Infectious Knee Pain: Types, Symptoms, Diagnosis, and Treatment

What Infections Cause Knee Pain?

Knee pain, while often associated with mechanical issues or injury, can also be a symptom of various infections, ranging from direct joint invasion (septic arthritis) to systemic illnesses that trigger an inflammatory response in the joint.

Understanding Infectious Arthritis

Infectious arthritis, also known as septic arthritis, is a serious condition characterized by an infection within the joint space. While any joint can be affected, the knee is a common site due to its large size and susceptibility to trauma. Pathogens can reach the knee joint through several routes:

• Hematogenous Spread: The most common route, where bacteria or other microorganisms travel through the bloodstream from an infection site elsewhere in the body (e.g., skin infection, urinary tract infection, pneumonia).
• Direct Inoculation: Occurs when bacteria are introduced directly into the joint, often due to penetrating injuries, injections (e.g., corticosteroid shots), or surgery (e.g., knee arthroscopy, total knee replacement).
• Contiguous Spread: Less common, where an infection from adjacent bone (osteomyelitis) or soft tissue spreads into the joint.

Once inside the joint, the pathogens multiply and trigger a severe inflammatory response, leading to rapid destruction of cartilage and bone if not treated promptly.

Types of Infections Affecting the Knee

Several types of infections can manifest as knee pain, each with distinct characteristics and causative agents:

• Bacterial Infections (Septic Arthritis)

  • Mechanism: These are the most common and urgent causes of infectious knee pain, leading to acute, purulent inflammation within the joint. Bacteria rapidly destroy articular cartilage.
  • Common Culprits:
    • Staphylococcus aureus: The most frequent cause in all age groups, often originating from skin infections, pneumonia, or endocarditis.
    • Streptococcus species: Another common cause, particularly in individuals with compromised immune systems.
    • Neisseria gonorrhoeae: Causes gonococcal arthritis, typically affecting young, sexually active individuals. It often presents as migratory polyarthralgia before localizing to one or two joints, frequently the knee.
    • Gram-negative bacilli: More common in older adults, intravenous drug users, or those with underlying medical conditions.
  • Risk Factors: Joint prosthesis, recent joint surgery, rheumatoid arthritis, diabetes, immunocompromised states, intravenous drug use, skin infections.

• Viral Infections

  • Mechanism: While less commonly causing direct joint infection, many viruses can trigger reactive arthritis or transient arthralgia (joint pain) as part of a systemic illness. The joint pain is often due to the body's immune response rather than direct viral replication in the joint.
  • Examples:
    • Parvovirus B19: Commonly causes "slapped cheek" rash in children but can cause acute, symmetrical polyarthralgia in adults, often affecting the knees, hands, and feet.
    • Rubella (German Measles): Can cause acute arthritis, especially in adult women, affecting knees, wrists, and fingers.
    • Hepatitis B and C: Can be associated with immune complex-mediated arthritis.
    • HIV: Can cause various musculoskeletal manifestations, including arthralgia and inflammatory arthritis.
    • Arboviruses (e.g., Chikungunya, Dengue, Zika): Known to cause severe and sometimes chronic joint pain, including in the knees, often in endemic regions.

• Fungal Infections

  • Mechanism: Fungal arthritis is rare but can occur, especially in immunocompromised individuals or those with systemic fungal infections. It typically causes chronic, monoarticular arthritis.
  • Examples: Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatitidis.
  • Presentation: Often insidious onset, mimicking other chronic inflammatory conditions.

• Lyme Disease (Spirochetal Infection)

  • Mechanism: Caused by the bacterium Borrelia burgdorferi, transmitted by ticks. Lyme arthritis is a late manifestation of the disease, often affecting large joints like the knee.
  • Presentation: Characterized by intermittent or persistent swelling and pain, often in one or both knees. It can recur over months or years if untreated.

• Tuberculosis (Mycobacterial Infection)

  • Mechanism: While more commonly affecting the lungs and spine (Pott's disease), tuberculosis can also spread to peripheral joints, including the knee. It causes a chronic, granulomatous infection.
  • Presentation: Insidious onset of pain, swelling, and stiffness, often with constitutional symptoms like fever and weight loss.

• Reactive Arthritis (formerly Reiter's Syndrome)

  • Mechanism: This is an autoimmune condition triggered by an infection elsewhere in the body (typically gastrointestinal or genitourinary), not a direct infection of the joint itself. The body's immune system mistakenly attacks its own joint tissues.
  • Common Preceding Infections: Chlamydia trachomatis (genitourinary), Salmonella, Shigella, Campylobacter, Yersinia (gastrointestinal).
  • Presentation: Typically asymmetrical arthritis, often affecting the knees and ankles, accompanied by other symptoms like urethritis (inflammation of the urethra), conjunctivitis (eye inflammation), and skin lesions.

Common Symptoms of Knee Infection

Regardless of the specific pathogen, infectious knee pain often presents with a characteristic set of symptoms:

• Acute, Severe Pain: The pain is often intense and worsens with movement or weight-bearing.
• Swelling: The knee joint appears visibly swollen due to fluid accumulation.
• Redness and Warmth: The skin over the affected knee may be red and feel warm to the touch.
• Limited Range of Motion: Pain and swelling restrict the ability to bend or straighten the knee.
• Fever and Chills: Systemic signs of infection are common, especially with bacterial septic arthritis.
• General Malaise: Feeling unwell, fatigued, or having body aches.
• Other Symptoms: Depending on the underlying infection, there may be associated rashes (Lyme, viral), urethral discharge (gonococcal, reactive), or gastrointestinal symptoms (reactive).

Diagnosis and Treatment

Prompt diagnosis and aggressive treatment are crucial for infectious knee pain, especially septic arthritis, to prevent permanent joint damage.

• Diagnosis:

  • Joint Aspiration (Arthrocentesis): The definitive diagnostic test, involving drawing fluid from the knee joint for analysis. The fluid is examined for white blood cell count (elevated in infection), Gram stain (to identify bacteria), and culture (to grow and identify the specific pathogen).
  • Blood Tests: Elevated inflammatory markers (Erythrocyte Sedimentation Rate - ESR, C-Reactive Protein - CRP) and white blood cell count can indicate infection.
  • Imaging: X-rays may show joint space widening or bone erosion in later stages. MRI can provide detailed images of soft tissues and bone.
  • Specific Tests: Serology for Lyme disease, viral titers, or cultures for specific pathogens if suspected.

• Treatment:

  • Antibiotics/Antifungals/Antivirals: Administering the appropriate antimicrobial agent (often intravenously initially) to target the identified pathogen.
  • Joint Drainage/Lavag: Often necessary to remove pus and reduce pressure within the joint. This can be done via repeated needle aspirations, arthroscopy, or open surgical drainage.
  • Pain Management: Analgesics and anti-inflammatory medications to manage pain and swelling.
  • Physical Therapy: Once the acute infection is controlled, rehabilitation is essential to restore joint function and range of motion.

When to Seek Medical Attention

Any sudden onset of severe knee pain accompanied by swelling, redness, warmth, or fever warrants immediate medical attention. Septic arthritis is a medical emergency that can lead to rapid and irreversible joint destruction within days if left untreated. Do not delay in seeking professional medical advice if you suspect an infection is causing your knee pain.