Froment’s Sign

The Froment’s Sign occurs due to weakness of the adductor pollicis muscle, which occurs when the patient has an ulnar nerve palsy. The adductor pollicis muscle has two heads: the transverse head and the oblique head. The transverse head originates from the anterior body of the third metacarpal. The oblique head originates from the base of the second and the third metacarpals as well as the trapezoid and capitate bones. The two heads of the adductor pollicis muscle then insert into the base of the proximal phalanx of the thumb and the ulnar sesamoid bone.

The muscle is innervated by the deep branch of the ulnar nerve. The adductor pollicis muscle function is to adduct the thumb. It is important in pinch strength. When the ulnar nerve is injured, the adductor pollicis function is lost and thumb adduction will not occur. The Froment’s sign is used to test the function of the adductor pollicis muscle. when pinching a piece of paper between the thumb and index finger against resistance, the thumb IP joint will flex if the adductor pollicis muscle is weak. The flexion of the thumb occurs by the flexor pollicis longus which is innervated by the median nerve. The flexor pollicis longus, which is innervated by the median nerve, substitutes the function of the adductor pollicis which is innervated by the ulnar nerve. The O.K. sign is different from the Froment’s sign. The Froment’s sign test is performed to determine the presence of an ulnar nerve injury. Bending the thumb when pinching a piece of paper is a sign of an ulnar nerve injury. The anterior interosseous nerve innervates the flexor pollicis longus muscle. The integrity of the anterior interosseous nerve is tested by performing the O.K. sign. When asking the patient to make the O.K. sign by touching the tips of the index finger and the thumb together, the integrity of the anterior interosseous nerve allows flexion of the distal phalanx of the thumb and index finger creating the classic O.K. sign. When an anterior interosseous nerve injury is present, the patient will be unable to bring together the distal phalanx of the thumb and the index finger. With anterior interosseous nerve injury, there will be no sensory deficit. It is interesting to note that when testing the patient and the patient is able to bend the thumb, then the patient either has a normal anterior interosseous nerve or the patient has an injury to the ulnar nerve. If the thumb does not bend, then this means that the patient has a normal ulnar nerve but an injured anterior interosseous nerve. If you see that a patient is able to do the O.K. sign, then the patient has a normal anterior interosseous nerve. Froment’s sign is performed to determine the presence of an ulnar nerve injury.

Author
Nabil Ebraheim, MD

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