What replacement fluid for the knees seems to give most relief for the longest period of time?

Depends on the MD: There are three fluids available.: synvisc, (hylan g-f 20) supartz, and hyalgan. It depends on which one your doctor uses. We now have limitations as to who can give them. I used to do it but now only ortho, in hawaii, can give them. They have their own limitations and side effects, especially synvisc, (hylan g-f 20) which uses formaldehyde as the carrier, but some have done well with it. They have to be in expert's hands.

They are all similar: Aside from frequency of injections, they all work about the same. If you have fluid on the knee from arthritis, it must be drained.