Finger locking

Patient: Dear sir:Thanking you a lot for your guidance..My mother has a problem that i want to describe it and i will become glad if you help me about it please.she has a problem about locking of her medius finger of her left hand..Sometimes when she bend all of her fingers and begin to straight them, they all become straight except her medius finger of her left hand..after this she straight it by her right hand..also there is not any pain with her finger..what may be the problem please help me and guide me..thanks a lot…

Doctor: