A 52-year-old man complains of pain and clicking in the area of his left temporomandibular joint. Objectively, the face is symmetrical, palpation of the medial and lateral pterygoid muscles is painful on the left. The mouth opening is limited to 1.5 cm between the cutting edges of the central incisors. X-ray shows smooth contours of the articular surfaces, the joint space is normal. What is the most likely diagnosis in this case?