In a recent article I described how occlusal appliances potentially impact muscle activity levels; in this article I will carry on that thought process to show how they impact the TMJs. It is important to recognize that the conclusions from the last article and this article are intimately linked. If an appliance significantly reducing muscle activity levels, it will also significantly reduce the load on the TMJ.
Where does joint pain come from?
Pain in the area surrounding the TMJ can be referred from several sources, particularly muscles. This is especially true of pain in front of the ear, over the joint. If the joint is asymptomatic, the appliance choice will be directed at solving the muscle problem. However, there are many patients who present with pain occurring from the joints.
The most common source of that pain is an internal derangement of the joint where the intra-articular disc is displaced in an anterior direction. The disc normally sits between the condyle and fossa; when the condyle compresses it under load, the disc is not innervated and discomfort does not exist. The back of the disc is attached to the head of the condyle by the ligamentous attachment and prevents the disc from being displaced, but if the ligament is torn or stretched the disc may displace. In addition, the posterior of the disc is attached to the back of the fossa through the highly innervated and vascular retrodiscal tissue. When a disc is displaced anteriorly this tissue is now moved across the head of the condyle and could possibly be compressed under joint load.
When does it hurt?
In reality disc displacement is fairly common, with research stating it may occur in one out of three adults. The majority of patients do not experience pain, but in some patients it can be very painful. The critical part of choosing an appliance design is to identify if the joint hurts when the condyle is going to a seated position, or if it hurts when the mandible is moving.
Understanding joint anatomy helps in understanding the pain. The disc appears clearly in most two dimensional diagrams of the joint, but actually has significant width in a medio-lateral dimension that extends completely across the head of the condyle. When discs displace, it is common for only a portion of the disc to displace anteriorly, leaving the rest of the disc in the correct position.
This explains why it is possible to have a displaced disc but no pain when the mandible is in one position but hurts in another. In addition, we know that most discs reduce as the condyle moves back onto the displaced disc during movement and when the condyle is back on the disc, no discomfort occurs.
How do occlusal appliances affect joint loading?
Choosing the correct appliance for a patient with joint pain requires an understanding of how appliances may impact joint loading, and therefore joint pain. The first possibility is to use an appliance that attempts to reduce overall muscle activity levels, such as an anterior-only appliance or full coverage appliance with anterior guidance, to reduce pain.
However, research says that the biggest factor in reducing joint loading is to provide posterior occlusal contacts. Even though the muscle activity levels may not be as low, the posterior occlusal contacts absorb most of the force of the muscle contraction which reduces the load on the joints. It is estimated that with anterior-only contact, 60 percent of the force from muscle contraction goes to load the joints; by adding occlusion back to the second molars, only 5 percent of the force of the muscle contraction reaches the joint.
What appliances to use?
Since posterior occlusal contacts significantly reduce joint loading, the appliances that tend to be most effective all have posterior occlusion. This means posterior-only appliances work as well as full coverage appliances. Again, the key is to know when the joints experience pain.
If the joint hurts when the condyles are going into a seated position, the appliance will need to provide posterior occlusion and be adjusted to provide an intercuspal occlusion with the mandible in an anterior or protruded position.
If the joint hurts with the mandible moving, the appliance will need to provide posterior occlusion when the mandible moves to the painful location. This often means removing any anterior guidance from the appliance if the pain is in the joint on the same side the mandible is moving towards. If the joint on the side the mandible is moving away from experiences pain, it may be necessary to create non-working contacts in the occlusion of the appliance to support the joint.