Serratus Anterior

Origin. Fleshy digitations arise from the outer surfaces of the upper eight or nine ribs (ribs 1 to 8, commonly extending to rib 9) and the intervening intercostal fascia. The first digitation arises from both the first and second ribs and the fascia between them, so there are typically eight or nine digitations from nine or ten rib attachments. The lower four interdigitate with the origin of external oblique, a constant and much-loved detail that also marks the plane at which the lower digitations are separated during flap harvest.

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Insertion. Every part inserts on the costal (deep) surface of the medial border, but the three parts differ in size and role:

• Superior part - ribs 1 and 2 to a triangular impression on the costal surface of the superior angle
• Intermediate part - ribs 2 to 4, a thin sheet along the costal surface of the medial border
• Inferior part - ribs 5 to 9, converging as a thick, powerful fan onto the costal surface of the inferior angle. It carries most of the muscle mass and generates most of the rotation

Why the costal insertion matters. Because the whole insertion is on the rib-facing surface of the scapula, serratus pulls the medial border toward the chest wall as well as forward around it. That is why its failure produces a wing rather than merely weakness: nothing else holds the vertebral border down.

Fibre orientation. The fibres run posteriorly and medially around the chest wall, the upper fibres almost horizontally, the lower obliquely upward and backward to the inferior angle.

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The long thoracic nerve (nerve of Bell) is the sole motor supply, and serratus anterior is its only muscle. The root value is C5, C6 and C7, with a C8 contribution described in a minority; "C5-6-7 raise your arm to heaven" is the line that fixes it. The nerve arises directly from the ventral rami in the neck, proximal to the formation of the trunks, so, like the dorsal scapular nerve, it is a proximal marker in plexus injury.

The scalene relationship. This is the classic examination point:

• The C5 and C6 contributions pierce the middle scalene, together with, or just below, the dorsal scapular nerve
• The C7 contribution passes anterior to the middle scalene and joins the other two distal to the muscle

Course. The united nerve passes behind the brachial plexus and the axillary vessels and crosses the outer border of the first rib (or over the second rib). It then descends on the outer (superficial, lateral) surface of serratus anterior in the mid-axillary line, deep only to the deep fascia and the overlying subcutaneous fat and skin.

Motor entry. Branches leave the nerve in sequence and enter the muscle from its superficial surface, supplying successive digitations from cranial to caudal. Because the branches to the upper digitations leave first, a harvest that takes only the lower digitations can preserve the upper supply.

Why the course matters. The nerve is long, thin and unaccompanied by a substantial artery, tethered proximally at the scalene and distally by its muscular branches, and it lies directly under the fascia on the side of the chest. That makes it uniquely vulnerable to traction, to direct injury, to compression at the middle scalene or beneath the fascial band at the level of the second rib, and to immune-mediated neuritis: in neuralgic amyotrophy it is one of the most frequently affected nerves. The individual causes are listed under Clinical Relevance.

Arterial supply. The flap surgeon needs to know which vessel feeds which digitations:

• Lateral thoracic artery (from the second part of the axillary artery) - the upper and intermediate digitations, descending on the lateral chest wall along the lateral border of pectoralis minor
• Serratus branch of the thoracodorsal artery (from the subscapular artery) - the lower three or four digitations. This is the pedicle of the serratus anterior flap, and the reason the lower digitations are the ones harvested
• Perforating branches of the posterior intercostal arteries - a segmental supply that must be divided when the muscle is islanded
• Thoracoacromial artery - contributes to the uppermost part in some individuals

Pedicle position. The thoracodorsal artery divides into a latissimus branch and a serratus branch at a variable point, typically within a few centimetres of its origin from the subscapular artery, high in the axilla behind latissimus. The serratus branch then runs distally on the surface of the muscle, immediately anterior and parallel to the long thoracic nerve. That parallel course is the central technical fact of flap harvest: the artery you want and the nerve you must preserve travel together, and they must be separated under loupe magnification.

A long pedicle. The subscapular-thoracodorsal-serratus axis gives a long pedicle of good calibre, the basis of the free serratus flap and of chimeric latissimus-serratus flaps sharing a single subscapular pedicle.

Venous drainage. Venae comitantes of the thoracodorsal and lateral thoracic vessels drain into the axillary vein.

Superficial (lateral). The deep fascia, with the long thoracic nerve running in the mid-axillary line. Latissimus dorsi lies posteriorly and pectoralis major and minor anteriorly, and in the axilla sit the axillary contents, including the thoracodorsal and lateral thoracic vessels and the axillary lymph nodes. With the upper ribs and intercostal muscles, serratus forms the medial wall of the axilla.

Deep (medial). Ribs 1 to 9 and the intercostal muscles, and beyond them the parietal pleura. The infraserratus (serratus anterior) scapulothoracic bursa lies between the muscle and the chest wall.

Between serratus and the scapula. Subscapularis covers the costal surface of the scapula and is separated from serratus by the supraserratus bursa. Serratus and subscapularis are the two gliding layers of the scapulothoracic articulation.

Structures at risk. Whoever operates in this plane meets these:

• Long thoracic nerve - in the mid-axillary line over the second to fifth ribs; it defines the danger zone of axillary and chest wall surgery
• Thoracodorsal nerve and vessels - posteriorly in the axilla, on the deep surface of latissimus dorsi
• Intercostobrachial nerve - crossing the axilla from the second intercostal space; its division gives inner-arm numbness after axillary clearance
• Pleura - immediately deep to the muscle and its ribs

Neuralgic amyotrophy (Parsonage-Turner syndrome) is the commonest single cause. Severe continuous pain, lasting around four weeks on average, is followed by painless weakness as the pain settles. It is frequently patchy and multifocal: long thoracic and/or suprascapular involvement in the upper and middle trunk distribution is the commonest pattern, at 71 per cent, and it often follows a viral illness, immunisation or surgery.

Other non-traumatic causes. Neuralgic amyotrophy aside, consider:

• Viral illness without frank neuralgic amyotrophy
• Repetitive activity - overhead sport, tennis, weightlifting and occupational overhead work
• Idiopathic, a genuine category after careful evaluation

Traumatic and positional. Ask about injury and posture:

• Traction injury - forced depression of the shoulder with the head bent to the opposite side; falls; road traffic collisions
• Heavy backpack or shoulder strap compression at the second rib
• Prolonged general anaesthesia in the lateral decubitus position with an inadequate axillary roll

Iatrogenic. Axillary lymph node dissection and mastectomy are the classic surgical cause. The rest of the list:

• First rib resection and scalenectomy for thoracic outlet syndrome
• Chest drain insertion and thoracoscopic port placement in the mid-axillary line
• Thoracotomy, latissimus dorsi flap harvest and transaxillary approaches
• Interscalene and supraclavicular block - the C5 and C6 contributions run in the middle scalene, so needle injury there is possible

Winging that is not a palsy must be excluded. Bony pseudo-wings and pain-inhibited winging are covered under the examination pitfalls; glenohumeral instability and painful cuff disease also produce a dyskinetic pseudo-winging. Facioscapulohumeral muscular dystrophy is bilateral, with facial and humeral involvement, and the trapezius and rhomboids are affected as well.

Natural history. Most long thoracic palsies, particularly those from neuralgic amyotrophy or traction, improve spontaneously. Recovery is typically apparent between 6 and 12 months and continues to 24 months. It is, however, less complete than traditionally taught: in a cohort of 246 patients with neuralgic amyotrophy, approximately two-thirds still had pain or paresis at three years or more, and a quarter of idiopathic cases had recurrent attacks.

Observe before you reconstruct. The standard of care remains observation with structured rehabilitation and serial EMG for 12 to 24 months before considering reconstruction, unless the nerve is known to have been divided. Reconstructing a palsy that would have recovered is a real and avoidable harm. Counsel the patient that residual deficit is common.

Investigation. The work-up confirms the lesion, maps its extent and looks for a structural cause:

• EMG and nerve conduction studies at 4 to 6 weeks, repeated at 3 to 6 monthly intervals. Look for fibrillations in serratus anterior, and for early nascent motor units as the marker of reinnervation
• Screen the rest of the plexus - trapezius, rhomboids, supraspinatus, infraspinatus, deltoid and the anterior interosseous distribution - because neuralgic amyotrophy is patchy and the finding changes the prognosis and the counselling
• Cervical spine MRI if there is any suggestion of a root lesion
• Chest radiograph and CT where the history suggests a chest wall or apical cause, or a bony pseudo-wing
• Shoulder MRI in the chronic case, to assess for secondary cuff pathology before any reconstruction

Rehabilitation while waiting. Maintaining full passive range to prevent adhesive capsulitis is the single most important intervention in the waiting period. The rest:

• Avoid aggressive overhead strengthening into a winging scapula, which simply reinforces the compensation and can provoke impingement
• Scapular bracing is available but poorly tolerated and has limited evidence
• Treat coexisting pain, and address the trapezius and levator overload that develops

Secondary subacromial impingement. Loss of scapular posterior tilt and upward rotation narrows the subacromial space during elevation. Treating the impingement without addressing the winging is futile, and a subacromial decompression in a winging scapula is a well-recognised way to make the shoulder worse.

Scapular dyskinesis without palsy. Far commoner than true palsy. Serratus is inhibited rather than denervated, often in the context of pain, the rhomboids and levator dominate, and the EMG is normal. Treatment is exclusively rehabilitation directed at serratus and lower trapezius activation.

Snapping scapula. Serratus anterior atrophy, from denervation or from disuse, reduces the cushioning layer between the scapula and the ribs and can precipitate scapulothoracic crepitus.

Serratus anterior plane block. Local anaesthetic is deposited in the fascial plane superficial or deep to serratus anterior at the fourth or fifth rib in the mid-axillary line, blocking the lateral cutaneous branches of the intercostal nerves; it is widely used for rib fractures, thoracotomy and breast surgery. The relevance here is anatomical. The injection is made in exactly the plane in which the long thoracic nerve runs, and transient winging after the block has been reported.

Facioscapulohumeral dystrophy. Bilateral, progressive, multi-muscle winging with facial weakness, scapular elevation with attempted abduction and a characteristic "trapezius hump". No tendon transfer helps because all the donors are affected, so scapulothoracic fusion is the operation, accepting the loss of terminal elevation.

Indications. The case for transfer:

• Symptomatic winging with functional limitation, persisting beyond 12 to 24 months with no reinnervation on serial EMG
• Preserved passive scapular and glenohumeral motion - a stiff shoulder must be mobilised first
• An intact, innervated pectoralis major (medial and lateral pectoral nerves; confirm on examination)
• A patient who understands that this is a stabilisation, not a restoration of normal serratus function

Why pectoralis major. The sternal head has a line of pull that runs anteriorly and medially across the chest wall to the inferior angle, closely approximating the vector of the inferior serratus digitations. It has sufficient power, and it is expendable in the sense that the clavicular head is retained. Split transfer with the clavicular head retained is the standard; taking the entire muscle is unnecessary and costs adduction power.

Steps. The operation in order:

1. Position - supine or lateral, with the arm draped free and the whole hemithorax, including the inferior angle, prepared.
2. Harvest - an axillary or inframammary incision exposes the sternal (lower) head of pectoralis major, which is detached from the humerus with a small cuff, preserving the clavicular head and the pectoral nerves entering from the deep surface medially.
3. Graft extension - the sternal head almost never reaches the inferior angle directly, so it is extended with a graft: classically autologous fascia lata, alternatively semitendinosus/gracilis autograft or Achilles tendon allograft. The graft is tubularised and sutured to the tendon end with a robust weave.
4. Tunnel - the construct is passed deep to the chest wall soft tissues, superficial to the ribs, around the lateral chest wall to a second incision over the inferior angle of the scapula.
5. Fixation - through two or three drill holes in the inferior angle, or with suture anchors, with the graft tensioned with the scapula held in the corrected, protracted position against the chest wall and the arm at the side. Under-tensioning is the commonest error; the construct stretches.
6. Aftercare - sling or abduction brace for 6 weeks with no active protraction, then passive and active-assisted range, active work from 3 months, and unrestricted activity at 6 to 12 months.

Two series, two numbers. In the original series of split pectoralis major transfer, 10 of 11 patients (91 per cent) had a satisfactory result at a mean of 41 months, the single failure being a recurrence attributed to non-compliance with rehabilitation. A later series of 15 patients reviewed at a mean of 64 months was far more sobering. Pain decreased in 11 and function improved in 10, but Rowe results were excellent in 2, good in 5, fair in 4 and poor in 4: excellent or good in only 7 of 15, 47 per cent. Outcomes were better in those achieving at least 60 degrees of postoperative external rotation.

Quote the second figure, and understand why it differs. The two numbers describe the same operation, and three things separate them, all pushing in the same direction:

• Follow-up is 23 months longer. A fascia lata-augmented transfer fails by gradual attenuation of the graft, so a series that stops at 41 months is measuring a construct that has not yet been asked the question.
• The outcome measure changed. "Satisfactory" is a surgeon's global impression; the Rowe, Constant and ASES scores are patient-weighted and will always read lower. The two numbers are not on the same scale and should never be presented as a trend.
• Two thirds of the later cohort (10 of 15) were worker's compensation claimants, a case-mix that depresses self-reported scores independently of the anatomical result.

What to say at consent. What survives both series is the split that matters: pain relief is the more reliable gain, function the less reliable one. Say that most patients return to their previous activities with minor adaptations, and that complete pain relief with full function is not the expected result.

Other dynamic transfers. Pectoralis minor transfer to the coracoid or the scapula is less powerful and less commonly used. Combined pectoralis major and minor transfer, or rhomboid transfer, are described in small series.

Scapulothoracic fusion is the salvage procedure, and the primary procedure in facioscapulohumeral dystrophy (see Related Conditions). It abolishes winging and stabilises the base for the deltoid at the cost of scapulothoracic motion, and therefore of the last 30 to 40 degrees of elevation; that permanent loss of terminal elevation must be part of the consent. Its complications:

• Pneumothorax and rib fracture during wire or cable passage; a post-operative chest radiograph is mandatory
• Non-union and hardware problems or failure, with recurrence of pain

Static stabilisation. Fascial or synthetic tethering of the scapula to the ribs or spinous processes stretches out over time and is reserved for the low-demand or medically unfit patient.

What is taken. Only the lower three or four digitations (from ribs 5 to 8 or 9), based on the serratus branch of the thoracodorsal artery from the subscapular system. These digitations are thin, pliable and well vascularised, ideal for contouring over irregular surfaces.

The critical technical point. The serratus branch runs parallel to and immediately anterior to the long thoracic nerve on the surface of the muscle, so harvest requires the two to be separated under magnification, and the nerve branches to the upper digitations must be preserved. Taking the upper digitations, or dividing the nerve above the level of the harvest, produces a donor-site winging scapula: a complication that is entirely avoidable and entirely the surgeon's fault.

Approach. A lateral chest wall incision along the anterior border of latissimus dorsi, with the arm abducted. Latissimus is retracted posteriorly, the thoracodorsal pedicle identified in the axilla, and the serratus branch traced distally onto the muscle. The chosen digitations are detached from the ribs, taking care of the intercostal perforators and of the pleura immediately deep to the rib origins.

Applications. The flap's uses:

• Free flap for coverage of the foot, hand and scalp, where a thin, pliable, well-contoured muscle flap is needed
• Pedicled flap for the chest wall, axilla and shoulder
• Chimeric flap with latissimus dorsi, scapular or parascapular skin, and rib, all on a single subscapular pedicle - the workhorse of complex composite reconstruction
• Functional muscle transfer, transplanted with its nerve branch and neurotised, for facial reanimation and for restoring elbow or finger flexion
• Serratus fascia flap - the fascia alone, for very thin coverage such as the dorsum of the hand

Winging is the characteristic and most-quoted donor complication, following over-harvest or injury to the long thoracic nerve or its proximal branches to the upper digitations. Where it does occur it is usually mild and improves over months.

What the flap series shows. Whitney's 100 consecutive free serratus transplants recorded no scapular winging and retained full shoulder movement. That is reassuring, but read it for what it is: winging was not noted, in a flap-survival paper with no stated scapular examination protocol and no scapular outcome measure. The paper does not describe restricting the harvest, so it is not the evidence for doing so; preserving the upper digitations and their nerve branches is a technical principle taught on anatomical grounds.

Other morbidity. Beyond winging, the donor site can give:

• Pneumothorax during detachment of the digitations from the ribs - the pleura is immediately deep
• Seroma or haematoma in the lateral chest wall dead space - eight cases in Whitney's series, within a 12 per cent donor-site complication rate
• Loss of the subscapular system for any later reconstruction on that side if the pedicle is used
• Shoulder stiffness from the lateral chest wall scar; early mobilisation matters

Consent. Name winging explicitly. A patient who develops a wing after a flap harvest and was never warned about it has a legitimate grievance, and the complication is well described.