There is an outfit in the States that specialises in offroad adapted bikes, google Lasher. A group of injured US Servicemen, Ridetorecovery.com use bikes that have been produced by these guys.
I've seen these in action and they look fantastic, three independent wheels with Hydraulic brakes. The adaption’s include hand power and multiple gears. There is some good footage on youtube.http://www.reviewjournal.com/news/wound ... ied-cycles
I rode with these boys and girls earlier in the year.