Many times I get asked if there is a connection between the vertical dimension of an occlusal appliance and it’s effectiveness. The answer is “yes” and “no”. There is scientific evidence that supports that the activity of the elevator muscles decreases as we open the vertical dimension of occlusion. The next questions to ask, are how the amount of vertical change affects the amount of muscle activity reduction and the duration of this affect. One of our goals in appliance therapy is to reduce the activity of the elevator muscles. This reduction directly affects the signs and symptoms associated with muscles, like headache as well as having a positive impact in reducing joint loading which is a percentage of the total force applied. With this in mind the fact that the muscle activity of the elevator muscles decreases as we open vertical dimension is most likely one of the reasons appliance therapy works in helping our patients feel better. It may also contribute to why during appliance therapy of short duration we see less evidence of attrition. It may not be that the person has stopped or reduced the amount of parafunction occurring, but that they are applying less force and therefore not notching the appliance significantly.
Elevator muscle activity decreases as we begin to open vertical, but beware that at some point the force starts to rise again until we approach maximum opening. The range of decreased activity seems to be more than 1mm of opening and less that the patients vertical dimension of rest. The other challenge to be aware of is that studies have shown this reduction in elevator muscle activity to be transient, and that the body adapts and the muscle force returns to baseline even at the new VDO. This phenomenon means that we can not count on only the effect of changing VDO to provide therapeutic benefit for our patients. Long lasting benefits come from designing an appliance at the appropriate condylar position and with the optimal occlusal design based on that patient’s joint, muscle and dental condition.
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