The pyogenic microbes may be directly introduced from without or may reach the bone by the blood, and hence two principal forms may be distinguished.

(A) Acute Traumatic Ostitis

This occurs chiefly in cases of Compound fracture where septic processes have occurred in the wound. It used to be very common before the introduction of antiseptic surgery. The suppurative inflammation extends between the broken ends of the bones, and amongst the surrounding soft tissues, and abscesses frequently form around the seat of injury. There may also be pus in the medullary cavity, disintegrating the bone-marrow. Extension of the septic process to the veins is not uncommon, so that thrombo-phlebitis and pyaemia result.

(B) Acute Infective Ostitis

This condition is described under various names, such as Acute periostitis, Malignant osteomyelitis, Epiphyseal ostitis, Infective ostitis, Pseudo-rheumatic ostitis, Typhus of the limbs. These names refer to special features presented by different cases. The disease consists in an acute inflammation which nearly always goes on to suppuration, and occurs without any externally apparent cause.

Causation

The essential cause here is a pyogenic microbe which in a number of cases has been identified as the Staphylococcus pyogenes aureus. Experiments on animals, already alluded to, show that when a bone is injured and cultures of this microbe are injected into the animal, these will settle in the injured bone and produce a suppurative inflammation. In man the bone is always so far predisposed in respect that the disease is one of Adolescents in whom the tissues are excessively active, and there is usually also the history of an injury inflicted before the beginning of the illness. The bones most frequently affected are those most exposed to injury. The fact that boys are much more frequently attacked than girls, points in the same direction. It may, perhaps, be said that when, by accident, the microbes in question are present in the blood, perhaps by absorption from the alimentary canal, an injury, otherwise trivial, may determine their settlement in a bone, and so the disease may be induced.

Character Of Lesions

The inflammatory process presents varying degrees of severity and extent, as well as considerable variety in its seat. The bones most frequently affected are the long bones, more especially the tibia, but it occurs in the pelvic bones, the vertebrae, especially the atlas, and in other situations.

In some cases the periosteum is chiefly affected, and to these the name of Acute periostitis is often given. The periosteum is swollen and hypersemic, and suppuration usually ensues rapidly. The pus accumulates between the periosteum and the bone, and the periosteum is sometimes gangrenous. The bone beneath frequently undergoes necrosis, but this result is by no means constant.

In other cases there is a more diffused inflammation, affecting the bone and medulla as well as the periosteum, or there may even be a special involvement of the medulla. In these cases pus is present in the medullary cavity and in the substance of the bone. According to Oilier the inflammation frequently localizes itself in the neighbourhood of the ossifying borders of the shafts of long bones. This region as it is next the epiphysis (though a portion of the diaphysis) he designates juxta-epiphyseal. The reason of this localization will be apparent from the fact that this is the region of greatest ossific activity.

Along with these local phenomena there is commonly more or less fever, and this may be very great in comparison with the local phenomena. The designation, ' typhus of the limbs,' points to the occurrence of such severe febrile phenomena.

Necrosis may occur in the several forms of this disease, but it is by no means uniformly present, its occurrence depending chiefly on local interference with the blood-supply, and perhaps also on the condition of the person affected. (See under Necrosis).

Pyaemia occasionally develops as a result of the extension of the septic process to the veins.

(C) Post-Febrile Ostitis

An acute ostitis in many respects similar to that just described, sometimes follows acute febrile conditions, such as acute rheumatism, typhoid fever, scarlet fever, measles, dysentery, etc.

(D) Abscess In Bone

Abscesses are met with in the cancellous tissue of the ends of the long bones, where they are probably the result of an infective or post-febrile ostitis. The abscess expands the end of the bone, which may attain large dimensions.