The first dorsal interosseous is the most prominent and easily tested interosseous muscle, forming the rounded muscle bulk of the first web space between the thumb and index metacarpals. It is the clinical standard for assessing intrinsic hand muscle function and for detecting early ulnar nerve palsy, as it is reliably affected early in ulnar neuropathy while hand function is still largely preserved. Its wasting produces the characteristic hollow first web space of advanced ulnar nerve palsy.
| Origin | Adjacent sides of the first and second metacarpals, two heads |
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| Insertion | Radial base of the index finger proximal phalanx and extensor expansion |
| Nerve Supply | Deep branch of the ulnar nerve (C8, T1) |
| Blood Supply | First palmar metacarpal artery from the radial artery |
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The first dorsal interosseous is uniquely important for precision pinch as it stabilises the index finger radially during lateral pinch against the thumb, preventing the index from buckling into adduction during pinch loading. Without it, precision pinch requires the index finger to be stabilised by other means.
Froment's sign is assessed at the first web space, the patient holds a sheet of paper between thumb and radially-stabilised index finger while the examiner pulls. Adductor pollicis weakness (ulnar nerve) forces thumb IP flexion using the FPL, and first dorsal interosseous weakness allows the index to adduct. Both are signs of ulnar nerve palsy at different anatomical levels. The first dorsal interosseous is the preferred needle EMG target for evaluating ulnar nerve function at the elbow.
The first dorsal interosseous is directly visible and palpable as the rounded muscle mass in the first web space, assessed by asking the patient to spread the index finger away from the thumb against resistance.
First dorsal interosseous and interosseous wasting from ulnar nerve palsy producing the hollow first web space and intrinsic minus deformity with claw positioning of the ring and little fingers.
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