Froment's sign is positive due to loss of -

Froment sign is positive due to the loss of adduction of thumb in ulnar nerve palsy . The patient is asked to grasp a between thumb & index finger .Normally , a person will grasp the book firmly with the thumb extended , takjng full advantage of adductor pollicis & first dorsal interosseous muscles. If the ulnar nerve is injured , the adductor pollicis will be paralysed & patient will hold the book using the flexor pollicis longus ( supplied by median nerve ) in place of adductor . This produces flexion at interphalengeal joint of the thumb .This becomes more pronounced if the examiner tries to pull out the book while the patient tries to hold it .This sign is froment sign / book test REF:Essential ohopedics, Maheshwari , 5th edition, pg.no. 68