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When doing a full-arch wax-up (left), why
wouldn't the dental lab technician mount models on a supended skull and
then manipulate the mandible to replicate masticatory movements?
Obviously, because it is simply not practical.
However, with the articulator sitting on the lab bench to the right, the technician is required to manually controll the degree of vertical pressure on the teeth, especially when replicating parafunctional grinding movements, also making it impractical. |
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| If all dental lab work were done on suspended articulators (above left), there would be a keen awareness of the parafunctional elevating forces that threaten everyday dentistry. However, due to practicality, dentistry stipulates a degree of constant clenching force and regulates it. |  |
The "clenching force" supplied by gravity
is a constant, thereby making the wax-ups in these examples vulnerable.
The technician therefore stipulates that clenching is a constant, and uses
an incisal guidepin to regulate and controll the clenching force (the tenchician's
"lab-NTI" device).
The result is an observation of an almost constant "state of occlusion", instead of the more realistic instantaneous moments of occluding. |
| In order
to accurately evaluate, diagnose and treat “occlusal problems”, a clear
understanding of what is intended as normal is essential. Only then
can various occlusal presentations be identified and categorized as to
what type of problem there is.
Without
elevation of the mandible, there is no occlusion (the jaw is suspended
at rest position).
In the normal system, once the teeth have been occluded, the temporalis ceases its active contraction. However, the mandible does not just automatically “spring” back open, it must be actively re-opened, or “depressed”, which is the job of the lateral pterygoid muscles. During normal chewing and swallowing, the instant the teeth are occluded, the lateral pterygoids are activated to re-open the mandible. Since advancing the condyle down the eminence is a relatively easy task (gravity is working with the lateral pterygoids and there is no other resistance to their activity), the lateral pterygoid muscles in comparison to the temporalis are considerably weaker in strength. With this basic acknowledgment of the normal function of the temporalis and lateral pterygoid, their parafunction and the resultant signs and symptoms from that parafunction become more obvious. Traditionally, dentists have been focused on the occluding scheme of the teeth and have stipulated that teeth may be in occlusion for periods longer than 0.2 seconds. Dentistry has been attempting to effect the frequency, duration or intensity of occluding by altering the resultant occlusal scheme. In the event occluding does persist beyond 0.2 seconds, the nature of the occluding scheme and the intensity of activity of the opposing muscles can allow for various presentations of signs and symptoms. By providing for an ideal condylar orientation and occluding scheme, dentistry can reduce (or eliminate) dental and temporomandibular problems, although clenching may persist allowing non-dental/non-joint symptoms to continue. An example of this methodology (from a popular text on occlusal problems*): “A premature occlusal contact that displaces the mandible distally can intensify the problem of disk derangement because it not only forces the jaw back, but also hyper activates masticatory muscles to incoordinated contraction. Thus it loads the condyle with extra force against the deranged disk and contributes to progressive further damage to both the disk and its connective attachments.” An analysis of the above statement reveals the stipulation of parafunctional occluding: “A premature occlusal contact that displaces the mandible distally...” The only way this can happen is if the temporalis continues its contraction following the initial contact of the teeth. During normal function, food is between the teeth, and any tooth contacting triggers simultaneous temporalis relaxation and lateral pterygoid contraction (thereby separating the teeth). “…can intensify the problem of disk derangement because it not only forces the jaw back…” The “it” referred to here is initial tooth contact when it is really continual temporalis contraction at work. “…but also hyper activates masticatory muscles to incoordinated contraction.” The temporalis is already hyperactive (otherwise, there would be no occluding after the first .2 seconds). The initial contact does not cause the temporalis to maintain contraction, in fact, it was supposed to signal the temporalis to cease its contraction. Once the contact exists, the lateral pterygoids are supposed to contract; however, they are met with the resistance of the occluded teeth (by way of the contracting temporalis). This appears to the practitioner as “incoordinated contraction” because both the elevator and depressor muscles are contacting with intensity simultaneously (lateral pterygoid muscles have no purpose to contract with intensity). So it is not the occlusal interference that causes the muscle incoordination, but the persistence of the elevator contraction. “Thus it loads the condyle with extra force against the deranged disk and contributes to progressive further damage to both the disk and its connective attachments.” The occluding contact (“it”, above) does not “load” the condyle with extra force. Force on the condyle is applied by the lateral pterygoid’s isometric contraction in an anterior and medial direction (the orientation of the muscle), due to the resistance of the occluded teeth. Normally, a lateral pterygoid contraction results in the discluding of teeth in the free advancement and sliding of the condyle down the eminence. The so-called neuromuscular school of occlusion utilizes a surface TENSing unit (transcutaneous electrical nerve stimulation) to alter the presenting length of the elevator musculature by "relaxing" these muscles to an "improved physiologic" state. Once this had been accomplished, a full coverage "splint" (or full mouth restoration) is provided to provide a masticatory surface compatible with the new vertical dimension. Resting vertical dimension is a function of the length of the elevator musculature, and that length is a function of the tone of the resident spindle fibers. Spindle fibers are innervated by the tone of the sympathetic nervous system, which can vary from day to day, or hour to hour. Spindle fibers are naturally in a state of slight tension in response to the pull of gravity. Surface TENSing may "relax" the spindles to increase the resting freeway space considerably. Given that the tension of the spindles has been indentified as the likely source of the tension and pain of myofacial dysfunction and tension-type headache, it is not surprising that surface TENSing can be highly therapeutic. However, the occlusal splint continues to provide a clenching surface. In some patients, the condylar orientation and new artifical occlusal surface reduces clenching parafunction, while in others, clenching simply perpetuates or intensifies, resulting in continuing headache symptoms. There is no predictability as to who will continue to clench with intensity, and who won't. Essentially, “occlusal problems” are a result of problems of occluding, a muscular parafunction. The frequency, duration and intensity of the muscular parafunction, not the scheme of the occluded teeth, or orientation of the condyles, dictates the resultant presenting signs and symptoms. * Dawson: Evaluation, Diagnosis, and Treatment of Occlusal Problems, second edition, p117 |