Froment signs are positive in injury of which nerve

Froment's sign is clinical testing of adductor pollicis .Give the patient a thin book and ask him to grasp it firmly between thumb and index fingers of both hands. If the muscle is healthy and acting normally, the thumb will be straight. but if paralyzed,the thumbs are flexed at IP joints. This is because adductor pollicis is not acting and flexor pollicis compensates for it. Reference: Textbook of anatomy upper limb and thorax, Visharm Singh, 2nd edition, page no.144.