Paget's disease. Light micrograph of a section through bone tissue in Paget's disease. Also called osteitis deformans, this is a painful disorder which causes excessive breakdown and formation of bone, causing bones to weaken, thicken, and become deformed. Here, the weakening in the bone (pink area) due to cycles of bone destruction and regrowth is seen as thin dark 'mosaic lines'. This weakening can lead to fracturing of the bone. The condition 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. Treatments include injection of a synthetic form of the hormone calcitonin.