Suggest Treatment For Swelling In The Knees

i have had problems with my knees for years on and off for years thell swell up and then go down the pain is constintly there but sometime i get stabbing pains the pain is just under the knee cap its anoying me now as its been going on for years i have had xrays on it although that was about 5 years ago and also had physio on then about 7 years ago what could it be???