From SCIO tests the weak knee issue coincides with heavy Lead or Mercury in blood. Chelaters help but copious Selenium will do the job, also SeME can be taken in much larger dosages than Selenium from yeast or Elemental.
Once the Iodine has shook loose the metals the Selenium seems to bind them nicely bypassing the kidney.
The knee reactivity appears to be due to the kidneys inability to process these metals without help.
BTW: Ankle issues appear to come indirectly from weak knee problems, the body attempts to use the ankles more to compensate for the weak knee.