Ectopic calcification, also known as ectopic ossification, is a deposition of calcium salts that occurs outside of skeletal tissue in addition to normal physiologic calcification. This change usually occurs at tendon attachments, ligaments, vascular walls, and bone fascia of the skeleton. Precipitation of calcium ions from the skeletal system often leads to the development of osteoporosis, so some scholars advocate ectopic calcification as an indirect sign of osteoporosis. In fibrous connective tissue, primitive cells proliferate actively with an abundant capillary network, and calcium salts are deposited to form bone. Mature ectopic ossification has the structure of bone, with an outer layer of fibrous connective tissue, an inner layer of osteoblasts, trabecular nodes and bone-like tissue, and an active primitive cell in the center. It is thought that heterotopic ossification is somewhat different from ossifying myositis, which is a condition in which muscle tissue is mechanized due to injury or bleeding, resulting in the formation of hard knots and contractures. There is usually a clear history of local injury. Local pain is not always evident, but there is some degree of restriction of movement. Ossifying myositis is not necessarily around the joint but is more concentrated within the muscle. The etiology of heterotopic ossification is not well understood, making prevention difficult. Its development may be related to over-activity of the limb early in the injury.