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

Two distinct types of acute osteomyelitis must be considered:

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).

Fig. 7.3 (left) Acute osteomyelitis of the ulna in a child. A The initial film taken 2 days after the onset does not show any abnormality. B Two weeks later a faint shadow along the radial side of the ulna denotes new bone formation beneath the raised periosteum.

Fig. 7.4 (right) Osteomyelitis of the femur in an infant, three weeks after onset. Note new bone outlining the raised periosteum, and area of rarefaction in metaphysis and epiphysis. The infection spread to the knee joint.

Radioisotope scanning with ^99mtechnetium is reserved for the diagnosis of bone infection in the less clinically accessible sites such as the hip, pelvis and spine. Accumulation of isotope depends upon the rate of bone turnover and its vascularity, so that in the early stages of disease inadequate blood supply may result in a ‘cold’ lesion. More commonly, within a few hours or days of the onset of symptoms there is an increased uptake of isotope, giving a ‘hot’ scan at the site of the bone lesion. In the more obscure low-grade infections associated with prostheses and other surgical implants in bone, scanning with re-injected leucocytes labelled with ^111mindium may provide improved diagnostic accuracy.

MRI. In many centres this has now superseded isotope scanning as it provides more anatomical information on the infection.

Investigations. Blood culture may be positive in the incipient stage but this is not an invariable feature. Later there is a marked polymorphonuclear leucocytosis. The C-reactive protein and erythrocyte sedimentation rate is increased.

Diagnosis. Acute osteomyelitis is to be distinguished from pyogenic arthritis of the adjacent joint by the following features:

Acute osteomyelitis may also be confused with rheumatic fever, and, in infants, with vitamin C deficiency (scurvy) (p. 80). Whenever possible the causative organism must be identified bacteriologically. It is important that blood for culture be taken before antibiotic therapy is commenced.

Complications. The important complications are:

Fig. 7.5 Two complications of osteomyelitis. A Pyogenic arthritis, which led in this case to bony ankylosis. B Arrest of epiphysial growth, with consequent shortening. The normal humerus is shown for comparison.

Acute osteomyelitis often passes into a state of chronic infection.

Treatment. Efficient treatment must be begun at the earliest possible moment.

General treatment. This is by rest in bed and systemic antibiotic therapy, started intravenously to ensure high blood levels. Initially, it is recommended that broad-spectrum antibiotics with good anti-Staphylococcus activity are used, such as a third-generation cephalosporin combined with a synthetic penicillin, but as soon as the causative organism has been identified the antibiotic to which it is most sensitive should be ordered. In cases where a multiple-resistant Staphylococcus aureus (MRSA) is suspected it may be appropriate to use vancomycin instead of the penicillin. Antibiotics should be continued for at least 4 weeks, even when the response has been rapid.

Local treatment. The question of operation and its timing is still controversial. Operation may be unnecessary if effective antibiotic treatment can be begun within 24 hours of the onset of symptoms, and this should always be the aim. But in practice diagnosis is not always so prompt, and in that event it seems wiser to undertake early operation, in order to release pus and to relieve pain, which is often severe. This should definitely be performed if there has not been a marked improvement to the antibiotic treatment within 48 hours. Release of pus may also reduce the risk of ischaemic bone necrosis; and furthermore it allows the organism to be identified and its sensitivity to antibiotics determined. An incision at the site of maximum tenderness is made down to the bone and subperiosteal pus is evacuated. It is advisable – though not always essential – to make one or two drill holes through the cortex to improve medullary drainage. In most cases the wound may safely be sutured. Thereafter the limb is splinted until the infection is overcome.

OSTEOMYELITIS COMPLICATING OPEN FRACTURE OR SURGICAL OPERATION

When acute infection complicates open fracture or surgical operation the organisms are introduced directly through the wound. Any part of the bone may be affected, depending upon the site of injury. Suppuration and necrosis occur as in haematogenous osteomyelitis, but the pus discharges through the primary wound rather than collecting under the periosteum. The infection often becomes chronic.

Clinical features. This type of osteomyelitis may occur in children or in adults. The temperature fails to settle after the primary treatment of the wound or rises a few days later. Pain is not a prominent feature because pus is not contained under pressure. Re-examination of the wound reveals a purulent discharge.

Radiographic features. In the early stages radiographs do not help significantly. Later there may be local rarefaction, and eventually sequestrum formation may be evident.

Treatment. The main principle of treatment is to secure free drainage through the wound, which may be enlarged if necessary for the purpose. Appropriate antibacterial drugs should be ordered. Later, any bone fragment that has sequestrated should be removed.

CHRONIC OSTEOMYELITIS (Chronic pyogenic osteomyelitis)

Chronic osteomyelitis is nearly always a sequel to acute osteomyelitis. Occasionally infection is subacute or chronic from the beginning.

Cause. As with acute osteomyelitis, the Staphylococcus is the usual causative organism, but streptococci, pneumococci, typhoid bacilli, or other bacteria may be responsible.

Pathology. It is commonest in the long bones. It is often confined to one end of the bone, but it may affect the whole length. The bone is thickened and generally denser than normal, though often honeycombed with granulation tissue, fibrous tissue, or pus. Sequestra are commonly present within cavities in the bone. Often a sinus track leads to the skin surface: the sinus tends to heal and break down recurrently, but if a sequestrum is present it never heals permanently.

Clinical features