Curriculum · Musculoskeletal / Orthopedic / Rheumatology
Septic arthritis
What it is
Septic arthritis is an infection inside a joint, either a native joint or a joint replacement. It is an emergency, because the infection can quickly begin to destroy the joint and cause systemic illness, and it carries a mortality of around 10%. It is a common and important complication of joint replacement: it occurs in around 1% of hip or knee replacements, and more often in revision surgery.
Causes and risk
Staphylococcus aureus is the most common causative organism. The other bacteria named are Neisseria gonorrhoea in sexually active individuals, Group A Streptococcus, most commonly Streptococcus pyogenes, Haemophilus influenza and Escherichia coli. The risk factors are grouped by route. Contiguous spread from a skin infection or cutaneous ulcers. Direct inoculation from a previous intraarticular injection, a prosthetic joint or recent joint surgery. Hematogenous spread in diabetes mellitus, human immunodeficiency virus infection, immunosuppressive medication, intravenous drug abuse, osteoarthritis, another cause of sepsis, a prosthetic joint, rheumatoid arthritis, and sexual activity for the gonococcal form. Age older than 80 years is also listed. A history of immunosuppression or trauma may be present. A worked case sets the same organisms against the patient in front of you. Staphylococcus aureus is the most common. Hemophilus influenzae is the one in infants, Neisseria gonorrhoeae the one in sexually active adults, and coagulase-negative staph the one after a prior joint replacement. So the age and the history pick the second name, but with no such lead the first answer stays Staphylococcus aureus.