Well, sitting in a chair anyhow forces the body into an unnatural position; crossing your legs means you're also placing pressure on the blood vessels and veins, and constricting circulation. Poor circulation can lead to a weakening of the vein wall, which is what ultimately leads to varicose veins.
However, its not the only cause, for sure: other factors are genetic predisposition and pregnancy. So if you've been pregnant, or if you have varicose sufferers in your family, its wise to avoid anything that constricts circulation, and is even good to put your legs up for a few minutes a day and allow gravity to reactivate circulation and prevent stagnation.
The pressure isn't only on the veins - its also on the joints, and, of course the knee. Sometimes when you sit in this position for a long time, you tend to shift your grounded leg outwards, which means your knee is basically taking most of the weight of the leg you've crossed over.
As for posture, because crossing your legs forces you to shift your weight more to one side, its a harder posture to maintain without leaning or slouching, which can lead especially to lower back pain and tension. So sitting for prolonged periods of time with your legs crossed can lead to bad posture and back issues.
Same applies to sleeping, though the extent of pressure depends on how you cross your legs. So try not to make it a habit - occasional leg crossing would probably not be disastrous to someone with no such health issues, but since you do have knee and back pain, you should definitely follow your chiropractor's advice and not cross your legs! Uncross them everytime you catch yourself doing it, and in a while the habit will go away.
Lots of Paleo love :-)
For the better part of twenty years I never crossed by legs because I could not cross my legs. They were too fat to be crossed. So yes, right or wrong or good or bad or paleo or not, I cross my legs every damn chance I get and it makes me happier than words can express.
I'm not sure about circulation, but it can--and will-- definitely mess up spine alignment and posture. Anecdotal evidence that I believe I've heard corroborated: I have far less back pain now when I keep my legs uncrossed, and this is independent of other posture changes. My chiropractor made a big point of telling me it's best to keep my legs uncrossed, because the spinal alignment problems, however minor, contribute to my chronic knee pain.
I just got home from having my L4/L5 disc surgically repaired this past Wednesday (after years of conservative therapy - only option left since I could hardly walk). My PT and one of my doctors said I should never cross my legs while sitting again as it can put undo pressure on spinal nerve(s). If you're feeling numbness in your legs, feet, or pain in the back of your hamstring or knee while sitting with legs crossed (on floor or in chair), it's due to your nerve being compressed. I suffered from severe sciatica for years, and it is NOT fun. So I would suggest NOT sitting that way if it bugs you.
Here's the real bitch with bad posture - bad posture will lead to more back/sciatic pain which leads to even worse posture which leads to even worse back/sciatic pain, and on and on. My back essentially collapsed on me due to a bulging and fairly degenerative disc last week after being stuck in this circular cycle of of an unhealthy back.
im pretty sure if crossing ones legs lead to spider veins then sitting on ones ass would do the same and my ass is spider vein free. therefore i must conclude the risk factor for spider veins must have a different cause. my ankles have plenty of spider veins. i think it is due to a weakness in the blood vessel.
Crossing for short periods of time (i.e., less than 3 minutes) is okay as long as you don't experience numbness or pain and support the knee when applicable. Allowing your body to get to its end range and working through that is important to actually improve circulation. I think there's a difference between 'active' crossing and 'lazy' crossing. Active would be intentionally stretching and supporting, while lazy would be letting the weight of your leg/knee just hang. So, yes, avoid pain and danger zones, but there's no reason you shouldn't be able to move your body around in natural movements.