You might want to pick up a couple of basics before beginning this short article if this is foreign territory for you. My earlier article, TMJ Pain: Whose Fault Is It? would be a reasonable place to start. Knowing even a simplified version of the anatomy will help you picture what occurs when the disc is gone.
When the articular disc is displaced long enough, it will eventually perforate or more likely fragment. When this occurs, the mandibular condyle will contact the inferior surface of the glenoid fossa. In simpler terms, the top of the lower jawbone—the mandibular condyle—will touch the temporal bone. Put those two bones in contact long enough and they will appear to be wearing away.
Does that condition also change the patient’s occlusion? Definitely!
The T-scan image above shows that with disc out of place or completely absent on the left side, most of the biting force shifted to the posterior teeth on the left. In more basic terms, with the natural shock absorber missing, those left side upper and lower back teeth are banging away on each other.
When you suspect TMJ pathology, I recommend CT scan of the joint or MRI or both.
If the disc is fragmented or missing, the MRI report from your friendly radiologist will say something like “Articular disc not discernible.”
When discussing this situation with a patient, you could say, “The top of your lower jawbone actually rubs against the temple bone of the skull and the two bones are wearing flat. The bone is actually going away and that can be painful.” Now I admit that’s an oversimplification and you may want to review the process of osteoclastic activity of bone under pressure. That will give you a more realistic picture; type ‘osteoclast’ in your favorite search engine. But if you want to just think about two bones grating against each other– that’s what it sounds like after all– that’s perfectly okay.
In extreme cases, surgery may be indicated. My observation is that artificial parts, titanium condyles for example, should be viewed as a compromise and the last option. Replacing the disc with fat pads from the hip has worked reasonably well in the hands of a skilled surgeon, but again last resort I believe.
In most cases, your patient with degenerative arthritis of the TMJ can be helped with a properly designed oral appliance, or appliances.
The approach I recommend is to center the condyle or condyles in the glenoid fossa as well as possible and create some space between the condyle and fossa. This accomplishes four important things
- .Pain is reduced and sometimes eliminated.
- Comfortable, stable occlusion can be achieved with the orthotic in place.
- With space between the condyle and temporal bone maintained, over time the body will produce new soft tissue—a pseudo disc—to replace the cartilage disc. Will this be a new and healthy cartilage disc? No. The patient should not expect to have a cartilage disc in this joint again. The new soft tissue will be more like scar tissue. But it will suffice. It will prevent the two bones from grinding against each other.
- The destructive degeneration of the bone stops in most cases. No more osteoclastic activity from bone-on-bone pressure.
When discussing this situation with your patient, be realistic. The joint (or joints) is arthritic and always will be. The degeneration that has occurred will not reverse; in other words, the lost bone cannot be expected to grow back*. But you should be able to stop the degeneration from worsening.
Once an oral orthotic is placed, and the condylar position looks reasonable with joint imaging, what then? First, I recommend you schedule the patient for regular visits so that you can ensure that the occlusion continues to be well balanced on the orthotic and that discomfort is decreasing and range of motion improving. At some point the condyle-glenoid fossa space should be maintained even if the orthotic is removed. This generally required several months at least. Nine months to one year is not unusual for this to occur. Because there is now some disc space, occlusion usually improves. But the practitioner will need to decide whether it improved enough. Occasionally the answer is ‘yes,’ and the patient can gradually wean off the appliance. More often, the answer is no. In that event, there are two choices: long term appliance wear or restorative dentistry. Many patients are happy to simply continue wearing the appliance. Their pain is gone or greatly reduced, they’re chewing well on what is essentially an overlay partial denture, and they know the destructive forces on the bones has stopped. Other patients will want to have restorative dentistry done to replace the orthotic. This often means full mouth reconstruction. Transitioning from oral appliance to restorative dentistry is a topic that I cannot cover in a short article, but I am looking at the possibility of a series of articles or a book-length treatment of that subject. Stay tuned.
When is the patient ready for restorative dentistry?
- When the TMJs can comfortably accept firm loading
- When the TMJs are stable, with no further degeneration. In my opinion, that requires repeating the CT scans for comparison.
*In rare instances I have seen the flattened, degenerative condyles return to a rounded contour, with the apparent regrowth of at least some of the bone. This can take years, and again I would not count on it. There are no statistics on the likelihood of this result.