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

Part I

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

Dentists today are very familiar with impression taking materials as well as recording bite relationships. Manufactures of these materials have answered the dentist call to accuracy and stability of these materials to help them reproducing the necessary information intra orally for the extra oral fabrication of crowns, bridges, prosthetics and even recording bony relationships. Yet, it is at this step that the dentists are often not aware of the importance of muscle physiology and establishing an optimal mandibular to maxillary relationship.

Taking a bite registration is a procedure that is commonly practiced in dentistry when the clinician needs to re-establishing the mandible to maxillary relationship. This relationship is often recorded and used for a single tooth indirect crown restoration, prosthetic reasons such as dentures and partial denture fabrication, when there is need for full mouth rehabilitation, when there are orthodontic/ orthopedic concerns and even when a paining patient is in distress with TMD/ myofascial pain disorders for an appliance (orthotic/ splint) due to the dysfunction of the craniomandibular/ temporomandibular joint complex.

Bite registrations or interocclusal recordings of the upper and lower teeth or more specifically the registration of the maxillo-mandibular relationship are often established by using the existing present supportive teeth with the accommodative wear patterns as landmarks to establish a bite registration. A manual manipulative technique either chin point technique or bi-manual manipulative methods of mechanically guiding the mandible to a particular jaw joint position, using shims and jigs to help stabilize the mandible and position it in a particular manner with or without the aid of joint radiographs to assist in visualizing TM Joint position have been used. These are methods that are commonly used in today's clinical setting to establish a bite relationship of the existing dental arches when diagnosing or treating the patient.

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Distracted from the Bio-Physiological Sciences

Dentists pride themselves in giving attention to details, making sure that the preps are neat and tidy. Accurate impressions are taken with all the margins completely observable. Models are poured to be bubble free and trimmed under a microscope for accuracy. The laboratory process again confirms microscopically the accuracy of marginal adaptation and contour of their crowns, yet we clinicians have never been encouraged to pay attention to muscle physiology that clearly brings the mandible with the accompanying teeth together to contact the upper teeth in an accurate physiologic manner. Our dental training has instructed us to mechanically record these relationships, often ignoring the importance of 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. Our traditional training that once started out in understanding anatomical sciences, physiology, neurophysiology, histology and all the sciences to support what we do as dentist has been distracted by the mechanical aspects at the end of our dental school training with mechanical devices (articulators and face bows) that have now taken precedence away from the actual bio-physiological aspects of dental sciences of understanding the interrelationship of all the entities that comprise the stomatognathic system (joints, muscles, nerves and teeth). It is no surprise to realize why many dentists have problems with "Occlusion" issues and end up having to adjust all the wonderful tooth anatomy away after meticulous being carved by the dental technician. What we started out to learn in our dental school regarding the physiological sciences to be applied in our clinical dentistry became diluted down with emphasis now placed on the mechanical approaches to dentistry to solve the bio-physiological problems.

Although mechanical mandibular and jaw joint recording devices have plagued our profession for years to help us understand the articulation of teeth and the movements of the mandible and joints, they fail to accurately and reliably model the bio-physiologic aspects of muscle activity, mandibular movements and their influences to the human masticatory and postural system.

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Limitations of Working Within a Non Physiologic Position

It is often common practice to work within the habitual/ acquired occlusal position since this position is already established and proven to seemingly function and work for the average patient and dentist. It is often found to be the least complicated mechanically. It is simple, easy and predictable since there is an obvious end point to where one can reference a bite relationship and restore a single tooth with a crown or filling or even a quadrant of dental restorations. Working within these established occlusal schemes have limitations though.

Some of the limitations in taking a bite in an accommodated or manipulated position are:

  1. Limits the clinician from establishing an optimal mandible to cranium relationship.
  2. Limits the clinician from bringing his patient to optimal facial cosmetics.
  3. Limits the clinician from establishing optimal dental aesthetics - optimal tooth morphology and dental architecture.
  4. Limits the patient from having an optimal stable neuromuscular occlusion.
  5. It reduces the chances of long-term patient comfort and muscular balance of the complete posturing system which includes the head, the mandible, the cervical region of the neck, the shoulder and pelvis and legs.

Establishing an Optimal Neuromuscular Position

Establishing an optimal neuromuscular bite does have some extremely important aspects that all clinicians should consider if trying to achieve optimal dental function, stability and harmony of the stomatognathic system.

  1. It allows the clinician to move outside the constraints of a worn dentition.
  2. It allows the clinician to do "ideal" dentistry free of established malocclusions, re-establishing a new occlusal scheme.
  3. It allows the clinician to develop and establish optimal dental aesthetics as far as tooth shape, contour, anatomy and morphology in both the anterior and posterior regions.
  4. It allows the clinician to establish optimal facial cosmetics due to a more harmonious muscular balance when an optimal physiologic mandibular position is found as well as the lower one third of the face which is often deficient vertically and aesthetically.
  5. It allows one to address the numerous musculoskeletal occlusal signs and symptoms which often go undetected such as: headaches, ear congestion feelings, ringing in the ears, pressure behind the eyes, teeth sensitivities, TMJ noise, masticatory muscle tenderness, neck and shoulder pain, to only name a few.

In short the neuromuscular position allows the clinician to diagnose and view the patients treatment plan outside the restrictions of the present occlusal scheme that can present with numerous hidden dental surprises and symptoms once treatment has begun.

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Basic Questions to Consider

The following questions then arise:

  1. Where is the optimal physiologic rest position of the mandible to the cranium?
  2. How does one determine the optimal physiologic position to take an accurate bite registration?
  3. What methods and techniques are available in taking such an accurate bite registration on patients that have no symptomology to patients who have severe symptoms and TM joint dysfunctional problems?

These basic and fundamental questions will be our focus of discussion in the next two issues of this publication.

Where is the optimal physiologic rest position of the mandible to the cranium?

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. 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.

Physiologic rest position can be defined as:

  1. The mandibular position vertically anterior-posteriorly and laterally when the head is in an upright postural position and the involved muscles, particularly the elevator and depressor muscles, are in equilibrium in tonic contraction.
  2. It is that position in space where minimal expenditure of muscle energy is needed along an isotonic path of mandibular closure that begins from the rest position of the mandible. This means that the extensor and depressor muscles that move the mandible are postured at a position that exert minimal electrical activity during resting modes.
  3. It is the reference position of the mandible from which diagnostic and therapeutic decisions are made.

A mandibular position can be influenced by numerous others factors as postural problems of the head, neck/ cervical region, and back region. Internal derangements of the temporomandibular joint, emotional factors and systemic health factors of the patient can also contribute to a compromised mandibular position. All of these factors must be considered, identified and diagnosed before establishing a physiologically rested mandibular position prior to taking an accurate bite registration.

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How does one determine the optimal physiologic position to take 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 mandibular tracking instrumentation 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 physiologic rest position cannot be found on dental articulator neither on any mechanical device that simulates mandibular movements. It can only be found on the human being, the patient who is to be treated. It can be found after establishing physiologically rested muscles which effect the mandibular position which is typically down and forward of the acquired accommodated centric occlusal position. Physiologic rest position of the mandible can be established after muscle stimulation via TENS and can be recorded accurately when the mandible is at its physiologic rest position consistently. It can also be measured, observed and recorded when implementing computerized diagnostic and treatment instrumentation using surface EMG, computerized mandibular scanning and after TENSing the masticatory muscles at a more sophisticated but predictable level. It is not found by assumptions, subjective feelings or educated guesses but can be quantified by measuring and confirmed by clinically applied bio-instrumentation.

An accurate bite registration can be found after understanding that the masticatory muscles that support the mandible must be relaxed physiologically to a stable rest position. Physiologically relaxing the musculature allows the mandible to posture both vertically, anterior- posteriorly and laterally. The pitch, the yaw and roll position of the mandible as in (aeronautical terminology) is also addressed removing the torques and strains from the compressed jaw joints and muscles. Establishing this normalized position is the most reliable starting position to begin recording the maxillary to mandibular relationship for an accurate restorative, prosthetic or orthodontic result.

In our next issue, I will discuss the methods and techniques 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.

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