Palmar Interossei of the Hand

General plan

Each palmar interosseous is unipennate, arising from one side of a single metacarpal — the metacarpal of the digit it moves — and running distally to insert into the same side of that digit, adducting it toward the middle finger.

Individual attachments

The disputed fourth palmar interosseous

• Many anatomists describe a fourth (or first, in an alternative numbering) palmar interosseous acting on the thumb, arising from the ulnar side of the first metacarpal and inserting into the ulnar sesamoid and the base of the thumb proximal phalanx.
• Where it is described, its function is subsumed within the oblique head of adductor pollicis, with which it is continuous.
• The safe answer in an examination is "three palmar interossei, with a variably described fourth acting on the thumb that is functionally part of adductor pollicis." Numbering the palmar interossei as one to three for index, ring and little is the convention most sources use.

Insertion — proportions matter

• Like the dorsal interossei, each palmar interosseous divides into a medial (bony) tendon to the base of the proximal phalanx and MCP capsule, and a lateral (tendinous) contribution to the lateral band of the extensor hood.
• The palmar interossei have a proportionally greater lateral band contribution and a smaller bony insertion than the dorsal interossei. They therefore contribute relatively more to interphalangeal extension and relatively less to raw MCP flexion power.
• The lumbricals sit at the extreme of this spectrum: they insert exclusively into the radial lateral band with no bony insertion at all, making them the pure IP extensors.

The spectrum, in one line

First dorsal interosseous (mostly bone, powerful abductor and MCP flexor) — other dorsal interossei — palmar interossei — lumbricals (entirely lateral band, pure IP extensors).

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

• All three palmar interossei are supplied by the deep motor branch of the ulnar nerve. There is no median contribution in the standard pattern.
• Course: the ulnar nerve divides in Guyon canal at the distal pole of the pisiform. The deep motor branch turns radially and dorsally around the hook of the hamate, passes between abductor digiti minimi and flexor digiti minimi brevis beneath the fibrous arch of the latter, pierces opponens digiti minimi, then runs radially across the palm on the palmar surface of the interossei, deep to the flexor tendons, in company with the deep palmar arch.
• Order of supply: abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi, ulnar two lumbricals, all interossei, deep head of flexor pollicis brevis, terminating in adductor pollicis.
• Each palmar interosseous receives its branch directly from the deep branch as it crosses, entering the muscle from its palmar surface at roughly the junction of the proximal and middle thirds.

Root values and localisation

• C8 and T1, via the lower trunk and medial cord.
• The third palmar interosseous is the muscle whose weakness produces Wartenberg sign — the little finger drifts into abduction because the palmar interosseous cannot adduct it against the radial-nerve-innervated extensor digiti minimi, which has an abduction moment at that joint.
• Electrodiagnosis does not usually record from the palmar interossei; the first dorsal interosseous and abductor digiti minimi are the conventional recording sites for the deep branch.

Variations

• Riche-Cannieu anastomosis (ulnar to median in the palm) can supply some interossei from the median recurrent branch; Martin-Gruber anastomosis (median to ulnar in the forearm, roughly 15-20% of limbs) can carry ulnar-destined motor fibres in the median nerve. Both produce electrodiagnostic confusion and occasionally preserved intrinsic function in a documented nerve division.

Arterial

• Deep palmar arch — the dominant supply, running directly on the palmar surface of the interossei with the deep motor nerve at the level of the metacarpal bases.
• Palmar metacarpal arteries from the deep arch, running distally in the intermetacarpal spaces to join the common palmar digital arteries near the web spaces.
• Perforating branches connect the palmar and dorsal metacarpal systems at the metacarpal bases and near the deep transverse metacarpal ligament.

Pedicle position

• Each muscle receives its pedicle proximally, from its palmar surface, in company with the motor branch at the level of the metacarpal base and proximal shaft.
• The deep palmar arch lies approximately 1 cm proximal to the superficial arch, which itself lies about 1 cm distal to the distal edge of the transverse carpal ligament, at Kaplan's cardinal line.

Surgical relevance

• Deep palmar dissection at the metacarpal bases is the most dangerous place in the palm precisely because the deep arch and the deep motor nerve travel together on the interossei — an injury here produces a purely motor ulnar palsy with brisk deep bleeding that is difficult to control.
• Palmar metacarpal artery perforator flaps are described for small palmar defects but are not commonly used; the more reliable local options come from the dorsal metacarpal system.

Palmar to the palmar interossei

• The deep motor branch of the ulnar nerve and the deep palmar arch lie directly on them.
• Adductor pollicis in the radial part of the palm.
• The long flexor tendons and the lumbricals, separated by the midpalmar and thenar spaces and their fascial septa.

Dorsal to the palmar interossei

• The dorsal interossei, and beyond them the metacarpals and the extensor tendons.

The deep transverse metacarpal ligament — the plane that decides operations

• Connects the palmar plates of the second to fifth MCP joints.
• Interossei (dorsal and palmar) pass DORSAL to it.
• Lumbricals pass VOLAR to it.
• Because a tendon must lie volar to the MCP axis to flex that joint, and the intermetacarpal ligament marks that plane, any anti-claw transfer must be routed volar to the deep transverse metacarpal ligament. Routed dorsal, it extends the MCP joint and makes the claw worse. This is the most examined single surgical rule in intrinsic reconstruction.

Compartments

• There are three palmar interosseous compartments within the ten-compartment model of the hand (four dorsal interosseous, three palmar interosseous, thenar, hypothenar, adductor pollicis).
• Two dorsal longitudinal incisions over the second and fourth metacarpals decompress all four dorsal and all three palmar interosseous compartments, by dissecting on both sides of each metacarpal.

Causes of the intrinsic-minus hand

• Ulnar nerve palsy — ring and little claw (radial two lumbricals are median-innervated and spare the index and middle).
• Combined low median and ulnar palsy — all four digits claw. This is the classic leprosy and combined wrist laceration picture.
• Lower trunk brachial plexus injury and C8-T1 root avulsion.
• Charcot-Marie-Tooth disease and other hereditary neuropathies.
• Compartment syndrome and Volkmann ischaemic contracture — though here the claw is often mixed with intrinsic contracture.
• Leprosy remains the largest global cause.

Why only the ulnar two digits in an ulnar palsy

The first and second lumbricals are median-innervated; the third and fourth are ulnar. All seven interossei are ulnar. In an isolated ulnar palsy the index and middle retain their lumbricals, which insert exclusively into the radial lateral band and are sufficient to prevent clawing in those digits. The ring and little lose both interossei and lumbricals and therefore claw.

Bouvier test and the operation it selects

Anti-claw procedures in detail

The staging principle

In a combined median and ulnar palsy the priorities are, in order: sensation and skin, then thumb opposition, then correction of claw, then pinch (adductorplasty and index abduction), then finger flexion power. Do not correct a claw in an insensate hand.

The intrinsic-plus hand

• Posture: MCP flexion with IP extension; the patient cannot make a fist.
• Causes: ischaemic contracture after compartment syndrome or crush injury, spasticity (stroke, cerebral palsy, brain injury), rheumatoid arthritis, prolonged immobilisation in the intrinsic-plus position, and post-traumatic fibrosis after metacarpal fracture fixation.
• Consequence: the PIP joint is held in extension, the lateral bands subluxate dorsally, and swan-neck deformity develops.

Diagnosis — Finochietto-Bunnell

• Method: hold the MCP joint in extension and passively flex the PIP; then hold the MCP in flexion and repeat.
• Positive: PIP flexion is more limited with the MCP extended, because MCP extension puts the intrinsics on maximal stretch.
• Negative: equally limited in both positions, indicating a PIP joint capsular contracture rather than intrinsic tightness.
• Do the test on each digit separately; contracture is often asymmetrical.

Treatment

• Early: intrinsic stretching (MCP extended, PIP flexed), serial static splinting, dynamic flexion splinting, and a committed therapy programme.
• Established: distal intrinsic release (Littler) — excision of the oblique fibres of the lateral bands distal to the MCP joint, preserving the central slip and the transverse fibres. Confirm adequacy on the table by repeating the Finochietto-Bunnell test.
• Severe post-ischaemic: proximal intrinsic release or intrinsic slide from the metacarpal origins.
• The warning: do not over-release. Abolishing intrinsic function converts an intrinsic-plus hand into an intrinsic-minus hand, which is functionally worse and much harder to reconstruct.

Swan-neck deformity

• Mechanism: PIP hyperextension with DIP flexion, from volar plate laxity at the PIP, intrinsic tightness pulling the lateral bands dorsally, or a mallet DIP shifting the extensor force proximally.
• In the rheumatoid hand the sequence is typically MCP volar subluxation, intrinsic tightness, then swan-neck.
• Treatment depends on PIP suppleness: a supple swan-neck is treated with a figure-of-eight (silver ring) splint, an FDS tenodesis, an oblique retinacular ligament reconstruction (Littler) or a lateral band relocation; a stiff swan-neck requires release and sometimes arthrodesis.

Ulnar drift in the rheumatoid hand

• The MCP joints subluxate volarly and the digits drift ulnarwards. The ulnar-side intrinsics — the ulnar interossei and abductor digiti minimi — become deforming forces, pulling the digits further ulnarwards once the radial sagittal bands and collateral ligaments have failed.
• Crossed intrinsic transfer: the ulnar interosseous tendon of the index, middle and ring is divided and transferred to the radial side of the adjacent ulnar digit, converting a deforming ulnar force into a correcting radial one. Performed at the time of MCP arthroplasty.
• The other components of the rheumatoid MCP operation: synovectomy, radial collateral ligament reconstruction, ulnar capsular release, extensor tendon centralisation, and correction of any wrist radial deviation, without which the fingers will drift again.

Hand compartment syndrome

• Three palmar interosseous compartments among the ten hand compartments.
• Diagnosis: pain on passive stretch of the intrinsics — passively flexing the MCP joints while extending the IP joints — in a tense swollen hand.
• Release: two dorsal longitudinal incisions over the second and fourth metacarpals decompress all four dorsal and all three palmar interosseous compartments; add thenar, hypothenar and adductor releases, and a carpal tunnel release if the median nerve is involved.
• Untreated: ischaemic intrinsic contracture — a hand fixed in intrinsic-plus, unable to grasp.

Metacarpal fracture and stiffness

• The interossei cross the fracture and their pull produces apex dorsal angulation in shaft and neck fractures.
• After dorsal plating, adhesion between the interossei and the plate is a leading cause of stiffness. Early mobilisation is the mitigation, and late plate removal with tenolysis plus distal intrinsic release is the salvage.
• Rotational malalignment is never acceptable: around 5 degrees of malrotation gives roughly 1.5 cm of overlap at the fingertip. Confirm by checking that all nails lie in the same plane and all flexed digits point toward the scaphoid tubercle.

Central ray amputation

• The palmar and dorsal interossei of the adjacent spaces are divided. The deep transverse metacarpal ligament must be reconstructed or the hand splays and grip is permanently weakened.