Osteomyelitis is an infectious inflammatory disease of bone, often of bacterial origin. Early diagnosis, antibiotic therapy, and possibly surgical management can control and even eradicate bone infection. Causative organisms vary depending on the portal of entry (direct inoculation versus hematogenous seeding) and the associated health status of the patient.
Patients with increased susceptibility to osteomyelitis include those with sickle cell anemia, chronic granulomatous disease, diabetes mellitus, and human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS). Although Staphylococcus aureus is the most common cause of osteomyelitis overall, patients with these chronic medical conditions are especially prone to infection by gram-negative organisms, including Pseudomonas aeruginosa, as well as by fungi and atypical mycobacteria.
The most common route of infection is direct inoculation due to injury. Hematogenous osteomyelitis secondary to bacteremia is usually a single organism infection, whereas direct penetration may involve multiple organisms. S. aureus is the causative organism in most cases of osteomyelitis.
The inflammatory process causes tissue necrosis and destruction of bony structure. Infection also obliterates vascular channels to the periosteum and intramedullary bone, leading to ischemia and areas of necrotic cortical bone, or sequestra. These sequestra are the hallmark of chronic infection, as the devitalized bone cannot be healed by the body's immune response. Surviving periosteum forms new bone, called an involucrum, which encases the dead bone. Draining sinuses form when purulence tracks to the skin surface through irregularities in the involucrum.