Adductor Pollicis Anatomy

Transverse Head (larger, distal)

• Origin: the palmar (anterior) surface and 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.
• Fibres run transversely and radially, converging on the thumb.

Oblique Head (proximal, deeper)

• Origin: the capitate, the palmar carpal ligaments, the bases of the second and third metacarpals, and frequently a slip from the sheath of the flexor carpi radialis tendon and the trapezoid.
• Some fibres arise from the deep transverse metacarpal ligament complex.
• A small deep slip of the oblique head 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.

Common Insertion

• The two heads converge into a single tendon containing the ulnar sesamoid of the thumb metacarpophalangeal (MCP) joint.
• Bony insertion: the ulnar side of the base of the proximal phalanx of the thumb, and the ulnar sesamoid itself.
• Aponeurotic insertion: 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 / dorsal apparatus. This is why adductor pollicis contributes to thumb IP extension and why its aponeurosis lies exactly superficial to the UCL.

The gap between the heads

• The two heads are separated distally 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.

Deep (Motor) Branch of the Ulnar Nerve — C8, T1

• The ulnar nerve divides in Guyon canal into a superficial (mainly sensory, plus palmaris brevis) and a deep motor branch.
• The deep branch turns dorsally and radially around the hook of hamate, passing between abductor digiti minimi and flexor digiti minimi brevis, then pierces the opponens digiti minimi or its fibrous arch.
• It 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.
• After supplying the hypothenar three, the ulnar two lumbricals, all interossei and the deep head of flexor pollicis brevis, 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.

Root values and localisation

• C8 and T1 via the lower trunk and medial cord.
• Adductor pollicis is therefore weak in: lower trunk plexopathy, thoracic outlet syndrome (true neurogenic), C8/T1 radiculopathy, cubital tunnel syndrome, Guyon canal zone 1 or zone 2 lesions, and motor neurone disease.
• It is normal in isolated median nerve palsy — the single most useful discriminator at the bedside.

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.
• This anastomosis explains an occasional patient with a complete ulnar nerve division who retains a negative Froment sign.

Arterial

• Deep palmar arch (dominant contribution) — formed by the terminal radial artery, which passes between the two heads of adductor pollicis and then between the heads of the first dorsal interosseous.
• Princeps pollicis artery — arises from the radial artery as it turns between the adductor heads; runs along the ulnar border of the first metacarpal deep to the oblique head.
• 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 from the common palmar digital arteries of the superficial arch.

Surgical relevance of the pedicle

• The pedicle 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 be subperiosteal and from radial to ulnar, hugging bone, or the arch is at risk.
• Adductor pollicis has been described as a local muscle flap for coverage of the 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.

Venous and lymphatic

• Venae comitantes accompany the deep arch and drain to the deep palmar venous arch and thence to the radial and ulnar venae comitantes.

Superficial (palmar) to the muscle

• The long flexor tendons of the index and middle fingers with their lumbricals.
• The superficial palmar arch and 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.

Deep (dorsal) to the muscle

• The interossei and the shafts of the second and third metacarpals.
• The first dorsal interosseous, immediately radial and dorsal — the two muscles sandwich the first web.

Structures at risk in and around the muscle

• Deep branch of the ulnar nerve — lies on the dorsal surface of the muscle's proximal portion.
• Deep palmar arch — lies between the heads and on the deep surface.
• Princeps pollicis artery — deep to the oblique head; divided inadvertently in aggressive adductor release.
• Ulnar digital nerve to the thumb — crosses superficial to the adductor insertion distally, at risk in ulnar collateral ligament repair.

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. Pus in the thenar space collapses the first web and produces a pseudo-adduction contracture: the classic clinical picture is a swollen, abducted thumb with a full first web.

Adductor pollicis in ulnar nerve lesions

• High lesion (elbow, cubital tunnel): adductor pollicis, all interossei, ulnar two lumbricals, 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.
• Low lesion (wrist, Guyon canal): FDP is spared, so the profundus pulls the distal interphalangeal (DIP) joints into flexion against paralysed intrinsics — 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. 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 adductor pollicis compound muscle action potential recording is useful when the deep branch is selectively involved.
• A normal abductor digiti minimi response with an abnormal first dorsal interosseous and adductor pollicis response localises the lesion to the deep motor branch distal to the hypothenar takeoff — Guyon canal zone 2 or a distal palmar lesion.

Thumb UCL rupture and the adductor aponeurosis

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. The proximal edge of the aponeurosis sits over the mid-portion of the ligament.

• 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. 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 lies superficial to the aponeurosis and can never reach its footprint. This is the Stener lesion (Bertil Stener, 1962).

Numbers and detection

• Reported in roughly 64-87% of complete UCL ruptures in the original and subsequent series.
• A Stener lesion cannot heal with immobilisation and is the principal indication for operative repair of an acute complete UCL tear.
• Clinical: a palpable tender lump proximal and ulnar to the MCP joint (the "Stener nodule") is the displaced ligament with the aponeurosis edge.
• Stress testing: 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 (tests the accessory collateral and volar plate) and 30 degrees flexion (tests the proper collateral). Obtain radiographs first to exclude a displaced avulsion fracture.
• Imaging: ultrasound in experienced hands and MRI both identify the displaced stump; the classic MRI appearance is the "yo-yo on a string" sign — the balled-up ligament proximal to the aponeurosis edge.

Repair

• 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. The retracted UCL is found immediately beneath it.
• Reattach 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. 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.

Grading of the thumb-in-palm deformity (House classification, 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.

Assessing whether the skin, the muscle or the joint is the problem

• 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/tone.
• If it does not open under anaesthesia but the skin is supple, the problem is muscle and fascia — release the adductor.
• If the skin blanches and tethers as a taut band, add a skin lengthening procedure (Z-plasty, four-flap Z-plasty, or a dorsal rotation flap).
• If the trapeziometacarpal joint itself will not move, the problem is joint contracture — capsulotomy or arthrodesis.

Adductor compartment syndrome

• The adductor pollicis occupies its own fascial compartment in the hand (the ten-compartment model of the hand: thenar, hypothenar, adductor, four dorsal interosseous, three palmar interosseous compartments).
• 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 ("collar-stud" abscess through the first web) present with a swollen, abducted thumb — 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 normal elbow examination should prompt MRI of the palm before a cubital tunnel release is booked.

Rheumatoid and osteoarthritic hand

• In advanced trapeziometacarpal osteoarthritis the thumb metacarpal adducts, the adductor shortens, and the MCP compensates by hyperextending. This is why first web release and MCP stabilisation are often needed at the time of trapeziectomy if the MCP hyperextends beyond 30 degrees; simple trapeziectomy alone will not correct a fixed adducted metacarpal.