Osteoarthritis: This is the most common cause of knee effusion, especially in older adults. The wear and tear of the joint lead to inflammation and fluid accumulation.

Ligament Sprains or Tears: Injuries to the ligaments surrounding the knee can cause effusion due to the resulting inflammation.
Meniscal Tears: Tears in the cartilage of the knee joint can lead to effusion, especially if the tear is acute.
Patellofemoral Pain Syndrome: This condition, often related to overuse or poor tracking of the patella, can cause knee effusion.
Bursitis: Inflammation of the bursae around the knee can lead to effusion.

Septic Arthritis: Although less common, this is a medical emergency that requires prompt diagnosis and treatment to prevent joint destruction and systemic infection.
Crystal Arthropathies (Gout or Pseudogout): These conditions can cause sudden, severe knee effusion and require specific treatment.
Hemarthrosis: Bleeding into the joint space, which could be due to trauma, anticoagulation therapy, or bleeding disorders, is a critical diagnosis not to miss.

Rheumatoid Arthritis: While not the most common cause, rheumatoid arthritis can lead to knee effusion, especially in those with systemic symptoms.
Lyme Arthritis: In endemic areas, Lyme disease can cause knee effusion, often presenting with a distinctive pattern of intermittent swelling.
Tuberculous Arthritis: A rare cause of knee effusion, especially in immunocompromised individuals or those from areas where tuberculosis is common.
Neoplastic Conditions: Very rarely, knee effusion can be the presenting symptom of a tumor within the knee joint or surrounding bone.