Illustration of bones affected by osteoporosis (top), osteopetrosis (middle) and Paget's disease (bottom). Osteoporosis is a condition in which bone density is reduced, weakening the bones and making them susceptible to fractures. Osteopetrosis, also known as marble bone disease, is a rare inherited disorder where the bones harden, becoming denser than normal. Mild osteopetrosis results in slightly more brittle bones, but severe cases can lead to growth deformities, blindness, deafness and paralysis due to the extra bone interfering with nerve function. Paget's disease, or osteitis deformans, is a chronic bone disease that results in enlarged or deformed bones. The rate of bone reabsorption and formation become out of balance, leading to structurally disorganised, weaker and enlarged bone.