You are in a GP practice and you are seeing a 68-year-old male who has presented with activity related joint pain in both knees that has got progressively worse over the last 12 months. The patient also states that the pain in his joints when he gets out of bed lasts no longer than 30 minutes. On examination you note a restricted range of movements in both knees and feel a grating sensation when you undertake passive range of motion. What is your main differential diagnosis?