What causes osteomyelitis?
Many different types of bacteria and viruses can cause osteomyelitis. The most common type of bacteria is called Staphylococcus aureus. Staphylococcus aureus, a bacterium, is the most common organism involved in osteomyelitis. Other types of organisms include the mycobacterium which causes tuberculosis, a type of Salmonella bacteria in patients
with sickle cell anemia, Pseudomonas aeurginosa in drug addicts, and organisms which usually reside in the gastrointestinal tract in the elderly. Extremely rarely, the viruses which cause chickenpox and smallpox have been found to cause a viral osteomyelitis.
Generally microorganisms may be disseminated to bone hematogenously (i.e., via the blood stream), spread contiguously to bone from local areas of infection, such as cellulitis, or be introduced by penetrating trauma. Leukocytes then enter the infected area, and in their attempt to engulf the infectious organisms, release enzymes that lyse bone. Pus spreads into the bone's blood vessels, impairing the flow, and areas of devitalized infected bone, known as sequestra, form the basis of a chronic infection. On histologic examination, these areas of necrotic bone are the basis for distinguishing between acute osteomyelitis and chronic osteomyelitis.
Infection of bone is produced by bloodborne organisms (hematogenous osteomyelitis); spread from infected tissue, including prosthetic joint infection; contaminated fractures; and bone surgery. The host responds to the presence of bacteria in the metaphysis with a local increase in vascular permeability, resulting in edema, increased vascularity and the influx of polymorphonuclear leukocytes. Pressure increases as pus collects and is confined within rigid bone. Exudation through Volkmann's canals and the haversian canal affords little relief, although the relatively inelastic periosteum may become elevated. The blood supply to the area of involvement is decreased secondary to the pressure; necrosis of the infected bone may result in the formation of a sequestrum. A protein-rich liquid containing inflammatory cells may collect in an adjacent joint but such effusions are sterile. After the vascular supply to the involved area has been interrupted and necrosis has occurred, the chronic phase of osteomyelitis is established. The residual dead bone acts as a foreign body, making the eradication of bacteria impossible until the sequestrum is removed.
If the infected area becomes well demarcated and the infection is contained, the acute inflammatory process may subside, leaving a subperiosteal accumulation of pus which may be discovered by tenderness on palpation. This relatively quiescent form of subperiosteal infection is termed a Brodie's abscess. After some time, there is deposition of new bone, the involucrum, under the elevated periosteum. In osteomyelitis of the spine, infection most often involves the vertebral body. It spreads readily through the anastomotic venous system to adjacent ligaments and vertebral bodies. It is common for more than one vertebral body to be involved. Pus may accumulate between the vertebral periosteum and dura mater, forming an extradural abscess. Compression of the spinal cord may result, yielding a paraplegia. If a subdural abscess ruptures into the subarachnoid space, meningitis results. |
