Why is the Juvederm Lasting So Long?

A couple of things to keep in mind.  One is that hyaluronic acid (HA) products like juvederm, restylane, perlane, and belotero behave differently in different facial sites.  So mobile areas like the lips will break down faster than less mobile areas like tear trough and cheek. Second, as Dr Dembny noted, is that juvederm tends to swell more than restylane in the tear trough so most docs avoid it's use here.

Two studies worth mentioning.  One was a follow up on patients injected with HA in the tear trough, which recognized persistent volume/product in the tear trough at the end of the study 24 mos after injection.  So you may still have product there, or...

The second study of HA injected in the nasolabial fold found that 18 mos after a second injection patients continued to note augmentation of the fold.  A biopsy was done which showed the filler was gone, replaced by new collagen.  So fillers clearly induce collagen formation as they dissolve.  How much volume of collagen compared to the volume of filler is unclear at this point though.  

    The results after 2 years may be the result of scar formation and collagen deposition in addition to filler.  If you like the results you should probably do nothing.  If you are unhappy and there is still product, enzymatic degrading can be performed.

Juvederm has a high concentration of Hyaluronic acid and it is heavily cross linked so this gives it more persistence than some of the other fillers.  It's not uncommon to have it last a year or more but You are lucky to have it look the same after 2 yrs!  I have no idea what the world record is but I do know of others with the same experience.