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Third Palmar Interosseous
Proximal attachments - To the radial side of the fifth metacarpal.

Distal attachments - To the radial aspect of the extensor apparatus of the fifth digit.

Surface anatomy - The bulk of this muscle belly cannot be effectively palpated as it lies palmarly between the fourth and fifth metacarpals.

Nerve supplyy - Deep branch of the ulnar nerve (C8, T 1 ).

Action

The three volar interossei adduct the fingers relative to the midline of the long finger. Through their attachments to the extensor apparatus on each finger, they contribute to flexion of the MP and extension of PIP and DIP joints. They also have a role in isolated rotation of a digit.

The third palmar interosseous ulnarly deviates (adducts) the little finger at the MP joint.

Ulnar palsy

The function of this muscle is lost after ulnar nerve injury and the subsequent wasting gives the hand a a skeletal appearance. The interossei have altogether a large cross sectional area and the combined muscle mass can exert a considerable force although the excursion is less than in the case of the long flexor tendons. Nevertheless the loss of interossei results in a marked reduction of grip strength.

High ulnar palsy or cubital tunnel syndrome: At the elbow the ulnar nerve is prone to injury behind the medial epicondyle. The consequences of such injury include those described for lesions at the wrist. Additionally, the medial half of flexor digitorum profundus and flexor carpi ulnaris are paralyzed or weakened and the dorsal and palmar cutaneous branches are affected. A similar pattern of functional deficit occurs if the nerve becomes entrapped as it passes through the tendinous arch of origin of flexor carpi ulnaris (cubital tunnel syndrome).

The interossei and lumbricals normally flex the metacarpophalangeal joints and extend the interphalangeal joints through their attachments to the extensor apparatus. The classic deformity in ulnar palsy is clawing of the ring and little fingers due to paralysis of these muscles.

Paradoxically, lesions of the ulnar nerve before the motor branch to the flexor digitorum profundus causes less clawing because of absence of tension in the profundus muscles. .

Froment's sign in the thumb is sometimes present in ulnar palsy. It occurs during pinching when the adductor is unable to provide its stabilizing support to the thumb MP and IP joints. The flexor pollicis longus hyperflexes the IP joint to substitute for the loss of the stabilizing adductor.