Hip joint infection and septic arthritis, although most common in growing children, is becoming more prevalent in the adult population due to the growing number of elderly, disabled, and immunosuppressed patients, and those with chronic systemic illnesses. The ramifications of sepsis in this major weightbearing joint can result in lifelong disability for a child, and can be responsible for the loss of independence and/or demise of an adult if not recognized and treated early. Evrard has reported a 13% incidence of mortality from hip joint infection in the adult.104 This seemingly high mortality rate in adults may be secondary to concurrent medical issues or underlying hip disease that make diagnosis difficult and often impede appropriate, early aggressive surgical treatment. Although the rate of pediatric hip infections is not likely to increase, society's growing elderly, debilitated, and immuno-
suppressed population will likely necessitate an increased awareness of hip joint sepsis and may change our "classical" accepted treatment methods.
The underlying cause of "virgin" hip joint sepsis is not straightforward, and cannot simply be attributed to the common daily occurrence of transient bacteremia. A multifactor-ial etiology combining a susceptible joint, a vulnerable patient, transient bacteremia, systemic infection, and/or local inoculation is most likely. Understanding joint sepsis requires defining the attributes of the "susceptible" joint and the mechanisms of joint infection. The common occurrence of hip sepsis in the pediatric population can be attributed to the susceptibility conferred by the vascularity in the developing proximal femur, the intracapsular location of the femoral metaphysis, and the propensity for injury or trauma in this age group. In adults, any acute or chronic disease process that locally or systemically affects the synovial tissue may increase the vulnerability to infection. Osteoathritis, inflammatory arthropathies, avascular necrosis, trauma, crystalline arthropathy, and neuropathic arthropathy have all been shown to affect the occurrence of infection. Immune system function also plays a major role in preventing joint sepsis. Any disease or chronic illness that results in or requires immuno-suppression clinically carries with it an increased incidence of hip infections.105-110
The mechanisms of joint infection include hematogenous spread, direct inoculation, previous surgery, and local extension. Understanding these mechanisms is of the utmost importance in determining the appropriate method of diagnosis and treatment. Hematogenous bacterial spread is one of the most prevalent mechanisms responsible for hip joint sepsis in both the pediatric and adult populations. Prior to physeal closure in children, blind-end arterial loops in the proximal femoral metaphyseal bone act as reservoirs that capture bacteria. Metaphyseal involvement results in seeding of the hip joint because of its intracapsular location. In adults, bac-teremia deposits organisms in the synovium that may then enter the joint directly or indirectly.111 Direct seeding occurs through synovial capillaries damaged by preexisting disease or degeneration. Indirect seeding is a result of synovial inflammation caused by the presence of pyogenic bacteria that release proteolytic enzymes, causing synovial breakdown and seeding of the joint fluid. Surgery or direct inoculation is the most common mechanism for infection of the adult hip. Local infection also commonly spreads to the hip joint, and should always be ruled out or completely evaluated prior to surgical treatment so the appropriate technique, approach, and debridment can be performed.
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