Necrosis refers to the destruction of large portions of bone, as a result of interference with the blood-supply or from the extension of disease from the periosteum or neighboring bone.

The dead piece remains as a sequestrum; part having been absorbed, the rest remains in a cavity surrounded by granulation tissue and pus. It acts as a foreign body, and constantly keeps up a suppurative reaction in the adjacent structures. A fistulous opening or cloaca, communicating with the exterior, generally forms, and the pus continues to discharge through it. This may continue for years if the sequestrum is so large that it cannot pass through the opening. When it does escape or become absorbed, healing takes place. Phosphorous necrosis of the maxillae is frequently observed in those who are employed in the manufacture of phosphorous matches. The process begins as a thickening of the periosteum, with suppuration, caries, and necrosis, which may be very widespread and destructive. The condition is favored by the presence of carious teeth.

Tuberculosis of the bones is most frequent in childhood and may begin during uterine life. The epiphyses of the long bones are most commonly involved, then the spinal column, the wrist-and the ankle-joints. The infection may be hematogenous from tuberculosis of other organs, or it may result from direct extension by contiguity. The disease may occur in a miliary form within the bone-marrow, or as a localized condition of the periosteum. The process extends more rapidly in the spongy bone and frequently involves the joints.

In the marrow cavities of the bone tubercles appear as small grayish areas surrounded by a zone of active hyperemia. These extend through the Haversian canals, involving compact as well as spongy layers. These areas undergo caseation and are accompanied by caries of the bones. The degenerated masses may so obstruct the blood-supply as to cause necrosis. As the bone is destroyed, broken-down masses are formed, and these may escape into the surrounding tissue, giving rise to a cold abscess. There is little inflammatory reaction; the degenerated tissue follows along the lines of least resistance, along muscle-sheaths, and may finally be evacuated at some distance from the seat of the disease. This is particularly common in Pott's disease, tubercular caries of the spine. Occasionally encapsulation and absorption take place, but this is unusual. The periosteum surrounding the degenerated area may undergo a marked new bone-formation.

Very extensive deformities may ensue in this disease, either as a result of destruction of the bone or from the formation of osteophytes.

In long-continued tubercular degeneration of the bone, extensive amyloid changes in the internal organs are common.

Syphilis of the bones may be either hereditary or acquired.

In congenital syphilis the chief bony lesion occurs between the epiphysis and the diaphysis. The zone of calcification is broad, uneven and white or reddish-white in color. The cartilage at this point is much thickened, and the calcified portions project irregularly into the marrow cavity. As the condition progresses the cartilage may become almost gelatinous, and finally there may form a distinct, irregular, yellowish line of granulation tissue separating the diaphysis and the cartilage. This change is usually most advanced in the lower end of the femur, then in the lower ends of the leg bones and of the forearm, then in the upper ends of the tibia, femur, and fibula. This is one of the characteristic signs of congenital syphilis.

Acquired syphilis generally involves the periosteum and occurs usually in the tertiary stage. It may appear as a periostitis of the skull, tibia, ulna, etc., with thickening due to hyperplasia. This may become gummatous, set up a superficial erosion and necrosis of adjacent bones, with quite extensive loss of tissue. This is not infrequent in the skull.

Leprosy generally begins in the marrow in the form of nodules, and sets up an osteomyelitis. ' In one form, lepra mutilans, there may be marked destruction and absorption of the phalanges.

Actinomycosis gives rise to an osteomyelitis with more or less caries and necrosis.