Osteomyelitis is an acute or chronic inflammatory process of the bone and its structures secondary to infection with pyogenic organisms.
Osteomyelitis may be localized or it may spread through the periosteum, cortex, marrow, and cancellous tissue. The bacterial pathogen varies on the basis of the patient's age and the mechanism of infection.
The following are the 2 primary categories of acute osteomyelitis: hematogenous osteomyelitis and direct or contiguous inoculation osteomyelitis.
Hematogenous osteomyelitis is an infection caused by bacterial seeding from the blood. Acute hematogenous osteomyelitis is characterized by an acute infection of the bone caused by the seeding of the bacteria within the bone from a remote source. This condition primarily occurs in children. The most common site is the rapidly growing and highly vascular metaphysis of growing bones. The apparent slowing or sludging of blood flow as the vessels make sharp angles at the distal metaphysis predisposes the vessels to thrombosis and the bone itself to localized necrosis and bacterial seeding.
Vertebral osteomyelitis at any age is most often a secondary complication of a remote infection with hematogenous seeding. In approximately one half of vertebral osteomyelitis cases, a source can be identified such as urinary tract or skin, and approximately one third may be diagnosed with endocarditis.  Acute hematogenous osteomyelitis, despite its name, may have a slow clinical development and insidious onset.
Direct or contiguous inoculation osteomyelitis is caused by direct contact of the tissue and bacteria during trauma or surgery. Direct inoculation (contiguous-focus) osteomyelitis is an infection in the bone secondary to the inoculation of organisms from direct trauma, spread from a contiguous focus of infection, or sepsis after a surgical procedure. Clinical manifestations of direct inoculation osteomyelitis are more localized than those of hematogenous osteomyelitis and tend to involve multiple organisms.
Additional categories include chronic osteomyelitis and osteomyelitis secondary to peripheral vascular disease. Chronic osteomyelitis persists or recurs, regardless of its initial cause and/or mechanism and despite aggressive intervention. Although listed as an etiology, peripheral vascular disease is actually a predisposing factor rather than a true cause of infection.
Disease states known to predispose patients to osteomyelitis include diabetes mellitus,  sickle cell disease, acquired immune deficiency syndrome (AIDS), intravenous (IV) drug abuse, alcoholism, chronic steroid use, immunosuppression, and chronic joint disease. In addition, the presence of a prosthetic orthopedic device is an independent risk factor, as is any recent orthopedic surgery or open fracture.
The overall prevalence is 1 case per 5,000 children. Neonatal prevalence is approximately 1 case per 1,000. The annual incidence in patients with sickle cell anemia is approximately 0.36%. The prevalence of osteomyelitis after foot puncture (as is seen in the image below) may be as high as 16% (30-40% in patients with diabetes). The incidence of vertebral osteomyelitis is approximately 2.4 cases per 100,000 population. 
The overall incidence is higher in developing countries.
Morbidity can be significant and can include localized spread of infection to associated soft tissues or joints; evolution to chronic infection, with pain and disability; amputation of the involved extremity; generalized infection; or sepsis. As many as 10-15% of patients with vertebral osteomyelitis develop neurologic findings or frank spinal-cord compression. As many as 30% of pediatric patients with long-bone osteomyelitis may develop deep venous thrombosis (DVT). The development of DVT may also be a marker for disseminated infection. [3, 4] Vascular complications appear to be more common with community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) than was previously recognized. 
Mortality rates are low, unless associated sepsis or an underlying serious medical condition is present.
No increased incidence of osteomyelitis is noted based on race.
Males are at increased relative risk, which increases through childhood, peaking in adolescence and falling to a low ratio in adults. 
In general, osteomyelitis has a bimodal age distribution. Acute hematogenous osteomyelitis is primarily a disease in children. Direct trauma and contiguous focus osteomyelitis are more common among adults and adolescents than in children. Vertebral osteomyelitis is more common in persons older than 45 years.
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