(click here for deep overbite protocol)
An anterior midline point stop (AMPS) allows for a point stop, typically perpendicular to the long axis of lower incisors, and ideally on the mesial incisal edges of the two lower central incisors (1). An AMPS can be custom made, or quickly retrofitted with a pre-made device, the NTI Tension Suppression System (from NTI-TSS, Inc.
at http://www.nti-tss.com).
The therapeutic goal is to suppress the intensity of temporalis contraction in all habitual mandibular positions. However, at the initiation of AMPS treatment, the full nature of the habitual parafunctional movements may not be appreciated.
The appropriate degree of vertical opening creating by the AMPS is a function of the patient's original degree of incisal overlap and historical degree of lateral parafunction. For the patient with >50% incisal overlap (2), there has usually been little history of lateral parafunctional movement (sometimes referred to as "locked in"). In this situation, the point stop of the AMPS is relatively small, creating a millimeter of two of freeway space between the molars (3). Although canine occlusion is possible, it is unlikely, given the patients lack of historical activity.
For those patients who are not "primary clenchers" as in (3), and who have a history of protrusive and /or excursive activity, the DE may be ramped to the palate (3a).
Otherwise, for the majority of patients, the AMPS must prevent posterior and canine occluding in all excursive and protrusive movements. A flattened cusp tip of the canine tooth demonstrates a historical clenching of the canine teeth in excursive movement. Regardless of the freeway space between the molars (5b), the AMPS must ensure that the canine teeth can not occlude in excursive movement (4). "Daylight" between the canine cusp tips is all that is required (~1mm). As the jaw opens from a slightly discluded state to ~14mm, temporalis contraction intensity increases (X) (carnivores require significant biting force when their mouth is opened as they puncture their prey with their canine teeth). Opening the vertical dimension more that necessary may result in a reflexive continuation, or excitation of, clenching activity.
Occasionally, even when the canine teeth are discluded in excursive movement, there will be a posterior contact (5c), commonly involving a palatal cusp. This reveals an obvious occlusal interference (the occluding of the canine teeth in excursive movement should allow for disclusion of the molars). Although modification and/or equilibration of the teeth is generally initiated as symptoms resolve from treatment, this scenario dictates that the interfering cusp(s) be altered to allow for disclusion in excursive movement at the onset of treatment.. Although the option exists of increasing the vertical dimension by adding acrylic to the Discluding Element (DE) of the AMPS, this may increase the vertical dimension too far, usually resulting in destabilization of the cervical vertebrae (the patient may complain of a much more stiff and sore neck) (Y).
Some patients demonstrate an aggressive habitual excursive movement, or may develop it following use of the AMPS. Following an initial relief of symptoms, some patients report a return of symptoms, sometimes with a slightly different presentation. It would not be unusual for the parafunctional activity of the musculature to "seek out" a method to maintain temporalis contraction intensity. Although it may have not been within the patient's original habitual activity, the practitioner must confirm that the patient is not clenching is such an excursive position that a lower canine is contacting the DE of the AMPS (ghost image at (6)). The patient may report "I don't do that...because it hurts", which may be the indication that is in fact happening during sleep. In this situation, and for class III occlusions (8, 9), the AMPS can be fabricated for the lower incisors, with the DE occluding on the maxillary incisors (6), typically avoiding canine contact. If the occlusal scheme allows for it, some practitioners prefer to use a mandibular AMPS for the patient's daytime use, for enhanced esthetics and compliance.
Once lateral excursive movements have been accounted for, the practitioner must confirm that in full protrusive and/or retrusive mandibular movements, the lower incisors can not "get behind" or "get in front of" the DE. The distal corner of the DE must be extended lingually at least one millimeter distal to the distal-most contact of the incisors on the DE (7).Fabrication for deep overbite
(click here for custom fabrication without using an NTI matrix)
If the patient presents with a nearly 100% overbite (A), first check and see if the canine cusp tips are still "pointed". If so, then the patient's parafunction is most likely clenching only and they rarely, if ever, move excursively. In this case, the standard NTI would open the patient's vertical too much and would cause or perpetuate symptoms.
Adapt/reline a daytime device and completely remove the DE (B). Place a "blob" of acrylic at the cingulum area of the two centrals (C). Place ~2mm of folded paper towel between the posteriors and have the patient clench on the towel, while the daytime device (and its blob of acrylic) are in place (molars in D). After the acrylic has cured, sculpt to create a vertical stop for the opposing incisors (D).
The typical initial concern would be that the patient might protrude, thereby encountering resistance, thus increasing parafunctional intensity. However, for the primary clenching patient with a deep overbite (and minimal overjet), their pattern typically is to clench without any forcible excursive movement.Protocol for flared incisors (above): Follow normal protocol for adaptation of matrix, without addressing the relationship between the DE and the opposing incisors. Reshape and add acrylic to the DE to make the occluding surface parallel to the maxillary occlusal plane. In extreme cases the long axis of the lower incisors may not be perpendicular to the DE. Doing so would slant the DE in such a way that it would provide resistance to protrusive movement. Due to the patient's pre-existing adaptation, there are usually no adverse effects.
