Amputation of the limb is reserved for grave cases, in which life is endangered by toxaemia, which is attributed to the primary lesion.  It may be called for later if the limb is likely to be useless, as, for example, when the whole shaft of the bone is dead without the formation of a new case, when the epiphyses are separated and displaced, and the joints are disorganised.

Flat bones, such as the skull or ilium, must be trephined and the pus cleared out from both aspects of the bone.  In the vertebrae, operative interference is usually restricted to opening and draining the associated abscess.

#Nature’s Effort at Repair.#—­In cases which are left to nature, and in which necrosis of bone has occurred, those portions of the periosteum and marrow which have retained their vitality resume their osteogenetic functions, often to an exaggerated degree.  Where the periosteum has been lifted up by an accumulation of pus, or is in contact with bone that is dead, it proceeds to form new bone with great activity, so that the dead shaft becomes surrounded by a sheath or case of new bone, known as the involucrum (Fig. 118).  Where the periosteum has been perforated by pus making its way to the surface, there are defects or holes in the involucrum, called cloacae.  As these correspond more or less in position to the sinuses in the skin, in passing a probe down one of the sinuses it usually passes through a cloaca and strikes the dead bone lying in the interior.  If the periosteum has been extensively destroyed, new bone may only be formed in patches, or not at all.  The dead bone is separated from the living by the agency of granulation tissue with its usual complements of phagocytes and osteoclasts, so that the sequestrum presents along its margins and on its deep surface a pitted, grooved, and worm-eaten appearance, except on the periosteal aspect, which is unaltered.  Ultimately the dead bone becomes loose and lies in a cavity a little larger than itself; the wall of the cavity is formed by the new case, lined with granulation tissue.  The separation of the sequestrum takes place more rapidly in the spongy bone of the ossifying junction than in the compact bone of the shaft.

When foci of suppuration have been scattered up and down the medullary cavity, and the bone has died in patches, several sequestra may be included by the new case; each portion of dead bone is slowly separated, and comes to lie in a cavity lined by granulations.

Even at a distance from the actual necrosis there is formation of new bone by the marrow; the medullary canal is often obliterated, and the bone becomes heavier and denser—­sclerosis; and the new bone which is deposited on the original shaft results in an increase in the girth of the bone—­hyperostosis.

[Illustration:  FIG. 118.—­Shaft of Femur after Acute Osteomyelitis.  The shaft has undergone extensive necrosis, and a shell of new bone has been formed by the periosteum.]