Periosteal reaction, also known as periosteal hyperplasia, is an increase in bone growth caused by stimulation of the periosteum and increased activity of osteoblasts in the inner layer of the periosteum, which usually indicates the presence of a lesion. Histologically, an increase in osteoblasts in the inner layer of the periosteum is seen, by new bone trabeculae. The periosteal reaction is not specific and is commonly seen in inflammation, tumors, trauma, and subperiosteal hemorrhage, as well as in normal growth and developmental stages. Etiology: The periosteum is wrapped around the outside of the bone and consists of an outer layer (fibrous layer) and an inner layer (cellular layer). In adults, the resting state periosteum is dominated by the fibrous layer with little cellular component, but in children during growth and development or when stimulated by various pathologies, the periosteum is thicker and the two layers are clearly delaminated. The normal periosteum is constantly undergoing the process of osteogenesis and resorption, and during the growth phase bone formation exceeds resorption, so the backbone is constantly thickening. In adulthood, osteogenesis and resorption reach a balance, and in old age resorption exceeds osteogenesis and osteoporosis occurs. In pathological states osteogenesis or resorption can be reactivated or intensified. The positive periosteal reaction is the thickening of the bone stem by attaching new bone to the surface of the bone stem, resulting in various forms of periosteal reaction, while the negative periosteal reaction (subperiosteal bone resorption) results in thinning of the bone, as in hyperparathyroidism.