A 57-year-old man complains of pain and creaking in his right temporomandibular joint when eating. In the evening the signs diminish. The pain is observed for the last 2-3 years. Objectively the face is symmetrical, mouth opening is reduced to 2.5 cm. Molars are missing on both jaws. X-ray shows deformed articular surfaces of the right temporomandibular joint. What is the most likely diagnosis?