Froment's sign is due to which nerve injury

froment's sign / book test : tests adductor pollicis muscle it is innervated by ulnar nerve. in ulnar nerve injury, when pt is asked to grasp a book firmly b/w thumb & other fingers of both hands, the terminal phalanx of thumb on paralysed side becomes flexed at interphalangeal joint (by flexor pollicis longus supplied by median nerve). The signs indicative of ulnar nerve palsy are as follows: Froment's sign: Jeanne's sign: