Pain Pumps

I asked a couple doctors and have done some research on a pain pump, but I have been told that I am too small and don't have enough "meat on my bones" to even consider one as an option. I haven't been able to find any requirements for that, and plan to ask my pain management doctor on Thursday, but I was just wondering if anyone has one, how they work (it's tough to decipher medical mumbo-jumbo while sober, much less while in Valium-kicked-in-relaxation mode!), and how well they work. I want to learn what I can because it may be a valid option later on. It would help with my constipation, that's for sure!