considered statistically significant for these pairwise comparisons.
To quantify cumulative pain burden over the 24 h observation period, the area under the curve (AUC) for both resting and activity VAS scores was computed for each participant using the trapezoidal rule, based on serial measurements obtained at 30 min, 6 h, 12 h, and 24 h postoperatively. Given the non-normal distribution and floor effects consistently observed in VAS data across all scheduled timepoints, group comparisons of AUC values were conducted using the Mann–Whitney U-test—a non-parametric method robust to violations of normality and appropriate for ordinal pain scores. This analysis was prespecified as an exploratory secondary endpoint in the statistical analysis plan, intended to provide a time-weighted, integrated assessment of analgesic effect complementary to the primary timepoint-specific analyses. Furthermore, 95% confidence intervals (CIs) for between-group median differences were estimated using the Hodges–Lehmann estimator for all primary and secondary outcomes, thereby quantifying the precision of treatment effect estimates in accordance with CONSORT 2010 (item 7a) and current standards for non-param. For all other primary and secondary endpoints, statistical significance was defined as two-tailed P < 0.05.
Results
Patient Characteristics and Baseline Data
A total of 68 patients were included in the final analysis, comprising 35 patients in the ISB group and 33 patients in the STB group (Figure 1). There were no statistically significant differences between the two groups regarding baseline demographic characteristics, including sex (P = 0.551), age (P = 0.730), height (P = 0.518), weight (P = 0.465), BMI (P = 0.668), and ASA physical status (P = 0.524). Furthermore, no significant difference was observed in the duration of surgery between the ISB and STB groups (P = 0.188). The detailed baseline characteristics of the study population are summarized in Table 1.
Figure 1.
The CONSORT flow diagram.
Table 1.
Baseline Demographic and Clinical Characteristics of the Patients
| Variables | ISB Group (n = 35) | STB Group (n = 33) | Statistics (t/Z/x2) | P-value |
|---|---|---|---|---|
| Sex | x2 = 0.355 | 0.551 | ||
| Female, n (%) | 22 (62.9%) | 23 (69.7%) | ||
| Male, n (%) | 13 (37.1%) | 10 (30.3%) | ||
| Age (years), Median [IQR] | 59.00 [12.00] | 59.00 [17.00] | Z = −0.345 | 0.730 |
| Height (cm), Mean ± SD | 163.71 ± 7.62 | 162.48 ± 7.98 | t = −0.650 | 0.518 |
| Weight (kg), Median [IQR] | 65.00 [15.00] | 60.00 [16.00] | Z = 0.731 | 0.465 |
| BMI (kg/m2), Median [IQR] | 24.22 [3.17] | 23.11 [4.53] | Z = 0.430 | 0.668 |
| ASA Physical Status, n (%) | Z = 0.637 | 0.524 | ||
| Class I | 0 (0.0%) | 1 (3.0%) | ||
| Class II | 31 (88.6%) | 29 (87.9%) | ||
| Class III | 4 (11.4%) | 3 (9.1%) | ||
| Operation Time (min), Median [IQR] |
150.00 [50.00] | 125.00 [55.00] | Z = 1.317 | 0.188 |
Notes: Data are presented as Mean ± SD, Median [IQR], or n (%). Inter-group comparisons were performed using the independent-samples t-test, Mann–Whitney U-test, or Pearson chi-square (x2) test, as appropriate. A two-tailed P < 0.05 was considered statistically significant.
Abbreviations: ISB, interscalene brachial plexus block; STB, superior trunk block; SD, standard deviation; IQR, interquartile range; BMI, body mass index; ASA, American Society of Anesthesiologists; x2, chi-square test statistic; t, independent t-test statistic; Z, Mann–Whitney U-test statistic.
Postoperative Pain Assessment
Postoperative pain intensity was evaluated using the VAS at both rest and during activity across multiple postoperative time points (30 min, 6 h, 12 h, and 24 h). No statistically significant differences were observed in either resting or activity VAS scores between the ISB and STB groups at any designated time points (all P > 0.0125 after Bonferroni correction).
At the early 30 min post-block peak period, both groups exhibited excellent initial analgesic depth, with identical median resting VAS scores of 0.00 (ISB: 0.00 [1.00] vs STB: 0.00 [1.00], median difference 0.00 [95%: 0.00, 0.00], Z = 0.560, P = 0.575). Similarly, at the 24 h mark, the median activity VAS score was 3.00 in both groups (ISB: 3.00 [2.00] vs STB: 3.00 [2.00], median difference 0.00 [95%: −1.00, 1.00], Z = 0.323, P = 0.746), indicating sustained and comparable long-term analgesia from the liposomal bupivacaine admixture. The comprehensive longitudinal pain trajectory data are displayed in Table 2.
Table 2.
Postoperative Resting and Activity VAS Scores Between the Two Groups Over 24 h
| Pain Assessment | ISB Group (n = 35) | STB Group (n= 33) | Median Difference (95% CI) | Statistics (Z) | P-value |
|---|---|---|---|---|---|
| Resting VAS | |||||
| 30 min | 0.00 [1.00] | 0.00 [1.00] | 0.00 [0.00, 0.00] | 0.560 | 0.575 |
| 6 h | 1.00 [2.00] | 1.00 [2.00] | 0.00 [0.00, 0.00] | 0.078 | 0.938 |
| 12 h | 2.00 [2.00] | 2.00 [2.00] | 0.00 [0.00, 1.00] | −0.776 | 0.438 |
| 24 h | 2.00 [2.00] | 2.00 [2.00] | 0.00 [−1.00, 1.00] | 0.145 | 0.885 |
| Activity VAS | |||||
| 30 min | 0.00 [1.00] | 0.00 [2.00] | 0.00 [0.00, 0.00] | −0.796 | 0.426 |
| 6 h | 1.00 [1.00] | 1.00 [2.00] | 0.00 [−1.00, 1.00] | 0.127 | 0.899 |
| 12 h | 2.00 [1.00] | 2.00 [1.00] | 0.00 [−1.00, 0.00] | 0.960 | 0.337 |
| 24 h | 3.00 [2.00] | 3.00 [2.00] | 0.00 [−1.00, 1.00] | 0.323 | 0.746 |
Notes: Data are expressed as Median [IQR]. Inter-group comparisons at each serial postoperative time point were performed using the non-parametric Mann–Whitney U-test, with the test statistic represented by Z scores. A Bonferroni-corrected two-tailed P value < 0.0125 was considered statistically significant.
Abbreviations: CI, confidence interval; ISB, interscalene brachial plexus block; STB, superior trunk block; VAS, visual analogue scale; IQR, interquartile range; Z, Mann–Whitney U-test statistic.
To further quantify cumulative pain burden over the 24 h observation period, we computed the area under the curve (AUC) for both resting and activity VAS scores using the trapezoidal rule, based on measurements obtained at 30 min, 6 h, 12 h, and 24 h postoperatively. As shown in Table 3, no statistically significant between-group differences were observed: for resting VAS AUC, the STB group (median [IQR] = 35.75 [38.50]) did not differ from the ISB group (35.75 [41.50], median difference 0.00 [95% CI: −11.75, 15.00], P = 0.800); for activity VAS AUC, the STB group (41.75 [33.63]) did not differ from the ISB group (41.75 [29.50], median difference −0.25 [95% CI: −17.50, 8.50], P = 0.806). These non-parametric AUC results—consistent with all primary and secondary timepoint-specific VAS comparisons—support the conclusion that integrated analgesic effect, reflecting total pain burden across the early postoperative period, was comparable between the two techniques.
Table 3.
Area Under the Curve (AUC) for VAS Scores Over 24 h
| Variable | ISB Group (n = 35) | STB Group (n = 33) | Median Difference (95% CI) | P value |
|---|---|---|---|---|
| AUC_Resting VAS | 35.75 [41.50] | 35.75 [38.50] | 0.00 [−11.75, 15.00] | 0.800 |
| AUC_Activity VAS | 41.75 [29.50] | 41.75 [33.63] | −0.25 [−17.50, 8.50] | 0.806 |
Notes: Data are presented as median [IQR]. AUC was calculated using the trapezoidal rule based on VAS measurements at 30 min, 6 h, 12 h, and 24 h postoperatively. Comparisons were performed using the Mann–Whitney U-test.
Abbreviations: CI, confidence interval; AUC, area under the curve; VAS, visual analogue scale; STB, superior trunk block; ISB, interscalene brachial plexus block; IQR, interquartile range.
Motor Block and Functional Recovery
The assessment of peripheral motor function and overall joint recovery demonstrated a significant clinical advantage for the novel superior trunk approach. MBS score, utilized to evaluate forearm and hand motor block, was significantly higher in the STB group compared to the ISB group (median difference 0.00 [95% CI: 0.00, 1.00], Z = −2.706, P = 0.007). This indicates a profound motor-sparing effect and superior sensory-motor dissociation achieved by the superior trunk block technique.
Conversely, regarding the early functional assessment at 24 h postoperatively, the UCLA shoulder assessment scores at the final follow-up were highly comparable between the two groups, with an identical median score of 30.00 (ISB: 30.00 [3.00] vs STB: 30.00 [7.00]; median difference −1.00 [95% CI: −3.00, 1.00], P = 0.391). This confirms that the motor-sparing benefits of the STB technique did not compromise the quality of early shoulder function at 24 h postoperatively. These outcomes are presented in detail in Table 4.
Table 4.
Postoperative Motor Block and Shoulder Function Recovery Between the Two Groups
| Outcomes | ISB Group (n = 35) |
STB Group (n = 33) |
Median Difference (95% CI) |
Statistics (Z) | P-value |
|---|---|---|---|---|---|
| MBS for forearm function, n (%) | 0.00 [0.00, 1.00] | −2.706 | 0.007 | ||
| Grade 4 (Normal strength) |
10 (28.6%) | 19 (57.6%) | |||
| Grade 3 (Decreased, against resistance) |
21 (60.0%) | 14 (42.4%) | |||
| Grade 2 (Against gravity only) |
4 (11.4%) | 0 (0.0%) | |||
| Grade 1 (Trace muscle activity) |
0 (0.0%) | 0 (0.0%) | |||
| Grade 0 (No activity) |
0 (0.0%) | 0 (0.0%) | |||
| UCLA shoulder score | 30.00 [3.00] | 30.00 [7.00] | −1.00 [−3.00, 1.00] | 0.858 | 0.391 |
Notes: Data are presented as n (%) for or Median [IQR]. Inter-group statistical comparisons for both outcomes were performed using the non-parametric Mann-Whitney U test, with the test statistic indicated by the Z score. A two-tailed P < 0.05 was considered statistically significant.
Abbreviations: CI, confidence interval; ISB, interscalene brachial plexus block; STB, superior trunk block; MBS, Modified Bromage Scale; UCLA, University of California at Los Angeles; IQR, interquartile range; Z, Mann-Whitney U test statistic.
Discussion
The primary objective of this randomized clinical trial was to compare the analgesic efficacy and motor-sparing profile of ultrasound-guided STB versus ISB using liposomal bupivacainein patients undergoing arthroscopic shoulder surgery. Our results demonstrated two key clinical findings: first, no statistically significant differences were observed in postoperative pain scores between the STB and ISB groups at any assessed time point within the first 24 h; second, the STB group demonstrated significantly greater preservation of the upper extremity motor function compared with the ISB group, as reflected by higher MBS scores at 24 h postoperatively (P = 0.007). Crucially, this motor-sparing advantage did not impair shoulder-specific functional recovery, as evidenced by equivalent UCLA shoulder scores at 24 h follow-up (median: 30.00 in both groups; P = 0.391). Collectively, these findings support the superior anatomical targeting of the STB for regional anesthesia in outpatient shoulder arthroscopy.
The observation of equivalent postoperative pain relief between the STB and ISB groups over the first 24 h is consistent with the shared sensory innervation of the shoulder joint by the suprascapular and axillary nerves-and the sustained local anesthetic effect conferred by liposomal bupivacaine at both anatomical targets. The shoulder joint receives its predominant sensory innervation from the suprascapular and axillary nerves, both of which arise directly from the upper trunk-formed by the convergence of the C5 and C6 nerve roots.22,23 The traditional ISB targets the C5 and C6 nerve roots at their proximal emergence from the transverse processes of the cervical vertebrae, whereas the STB targets the upper trunk-formed by the convergence of C5 and C6-directly at the level where it lies deep to the posterior belly of the omohyoid muscle.24,25 Therefore, the STB targets the upper trunk-the primary source of sensory innervation to the shoulder joint via the suprascapular and axillary nerves-and thus provides comprehensive postoperative analgesia for arthroscopic shoulder procedures. When combined with liposomal bupivacaine-a sustained-release formulation that delivers bupivacaine gradually over approximately 72 h via multivesicular liposome technology-both STB and ISB produced profound and sustained analgesia. This was evidenced by median resting VAS scores of 0.00 at 30 min postoperatively and well-controlled activity-related VAS scores at 24 h in both groups.
The most clinically relevant finding of our study was the statistically significant difference in the distribution of postoperative motor block between the STB and ISB groups (P = 0.007). Patients receiving STB demonstrated selective preservation of the upper extremity motor function while achieving effective shoulder analgesia. This difference in motor impact is attributable to the distinct local anesthetic spread patterns associated with each technique, which are critically influenced by the injectate volume and the fascial anatomy at each target site. In the ISB, injection of 20 mL local anesthetic into the interscalene groove typically results in caudal spread along the prevertebral fascia, leading to consistent involvement of the middle and lower trunks-including the C8 and T1 nerve roots-that innervate intrinsic upper extremity muscles. Consequently, patients receiving ISB commonly experience transient upper extremity motor impairment-including inability to actively elevate the arm or grasp objects- contributing to functional limitation and patient-reported anxiety. In contrast, the STB targets the upper trunk at a site anatomically remote from the lower cervical nerve roots (C8-T1).26 Although the same 20 mL volume is used, the fascial boundaries surrounding the upper trunk limit caudal spread of local anesthetic, thereby sparing the C8 and T1 nerve roots and preserving motor function in the upper extremity. This anatomically constrained drug distribution, where volume ensures efficacy but fascial planes determine selectivity, underlies the superior preservation of motor function observed in the STB group.
In the context of ambulatory surgery and Enhanced Recovery After Surgery (ERAS) protocols, preservation of non-surgical-site motor function is a key perioperative objective.27 Maintaining active mobility of the forearm and hand-without compromising surgical analgesia-confers tangible clinical benefits for outpatients undergoing shoulder arthroscopy.28 Patients with preserved forearm and hand motor function can independently perform essential postoperative activities of daily living (ADLs) immediately after surgery-including operating electronic devices, self-feeding, oral hydration, and personal hygiene management. This preservation supports greater patient autonomy, reduces reliance on nursing staff or informal caregivers, and mitigates risks associated with postoperative functional dependence. Moreover, preservation of forearm and hand motor function mitigates the risk of iatrogenic traction injury to the brachial plexus during intraoperative positioning and postoperative mobilization-particularly when patients transition from supine to seated or ambulatory positions-thereby enhancing perioperative safety.29
Several limitations of this study warrant acknowledgment. First, although the sample size (N = 68) was adequate to detect the prespecified differences in primary and secondary endpoints, it remains underpowered to reliably estimate the incidence of rare adverse events, and the analytical approach diverged from the original sample size calculation—which was based on a two-sample t-test assuming normality—because the VAS data exhibited non-normality and floor effects, necessitating the use of the Mann–Whitney U-test; this deviation may have affected the achieved statistical power. Second, although motor-sparing effects were rigorously assessed using the MBS score, diaphragmatic excursion was not quantified, and safety endpoints and adverse event monitoring were not prespecified, precluding definitive comparative evaluation of safety profiles between the two techniques, particularly regarding hemidiaphragmatic paresis. Third, the observation period was restricted to 24 h postoperatively, although liposomal bupivacaine is designed to provide analgesia for up to 72 h; this window was deliberately chosen to focus on the clinically most critical early recovery phase, but it precludes evaluation of the complete analgesic duration, potential rebound hyperalgesia, and motor recovery beyond the first 24 h, and the UCLA score assessed at 24 h is insufficient to evaluate true long-term functional recovery. Fourth, analgesic drug consumption, the need for rescue analgesia, the incidence of rebound pain, and the time to first analgesic request were not systematically recorded, and sensory block onset and block failure were not formally assessed using standardized quantitative scales, nor were the time intervals from block completion to anesthesia induction and surgical incision recorded, as these parameters were not prespecified. Fifth, although rigorous blinding procedures were implemented for patients and outcome assessors, the possibility of inadvertent unblinding due to observable differences in motor function cannot be entirely excluded. Finally, this was a single-center study conducted by a single surgical and anesthetic team, which may limit the generalizability of our findings to other institutions with different patient populations, surgical techniques, or anesthetic practices.
Conclusion
In conclusion, ultrasound-guided superior trunk block with liposomal bupivacaine provides postoperative analgesia comparable to that achieved with the interscalene block, as no statistically significant differences in VAS scores were observed between the two groups within the 24 h postoperative period. Crucially, the superior trunk approach significantly preserves motor function in the upper extremity, thereby conferring a clinically meaningful motor-sparing advantage without compromising early shoulder function. This anatomically targeted technique aligns with core ERAS principles and represents a more functional option for outpatient shoulder surgery, although its potential safety advantages remain to be validated in future studies.
Acknowledgment
The authors would like to acknowledge professor Jia Zhan for her specialized guidance.
Funding Statement
This study was supported by Special Fund for Scientific Research from Hubei Chen Xiaoping Science and Technology Development Foundation (CXPJJH12000005-07-78). This study was supported by the 2025 Clinical Research Special Project of Zhongnan Hospital of Wuhan University (2025LCYJZX-MS001).
Data Sharing Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request. Individual de-identified participant data-including demographic characteristics, surgical and anesthetic information, intervention details, postoperative VAS scores, MBS scores, and UCLA shoulder assessment outcomes-will be shared. The study protocol, statistical analysis plan, informed consent form, data dictionary, case report forms, and analysis code will also be accessible.
Data will be made available to qualified researchers beginning six months after the official publication date and will remain accessible for three years thereafter. Access will be granted exclusively to researchers who submit a methodologically rigorous and ethically approved proposal that clearly articulates specific, hypothesis-driven analytical objectives aligned with the study’s scope. All proposals and data access requests must be submitted to the corresponding author, Professor Zheng, at fengzheng@whu.edu.cn. Prior to data release, applicants must execute a legally binding Data Access Agreement that explicitly safeguards participant privacy, ensures strict confidentiality, and prohibits any commercial use.
Disclosure
The authors report no conflicts of interest in this work.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon reasonable request. Individual de-identified participant data-including demographic characteristics, surgical and anesthetic information, intervention details, postoperative VAS scores, MBS scores, and UCLA shoulder assessment outcomes-will be shared. The study protocol, statistical analysis plan, informed consent form, data dictionary, case report forms, and analysis code will also be accessible.
Data will be made available to qualified researchers beginning six months after the official publication date and will remain accessible for three years thereafter. Access will be granted exclusively to researchers who submit a methodologically rigorous and ethically approved proposal that clearly articulates specific, hypothesis-driven analytical objectives aligned with the study’s scope. All proposals and data access requests must be submitted to the corresponding author, Professor Zheng, at fengzheng@whu.edu.cn. Prior to data release, applicants must execute a legally binding Data Access Agreement that explicitly safeguards participant privacy, ensures strict confidentiality, and prohibits any commercial use.