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Abstract
Background and objectives Acromial branches of the lateral pectoral and suprascapular nerves have been proposed as targets for diagnostic block and radiofrequency ablation to treat superior shoulder pain; however, the nerve capture rates of these procedures have not been investigated. The objectives of this study were to use dissection and 3D modeling technology to determine the course of these acromial branches, relative to anatomical landmarks, and to evaluate nerve capture rates using ultrasound-guided dye injection and lesion simulation.
Methods Ultrasound-guided dye injections, targeting the superior surface of coracoid process and floor of supraspinous fossa, were performed (n=5). Furthermore, needles targeting the superior and posterior surfaces of the coracoid process were placed under ultrasound guidance to simulate needle electrode position (n=5). Specimens were dissected, digitized, and modeled to determine capture rates of acromial branches of lateral pectoral and suprascapular nerves.
Results The course of acromial branches of lateral pectoral and suprascapular nerves were documented. Dye spread capture rates: acromial branches of lateral pectoral and suprascapular nerves were captured in all specimens. Lesion simulation capture rates: (1) when targeting superior surface of coracoid process, the entire acromial branch of lateral pectoral nerve was captured in all specimens and (2) when targeting posterior surface of coracoid process, the acromioclavicular and bursal branches of acromial branch of suprascapular nerve were captured in all specimens; coracoclavicular branch was captured in 3/5 specimens.
Conclusions This study supports the anatomical feasibility of ultrasound-guided targeting of the acromial branches of lateral pectoral and suprascapular nerves. Further clinical investigation is required.
- chronic pain
- pain management
- nerve block
- ultrasonography
- upper extremity
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Introduction
In the USA, between 2013 and 2015, 78.9 million individuals self-reported chronic joint pain.1 The shoulder joint was the second most common site affected in adults age 18 and older, with a rate of 24 per 100 persons.1 Chronic shoulder pain can significantly impair the ability to perform activities of daily living, by limiting the range of motion of the joint, which represent 60% of the function of the upper extremity.2 The lateral pectoral nerve (LPN) and suprascapular nerve (SSN) have been implicated for diagnostic block and radiofrequency ablation (RFA) procedures to manage chronic superior shoulder joint pain.3–6 The nerve capture rates of shoulder diagnostic blocks are of clinical significance as it prognosticates the outcome of the ablation procedure for the articular nerves.
Landmarks used for targeting the acromial branches of LPN and SSN for RFA have been documented in the previous literature.3–10 Four studies were found where the acromial branch of LPN was localized using the superior surface of the coracoid process and the acromial branch of the thoracoacromial artery.7–10 Targeting only the acromial branch of SSN has not been investigated although the entire SSN, including motor and sensory branches, has been targeted either at the suprascapular notch or the supraspinous fossa.11 12 In our previous studies, we described different branches of the SSN on entering the suprascapular notch.10 13 It is unclear whether a diagnostic block at the supraspinous fossa will capture the acromial branch of SSN, which determines its prognosticate value.
In previous cadaveric studies, the acromial branch of SSN was reported to pass deep to the suprascapular ligament (SSL) and then course laterally along the posterior surface of the coracoid process, prior to terminating in the acromioclavicular (AC) joint.7 10 13–15 In our previous SSN denervation needle placement study, the posterior surface of the coracoid process was proposed as a possible landmark to target the acromial branch of SSN, although it has not been investigated.13
In the literature, the nerve capture rates of diagnostic blocks and the anatomical feasibility of RFA techniques specifically targeting the acromial branches of LPN and SSN have not been investigated in 3D. The objectives, of this cadaveric study, were to use dissection and 3D modeling technology to determine: (1) the course of the acromial branches of LPN and SSN, relative to anatomical landmarks visible with ultrasound, (2) the nerve capture rates using ultrasound-guided dye injection at the superior surface of the coracoid process and the supraspinous fossa/midpoint between the suprascapular and spinoglenoid notches and (3) the nerve capture rates following ultrasound-guided RF lesion simulation at the superior and posterior surfaces of the coracoid process.
Methods
Ten lightly embalmed cadaveric specimens, with a mean age of 80.4±14.5 years (5F/5M), were used; dye injection study (n=5) and lesion simulation study (n=5). Specimens with visible signs of injury, pathology or previous surgery were excluded.
Ultrasound-guided dye injection protocol
Targeting of the acromial branch of LPN was carried out using a high-frequency (6-15MHz) linear ultrasound probe (Edge I, Sonosite Fujifilm, Toronto, ON, Canada). The probe was placed in a coronal oblique direction on the anterosuperior aspect of the shoulder to identify the coracoid process medially, the anteromedial edge of the acromion laterally, and the coracoacromial ligament coursing between these structures. The probe was then rotated medially approximately 90 degrees and translated anteromedially (figure 1A) until the superior surfaces of the coracoid process and lateral third of the clavicle were visualized (figure 2). At this site, a trilaminar arrangement of tissues was visualized (figure 2B,C). This included (1) clavicular (anterior) part of the deltoid muscle, superficially, (2) neurovascular bundle consisting of the acromial branches of LPN and corresponding vessels surrounded by fatty connective tissue, located centrally and (3) trapezoid part of the coracoclavicular ligament located deeply. A 25G 1½-inch needle was inserted in-plane and advanced until the needle tip reached the superior surface of the coracoid process, just anterior to the neurovascular bundle. At this site, 0.8 mL of methylene blue dye was injected to evaluate dye spread, simulating a diagnostic block. The articular branches that would be captured with this block would be stained with the methylene blue dye.
Ultrasound probe and needle positioning. (A) Lateral view. (B) Posterolateral view.
Localization of the acromial branch of lateral pectoral nerve at the superior surface of coracoid process. (A) Cadaveric dissection. White box indicates ultrasound probe position. (B) Schematic illustration of relationship of structures visualized with ultrasound using the probe position in A (white box). Needle tip position at the periosteum of the coracoid process is indicated with pink diamond. (C) Ultrasound scan of cadaveric specimen as indicated in A. The white circle is surrounding the neurovascular bundle. Note the trilaminar arrangement of tissues (B and C): layer 1, anterior deltoid; layer 2, neurovascular bundle with acromial branch of lateral pectoral nerve; and layer 3, coracoclavicular ligament. Reprinted with permission from Philip Peng Educational Series. AC, acromion; AcBr, acromial branch; CL, clavicle; LPN, lateral pectoral nerve; S, supraspinatus; SSN, suprascapular nerve; T, trapezius. *Tp of coracoid process.
A linear high-frequency (6–15MHz) probe was placed on the shoulder above the supraspinous fossa. The probe was translated along the floor of the supraspinous fossa in a posterolateral direction from the suprascapular notch to the spinoglenoid notch to identify the midpoint between the notches (figure 3). This technique was carried out as previously described.13 At the midpoint, a 25G 2-inch needle was inserted from a medial to lateral direction. The needle was advanced until the floor of the supraspinous fossa was contacted, and 0.8 cc of methylene blue dye was injected. In our previous study, we found that a simulated lesion at midpoint between suprascapular and spinoglenoid notches did not cover the acromial branch of the SSN.13 However, it was unclear whether a diagnostic block performed at the midpoint would capture the acromial branch resulting in false-positive result; therefore, we chose a small volume for this investigation.
Localization of the origin of articular branches of suprascapular nerve at the midpoint between the suprascapular and spinoglenoid notches. (A) Cadaveric dissection. Orientation of the ultrasound probe is indicated by white box. (B) Ultrasound scan of the midpoint between the suprascapular and spinoglenoid notches. Needle tip position at the periosteum of the supraspinous fossa is indicated with pink diamond. (C) Schematic diagram of structures viewed in ultrasound scan. Reprinted with permission from Philip Peng Educational Series. AcBr, acromial branch; F, adipose tissue; LT, lateral trunk of suprascapular nerve; MT, medial trunk of suprascapular nerve; SS, spine of scapula; SSN, suprascapular nerve. *Tip of coracoid process.
Ultrasound-guided lesion simulation protocol
Targeting of the acromial branch of LPN was carried out using the same methodology described in the ultrasound-guided dye injection scanning protocol above. Once the trilaminar arrangement was clearly visualized (figure 2C), a 18G 100 mm OWL RF cannula (Diros Technology, Markham, ON, Canada) was inserted in-plane in cephalad-to-caudal direction and advanced until the needle tip reached the superior surface of the coracoid process, just anterior to the neurovascular bundle. The needle tip was gently secured with a small rubber mallet into the superior surface of the coracoid process to prevent movement of needle postplacement.
To target the acromial branch of SSN, a linear high-frequency (6-15MHz) probe was initially placed on the shoulder above the supraspinous fossa and then tilted forward (figures 1B and 4). Once the suprascapular notch was visualized, a 18G 100 mm needle was inserted out-of-plane just lateral to the notch, targeting the posterior surface of the coracoid process, which lies deep to the lateral end of the clavicle (figure 4E,F). The needle tip was gently secured with a small rubber mallet into the posterior surface of the coracoid process to prevent movement of the needle postplacement.
Localization of the acromial branch of suprascapular nerve coursing along the posterior surface of coracoid process. (A–C). Serial dissection of the suprascapular nerve passing deep to the suprascapular ligament (*). Probe position is indicated by the white box. (A) Skin and superficial fascia have been removed. (B) Trapezius muscle has been removed. (C) Supraspinatus muscle has been partially reflected posteriorly to reveal the suprascapular nerve. (D) Enlargement of C to demonstrate the relationship of the suprascapular nerve to the clavicle, supraspinatus and suprascapular ligament. (E) Ultrasound scan showing the structures in serial dissection A–C. (F) Schematic diagram of ultrasound scan in E. Needle tip position at the periosteum of the supraspinous fossa is indicated with pink diamond. Reprinted with permission from Philip Peng Educational Series. A, suprascapular artery; AC, acromion; AcBr, acromial branch; CL, clavicle; F, adipose tissue; LT, lateral trunk of suprascapular nerve; MT, medial trunk of suprascapular nerve; S, supraspinatus; SSN, suprascapular nerve; T, Trapezius; V, suprascapular vein.
Dissection and digitization
The skin and subcutaneous tissues were removed from all specimens. This exposed the underlying musculature that will be the starting point of dissection for the dye injection and lesion simulation studies. The digitization and 3D model reconstruction methodology are described in a previously published paper from our laboratory.16
Dye injection study
In the dye-injected specimens to reveal the area of dye spread relative to the acromial branch of LPN, the anterior deltoid was exposed and reflected laterally. The neurovascular bundle, including the acromial branch of LPN, was exposed as its coursed superior to the coracoid process. The acromial branch of LPN was separated from the accompanying vessels and traced proximally, to its origin from the lateral cord of the brachial plexus, and distally to its termination. Next, to expose the dye spread relative to the acromial and articular branches of SSN, the trapezius was removed to expose the supraspinatus. Following the exposure of supraspinatus, the main trunk of SSN was identified anterior to the suprascapular notch and SSL. The SSN was traced posteriorly into the supraspinous fossa deep to the supraspinatus. Fiber bundles of supraspinatus were delineated and excised until the full extent of dye spread could be visualized. Next, the SSN and its branches were separated from the suprascapular vessels and traced to their termination.
Lesion simulation study
Following needle placement, to target acromial branches of LPN and SSN, the needle tips were gently secured with a small rubber mallet into the superior and posterior surfaces of the coracoid process, respectively. By securing the needle into bone, it enabled the preservation of the needle trajectory during dissection and digitization. The acromial branches of LPN and SSN were dissected using the same protocols described above in the injection part of this study. For the needle targeting the acromial branch of SSN, the trajectory of the needle was exposed as fiber bundles of supraspinatus were removed. Similarly, the needle targeting the acromial branch of LPN was exposed after reflection of the anterior deltoid.
Digitization of the acromial branches of LPN and SSN, needle tips, and bony landmarks were carried out using a MicroScribeTM G2X Digitizer with accuracy of 0.23 mm (Immersion Corporation, San Jose, California, USA) along with custom software developed in our laboratory. The RF needle trajectory and tip location were digitized first followed by the acromial branch of LPN, and acromial and articular branches of SSN. Nerve branches were digitized in 1–2 mm increments from their origin to their termination. Next, the coracoid process, acromion, and clavicle were digitized using a 4 mm grid pattern. Following digitization, each specimen was skeletonized. The surface of the scapula, clavicle, proximal humerus, capsules of the AC and glenohumeral joints, and the coracoacromial and coracoclavicular ligaments were scanned using a FARO Laser ScanArm with accuracy of 75 µm (FARO Technologies, Lake Mary, Florida, USA).
3D modeling
Cartesian coordinate data of RF needle trajectory, acromial branches of LPN, acromial and articular branches of SSN, bones, ligaments and joint capsules collected using digitization and laser scanning were registered and modeled using Autodesk Maya (Autodesk, San Rafael, California, USA). Since both the digitization and laser scanning datasets are composed of x, y, z data points in 3D space, they can be registered using Autodesk Maya.
To replicate the volume of the lesion, an oval with radius of 5 mm was modeled based on RF needle manufacturer data (Diros Technologies, Markham, ON, Canada). The oval lesion was positioned with the needle tip at its center (figure 5).
Steps to modeling and registrating the needle and lesion volume. (A) Digitized needle. The needle tip is indicated with red arrow. (B) Digitized data of needle modeled as cylinder. (C) Lesion volume. This was modeled based on manufacturer’s dimensions (Diros Technology Inc.). (D) Wireframe model of needle (B) and lesion (C). (E) Rendered 3D model.
Data analysis
The dissected specimens and reconstructed 3D models from the dye injection and lesion simulation portions of this study were used to determine nerve capture rates.
Dye injection study
Using the dissected specimens, the stained branches were recorded and the overall frequency of staining was quantified. The extent of dye spread was documented and described in relationship to bony and soft tissue landmarks. Implications to clinical practice based on the frequency and pattern of nerve staining were summarized.
Lesion simulation study
The 3D model of each specimen was used to define the (1) spatial location (medial, lateral, posterior, and anterior) of RF needle tips to the acromial branch of LPN and acromial and articular branches of SSN and (2) nerve capture rates as determined by the presence of the nerve within the geometric lesion volumes. Implications of findings to clinical practice were summarized.
Results
The course of the acromial branches of LPN and SSN, as documented in the dissected specimens, will be described first. This will be followed by the results of the dye injection and lesion simulation studies.
Course of nerves
The acromial branch of LPN arises in the deltopectoral triangle, inferomedial to the coracoid process. It is accompanied by the acromial branch of thoracoacromial artery that originates from the thoracoacromial trunk. This neurovascular bundle courses along the superior surface of the coracoid process, deep to the clavicular part of deltoid, to supply the AC joint, coracoacromial and coracoclavicular ligaments (figures 6 and 7). The coracoacromial branch continued beyond the ligament to also supply the region of the subacromial (subdeltoid) bursa (figure 6A,C).
Dye spread following injection at the superior surface of coracoid process and midpoint between the suprascapular and spinoglenoid notches. (A) Overview. (B) Capture of branches of the suprascapular nerve (red inset). (C) Capture of branches of the acromial branch of lateral pectoral nerve (white inset). Reprinted with permission from Philip Peng Educational Series. AC, acromion, br, branch; CL, lateral clavicle; CP, coracoid process; LT, lateral trunk of suprascapular nerve; MT, medial trunk of suprascapular nerve; S, supraspinatus; SSN, suprascapular nerve.
Needle tip position following placement at the superior surface of coracoid process to capture the acromial branch of lateral pectoral nerve. (A–C) Position of needle tip relative to the acromial branch of lateral pectoral nerve in three different specimens. (D–F) Dissection and 3D models of the acromial branch of lateral pectoral nerve without (E) and with (F) lesion volume. Reprinted with permission from Philip Peng Educational Series. AC, acromion process; Ant, anterior; r, branch; CL, lateral clavicle; CP, coracoid process; D, deltoid (reflected), H, humerus; L, lateral; M, medial; Post, posterior; T, trapezius.
The SSN gave off the acromial branch just anterior or at the suprascapular notch as the nerve passed deep to the SSL. In all 10 specimens, the acromial branch coursed laterally between the anterior margin of the supraspinatus and posterior surface of the coracoid process to reach the region of the AC joint (figure 8). Branches to the coracoclavicular ligament were given off medially, followed by branches to the region of the subacromial bursa, prior to terminating in the AC joint laterally. Additionally, in four specimens, the acromial branch of SSN gave off articular (glenohumeral) branches that coursed deep to the supraspinatus to supply the anterosuperior/superior glenohumeral joint (figure 8D).
Needle tip position following placement at the posterior surface of coracoid process to capture the acromial branch of suprascapular nerve. (A–C) Position of needle tip relative to the acromial branch of suprascapular nerve in three different specimens. (D–F) Dissection and 3D models of the acromial branch of suprascapular nerve without (E) and with (F) lesion volume. Reprinted with permission from Philip Peng Educational Series. AC, acromion process; Ant, anterior; Asterisk, coracoclavicular ligament; blue arrow, acromioclavicular joint; br, branch; CL, lateral clavicle; L, lateral; LT, lateral trunk of suprascapular nerve; M, medial; MT, medial trunk of suprascapular nerve; Post, posterior; S, supraspinatus; SS, spine of scapula; SSF, supraspinous fossa; SSN, suprascapular nerve; T, trapezius (reflected posteriorly).
Dye injection study
When the needle was placed at the superior surface of the coracoid process, the injected dye was found to spread in a plane between the clavicular part of deltoid and the coracoid process/coracoacromial ligament (figure 6A). In all five specimens, the acromial branch of LPN was captured as it coursed along the superior surface of the coracoid process. The dye spread extended to the deltopectoral triangle, the lateral third of clavicle, the fascia superficial to the superior border of subscapularis, and the coracoclavicular/coracoacromial ligaments. Branches of the LPN to the coracoacromial ligament, coracoclavicular ligament, and AC joint were also in the area of dye spread (figure 6C).
Dye injected to target the midpoint between the suprascapular and spinoglenoid notches was found to spread in a plane between the supraspinatus and supraspinous fossa. In all specimens, the dye spread extended from the suprascapular notch to the spinoglenoid notch to capture the entire SSN including the medial/lateral trunks, the articular branches originating from the lateral trunk, and the acromial branch (figure 6B).
Lesion simulation study
The acromial branch of LPN (including its branches to the coracoacromial ligament, coracoclavicular ligament, and AC joint) was captured in all five specimens. The needle tip following ultrasound-guided placement was located posterior to the acromial branch of LPN in three specimens (figure 7B,C) and anterior in two specimens (figure 7A,D). In one specimen, the needle was placed about 2 mm lateral to the coracoid process but still resulted in capture of the acromial branch of LPN prior to its division into terminal branches (figure 7A).
When targeting the acromial branch of SSN, the needle tip was found just inferior to the coracoclavicular ligament at the site of the articular/ligamentous branches (figure 8). The AC and bursal branches were captured in all specimens; however, the coracoclavicular branch was captured in only three specimens. In the two specimens, where the coracoclavicular branch was spared, the needle was placed more laterally (figure 8B).
The location of the acromial branches of LPN/SSN and RF lesions targeting the superior and posterior surfaces of the coracoid process have been correlated with standard fluoroscopic images (figure 9). This enables correlation of the 3D models with anatomical landmarks visible with fluoroscopy.
Fluoroscopic targeting. (A) Acromial branch (red) of suprascapular nerve (yellow). (B) Acromial branch of lateral pectoral nerve (blue). Reprinted with permission from Philip Peng Educational Series. AC, acromion; Asterisk, tip of coracoid process; bold arrow, suprascapular notch; CL, lateral clavicle; H, humerus; orange oval, lesion volume; green line, outline of suprascapular notch; red dash line, outline of tip of coracoid process; red X, needle tip position; SS, spine of scapula.
Discussion
This anatomical study used dissection and 3D modeling technology to assess the course of the acromial branches of LPN and SSN. Furthermore, ultrasound-guided dye injection and lesion simulation were performed and nerve capture of the acromial branches of LPN and SSN was quantified.
The acromial branch of LPN was found to innervate the coracoacromial ligament, coracoclavicular ligament, AC joint, and region of the subacromial bursa consistent with the previous literature.3 4 7 8 10 In the current study, further cadaveric investigation enabled identification of the individual branches arising from the acromial branch of LPN and digitization of their course. Knowledge of the 3D course of the acromial branch of LPN and its components provide an evidence-based approach to document nerve capture rates. In the dye injection and lesion simulation studies, replicating clinically used diagnostic block/RFA protocols that target the coracoid process, the acromial branch of LPN and its components were captured in all specimens. Clinically, targeting the acromial branch of LPN for denervation has shown promise. Dellon et al, when describing prognosis of patients following denervation of LPN stated “… expect an excellent chance of having shoulder function restored after anterior shoulder denervation.”3 Furthermore, in a recent case report of thermal RFA of the articular branch of LPN, targeting the coracoid process, Eckmann et al, reported “substantial relief of resting and dynamic deep anterior shoulder pain beyond 3 months.”4 The nerve capture as reported in this study provides anatomical evidence supporting these clinical observations.
In the current study, the SSN was found to give off an acromial branch prior to bifurcating into the lateral and medial trunks. The acromial branch of SSN coursed laterally along the posterior surface of the coracoid process and supplied branches to the coracoclavicular ligament, AC joint, and region of the subacromial bursa consistent with previous literature.7 10 14 15 In addition, in four specimens, the acromial branch of SSN supplied innervation to the anterosuperior/superior glenohumeral joint capsule, which has also been previously documented.7 10 17 Interestingly, the dye injection study, targeting the midpoint between the suprascapular and spinoglenoid notches, resulted in the capture of both trunks including the articular and the acromial branches of SSN. While the diagnostic block covers all branches of the SSN, in the supraspinous fossa, it does not discriminate the articular branches of the lateral trunk from the acromial branch. Since a lesion at the midpoint between the suprascapular and spinoglenoid notches does not cover the acromial branch of SSN,13 a separate lesion, specifically targeting the posterior surface of the coracoid process, is needed. The results of the current study suggest that a 3-needle placement configuration could be most optimal to ensure capture of the acromial branch of LPN and acromial and articular branches of SSN. The first needle targeting, the superior surface of the coracoid process, would capture the acromial branch of LPN; the second, at the posterior surface of coracoid process, would capture the acromial branch of SSN and when present articular branches to the anterosuperior/superior glenohumeral joint capsule; and, the third, placed at the midpoint between the suprascapular and spinoglenoid notches, would capture the articular branches of SSN originating from the lateral trunk that supply the posterior glenohumeral joint10 and subacromial bursa.15
The acromial branches of LPN and SSN, in the current study, were found to innervate pain-generating structures including the subacromial bursa, coracoclavicular ligament, and the AC ligament and joint.18 Therefore, the acromial branches of LPN and SSN could serve as targets for managing chronic shoulder pain to avoid resorting to the use of opioid. The three-needle placement configuration, proposed in this study, would capture the area with highest density of nociceptor in the shoulder: the anterosuperior portion of subacromial bursa, AC and coracoclavicular ligaments,18 19 while sparing the medial trunk of SSN that innervate the supraspinatus. Further clinical investigation is required to confirm the analgesic effectiveness of this proposed approach.
Limitations of this study include a small sample size; thus, not accounting for all possible anatomical variations. In the literature, animal tissue has been used to determine lesion volumes,20 21 and, thus, may not reflect lesion volumes in vivo. In addition, the spread of injectate, as in all cadaveric studies, may not exactly replicate local anesthetic spread in patients; however, specimens prepared with the lightly embalmed technique have tissue qualities similar to that of live subjects with minimal structural distortion.22–24 As with all proof of concept anatomical studies, further clinical investigation is needed to determine analgesic effect.
In conclusion, this cadaveric dye injection/lesion simulation study supports the feasibility of ultrasound-guided targeting of the acromial branch of LPN along the superior surface of the coracoid process and the acromial branch of SSN at the posterior surface of the coracoid process. Further clinical investigation is required.
Acknowledgments
The authors wish to thank Ian Bell and Benjamin Kozlowski for their valuable technical assistance. We also wish to thank the individuals who donate their bodies and tissue for the advancement of education and research.
References
Footnotes
Contributors JT, PP, AA, and NM contributed to the experimental design, data acquisition, analysis of data, drafting, and revising the manuscript critically for important intellectual content.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests PP received equipment support from Sonosite Fujifilm Canada. AA is an Anatomy Faculty with Allergan Academy of Excellence.
Patient consent for publication Not required.
Ethics approval This cadaveric study includes nonidentifiable dissection images and the use was approved by the University of Toronto Health Sciences Research Ethics Board (#27210).
Provenance and peer review Not commissioned; externally peer reviewed.
Data availability statement This is a cadaveric study and all data relevant to this study are included in the article or uploaded as supplementary information.