Osteoarthritis (OA): Given the chronic nature of the effusion and the absence of trauma, OA is a leading consideration. The inflammatory fluid analysis supports this diagnosis, as OA can cause chronic inflammation.

Rheumatoid Arthritis (RA): This autoimmune disorder can cause chronic knee effusions with inflammatory fluid analysis results similar to those described. The lack of systemic symptoms or other joint involvement does not rule out RA.
Psoriatic Arthritis: Similar to RA, psoriatic arthritis can cause knee effusions with inflammatory fluid. A history of psoriasis or skin lesions would support this diagnosis.
Gout or Pseudogout (without crystals at the time of analysis): Although the fluid analysis was negative for crystals, it's possible that crystals were not present at the time of the tap or were missed. These conditions can cause chronic effusions and should be considered, especially if there are recurrent episodes.

Septic Arthritis (despite negative bacterial culture): It's crucial to consider septic arthritis, even with a negative culture, as the consequences of missing this diagnosis can be severe. Repeat fluid analysis or blood cultures might be necessary.
Tuberculous Arthritis: This rare form of arthritis can present with chronic effusion and might not always have a clear infectious history. It's essential to consider in endemic areas or in patients with risk factors.
Lymphoma or Other Malignancies: Although rare, malignancies can cause chronic effusions. A high index of suspicion is necessary, especially in older adults or those with systemic symptoms.

Hemophilic Arthropathy: In patients with hemophilia, recurrent joint bleeds can lead to chronic effusions and arthritis. This would be considered in patients with a known history of hemophilia.
Sarcoidosis: This systemic disease can cause arthritis and chronic effusions. Diagnosis would require systemic symptoms or findings suggestive of sarcoidosis.
Villonecrotic Synovitis (Pigmented Villonodular Synovitis, PVNS): A rare condition characterized by benign proliferation of the synovium, leading to chronic effusions and potentially joint damage. Diagnosis is often made after surgical synovectomy and histological examination.