hi I think you are asking this for a mother. based on the fact that she is 51 years old and is having pain in knee joint , she is mostly suffering from condition called  osteoarthritis. osteoarthritis occurs commonly with age and can cause a lot of pain and difficulty with respect to using the joint properly. however with some daily Lifestyle precautions as well as intermittent  short courses of anti inflammatory medication either oral or in the form of creams, many patients are able to lead normal life. you need to meet a Rheumatologist. who will examine her physically and confirm that she is suffering from osteoarthritis as I suspect or some other Arthritis. once it is  confirmed the necessary Lifestyle precautions as well as the anti-inflammatory medications can be prescribed to you by the same doctor.  hope this helps also get more information regarding osteoarthritis from my website www.rheumatologybangalore.com