Periostitis may be acute or chronic. In the simple or acute form the periosteum becomes swollen, hyperemic, and infiltrated with blood. The process may stop there, but if infection has taken place, pus collects in the inner layers of the periosteum, and between it and the bone. This accumulation of pus may dissect the membrane from the bone and form a subperiosteal abscess. The pus may be absorbed, become dry and cheesy or burst through the periosteum and cause abscesses in the adjacent tissues. In such a case the nutrition of the bone is interfered with, and the osseous tissue begins to undergo destruction - necrosis. Large portions may separate and be discharged as sequestra through a superficial opening. The process may extend into and involve the marrow cavity, and death may result from septicemia or pyemia. The so-called malignant periostitis is merely a very severe purulent periostitis of considerable extent, and accompanied by marked destruction of the bone.

Chronic periostitis may have originated as such, or it may have followed an acute form. It may be fibrous, in which the periosteum becomes very greatly thickened and adheres very closely to the bone. Ossifying periostitis is a condition in which there is the formation of loose bony tissue from the inner layers of the periosteum. It is found in pregnancy, in tumors, and in syphilis and tuberculosis of the bones.

The bony masses are called osteophytes and at first have a loose spongy character and are not firmly attached to the old bone. Later, however, layers of dense compact bone are deposited within the medullary spaces until the entire mass may become as compact or even more so than the normal bone. At the same time they become firmly attached.