[Periosteal reaction].

The periosteal membrane covers the cortical bone except for the articular surface. The deep layer of the periosteum contains bone-forming mesenchymal cells, capillaries, and nerves. This layer is more active in infants than in adults. Prostaglandin osteopathy, infantile cortical hyperostosis, hypervitaminosis A, and congenital syphilis are examples of periostitis in infants. Incidental asymptomatic periosteal reactions are usually either physiological changes or cortical bone irregularities simulating periostitis. On the other hand, symptomatic periosteal reactions of single bone, such as bone tumor, tumor-like lesion, infection, and trauma, are always pathologic. Careful radiological analysis of periosteal reactions is needed to evaluate the activity and aggressiveness of the lesions. Periosteal reactions of multiple bones usually show solid smooth or undulating patterns. They include pachydermoperiostosis, secondary hypertrophic osteoarthropathy, vascular insufficiency, renal osteodystrophy, and thyroid acropachy. These are usually skeletal manifestations of systemic disorders.