Osteomyeltis is an infection of bone. Osteomyelitis can be caused by hematogenous spread, contiguous spread from adjacent soft tissues or joints, or direct inoculation (ie: during trauma or surgery)
In addition to bacteria, the focus of osteomyelitis contains dead bone, local inflammation and granulation tissue.
Microbiology most often reveals Staph aureus infection, although Staph epidermis and gram negative rods are implicated in a minority of cases.
Bone is normally resistant to infection. In order to overcome the normal immune system, bacteria must be present in large number (high inoculum), be able to adhere to the bone surface or bone ischemia must be present to inhibit to a local immune effort. Bone trauma, especially open fracture (thus exposing bone to environmental pathogens), establishes these conditions necessary for chronic infection. The site is most frequently the tibia, as this is the most common site of open fractures. (Non-union of the fractured bone, possibly associated with recurrent infection, is a major complication.)
Typical symptoms include local bone pain, erythema, swelling as well as systemic symptoms such as fever. A classic syndrome of cyclic pain correlating with the progression of bone necrosis has been described, although it is not a sensitive finding.
On exam, signs of local infection may be seen, possibly including small recurrent sinus tracts draining to the skin (actually communicating with underlying infected bone).
Abscesses form within the necrotic bone, expanding contiguously into bone and soft tissue, Sinus tracts may develop, which can lead to the skin and be seen as superficial orifices on exam.
Bone grows over the infection, producing a “sequestrum”— the island of bone shown below. The infected bone effectively gets walled off from the circulation.
!worddav105cfd12dc27921bf7d46f782bf47e3a.png|align=left!The sequestering of the sequestrum is a positive adaptation in evolution: it keeps that bad segregated from the good. Evolution did not anticipate that we would have medicine to deliver, and today this wall-it-off strategy is a problem. Antibiotics can’t reach their target. As such, surgical debridement is often needed.
Poor nutrition and excess alcohol intake are thought to predispose at-risk patients to developing post-traumatic osteomyelitis. Additionally, patients with diabetes may develop vascular insufficiency, which would also put them at higher risk for developing osteomyelitis in the right setting.
Imaging: Plain films may show characteristic changes after 10-14 days (at which point necrotic changes, including the involucrum may be evident); three phase bone scan is high sensitivity (>90%) but low specificity (<10%); radiolabeled leukocyte imaging is the best nuclear medicine choice for non-vertebral osteo. It too has high sensitivity but higher specificity. MRI shows decreased intensity of marrow on T1 imaging at foci of infection. A bright signal on T2 may be seen, representing granulation tissue.
WBC, ESR, CRP are all nonspecific markers of inflammation or infection. But they may be normal in chronic osteomyelitis. In other words, not much help. Cultures of the debrided tissue are helpful in identifying organisms.
It is of course better to prevent (rather than treat) acute osteomyelitis; and it is better to treat before the bone dies and there is progression to “chronic” disease. Surgical debridement of the involved bone and soft tissue (debrided until normal bleeding is observed) is definitive. However, beyond debridement, the surgeon must decide how to stabilize the bone and fill the resulting “dead space.” Some form of rigid fixation had been shown in animals to improve bone union following debridement, and there is evidence supporting both internal and external methods of fixation. At the time of debridement, antibiotic-containing beads may be implanted in this space for up to 30 days (and subsequently removed).
Systemic antibiotics are used as adjuncts to surgery to help prevent infection in the healing of the operative wound and to prevent spread. The antibiotic used is based on culture results.
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