Osteoarthritis (OA): Given the mild to moderate arthritic changes noted on the images and the presence of a small to moderate suprapatellar effusion, OA is the most likely diagnosis. The chronic nature of the pain (12 months) without a history of injury also supports this diagnosis.

Patellofemoral Pain Syndrome: Although there's no patellar skyline view, the presence of arthritic changes and effusion could also suggest patellofemoral pain syndrome, especially if the patient has anterior knee pain.
Meniscal Tear: A meniscal tear could cause chronic knee pain and effusion, even without a history of injury, especially if the tear is degenerative.
Ligamentous Sprain: A chronic ligamentous sprain could also present with chronic pain and effusion, although this is less likely without a history of injury.

Septic Arthritis: Although less likely, septic arthritis is a serious condition that requires prompt diagnosis and treatment. The presence of an effusion could be a sign of septic arthritis, especially if the patient has systemic symptoms such as fever.
Tumor: A bone or soft tissue tumor could cause chronic pain and effusion, and although rare, it's a diagnosis that should not be missed.
Osteonecrosis: Osteonecrosis of the knee could cause chronic pain and arthritic changes, and is a diagnosis that should be considered, especially if the patient has risk factors such as steroid use or trauma.

Hemophilic Arthropathy: If the patient has a history of hemophilia, hemophilic arthropathy could be a consideration, although this is rare.
Gout or Pseudogout: Although less likely without a history of acute attacks, gout or pseudogout could cause chronic knee pain and effusion, especially if the patient has risk factors such as kidney disease.
Neuropathic Pain: Neuropathic pain could cause chronic knee pain, although this is rare and would typically require other symptoms such as numbness or tingling.