INFECTIONS THROUGH THE BLOOD-STREAM

#Diseases caused by the Staphylococcus Aureus.#—­As the majority of pyogenic diseases are due to infection with the staphylococcus aureus, these will be described first.

#Acute osteomyelitis# is a suppurative process beginning in the marrow and tending to spread to the periosteum.  The disease is common in children, but is rare after the skeleton has attained maturity.  Boys are affected more often than girls, in the proportion of three to one, probably because they are more liable to exposure, to injury, and to violent exertion.

Etiology.—­Staphylococci gain access to the blood-stream in various ways, it may be through the skin or through a mucous surface.

Such conditions as, for example, a blow, some extra exertion such as a long walk, or exposure to cold, as in wading, may act as localising factors.

The long bones are chiefly affected, and the commonest sites are:  either end of the tibia and the lower end of the femur; the other bones of the skeleton are affected in rare instances.

Pathology.—­The disease commences and is most intense in the marrow of the ossifying junction at one end of the diaphysis; it may commence at both ends simultaneously—­bipolar osteomyelitis; or, commencing at one end, may spread to the other.

The changes observed are those of intense engorgement of the marrow, going on to greenish-yellow purulent infiltration.  Where the process is most advanced—­that is, at the ossifying junction—­there are evidences of absorption of the framework of the bone; the marrow spaces and Haversian canals undergo enlargement and become filled with greenish-yellow pus.  This rarefaction of the spongy bone is the earliest change seen with the X-rays.

The process may remain localised to the ossifying junction, but usually spreads along the medullary canal for a varying distance, and also extends to the periosteum by way of the enlarged Haversian canals.  The pus accumulates under the periosteum and lifts it up from the bone.  The extent of spread in the medullary canal and beneath the periosteum is in close correspondence.  The periosteum of the diaphysis is easily separated—­hence the facility with which the pus spreads along the shaft; but in the region of the ossifying junction it is raised with difficulty because of its intimate connection with the epiphysial cartilage.  Less frequently there is more than one collection of pus under the periosteum, each being derived from a focus of suppuration in the subjacent marrow.  The pus perforates the periosteum, and makes its way to the surface by the easiest anatomical route, and discharges externally, forming one or more sinuses through which fresh infection may take place.  The infection may spread to the adjacent joint, either directly through the epiphysis and articular cartilage, or along the deep layer of the periosteum and its continuation—­the capsular ligament.  When the epiphysis is intra-articular, as, for example, in the head of the femur, the pus when it reaches the surface of the bone necessarily erupts directly into the joint.