Modified longitudinal intermediate cervical plexus block for supraclavicular nerve analgesia: a randomized, double-blind trial in healthy volunteers

Abstract

Background

The superficial cervical plexus block (SCPB) is routinely employed for clavicle surgery due to the dominant innervation of the surgical field by the supraclavicular nerves (SN). However, evidence directly comparing the blocking effect of different SCPB approaches—particularly the novel modified longitudinal intermediate cervical plexus block (MU)—on SN coverage and phrenic nerve safety remains limited. This randomized, double-blind, controlled trial in healthy volunteers aimed to evaluate the efficacy of MU relative to landmark-based and ultrasound-guided intermediate approaches in achieving SN blockade at 20 min, while assessing diaphragmatic paralysis as a key safety outcome. We hypothesized that MU would enhance SN blockade success.

Methods

In this randomized, double-blind, controlled volunteer trial, 45 healthy adults received one right-sided SCPB using one of three approaches: a landmark-based SCPB within the superficial cervical fascia (LM), an ultrasound-guided intermediate cervical plexus block at the C4 transverse process level (US), or the MU. All blocks used 10 mL of 0.5% ropivacaine. Blockade of superficial cervical plexus branches was assessed at 5, 10, 15, and 20 min. The primary outcome was supraclavicular nerve block (SNB) success within 20 min. Secondary outcomes included hemidiaphragmatic excursion and ordered grades of paralysis during quiet and deep breathing, oxygen saturation, and complications.

Results

SNB success rates at 20 min were 0% (LM), 40.0% (US), and 73.33% (MU) (p < 0.001). During quiet breathing, complete diaphragmatic paralysis occurred in 0% (LM), 0% (US), and 33.33% (MU), with partial paralysis rates of 13.33%, 53.33%, and 26.67%, respectively (p = 0.005). Deep breathing results mirrored this trend. Oxygen saturation and complication rates showed no significant intergroup differences.

Conclusion

In this volunteer study, the MU approach resulted in a higher rate of SN sensory block at 20 min compared to LM and US techniques, but was associated with an increased incidence of diaphragmatic paralysis. These findings warrant further investigation in surgical populations.

Trial registration number

ChiCTR2500104036 (Retrospective registration. Registration date: June 10, 2025).

Supplementary Information

The online version contains supplementary material available at 10.1186/s12871-026-03723-0.

Introduction

SCP provides cutaneous innervation to the auricular, cervical, mandibular-angle, shoulder, and clavicular regions [1, 2]. SCPB is widely employed for thyroidectomy, carotid endarterectomy, anterior cervical spine surgery, orthognathic surgery, and clavicular procedures [3–7]. For clavicular fixation, reliable coverage of SN is essential; accordingly, SCPB is often combined with interscalene or supraclavicular brachial plexus block. Clinical series have reported that targeted SNB combined with superior-trunk blockade facilitates anesthesia for clavicular fixation [8, 9]. Cadaveric studies further show that the SN innervates not only the skin superior to the clavicle but also the sternoclavicular and acromioclavicular joints [10], consistent with clinical observations [11, 12].

SCPB can be performed via a landmark-based SCPB within LM and US, or MU. When regional anesthesia is used as a sole technique, combined cervical-brachial plexus coverage is usually required, with SCPB plus interscalene block being the most common and well-supported regimen [13]. However, the relative efficacy of these SCPB approaches for SN coverage remains uncertain. We therefore conducted a prospective, randomized, double-blind, controlled trial comparing LM, US, and MU with respect to 20-minute SNB success and phrenic safety. Our a priori hypothesis was that MU would increase SNB success at 20 min.

Methods

Study design​

This study was a randomized controlled trial (RCT). We conducted SCPB using different approaches on healthy volunteers, with the primary objective of evaluating the success rate of SNB.

The trial was registered with the Chinese Clinical Trial Registry on June 10, 2025 (Registration Number: ChiCTR2500104036; URL:https://www.chictr.org.cn). And the recruitment of subjects was initiated on June 10, 2025. The reporting of this study followed the Consolidated Standards of Reporting Trials (CONSORT-2010) guidelines. After obtaining approval (2025-lunshenpi-013-01) from the Ethics Committee of the First People’s Hospital of Neijiang and written informed consent from all volunteers, we prospectively recruited 45 healthy volunteers to participate in the study. Patient enrollment commenced on April 15, 2025.

Enrollment​

The inclusion criteria were set as follows: age between 18 and 60 years, American Society of Anesthesiologists (ASA) physical status classification of I, and body mass index (BMI) ranging from 20 to 28 kg/m². Exclusion criteria included infection at the puncture site, coagulation dysfunction, hepatic or renal failure, and allergy to local anesthetics. These volunteers were assigned to receive either the LM, US, or MU approach based on a computer-generated random list. This information was sealed in opaque envelopes, accessible only to the anesthesiologist performing the block. Patients and outcome assessors were blinded to group allocation. The statistician performing the data analysis was also blinded to group identity; data were analyzed with groups labeled as A, B, and C to conceal their correspondence to the LM, US, and MU interventions.

Regional anesthesia

All procedures were performed by a single experienced operator (J.F.), who did not participate in data collection. Demographic and screening data (age, sex, height, weight, sensory abnormalities, relevant laboratory results, and contact information) were recorded according to the inclusion/exclusion criteria. After volunteers entered the anesthesia preparation room, standard ASA monitoring was implemented, and no sedatives were administered during the nerve blockade procedure. The volunteer was positioned supine with the head turned contralaterally. A large sterile drape was placed to cover the entire neck and shoulder area, including the ultrasound screen (for three groups) and the operator’s hands. The volunteer was also instructed to close their eyes during the block procedure. The same skin preparation and draping technique were used for all participants, regardless of group assignment. The sensation of the skin prep and the initial local anesthetic skin wheal was similar across all techniques. Subsequently, 10 mL of 0.5% ropivacaine was injected, with all blocks performed on the right side following standard skin disinfection and the operator’s use of sterile gloves. The outcome assessor, who performed the sensory and diaphragmatic testing, was not present in the procedure room during the block performance. They entered the room only after the procedure was complete, all equipment (including the ultrasound machine) had been removed, and the sterile drape was taken down. The puncture site was covered with a small, standard adhesive bandage, identical for all participants, which concealed any potential differences in puncture location. The assessor therefore had no visual cues regarding the technique used.

Right diaphragmatic excursion was measured with M-mode ultrasonography during quiet and deep breathing before the block and 20 min after the block (Fig. 1) to confirm the presence or absence of diaphragmatic paralysis [14]. Compared to pre-block measurements, diaphragmatic paralysis was graded as follows: Grade 0 (no paralysis), reduction in excursion < 25%; Grade 1 (partial paralysis), reduction ≥ 25%; Grade 2 (complete paralysis), reduction ≥ 75%, or absent/paradoxical motion [15] .

Fig. 1.

Measurement of right diaphragmatic excursion using M-mode ultrasonography

LM group (Fig. 2): The insertion point was the midpoint of the posterior border of the sternocleidomastoid muscle (SCM). If the SCM was palpated unclearly, the volunteer was instructed to lift their head to tense the muscle, delineating its posterior border visible. To avoid excessive depth, the puncture depth was limited to ≤ 2 cm. The needle was advanced slowly until a loss of resistance indicated passage through the platysma. After confirming no blood aspiration, 1 mL of local anesthetic was injected deep to the platysma muscle. Subsequently, 5 mL of local anesthetic was injected separately in two directions, including toward the clavicle and horizontally anteriorly [16].

Fig. 2.

Two puncture directions for landmark-based SCPB

US group (Fig. 3): A high-frequency linear probe (6–13 MHz) was used to ultrasonographically locate the cervical neural pathway at the C4 transverse process level, confirming its position between the superficial cervical fascia and prevertebral fascia, deep to the SCM. Using an in-plane lateral-to-medial approach, 10 mL of local anesthetic was injected horizontally within the cervical neural pathway [17] .

Fig. 3.

Ultrasound-guided intermediate cervical plexus block at the C4 transverse process level.(A, Arteria carotis communis; V, Internal jugular vein; C4 TP, C4 transverse process; SCM, Sternocleidomastoid muscle; GaN, greater auricular nerve; LCa, Longus capitis muscle; LCo, Longus colli muscle; LsCa, longissimus capitis muscle; LS, levator scapulae muscle; SECM, semispinalis capitis muscle; SPL, splenius capitis muscle; Yellow arrow, Cervical neural pathway)

MU group (Fig. 4): A 6–13 MHz linear probe was used. The C4 transverse process was identified first; the probe was then rotated 90° and shifted laterally to the lateral aspect of the transverse process to minimize selective cervical nerve root blockade. The superficial cervical fascia and prevertebral fascia deep to the SCM were clearly identified. An in-plane proximal-to-distal approach was used to inject 10 mL of local anesthetic within the cervical neural plane [18].

Fig. 4.

Ultrasound-guided modified longitudinal intermediate cervical plexus block (C4 TP pt , Posterior tubercle of the transverse process of C4; SCM, Sternocleidomastoid muscle; LsCa, Longissimus capitis muscle; LS, levator scapulae muscle; Yellow arrow, Cervical neural pathway)

Primary outcome measure

The primary endpoint was SNB success at 20 min after injection. SNB success was defined as a sensory grade of 2 (complete loss of sharp pain sensation) or 3 (loss of touch sensation) assessed at all three designated clavicular test points (acromial end, mid-clavicle, and sternoclavicular joint) on the blocked side, confirmed at the 20-minute time point. A block was considered unsuccessful if any of the three test points exhibited a sensory grade of 0 (normal sensation) or 1 (diminished sensation). Sensation was graded on a 4-point scale: Grade 0: No sensory reduction (normal sensation), Grade 1: Diminished sharp pain sensation, Grade 2: Complete loss of sharp pain sensation, Grade 3: Loss of touch sensation. A sensory grade of 2 or higher was defined as a successful block.

Secondary outcome measures included

Recording the success rates of blocking the transverse cervical nerve (TcN), great auricular nerve (GaN), and lesser occipital nerve (LoN); TcN: Sensory testing will be performed in the anterior cervical triangle, specifically over the skin covering the mid-portion of the SCM and the adjacent anterior neck region. This aligns with its cutaneous innervation to the anterior neck. GaN: Sensory testing will be performed in the anterior cervical triangle, specifically over the skin covering the mid-portion of the SCM and the adjacent anterior neck region. This aligns with its cutaneous innervation to the anterior neck. LoN: Testing will be conducted over the posterolateral aspect of the neck and the scalp behind the ear, reflecting its distribution to the occipital region. As described for the primary outcome, sensory testing will use a pinprick comparison between the blocked side and the contralateral homologous area. Monitoring the incidence of diaphragmatic paralysis; Measuring the distance between the midpoint of the posterior border of the SCM identified by surface landmark and that located under ultrasound guidance (Fig. 5) for all patients; Systematically observing and documenting the occurrence of anesthesia-related complications, such as local anesthetic systemic toxicity, stellate ganglion block, and recurrent laryngeal nerve block, in all volunteers.

Fig. 5.

The midpoint of the posterior border of the SCM localized via surface anatomy at the C4 transverse process level versus ultrasound-guided localization (The cross symbol represents the midpoint of the posterior border of the SCM located by surface anatomy; the five-pointed star represents the midpoint of the posterior border of the SCM located by ultrasonography.)

Sample size calculation​

The success rate of SNB in the study participants was the primary outcome measure. Based on preliminary trial results, the success rates were 0% in the LM group, 30% in the US group, and 70% in the MU group. With a Type I error rate set at α = 0.05/3 = 0.0167 (adjusted for multiple comparisons), a statistical power (1-β) of 80%, and a 1:1:1 sample size ratio across the three groups, the total sample size was calculated as 34 participants using PASS 15 software. Accounting for a 20% attrition and refusal rate, the final required sample size was at least 45 participants in total, with at least 15 participants per group.

Statistical analysis​

All analyses were conducted using IBM SPSS Statistics for Windows, version 17.0 (IBM Corp., Armonk, NY, USA), and all tests were two-sided. Unless otherwise specified, p < 0.05 was considered statistically significant; for prespecified pairwise comparisons of the primary outcome, p < 0.0167 was required after Bonferroni correction. Descriptive statistics were used to summarize baseline characteristics and outcomes. Categorical variables are presented as counts and percentages, and continuous variables as mean ± standard deviation (SD) or median with interquartile range (IQR), as appropriate. Normality of continuous variables (e.g., diaphragmatic excursion) was assessed using the Shapiro–Wilk test and inspection of summary statistics and plots. Between-group comparisons across the three treatment arms were selected according to the measurement scale and distribution of each variable. Categorical outcomes (e.g., SNB success, sex, complications) were compared using Pearson chi-square or Fisher’s exact tests, with Fisher’s exact used when expected cell counts were < 5. Where relevant, absolute risk differences with 95% confidence intervals (CIs) were reported. For significant overall tests, post-hoc pairwise comparisons of categorical variables were performed using Fisher’s exact test, and p-values were adjusted for multiple comparisons using both Bonferroni and Holm-Bonferroni methods. Continuous outcomes (e.g., age, BMI, diaphragmatic excursion) were analyzed using one-way analysis of variance (ANOVA) when parametric assumptions were met and the Kruskal–Wallis test otherwise; significant overall tests were followed by Bonferroni-adjusted pairwise comparisons. Ordinal outcomes (grades of diaphragmatic paralysis: 0/1/2) were analyzed primarily using an ordinal (proportional-odds) logistic regression model, with chi-square or Fisher’s exact tests used in a sensitivity analysis.

Results​​

The baseline characteristics of the 45 enrolled volunteers were well-balanced across the three intervention groups, with no statistically significant differences observed (Table 1). As detailed in the CONSORT flow diagram (Fig. 6), 52 volunteers were assessed for eligibility.

Table 1.

Baseline characteristics of volunteers by group

Characteristic	LM (n = 15)	US (n = 15)	MU (n = 15)	Overall (N = 45)	P (overall)
gender, n(%) Female	9(60.0)	11(73.3)	10(66.7)	30 (66.67)	0.741
male	6 (40.00)	4 (26.67)	5 (33.33)	15 (33.33)
Age, years, mean ± SD	43.5 ± 8.5	46.9 ± 6.5	44.3 ± 8.3	44.9 ± 7.8	0.459²
BMI, kg/m², mean ± SD	23.2 ± 3.4	23.7 ± 1.8	23.0 ± 2.3	23.3 ± 2.5	0.769²
Distance from surface landmark to posterior SCM midpoint located under ultrasound guidance (mm), median (P25–P75)	16.0 (15.0–18.0)	16.0 (15.0–17.0)	16.0 (15.0–18.5)	16.0 (15.0–18.0)	0.994³

Abbreviations SCM sternocleidomastoid, IQR interquartile range

Randomized groups: LM, US, MU; values are presented as mean ± SD or median (P25–P75) as indicated

Fig. 6.

shows the CONSORT flow diagram of enrollment, intervention allocation, follow-up, and data analysis for this study

Primary outcome measurements (Tables 2 and 3)

Table 2.

Block Success Rate of SCPB at Different Time Points Among Groups​​

Group	Nerve	5 min Grading scale (0/1/2/3)	10 min Grading scale (0/1/2/3)	15 min Grading scale (0/1/2/3)	20 min Grading scale (0/1/2/3)
LM group (n = 15)	SN	14/1/0/0	13/2/0/0	11/4/0/0	11/4/0/0
	LoN	10/5/0/0	10/5/0/0	4/10/0/0	3/12/0/0
	TcN	10/5/0/0	3/9/3/0	0/0/14/1	0/0/14/1
	GaN	4/7/4/0	5/8/2/0	0/3/10/2	0/3/10/2
US group (n = 15)	SN	10/5/0/0	6/7/2/0	5/6/4/0	5/5/5/0
	LoN	0/7/7/1	0/0/12/3	0/0/12/3	0/0/11/4
	TcN	1/10/4/0	0/6/7/2	0/5/8/2	0/2/11/2
	GaN	1/9/5/0	0/3/10/2	0/1/11/3	0/1/13/1
MU group (n = 15)	SN	4/7/4/0	1/6/8/0	1/3/11/0	1/3/11/0
	LoN	1/7/7/0	0/5/10/0	0/3/12/0	0/2/13/0
	TcN	0/6/9/0	0/3/12/0	0/2/13/0	0/2/12/1
	GaN	2/8/5/0	1/8/6/0	1/6/8/0	0/7/8/0

Abbreviations GaN Great Auricular Nerve, LoN Lesser Occipital Nerve, TcN Transverse Cervical Nerve, SN Supraclavicular Nerve

Grading scale: 0, no block; 1, partial; 2, effective; 3, complete. Values are counts per category (0/1/2/3). N = 15 per group

Table 3.

Block Success Rate (grade ≥ 2) of SCPB at Different Time Points Among Groups

Characteristics	LM group (n = 15)	US group (n = 15)	MU group (n = 15)	P value (overall)	Adjusted P values (LM vs. US / LM vs. MU / US vs. MU)
SN
5 min	0 (0.0%)	0 (0.0%)	4 (26.7%)	0.027	1.000/ <0.003/0.099
10 min	0 (0.0%)	2 (13.3%)	8 (53.3%)	0.001	1.000/<0.003/0.151
15 min	0 (0.0%)	4 (26.7%)	11 (73.3%)	< 0.001	0.299/<0.003/0.045
20 min	0 (0.0%)	6 (40.0%)	11 (73.3%)	< 0.001	0.051/<0.003/0.417
LoN
5 min	0 (0.0%)	8 (53.3%)	7 (46.7%)	< 0.001	< 0.003/0.003/1.000
10 min	0 (0.0%)	15 (100.0%)	10 (66.7%)	< 0.001	< 0.003/0.003/0.396
15 min	0 (0.0%)	15 (100.0%)	12 (80.0%)	< 0.001	< 0.003/<0.003/0.672
20 min	0 (0.0%)	15 (100.0%)	13 (86.7%)	< 0.001	< 0.003/<0.003/1.000
TcN
5 min	3 (20.0%)	4 (26.7%)	9 (60.0%)	0.053	1.000/0.181/0.417
10 min	12 (80.0%)	9 (60.0%)	12 (80.0%)	0.823	n/a
15 min	15 (100.0%)	10 (66.7%)	13 (86.7%)	0.053	0.127/1.000/1.000
20 min	15 (100.0%)	13 (86.7%)	13 (86.7%)	0.334	n/a
GaN
5 min	4 (26.7%)	5 (33.3%)	5 (33.3%)	0.902	n/a
10 min	10 (66.7%)	12 (80.0%)	6 (40.0%)	0.071	1.000/0.861/0.081
15 min	12 (80.0%)	14 (93.3%)	8 (53.3%)	0.038	1.000/0.735/0.105
20 min	12 (80.0%)	14 (93.3%)	8 (53.3%)	0.038	1.000/ 0.735/0.105

Abbreviations SCPB superficial cervical plexus block, GaN great auricular nerve, LoN lesser occipital nerve, TcN transverse cervical nerve, SN supraclavicular nerve

At the 20-minute test point for SNB: In the LM group, there were 0 cases of successful block, 4 cases of partial block, and 11 cases of block failure, resulting in a success rate of 0%. In the US group, there were 6 cases of complete block, 5 cases of partial block, and 4 cases of block failure, yielding a success rate of 40%. In the MU group, there were 11 cases of complete block, 3 cases of partial block, and 1 case of block failure, achieving a success rate of 73.3%. An overall chi-square test revealed a significant difference in SNB success rates among the three groups (χ² = [17.651], p < [0.0001]). Post-hoc pairwise comparisons with Bonferroni correction (significance threshold set at p < 0.0167) demonstrated that the success rate in the MU group was significantly higher than that in the LM group (adjusted p < 0.001). However, no significant differences were observed between the MU and US groups (adjusted p = 0.417) or between the US and LM groups (adjusted p = 0.051).

Secondary outcome measurements (Tables 2 and 3)

At the 20-minute assessment

GaN. In the LM group, 12 of 15 participants achieved complete block with 3 partial blocks (success 80.0%). In the US group, 14 of 15 achieved complete block with 1 partial and no failures (success 93.3%). In the MU group, 8 of 15 achieved complete block with 7 partial blocks (success 53.3%). Group differences were significant (p = 0.017), with MU lower than both US and LM.

LoN. In the LM group, no complete blocks were observed (0/15), with 12 partial and 3 failed blocks (success 0%). The US group had complete blocks in all participants (15/15; 100%). The MU group had 13 complete and 2 partial blocks (13/15; 86.7%). Differences were significant ( p < 0.001); LM was inferior to US and MU.

TcN. The LM group achieved complete block in all participants (15/15; 100%). The US and MU groups each had 13 complete and 2 partial blocks (13/15; 86.7%). Success rates did not differ significantly between groups ( p = 0.334).

Diaphragmatic function analysis (Table 4)

Table 4.

Diaphragmatic Function Analysis

Characteristics	LM group (n = 15)	US group (n = 15)	MU group (n = 15)	P value (overall)	Adjusted P values (LM vs. US / LM vs. MU / US vs. MU)
Pre-block of diaphragmatic excursion during quiet respiration, M (P25, P75) / median (IQR) [mm]	22.3 (18.5–23.8)	20.0 (18.8–22.3)	18.7 (17.8–20.3)	0.128	0.927/0.669/ 1.000
20 min post-block of diaphragmatic excursion during quiet respiration, M (P25, P75) / median (IQR) [mm]	18.9 (17.1–22.6)	14.0 (11.0–15.6)	11.5 (4.7–15.9)	< 0.001	< 0.003/0.003/1.000
Grading of diaphragmatic paralysis during quiet respiration, no.(%)				0.028	0.441/0.099/1.000
0	13 (86.7%)	7 (46.7%)	7 (46.7%)
1	2 (13.3%)	8 (53.3%)	4 (26.7%)
2	0 (0.0%)	0 (0.0%)	4 (26.7%)
Pre-block of diaphragmatic excursion during deep respiration, M (P25, P75) / median (IQR) [mm]	40.5 (38.9–44.9)	43.0 (36.0–45.0)	46.5 (38.7–49.5)	0.332	1.000/1.000/1.000
20 min post-block of diaphragmatic excursion during deep respiration, M (P25, P75) / median (IQR) [mm]	39.3 (36.0–43.7)	31.0 (25.5–34.7)	29.5 (13.3–36.6)	0.009	0.045/0.096/1.000
Grading of diaphragmatic paralysis during deep respiration, no.(%)†				0.016	0.906/0.036/0.087
0	13 (86.7%)	8 (53.3%)	6 (40.0%)
1	2 (13.3%)	7 (46.7%)	5 (33.3%)
2	0 (0.0%)	0 (0.0%)	4 (26.7%)

Values are median (P25–P75) for excursions and n (%) for grading. Overall P (continuous): Kruskal–Wallis with pairwise Mann–Whitney U (Holm). Overall P (ordinal categorical): 3 × 3 chi-square; pairwise P for letters: Fisher exact on ≥ 2 vs. < 2 with Holm–Bonferroni

† Deep-respiration grading had original grades 2 & 3 collapsed into “2 (≥ 2)” to match the three-row template

At 20 min post-block, during quiet breathing, partial diaphragmatic paralysis occurred in 13.33% (LM), 53.33% (US), and 26.67% (MU), while complete paralysis occurred in 0%, 0%, and 33.33%, respectively; group differences were significant (p = 0.028). During deep breathing, partial paralysis was 13.33% (LM), 46.67% (US), and 33.33% (MU), and complete paralysis was 0%, 0%, and 26.67%, respectively (p = 0.016).

The distances from the surface localization point to the ultrasound-identified midpoint of the posterior border of the sternocleidomastoid were 16.0 (15.0, 18.0) mm in LM, 16.0 (15.0, 17.0) mm in US, and 16.0 (15.0, 18.5) mm in MU; differences were not statistically significant. Oxygen saturation and the incidence of complications did not differ between groups.

Discussion

In this study, we compared three SCPB approaches—LM, US and MU—with respect to SN coverage and phrenic safety. The MU approach achieved a higher SN block (SNB) success rate (73.3%) than US (40.0%) and LM (0%), suggesting that longitudinal injection within the intermediate plane (between the superficial cervical and prevertebral fasciae) promotes caudal spread along this fascial channel and improves coverage of the SN origin and course. These results differ substantially from Tran et al. [19] (LM 80%, US 100%). In addition to discrepancies between surface “midpoint” landmarks and true anatomy and the tendency of single-injection techniques to bias spread medially, differences in study populations, endpoint definitions, dose and concentration, needle plane and trajectory, and ultrasound settings/equipment may also have contributed. Additionally, our definition of a successful block (Grade 2–3) is likely more stringent than the criteria employed by Tran et al., which may contribute to the discrepancy in success rates. These explanations are exploratory and require confirmation in head-to-head trials under harmonized protocols.

The LM failures appear primarily related to surface landmark error. Ultrasound showed that the palpated “midpoint” on the posterior border of the SCM was more medial and cranial than the true midpoint. This was supported by the surface-to-ultrasound midpoint distances (LM 16.0 [15.0–18.0] mm; US 16.0 [15.0–17.0] mm; MU 16.0 [15.0–18.5] mm). Mislocalization likely leaves the needle tip subcutaneous or intramuscular, preventing deposition in the intermediate plane. Although the US used ultrasound guidance, a single C4-level injection may favor medial rather than cranial or caudal spread and thus insufficiently bathe the more caudal SN origin. Consistent with this interpretation, Kim et al. [20] demonstrated on cadaveric necks that the four superficial cervical plexus branches, while emerging in a constant order along the SCM posterior border, do not arise from a single midpoint. Because the SN is the most caudal branch, a midpoint-only injection is more likely to fully block the great auricular and transverse cervical nerves while under-covering the SN. Our observations align with this anatomy and with the lower SN success reported by Calderon et al.[21]

The branch-specific findings were consistent with nerve trajectories. MU produced a lower success rate for the GaN (53.3%) than LM (80.0%) and US (93.3%), whereas for the LoN, LM (0%) was inferior to US (100%) and MU (86.7%). TcN success was similar across groups (LM 100%, US and MU 86.7%). Mechanistically, MU’s longitudinal injection favors caudal spread and may provide insufficient coverage of the most cranial GaN; US at C4 may constrain spread medially owing to needle trajectory; and LM’s subcutaneous fan-shaped injection may be susceptible to compartmentalization by the superficial cervical fascia, resulting in discontinuous distribution. Given that the GaN ascends cranially, the LoN courses more posteriorly, and the TcN runs medially over the SCM, these differences are anatomically plausible.

Safety signals indicate a trade-off between efficacy and phrenic involvement. MU was associated with higher hemidiaphragmatic paralysis rates (complete paralysis 33.33% during quiet breathing and 26.67% during deep breathing; both P < 0.01). The longitudinal injection technique of the MU method may facilitate a deeper spread of local anesthetic into the prevertebral space, potentially increasing the likelihood of diffusion to the phrenic nerve region. The phrenic nerve arises primarily from C3–C5, initially coursing on the anterior scalene deep to the prevertebral fascia. Cadaveric findings are conflicting. Pandit et al. [22] observed dye tracking through fascial weak points into deep cervical spaces, staining the phrenic nerve and cervical roots. In contrast, Seidel et al. [23] injected within the “C2–C4 compartment” (between the lamina superficialis and lamina praevertebralis) and found no staining of the phrenic nerve deep to the prevertebral fascia. These discrepancies suggest inter-individual variability in fascial continuity/permeability [24]. Clinically, dose reduction, multi-point or layered injections, more lateral needle placement, and strategies to limit medial spread may help preserve SN coverage while mitigating phrenic involvement. Notably, despite measurable diaphragmatic impairment with MU, SpO₂ did not differ from baseline in these healthy volunteers, consistent with compensation by accessory respiratory muscles; however, this should not be extrapolated to patients with limited respiratory reserve (e.g., COPD, obesity, neuromuscular disease). MU should be used cautiously in such populations, and bilateral blocks should be avoided to reduce the risk of acute respiratory failure.

Clinical implications. Technique selection should balance SN coverage against phrenic safety. MU may be considered for unilateral procedures in patients with adequate respiratory reserve who require robust coverage (e.g., prolonged open reduction and internal fixation), particularly when general anesthesia is contraindicated or regional anesthesia is preferred. When minimizing phrenic involvement is paramount, US/LM may be favored. Broader adoption of MU will depend on validation using lower volumes and/or multi-point injections and evaluation in real-world clinical cohorts. The high success rate of the LM and US approaches for the great auricular and TcNs makes them highly relevant for surgeries involving the anterior and lateral neck, such as thyroidectomy or carotid endarterectomy. The SNB with the MU technique confirms its primary indication for clavicular surgery, where reliable SN coverage is paramount. Where available, effect sizes with 95% confidence intervals should inform choice alongside statistical significance. In recent years, selective SNB has gradually become one of the optimal choices for clavicle fracture surgery, and when combined with a clavipectoral fascia plane block, it can prevent the occurrence of phrenic nerve paralysis [25–27].

This study has several limitations. First, our findings are primarily hypothesis-generating, derived from a small volunteer sample without surgical endpoints. Therefore, future studies in patients undergoing clavicular procedures should integrate clinical outcomes such as analgesic consumption and surgical conditions to determine the actual value of MU within combined anesthetic techniques. Second, the sample size was limited, warranting larger trials to improve precision and enable subgroup analyses. Third, block efficacy was assessed only at 20 min, precluding characterization of onset and peak dynamics. Additionally, this study did not evaluate the duration of sensory block, which is critical for both surgical anesthesia and postoperative analgesia, representing an important direction for future research. Future studies in patient populations should systematically evaluate dose and injection strategy, incorporate objective respiratory mechanics, and define the optimal balance between SN coverage and phrenic safety. The safety assessment in this study primarily relied on diaphragmatic excursion and oxygen saturation, which effectively detect phrenic nerve blockade but did not include pulmonary function tests or dyspnea scales. This may limit the clinical predictive value for patients with impaired respiratory reserve. In healthy volunteers, unilateral diaphragmatic paralysis can often be compensated by accessory respiratory muscles, but future studies should integrate multidimensional respiratory indicators to enhance reliability. Although oxygen saturation did not show a significant decrease, this may mask potential ventilation changes due to compensatory mechanisms in healthy individuals. Therefore, the extrapolation of these safety findings to clinical surgical populations requires validation in studies that include integrated respiratory monitoring and patient-centered outcomes.

Conclusions

This single-center volunteer study suggests that the MU technique may enhance sensory blockade of the SN compared to traditional approaches under controlled conditions. However, its utility as a component of multimodal regional anesthesia for clavicular surgery requires validation in clinical cohorts, where factors such as surgical pain, opioid use, and combination with other blocks (e.g., brachial plexus or clavipectoral fascial blocks) must be assessed. However, its associated risk of phrenic nerve blockade necessitates individualized clinical decision-making. MU may be the preferred option for patients with adequate respiratory reserve; whereas in those with limited reserve or at risk of respiratory failure, it should be used cautiously and alternatives considered. Future research should validate these techniques in actual patient populations and explore innovative strategies to reduce the incidence of diaphragmatic paralysis, ultimately achieving the dual optimization of safety and efficacy in regional anesthesia for the clavicular region.

Authors’ contributions

Study design: FJ, NN-LData acquisition: HX-X, FJ, NN-LDraft of manuscript: FJ, NN-L, HY-W, WT, XQ-G, YYStatistical analysis: HY-W, WT, ZYAnalysis and interpretation of data: all authorsManuscript revision and final approval of the version to be published: all authors.

Funding

This study was Supported by Health Commission of Sichuan Province Medical Science and Technology Program, China (No.24WSXT048). The funding bodies had no role in the design of the study, the collection, analysis, or interpretation of the data, or writing the manuscript.

Data availability

Data are available upon reasonable request. Study protocols and raw data are available upon reasonable request from the corresponding author.

Declarations

Ethics approval and consent to participate

This work has been carried out by the Declaration of Helsinki (2013) of the World Medical Association. This study was approved by the Ethics Committee of Neijiang First People’s Hospital, and all participants provided written informed consent.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.