Flexor Pollicis Brevis Anatomy

Origin. The superficial head arises from the distal border of the flexor retinaculum (transverse carpal ligament) and the tubercle of the trapezium; it is the larger head and lies ulnar to abductor pollicis brevis. The deep head arises from 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, a figure given with the innervation data in the next tab.

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 is that the radial sesamoid receives flexor pollicis brevis and the ulnar sesamoid receives adductor pollicis; it 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. The order also gives the nerve of each:

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)

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The textbook pattern. The superficial head is supplied by the recurrent (thenar) motor branch of the median nerve and the deep head by the deep branch of the ulnar nerve, both from 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. Even the superficial head is not reliably pure median, and the absence of the deep head is itself a variant.

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. 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. It has been found in a meaningful minority of hands at dissection: one anatomical study of 68 palmar regions from 34 cadavers found an ulnar-median anastomosis in 13.

Why it confuses. The anastomosis 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. Preserved thumb function therefore does not exclude a complete median nerve division: test sensation, test abductor pollicis brevis specifically, and if the picture remains inconsistent, name the anastomosis rather than doubting the injury.

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. Riche-Cannieu has cousins at other levels that produce the same category of surprise, and Berrettini communications do it for sensation:

Arterial supply. The principal supply to the superficial head is the superficial palmar arch and its branches. 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, and the deep palmar arch contributes to the deep head as well.

The superficial palmar branch of the radial artery crosses the proximal thenar region and supplies the proximal part of the muscle. It is the vessel encountered and often ligated in the extended volar approach to the scaphoid, and the one most often met at the radial end of a carpal tunnel incision extended proximally; recognise and ligate it rather than avulse it.

Why it matters. The thenar eminence is richly vascularised, which is why thenar space infections spread readily and why a thenar flap for fingertip reconstruction is reliable.

Around the muscle. Radial and superficial lie abductor pollicis brevis and then skin, with opponens pollicis deep to APB along the metacarpal. Ulnar and deep lies 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 runs the flexor pollicis longus tendon, and proximally lie 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. 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 was 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 (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 are Poisel's, not Lanz's. The percentages quoted for the course of the thenar branch come from 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.

• 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 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.

Thenar wasting in carpal tunnel syndrome. Advanced median compression wastes 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.

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, and in a transligamentous branch the nerve crosses the very structure being divided. A bifid nerve is at particular risk during a limited-incision or endoscopic technique; 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. 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. The deficit is 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 is 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 is by opponensplasty, with the donor chosen by lesion level as set out under Surgical Relevance.

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. The donors that serve a low palsy are therefore gone; the alternatives are set out under Surgical Relevance.

The all-ulnar hand. In its complete form, a Riche-Cannieu anastomosis supplies all the intrinsic muscles from the ulnar nerve, and a complete median nerve lesion then produces sensory loss with essentially no intrinsic motor deficit. Electrodiagnostically, 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 is to 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 are adductor pollicis, the first dorsal interosseous and 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.

The surgery. All three limbs are usually needed:

1. Release the deforming forces - release or lengthen adductor pollicis and the flexor pollicis brevis origin from the flexor retinaculum and trapezium, 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

The dissection runs directly across the territory of the recurrent motor branch, which must be protected throughout along with the neurovascular bundles. Counsel that 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 is a potential space deep to the flexor tendons of the index finger and superficial to adductor pollicis, bounded ulnarly by the midpalmar septum. An abscess there presents as a painful, swollen thenar eminence with the thumb held abducted, a tense first web space and pain on passive thumb adduction, and requires formal drainage. Drainage may be through a volar (thenar crease) incision, a dorsal first web incision, or both; avoid crossing the first web space transversely, because a contracture there is disabling, and protect the recurrent motor branch and the digital nerves.

Thumb 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 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.

Every rule of open carpal tunnel release derives from the variability of the recurrent motor branch and of the median nerve itself. You cannot know in advance which patient has a transligamentous branch, so the technique is built to make the variation irrelevant.

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. This line 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. Never cross the wrist crease transversely; cross obliquely if extension is required.

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, protecting the contents with a blunt instrument passed beneath the ligament. 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. Confirm complete distal release by seeing the fat pad of the superficial palmar arch beyond the distal edge of the ligament, and complete proximal release by passing an instrument freely into the distal forearm; never cut what you cannot see.

Inspection. Look for the Lanz variants, a bifid nerve, a persistent median artery and accessory branches at the distal tunnel or proximal to it, 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, chosen by lesion level. Check what is still innervated before choosing a donor. In a low median palsy, flexor digitorum superficialis and palmaris longus are available, because both are median-innervated muscles that remain functional above the lesion. 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, which also restores thenar bulk and is attractive in children. 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. Transfers such as flexor digitorum superficialis of the middle or ring finger to adductor pollicis, routed through the second or third intermetacarpal space, or an extensor-based transfer; first dorsal interosseous reinforcement with an extensor-based transfer is added where index abduction is required for pinch stability. Where the thumb metacarpophalangeal joint collapses into hyperextension, arthrodesis of the joint converts a collapsing pinch into a strong one and is often more valuable than adding a motor.