I have had nine back fusions and have disc disease. I also have two knee replacements. I have a mattress I bought about eight years ago and it got so bad I bought a two-inch foamtopper from Walmart.
This mattress came with coil springs I ended up sleeping like a hammock in the center and could hardly move after crawling out. I put a 3/4 inch piece of plywood under the mattress but even with the topper on it still has a hole in the center. I have to sleep on my side. I also have both rotor cups in my shoulders worn out and because of knee replacements and all L vertebrae fused I need to invest in something. I only want a full-size bed because I have many expensive sheets and made many quilts. No need for a queen but full size seems hard to find. Any suggestions?
Hi Farmgal41,
Certainly, you’ve had your share of physical issues.
The failure of your mattress is internal so anything you put either on top or underneath isn’t going to fix the problem of failed internal components.
Based on your issues, it appears obvious that you require a mattress that will place your spine in a level and stress-free position. In addition, you need something that is density appropriate for your weight and will cushion your shoulders. There are a couple of companies on the forum that can help you. SleepEZ @Sleep_EZ, Arizona Premium Mattress, @Expert-ArizonaPr, and Custom Sleep Technology, @CST.
I would suggest that you reach out to one or more of these companies to investigate the solutions they may have for you.
I hope this helps.
Bob
Hi Farmgal41 and welcome to the Mattress Underground 
Whoa, that is a lot of physical issues! I am very sorry you are dealing with all that! I am not a doctor or physio-therapist, so I’ll say at the start that you may want to discuss any potential sleeping solutions with your physician, as there are a lot of interconnecting issues here. Thanks @CST for your great suggestions!
Just for the basics: keep in mind that any mattress you chose will be based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) and your PPP (Posture & alignment, Pressuret relief, and Personal preferences). Your spinal and knee issues complicate matters. You need a mattress with proper support, but without knowing your Stats, from what you say, you would likely want a firmer mattress.
For foams, We suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support.
Many of the Trusted Members of the site offer a variety of foam and foam hybrid mattresses at different price points, and many carry full- size mattresses as you desire. Giving any of these a call and providing your Ststs, PPP, and mattress history will help them to provide tyou with good quality sleep solutions; and If you want to provide your stats and any mattresses you are looking at here, we can possible comment on their durability and support for you. Your situation is a bit more complicated due to your surgeries, but a supportive comfortable mattress that holds up over time is possible with a little consideration of your specific needs!
~ Basilio
Hello Farmgal41, welcome to the Mattress Underground.
I agree with what everyone said here. Talking to your physician and a good mattress specialist can help. I wanted to add that there should be options in full size for you; sometimes that size is called ‘double’ as I’m sure you know. Looking for both terms may turn up more options.
Trying out zoning (different firmness for shoulder, hips, knees) or a mattress topper on top of a firm mattress that would support you may be helpful, but I am not a doctor or physical therapist and you know your body the best. With all of the challenges you have, you are right, investing in something solid and of good quality will help you. All the best to you as you continue your search!
Katie