Neither low bone density or osteoporosis need stop you from practicing Pilates movements. As long as you are careful about certain movements so that your spine, your ribs and your hips are not under unhealthy stress, this can be a safe and beneficial form of exercise. The guidance of a knowledgeable instructor can be invaluable.
Karen Kemmis, PT, DPT, MS has some suggestions to help you practice Pilates movements safely.
"If you have low bone density or osteoporosis, you should avoid Pilates movements that involve flexion of the spine and cannot be modified. Some of the exercises to avoid include: roll up, rolling like a ball, seal, criss cross, teaser, roll over, shoulder stand, saw, spine stretch, jackknife, bicycle, boomerang, double leg stretch, single leg stretch, open leg rocker, crab, corkscrew and neck pull."
If you have low bone density or osteoporosis, extension exercises like double leg kicks, breast stroke and breast stroke prep are the best ones for you. Side bending and twisting should be done gently and sparingly.
http://forums.webmd.com/3/osteoporosis-exchange/forum/363?ecd=wnl_hbn_062110