Paget's disease. Light micrograph of bone tissue from the femur (thigh bone) affected by Paget's disease. Paget's disease (also called osteitis deformans) is a painful disorder which causes excessive breakdown and formation of bone, causing bones to weaken, thicken, and become deformed. It usually occurs in people over the age of 40. Complications include arthritis and, in rare cases, bone cancer (osteosarcoma). Usually the pelvis, collarbone, backbone, and leg bones are affected. Paget's disease may run in families. Treatments include injection of a synthetic form of the hormone calcitonin. Magnification unknown.