What is septic arthritis? What are its causes?
Septic arthritis is a bacterial (and less commonly fungal or mycobacterial) infection of a joint. It can be caused by hematogenous spread (most common), direct inoculation (ie: trauma, during surgery) or contiguous spread from infected periarticular tissue.
Here cellulitis near the knee (from a puncture wound originally) is at risk of spreading into the joint
The most common site is the knee, but the wrist and hip, among others, can be affected.
Note the large space for infected joint fluid in the knee:
Soft tissue infections of the tendons and discs are considered separately. Also, the condition in pediatrics has a different pattern of presentation (eg the hip is often the site of infection in infants) and predicted course.
Most cases of acute septic arthritis are caused by staphylococcus. Certain sites (foot puncture wounds = Pseudomonas) and certain patients (those with sickle cell disease = salmonella) are susceptible to other characteristic organisms.
Infections can be iatrogenic (as suggested below by a glove-less injector, who probably did not wash his hands either)
Septic arthritis may not cause systemic effects. The patient will have pain especially on motion of the joint
Note that the hip has so little room for joint fluid build up, it may be very painful even without motion
The diagnosis is made by aspirating the fluid and sending it to the lab for cell count and culture / gram stain
Elevated risk for septic arthritis is seen in patients with joint implants, known infection elsewhere, a history of drug abuse, immune suppression and chronic diseases (such as diabetes or rheumatoid arthritis.
It is possible to get an autoimmune arthritis after a systemic infection; but in that case the joint space itself is sterile. This condition is termed "reactive arthritis" and is grouped under the rubric of rheumatological, not infectious, diseases.