Pathological fracture of the shaft may occur at the site of necrosis, when the new case is incapable of resisting the strain put upon it, and is most frequently met with in the shaft of the femur.  Short of fracture, there may be bending or curving of the new case, and this results in deformity and shortening of the limb (Fig. 119).

The extrusion of a sequestrum may occur, provided there is a cloaca large enough to allow of its escape, but the surgeon has usually to interfere by performing the operation of sequestrectomy.  Displacement or partial extrusion of the dead bone may cause complications, as when a sequestrum derived from the trigone of the femur perforates the popliteal artery or the cavity of the knee-joint, or a sequestrum of the pelvis perforates the wall of the urinary bladder.

The extent to which bone which has been lost is reproduced varies in different parts of the skeleton:  while the long bones, the scapula, the mandible, and other bones which are developed in cartilage are almost completely re-formed, bones which are entirely developed in membrane, such as the flat bones of the skull and the maxilla, are not reproduced.

[Illustration:  FIG. 119.—­Femur and Tibia showing results of Acute Osteomyelitis affecting Trigone of Femur; sequestrum partly surrounded by new case; backward displacement of lower epiphysis and implication of knee-joint.]

It may be instructive to describe the X-ray appearances of a long bone that has passed through an attack of acute osteomyelitis severe enough to have caused necrosis of part of the diaphysis.  The shadow of the dead bone is seen in the position of the original shaft which it represents; it is of the same shape and density as the original shaft, while its margins present an irregular contour from the erosion concerned in its separation.  The sequestrum is separated from the living bone by a clear zone which corresponds to the layer of granulations lining the cavity in which it lies.  This clear zone separating the shadow of the dead bone from that of the living bone by which it is surrounded is conclusive evidence of a sequestrum.  The medullary canal in the vicinity of the sequestrum being obliterated, is represented by a shadow of varying density, continuous with that of the surrounding bone.  The shadow of the new case or involucrum with its wavy contour is also in evidence, with its openings or cloacae, and is mainly responsible for the increase in the diameter of the bone.

The skiagram may also show separation and displacement of the adjacent epiphysis and destruction of the articular surfaces or dislocation of the joint.

Sequelae of Acute Suppurative Osteomyelitis.—­The commonest sequel is the presence of a sequestrum with one or more discharging sinuses; owing to the abundant formation of scar tissue these sinuses have rigid edges which are usually depressed and adherent to the bone.