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The flexor pollicis brevis muscle is one of the thenar muscles of the hand. The superficial head of this muscle is the more superficially situated part of the entire muscle and it is separated from the deep head of the muscle by the tendon of the flexor pollicis longus muscle. The superficial head has its origin on the flexor retinaculum and inserts, via its tendon, at the base of the proximal phalanx of the thumb via the radial sesamoid bone. It is innervated by the thenar branch of the median nerve (C8, T1), also known as the recurrent branch of the median nerve. As the name suggests, the main function of this muscle is to flex the thumb at the carpometacarpal joint. However, it also facilitates extension at the interphalangeal joint of the thumb.
In patients with suspected ulnar nerve palsy or those affected by an ulnar lesion, the clinician performs a test known as Froment's sign where the patient is asked to grip a sheet of paper using only their index finger and thumb. The clinician then tries to pull the sheet of paper away, asking the patient to try and hold on to it as strongly as possible. Individuals with ulnar nerve palsy will have a weakened adductor pollicis and will therefore be forced to compensate by using the flexor pollicis brevis muscle instead, which is supplied by the median nerve. This compensation can be visually recognized by the flexion of the carpometacarpal and interphalangeal joints of the thumb, indicating a positive Froment's sign.
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