Adductor Pollicis Anatomy

Transverse head (larger, distal). Arises from the palmar (anterior) surface of the distal two-thirds of the shaft of the third metacarpal, along a ridge that runs from just distal to the base to the metacarpal neck. Its fibres run transversely and radially, converging on the thumb.

Oblique head (proximal, deeper). Arises from the capitate, the palmar carpal ligaments and the bases of the second and third metacarpals, frequently with a slip from the sheath of the flexor carpi radialis tendon and the trapezoid, and with some fibres from the deep transverse metacarpal ligament complex. A small deep slip passes to the radial sesamoid and blends with flexor pollicis brevis; this is the source of much of the confusion about which "head" of FPB is ulnar-innervated.

Insertion. The two heads converge into a single tendon containing the ulnar sesamoid of the thumb metacarpophalangeal (MCP) joint, and insert on the ulnar side of the base of the proximal phalanx and on the sesamoid itself. A broad fibrous sheet, the adductor aponeurosis, sweeps dorsally over the ulnar aspect of the MCP joint to blend with the extensor pollicis longus (EPL) hood, which is why adductor pollicis contributes to thumb IP extension and why its aponeurosis lies exactly superficial to the UCL.

The gap between the heads. Distally the two heads are separated by a triangular defect through which the radial artery passes from the first dorsal interosseous space into the palm to complete the deep palmar arch, and through which the princeps pollicis artery arises.

The deep branch. The ulnar nerve divides in Guyon canal into a superficial branch, mainly sensory plus palmaris brevis, and a deep motor branch. The deep branch turns dorsally and radially around the hook of hamate, passes between abductor digiti minimi and flexor digiti minimi brevis, pierces opponens digiti minimi or its fibrous arch, and then arcs radially across the palm deep to the long flexor tendons and lumbricals but superficial to the interossei and the metacarpals, travelling with the deep palmar arch.

Its territory. On the way it supplies the hypothenar three (abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi), the ulnar two lumbricals (ring and little), the palmar interossei (three or four) and all four dorsal interossei, and the deep head of flexor pollicis brevis: everything intrinsic in the hand except the radial two lumbricals and the superficial thenar three. It terminates in adductor pollicis, usually entering the deep (dorsal) surface of the oblique head first, at about the level of the third metacarpal base to mid-shaft, and sending a distal twig into the transverse head.

Roots and localisation. C8 and T1, via the lower trunk and medial cord. Adductor pollicis is therefore weak in:

• Lower trunk plexopathy
• True neurogenic thoracic outlet syndrome
• C8/T1 radiculopathy
• Cubital tunnel syndrome
• Guyon canal zone 1 or zone 2 lesions
• Motor neurone disease

It is normal in isolated median nerve palsy, the single most useful discriminator at the bedside. An ulnar palsy patient can still make a fist but cannot pinch; a median palsy patient can still pinch, weakly, with adductor, but cannot oppose.

Variations. A Riche-Cannieu anastomosis between the recurrent median motor branch and the deep ulnar branch in the thenar region can leave adductor pollicis apparently median-supplied, the so-called "all-ulnar hand" in reverse. Prevalence in cadaveric series is high, frequently quoted around 50-77% for some communicating branch, though a functionally complete crossover is rare. It explains an occasional patient with a complete ulnar nerve division who retains a negative Froment sign.

Arteries. The dominant contribution is the deep palmar arch, formed by the terminal radial artery as it passes between the heads of the first dorsal interosseous and then between the two heads of adductor pollicis. The princeps pollicis artery arises from the radial artery as it turns between the adductor heads and runs along the ulnar border of the first metacarpal deep to the oblique head; the radialis indicis artery, often a common trunk with princeps pollicis, supplies the transverse head en route to the radial side of the index. Additional twigs come from the common palmar digital arteries of the superficial arch. Venae comitantes accompany the deep arch and drain to the deep palmar venous arch and thence to the radial and ulnar venae comitantes.

The pedicle. It enters the muscle from its deep (dorsal) surface near the third metacarpal, in company with the deep ulnar motor branch. Any release of the transverse head from the third metacarpal shaft must therefore be subperiosteal and from radial to ulnar, hugging bone, or the arch is at risk.

As a flap. Adductor pollicis has been described as a local muscle flap for coverage of first web and thenar defects and as a source of vascularised tissue, but the constant intimacy of arch and motor nerve means the donor site is unforgiving.

Palmar to it. The long flexor tendons of the index and middle fingers with their lumbricals, and the superficial palmar arch with the common digital nerves and vessels. Distally the thenar muscles (abductor pollicis brevis, flexor pollicis brevis, opponens pollicis) lie radial and superficial to adductor pollicis, separated from it by the thenar septum and the adductor–thenar fascial plane.

Dorsal to it. The interossei and the shafts of the second and third metacarpals. The first dorsal interosseous lies immediately radial and dorsal, so the two muscles sandwich the first web.

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The fascial spaces. The midpalmar space lies palmar to the interossei and the third and fourth metacarpals, deep to the flexor tendons; adductor pollicis forms part of its radial boundary at the thenar septum, which attaches to the third metacarpal along the origin of the transverse head. The thenar space lies radial to the septum, palmar to adductor pollicis. The septum separates the two spaces and dictates which incision drains which.

High versus low lesion. In a high lesion, at the elbow or cubital tunnel, adductor pollicis, all interossei, the ulnar two lumbricals, the hypothenar muscles and flexor digitorum profundus (FDP) to ring and little are affected; clawing is less marked because the FDP is also weak, the ulnar paradox. In a low lesion, at the wrist or Guyon canal, FDP is spared, so the profundus pulls the distal interphalangeal joints into flexion against paralysed intrinsics and clawing is more marked. Froment sign is present in both.

Recovery order. Because adductor pollicis is the terminal muscle of the deep branch, it is the last to reinnervate. A persistent Froment sign at 12-18 months after a proximal repair does not mean failure of the repair, but it is a poor prognostic sign for pinch.

Electrodiagnosis. The first dorsal interosseous and abductor digiti minimi are the conventional recording sites, but an adductor pollicis compound muscle action potential is useful when the deep branch is selectively involved. A normal abductor digiti minimi response with abnormal first dorsal interosseous and adductor pollicis responses localises the lesion to the deep motor branch distal to the hypothenar takeoff: Guyon canal zone 2 or a distal palmar lesion.

The anatomy. The ulnar collateral ligament of the thumb MCP joint runs from the metacarpal head to the ulnar base of the proximal phalanx, deep to the adductor aponeurosis, and the proximal edge of the aponeurosis sits over the mid-portion of the ligament.

The lesion. In forced radial deviation ("skier's thumb", "gamekeeper's thumb") the UCL usually avulses distally from the base of the proximal phalanx. If the tear is complete the torn distal stump retracts proximally, and as the thumb springs back into position the proximal edge of the adductor aponeurosis slides distally and comes to lie between the retracted ligament and its bony insertion. The ligament now sits superficial to the aponeurosis and can never reach its footprint: the Stener lesion (Bertil Stener, 1962), reported in roughly 64-87% of complete UCL ruptures in the original and subsequent series. It cannot heal with immobilisation and is the principal indication for operative repair of an acute complete UCL tear.

Detection. A palpable tender lump proximal and ulnar to the MCP joint, the "Stener nodule", is the displaced ligament with the aponeurosis edge. Obtain radiographs first to exclude a displaced avulsion fracture, then stress the joint:

• Greater than 30-35 degrees of radial opening in 30 degrees of MCP flexion, or greater than 15-20 degrees more than the contralateral side, indicates complete rupture
• Test in both extension, which tests the accessory collateral and volar plate, and 30 degrees flexion, which tests the proper collateral

Ultrasound in experienced hands and MRI both identify the displaced stump; the classic MRI appearance is the "yo-yo on a string", the balled-up ligament proximal to the aponeurosis edge.

Repair. Through a dorsoulnar incision over the MCP joint, identify and protect the dorsal branches of the radial nerve crossing the field; they are the commonest iatrogenic casualty. Incise the adductor aponeurosis 2-3 mm dorsal to its junction with the EPL hood, leaving a cuff for repair, and the retracted UCL is found immediately beneath it. Reattach the ligament to the phalangeal footprint with a suture anchor or bone tunnel, then repair the aponeurosis: failure to repair it leaves a weak, painful ulnar-sided MCP.

First web space (adduction) contracture. A shortened adductor pollicis plus first dorsal interosseous fascia narrows the first web and rotates the thumb into the palm. The causes:

• Spasticity: cerebral palsy, stroke, traumatic brain injury, the thumb-in-palm deformity
• Ulnar nerve palsy: the paralysed adductor fibroses in a shortened position while the thumb sits adducted
• Trauma and burns: direct scarring, compartment syndrome of the adductor compartment, Volkmann-type ischaemic contracture
• Post-traumatic: malunited first or second metacarpal fractures, prolonged immobilisation in adduction
• Dupuytren disease of the first web (the commissural and proximal commissural ligaments), and arthrogryposis

House classification of the thumb-in-palm deformity, for spasticity:

• Type I: simple metacarpal adduction, MCP and IP normal
• Type II: metacarpal adduction with MCP flexion
• Type III: metacarpal adduction with MCP hyperextension or instability
• Type IV: metacarpal adduction with MCP and IP flexion

Skin, muscle or joint. With the wrist and MCP joints in neutral, passively abduct the thumb. If the web opens fully under anaesthesia but not awake, the problem is spasticity or tone. If it does not open under anaesthesia but the skin is supple, the problem is muscle and fascia, and the adductor is released. If the skin blanches and tethers as a taut band, a skin lengthening procedure is added (Z-plasty, four-flap Z-plasty or a dorsal rotation flap). If the trapeziometacarpal joint itself will not move, the problem is joint contracture, which needs capsulotomy or arthrodesis.

Adductor compartment syndrome. Adductor pollicis occupies its own fascial compartment in the ten-compartment model of the hand (thenar, hypothenar, adductor, four dorsal interosseous and three palmar interosseous compartments). A missed adductor compartment syndrome after crush injury or high-pressure injection produces a fixed adduction contracture. Release is through a dorsal incision over the first web, separate from the thenar and dorsal interosseous releases.

Infection. Thenar space and first web space infections, the "collar-stud" abscess through the first web, present with a swollen, abducted thumb and a full first web: the abscess pushes the thumb out, in contrast with the adducted posture of a contracture. Drainage is usually combined dorsal and palmar.

Tumour and mass. Deep palmar lipoma, giant cell tumour of the tendon sheath and neurogenic tumours of the deep motor branch present as painless first web fullness with progressive loss of pinch, and are commonly mistaken for early ulnar neuropathy at the elbow. A progressive, painless, purely motor ulnar deficit with normal sensation and a normal elbow examination should prompt MRI of the palm before a cubital tunnel release is booked.

The rheumatoid and osteoarthritic hand. In advanced trapeziometacarpal osteoarthritis the thumb metacarpal adducts, the adductor shortens and the MCP compensates by hyperextending. A fixed adduction contracture converts a good trapeziectomy into a poor result, so the adductor must be assessed before any trapeziometacarpal arthroplasty or arthrodesis: first web release and MCP stabilisation are often needed at the time of trapeziectomy if the MCP hyperextends beyond 30 degrees, because simple trapeziectomy alone will not correct a fixed adducted metacarpal.