Osteomyelitis

Osteomyelitis is an infection of bone or bone marrow, usually caused by pyogenic bacteria or mycobacteria. It can be usefully subclassified on the basis of the causative organism, the route, duration and anatomic location of the infection.

Presentation

Generally, microorganisms may infect bone through one or more of three basic methods: via the bloodstream, contiguously from local areas of infection (as in cellulitis), or penetrating trauma, including iatrogenic causes such as joint replacements or internal fixation of fractures or root-canaled teeth. Once the bone is infected, leukocytes enter the infected area, and in their attempt to engulf the infectious organisms, release enzymes that lyse the bone. Pus spreads into the bone's blood vessels, impairing their flow, and areas of devitalized infected bone, known as sequestra, form the basis of a chronic infection. Often, the body will try to create new bone around the area of necrosis. The resulting new bone is often called an involucrum. On histologic examination, these areas of necrotic bone are the basis for distinguishing between acute osteomyelitis and chronic osteomyelitis. Osteomyelitis is an infective process which encompasses all of the bone (osseous) components, including the bone marrow. When it is chronic it can lead to bone sclerosis and deformity.

In infants, the infection can spread to the joint and cause arthritis. In children, large subperiosteal abscesses can form because the periosteum is loosely attached to the surface of the bone.

Because of the particulars of their blood supply, the tibia, femur, humerus, vertebra, the maxilla, and the mandibular bodies are especially susceptible to osteomyelitis. However, abscesses of any bone may be precipitated by trauma to the affected area. Many infections are caused by Staphylococcus aureus, a member of the normal flora found on the skin and mucous membranes.

Etiology

In children, the long bones are usually affected. In adults, the vertebrae and the pelvis are most commonly affected.

Acute osteomyelitis almost invariably occurs in children. When adults are affected, it may be because of compromised host resistance due to debilitation, intravenous drug abuse, infectious root-canaled teeth, or other disease or drugs (e.g. immunosuppressive therapy).

Osteomyelitis is a secondary complication in 1-3% of patients with pulmonary tuberculosis. In this case, the bacteria generally spread to the bone through the circulatory system, first infecting the synovium (due to its higher oxygen concentration) before spreading to the adjacent bone. In tubercular osteomyelitis, the long bones and vertebrae are the ones which tend to be affected

Treatment

Osteomyelitis often requires prolonged antibiotic therapy, with a course lasting a matter of weeks or months. A PICC line or central venous catheter is often placed for this purpose. Osteomyelitis also may require surgical debridement. Severe cases may lead to the loss of a limb. Initial first line antibiotic choice is determined by the patient's history and regional differences in common infective organisms.

In 1875, American artist Thomas Eakins depicted a surgical procedure for osteomyelitis at Jefferson Medical College, in a famous oil painting titled The Gross Clinic.

Prior to the widespread availability and use of antibiotics, blow fly larvae were sometimes deliberately introduced to the wounds to feed on the infected material, effectively scouring them clean.

Hyperbaric oxygen therapy has been shown to be a useful adjunct to the treatment of refractory osteomyelitis. A treatment lasting 42 days is practiced in a number of facilities.

Causes

Staphylococcus aureus is the organism most commonly isolated from all forms of osteomyelitis.

Bloodstream-sourced osteomyelitis is seen most frequently in children, and nearly 90% of cases are caused by Staphylococcus aureus. In infants, S. aureus, Group B streptococci (most common) and Escherichia coli are commonly isolated; in children from 1 to 16 years of age, S. aureus, Streptococcus pyogenes, and Haemophilus influenzae are common. In some subpopulations, including intravenous drug users and splenectomized patients, Gram-negative bacteria, including enteric bacteria, are significant pathogens.

The most common form of the disease in adults is caused by injury exposing the bone to local infection. Staphylococcus aureus is again the most common organism seen in osteomyelitis seeded from areas of contiguous infection, but anaerobes and Gram-negative organisms, including Pseudomonas aeruginosa, E. coli, and Serratia marcescens, are also common, and mixed infections are the rule rather than the exception.

Systemic mycotic (fungal) infections may also cause osteomyelitis. The two most common pathogens involved in such infections are Blastomyces dermatitidis and Coccidioides immitis.

In osteomyelitis involving the vertebral bodies, about half the cases are due to Staphylococcus aureus, and the other half are due to tuberculosis (spread hematogenously from the lungs). Tubercular osteomyelitis of the spine was so common before the initiation of effective antitubercular therapy that it acquired a special name, Pott's disease, by which it is sometimes still known. The Burkholderia cepacia complex have been implicated in vertebral osteomyelitis in intravenous drug abusers.

Diagnosis of osteomyelitis is often based on radiologic results showing a lytic center with a ring of sclerosis, though bone cultures are normally required to identify the specific pathogen.

Factors that may commonly complicate osteomyelitis are fractures of the bone, amyloidosis, endocarditis, or sepsis.