Synergy Health Partners

Septic Arthritis in the Knee


Septic arthritis, also known as infectious arthritis, is a severe infection in a joint. It occurs when bacteria, viruses, or fungi invade the joint space, leading to inflammation and damage. The knee is a common site for septic arthritis, and it requires prompt medical attention to prevent permanent joint damage and systemic spread of the infection.


Septic arthritis is usually caused by bacterial infection, with Staphylococcus aureus being the most common pathogen. Other bacteria, such as Streptococcus species and Gram-negative bacteria, can also cause septic arthritis. Causes include:

  • Direct Introduction: Through a joint injury, surgical procedures, or injections.
  • Hematogenous Spread: Bacteria entering the bloodstream from another infection site, such as skin infections, urinary tract infections, or respiratory infections.
  • Underlying Conditions: Rheumatoid arthritis, osteoarthritis, diabetes, or immunosuppressive conditions that increase susceptibility to infections.


Symptoms of septic arthritis in the knee include:

  • Severe Joint Pain: Intense pain in the knee, worsening with movement.
  • Swelling: Significant swelling and warmth over the affected knee.
  • Redness: Redness or discoloration of the skin over the joint.
  • Fever: High fever and chills.
  • Limited Range of Motion: Difficulty moving the knee due to pain and swelling.
  • General Malaise: Feeling generally unwell or fatigued.


Diagnosis of septic arthritis involves a combination of clinical evaluation and laboratory tests:

  • Medical History and Physical Examination: Assessment of symptoms, medical history, and physical examination to identify signs of joint infection.
  • Joint Aspiration (Arthrocentesis): Removing synovial fluid from the affected knee for analysis. This is the definitive diagnostic test, where the fluid is examined for white blood cells, crystals, and cultured for microorganisms.
  • Blood Tests: Elevated white blood cell count, increased inflammatory markers (CRP and ESR), and blood cultures to identify the causative organism.
  • Imaging Studies: X-rays, ultrasound, or MRI to assess joint damage and rule out other conditions.


Treatment for septic arthritis is urgent and typically involves:

  • Antibiotic Therapy: Intravenous antibiotics are started immediately, often before the specific pathogen is identified. The choice of antibiotic is initially broad-spectrum and may be adjusted based on culture results. Duration is usually 2-6 weeks, starting with intravenous administration followed by oral antibiotics.
  • Joint Drainage: Repeated aspiration or surgical drainage of the infected synovial fluid to reduce bacterial load and relieve pressure. Options include:
    • Arthrocentesis: Repeated needle aspiration.
    • Arthroscopic Surgery: Minimally invasive surgery to wash out the joint.
    • Open Surgery: In severe cases, open surgical drainage may be required.
  • Supportive Care: Pain management with NSAIDs or analgesics, rest, and immobilization of the affected knee.


The prognosis for septic arthritis depends on the promptness of treatment and the underlying health of the patient. Early diagnosis and aggressive treatment typically result in good outcomes, with many patients recovering full joint function. Delayed treatment can lead to complications such as joint destruction, osteomyelitis, or systemic sepsis, which can be life-threatening.


Preventing septic arthritis involves:

  • Prompt Treatment of Infections: Treating bacterial infections early to prevent hematogenous spread.
  • Sterile Techniques: Ensuring sterile techniques during joint injections or surgeries.
  • Managing Underlying Conditions: Controlling chronic illnesses like diabetes and arthritis that predispose individuals to infections.
  • Vaccinations: Keeping vaccinations up-to-date to prevent infections that could lead to septic arthritis.


Septic arthritis in the knee is a medical emergency requiring immediate intervention to prevent severe joint damage and systemic infection. If you experience symptoms of septic arthritis, such as severe knee pain, swelling, redness, and fever, seek medical attention promptly. Early diagnosis and appropriate treatment are crucial for optimal recovery and preventing complications.