Flexor Digiti Minimi Brevis

Origin

• The hook (hamulus) of the hamate, on its palmar and radial surface.
• The adjacent palmar surface of the flexor retinaculum (transverse carpal ligament).
• The origin is partly fibrous and arch-shaped, spanning from the hook to the retinaculum. The deep motor branch of the ulnar nerve and the deep branch of the ulnar artery pass beneath this arch as they turn radially and dorsally. This arch is the single most important surgical fact about the muscle.

Insertion

• The ulnar side of the base of the proximal phalanx of the little finger, blending with the insertion of abductor digiti minimi.
• Slips extend into the extensor expansion (dorsal apparatus) of the little finger, so the muscle contributes to interphalangeal extension in the same way an interosseous does.

Relationship to abductor digiti minimi

• The two muscles share an insertion, are frequently continuous at their origins, and act as a functional unit.
• Abductor digiti minimi originates from the pisiform, the pisohamate ligament and the flexor carpi ulnaris tendon; flexor digiti minimi brevis originates from the hook of hamate and the retinaculum. The origins are the way to tell them apart on dissection: pisiform is abductor, hook is flexor.

Variations

• Absent or vestigial in a significant minority of hands; reported absence rates vary widely between series.
• Fused with abductor digiti minimi so that no separate belly is identifiable — the commonest variant.
• Occasional accessory slips from the pisohamate ligament or from a persistent median artery sheath.
• Clinical implication of absence: there is no functional deficit, and the finding at operation should not be reported as pathological.

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

• The ulnar nerve enters Guyon canal at the pisiform, lying ulnar and deep to the ulnar artery.
• It divides at the level of the distal pole of the pisiform into a superficial branch (sensory plus palmaris brevis) and the deep motor branch.
• The deep motor branch turns radially and dorsally around the hook of the hamate, passing between abductor digiti minimi and flexor digiti minimi brevis and beneath the fibrous arch of origin of flexor digiti minimi brevis, then pierces opponens digiti minimi or passes beneath a fibrous arch within it.
• Order of supply: abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi, then the ulnar two lumbricals, all interossei, the deep head of flexor pollicis brevis, and finally adductor pollicis.

Motor point

• The branch to flexor digiti minimi brevis is short and enters the deep surface of the muscle within 1 cm of the hook of hamate, so the muscle is denervated by any lesion at or proximal to the hook.

Why the order matters

• Because the hypothenar three are supplied first, a lesion distal to their branches but within the palm will spare them entirely while paralysing the interossei and adductor pollicis. That is the electrodiagnostic pattern of a distal zone 2 or mid-palmar lesion: normal abductor digiti minimi compound muscle action potential with an absent or reduced first dorsal interosseous response.
• A lesion at or proximal to the hook takes out the hypothenar three as well, so abductor digiti minimi is also abnormal.

Root values

• C8 and T1, via the lower trunk and medial cord.
• Martin-Gruber anastomosis (median-to-ulnar in the forearm, present in roughly 15-20% of limbs) can carry fibres destined for the hypothenar and interosseous muscles, and is a classic cause of confusing conduction studies.

Arterial

• The deep branch of the ulnar artery, which accompanies the deep motor nerve beneath the fibrous arch — the artery and nerve are inseparable at this point.
• Direct branches from the ulnar artery within Guyon canal.
• Distal contributions from the superficial palmar arch and the fourth common palmar digital artery.

Pedicle position

• The dominant pedicle enters the proximal third of the muscle from its deep and radial aspect, within about 1 cm of the hook of hamate, together with the motor branch.
• This is the reason a hypothenar fat pad flap must be raised superficial to the hypothenar muscle fascia — dropping into the muscle at its proximal edge threatens both the deep nerve and the deep artery.

The vascular hazard next door

• The ulnar artery crosses the hook of hamate superficially and unprotected. Repetitive blunt trauma at this exact point produces hypothenar hammer syndrome, with thrombosis or aneurysm of the ulnar artery and embolisation to the ulnar digits. The muscle itself is not injured; the artery lying on top of it is.

Superficial (palmar)

• Palmaris brevis and the subcutaneous fat of the hypothenar eminence.
• The superficial branch of the ulnar nerve and the ulnar artery proximally, and the ulnar digital nerve and artery to the little finger distally.

Deep (dorsal)

• Opponens digiti minimi and, beneath it, the fifth metacarpal.
• The deep motor branch of the ulnar nerve and the deep branch of the ulnar artery, passing beneath the fibrous arch.

Ulnar

• Abductor digiti minimi, often inseparable.

Radial

• The long flexor tendons of the little and ring fingers and, deep to them, the hypothenar (ulnar) septum separating the hypothenar compartment from the midpalmar space.

Guyon canal — recite the boundaries

• Roof: the volar (palmar) carpal ligament, and distally palmaris brevis and hypothenar fat.
• Floor: the transverse carpal ligament, the pisohamate and pisometacarpal ligaments, and distally opponens digiti minimi.
• Ulnar wall: the pisiform and the origin of abductor digiti minimi.
• Radial wall: the hook of the hamate.
• Contents: ulnar nerve and ulnar artery with venae comitantes; the nerve is ulnar and deep to the artery.
• Length: approximately 4-4.5 cm, from the proximal edge of the palmar carpal ligament to the fibrous arch of the hypothenar muscles — and it is that distal fibrous arch, formed principally by flexor digiti minimi brevis, that marks the distal end of the canal.

Ulnar tunnel syndrome by zone

The critical negative

Dorsal ulnar hand sensation is normal in every Guyon canal zone. The dorsal cutaneous branch arises 5-8 cm proximal to the ulnar styloid and passes dorsally beneath flexor carpi ulnaris, entirely outside the canal. A patient with numbness on the dorsum of the ulnar hand has a lesion at or proximal to the distal forearm — most often the cubital tunnel.

Causes, in the order you should think of them

1. Ganglion from the pisotriquetral or hamate-metacarpal joint — the commonest identified cause.
2. Hook of hamate fracture or non-union — sporting injury, missed on plain films, diagnosed on CT.
3. Repetitive external compression — cyclist's palsy, vibrating tools, wheelchair use.
4. Anomalous muscle — accessory abductor digiti minimi (roughly 20-25% of limbs), hypertrophic palmaris brevis, an accessory flexor digiti minimi slip.
5. Ulnar artery pathology — thrombosis or aneurysm from hypothenar hammer syndrome.
6. Fracture — distal radius, pisiform, fifth carpometacarpal fracture-dislocation.
7. Idiopathic, with fibrous arch compression.

Imaging rule

Image every pure motor ulnar palsy before operating. Radiographs including a carpal tunnel view, then ultrasound or MRI of the wrist and palm, and CT if a hook fracture is suspected.

The intrinsic-minus (claw) hand

• Posture: MCP hyperextension with PIP and DIP flexion in the ring and little fingers (ulnar palsy) or in all four fingers (combined median and ulnar palsy).
• Mechanism: loss of the intrinsic muscles removes the MCP flexion and IP extension moment; extensor digitorum communis hyperextends the MCP and the long flexors curl the IP joints.
• Why only ring and little in ulnar palsy: the radial two lumbricals are median-innervated and preserve the index and middle.

Bouvier test — the decision point

• Passively hold the MCP joints in flexion and ask the patient to extend the PIP joints.
• Positive (correctable): the PIP joints extend fully. The extensor mechanism is intact and simply needs an MCP flexion block. A static procedure — Zancolli capsulodesis, Parkes or a bone block — or a simple dynamic transfer will work.
• Negative (non-correctable): the PIP joints do not extend even with the MCP blocked. The lateral bands are attenuated or the PIP joints are contracted; a dynamic transfer inserting into the lateral bands is required (a modified Stiles-Bunnell or Brand transfer), after releasing any fixed contracture.

Anti-claw procedures

• Static: Zancolli lasso (flexor digitorum superficialis looped around the A1 pulley), volar plate capsulodesis, bone block, MCP volar tenodesis.
• Dynamic: Brand transfer (extensor carpi radialis brevis or longus with tendon grafts, routed volar to the intermetacarpal ligament to the lateral bands), modified Stiles-Bunnell (flexor digitorum superficialis of the ring finger split and routed to the lateral bands), flexor carpi radialis with grafts.
• Routing rule: the transfer must pass volar to the deep transverse metacarpal ligament to produce MCP flexion. Routed dorsal to it, the transfer extends the MCP and makes the claw worse.
• Beware the FDS harvest: taking the ring FDS in a hand where the profundus is weak can produce a swan-neck deformity at the donor PIP, and taking it too distally causes quadriga.

Hypothenar hammer syndrome

• The ulnar artery crossing the hook of hamate is the injured structure; the hypothenar muscles are the anatomical bed.
• Unilateral, dominant hand, ulnar digits affected, thumb spared in a manual worker. Distinguish from Raynaud phenomenon, which is bilateral, symmetrical and includes the thumb.
• Allen test, duplex ultrasound and angiography; treatment is activity modification, smoking cessation, vasodilators and antiplatelets for mild disease, and resection with interposition vein graft for aneurysm or embolic disease.

Hypothenar compartment syndrome

• The hypothenar muscles occupy their own fascial compartment in the ten-compartment model of the hand.
• Released through a longitudinal incision along the ulnar border of the fifth metacarpal, separate from the thenar, adductor and interosseous releases.

Hypothenar hypertrophy and pseudotumour

• A prominent hypothenar mass in a manual worker is more often muscle hypertrophy or an anomalous muscle than a tumour, but a pulsatile mass is an ulnar artery aneurysm until proven otherwise, and a firm, mobile, non-pulsatile mass is most often a ganglion.

Little finger amputation and ray resection

• Fifth ray amputation detaches all three hypothenar muscles. Grip strength on the ulnar side falls measurably, and the palm loses its ulnar contour.
• Technical point: transferring the insertion of abductor digiti minimi (and where present flexor digiti minimi brevis) to the ring finger, or accepting the loss, must be planned explicitly; and the deep motor branch runs immediately deep to the field at the base of the fifth metacarpal.