Froment's Sign

The Froment’s sign occurs due to weakness of the adductor pollicis muscle in ulnar nerve palsy. The adductor pollicis muscle has two heads:

1. Transverse Head
   1. Originiates from the anterior body of the third metacarpal
2. Oblique Head
   1. 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 bones. The muscle is innervated by the deep branch of the ulnar nerve. The function of the adductor pollicis muscle 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 Foment’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.