Septic Arthritis

Septic arthritis is a condition of joint inflammation and damage caused by infection – bacterial, usually, but fungi, mycobacteria, and viruses can be responsible. Septic arthritis typically involves the knee or large other joints such as the hip and shoulder, though any joint could be affected. The knee joint is most commonly affected in adults; among children, the hip is the most common site. The bacteria responsible for septic arthritis vary by the age of the patient, though Staphylococcus aureus is a common pathogen causing septic arthritis in all age groups. Septic arthritis can lead to permanent joint damage unless treated expeditiously with irrigation of the joint and antibiotic coverage.



Structure and Function

A microbe can infiltrate the joint via three methods. The most common mechanism is hematogenous spread: because the articular capsule lacks a basement membrane, the joint space is more susceptible to direct invasion from organisms leaving the bloodstream. Direct inoculation of the joint space can occur either through trauma or iatrogenic causes, such as surgeries or injections. Infection can also spread to the joint from osteomyelitis in nearby bones or cellulitis of the skin overlying the joint (Figure 1).


Figure 1: Cellulitis near the knee. Infection of the skin adjacent to a joint can be a source of septic arthritis via contiguous spread.

When the body senses the infection, it mounts an acute inflammatory response. Local macrophages and dendritic cells are activated. These cells release molecular signals to increase blood vessel permeability and recruit neutrophils. Leukocytes release proteolytic enzymes to kill the invading organism, but these enzymes are also toxic to the articular cartilage. Thus, treatment involves removing the joint fluid, as well as antibiotic directed against the offending organisms.


The bacteria responsible for septic arthritis vary by the age of the patient. In children under one month of age, the common causes are Staphylococcus aureus, Group B strep, Gram-negative organisms, and Streptococcus pneumoniae. In children between one month and three years of age, Staphylococcus aureus and Streptococcus pneumoniae are also common, but Streptococcus pyogenes, Kingella kingae, and Haemophilus influenza Type B are seen. These bacteria, with the exception of Kingella kingae, are seen in children older than three years of age as well. In adolescents, the common causes are Staphylococcus aureus, Neisseria gonorrhoeae, Streptococcus pneumoniae, and Streptococcus pyogenes.


Among adults, the most common organism is Staphylococcus aureus, which accounts for a majority. Neisseria gonorrhea account for ~20% of cases and gram-negative bacilli such as E coli, klebsiella and Enterobacter account for another 15%.


Patient Presentation

In pediatric septic arthritis, patients typically present with acute onset guarding of a joint. Initially, pain is often poorly localized. A history of mild trauma is common (and might be coincidental). If the lower extremity is involved, patients often have a limp or will refuse to bear weight. If the upper extremity is involved, patients might refuse to use the extremity. Patients typically also have systemic symptoms, such as malaise, fever, and poor appetite.


On physical exam, the child will often appear ill and will tend to hold the affected joint in a position to accommodate joint distention. Patients with septic arthritis of the hip tend to hold the hip in a flexed, abducted, and externally rotated position, whereas if the knee is involved, the joint is held in a slightly flexed position. Children are typically apprehensive, and resist attempts to examine the affected extremity. Any movement of the joint is typically painful. The joint is often tender to palpation.


Septic arthritis in the adult typically presents as an acute condition of a single joint (monoarthropathy), usually the knee, with joint pain, swelling (see Figure 2), warmth, and restricted movement.


Figure 2: A patient with septic arthritis of the right knee. The effusion filling the supra-patellar space is easily seen (red lines), especially in comparison to the normal contralateral side. (Image courtesy of

Adults suffering from septic arthritis also tend to hold the joint in a rigid position where the joint space is maximal. This minimizes the pressure from the effusion. Passive motion by the examiner will be very painful.


Fever can be present in septic arthritis, especially if the mode of infection is hematogenous seeding, but in 40% of cases fever will be absent: normal temperature does rule out the presence of septic arthritis.


Polyarticular infections occur in 20% of cases of septic arthritis. These cases of septic arthritis involving multiple joints are more commonly seen in patients with rheumatoid arthritis or underlying immune compromise.



Objective Evidence

Objective evidence related to the diagnosis of septic arthritis comes from three sources: blood tests; synovial fluid aspiration and examination; and medical imaging.


Inflammatory markers such as the peripheral white blood cell count, erythrocyte sedimentation rate (ESR), and C-reactive protein levels (CRP) can be increased in septic arthritis but are non-specific and also may be normal early in the course of disease.


Blood cultures are positive in ~25% of cases septic arthritis. Although blood cultures neither confirm normal or exclude the diagnosis of septic arthritis, positive results can be used to confirm the infectious pathogen and tailor anti-microbial therapy.


Joints that are suspected of having an infection should be aspirated (Figure 3). 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. The synovial fluid aspirate of a septic joint is often yellow and turbid, though the appearance of the fluid is not diagnostic.


Figure 3: Aspiration of the knee joint. The patella is outlined in red. As seen the MRI figure below, the joint cavity extends well above the patella, such that fluid can be removed with a needle placed well away from the cartilage surfaces.

The gold standard for diagnosing septic arthritis is a positive bacterial culture of synovial fluid obtained from a joint aspiration. On the other hand, definitive culture results may not be known for 24 to 48 hours, and (owing to the cartilage-destructive properties of white cells in infected joint fluid) waiting that long to treat can be harmful. Thus, information that could be obtained immediately – namely, the number of white cells per microliter – is used as a proxy.


A common clinical decision rule is that a synovial white blood cell count of 50,000 cells should be treated as if an active infection were present. As shown in Figure 4, there is an overlap in cell counts seen in people without infections and counts seen in people with infections. Thus, there is an inevitable sensitivity-specificity trade-off.


Figure 4: A hypothetical distribution of white cell counts in both normal and septic joints. As shown, if a threshold corresponding to the orange arrow were used to signify an infection, no cases will be missed, but that would require treating more than half of the normal population! On the other hand, a threshold suggested by the green arrow would eliminate all false positives, but at the price of failing to treat more than half of the truly infected. In practice, a compromise value, as shown by the purple arrow, is often used. This value is adjusted to suit the clinical scenario.

Although an elevated white blood cell count in the synovial fluid is suggestive of septic arthritis, it can also be seen in other inflammatory conditions like rheumatoid arthritis. Accordingly, it may be helpful examine the fraction of white cells that are polymorphonuclear leukocytes. When fluid that has a borderline total count is composed of more than 90% polymorphonuclear leukocytes, it is highly likely that an infection is present.


The Kocher criteria can be used to assist with diagnosis of pediatric septic arthritis of the hip. The four criteria are 1) a history of fever over 38.5 degrees Celsius, 2) an inability to bear weight on the affected extremity, 3) an ESR greater than 40 mm/hr, and 4) a white count greater than 12,000 cells/microliter. If the patient has three of these factors, the probability of septic arthritis is more than 90%. If all four are present, it’s a virtual certainty.


Analyzing the fluid for crystals can rule out a crystal induced arthropathy (though a joint with gout or pseudogout can be infected as well). Gram staining and culturing the fluid will aid in tailoring the appropriate anti-microbial therapy. Gram stains are positive ~70% in nongonococcal septic arthritis cases. Cultures may be negative in the setting of recent antibiotic use or with certain pathogens such as gonococcus.


Radiographs finding for septic arthritis are non-specific. As septic arthritis progress and causes more damage and inflammation, x-rays will reveal increased joint space and joint effusion. Later in the course, juxta-articular osteopenia from the hyperemia, joint space narrowing from cartilage destruction, and destruction of subchondral bone become evident on plain x-rays.


Ultrasound and MRI are useful in evaluating for the presence of joint effusions (see Figures 5 and 6).


Figure 5: This MRI was obtained for the patient shown in Figure 2. As shown, there is a large effusion. (The extent of the effusion is seen more easily in the call-out figure to the right, which is a reproduction of the figure to the left, with the joint fluid highlighted in blue.) (Image courtesy of

Figure 6: Ultrasound of a patient with a large hip effusion. Note the elevation of the thickened capsule off the anterior femoral neck. (Image courtesy of Dan Miller, MD.)



The overall incidence of septic arthritis ranges from 4 to 29 cases per 100,000 persons around the world. The incidence in the United States is ~8 cases per 100,000 persons. While people of any age can have septic arthritis, about half of the cases in adults are in people 65 years of age or older.



Differential Diagnosis

In children with suspected septic arthritis, other diagnoses to consider include transient synovitis, hemarthrosis, inflammatory diseases, Legg-Calve-Perthes disease, and neoplastic processes. Hemarthrosis can occur secondary to hemophilia or trauma. Other infectious etiologies to consider include osteomyelitis, pyomyositis, and Lyme disease. Inflammatory diseases to consider include juvenile idiopathic arthritis, reactive arthritis, and rheumatic fever. Neoplastic processes to consider include leukemia and pigmented villonodular synovitis (PVNS).


Acute monoarthropathy in the adult has a broad differential. These can be split into infectious and non-infectious causes. Infectious causes include septic arthritis, septic bursitis, and overlying cellulitis. It is important to discern if the infection in intra-articular before proceeding with joint aspiration. Sticking a needle through a cellulitis into a joint and causing septic arthritis is considered poor form, to say the least.


Non-infectious causes of acute monoarthropathy include crystal arthropathy (gout and pseudogout), reactive arthritis, rheumatoid arthritis, and osteoarthritis. Joint aspiration is required to discern septic arthritis from a crystal arthropathy. Specific tests for Lyme disease are helpful, though for this diagnosis, the history is very important as well.



Red Flags

Acute onset pain and reluctance to move a joint is a red flag finding suggesting septic arthritis.


Any recent history of invasive procedures or trauma near the affected joint, IV drug use, or an immunosuppressed state raises suspicion for septic arthritis as well.


Treatment Options and Outcomes

Septic arthritis should be treated urgently. The treatment for septic arthritis of any joint is drainage of the fluid and initiation of antibiotic coverage. Surgeons tend to believe that surgery (arthroscopy or open arthrotomy) is needed (see Figure 7). Internists are more likely to claim that serial needle aspirations will suffice. Both are demonstrably effective.


Figure 7: Arthroscopic photo a of shoulder joint, taken during irrigation for septic arthritis. There is synovial inflammation and [white] fibrin deposits seen to the right. (Image courtesy of Knee Surg Sports Traumatol Arthrosc. 2021; 29(10): 3149–3158. doi: 10.1007/s00167-021-06525-8)

Empiric antibiotics need to be started after joint aspiration is completed. Vancomycin provides broad coverage for gram-positive bacteria. If the gram-negative bacteria are suspected or identified on gram stain, a 3rd or 4th generation cephalosporin should be added. If the patient has a history of IV drug abuse, and Pseudomonas coverage is needed, a 3rd or 4th generation cephalosporin with an aminoglycoside like gentamicin should be administered. Antibiotics can be tailored to a specific pathogen if and when one is identified on synovial fluid cultures.


Serial synovial fluid analyses can be monitored for white blood cell counts reverting to normal; additional cultures can be obtained from that fluid as well.


The best duration of antibiotic treatment is not known with certainty. Many physicians treat septic arthritis with intravenous antibiotics for 2 weeks followed by another 2 weeks of oral therapy. Physical therapy is often recommended to regain strength and full range of motion.


Outcomes from treatment depend on host factors, the offending organism and timing. About 50% of normal hosts will completely recover. Patients with underlying joint disease develop functional impairment in about 1/3rd of cases. In-hospital mortality rates of 15% or greater have been cited, though it may be that the septic arthritis is simply a manifestation of terminal decline (i.e., not the true cause of the patient’s demise).



Risk Factors and Prevention

Risks for septic arthritis include age >80 years old, diabetes mellitus, immunosuppressed states, immunosuppressive medications, underlying arthritis but especially rheumatoid arthritis, recent exposure of the joint from trauma or surgery, and other infections, either nearby (e.g., cellulitis and osteomyelitis) or distal. As seen, very few of these factors are “modifiable,” and thus the true role of prevention is to prevent complications of septic arthritis by timely diagnosis and treatment.




Septic arthritis in a joint with a prior arthroplasty is of special concern because unless the infection is caught early, it is extremely difficult to eradicate the bacteria and prevent failure of the arthroplasty. Most cases of prosthetic joint infections found within the first weeks after surgery are caused by seeding at the time of implantation. Late cases are usually secondary to hematogenous spread from oral, urinary tract or visceral infections. (This topic is beyond the scope of this volume but the interested reader might wish to consult an open-access review such as this one: Li, C. et al. Twenty common errors in the diagnosis and treatment of periprosthetic joint infection. International Orthopaedics (SICOT) 44, 3–14 (2020).


Staphylococcus aureus is the most common organism causing septic arthritis. Some elements of the patient’s history can suggest other organisms, as shown in the table. 

Key Terms

Septic arthritis, joint aspiration, surgical washout




Recognize signs and symptoms of septic arthritis. Perform joint aspiration under sterile technique.




Meg’s notes

Surgical washout is not mentioned until the key terms. 

is there an original Figure 2 or 5? Sizing is rather small on this.