Osteoarthritis with possible meniscal tear or ligament sprain: Given the patient's age and the history of a fall, it's plausible that the injury exacerbated existing osteoarthritis or caused a meniscal tear or ligament sprain, leading to swelling and pain.

Hemarthrosis: A fall could cause bleeding into the joint space, especially if there was trauma to the knee.
Bursitis: Inflammation of the bursae around the knee joint could be caused by the fall or overuse in compensating for the injury.
Tendinitis: Tendons around the knee could be inflamed due to the fall or subsequent altered gait.
Fracture: Although it might seem obvious, a fracture, especially of the patella or tibial plateau, could be a cause of swelling and should be considered, especially if there was significant trauma.

Deep Vein Thrombosis (DVT): Given the patient's age and recent history of immobility following a fall, DVT is a critical diagnosis not to miss, as it can lead to pulmonary embolism.
Infection (septic arthritis): Especially if the patient has been immobile or has compromised skin integrity, infection of the joint is a serious and potentially deadly condition that must be ruled out.
Osteonecrosis: If the blood supply to the bone was compromised during the fall, osteonecrosis could occur, leading to bone death and potentially severe arthritis.

Tumor (primary or metastatic): Although rare, a tumor could cause swelling and pain in the knee, especially in an older adult.
Rheumatologic conditions (e.g., rheumatoid arthritis, lupus): These conditions can cause knee swelling but would typically be accompanied by other systemic symptoms.
Complex regional pain syndrome (CRPS): A condition characterized by chronic pain, inflammation, and hypersensitivity following an injury, which could be considered if other diagnoses are ruled out and symptoms persist.