Thank you for your query at AskTheDoctor.com
I understand your concern.
Yes, silk mattresses are good for spine, you just need to ensure you buy from a good quality mattress company.
Foam mattresses are good too, however they are not as durable as cotton over the longer run. And some people just can't adjust to foam mattresses and do complain of considerable pain and discomfort. So your analysis is right. As long as you buy a good brand cotton mattress, you should be fine.
Hope this was helpful,
Feel free to discuss further,
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