A general rule-of-thumb is to have the occluding surface of the Discluding Element parallel with the maxillary occlusal plane...
(the below animation plays for 16 seconds and then repeats).
More insight on the significane of minimizing the VDO in extreme protrusive:
By creating too much vertical opening with the NTI-tss (left), the condyle may experience excessive rotation during parafunctional excursive movement, thereby exposing it to excess strain. The patient with "too much vertical" reports discomfort when clenching excursively with the NTI-tss in place, but is not uncomfortable when clenching in centric.The requirement to suppress muscular intensity is lack of canine and posterior contact, and the requirement for minimal joint strain is minimal (if any) rotation during clencing. Therefore, for the patient who may be clenching in an excursive position, the DE should always be modified to provide only enough vertical opening to keep the canine teeth discluded when in excursive position (right).
In this scenario, an excursive position allows for canine-to-canine contact with the NTI-tss in place, which is creating excessive vertical dimension (center, arrows), meaning on the opposite side, the condyle is considerably unnecessilarily rotated during translation (right). The more rotated the condyle is, the more strain it is subject to. With the canine able to occlude, clenching intensity on the ipsilateral side may persist (left). The contra-indicated remedy would be to increase the height of the Discluding Element.
By reducing the cusp tips of the occluding canines and the height of the DE, vertical dimension is closed to a more therapeutic level. This has decreased the degree of rotation of the contra-lateral condyle, and eliminated the resistance utilized by the lateral pterygoid to strain its condyle. Without the canine contacts, clenching intensity is significantly reduced.
In the above diagram, the NTI-tss device has been adapted. (note how the Discluding Element has been centered over the mandibular midline)
In an excursive movement on the day of delivery, note the disclusion of the posterior teeth...
Two weeks later, a left excursive movement (above) reveals an occlusal interference with a maxillary palatal cusp and a mandibular distal-lingual cusp. This may not be evident upon original delivery. The NTI may allow a condyle to seat more posteriorly/superiorly in the fossa, thereby allowing the patient to eventually "find" this mandibular position and continue clenching activity, possibly resulting in an increase in symptoms. THEY MAY ALSO REPORT THAT THEIR LOWER INCISORS ARE SUDDENLY SORE. This is due to the increased clenching intensity that the posterior contact has allowed. In this example, the translating right condyle is strained due to the chronic contraction of its lateral pterygoid with resistance, while clenching intensity of the left temporalis persists. (the above photo was digitally altered for demonstration purposes).
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The indicated remedy is to reduce the cusp tip(s) of the interfering teeth. Increasing the vertical dimension of the DE to discluded the interfering teeth is contra-indicated. Doing so would simply further translate the right condyle (in this example), placing it under an even greater strain when the patient continues excursive clenching behavior.
Lack of simultaneous contact on the central incisors:
If upon initial closure, the two lower centrals do not contact the DE simultaneously (first photo), the patient will exert an unconscious effort to "equilibrate" the forces (second photo), by shifting their jaw upon initial contact. Either the teeth (last photo) or the DE should be altered to allow forces on both centrals upon initial contact. (note: This example was originally composed prior to the development of the lower "IG" (Incisal Guidance) device. The IG is now the preferred design)
The most important reason to make a lower NTI (b) (the original Standard device is pictured), is to avoid (a) having a lower canine contact the DE of the upper device, which can cause or perpetuate symptoms. (note: the DE of the lower device pictured in (b) may be "getting stuck" on the distal/incisal corner of the upper central incisor. The dome shape is the IG device was designed to prevent this occurrence, BELOW)
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b)
