Twin Block Appliance to Reduce 8mm Overjet in Adult?

All a functional appliance will do when used by an adult is move teeth...no bone changes...If you had such a large overjet after braces I'm guessing that you have a very severe skeletal problem.

  There is a limit to how far you can move the teeth to compensate for a poor bone pattern...at some point surgery is the only way to correct severe problems.

There are a few doctors who believe functional appliances work for adults...I would be very leery of this approach..In 30 plus years of practice I have never seen it work!