I have hyper-extensability of the metacarpaphalangeal (or however you spell it) joints. That's fingers for the less medically inclined. I also have it in my hips and knees. As it was explained to me, the tendons/ligaments are longer than normal and don't hold the joint in place as they should. Therefore the joint moves/flexes more than normal. It does lead to greater wear and tear on the joint and thus osteoarthritis and joint replacements.
BTW - the boys in grammar school were always impressed that I could make Frankenstein fingers.
BTW # 2 - double-jointed hips makes for better sex.
I scared my Dad with my fingers as a kid.
My flexibility is probably why one of my nerves can get blocked easily. The ligaments are too long indeed. And the bones not kept in place well enough.
Training the muscles, to form a kind of muscle corset to keep the bones in place helped.
Over the years I am getting less flexible btw.
