Palmaris Brevis Anatomy

Palmaris brevis is a thin, quadrilateral sheet of muscle lying immediately beneath the skin of the ulnar palm. It is a dermal muscle, one of very few in the human body that inserts into skin rather than bone or tendon, and its entire function is to manipulate the soft tissue envelope of the hypothenar eminence rather than to move a joint.

Its clinical importance derives entirely from where it lies and what supplies it. It roofs Guyon canal, so every operation on the distal ulnar tunnel goes through it. Its nerve supply makes it a localising sign for ulnar lesions. And it sits directly over the segment of the ulnar artery injured in hypothenar hammer syndrome, where it can both contribute to arterial compression and, raised as a turnover flap, provide the vascularised cover that the reconstruction needs.

Origin. The muscle arises from the ulnar border of the palmar aponeurosis and from the ulnar margin of the flexor retinaculum (transverse carpal ligament). Some fibres arise from the fascia over the pisiform region and from the antebrachial fascia as it continues into the palm.

Insertion. It inserts into the dermis of the skin along the ulnar border of the palm, over the hypothenar eminence, and has no bony or tendinous insertion at all. That is its defining feature: every other hand muscle inserts on bone or tendon, and it places the muscle in the same functional category as platysma.

Form and position. A thin quadrilateral sheet, typically 2 to 3 cm across, running transversely across the proximal hypothenar eminence immediately deep to the skin and subcutaneous fat. It lies superficial to the ulnar neurovascular bundle and the hypothenar muscles, the most superficial muscle of the ulnar palm. It is frequently thin, occasionally hypoplastic and sometimes absent, and its size varies considerably between individuals.

Part of the palm's anchoring system. Arising from the palmar aponeurosis and inserting into skin, palmaris brevis belongs to the skin-fascia anchoring system of the palm, alongside the vertical fascial septa and the natatory and pretendinous bands. This is why the ulnar palm has firm, tethered, relatively immobile skin adapted to shear loading during grip, and why an ulnar-sided Dupuytren cord produces puckered, dimpled hypothenar skin.

Nerve supply. The superficial branch of the ulnar nerve, root values C8 and T1. The ulnar nerve divides within Guyon canal into this superficial branch, mainly sensory to the ulnar one and a half digits, and a deep motor branch. Palmaris brevis is the only muscle supplied by the superficial branch; its branch leaves at or immediately after the bifurcation, at the very proximal end of the division, and enters the deep surface of the muscle. Every other ulnar-innervated intrinsic is supplied by the deep motor branch:

• The interossei
• The ulnar two lumbricals
• Adductor pollicis
• The deep head of flexor pollicis brevis
• The hypothenar group: abductor, flexor and opponens digiti minimi

Not under voluntary control. Most people cannot contract palmaris brevis on command. It fires as part of grip and palmar cupping rather than as an isolated voluntary act, an observation made explicitly in the original description of palmaris brevis spasm syndrome. It matters twice: at examination you observe the muscle rather than instruct it, and in the spasm syndrome the patient cannot suppress the contraction, which is part of the reason the syndrome exists at all and why it is so visible.

Blood supply. Direct muscular branches come from the ulnar artery as it traverses Guyon canal and from the superficial palmar arch into which it continues, with additional supply from the deep palmar branch of the ulnar artery and small perforators to the overlying skin. The muscle therefore has a segmental, proximally reliable pedicle from the ulnar artery immediately deep to it, which is what allows it to be raised as a turnover flap.

Guyon canal. Palmaris brevis is the distal, muscular part of the roof. The canal is approximately 4 to 4.5 cm long:

• Roof: the volar carpal ligament and palmaris brevis
• Floor: the transverse carpal ligament and the pisohamate ligament, with the hypothenar muscles distally
• Ulnar wall: the pisiform and the flexor carpi ulnaris tendon
• Radial wall: the hook of the hamate

Contents. Immediately deep to palmaris brevis lie the ulnar nerve and the ulnar artery, with the artery radial to the nerve, and deep to them abductor digiti minimi, flexor digiti minimi brevis and opponens digiti minimi. The nerve bifurcates within the canal. The deep motor branch then curves dorsally and radially around the hook of the hamate, between the origins of abductor digiti minimi and flexor digiti minimi brevis, and beneath the fibrous arch of the hypothenar muscles, a discrete second compression point.

The artery at the hook. The distal ulnar artery crosses the hook of the hamate immediately deep to palmaris brevis, where it is relatively unprotected against the bone. This is precisely the segment injured by repetitive blunt trauma to the hypothenar eminence in hypothenar hammer syndrome.

The zones. Gross and Gelberman divided the canal into three zones by the internal topography of the nerve. All the reviewed cases of combined motor and sensory deficit arose in zone 1, and zone 2 lesions produced intrinsic paralysis, with hypothenar involvement determined by the exact level within the zone. Combined loss most often followed compression from deep to the nerve; pure sensory deficits followed compression superficial to it.

The palmaris brevis sign. As the marker of superficial branch integrity, palmaris brevis adds a further level of resolution to the zonal scheme. Paralysed intrinsics with hypothenar skin that still dimples on grip mean the superficial branch is working and the lesion sits on the deep motor branch, zone 2. When palmaris brevis is out with the intrinsics and there is sensory loss, the lesion is proximal to the bifurcation, zone 1.

Sensory loss alone, with palmaris brevis and the intrinsics intact, places the lesion on the superficial branch distal to the palmaris brevis branch, zone 3.

Wrist or elbow. Weak flexor carpi ulnaris and weak ring and little profundus, with reduced dorsal ulnar hand sensation, put the lesion at the elbow and not the wrist at all. The dorsal cutaneous branch leaves 5 to 8 cm proximal to the ulnar styloid, and both forearm muscles are supplied above the wrist.

Actions. Palmaris brevis wrinkles and dimples the skin of the ulnar palm, drawing it toward the midline, and deepens the hollow of the palm, improving its conformity around a grasped object. During power grip it anchors the ulnar palmar skin against shear, so the skin does not slide over the underlying structures when a tool is gripped and twisted. It also cushions and protects the ulnar neurovascular bundle when body weight or tool pressure is taken through the hypothenar eminence.

Why a skin muscle earns its place. Grip is not only about tendon force; it depends on friction and skin stability at the interface. A palm whose skin slides freely over the deep structures grips poorly and blisters, and palmaris brevis, with the vertical fascial septa, converts loose subcutaneous tissue into a stable, tethered gripping surface.

Protecting the bundle. When the heel of the hand presses against a handle, the contracted muscle and the tethered fat pad spread the load away from the artery crossing the hook of the hamate. Where that protection is repeatedly overwhelmed, in a mechanic using the palm as a hammer, a cyclist on the bars or a golfer gripping a club, the artery is the structure that fails.

What its loss produces. No functional deficit that a patient will notice. Palmaris brevis can be divided or excised without measurable loss of grip or dexterity, which is what makes both ulnar tunnel release and the turnover flap acceptable operations. The only appreciable change is loss of hypothenar dimpling, a sign rather than a symptom.

Landmarks. The pisiform is the palpable landmark at the proximal hypothenar eminence. The hook of the hamate lies approximately 1 to 2 cm distal and radial to it and is palpable with deep pressure, a manoeuvre uncomfortable even in a normal hand. The hook is where the deep motor branch and the ulnar artery are most vulnerable. Palmaris brevis lies in the skin between and just distal to these two landmarks, running transversely. Kaplan cardinal line, drawn from the apex of the first web space parallel to the proximal palmar crease, crosses the hypothenar region and helps localise the deep branch and the superficial palmar arch.

Observing the muscle. Do not ask the patient to contract it. Ask for a firm grip, or ask them to cup the palm, and watch the skin over the proximal hypothenar eminence: visible transverse wrinkling and dimpling means the muscle, and therefore the superficial branch of the ulnar nerve, is working. Compare with the other hand. Asymmetry is the useful finding, because a thin or hypoplastic muscle dimples poorly in an entirely normal hand.

Iatrogenic injury. In the palmaris brevis field the structures lie millimetres beneath the skin, immediately deep to the muscle:

• Ulnar nerve or artery division during a hypothenar incision made without identifying the pisiform first, a fat pad flap harvest, or drainage of a hypothenar abscess
• Deep motor branch injury at the hook of the hamate during hook excision, ganglion dissection, pisiform excision or Guyon canal exploration. It produces intrinsic paralysis with entirely normal sensation, a devastating result easily attributed to something else if the surgeon does not consider it. Trace the branch before excising bone
• Superficial branch injury, producing ulnar digital numbness and loss of palmaris brevis function
• A painful scar directly over the nerve from a transverse incision across the wrist crease

Failures of Guyon canal surgery. These include:

• Incomplete release of the fibrous arch of the hypothenar muscles, leaving a zone 2 compression of the deep motor branch untreated
• Failure to identify and treat the underlying cause: a ganglion, hook nonunion or arterial lesion. Simply opening the roof does not treat a space-occupying lesion, and decompressing a canal that was never structurally compressed is the specific error to avoid in palmaris brevis spasm syndrome
• Missing a coexisting proximal lesion: a cubital tunnel syndrome or a cervical radiculopathy in a double-crush pattern

Flap-related complications. Those of the turnover flap include:

• Pedicle injury during elevation, devascularising the flap
• Inadequate coverage where the muscle is hypoplastic or absent; assess its bulk before committing to the flap as the coverage plan
• Tension on the flap, compressing the very structures it was intended to protect

After ulnar artery reconstruction. Vascular complications include:

• Graft thrombosis, particularly if the causative activity continues or the patient smokes
• Recurrent ischaemia from continued occupational trauma; activity modification is part of the treatment, not an optional extra
• Persistent digital ischaemia from embolic material already lodged distally before the reconstruction

The cost of doing nothing. Unrecognised and untreated hypothenar hammer syndrome progresses to digital ulceration and tissue loss. The condition is potentially curable, and its recognition depends on someone taking an occupational history and performing an Allen test.

Related pages. The canal this muscle roofs: Ulnar Tunnel Syndrome (Guyon Canal) and Ulnar Nerve Anatomy - and Ulnar Nerve Palsy for the deficits the zonal framework predicts. The vascular lesion beneath it: Hypothenar Hammer Syndrome, Hamate Fractures for the bone the artery is struck against, and Raynaud Phenomenon of the Hand, which is the diagnosis it is mistaken for. The rest of the hypothenar group: Abductor Digiti Minimi, Flexor Digiti Minimi Brevis and Opponens Digiti Minimi. The muscle it is constantly confused with by name: Palmaris Longus - a wrist flexor and graft donor, not a hypothenar muscle. A lump in this territory: Palmar Lump. The other cause of intrinsic wasting: Cubital Tunnel Syndrome.

Anatomical variation across populations

• Palmaris brevis varies considerably in size and is described as hypoplastic or absent in a proportion of hands in cadaveric series. A poorly developed muscle gives weak hypothenar dimpling in an entirely normal hand, so the palmaris brevis sign must always be interpreted by comparison with the contralateral side.
• Accessory and anomalous fibres are recognised, and the reported case of fibres forming a sling around the ulnar artery at the hook of the hamate establishes that these variants can be clinically significant rather than merely curious.
• Anomalous muscles crossing Guyon canal more generally - including an accessory abductor digiti minimi and a palmaris profundus - are described in a small percentage of hands and are a recognised cause of ulnar tunnel syndrome in young patients with no other pathology.

Differences in described technique between sources

Evidence positions worth stating

• Palmaris brevis spasm syndrome is supported only by small case series - five patients in the original description and five in the occupational series, with earlier isolated case reports. The defensible position is that it is a benign, EMG-confirmed focal hyperactivity phenomenon managed conservatively, and that decompressive surgery has no role without a demonstrated structural lesion.
• Guyon canal decompression for genuinely idiopathic compression is uncommon and generally unnecessary; both the evidence and the anatomy point to finding and treating the causative lesion, most often a ganglion, a hook of hamate nonunion or an arterial lesion.
• Hypothenar hammer syndrome: there is no randomised evidence comparing ligation with vein graft interposition. The pragmatic position is resection with reversed vein graft where there is embolisation, aneurysm or critical ischaemia, and ligation only where a complete arch and adequate radial perfusion have been demonstrated rather than assumed.

High- versus limited-resource practice

• Well-resourced settings: high-resolution ultrasound and MRI to identify ganglia, anomalous muscles and arterial lesions before surgery; formal angiography; microvascular vein grafting; EMG for spasm syndromes; botulinum toxin for refractory spasm.
• Limited-resource settings: the diagnosis and localisation are almost entirely clinical, and remarkably good - flexor carpi ulnaris strength, dorsal ulnar sensation and the palmaris brevis sign together level an ulnar lesion with no investigation at all. The Allen test costs nothing and identifies the arterial cause. Where vein grafting is unavailable, ulnar artery ligation with a demonstrated complete arch is a reasonable option, and activity modification remains the single most effective intervention in hypothenar hammer syndrome anywhere in the world.