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. The main symptom is usually a purulent discharge from a sinus over the affected bone. In other cases pain is the predominant feature which brings the patient to the doctor. Discharge of pus may be continuous or intermittent. Reappearance of a sinus that has been healed for some time is heralded by local pain, pyrexia, and the formation of an abscess. This is termed a ‘flare-up’, or ‘flare’, of infection.

On examination the bone is palpably thickened, and there are nearly always a number of overlying scars or sinuses.

Imaging. Radiographic examination: The bone is often thickened and shows irregular and patchy sclerosis which may give a honeycombed appearance. If a sequestrum is present it is seen as a dense loose fragment, with irregular but sharply demarcated edges, lying within a cavity in the bone (Fig. 7.6). Radioisotope scanning may show increased uptake in the vicinity of the lesion. In diffuse disease MRI and CT scanning may be of value for localisation of abscess cavities and sequestra, thus allowing accurate planning of operative treatment.

Fig. 7.6 Extensive chronic osteomyelitis of the tibia. The upper part of the shaft is thickened and shows patchy sclerosis. Two cavities are evident, each containing a sequestrum.

Complications. If the bone has been much weakened by disease pathological fracture may occur. Rarely, amyloid disease may complicate long-continued chronic osteomyelitis with persistent discharge of pus. Another rare complication in cases of many years’ duration is the development of squamous celled carcinoma in a sinus.

Treatment. An acute flare-up of chronic osteomyelitis often subsides with rest and antibiotics. If an abscess forms outside the bone it must be drained. If there is a persistent and profuse discharge of pus a more extensive operation is advised. The aim should be to remove fragments of infected dead bone (sequestra) and to open up or ‘saucerise’ abscess cavities by chiselling away the overlying bone. Sometimes it is possible to obliterate a cavity with a flap of muscle, or to exteriorise it and line its walls directly with split-skin grafts. The principles of treatment are (1) remove dead and foreign material, (2) obliterate dead space, (3) if necessary, stabilise the skeleton, (4) obtain soft tissue cover, (5) if necessary, reconstruction of the bone defect, (6) possible appropriate antibiotic cover.

Brodie’s abscess¹ (chronic bone abscess)

This is a special form of chronic osteomyelitis which arises insidiously, without a preceding acute attack. There is a localised abscess within the bone, often near the site of the metaphysis. A deep ‘boring’ pain is the predominant symptom.

Imaging. Radiographically, the lesion is seen as a circular or oval cavity surrounded by a zone of sclerosis (Fig. 7.7A), but the site and extent of the lesion can be shown more accurately on an MRI scan (Fig. 7.7B). The rest of the bone is normal.

Fig. 7.7 A Radiograph of tibia with Brodie’s abscess. There are two lucent areas in the metaphysis with a sclerotic margin and some generalised surrounding bone sclerosis. B Axial MRI scan contrast enhanced image of same tibia. There is a central area of low signal (fluid) which is surrounded by a bright rim of contrast enhancement. There is generalised marrow oedema (bright signal). These findings are diagnostic of a bone abscess.

Treatment is by operation. The cavity is de-roofed and the pus evacuated. Whenever possible the cavity should be filled with a muscle flap to obliterate the dead space.

TUBERCULOUS INFECTION OF BONE

Tuberculous infection of bone is uncommon except in the vertebral bodies and in association with tuberculous infection of joints. Occasionally it occurs as an isolated lesion in a long bone or in a bone of the hand or foot.

Pathology. Tubercle bacilli reach the bone either through the blood stream or by direct extension from an adjacent focus of infection in joint or soft tissue. There is a typical tuberculous inflammatory reaction. Part of the bone is destroyed and replaced by granulation tissue. A tuberculous abscess is commonly formed; it tracks beneath the soft tissues or towards the surface of the body. With treatment there is a tendency to healing, with fibrosis.

Tuberculosis of a vertebra

The infection typically affects the vertebral body. It may arise initially in the bone (Fig. 7.8A) or it may spread to the vertebra from the adjacent intervertebral disc. Tuberculous vertebral bodies collapse anteriorly but often retain their full depth behind, thereby becoming wedge-shaped (Fig. 7.8B). An abscess usually tracks downwards along the vertebral column; it may also extend backwards towards the spinal canal, where it may interfere with the function of the spinal cord.

Fig. 7.8 Tuberculosis beginning in a vertebral body. The infection starts close to the anterior border and adjacent to an intervertebral disc A. It soon involves the disc and may spread to adjoining vertebrae. The bone destruction is most marked anteriorly; so the affected vertebral bodies become wedge-shaped B.

Juxta-articular tuberculosis

The articular ends of bones are frequently eroded by tuberculosis beginning primarily in the joint. Less often there is an isolated focus of infection within the bone (Fig. 7.9). From such a lesion the infection may spread eventually to the neighbouring joint.

Fig. 7.9 Juxta-articular tuberculous focus in the neck of the femur. There is no sclerosis of the surrounding bone. If unchecked by treatment, such a focus of infection may spread to involve the joint.

Bony tuberculosis in the hand or foot

The metacarpals or phalanges are the bones most commonly affected (tuberculous dactylitis). Characteristically the bone is enlarged by a fusiform swelling which at first represents thickened and raised periosteum. Later, much of the original bone is destroyed, but at the same time new bone is laid down beneath the expanded periosteum, giving the affected metacarpal or phalanx a ‘distended’ appearance (Fig. 7.10). Similar changes may affect a bone of the foot, or occasionally a long bone.

Fig. 7.10 Tuberculous dactylitis. The affected phalanx has been ‘distended’ by destruction of the original cortex and the laying down of new cortical bone beneath the expanded periosteum.

Clinical features. There is usually evidence of constitutional ill health. The local symptoms and signs depend upon the site of the infection. In general, pain is the initial symptom; and at most sites it is associated with obvious swelling and often with the formation of a ‘cold’ abscess. When the bone lesion is associated with tuberculous joint disease the joint symptoms predominate (see sections on the individual joints).

Imaging. The typical radiographic features of tuberculous infection of bone are:

1 diffuse rarefaction around the site of infection

2 erosion or ‘eating away’ of bone, leaving a fluffy, ill-defined outline with no suggestion of a surrounding zone of sclerosis

3 in many cases a shadow in the soft tissues, denoting abscess formation.

On radioisotope scanning there is increased uptake of isotope in the vicinity of the lesion.

Investigations. Except in the case of quite small lesions the erythrocyte sedimentation rate is raised. The Mantoux test is positive. Aspirated pus is yellow and creamy: only occasionally can organisms be identified by direct examination, but culture may prove its tuberculous nature. Biopsy of affected bone or surrounding soft tissue will show the histological features of tuberculosis.

Diagnosis. The diagnosis can often be presumed with fair certainty from a consideration of the history, clinical features, and radiographic findings. Features that lend support are a history of contact with tuberculosis, a positive Mantoux test (particularly in children), a raised erythrocyte sedimentation rate, and evidence of a tuberculous lesion elsewhere. The diagnosis can be proved only by identifying the causative organism or by demonstration of the typical histological features in excised fragments of tissue.

Treatment. In most instances tuberculosis of bone is associated with infection of a joint, and the treatment is mainly that of the joint lesion (tuberculous arthritis, p. 98). The treatment of an isolated tuberculous focus in bone is along similar lines, main reliance being placed upon a combination of antituberculous drugs, usually rifampicin and isoniazid for a period of six months, as outlined on page 102. Local collections of pus should be removed by aspiration or, sometimes, by operative drainage followed by immediate suture of the wound. Resolution is indicated by improvement in the general health and weight, decrease of erythrocyte sedimentation rate, and improved radiographic appearance.

SYPHILITIC INFECTION OF BONE

In Western countries syphilitic infection of bone is now seen only rarely, but it is still found in some parts of the world, and it is important that the possibility of its occurrence should be borne constantly in mind. Bone changes are a late manifestation of acquired syphilis, but they may appear early in life in patients with congenital syphilis. Similar bone changes may occur in yaws, a related disease seen commonly in many tropical countries.

Syphilis of bone can take many forms, but the two commonest types are syphilitic metaphysitis of infants, and osteo-periostitis (combined osteitis and periostitis) in children or adults.

Syphilitic metaphysitis presents as severe local limb pain in young infants in the first six months of life with congenital syphilis. It can affect several epiphyses, with replacement of the adjacent metaphysis by granulation tissue. These appear as a zone of sclerosis on radiographs and the Wasserman reaction is positive, though unlike osteomyelitis there is no leucocytosis. Treatment by intensive antisyphilitic drugs is rapidly effective.

Syphilitic osteo-periostitis occurs when the diaphysis or body of a bone is infected by syphilis. There is usually a combination of osteitis and periostitis, although one or other may predominate. Osteo-periostitis often occurs with metaphysitis in infants (Fig. 7.11A); it may occur separately in older children with congenital syphilis (Fig. 7.11B), or in adults with acquired syphilis.

Fig. 7.11 Two examples of congenital syphilis of bone. A Metaphysitis in an infant. Note white lines at ends of metaphyses, with adjacent zones of rarefaction. B Osteo-periostitis in a child. New bone has been laid down in layers under the periosteum.

Clinical features. Deep boring pain, worse at night, and swelling are the predominant symptoms. On examination there is either a localised fusiform swelling over the shaft of the bone or a diffuse thickening of the whole length of the bone.

Radiographic features. Syphilitic bone disease is represented by a variety of radiographic changes ranging from severe osteoporosis to dense sclerosis and may be easily confused with a malignant bone tumour. Occasionally the predominant change is bone destruction without new bone formation.

Investigations. The Wassermann reaction is positive.

Treatment. The bone lesions usually respond well to intensive antisyphilitic measures.