Septic arthritis is a process that occurs when a joint becomes infected and, if left untreated, can result in local destruction of the affected joint as the host response to the infection (killing the bacteria) damages the affected joint (i.e., also kills the cartilage).
Etiology and Risk Factors
Most cases of septic arthritis are caused by a bacterial infection of the joint; fungal or mycobacterial etiologies are possible but rare.
There are two ways that bacteria make it into the joint: hematogenous spread (that is, carried by the blood) from distant sites of infection and direct inoculation (that is, physically introduced into the joint space). Bacteria can be directly inoculated most commonly from a trauma, but surgery or contiguous spread from nearby infected tissue (including bone) is possible, too.
Cellulitis over a joint (Figure 1) is a common example of contiguous spread, with infectious seeding from the skin and soft tissues into the nearby joint space.
Figure 1: Cellulitis over the knee may contribute to septic arthritis. (Courtesy: https://www.orthopaedicsone.com/x/SwFxB)
Risk factors for developing septic arthritis include the presence of joint implants, known infection elsewhere in the body, a history of injection drug use, and medications or conditions that cause immune suppression (such as diabetes or rheumatoid arthritis).
Staphylococcus species of bacteria are responsible for most cases of acute septic arthritis. Certain sites (foot puncture wounds = Pseudomonas) and certain patients (those with sickle cell disease = Salmonella) are susceptible to other characteristic organisms.
It is important to remember that septic arthritis may not cause systemic effects, such as fever and chills, seen in other infectious diseases. Instead, the patient typically presents with pain in the affected joint. This pain is exacerbated by even slight movement of the joint.
Often, the joint is warm to the touch with a noticeable effusion (Figure 2 and 3).
Figure 2: An MRI highlighting the space holding synovial fluid (in pink) in the knee. As shown, the synovial cavity includes an area proximal to the patella (blue star) that may be used to introduce a needle for aspiration that is unlikely to hit any cartilage surface. (Courtesy: modified from radiopaedia.org/cases/41239)
Figure 3: Picture of a knee with effusion. (Courtesy: https://en.wikipedia.org/wiki/Knee_effusion)
Diagnosis and Management
Diagnosis of septic arthritis requires aspirating the joint fluid and sending it to the lab for cell count, culture, and gram stain. The aspirate of a septic joint is often yellow and turbid, as shown in Figure 4. Removing the joint fluid can provide some pain relief - the pressure is dissipated - but that relief is a secondary benefit, and should not suggest that the condition is fully addressed.
Figure 4: Synovial fluid aspirate from a septic joint. (Courtesy: https://www.orthopaedicsone.com/x/SwFxB)
When left untreated, septic arthritis can lead to destruction of the joint, as the enzymes released to kill the bacteria are highly toxic to the cartilage as well. Treatment includes antibiotics and “washing out” the joint, which can be completed surgically or with serial aspirations of the synovial fluid. (More information on diagnosis and management of septic arthritis is provided in OTHER QUESTION)
*** Please note that the wording of the question was deliberate: discussed here are the causes and consequences of adult septic arthritis. Pediatric septic arthritis is of course somewhat similar to the adult form, but has sufficient differences to be worthy of its own designation. Please see the chapter on Pediatric Infections for more details.