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.

The dual insertion. Each dorsal interosseous divides into two tendons, and that split is the key to everything the muscle does.

• The 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 produces abduction and MCP flexion.
• The 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 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.

The proportion 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, with no bony insertion at all, which is why they are the pure IP extensors.

The first dorsal interosseous. 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. The two heads are separated by a triangular gap. 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 and 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. The roots are C8 and T1, via the lower trunk and medial cord.

The 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, 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. It crosses the palm roughly 1-2 cm distal to the distal edge of the transverse carpal ligament, and it is at risk in deep palmar dissection, in palmar approaches to the metacarpal bases, and in fifth carpometacarpal fracture-dislocation.

Order of supply. Abductor digiti minimi, flexor digiti minimi brevis, opponens digiti minimi, the ulnar two lumbricals, all interossei (palmar and dorsal), the 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.

Electrodiagnosis and localisation. 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, and 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 supply. The deep palmar arch is the dominant source, running on the palmar surface of the interossei at the level of the metacarpal bases with the deep motor nerve. It lies approximately 1 cm proximal to the superficial arch, which is itself about 1 cm distal to the transverse carpal ligament at Kaplan's cardinal line. The remaining vessels:

• 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 connecting the dorsal and palmar metacarpal systems at the metacarpal bases and at the level of the deep transverse metacarpal ligament

The radial artery. After crossing the floor of the anatomical snuffbox and running dorsally over the scaphoid and trapezium, the radial artery dives between the two heads of the first dorsal interosseous into the palm. It then passes between the two heads of adductor pollicis, gives the princeps pollicis and the radialis indicis, and completes the deep palmar arch by anastomosing with the deep branch of the ulnar artery.

Why that matters at operation. First web release, aggressive release of the first dorsal interosseous origin from the first metacarpal, and dissection in the first intermetacarpal space all threaten the radial artery at the point where it is least accessible for control, and bleeding here is deep and hard to control. Release on the first metacarpal must be subperiosteal.

The first dorsal metacarpal artery. It arises from the radial artery immediately before the artery dives between the heads, and runs distally on or within the fascia over the first dorsal interosseous toward the dorsum of the index. It is the pedicle of the kite flap (first dorsal metacarpal artery island flap) used for thumb dorsal and pulp reconstruction. The flap is raised with the fascia of the first dorsal interosseous included in the pedicle precisely to capture the vessel reliably; skeletonising the vessel risks flap failure.

Dorsal to the interossei lie the extensor tendons (extensor digitorum communis, extensor indicis proprius, extensor digiti minimi) with the juncturae tendinum connecting them, the dorsal metacarpal arteries and the dorsal venous network, and, in the subcutaneous plane, the superficial branch of the radial nerve and the dorsal cutaneous branch of the ulnar nerve. Two or three constant branches of the superficial branch of the radial nerve cross the dorsal first web subcutaneously; injury produces a painful neuroma that ruins an otherwise good result, so dorsal flaps are elevated in the subdermal plane under direct vision.

Palmar to the interossei lie the deep motor branch of the ulnar nerve and the deep palmar arch, directly on the muscles; adductor pollicis in the first and second spaces; and 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. It 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.

Compartments. The hand has ten compartments in the standard model: four dorsal interosseous, three palmar interosseous, thenar, hypothenar and adductor pollicis. Some models add the carpal tunnel and the individual digits. Each dorsal interosseous therefore has its own fascial envelope, and hand compartment syndrome requires release of each; the incisions are set out under Surgical Relevance.

The claw hand. The posture is MCP hyperextension with PIP and DIP flexion, in the ring and little fingers in an ulnar palsy and 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.

The 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
2. Abductor digiti minimi, power; separates lesions at or proximal to the hook from those distal
3. Froment sign, for adductor pollicis
4. Jeanne sign, thumb MCP hyperextension
5. Wartenberg sign, little finger abduction
6. Clawing at rest, and the Bouvier test for correctability
7. Egawa sign, middle finger deviation
8. Flexor digitorum profundus to ring and little, which separates high from low
9. Sensation: palmar little and ulnar ring (superficial branch), and the 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. The pattern of loss places the lesion:

• 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. 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. The Finochietto-Bunnell test makes the diagnosis. The causes:

• Ischaemic contracture after compartment syndrome or crush
• Spasticity (stroke, cerebral palsy, traumatic brain injury)
• Rheumatoid arthritis
• Prolonged immobilisation in the intrinsic-plus position
• Post-traumatic fibrosis

What it costs. The PIP joint is held in extension and drifts into swan-neck deformity, and grip is lost because the fingers cannot curl. Treatment is stretching and splinting early and, for established contracture, distal intrinsic release; in severe ischaemic contracture a proximal intrinsic slide or muscle release at the metacarpal origin may be needed. Both operations are described under Surgical Relevance.

Hand compartment syndrome. The hand is tense and swollen and rests in the intrinsic-minus posture of MCP extension with IP flexion. Passive stretch into the intrinsic-plus position, MCP flexion with IP extension, reproduces pain, and that is the diagnostic manoeuvre. Untreated, the outcome is ischaemic intrinsic contracture, a hand fixed in intrinsic-plus, and permanent loss of grasp.

The 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, described under Surgical Relevance, is used to counteract ulnar drift at MCP arthroplasty.

Interosseous 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. It is told apart from a tendon rupture because 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 fractures. The intrinsics deform them: their pull across the fracture produces apex dorsal angulation in shaft and neck fractures. Shaft fractures also produce interosseous scarring and adherence, and the resulting intrinsic tightness is a common and underappreciated cause of stiffness after fixation; the approaches and their consequences are under Surgical Relevance.

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.