The Froment’s sign occurs when a patient has weakness of the adductor pollicis muscle which is due to ulnar nerve palsy. Here, you can see the anatomy of the ulnar nerve innervation of the adductor pollicis muscle, and this is very important. The muscle is innervated by the deep branch of the ulnar nerve.
The adductor pollicis muscle has two heads. The first is the transverse head, which originates from the anterior body of the third metacarpal. The second 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 adductor pollicis’ function is to adduct the thumb. This movement is important for pinch strength. When the ulnar nerve is injured, the adductor pollicis function is lost and thus, adduction of the thumb will not occur.
The Froment’s sign is used to test the function of the adductor pollicis muscle. The Froment’s sign will become apparent when pinching a piece of paper between the thumb and index finger against resistance, and the thump 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 substitutes the function of the adductor pollicis, when the ulnar nerve is injured and the adductor pollicis muscle cannot function properly.
Froment's Sign Vs. OK Sign
The OK sign is different form the Froment’s sign. The Froment’s sign test is performed to determine the presence of an ulnar nerve injury, while the OK sign test is performed to test the function of the anterior interosseous nerve. The Froment’s sign is positive for ulnar enrev injury when bending the thumb when pinching a piece of paper. The anterior interosseous nerve innervates the flexor pollicis longus muscle, and the integrity of the anterior interosseous nerve is tested by performing the OK sign. When asking the patient to make the OK 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 OK sign. But, when there is an interosseous nerve injury present, the patient will not be able to bring the distal phalanx of the thumb and index finger together, and they will not be able to make the OK sign.
With an anterior interosseous nerve injury, there will be no sensory deficit. However, it should be noted that when the patient is able to bend the thumb to make the OK sign, then the patient has a normal anterior interosseous nerve, but still may have an injury to the ulnar nerve. On the other hand, if the thumb does not bend when making the OK sign, this means the patient has a normal ulnar nerve but an injured anterior interosseous nerve.
So, if a patient is able to do the OK sign, they have a normal anterior interosseous nerve, but if the patient grips a piece of paper between the thumb and the index finger with a bent thumb, then the patient has an ulnar nerve injury.
