Dorsal Interossei of the Hand

General plan

Each dorsal interosseous is bipennate, arising by two heads from the adjacent sides of the two metacarpals that bound its intermetacarpal space. The two heads are separated proximally by a gap through which a vessel passes.

Individual attachments

The dual insertion — the key to everything

Each dorsal interosseous divides into two tendons:

• Medial (deep) tendon — inserts into the tubercle at the base of the proximal phalanx and into the MCP joint capsule and the accessory collateral ligament. This is the bony insertion and it is the one that produces abduction and MCP flexion.
• Lateral (superficial) tendon — passes into the lateral band of the extensor hood, running distally to join the conjoined lateral band and thence the terminal tendon. This is the tendinous insertion and it is the one that produces interphalangeal extension.
• The lateral tendon also contributes to the transverse fibres of the extensor hood (the sagittal band region and the transverse lamina), which help depress the extensor tendon and flex the MCP joint.

Proportion of bone versus hood insertion varies by muscle: the first dorsal interosseous has a dominant bony insertion (making it a powerful abductor and MCP flexor with relatively little IP extension), whereas the second, third and fourth have a proportionally greater hood insertion. The lumbricals, by contrast, insert entirely into the lateral band — no bony insertion at all — which is why they are the pure IP extensors.

The first dorsal interosseous is special

• Its physiological cross-sectional area, with that of adductor pollicis, is comparable to an extrinsic muscle and much greater than the other intrinsics.
• Its two heads are separated by a triangular gap through which the radial artery passes from the dorsum into the palm to complete the deep palmar arch.
• Its superficial head arises from the first metacarpal and its deep head from the second, and it is often described as having a distinct "oblique" and "transverse" fibre orientation.
• Its bulk on the dorsal aspect of the first web makes it the muscle whose wasting is visible from across the room.

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

• All four dorsal interossei are supplied by the deep motor branch of the ulnar nerve. There is no exception and no median contribution in the standard pattern.
• Course to reach them: 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, and 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 (palmar and dorsal), deep head of flexor pollicis brevis, and terminally adductor pollicis.
• Each interosseous receives its branch from the palmar surface, entering the muscle at roughly the junction of its proximal and middle thirds.
• The first dorsal interosseous is supplied by a branch that continues radially with the terminal deep branch and enters the muscle close to where the deep branch reaches adductor pollicis; the two muscles are effectively co-terminal in the nerve's course.

Root values and localisation

• C8 and T1, via the lower trunk and medial cord.
• Electrodiagnosis: the first dorsal interosseous is the standard recording muscle for the deep ulnar motor branch, alongside abductor digiti minimi. A normal abductor digiti minimi with an abnormal first dorsal interosseous localises to the deep branch distal to the hypothenar takeoff — Guyon canal zone 2 or a mid-palmar lesion.

Variations

• A Riche-Cannieu anastomosis in the palm can supply some interossei from the median recurrent branch; a Martin-Gruber anastomosis in the forearm can carry ulnar-destined motor fibres in the median nerve. Both produce electrodiagnostic confusion and occasionally an unexpectedly preserved intrinsic in a documented nerve division.
• Accessory slips and fusion between adjacent interossei are common but functionally silent.

Arterial

• Deep palmar arch — the dominant supply, running on the palmar surface of the interossei at the level of the metacarpal bases, with the deep motor nerve.
• Palmar metacarpal arteries from the deep arch, running in the intermetacarpal spaces.
• Dorsal metacarpal arteries from the dorsal carpal arch, supplying the dorsal aspect.
• Perforating branches connect the dorsal and palmar metacarpal systems at the metacarpal bases and at the level of the deep transverse metacarpal ligament.

The first dorsal interosseous pedicle and the radial artery

• The radial artery crosses the anatomical snuffbox floor, runs dorsally over the scaphoid and trapezium, then dives between the two heads of the first dorsal interosseous into the palm, where it gives the princeps pollicis and the radialis indicis and completes the deep palmar arch by anastomosing with the deep branch of the ulnar artery.
• The first dorsal metacarpal artery (FDMA) arises from the radial artery just before it dives, and is the pedicle of the kite flap (first dorsal metacarpal artery island flap) used for thumb dorsal and pulp reconstruction. It runs on or within the fascia over the first dorsal interosseous — the flap is raised with the fascia of the first dorsal interosseous precisely to capture the vessel reliably.

Surgical implication

• Dissection between the heads of the first dorsal interosseous, or aggressive release of its origin from the first metacarpal, threatens the radial artery at the point where it is least accessible for control.
• When raising a kite flap, include the fascia over the first dorsal interosseous in the pedicle; skeletonising the vessel risks flap failure.

Dorsal to the interossei

• The extensor tendons (extensor digitorum communis, extensor indicis proprius, extensor digiti minimi) and the juncturae tendinum connecting them.
• The dorsal metacarpal arteries and the dorsal venous network.
• The superficial branch of the radial nerve and the dorsal cutaneous branch of the ulnar nerve in the subcutaneous plane.

Palmar to the interossei

• The deep motor branch of the ulnar nerve and the deep palmar arch, lying directly on the muscles.
• The adductor pollicis in the first and second spaces.
• The long flexor tendons and the lumbricals, separated from the interossei by the midpalmar and thenar spaces and their fascial septa.

The deep transverse metacarpal ligament — the dividing line

• This ligament connects the palmar plates of the second to fifth MCP joints.
• The interossei pass DORSAL to it; the lumbricals pass VOLAR to it.
• This is the single most important spatial fact in intrinsic surgery, because any tendon transfer intended to flex the MCP joint must be routed volar to this ligament. Routed dorsal to it, the transfer extends the MCP and makes a claw worse.

Interosseous compartments

• The hand has ten compartments in the standard model: four dorsal interosseous, three palmar interosseous, thenar, hypothenar and adductor pollicis.
• Each dorsal interosseous therefore has its own fascial envelope, and hand compartment syndrome requires release of each. Two dorsal longitudinal incisions, over the second and fourth metacarpals, allow access to all four dorsal and all three palmar interosseous compartments.

The claw hand

• Posture: MCP hyperextension with PIP and DIP flexion in the ring and little fingers in an ulnar palsy; in all four fingers in a combined median and ulnar (low) palsy.
• The index and middle are spared in isolated ulnar palsy because the radial two lumbricals are median-innervated.
• Ulnar paradox: a low lesion claws more than a high one, because in a high lesion the flexor digitorum profundus to the ring and little is also paralysed and cannot curl the IP joints. Clawing therefore worsens as a high lesion recovers.

The full ulnar motor examination

Recite it in this order and you will not miss anything:

1. First dorsal interosseous — bulk and power. The earliest sign.
2. Abductor digiti minimi — power. Separates lesions at or proximal to the hook from those distal.
3. Froment sign — adductor pollicis.
4. Jeanne sign — thumb MCP hyperextension.
5. Wartenberg sign — little finger abduction.
6. Clawing at rest, and Bouvier test for correctability.
7. Egawa sign — middle finger deviation.
8. Flexor digitorum profundus to ring and little — separates high from low.
9. Sensation: palmar little and ulnar ring (superficial branch), and dorsal ulnar hand (dorsal cutaneous branch, arising 5-8 cm proximal to the ulnar styloid) — the discriminator for lesions at or above the distal forearm.

Levels

• Elbow (cubital tunnel): all of the above plus flexor digitorum profundus weakness and dorsal ulnar sensory loss.
• Guyon zone 1: motor and palmar sensory loss, dorsal sensation normal.
• Guyon zone 2: pure motor, sensation entirely normal.
• Guyon zone 3: pure sensory.

Intrinsic contracture (intrinsic-plus)

The opposite of clawing: the intrinsics are shortened, holding the hand in MCP flexion with IP extension — the "intrinsic-plus" or "swan-neck-prone" hand.

• Causes: ischaemic contracture after compartment syndrome or crush, spasticity (stroke, cerebral palsy, traumatic brain injury), rheumatoid arthritis, prolonged immobilisation in the intrinsic-plus position, and post-traumatic fibrosis.
• Diagnosis: the Finochietto-Bunnell test as above.
• Consequence: the PIP joint is held in extension and drifts into swan-neck deformity; grip is lost because the fingers cannot curl.
• Treatment: stretching and splinting early; intrinsic release (distal intrinsic release, Littler) — excision of the oblique fibres of the lateral bands distal to the MCP joint, leaving the central slip and the transverse fibres intact — for established contracture. In severe ischaemic contracture a proximal intrinsic slide or muscle release at the metacarpal origin may be needed.

Hand compartment syndrome

• The ten compartments: four dorsal interosseous, three palmar interosseous, thenar, hypothenar and adductor pollicis. Some models add the carpal tunnel and the individual digits.
• Presentation: a tense, swollen hand held in the intrinsic-minus posture (MCP extension, IP flexion) — because swollen intrinsics are painful on stretch and the hand adopts the position that shortens them... but note the classic teaching that the hand at rest in compartment syndrome adopts MCP extension with IP flexion, and passive stretch into the intrinsic-plus position (MCP flexion with IP extension) reproduces pain — that is the diagnostic manoeuvre.
• Release: two dorsal longitudinal incisions over the second and fourth metacarpals decompress all four dorsal and all three palmar interosseous compartments; add a radial thenar incision, an ulnar hypothenar incision along the fifth metacarpal, and a dorsal first web incision for the adductor compartment. Carpal tunnel release is added if there is median compression.
• Untreated: ischaemic intrinsic contracture, a hand fixed in intrinsic-plus, and permanent loss of grasp.

Rheumatoid hand

• Intrinsic tightness contributes to swan-neck deformity and to ulnar drift at the MCP joints, in which the ulnar-side intrinsics (the ulnar interossei and abductor digiti minimi) become deforming forces.
• Crossed intrinsic transfer — transferring the ulnar interosseous tendon of one finger to the radial side of the adjacent ulnar finger — is used to counteract ulnar drift at MCP arthroplasty.

Interosseous muscle tumours and masses

• Intramuscular haemangioma, lipoma and, rarely, sarcoma present as a firm dorsal intermetacarpal mass with progressive loss of finger abduction. Any enlarging, firm, deep mass in the hand deserves MRI before biopsy, and biopsy should be planned so the tract can be excised with the definitive resection.

Boxer's knuckle and sagittal band rupture

• The sagittal bands are part of the extensor hood into which the interossei insert. Rupture of the radial sagittal band, usually at the middle finger, allows the extensor tendon to subluxate ulnarwards and produces an extension lag with a snapping sensation.
• Differentiate from a tendon rupture: the patient can maintain extension once the finger is passively extended (the intrinsics extend the IP joints and the tendon reduces), but cannot initiate extension from a flexed position.
• Acute cases splint in MCP extension for 4-6 weeks; chronic cases need sagittal band reconstruction.

Metacarpal fracture and interosseous injury

• Metacarpal shaft fractures produce interosseous scarring and adherence; the resulting intrinsic tightness is a common and underappreciated cause of stiffness after fixation.
• The intrinsics deform metacarpal fractures: their pull across the fracture produces apex dorsal angulation in shaft and neck fractures.
• Fixation of the second and fifth metacarpals approaches from the radial and ulnar borders respectively; fixation of the third and fourth requires elevating the interosseous origins, and reattachment is not required but the resulting scar is a source of stiffness. Early mobilisation is the mitigation.

First dorsal interosseous transfer

• The first dorsal interosseous can be augmented in ulnar palsy to restore the index post for key pinch — extensor carpi radialis longus with a graft, an accessory abductor pollicis longus slip, or a brachioradialis transfer are described, inserting into the first dorsal interosseous tendon at the radial base of the index proximal phalanx.
• Restoring index abduction is as important as restoring thumb adduction in reconstructing key pinch; a strong adductorplasty against a collapsing index post achieves little.