Aetiology and Pathogenesis

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Based upon clinical manifestations, osteomyelitis can be subdivided into the following subtypes:

1. Acute or haematogenous osteomyelitis

2. Subacute or focal osteomyelitis

3. Chronic osteomyelitis

4. Postoperative osteomyelitis

5. Neonatal osteomyelitis

Acute or haematogenous osteomyelitis is the most frequently encountered type of osteomyelitis during childhood (Nelson, 1997; Gutierrez, 2005). Usually it is seen in children under the age of 5 years who have no specific medical history. They present with acute systemic complaints as a result of bacteria or bacterial products reaching the bloodstream. In the majority of cases, S. aureus is found as the causative micro-organism. Sometimes group A streptococci or pneumococci are isolated, especially in younger children. Before routine vaccination against Haemophilus influenzae type b (Hib), this micro-organism played an important role. However through high vaccination coverage, Hib is rarely isolated in cases of osteomyelitis nowadays.

When patients have underlying diseases, other micro-organisms may be isolated, e.g. association between sickle cell anaemia and Salmonella (Burnett et al.,

The bone is infected by bacteria that have spread through the bloodstream. In 30% of cases a history of trauma is present shortly before the onset of the infection. Sometimes boils or other staphylococcal skin-infections might have preceded osteomyelitis. Acute osteomyelitis is sometimes seen in association with chicken-pox and secondary impetigo. In these cases Group A streptococci are frequently isolated.

The primary focus of infection is usually the metaphysis. Several factors are responsible for this preference. Firstly, the metaphyses of growing children are well supplied with blood and blood vessels. Secondly, the slow blood flow through the strongly curved capillary network gives the micro-organisms the opportunity to bind to the endothelium and penetrate through the fenestrated endothelial cells into the bone tissue. Additionally, the normal phagocytosing function of the endothelial cells is absent in the metaphyseal blood vessels. Thirdly, there are hardly any immune-cells (e.g., macrophages) present in the metaphysis to respond to the incoming micro-organisms (Sonnen and Henry, 1996). Furthermore, S. aureus possesses an adherence factor for fibronectin, a protein that is widely distributed within bone and connective tissue. Using this adherence factor S. aureus may easily stick to bone tissue in the metaphyseal region.

Characteristic of the course of osteomyelitis is the spread of the inflammatory exudate to the subperiostal areas through the canals of Volkmann and the Haversian system (Sonnen and Henry, 1996). When the subperiostal pressure increases, the blood flow to the metaphysis and the bone cortex becomes endangered. This eventually leads to bone necrosis and the formation of a sequestrum.

Since there is a close relationship between the metaphysis and the adjacent joint, the infection may easily spread to the joint as well. Sometimes arthritis is the clinical presentation of an osteomyelitis in the adjacent bone, especially when S. aureus is isolated.

Subacute or focal osteomyelitis usually occurs after human or animal bites or puncturing of the skin by sharp materials (broken glass or nails). Pathogens reach the bone tissue directly from the surface. The spectrum of bacteria involved is broader and includes Pseudomonas aeruginosa, S. aureus and anaerobic microorganisms. Clinical symptoms are usually limited to local swelling, pain and redness. General symptoms are frequently lacking (Jacobs et al., 1989).

Chronic osteomyelitis is a specific type of osteomyelitis in which the primary infection has not been diagnosed or has been insufficiently treated. Smouldering inflammation affects the bone and leads to its destruction. Formation of bone sequestrum, pathological fractures and severe growth retardation are the consequence. Surgical treatment is an essential part of the treatment.

Postoperative osteomyelitis is a type of osteomyelitis that occurs in superficial localised bones (e.g., sternum) after a surgical procedure. Micro-organisms that live on or in the skin cause the infection. In addition to pathogens such as S. aureus and P. aeruginosa, other micro-organisms may be encountered like S epidermidis. Patients with diabetes mellitus or corticosteroid treatment do have a higher risk of developing this type of osteomyelitis.

Neonatal osteomyelitis is caused by the same pathogens as found in neonatal sepsis. Frequently, Group B streptococci are isolated, but E. coli, S. aureus or Candida albicans are also found as causative pathogens. Communication between blood vessels of the metaphysis and the epiphysis lead to progressive spread of the infection to the epiphysis itself and to the adjacent joint. In comparison with osteomyelitis in the older child, this type is more aggressive in its clinical course. In 40% of cases multiple foci of infections are demonstrated on Tc-scans (Asmar, 1992).

Septic arthritis is an infection of the synovial space. As in osteomyelitis, three main routes of infection exist: haematogenous spread through the capillary vascular network of the synovium, by contiguous spread (e.g., osteomyelitis) or after trauma or surgery. It is a condition that needs immediate treatment. Delay in diagnostic procedures and treatment leads to destruction of the joint cartilage and cause irreparable damage.

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