Osteoarthritis of the knee

A 66-year-old woman who is overweight reports bilateral knee pain of gradual onset during the past several months that increasingly has limited her activities. Last week, when walking down the stairs, she nearly fell when her knee gave way. She does not recall having injured her knee, and she has no morning stiffness and no pain in other joints. She has tried taking up to eight extra-strength ( 500 mg each) acetaminophen tablets daily without success and has never had ulcers or stomach bleeding. How should the patient be evaluated and treated?