Flexor Pollicis Brevis Anatomy

Origin - two heads

Superficial head

• The distal border of the flexor retinaculum (transverse carpal ligament).
• The tubercle of the trapezium.
• It is the larger head and lies superficially, ulnar to abductor pollicis brevis.

Deep head

• The trapezoid and capitate, and the adjacent palmar ligaments of the distal carpal row.
• It is smaller, sometimes considered part of the adductor pollicis complex, and is absent in a proportion of hands - reported absent in 14 per cent of 60 dissected limbs.

The relationship between the heads

• The two heads converge on a common insertion, and the flexor pollicis longus tendon passes BETWEEN them on its way from the carpal tunnel to the thumb. This is the anatomical basis of thumb zone III, the thenar segment in which a divided FPL retracts and must be retrieved.

Insertion

• The radial side of the base of the proximal phalanx of the thumb, through the RADIAL SESAMOID of the metacarpophalangeal joint.
• A slip continues into the extensor apparatus of the thumb, giving FPB a secondary role in interphalangeal extension in the same way the lumbricals act in the fingers.
• The sesamoid rule: the radial sesamoid receives flexor pollicis brevis and the ulnar sesamoid receives adductor pollicis. This matters in thumb metacarpophalangeal collateral injury, where the sesamoid position on radiographs indicates the integrity of the volar plate and sesamoid complex.

The thenar layers - superficial to deep

1. Abductor pollicis brevis - most superficial and most radial; purely median in almost all hands.
2. Flexor pollicis brevis, superficial head - deep and ulnar to APB; median (recurrent branch).
3. Opponens pollicis - deep to APB, inserting along the shaft of the thumb metacarpal; median.
4. Flexor pollicis brevis, deep head - deeper still; ulnar (deep branch).
5. Adductor pollicis - deepest and most ulnar, with transverse and oblique heads; ulnar (deep branch).

The textbook pattern

• Superficial head: the RECURRENT (thenar) MOTOR BRANCH of the median nerve, roots C8 and T1.
• Deep head: the DEEP BRANCH of the ulnar nerve, roots C8 and T1.

The actual pattern, from dissection

A study of 60 limbs from 30 cadavers reported:

The authors' conclusion is worth quoting in a viva: the pattern that should be considered normal is a superficial head supplied by the median nerve and a deep head supplied by BOTH the ulnar and the median nerve.

The Riche-Cannieu anastomosis

• A communication between the deep branch of the ulnar nerve and the recurrent (thenar) branch of the median nerve, lying within the substance of the thenar muscles, often around the deep head of FPB and adductor pollicis.
• Clinical significance: where it is substantial, all or part of thenar motor function can be preserved by the ulnar nerve despite a complete median nerve lesion - the all-ulnar hand. The reverse also occurs, with median fibres supplying muscles normally regarded as ulnar.
• Why it matters: it produces clinical, surgical and electrodiagnostic findings that appear contradictory. A reported case described a very long anastomosis running from the proximal deep ulnar branch destined for adductor pollicis to a ramus of the recurrent median branch supplying the superficial head of FPB.
• The examination point: if asked why a patient with a divided median nerve at the wrist still has thumb function, the answer has three parts - the ulnar-innervated deep head of FPB, the ulnar-innervated adductor pollicis substituting for a lost thenar motor unit, and a Riche-Cannieu anastomosis.

The family of anastomoses

Arterial supply

• The superficial palmar arch and its branches - the principal supply to the superficial head.
• The princeps pollicis artery, a branch of the radial artery in the first web space, and the radialis indicis contribute to the deep head and the adjacent adductor.
• The superficial palmar branch of the radial artery, which crosses the proximal thenar region, supplies the proximal part of the muscle and is the vessel encountered and often ligated in the extended volar approach to the scaphoid.
• The deep palmar arch contributes to the deep head.

Practical points

• The thenar eminence is richly vascularised, which is why thenar space infections spread readily and why a thenar flap for fingertip reconstruction is reliable.
• The superficial palmar branch of the radial artery is the vessel most often encountered at the radial end of a carpal tunnel incision extended proximally - recognise and ligate rather than avulse.
• The thenar space is a potential space deep to the flexor tendons of the index finger and superficial to adductor pollicis, bounded ulnarly by the midpalmar septum. A thenar space abscess produces a swollen, abducted thumb with a tense first web space and requires formal drainage.

Around the muscle

• Radial and superficial: abductor pollicis brevis, then skin. Opponens pollicis lies deep to APB along the metacarpal.
• Ulnar and deep: adductor pollicis, with the deep branch of the ulnar nerve and the deep palmar arch running transversely across the palm deep to the flexor tendons and superficial to the interossei and the metacarpals.
• Between the heads: the flexor pollicis longus tendon.
• Proximal: the distal border of the transverse carpal ligament and the distal end of the carpal tunnel, from which the recurrent motor branch emerges.

The recurrent motor branch - where it is and what varies

• It arises from the radial side of the median nerve at or just distal to the distal border of the transverse carpal ligament, and turns sharply radially and proximally (hence recurrent) to enter the deep surface of the superficial head of FPB and the adjacent thenar muscles.
• The Lanz classification of median nerve variation in the carpal tunnel, derived from 246 operatively explored carpal tunnels with 29 variations identified:
  • Group I: variation in the course of the thenar branch.
  • Group II: accessory branches at the distal portion of the carpal tunnel (found in 18 hands in the original series).
  • Group III: high division of the median nerve (7 hands) - the bifid median nerve, often with a persistent median artery.
  • Group IV: accessory branches proximal to the carpal canal (4 hands).
• The course variants - Poisel's 100 cadaver hands, quoted inside Lanz's paper and almost always mis-credited to Lanz himself. Lanz's own material is the 246 operative explorations above; these percentages are a separate cadaveric series:
  • Extraligamentous 46 per cent - the branch leaves distal to the ligament and curves back over its distal edge. The classical and safest pattern.
  • Subligamentous 31 per cent - the branch arises within the tunnel and runs distally beneath the ligament before turning radially.
  • Transligamentous 23 per cent - the branch pierces the transverse carpal ligament to reach the thenar muscles. This is the variant that is divided by a knife passed blindly through the ligament in its radial half.
• The conclusion of that paper is the operative rule: approach the median nerve from the ULNAR side when opening the carpal tunnel.

Surface landmarks

• Kaplan cardinal line: drawn from the apex of the first web space, parallel to the proximal palmar crease, toward the hook of the hamate. The recurrent motor branch and the superficial palmar arch lie in relation to this line, and it is the standard reference for the distal limit of a carpal tunnel incision.
• The safe incision axis is in line with the radial border of the ring finger - ulnar to the palmaris longus axis proximally and ulnar to the recurrent branch distally.

Thenar wasting in carpal tunnel syndrome

• Advanced median compression produces wasting of the median-innervated thenar muscles - abductor pollicis brevis, opponens pollicis and the superficial head of flexor pollicis brevis - while the ulnar-innervated deep head and adductor pollicis are spared.
• The clinical picture is a flattened, hollowed thenar eminence with loss of palmar abduction and opposition, with sensory symptoms that may by then have become constant or, in long-standing cases, have paradoxically diminished.
• Recovery of motor function after release is unreliable in the presence of established wasting and an absent motor response - which is the indication to consider a simultaneous opponensplasty.

The safe zone for carpal tunnel release

Every rule derives from the variability of the recurrent motor branch and of the median nerve itself:

1. Incision in line with the radial border of the ring finger, that is, ULNAR to the palmaris longus axis proximally. This keeps away from the median nerve (deep and radial to palmaris longus), from its palmar cutaneous branch (which runs between palmaris longus and flexor carpi radialis) and from the recurrent motor branch (which lies radially).
2. Do not cross the wrist crease transversely. Cross obliquely if extension is required.
3. Divide the transverse carpal ligament at its ULNAR margin, immediately radial to the hook of the hamate. This is the single most protective step: it moves the plane of division away from both the median nerve and every configuration of the recurrent branch.
4. Visualise the distal edge of the ligament and the fat pad of the superficial palmar arch to confirm complete distal release, and pass an instrument freely into the distal forearm to confirm proximal release.
5. Be alert to the Lanz variants: accessory branches at the distal tunnel, a high division of the nerve (bifid median nerve, often with a persistent median artery) and accessory proximal branches. A bifid nerve is at particular risk during a limited-incision or endoscopic technique.

Endoscopic and limited-incision techniques

• These reduce scar tenderness and may speed return to work, but they rely on the surgeon not seeing the recurrent branch. In a transligamentous branch - 23 per cent of hands - the branch crosses the very structure being divided.
• Preoperative ultrasound identifies a bifid median nerve and a persistent median artery and is a reasonable adjunct where a limited technique is planned.
• Where anatomy is atypical, revision surgery is planned, or a mass is suspected, open release with direct visualisation is the safe choice.

Revision carpal tunnel release

• Indications include persistent, recurrent or new symptoms. New motor loss with intact sensation after a release should raise immediate suspicion of a recurrent branch injury.
• At revision the nerve is identified in virgin tissue proximal to the previous incision, then followed distally through the scar.
• A hypothenar fat pad flap is a recognised adjunct for coverage of a scarred median nerve; the ulnar bundle must be protected during its elevation.

Low median nerve palsy

• Deficit: loss of thumb palmar abduction and opposition, thenar wasting, and sensory loss in the thumb, index, middle and radial half of the ring finger, including the palm if the lesion is proximal to the palmar cutaneous branch.
• What is preserved: the ulnar-innervated deep head of FPB and adductor pollicis, so a degree of thumb flexion and adduction remains - and where a Riche-Cannieu anastomosis exists, far more than expected.
• Reconstruction: opponensplasty. In a low lesion, flexor digitorum superficialis of the ring finger (Bunnell, or the Royle-Thompson modification) or palmaris longus with a strip of palmar fascia (Camitz) are available, because both are median-innervated muscles that remain functional above the lesion.

High median nerve palsy

• Adds loss of the extrinsic median muscles - pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor pollicis longus, index and middle profundus and pronator quadratus.
• Neither FDS nor palmaris longus is available as a donor. Use extensor indicis proprius routed around the ulnar border of the wrist, or an abductor digiti minimi (Huber) transfer, which also restores thenar bulk and is attractive in children.

Ulnar palsy and the thumb

• Loss of adductor pollicis, the first dorsal interosseous and the deep head of FPB produces a weak key pinch with the Froment sign (thumb IP flexion substituting) and the Jeanne sign (MCP hyperextension).
• Reconstruction of key pinch uses transfers such as FDS of the middle or ring finger to adductor pollicis routed through an intermetacarpal space, or an extensor-based transfer, sometimes combined with thumb metacarpophalangeal arthrodesis to eliminate the collapsing joint.

The all-ulnar hand

• In its complete form, a Riche-Cannieu anastomosis supplies all the intrinsic muscles from the ulnar nerve. A complete median nerve lesion then produces sensory loss with essentially no intrinsic motor deficit.
• Electrodiagnostic consequence: stimulating the median nerve produces little or no thenar response even in an intact nerve, and stimulating the ulnar nerve produces a thenar response. Without knowing the anastomosis, this looks like a severe median lesion in a normal hand, or a normal hand in a severe lesion, depending on which way the error falls.
• The correct response in a viva: name the anastomosis, explain that it is a normal variant rather than pathology, and stress that clinical correlation and testing of APB specifically must override an isolated confusing electrodiagnostic result.

Thumb-in-palm deformity

• Seen in spastic cerebral palsy and after stroke or brain injury. The thumb is held flexed and adducted across the palm, obstructing grasp, hygiene and appearance.
• The deforming forces: adductor pollicis, the first dorsal interosseous, flexor pollicis brevis and, where the interphalangeal joint is also flexed, flexor pollicis longus. Weakness or absence of the extensor and abductor forces compounds it.
• Assessment must define which muscles are tight and whether the deformity is passively correctable, whether the metacarpophalangeal joint is unstable in hyperextension, and whether the patient has voluntary control and useful sensation.
• Surgical principles - all three limbs are usually needed:
  1. Release the deforming forces: release or lengthen adductor pollicis and the flexor pollicis brevis origin, and release the first dorsal interosseous origin as required, with a first web space release or Z-plasty for a contracted web.
  2. Augment the weak forces: transfer to reinforce extensor pollicis longus or abductor pollicis longus.
  3. Stabilise the unstable joint: metacarpophalangeal joint capsulodesis or arthrodesis where the joint hyperextends.
• Counselling: appearance, hygiene and passive positioning improve reliably; functional grasp improves only in patients with voluntary control and preserved sensation.

Thenar space infection

• The thenar space lies deep to the index flexor tendons and superficial to adductor pollicis, bounded ulnarly by the midpalmar septum.
• Presentation: a painful, swollen thenar eminence with the thumb held abducted, a tense first web space, and pain on passive thumb adduction.
• Drainage may be through a volar (thenar crease) incision, a dorsal first web incision, or both. Avoid crossing the first web space transversely - a contracture there is disabling. Protect the recurrent motor branch and the digital nerves.

Thenar hypoplasia and congenital variants

• Thumb hypoplasia (Blauth classification) includes deficiency of the thenar musculature. Blauth type II, with thenar hypoplasia, an unstable ulnar collateral ligament and a first web contracture, is treated by opponensplasty, ulnar collateral ligament reconstruction and web release; type IIIB and above, with an unstable carpometacarpal joint, is treated by pollicisation.
• Absence of the deep head of FPB is a recognised isolated variant, reported in 14 per cent of dissected limbs, and is of no functional consequence.

Basal joint arthritis and the thenar muscles

• The thenar intrinsics, FPB among them, contribute to the adduction and metacarpophalangeal hyperextension deformity of advanced thumb carpometacarpal arthritis.
• Where metacarpophalangeal hyperextension exceeds about 30 degrees at the time of trapeziectomy, it should be addressed - by volar capsulodesis, sesamoid arthrodesis or metacarpophalangeal arthrodesis - or the deformity will persist and compromise the arthroplasty.

Open carpal tunnel release - the anatomy of each step

Incision. Longitudinal, in line with the radial border of the ring finger, from just distal to the distal wrist crease toward Kaplan cardinal line, 3 to 4 cm. Never transverse across the crease.

• Why this line: it is ulnar to the palmaris longus axis, and therefore ulnar to the median nerve (deep and radial to palmaris longus), ulnar to the palmar cutaneous branch (which runs between palmaris longus and flexor carpi radialis) and ulnar to every configuration of the recurrent motor branch.

Superficial dissection. Through skin and subcutaneous fat to the palmar fascia, then through the palmar fascia in line with the incision. Small palmar cutaneous twigs crossing the field should be preserved where possible.

Division of the transverse carpal ligament.

• Divide at its ULNAR margin, immediately radial to the hook of the hamate, under direct vision.
• Protect the contents with a blunt instrument passed beneath the ligament.
• Confirm complete DISTAL release by seeing the fat pad of the superficial palmar arch beyond the distal edge of the ligament; confirm complete PROXIMAL release by passing an instrument freely into the distal forearm.

Inspection. Look for the Lanz variants - a bifid nerve, a persistent median artery, accessory branches at the distal tunnel - and for a space-occupying lesion such as tenosynovium, a ganglion or an anomalous muscle belly.

Closure. Skin only. Do not repair the ligament.

If the recurrent branch is seen

• An extraligamentous branch curving back over the distal edge is the common and reassuring finding.
• A transligamentous branch encountered piercing the ligament must be protected and the ligament divided around it, ulnar to its course.
• If a branch is inadvertently divided, repair it primarily under magnification at the same sitting - this is far more likely to give a useful result than delayed reconstruction, and delayed diagnosis is the usual reason it is not done.

Opponensplasty - choosing by lesion level

The rule that ties them together

Check what is still innervated before choosing a donor. In a low median palsy, FDS and palmaris longus are available. In a high median palsy they are denervated and the donor must come from outside the median territory - extensor indicis proprius or abductor digiti minimi. Choosing an FDS opponensplasty in a high median palsy is one of the most reliably punished errors in an upper limb viva.

Restoring key pinch in ulnar palsy

• FDS of the middle or ring finger to adductor pollicis, routed through the second or third intermetacarpal space.
• Thumb metacarpophalangeal arthrodesis where the joint collapses into hyperextension - stabilising the joint converts a collapsing pinch into a strong one and is often more valuable than adding a motor.
• First dorsal interosseous reinforcement with an extensor-based transfer where index abduction is required for pinch stability.

Thumb-in-palm surgery

• Release of the adductor pollicis and the flexor pollicis brevis origin from the flexor retinaculum and trapezium, plus first dorsal interosseous release and web space Z-plasty as required.
• Augment the extensor and abductor side by transfer.
• Stabilise the metacarpophalangeal joint with capsulodesis or arthrodesis where it hyperextends.
• Protect the recurrent motor branch and the neurovascular bundles throughout - the dissection is directly across the territory of the recurrent branch.