In this series I have previously described the common appliance options available, as well as the most common reasons an appliance may be prescribed.

I'll now start describing how the different appliances may impact muscle activity levels. It is critical that clinicians realize one of the great challenges of occlusion, and particularly appliance therapy: Different patients may respond differently to the same appliance. For example, many patients will reduce their clenching and grinding activity when prescribed an anterior only appliance, but others may actually increase their clenching activity. It is for this reason that we can't say absolutely what any appliance will do since it is very patient dependent.

It is important to realize that if we are talking about pain management, often the patient gets better regardless of which appliance is used. In fact, Laskin and Greene in the 1970s provided patients in pain mock appliances; they didn't alter the occlusion in any way and a significant percentage of patients got improvement. They additionally did mock equilibrations, where they used a smooth bur so nothing actually got adjusted and a significant percentage of patients reported relief.

Research for different appliance designs and muscle activity levels: Different occlusal appliances and their impact on muscle activity have been investigated for decades. This research is typically done by placing surface electrodes over the masseter and temporalis muscles and some studies include the digastrics, trapezius and sternocleidomastoid muscles as well. The patient is then asked to clench and move while the electrodes record the electrical activity from the different muscles. This may first be done on the natural teeth to get a baseline level and then an appliance is inserted and the recording process is repeated. In most studies the appliance is then altered to provide a different occlusal scheme and the process is repeated.

The risk of interpreting the research data: The research performed as I described above is the basis for many of our beliefs about how different appliances effect muscles, but it is important to realize there are risks in utilizing the data as gospel for all patients. For one thing the patients in these studies are all awake and conscious of what they are doing during the process. Second, it is a temporary and artificial change to their occlusion; this doesn't mean the changes in muscle activity levels seen with different occlusal designs would continue over a long time period. This is born out by the fact that there are patients who experience significant relief of muscle symptoms from an appliance and a few months later no longer get relief from the same appliance but do from a different design.

And finally, research done on 10 or 20 patients doesn't mean everyone will behave the same following the same treatment. In Part II I'll go over more details about what the research says, as well as, clenching, excursions and lateral pterygoid activity.