Osteoporosis can lead to a condition called dowager's hump. This occurs when the spine, due to diminished mass and strength, begins to compress, resulting in an outward curve of the upper vertebrae in the spine.
Another disease that can affect the bones is bone cancer. Bone cancer most often spreads to the bone from other parts of the body, but it can also start in the bone. When it begins in the bone, it's known as primary bone cancer. Fortunately, primary bone cancer is pretty uncommon -- there are about 2,300 new cases discovered each year [source: National Cancer Institute]. When found, bone cancer can be treated by surgically removing the tumor from the bone or using chemotherapy, radiation therapy or cryosurgery (killing cancerous cells by freezing them with liquid nitrogen).
Osteonecrosis is a condition in which bones no longer receive the blood they need to survive, leading to bone death and degeneration. The cause of the disease isn't currently known. Most cases require surgical intervention, and doctors can graft healthy bone onto the diseased portions, attempt to restore blood flow or replace joints with mechanical joints.
Osteogenesis imperfecta is an inherited disease that causes bones to be especially brittle. A faulty gene leaves the body unable to produce collagen normally. (For more, read How Osteogenesis Imperfecta Works.)
Paget's disease of bone affects about 1 million people in the United States and tends to show up more often in people with Northern European ancestry [source: National Institute of Arthritis and Musculoskeletal and Skin Diseases]. Paget's disease of bone affects seemingly random bones in the body, making them grow too large and structurally unstable. While the disease can affect any bone, it most often affects the pelvis, skull, spine and leg bones.
See the next section for lots more information about bones and the human body.