What are the most common causes in disorders of the tmj specifically when a patient has pain when trying to open widely the lower jaw?

Causes: Included, but not limited to traumatic joint damage, stretched ligaments, bruxism/clenching, malocclusion, lack of anterior guidance, para functional habits, occlusal interferences, anatomy of the joint itself, and many other causes. If you have a problem, seek treatment before chronic dysfunction becomes acute. Start with your Dentist.

Muscle Pain: Doctors sometimes refer to the condition as "myofascial pain disorder". so avoiding opening the joint too wide and soft diet and NSAIDs OTC until the pain settles.