Before you buy jointed stirrups, please get a physical and find out WHY your back is hurting. Jointed stirrups aren't a cure-all for aches and pains.
That being said, they CAN help if you have ankle/hip/knee pain. I've had one too many horse riding wrecks over the years and my lower back, hips, knees, and ankles have made it painful for me to ride in hard irons.
I switched over to jointed about 10 years ago, and now I can ride for hours with little pain, compared to only an hour before I have to get off the horse in hard irons.
Plus, if you decide to go jointed don't spend a lot of money on your first pair, in case you hate them. Jointed stirrups are one of those things you either love or hate; there doesn't seem to be any in between.
It's not 'cheating' to use jointed stirrups. I've seen many, many people using them in the show ring now.
Nothing that keeps you on a horse and riding should be considered 'cheating'.