Appliance Fit and Bulk
The excessive tightness and unnecessary bulk of commercially made oral appliances not only makes them uncomfortable, it also exacerbates some of the same problems that the appliance is intended to solve. Therefore, to make them easier to wear and more effective, we make all our oral appliances in our own in-house dental lab using a block-out technique to ensure a passive fit and a tongue fitting inner contour that promotes healthy tongue posture.
Commercial dental laboratories make their appliances thick, because they worry about breakage; but thick appliances can exacerbate TMJ disorders by adversely affecting tongue posture. Since the tongue has no bone of its own, its base of operation is the lower jawbone. Consequently, anything that displaces tongue posture also displaces lower jawbone posture. In experimental studies, placing a block of acrylic in the palates of growing monkeys caused them to grow long narrow faces, because it caused them to hold their tongues downward and backward to avoid the block, which also caused them to hold their lower jawbones downward and backward. A bulky appliance can have a similar effect.
We don't need to make our oral appliances any thicker than necessary, because we can easily deal with breakages. In almost all cases, we can repair them for a small fee in one appointment using our in-house lab. Most people would rather wear an oral appliance that is thin enough to be comfortable rather than one thick enough to withstand accidentally sitting or stepping on it.
In addition, to further promote healthy tongue posture, we carve out a hollow for the tip of the tongue just behind the upper front teeth, where the tip of the tongue should rest. This "tongue tip hollow" encourages the tongue to maintain a healthy resting posture during sleep.
Commercial dental laboratories also make their appliances too tight, especially around the front teeth. Plastics all shrink when they set, usually about 3%. Because the upper dental arch is horseshoe shaped, shrinkage of plastic in an upper dental appliance squeezes the teeth inward, especially the front teeth in the middle of the horseshoe. The resulting pressure on teeth increases jaw muscle tension, just as any source of pain, including TMJ inflammation. Thus the tightness of the appliance works against our treatment goals. Increases in jaw muscle resting tension are cumulative. Those caused by pain or pressure around the teeth add to those caused by an inflamed TMJ or an unstable bite.
Eventually the pressure of a tight appliance is usually relieved by the inadvertent tooth movement it creates. At first, the teeth rebound every day, shifting back and forth between daytime and nightime positions, but over time they shift positions until they fit the appliance. This accidental orthodontics makes the appliance more comfortable, but in most people it moves the teeth in the worst possible direction, because the inward shifting of the upper front teeth increases the steepness of the overbite and can thereby increase the retrusive strain on the lower jawbone.
The dental industry has been unable to solve the shrinkage problem. Some dental companies market a plastic that can be softened in hot water just before placing it in the mouth, which only reduces the pain of inserting the appliance each night. It doesn't prevent the increased jaw muscle resting tensions or the adverse orthodontics that occur in response to the tightness. Other dental companies market a plastic that has an inner layer of rubber, which is more comfortable against the teeth than hard plastic; but it requires additional thickness without providing additional strength, and it frequently delaminates because rubber is difficult to attach to plastic.
Some dentists try to avoid the tightness problem by making all their oral appliances fit on the lower teeth. Lower teeth are less sensitive to pressure than upper teeth; because the lower teeth are all housed in one thick piece of cortical bone, while the upper teeth are housed in two thin membrane bones connected by a long suture. However, lower appliances worn at night can destabilize bites by confining the forces of nocturnal bruxing to the back teeth. These forces drive the back teeth into the bones (intrude) while the front teeth get longer and supererupt (extrude). The resulting increase in overbite can destabilize a natural bite and create the need for dental work just to re-establish a stable natural bite. To prevent such problems, oral appliances should only fit on the lower teeth in some people with class 3 (underbite) malocclusions.
We use a special block-out process during the fabrication of the appliance to create a perfectly passive fit. Leaving the teeth in their rest positions makes the appliance comfortable enough to wear every night on a long term basis and prevents pressure on the teeth from reflexively increasing jaw muscle resting tensions or causing adverse tooth movement, both of which move us away from our goals.