INTRODUCTION TO PERIPHERAL NERVE DISORDERS OF THE SHOULDER GIRDLE
Peripheral nerve entrapments and injuries around the shoulder girdle present a complex diagnostic and therapeutic challenge for the orthopedic surgeon. The intricate neurovascular anatomy of the shoulder—specifically involving the suprascapular, axillary, and long thoracic nerves—demands a profound understanding of biomechanics, topographic anatomy, and surgical exposures.
Pathologies ranging from suprascapular nerve entrapment at the transverse scapular ligament to quadrilateral space syndrome and serratus anterior paralysis require meticulous clinical evaluation. This masterclass expands upon the foundational principles of shoulder neuroanatomy, detailing the posterior approach for the division of the transverse scapular ligament (as described by Swafford and Lichtman), and provides an evidence-based framework for managing axillary and long thoracic nerve dysfunction.
PART I: SUPRASCAPULAR NERVE ENTRAPMENT AND THE POSTERIOR APPROACH
The suprascapular nerve arises from the upper trunk of the brachial plexus (C5, C6), coursing through the posterior triangle of the neck before passing beneath the transverse scapular ligament at the suprascapular notch. It provides critical motor innervation to the supraspinatus and infraspinatus muscles, alongside sensory branches to the acromioclavicular and glenohumeral joints. Entrapment at the suprascapular notch typically results in deep, poorly localized posterior shoulder pain, accompanied by weakness in abduction and external rotation.
Indications for Surgical Decompression
Surgical release of the transverse scapular ligament is indicated in patients who demonstrate:
* Refractory posterior shoulder pain failing 3 to 6 months of conservative management (NSAIDs, physical therapy, image-guided corticosteroid injections).
* Progressive atrophy of the supraspinatus and infraspinatus muscles.
* Electromyographic (EMG) and nerve conduction velocity (NCV) evidence of denervation or delayed conduction across the suprascapular notch.
* Magnetic Resonance Imaging (MRI) confirming nerve compression without an intra-articular etiology (e.g., massive rotator cuff tear) that would otherwise dictate a different surgical algorithm.
Preoperative Planning and Positioning
Meticulous patient positioning is paramount for optimal exposure and safety during the posterior approach.
- Positioning: The patient is placed in the prone position on a radiolucent operating table. Chest rolls or a specialized prone frame are utilized to ensure unrestricted abdominal excursion and optimal ventilation.
- Arm Placement: The operative arm is draped free to allow for intraoperative manipulation. Slight internal rotation and adduction of the arm can help distract the scapula laterally, opening the posterior surgical corridor.
- Landmarking: The spine of the scapula, the acromion, and the medial border of the scapula are palpated and marked.
🔪 Surgical Pearl: The "Army Over Navy" Rule
When dissecting the suprascapular notch, always remember the anatomical relationship: the suprascapular Artery passes over the transverse scapular ligament, while the suprascapular Nerve passes under the ligament through the notch. Meticulous hemostasis and careful retraction of the artery are mandatory before dividing the ligament.
Surgical Technique: The Swafford and Lichtman Approach
The posterior approach provides direct, anatomic access to the suprascapular notch. The steps are as follows:
- Incision: Make a transverse incision approximately 3 cm superior to, and parallel with, the spine of the scapula. The incision should be centered over the anticipated location of the suprascapular notch (typically at the junction of the medial two-thirds and lateral one-third of the scapular spine).
- Superficial Dissection: Incise the subcutaneous tissues to expose the deep fascia overlying the trapezius muscle.
- Trapezius Elevation: Split the trapezius muscle fibers bluntly in line with their course, or elevate the trapezius subperiosteally from the spine of the scapula, depending on the required exposure. Retract the trapezius superiorly and medially to expose the underlying supraspinatus muscle fascia.
- Supraspinatus Mobilization: Carefully elevate the supraspinatus muscle from its fossa. Dissect superior and inferior to the muscle belly to identify the neurovascular bundle. Retract the supraspinatus posteriorly and inferiorly.
- Identification of the Notch: Palpate the superior border of the scapula to locate the suprascapular notch. Clear the overlying fibrofatty tissue using blunt dissection (e.g., a peanut sponge or Kittner).
- Ligament Release: Identify the transverse scapular ligament. Protect the suprascapular artery, which runs superior to the ligament. Introduce a blunt nerve hook or a right-angle clamp beneath the ligament to protect the underlying suprascapular nerve. Divide the ligament sharply with a scalpel or dissecting scissors.
- Bony Decompression: Inspect the notch. If the bony architecture is narrow or osteophytic (a common anatomical variant), use a 2 mm or 3 mm Kerrison rongeur to enlarge the notch.
- Smoothing the Margins: It is critical to smooth the bony edges of the enlarged notch with a fine rasp or burr to prevent secondary frictional neuritis or delayed mechanical damage to the nerve.
Spinoglenoid Notch Exploration
If no definite entrapment or anatomical stricture is identified at the suprascapular notch, or if the patient presents with isolated infraspinatus atrophy, the surgeon must follow the nerve distally.
The nerve courses laterally around the base of the scapular spine through the spinoglenoid notch to innervate the infraspinatus. Entrapment here is frequently caused by the spinoglenoid ligament (inferior transverse scapular ligament) or by paralabral cysts arising from superior labral anterior-posterior (SLAP) tears. The dissection must be extended laterally to exclude entrapment in this region, releasing the spinoglenoid ligament or decompressing any cystic structures as necessary.
Postoperative Protocol and Outcomes
Postoperatively, the patient is placed in a standard shoulder immobilizer for comfort for 1 to 2 weeks. Passive and active-assisted range of motion (ROM) exercises are initiated early to prevent adhesive capsulitis. Strengthening of the rotator cuff begins at 4 to 6 weeks.
Return of function after release varies significantly based on the chronicity of the compression and the degree of preoperative axonal loss. While pain relief is often immediate and dramatic, motor recovery of the supraspinatus and infraspinatus may take 6 to 12 months. Reports regarding the results of primary suture or grafting of a transected suprascapular nerve remain inconclusive, emphasizing the importance of early decompression before irreversible Wallerian degeneration occurs.
PART II: THE AXILLARY NERVE AND QUADRILATERAL SPACE SYNDROME
The axillary nerve is a terminal branch of the posterior cord of the brachial plexus (C5, C6). It is highly susceptible to injury due to its relatively tethered course around the proximal humerus.
Detailed Topographic Anatomy
The axillary nerve courses inferior to the subscapular and thoracodorsal nerves at the level of the humeral head. It exits the axilla posteriorly by winding around the surgical neck of the humerus, passing through the quadrilateral space.
The boundaries of the quadrilateral space are:
* Superior: Teres minor (and the inferior capsule of the glenohumeral joint).
* Inferior: Teres major.
* Medial: Long head of the triceps brachii.
* Lateral: Surgical neck of the humerus.
Upon exiting the quadrilateral space, the nerve divides to supply the deltoid and teres minor muscles.
The Ball et al. Anatomical Study:
Ball and colleagues provided a seminal description of the axillary nerve's branching pattern. They emphasized a distinct posterior branch that innervates the teres minor and the posterior third of the deltoid. This branch also provides cutaneous innervation to the lateral shoulder via the superior lateral brachial cutaneous nerve.
Crucially for surgical approaches, the cutaneous portion traverses the deep fascia between 6.3 to 10.9 cm below the posterolateral corner of the acromion, along the medial border of the deltoid. The anterior branch continues anterolaterally, wrapping around the surgical neck to supply the anterior and middle heads of the deltoid. Notably, in 5 of 19 cadaveric specimens studied by Ball et al., the posterior deltoid received its sole innervation from the anterior branch, highlighting significant anatomical variability that surgeons must respect during deltoid-splitting approaches.
Mechanisms of Injury
The axillary nerve is most frequently injured just proximal to or within the quadrilateral space. Common etiologies include:
* Trauma: Anterior glenohumeral dislocations and proximal humerus fractures.
* Penetrating Wounds & Direct Blows: High-energy trauma to the lateral shoulder.
* Iatrogenic: Injury during posterior approaches to the shoulder, inferior capsular releases during shoulder arthroscopy, or misplaced intramuscular deltoid injections.
Quadrilateral Space Syndrome (QSS)
Rarely, chronic compression of the axillary nerve (or its major branches) and the posterior humeral circumflex artery occurs within the quadrilateral space. This is known as Quadrilateral Space Syndrome.
QSS typically affects young, active individuals (often overhead athletes). It is caused by fibrous bands within the space or hypertrophy of the surrounding musculature. Patients present with chronic, poorly localized posterior shoulder pain and paresthesias that are characteristically aggravated by forward flexion, or abduction and external rotation of the humerus (the ABER position).
Clinical Examination and Diagnosis
Because a lesion of the axillary nerve sometimes does not cause profound anesthesia (due to overlapping dermatomes), the diagnosis must rest heavily on the presence or absence of motor function in the deltoid muscle.
🚨 Clinical Pitfall: Trick Movements in Deltoid Paralysis
Usually, deltoid paralysis is easily detected by the patient's inability to actively abduct the arm. However, it is well documented that full active abduction of the arm is possible even in the presence of complete deltoid paralysis. This "trick movement" is achieved through the compensatory action of the supraspinatus muscle combined with exaggerated rotation of the scapula.
Therefore, it is absolutely essential to visually observe and manually palpate the anterior, middle, and posterior bellies of the deltoid muscle for active contraction during the examination. Electrical stimulation of the nerve in situ, alongside formal EMG/NCV testing, is the gold standard for confirming the diagnosis and localizing the lesion. In suspected QSS, MR Angiography (MRA) or conventional arteriography may demonstrate occlusion of the posterior humeral circumflex artery when the arm is placed in the ABER position.
PART III: THE LONG THORACIC NERVE AND SERRATUS ANTERIOR PARALYSIS
The long thoracic nerve of Bell arises from the anterior rami of C5, C6, and C7. Unlike most motor nerves, it courses superficial to the muscle it innervates—the serratus anterior—making it uniquely vulnerable to mechanical injury.
Pathophysiology of Injury
Paralysis of the serratus anterior muscle alone is occasionally seen and is almost exclusively due to long thoracic nerve dysfunction. Injuries can result from:
* Direct Trauma: Sharp penetrating injuries or blunt trauma to the lateral chest wall.
* Traction Injuries: Occur when the head is forced acutely away from the shoulder, or when the shoulder is violently depressed (e.g., carrying heavy weights or backpacks).
* Iatrogenic/Positioning: Placing patients in the Trendelenburg position with poorly padded shoulder braces can compress the supraclavicular area, crushing the nerve against the first rib. It is also at risk during axillary lymph node dissections.
* Atraumatic/Medical: Exposure to cold, viral infections, and neuralgic amyotrophy (Parsonage-Turner Syndrome).
Clinical Presentation
The serratus anterior is the primary protractor and upward rotator of the scapula. When paralyzed, the biomechanics of the shoulder girdle are severely disrupted.
* The patient cannot fully flex or abduct the arm above the level of the shoulder anteriorly, as the scapula fails to upwardly rotate to clear the acromion from the greater tuberosity.
* Scapular Winging: When the patient attempts to exert forward pushing movements with the hands against a wall (the "wall push-up test"), classic medial "winging" of the scapula occurs. The vertebral border and the inferior angle of the scapula lift away from the thoracic cage and become unduly prominent.
Management and Prognosis
The initial management of long thoracic nerve palsy is almost universally conservative, particularly when the nerve has been stretched or contused rather than sharply severed.
- Conservative Care: It is usually sufficient to immobilize the shoulder girdle in slight extension with the arm resting against the chest during the acute painful phase. Aggressive physical therapy must be instituted to maintain passive ROM. Contractures of the shoulder, elbow, and wrist must be strictly avoided while awaiting neural regeneration.
- Timeline for Recovery: According to Sunderland's classification of nerve injuries, a neurapraxia or axonotmesis of the long thoracic nerve may take anywhere from 3 to 12 months to recover. Serial clinical examinations and EMG studies at 3-month intervals are recommended to track reinnervation.
Surgical Reconstruction
If paralysis persists beyond 12 to 18 months with no EMG evidence of recovery, or if the nerve has been definitively severed (e.g., during tumor resection), the prognosis for spontaneous recovery is poor.
There are no significant, reliable reports of successful outcomes following direct primary suture or nerve grafting of the long thoracic nerve due to its length and the diffuse nature of its motor endplates. Consequently, reconstructive salvage operations are indicated. The gold standard for isolated serratus anterior paralysis is dynamic muscle transfer. The most common procedure is the transfer of the sternal head of the pectoralis major, extended with a fascia lata autograft, to the inferior angle of the scapula (as detailed in Chapter 34 of operative texts). This transfer effectively restores scapular protraction and stabilizes the medial border against the thoracic wall, significantly improving overhead function.
