Osteomyelitis (OM) is defined as an infection of the bone marrow and adjacent osseous structures with potential surrounding soft tissue extent. Staphylococcus aureus is the causative organism in up to 80% of cases [1,2]. Clinical symptoms include fever, malaise, localized pain, swelling and redness, although they may be minimal or absent [1,3].

Three main routes for the spread of OM have been described:

1. Haematogenous spread (most frequent in children): Blood-borne organisms are deposited in the medullary cavity and form a nidus of infection. In long bones, the region which is most affected is the metaphysis (due to discontinuity in the endothelial lining of the vessel walls and slow-flowing blood). In flat bones, the equivalent regions are the bony-cartilaginous junctions. The nature of the blood supply to bone depends on the age of the patient [1-3]. (Figure 1)
   

   Fig 1: Figure 1. Changes of the blood supply to bone depending on the age of the...
    
2. Direct inoculation: It can occur due to open fractures, insertion of metallic implants or joint prostheses, human or animal bites and puncture wounds. Relatively common in hands and feet. [4] (Figure 2)
   

   Fig 2: Figure 2. Contiguous spread in Osteomyelitis. 40-year-old paraplegic man with...
3. Contiguous spread (most frequent in adults): It is related to transmission of infection through the skin or from adjacent tissues. Most common scenarios: diabetic pedal infection and pelvic OM in paralyzed patients. [1] (Figure 3)
   

   Fig 3: Figure 3. Osteomyelitis caused by direct implantation. A 60-year-old man...
    

Generally we can distinguish three clinical stages (Figure 4) although in clinical practice there may be overlap between these stages:

1. Acute OM: Bacterial proliferation within the bone induces an acute suppurative response. Intramedullary pus can form a Brodie abscess (intramedullary abscess with surrounding sclerosis) or spread and through the cloaca to a subperiosteal abscess. [2,3]
2. Subacute OM
3. Chronic OM: Result of osteonecrosis, caused by disruption of the intraosseous and periosteal blood supply during the acute stage.  When a dead bone fragment becomes separated from viable bone and is known as a sequestrum. When the sequestrum becomes surrounded by granulation tissue/reactive new bone, it is known as an involucrum. [1]

Fig 4: Figure 4. Clinical stages of osteomyellitis