Serratus Anterior

Origin

• Fleshy digitations 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 digitations interdigitate with the origin of external oblique - a constant and much-loved anatomical detail that also marks the plane at which the lower digitations are separated during flap harvest.

Insertion (all on the COSTAL - deep - surface of the medial border)

• Superior part: ribs 1 and 2 to a triangular impression on the costal surface of the superior angle.
• Intermediate part: ribs 2 to 4 as 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. This part carries most of the muscle mass and generates most of the rotation.

The consequence of a costal-surface insertion

Because the whole insertion is on the deep (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

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

Long thoracic nerve (nerve of Bell) - the sole supply

• Root value: C5, C6 and C7, with a C8 contribution described in a minority.
• Origin: directly from the ventral rami of the roots, in the neck, proximal to the formation of the trunks. Like the dorsal scapular nerve, this makes it a proximal marker in plexus injury.
• Relationship to the scalenes - the classic examination point:
  • The C5 and C6 contributions pierce the middle scalene muscle (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, crosses the outer border of the first rib (or over the second rib), and 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 skin and subcutaneous fat.
• Length: approximately 22 to 24 cm - the longest motor nerve of the upper limb relative to its calibre and the most superficially placed along its course.
• Motor entry: branches leave the nerve sequentially 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 this course matters

A nerve that is long, thin, unaccompanied by a substantial artery, tethered proximally at the scalene and distally by its muscular branches, and lying directly under the fascia on the side of the chest is uniquely vulnerable to:

• Traction (sudden depression of the shoulder with the head forced away, heavy backpack, prolonged surgical positioning).
• Direct injury (axillary node dissection, first rib resection, chest drain insertion, thoracoscopic ports, mastectomy, transaxillary approaches).
• Compression at the middle scalene or beneath the fascial band at the level of the second rib.
• Immune-mediated neuritis (neuralgic amyotrophy), for which it is one of the most frequently affected nerves.

Named vessels

• Lateral thoracic artery (from the second part of the axillary artery) - supplies 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) - supplies the lower three or four digitations. This is the pedicle of the serratus anterior flap, and it is the reason the lower digitations are the ones harvested.
• Perforating branches of the posterior intercostal arteries - segmental supply that must be divided when the muscle is islanded.
• Thoracoacromial artery contributes to the uppermost part in some individuals.

Pedicle position - the flap surgeon's landmarks

• The thoracodorsal artery divides into a branch to latissimus dorsi and a serratus branch at a variable point, typically within a few centimetres of the origin of the thoracodorsal artery 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 serratus flap harvest: the artery you want and the nerve you must preserve travel together. They must be separated under loupe magnification.
• The subscapular-thoracodorsal-serratus axis provides 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, draining into the axillary vein.

Superficial (lateral) to serratus anterior

• Deep fascia and the long thoracic nerve running in the mid-axillary line.
• Latissimus dorsi posteriorly, pectoralis major and minor anteriorly, and in the axilla the axillary contents including the thoracodorsal and lateral thoracic vessels and the axillary lymph nodes.
• The muscle forms the medial wall of the axilla together with the upper ribs and intercostal muscles.

Deep (medial) to serratus anterior

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

Between serratus anterior and the scapula

• Subscapularis on the costal surface of the scapula, separated from serratus by the supraserratus bursa. Serratus and subscapularis form the two gliding layers of the scapulothoracic articulation.

Structures at risk

• Long thoracic nerve in the mid-axillary line, over the second to fifth ribs - the structure that 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 - the sensory nerve whose division gives inner-arm numbness after axillary clearance.
• Pleura, immediately deep to the muscle and its ribs.

Non-traumatic

• Neuralgic amyotrophy (Parsonage-Turner syndrome) - the commonest single cause. Severe continuous pain lasting around four weeks on average, followed by painless weakness as the pain settles. 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.
• 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

• 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 - the classic surgical cause.
• 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.

Non-neurogenic causes of winging that must be excluded

• Scapular osteochondroma or exostosis (a fixed pseudo-wing).
• Malunited scapular or rib fracture.
• Glenohumeral instability and painful cuff disease producing dyskinetic pseudo-winging.
• Facioscapulohumeral muscular dystrophy - bilateral, with facial and humeral involvement, and with the trapezius and rhomboids also affected.

Natural history

Most long thoracic palsies, particularly those from neuralgic amyotrophy or traction, improve spontaneously, and recovery is typically apparent between 6 and 12 months, continuing to 24 months. Recovery 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. The standard of care remains observation with structured rehabilitation for 12 to 24 months before considering reconstruction, unless the nerve is known to have been divided - but counsel the patient that residual deficit is common.

Investigation

• 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, 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

• Maintain full passive range to prevent adhesive capsulitis - this is the single most important intervention in the waiting period.
• 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, and the rhomboids and levator dominate. 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

A regional anaesthetic technique in which 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; scapulothoracic fusion is the operation, accepting the loss of terminal elevation.

Indication

• 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 of pectoralis major 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.

Steps

• Position: supine or lateral, with the arm draped free and the whole hemithorax including the inferior angle prepared.
• 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.
• 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.
• 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.
• 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.
• 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.

Outcomes

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 - with better outcomes in those achieving at least 60 degrees of postoperative external rotation.

Quote the second figure, and understand why it is different. Ninety-one and forty-seven per cent describe the same operation, and three things separate them, all of which push 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. These 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 survives both series is the split that matters at consent: 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 - less powerful and less commonly used.
• Combined pectoralis major and minor, or rhomboid transfer, are described in small series.
• Split pectoralis major with the clavicular head retained is the standard; taking the entire muscle is unnecessary and costs adduction power.

Scapulothoracic fusion

• The salvage procedure, and the primary procedure in facioscapulohumeral dystrophy where all periscapular donors are affected.
• 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.
• Complications: pneumothorax, rib fracture, non-union and hardware problems. A post-operative chest radiograph is mandatory.

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 of the thoracodorsal artery runs parallel to and immediately anterior to the long thoracic nerve on the surface of the muscle. Harvest requires them 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

• 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

The characteristic and most-quoted donor complication. It follows either taking too many digitations or injuring the long thoracic nerve or its proximal branches. Restricting the harvest to the lower three or four digitations with meticulous nerve preservation keeps the incidence very low - a 100-case consecutive series reported no winging at all and full retained shoulder movement; where winging does occur it is usually mild and improves over months.

Other morbidity

• Pneumothorax during detachment of the digitations from the ribs - the pleura is immediately deep.
• Seroma in the lateral chest wall dead space.
• Loss of the subscapular system for future reconstruction 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.