Hi ! I have gone through your question and i understand your concern. I would say that calcium supplement are safe while you take the right dosage .The Calcium intake should not be more than 1500 miligrams Calcium per day (This include calcium you take from food also from the suplement ) . I would say that you have been developing Osteoarthritis(OA) cause of your fracture in both knees .OA is a condition that we can not stop,the thing we can do is to treat the symptoms -If you have pain you can take some painkillers such Paracetamol or Ibuprofen 3Xday - with an antiacid drug (Ranitidine 150mg twice a day to prevent stomach problem)., or Celekoksib. Keep moving , exsercise is good even if its hurtfull,massage ,streching - I would recomed to do Xray of your joint to see the condition, Wish you good health . If you have any other questions please feel free to ask me .