Hi crstroup,
It’s certainly possible that a mattress can aggravate knee pain because like any other joint in the body it needs to rest in it’s natural alignment and not in the outside of its natural range of motion. If your hips or thighs are sinking down too far when you are side sleeping then it can cause the lower extremities which are lighter to be higher than the upper leg which could flex the knees sideways and cause stress in the knee joint. In some zoned mattresses there is a firmer zone under the knee for this very reason. Back sleepers can also benefit from a pillow under the knees to flex the knees which can also help with tension or pain in the lower back.
Lower back pain can also be from a mattress that has comfort layers that are too thick and soft which can cause the pelvis to tilt and bend the spine out of its natural alignment as well.
The Elegance would probably be closest to the PLB Beautiful which is a very soft mattress and there are many people that are in a higher weight range that love the “feel” and softness of it but don’t do as well with alignment because the upper layers are quite thick and soft. It’s not uncommon at all to see this in my experience (and several forum members have had similar back alignment issues with the Beautiful). Of course it would also depend on physiology because each person can have a different weight distribution and sink into a mattress differently depending on the shape, surface area, and weight of each part of their body.
I don’t know the specific makeup of the Healthy Back mattresses because in the past they have used their own design that was similar but not the same as the PLB mattresses and there have been several versions of the specs posted including post #1 here and post #11 here. You can compare this to the version 3 designs of the Pure Latex bliss which are in post #2 here and the version 2 PLB designs which are in post #1 here (which at least until very recently are still being made in some areas of the country). Version 1 was the same as version 2 except they used an inch of firm polyfoam on the bottom layer instead of the latex. The specs you posted are the version 2 Beautiful specs but this can’t be correct for the Elegance because they use the fast response talalay in the top layer and doesn’t come in 19 ILD (only 15, 21, and 27 ILD).
If I had to guess I would say it was most likely the same as the version 2 Beautiful except they replaced the top inch of 19 ILD latex with 15 ILD talalay GL fast response (which used to be called Celsion and PLB calls Active Fusion fast response) but they are the only source of information that can verify this.
Either way the Elegance has at least 5" of soft latex (or softer) in the top layers
Phoenix