Like the PP says, no carrier can cause Hip dysplasia, but carriers which hang babies by the crotch will exasperate the preexisting condition.
If you look up Hip dysplasia in infants, you will see that their legs are splinted in a wide-spread position with knees bent. The NEWEST research shows that this very wide position isn't the best either, but the a narrower spread is the best for correcting Hip dysplasia, just like when a child is in a hip carry with their knees up.
At the International Babywearing Conference in Chicago, I learned that a hip carry (switching sides for even amounts of time on each hip) is therapeutic because when you walk, your hip acts as a massage while the baby's hips are in the correct position.
Whatever carrier you use, you should be looking to see if the baby's knees are above the level of the bum. If the legs are allowed to hang straight down, then you should try to adjust the baby into the correct seated position; With some carriers, the correct position is not possible.
Why not have the legs straight down? There are certain stages to the spinal development of infants. Newborns, used to the fetal position, need to be allowed to curve their spines into a 'C' position. If their legs hang down, their spines curve the opposite direction.
Some babies who have experienced birth trauma, or have other contributing factors, have what is called High Tone. This causes them to feel "stiff" and arch their backs. This can be treated with CranioSacral Therapy, Chiropractic adjustment, and other gentle hands-on techniques. On the other end of the spectrum, babies with extremely Low Tone are way too floppy and do not have muscular reactions or control. The same treatments can apply in this situation.
Many parents interpret distress, stiffness, and arching to be the preference of the baby not to be held or worn. This in not the case. In fact, there might be Sensory Integration issues which need to be treated so that the baby will no longer be overwhelmed and unable to cope with normal amounts of stimulation.
Another reason not to allow the legs to hang straight down is that the carrier would then put undue pressure on the Pubis and Pubic symphysis, possibly compress the Femoral arteries and nerves, and promote lateral curvature of the spine.
These multiple reasons no doubt impact the development of a child, but by no means are crippling. Bjorn's and Snuggli's aren't the most comfortable things for babies or parents, but they are not as dangerous to health as bag-style slings (http://babyslingsafety.blogspot.com/
) which can lead to infant positional asphyxiation.
Here is a link to a very long PDF about natural parenting. Page #107-108 have pictures of natural hip positioning. This dissertation is a really good one. If you have the time, please read it:http://www.epjournal.net/filestore/ep05102183.pdf
Here are more links about correct ergonomic positioning:http://www.storchenwiege.com/babycarrierresearch.htmhttp://www.continuum-concept.org/rea...nalStress.html
(<-- already referenced in a PP)http://www.thebabywearer.com/article...ositioning.pdfhttp://www.frogmama.com/articles.php?article=10
I hope that helps!