Acute haematogenous osteomyelitis

Commonly affected sites are metaphyseal regions with high vascular flow, in particular:

• Distal and proximal femur

• Proximal tibia

• Proximal humerus

• Distal radius

Pathological changes:

1. The bacteria (in this case S. Aureus) lodge in Haversian canals or marrow spaces  elicit an acute inflammatory response.

Trauma may be an important predisposing factor to the development of bacterial osteomyelitis, as it is associated with stasis and or thrombosis, thus providing a nidus for infection.

2. Acute inflammatory response (oedema)   intracompartmental pressure, which compromises blood supply to osteocytes eventually  necrosis of bone.

3. Infected and inflammatory focus is walled off by granulation tissue and then dense fibrous tissue, the result of which is the formation of an abscess containing necrotic bone known as a sequestrum.

[Abscess + necrotic bone = sequestrum]

4. Exudate passes through the cortex and in children elevates the periosteum, or when the infection is localized an abscess (Brodie’s abscess) forms.

5. The formation of new reactive bone about the abscess, a further attempt to isolate the infection with a shell of bone, results in the formation of an enveloping mass of new reactive bone, i.e. involucrum.

Outcome: is determined by:

• Anatomic location

• Host response / resistance

• Number and relative virulence of the pathogens

• Effectiveness of therapy

Ideally with resolution and repair, the sequestrum is resolved, the organisms are dealt with by the immune and inflammatory systems and the defects fill with granulation tissue, which ultimately undergoes intramembranous ossification and healing.

Complications: if the host response is inadequate, the sequestrum and bacteria will persist within a non-healing cavity and result in disease chronicity.

Chronic osteomyelitis develops following inadequate antibiotic therapy, inadequate surgical removal of necrotic sequestrum or in immunocompromised individuals.

Necrotic bone within the sequestrum provides a haven for bacteria. The infection may gradually spread, extending beyond the confines of the involucrum and bone, into the adjoining soft tissues. Having done so, the infection will dissect its way to the skin surface forming draining sinus tracts. The latter may undergo malignant transformation i.e. SCC, or Marjolin’s ulcer. Amyloidosis is another potential complication of chronic osteomyelitis.

Category: Orthopedics Notes