Appliance Fit and Thickness
SUMMARY
The excessive tightness and unnecessary thickness of all commercially made oral appliances today, including those made by your dentist through a dental lab, can exacerbate some of the same problems they are intended to solve. The tightness produces pressure on the teeth, which reflexively increases jaw muscle tension. The bulkiness causes the tongue to shift down and back away from the bulk in front, which pulls the mandible down and back. To prevent these problems, we make all our appliances with a passive fit, minimal thickness, and a large carved out space for the tongue tip just behind the upper front teeth.
EXCESSIVE TIGHTNESS occurs because plastics shrink when they set, which squeezes the teeth inward, especially the front teeth in the middle of the squeeze. The pressure on teeth automatically increases jaw muscle tension, because noxious sensory signaling from any joint increases the tension in the muscles which cross that joint, and the jaw muscles cross the TMJs. In this manner, pressure on teeth causes automatic jaw muscle tension the same way a swollen ankle causes limping. The increased jaw muscle tension caused by the tightness of an oral appliance adds to the jaw muscle tension caused by an inflamed TMJ or an unstable bite.
Even slight pressure on teeth can trigger jaw muscle tension, because the system is so delicate. Each tooth at rest is suspended in a sophisticated shock absorbing mechanism which contains so many sensory nerves that they occupy an oversized portion of the brain. When teeth remain in the middle of their sockets, those tissues remain passive. When a tooth is held on one side of the socket all night from the pressure of an oral appliance, those tissues undergo reflex adaptive responses designed to make the socket once again fit the tooth position. One of those reflex adaptive responses is increased jaw muscle tonus.
Each tooth also has a range of motion within its socket up-and-down, side to side, and even mesio-distally (front to back) against the adjacent teeth. The functional movements of the teeth throughout that range of motion help drain the waste products from their sockets. An appliance that "hugs" a tooth restricts its range of motion and thereby also its circulation. Tightness is not required for appliances to stay in, comfort is. An appliance only needs to be tight enough to resist the forces of gravity.
Some appliances that have been worn for many years have become passive, because they have moved the teeth to fit the appliance. This accidental orthodontics makes the appliance more comfortable, but it also tips the front teeth inward, which increases the steepness of the overbite, which is part of the root cause of your problem.
Some dentists try to avoid complaints of tightness by making their oral appliances fit on the lower teeth, which are much less sensitive to pressure than the 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 pressure-sensitive suture. However, lower appliances worn at night can destabilize bites by confining the forces of nocturnal bruxing to the molars and premolars, which drives them back into the bone (intrusion) and makes them shorter, while allowing the front teeth to super-erupt (extrusion) and become taller.
Dental companies have failed to compensate for the shrinkage of acrylic. Some companies market an acrylic that can be softened in hot water just before placing it in the mouth, which reduces the pain on initial insertion of the oral appliance, but doesn't reduce the pressure on teeth all night or the adverse orthodontics it can cause. Other dental companies market a plastic that has an inner layer of rubber. The rubber lining allows the individual teeth at least a slight range of motion, which feels more comfortable; but it often delaminates, and it requires additional thickness, which also works against our goals by interfering with healthy tongue posture, as described in the section on excessive thickness below.
We use a special block-out process during the fabrication of each appliance to create a perfectly passive fit. As soon as the appliance is in place, it should apply no pressure to the teeth. It should almost feel like there is nothing in your mouth. Soon dental labs will be able to use digital scanning and milling or 3D printing to create appliances with a perfectly passive fit, but not yet in 2025.
EXCESSIVE THICKNESS, especially at the front of the palate, shifts tongue posture backward to avoid the bulk in front, which also shifts mandible posture backward, because the mandible functions as the bone for the tongue. Experimentally cementing a block of acrylic in the front of the palates of monkeys caused them to grow long narrow faces, because they acquired a lowered tongue posture to avoid the block, which caused the mandible to grow in that direction. In humans, when the mandible lowers, it also rotates backward, which restricts airway passage and causes forward head posture as an airway-preserving adaptation. However, nearly all currently made appliances have a great deal of unnecessary acrylic in the front of the palate, because commercial dental labs make their appliances thick enough to prevent accidental breakage. We don't need to make our oral appliances thick, because we can repair breaks during a single office visit for a small lab fee. Most people would rather have their oral appliance thin enough to be as comfortable and effective as possible rather than thick enough to withstand accidentally sitting or stepping on it. In addition, we carve out the front of each appliance to fit the tip of the tongue (tongue tip hollow) so the tongue can maintain a healthy posture during sleep.
