My wife has one replacement hip, the rest of her joints are by now pretty much shot although, mercifully, she has (so far) no back problems. At one point at least two decades ago I thought I was perhaps about to develop a bad back. We sleep on very firm mattresses (the give is akin to the firmest rear car seat cushion) which are on a rigidly flat surface. Both of us sleep very soundly and both of us attribute the absence of back pain to our mattresses. Avoid springs in any shape or form because they lose tone over time, some very rapidly. Our mattresses are made of composite foam, the type that has bits of different types and colours glued/moulded into a block which is then sliced into sheets. Ours are about 5cm in thickness. We do not like memory foam and have never owned anything like that. Our mattresses are now about 20 years old and are as good as they were when new. At first I feared I had made a mistake because they are so hard and immediately after delivery tried lying on one not intending to sleep, just see if I would get pressure point irritation - I woke up about 15 minutes later and was entirely reassured that this was good stuff. A visiting relative once asked me why one sleeps so well on our mattresses - this was over a decade after they visited us and he had asked this then but now had forgotten the explanation, I guess.