Hi candiceneedham,
Unfortunately there is no specific mattress or material that is “best for backs” in general terms … and a mattress that is a good match for one person that has back issues may be completely unsuitable for someone else with similar back issues to sleep on. In very general terms people with back issues will tend to do better with mattresses that are in a medium or medium firm range but there are also exceptions to every “theory” based on “averages” and there is also no “standard” definition or consensus of opinion for firmness ratings and different manufacturers can rate their mattresses very differently than others.
Different people can also have very different perceptions of firmness and softness compared to others as well and a mattress that is firm for one can feel like “medium” for someone else or even “soft” for someone else depending on their body type, sleeping style, physiology, their frame of reference based on what they are used to, and their individual sensitivity and perceptions. This is all relative and is as much an art as a science. There are also different types of firmness and softness that different people may be sensitive to that can affect how they “rate” a mattress (see post #15 here).
What this “translates” into is that the most reliable way to know whether any mattress is a good match for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) is based on your own careful testing or personal experience. If you have less lordosis in your lumbar spine (less of a lumbar curve) then you may benefit from a thinner comfort layer (which will generally be a firmer mattress as well because you will feel more of the firmness of the deeper layers “through” the thinner comfort layer) because there are less “curves” that the mattress needs to fill in but again it’s always more effective to go by your own personal testing or experience than to try and choose a mattress based on specs (either yours or a mattress).
Just in case you haven’t read it yet the best place to start your research is the mattress shopping tutorial here which has all the basic information, steps, and guidelines that can help you make the best possible choice … and perhaps more importantly help you know how and why to avoid the worst ones.
Two of the more important links in the tutorial that I would especially make sure you’ve read are post #2 here which has more about the different ways to buy a suitable mattress that is the best “match” for you in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences) that can help you assess and minimize the risks of making a choice that doesn’t turn out as well as you hoped for that are involved in each of them and post #13 here which has more about the most important parts of the “value” of a mattress purchase which can help you make more meaningful comparisons between mattresses in terms of suitability, durability, and value.
There is also more about primary or “deep” support and secondary or “surface” support and their relationship to firmness and pressure relief and the “roles” of different layers in a mattress in post #2 here and in post #4 here that may also be helpful in clarifying the difference between “support” and “pressure relief” and “feel” but this may also be more than you really need to know.
Phoenix