7 Infections of bone and joints

INFECTIONS OF BONE

Infection of bone by pyogenic organisms is termed osteomyelitis.¹ It occurs in acute and chronic forms. The only other infections of bone with which the student in Western countries need concern himself are tuberculous infections, although syphilitic infections and fungal infections may still occur occasionally in other parts of the world.

ACUTE OSTEOMYELITIS (Acute pyogenic infection of bone; acute osteitis)

Two distinct types of acute osteomyelitis must be considered:

1 haematogenous osteomyelitis, a disease mainly of childhood in which organisms reach the bone through the blood stream

2 osteomyelitis complicating open fracture or surgical operation, in which organisms gain entry directly through the wound.

The two types are sufficiently distinct to require separate descriptions.

ACUTE HAEMATOGENOUS OSTEOMYELITIS

Acute haematogenous osteomyelitis is one of the important diseases of childhood. Only about 5% of cases occur in adults. Early diagnosis is especially important because a satisfactory outcome depends upon prompt and efficient treatment.

Cause. It is caused by infection of the bone with pyogenic organisms – usually Staphylococcus aureus, though a variety of other organisms are occasionally responsible, including Salmonella. A minor injury to a bone may render it vulnerable to infection by organisms circulating in the blood.

Pathology. Organisms reach the bone through the blood stream from a septic focus elsewhere in the body – for instance from a boil in the skin. In a rare atypical form in adults infection reaches the vertebral column through the spinal venous plexus from an infected intrapelvic lesion.

In the usual childhood manifestation, the infection begins in the metaphysis of a long bone, which must be presumed to form a productive medium for bacterial growth (Fig. 7.1A); thence it may spread to involve a large part of the bone. The organisms induce an acute inflammatory reaction, but the marshalling of the body’s defensive forces is greatly handicapped in bone because its rigid structure does not allow swelling. Pus is formed and soon finds its way to the surface of the bone where it forms a subperiosteal abscess (Fig. 7.1B); later the abscess may burst into the soft tissues and may eventually reach the surface to form a sinus.

Fig. 7.1 The natural evolution of a focus of osteomyelitis. A Initial lesion in the metaphysis. B Pus has escaped to the surface of the bone and formed a subperiosteal abscess. Part of the bone has lost its blood supply from septic thrombosis of vessels. C The devitalised area eventually separates as a sequestrum. Meanwhile new bone (involucrum) is formed beneath the stripped-up periosteum; it is perforated by sinuses through which pus escapes. This is the stage of chronic osteomyelitis. With prompt treatment the disease can often be arrested at the stage shown in A.

Often the blood supply to a part of the bone is cut off by septic thrombosis of the vessels (Fig. 7.1B). The ischaemic bone dies and eventually separates from the surrounding living bone as a sequestrum (Fig. 7.1C). Meanwhile new bone is laid down beneath the stripped-up periosteum, forming an investing layer known as the involucrum (Fig. 7.1C).

The epiphysial cartilage plate is a barrier to the spread of infection, but if the affected metaphysis lies partly within a joint cavity the joint is liable to become infected (acute pyogenic arthritis). Metaphyses that lie wholly or partly within a joint cavity include the upper metaphysis of the humerus, all the metaphyses at the elbow, and the upper and lower metaphyses of the femur (Fig. 7.2). Even when the joint is not infected it may swell from an effusion of clear fluid (sympathetic effusion).

Fig. 7.2 The metaphyses shown in black are wholly or partly intracapsular. Infection at one of these sites is liable to involve the adjacent joint, with consequent pyogenic arthritis.

With efficient treatment, the infection may be aborted in its earliest phase. But when it has progressed to the stage of septic thrombosis and death of bone it almost inevitably passes into a state of chronic osteomyelitis.

Clinical features. Except for rare exceptions acute haematogenous osteomyelitis is confined to children, especially boys. The bones most commonly affected are the tibia, the femur and the humerus. The onset is rapid. The child complains of feeling ill, and of severe pain over the affected bone. There may be a history of recent boils or of a minor injury.

On examination there is obvious constitutional illness with pyrexia. Locally there is exquisite tenderness over the affected bone. The area of tenderness is clearly circumscribed; it is usually near the end of the bone in the metaphysial region. The overlying skin is warmer than normal, and often the soft tissues are indurated; later a fluctuant abscess may be present. The neighbouring joint is sometimes distended with clear fluid, but a good range of movement is retained unless the infection has spread to the joint (septic arthritis): in that event movement would be greatly restricted.

Imaging. Radiographic examination in the early stage does not show any alteration from the normal (Fig. 7.3A). Only after two or three weeks do visible changes appear, and they may never do so if efficient treatment is started very early. The important changes are diffuse rarefaction of the metaphysial area and new bone outlining the raised periosteum (Figs 7.3B and 7.4).