Osteoporosis is a condition that causes bones to lose mass / strength and therefore break more easily. It develops slowly over several years, without causing pain, and is often only diagnosed when a fall or sudden impact causes a bone to fracture. It commonly causes injuries to wrists, hips and spinal bones (vertebrae) although breaks can happen in other bones too.
You can view an article by The Royal Osteoporosis Society, originally printed in TalkBack magazine, here.
There is also an NHS video, made in association with InstructorLive, on Pilates for Osteoporosis that you can access on YouTube