”Imagine” – John Lennon
Imagine a perfect occlusion
A mandible of match below
Nothing to grin or gag for
Above us only sky
Imagine, no bite interferences,
it’s easy if you try.
There’s no stop in chewing
The mandible just glides
Imagine all the people
Chewing gum at ease
You may say I’m a dreamer,
but I’m not the only one.
Examining my patient’s lateral excursions with the Tscan helps me to identify posterior tooth contacts that interfere with the smooth mediotrusive glide of the mandible in their chewing movements. I have realized that the final mediotrusive jaw-movement in the gum-chewing cycle, for many of us, consists of many temporally alternating bursts and quiet periods of directionally opposing muscles. The fewer the number of interfering back-tooth-contacts there are, the easier the chewing process is.
Watching the occlusal contact forces developing to different sets of teeth in the process of laterotrusion, I have learned to understand how the antagonism of muscles of conflicting orientation slows down the velocity of the mediotrusive and laterotrusive movements of the mandible. It is the number of these back-tooth interferences that determines why some individuals are more confident with their gum chewing, for others it is hell.
Mutual exclusion of the activity of antagonistic muscles must be universal to jaw-closures.
Imagine an ideal world, where, during the mediotrusive glide of the mandible, there would not be any interfering back tooth contacts. There would just be the steady flow of tactile sensory information coming from the working side canine teeth and the anterior region. The ideal medial pterygoid would be active, unhindered and triumphant, bringing the mandible home swiftly to the maximum intercuspal position (MIP), whilst the other, more vertically oriented muscles of jaw-closing would ideally be at rest.
At the absolute ideal instant, as the ideal mandible would hit its ideal home – at the MIP, the amount of occluding back-tooth contacts would suddenly increase. The sensory information from the back-teeth from the left and right side of the dentition would increase dramatically. There would be an overflow of sensory information coming from the back teeth. That’s a lot of orders for the motor ganglia to once again start the left and right masseter and temporalis muscles going. The tiny amount of a different kind of sensory information from the anterior teeth would be overwhelmed at the instant. Now it would be the time for the medial pterygoid to be silent.
It does not matter, whether the mandible is homing to the MIP from a lateral excursion. The same things happen in an habitual, non-manipulated, vertical down-up jaw-closing. The interesting thing is the beautifully organized interplay with the muscles. The different orientation of muscles necessitates that their activity cannot be happening at the same time for all of them. It is my logical assumption that the activity of the medial pterygoid must emanate from the tactile sensory signals of the anterior dentition.
My narrative of the final milliseconds of the chewing cycle is based on what I can see happen in a Tscan recording. Anyone who has got a Tscan can easily check out what happens when the mandible hits the MIP in habitual closing movement. The computer software gives a mark “A” for the time-point of the first point of occlusion contact. In an unassisted closure of the mandible the pull of the muscles is the only determinant for the movement of the mandible. The sensors of the muscle spindles may be involved to guide the movement but as long as the upper and lower jaw teeth do not touch each other there is no sensory information coming from the teeth. Only after the time-point “A” a flow of sensory information starts emanating from the teeth.
For almost any person with an undisturbed, vaguely “normal” bite, it is a tooth-pair from the anterior area that hits first. The first occluding pair of contact very seldom appears on the back-teeth, except for patients with gross back-tooth prematurities. For subjects with more or less “normal” curve of Spee, the normal course of events is that the anterior teeth hit first. The anterior teeth, sort of, prepare the ground, after which, the more forcible back-tooth contacts are encouraged to take effect. This is what I think is the normal conduct of a natural occlusion in vivo.
During these initial milliseconds, after the time-point “A”, there may be some pairs of teeth that occlude more forcibly relative to other pairs of teeth. Without a Tscan it is impossible to see where these excessive points of contact force happen. Occlusion foil ink marks help to identify the occlusion points of contacts that appear on the Tscan software. The appearance of the ink spots, however, correlates poorly with the force.
In the anterior area often, but not always, there appears to be tiny lines on the palatal aspect of the maxillary incisors and canines. These lines are due to the initial forced gliding of the mandible into retro-gnathic, or worse, lateral direction. These line-drawings are often less than a few decimals of a millimeter, nevertheless these interferences profoundly effect a forced deflection of the as yet occluding dentition out of the position that otherwise would be preferred by the muscles. The Tscan software detects and displays such force outliers. For a clinician the force outliers on the anterior teeth are a powerful sign of trouble in jaw closing. Rapidly increasing growth of force on the anterior dentition makes the control system of jaw closure alarmed in about as short a time as it takes for a nerve impulse from the periodontium of an anterior teeth to be transported, via myelinated nerve fibers, to the ganglia of the brain stem. The sensory signal from the anterior teeth sends a very rapid warning signal, that kills the vigorous activity of the temporalis and masseter muscles. As long as there are prematurities (i.e. force outliers) of the anterior teeth, considerably long time can occur between the initial tooth contact (point ”A”) until the moment, when all the tooth contacts are finally secured to lock each other at the MIP. A slight adjustment of a such prematurity in the anterior area immediately makes a therapeutic intervention. The patients normally notice the change themselves. Tapping the teeth together, they say, goes in a more relaxed way. In contrast, leveling off any prematurities of the back teeth would usually go unnoticed by patients. It is my experience, and I would like to stress that it is quite mandatory not to touch any of the high force contact points of the premolar and molar area in the initial phase of adjustment of occlusion.
The anterior teeth, that become subjected to rapidly growing relative force, as compared to their neighbouring teeth in habitual jaw-closing, can also be identified by holding the examiners fingertips on the facial side of the anterior teeth while the patient taps his/her teeth together. The fremitus is coincident with the force pillars that appear on the computer screen in the Tscan 3-D graph. Usually, to attain force equilibrium within the six anterior teeth pairs only requires a minimum amount of adjusting. It is my way of removing tooth/filling -material with a diamond bur to start from the most incisal end of the tiny ink lines of excessive force on the palatal of the front teeth. The ink foil markings should be repeated until the front teeth contacts should appear as dots, not lines. Consequently, no more fremitus should be felt by the fingertips of the examiner. With a Tscan, the anterior tooth contacts should be light, evenly distributed and barely discernible with the set level of voltage of the sensor. Most of the total force becomes to rest on the back teeth. Milliseconds later, another workshift-change of the muscles occurs again. A powerful clench can be observed on the total force curve on the time vs. force graph. The mandible is securely at home and the temporalis and masseter muscles are triumphant again.
To conclude, the description above ends my narrative of what I imagine takes place during the final milliseconds of the chewing cycle. This article also offers an outline of a therapeutic clinical procedure that should precede any other interventions to change the occlusion of our patients.