Evaluation of Swollen/Red Knee

When evaluating a swollen or red knee, it's crucial to consider a wide range of potential causes to ensure accurate diagnosis and appropriate treatment. The differential diagnosis can be organized into the following categories:

• Single Most Likely Diagnosis
  • Osteoarthritis: This is a common condition, especially in older adults, characterized by wear and tear of the joint cartilage. Symptoms include pain, stiffness, and swelling, making it a likely cause for a swollen or red knee.
• Other Likely Diagnoses
  • Meniscal tear: A tear in the cartilage of the knee can cause pain, swelling, and locking of the knee, making it a plausible diagnosis.
  • Ligament sprain: Injuries to the ligaments surrounding the knee can lead to swelling, pain, and instability.
  • Bursitis: Inflammation of the fluid-filled sacs (bursae) that cushion the knee can cause swelling and redness.
  • Tendinitis: Inflammation of the tendons, which connect muscles to bones, can cause pain and swelling around the knee.
• Do Not Miss Diagnoses
  • Septic arthritis: A bacterial infection within the joint space, which is a medical emergency requiring prompt antibiotic treatment to prevent joint destruction.
  • Deep vein thrombosis (DVT): A blood clot in the deep veins of the leg, which can cause swelling and redness and has the potential to lead to a life-threatening pulmonary embolism if not treated.
  • Osteonecrosis: Death of bone tissue due to a lack of blood supply, which can cause knee pain and swelling and may lead to bone collapse if not addressed.
• Rare Diagnoses
  • Rheumatoid arthritis: An autoimmune disorder that can cause inflammation and swelling in multiple joints, including the knee, though it typically presents with symptoms in other joints as well.
  • Gout: A form of arthritis characterized by sudden, severe attacks of pain, swelling, redness, and tenderness, often occurring at the base of the big toe but can also affect the knee.
  • Pseudogout (Calcium Pyrophosphate Deposition Disease, CPPD): A type of arthritis that causes sudden, severe joint pain, typically in the knee, due to the deposition of calcium pyrophosphate dihydrate crystals within the joint.
  • Hemarthrosis: Bleeding into the joint space, which can be due to trauma, bleeding disorders, or other conditions, and can cause sudden swelling and pain.

Each of these diagnoses has distinct features and requires a careful history, physical examination, and sometimes additional diagnostic tests to confirm the cause of a swollen or red knee.