Identify a Physiologic Mandibular Rest Position -
The Key to Taking an
Accurate Bite

Part II of II

By Clayton A. Chan, D.D.S., F.I.C.C.M.O.

In this issue, I will discuss the methods and techniques using neuromuscular instrumentation that can be used to record an accurate bite registration with patients, relating the mandible to the maxilla. The history and present day methods of bite registration techniques will be further discussed in light of the patient's musculoskeletal occlusal signs and symptoms.

Dentistry is a profession that has prided itself to giving attention to detail, especially in the areas of dental mechanics as it pertains to tooth repair. As was pointed out in the previous issue our profession has been distracted from the basic bio-physiologic laws of mandibular movement and function to the mechanical aspects of dentistry.

Our dental training has instructed us to "mechanically" record these maxillary to mandibular relationships, often ignoring the importance of bio-physiology establishing an optimal mandibular position when muscles are at the most relaxed and unstrained position. It has also diminished the importance of muscle physiology and the importance of establishing an optimal normalized joint position, which impacts the occlusal position and how anterior as well as posterior teeth come together.

An astute clinician realizes that many musculoskeletal occlusal signs and symptoms effects the head, mandible, neck and cervical region. These signs and symptoms also have a correlation to the musculature, the neurology, bones and teeth. Some of the signs and symptoms are: Headaches that maybe present in the temporal or behind the head/ neck region, facial pain, muscular pain of the upper and lower jaws, ear congestion feelings, ringing in the ears, pressure behind the eyes, shoulder aches and tension, tingling in the hands and fingers as well as teeth sensitivities are only a few to mention that are often overlooked and ignored as medical problems having nothing to do with dentistry. Why do we palpate the muscles in the clinical examination? Why do we do a TMJ examinations and joint palpation? What meaning do these protocols have unless the clinician is able to understand how they relate to the stomatognathic system and occlusion. Are we treating teeth mechanically or being physicians of the mouth understanding the biology and physiology of this remarkable dynamic system?

Three basic and fundamental questions are the focus of discussion. The previous article raised and answered the following questions:

Where is the optimal physiologic rest position of the mandible to the cranium?
How does one determine the optimal physiologic position to take an accurate bite registration?

The neuromuscular paradigm of finding the mandibular position of optimal physiologic rest is based on muscle physiologic laws that muscle efficiency is generated at a length that corresponds to a maximal overlap of actin and myosin filaments, which represents an optimal crossing bridging effect. Nordin, in his book, "Basic Biomechanics of the Musculoskeletal System" and Grossman in his article, "Review of the length associated changes in muscle: experimental evidence and clinical implications", clearly document these principles of muscle physiology.

Physiologic rest of the mandible is where the muscle fibers are normally near a passive resting length fulfilling the laws of muscle physiology and posture. These known physiologic muscle laws are the basis to the neuromuscular approach in establishing an accurate bite registration.

The beginning reference position of rest and the isotonic closing path of the mandible can be measured using surface electromyography (sEMG). The physiologic rest position can also be confirmed by computerized mandibular scanning instrumentation (CMS) as a means to visually verify positional changes from a habitual accommodative rest position to a physiologic rest position usually after muscle stimulation via TENS (transcutaneous electro-neuro stimulation).

The third question, which pertains to identifying an accurate bite registration, is:

What methods and techniques are available in taking such an accurate bite registration on patients that have no symptomology to those who have severe symptoms and TM joint dysfunctional problems?

There are two approaches that are taught at LVI where an accurate bite can be taken that addresses the laws of muscle physiology. The first, is using ultra low frequency TENS- J4 Myomonitor (Myotronics-Noromed, Tukwila, WA) as a stand-alone unit to relax the masticatory muscles only. The second method is using both the TENS - Myomonitor in conjunction with the simultaneous aid of neuromuscular instrumentation (K6-I, kineseograph) with computerized mandibular scanning (CMS), electromyography (EMG) and sonographic capability to visualize and record mandibular position and masticatory muscle activity simultaneously.

Using the MYOMONITOR (TENS) the First Step to Relaxing the Masticatory Muscles

Using ultra low frequency TENS has been found to be an effective but basic means to establish a rested mandibular position after Myomonitor stimulus of 45-60 minutes of time with most patients. This stimulus when applied over the coronoid process bilaterally will stimulate neurally the nerve trunks of the fifth and seventh cranial nerves at the superior mandibular notch percutaneously. Numerous scientific studies have documented these facts (Choi and Mitani, 1973, Williams and Marshall 1986). At LVI we recommend using the Myomonitor (TENS) with a three-electrode configuration with a common ground that delivers a more optimal relaxation of all masticatory and cervical muscles bilaterally.

The use of the J4 Myomonitor is an excellent and efficient means to relax all the masticatory muscles before taking a bite registration, especially when the clinician has accessed that a more optimal mandibular to maxillary relationship is necessary in doing more definitive restorative treatment. The indications for the use of the Myomonitor are:

Relaxation prior to occlusal registration
Muscle relaxation for TMD diagnosis and treatment
Muscle relaxation prior to restorative procedures
Increase muscle circulation and lymphatic drainage
Muscle relaxation prior to adjusting the occlusion
Muscle relaxation to assist orthodontic diagnosis and treatment
Border molding for full denture impressions

By relaxing all the masticatory muscles via low frequency TENS the clinician is now able to establish an optimal mandibular position in space in six dimensions (vertical, antero-posterior, lateral, pitch, yaw and roll) prior to taking a bite registration for optimal diagnosis and treatment. The posterior region of the mandible which is often over looked is now better aligned, decompressing the temporomandibular complex to a more normalized position, thus removing the torques and strains that have been present.

With the use of metric calipers one can measure the vertical dimension manually between a point on the nose to a point on the chin after TENS. When taking the bite registration the Myomonitor stimulus guides the jaw through space and allows the mandible to rest at a vertical position that satisfies the physiologic requirements of the masticatory system. A 1-2 millimeter freeway space along the unseen path of mandibular closure (neuromuscular trajectory) is also computed to relate an appropriate interocclusal relationship between the maxilla to mandible when recording a bite registration to establish a plane of occlusion. Any registration material of choice that does not induce torque or muscle strain can be used.

It should be noted that the Myomonitor is only a device to relax hypertonic musculature bilaterally. It is not a recording or measuring instrument.

Taking a Guided Bite Registration Using Simultaneous TENS, CMS and EMG

The second and most optimal means to taking an accurate bite registration uses both the ultra low frequency TENS as well as the sophistication of computerized mandibular jaw tracking technology with electromyographic (EMG) capability to monitor the details of mandibular movements, function and rest. Visualizing where the mandible is in space in three dimensions gives the clinician the added edge to optimize his diagnostic and treatment capabilities to see the mandibular path of closure when taking a bite registration. Both the vertical, sagittal and frontal parameters can be monitored via CMS instrumentation. The mandible is guided without strain to a myocentric target along the neuromuscular path of trajectory where the masticatory muscles are most relaxed as confirmed objectively with simultaneous EMG in real time after the use of the Myomonitor. Over the past 20 years the neuromuscular clinicians have found the benefits of visually observing physiologic responses that effect the posturing of the mandible, head and neck. Implementing these diagnostic and treatment tools into a clinical setting now brings the former guess work into an objective quantifiable realm for optimal occlusal results.

Computerized Mandibular Scanning (CMS) - Visualizing Mandibular Position

Computerized mandibular scanning (CMS) or jaw tracking instrumentation allows the clinician to:

Track mandibular movement and position.
Identification of interocclusal distance and freeway space.
Monitors the position of the jaw in three dimensions.
Represents the spatial position of the mandibular incisal edge relative to the skull.
Measures the rotational movement in the frontal and sagittal planes confirming any neuromuscular dysfunction.
Displays multi-dimensional assessment of torquing movements used to differentiate between contributing factors of a pathologic position to a non-pathologic position on opening and closing of the mandible.

Surfacing Electromyography (sEMG) - Visualizing Rested Muscles

Surface electromyography (sEMG) is effective to:

More specifically delineate and define hypertonic musculature.
Differentially diagnose between intra-capsular interference (meniscal) and extra-capsular interference (influence of the surrounding hypertonic muscular matrix) to determine predominant dysfunctions.
Aids in defining the etiology of predominate neuromuscular dysfunctions to preclude misdirected palliative treatment regimens.

Combining all three modalities of TENS, CMS and EMG simultaneous allows the clinician to visualize mandibular responses and movements more discriminately allowing the clinician to better assess objectively where the mandible should be positioned when taking a bite registration. Having this capability to see finer physiologic responses without human interferences and record a mandibular to maxillary relationship without the intrusion of the traditional manual jaw manipulative techniques certainly minimizes human error and unwanted pathologies.

Both bite registration techniques after TENS are taken with the patient in an up right position to allow all the supportive muscles of the head, neck and cervical region to rest more calmly. At no time does the dentist or the patient need to manipulate or maneuver the jaw into place with strain. A low frequency bilateral stimulus from the Myomonitor allows the mandible to posture in space at a physiologic rested position, unique only to that patient. The vertical, the sagittal and frontal position of the mandible can be accurately monitored via computer and aligned to the skull meeting all the physiologic requirements that are necessary for an optimal result. An isotonic path of mandibular closure results during TENS and a bite registration material can be injected over the occlusal surfaces allowing the jaw to close calmly along the neuromuscular trajectory to a physiologic rest. EMG data is monitored objectively as it relates to the mandibular position simultaneously for best results.

It should be noted that using neuromuscular instrumentation does not remove the necessity of the doctor's skill, knowledge and insightful expertise of each particular patients condition and need that is necessary in the decision making process of taking an accurate bite registration.

Any bite registration material can be effective, as long it does not induce torques or mandibular muscle strain. Capturing an accurate bite registration in all three dimensions can now be accomplished more precisely and physiologically with the aid of today's neuromuscular technology.

Clayton A. Chan, DDS, FICCMO
http://www.tmjcare@drclaytonchan.com

References:

Myotronics-Noromed, Inc, 15425-53 Avenue South, Tukwila, WA 98188 (800) 426-0316 . FAX (206) 243-3625 . e-mail: myotronics@AOL.com http://www.myotronics.com
Nordin M and Frankel VH. Basic Biomechanics of the musculoskeletal System. 2nd ed. Philadelphia: Lea and Febiger, 1989.
Gossman MR, Sahrman SA and Rose SJ. Review of the length associated changes in muscle: experimental evidence and clinical implications. Phys Ther. 62:1791-1808, 1982.

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