[What is Paget's disease?]

Paget's disease of bone is characterized by the progressive and extensive replacement, in one or several bones, of normal bone tissue by a bone tissue of rough and irregular structure, the excessive and disorderly renewal of which gradually produces hyperdensity and hypertrophy of the bones involved. The condition results from the action of abnormal and overactive osteoclasts containing virus-like intranuclear and intracytoplasmic inclusions. The disease profoundly alters the physiology of bones in the region affected. The viral-looking inclusions that enter the osteoclasts seem to induce a loss of control of bone renewal and remodelling, as shown by histology and radiology. The resorption-formation mechanism persists but it is unbalanced, facilitating bone resorption during two to five years, then bone formation. In that way, subjects with Paget's disease, who have reached the age of physiological osteopenia, show an often important increase in bone density and mass. Paget's disease of bone is a disease grafted on the skeleton: it is partly dependent on the skeletal status of the host. The activity of Paget's disease is evaluated by measuring the ratio of plasma alkaline phosphatase levels to the volumes of normal and pagetic bones; the author suggests a mathematical formula for calculating this ratio.