Septic Arthritis

 

Written By: Rebecca Zhang, MD; Edited by: Timothy Khowong, MD, MSEd

 

Background:

Septic arthritis is a serious condition that affects the joint space, either in native or prosthetic joints. This infection can have significant implications if not promptly addressed. In this post, we’ll delve into the epidemiology, risk factors, etiology, presentation, diagnosis, treatment, and potential complications associated with septic arthritis.

Epidemiology and Risk Factors

Definition: Septic arthritis is an infection of the joint space, which can occur in both natural and artificial joints.

Anatomic Location: The most commonly affected joints are the knee (~50%), hip, and shoulder.1 These areas are particularly vulnerable due to their structure and the frequency with which they bear weight and stress.

Risk Factors: Several factors increase the likelihood of developing septic arthritis:

  • Age: Individuals over 80 years old are at higher risk.

  • Comorbidities: Conditions such as diabetes mellitus (DM), rheumatoid arthritis (RA), cirrhosis, and HIV can predispose individuals to this infection.

  • Intravenous Drug Use (IVDU): Drug users are at a higher risk due to potential needle-associated infections.

  • Crystal Arthropathy: Diseases like gout can also increase susceptibility.

  • Endocarditis: An infection of the heart valves can lead to septic arthritis as bacteria spread through the bloodstream.

Etiology

Septic arthritis can arise through several mechanisms:

  • Hematogenous Seeding: Bacteria spread through the bloodstream and infect the synovial membrane of the joint.

  • Direct Inoculation: Infection can occur through direct entry into the joint, such as from a puncture wound or surgical procedure.

  • Extension from Adjacent Tissues: An infection from nearby tissues can extend into the joint space.

Presentation

Signs and Symptoms: The hallmark of septic arthritis is a hot, swollen joint. This condition should be the first consideration when a patient presents with these symptoms, especially if they have relevant risk factors.

Physical Exam: On examination, the affected joint may appear erythematous, warm, and tender, with noticeable effusion. Patients often experience significant pain and difficulty bearing weight on the affected joint.

Ultrasound Guided Approach to Knee Arthrocentesis:

So how do we get to the synovial fluid? You can use ultrasound to guide needle aspiration of a joint if you didn’t want to do it blind. ACEP’s sonoguide recommends using the linear transducer and by placing the probe in the sagittal plane, you should be able to identify the quadriceps tendon attached to the patella. Deep to the tendon and proximal to the patella you should be able to identify the suprapatellar fat pad superficial to the femur. This will be displaced by joint effusion when a significant amount of fluid exists in this space, which is where you can aim.

Diagnosis

Differential Diagnosis for a Hot Swollen Knee:

  • Infectious Causes:

    • Septic arthritis

    • Septic bursitis

    • Gonococcal arthritis

  • Noninfectious Causes:

    • Crystal-induced arthritis (e.g., gout or pseudogout)

Arthrocentesis: This procedure is crucial for diagnosis. A definitive diagnosis is made when the synovial fluid tests positive for bacteria through Gram stain and/or culture, and blood cultures may also be positive.

Laboratory Findings:

  • White Blood Cell Count (WBC): Typically greater than 10,000 cells/mm³ with a predominance of polymorphonuclear cells (PMNs).4

  • C-Reactive Protein (CRP): Levels greater than 1 mg/dL indicate inflammation.

  • Serum Procalcitonin: Elevated levels (greater than 0.2-0.3 ng/mL) can suggest bacterial infection.

Imaging:

Imaging normally plays an adjunct role in the diagnosis of septic arthritis, as tapping the joint is gold standard. If you choose to get imaging, these may be some of the things you can see.

An example of untreated septic arthritis, which can lead to destruction of the subchondral bone on both sides of the joint. Displayed in this image is complete loss of the right superior femoral acetabular joint space.

Treatment and Complications

Treatment:

  • Serial Arthrocentesis: This is often necessary to relieve pressure and reduce inflammation. Monitoring involves assessing the reduction in cell count and PMNs in the synovial fluid from recurrent samples.

  • IV Antibiotics: Broad-spectrum antibiotics are administered initially, with adjustments based on culture results.3

Complications:

  • Osteomyelitis: The infection can spread to the bone.

  • Sepsis: A severe systemic response to infection.

  • Growth Arrest: In children, septic arthritis can impact bone growth and development.5

Conclusion

Septic arthritis is a severe condition requiring timely diagnosis and intervention. Understanding the risk factors, signs, and appropriate management strategies is essential for preventing complications and ensuring optimal recovery.

 
 

References:

  1. Horowitz DL, Katzap E, Horowitz S, Barilla-LaBarca ML. Approach to septic arthritis. Am Fam Physician. 2011 Sep 15;84(6):653-60. PMID: 21916390.

  2. https://www.acep.org/sonoguide/procedures/arthrocentesis

  3. Gilbert, DN; Chambers, HF. Sanford Guide to Antimicrobial Therapy 2020. 

  4. Costales C, Butler-Wu SM2018.A Real Pain: Diagnostic Quandaries and Septic Arthritis. J Clin Microbiol56:10.1128/jcm.01358-17.https://doi.org/10.1128/jcm.01358-17 Antimicrobial Therapy, Inc; 2020

Sheth, U. (n.d.). Septic arthritis - adult. Orthobullets. https://www.orthobullets.com/trauma/1058/septic-arthritis--adult

 
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