There are many causes for TMD such as:
1. Unbalanced occlusion or 'Bad Bite'
2. Stress (emotional or work/school related)
3. Injury or trauma (this could have been an injury as obvious as a blow to the jaw with a fist or something as subtle as a whiplash injury with direct trauma to the head or jaw.)
4. Teeth grinding or Bruxism
A 'Bad Bite' could be caused by any of the following:
1. Missing teeth
2. Crowded or 'crooked' teeth
3. Worn down teeth
4. Old crowns and bridges
5. Unbalanced dentures
When teeth are missing, or out of alignment, it causes the jaw to shift positions and the muscles to work harder to chew, swallow, bite, etc. and eventually will cause: muscle spasms, tension and pain.
Muscle tension and misplaced TMJ could cause the TMJ disc to be pulled out of place which will result in pain, clicking and popping in the TMJ.
If the disc is displaced for a long time, the bones will start rubbing against each other and some damage will happen to the bone, this is called Osteoarthritis.
If there is inflammation in other joints or bones of the body, it is called Osteoarthritis (Arthritis) which may involve the jaw bone and the TMJ. Some damage to the bone may be evident on the x-rays.