As practitioners, many of us have had at one time or another had to make diagnostic and treatment decisions involving the maxillo-mandibular relationship for our patients. Whether we are performing anterior esthetics, full mouth rehabilitation, orthodontics or just want to replace edentulous areas with a removable partial denture consideration of mandibular positioning must be part of the diagnostic and treatment protocol. There have been various “occlusion camps” each with their own philosophy of mandibular positioning all in an effort to find the “optimum” maxillo-mandibular relationship in 3-dimensions.
One well recognized camp centers itself around centric relation which of course is a condylar border theory. This theory, primarily taught by Dr. Peter Dawson, is based on the desirability of an occlusal position with mechanical repetitiveness. And since repetitiveness occurs along borders, centric relation has to be a border position. This position was originally the upper-most rear-most position but more recently the upper-most forward-most position of the condyle in the mandibular fossa.
Another camp has centered itself around the radiographic or the 4 – 7″ theory. This theory was popularized by Dr. Harold Gelb of New York, NY. He divided the mandibular fossa into eight sections and stated that the proper mandibular position would allow for the condyle to occupy the two portions designated as 4 and 7″ (Fig. 1).
A third camp and a commonly used theory is that of centric occlusion. Here the practitioner attempts to maintain the current maxillo-mandibular relationship of the patient. Possibly, this theory has been popular because some do not embrace centric relation and are unfamiliar or unable to obtain proper temporomandibular joint radiographs to determine the 4 – 7″ position. Others may feel that the patient has acquired that centric occlusion position for a reason and it is not to be changed.
Another camp is once again coming to the forefront and its theory is the subject of much interest both in journals and educational facilities. This is the physiologic theory. This theory holds that occlusion is interrelated to the musculature and the welfare of each is interdependent. Traditionally, the temporomandibular joint acted as an accessory organ that allowed movement to occur when muscles contracted and acted as a fulcrum to help muscles apply a load.1 Today, the temporomandibular joint is more critically evaluated and its health, or lack of, is a critical component of occlusal repositioning. To achieve physiologic occlusion, the optimum rest position of the mandible is first found. This is the position of the mandible when the muscles of mastication are in their most relaxed and efficient state. It is from this rest position that isotonic closure of the mandible through freeway space can be made to a stable base, which is then called physiologic occlusion.
RECORDING PHYSIOLOGIC OCCLUSION
There are three common means to achieve neuromuscular occlusion. The first is a registration technique called the swallow bite. This technique was first popularized by Dr. Willie May and most recently has been taught by Dr. Jim Carlson of Las Vegas, NV. The swallow bite is a dynamic registration that is extremely effective for recording the optimal vertical dimension. This is the vertical dimension when the posterior teeth contact, the condyles are seated against the disk without pathologic rotation and the muscles are at their optimum physiological working length.2 For this technique a soft bolus of wax is placed on the lower first molars, the mandible is centered and stabilized and the patient swallows 5 to 8 times during one minute. This allows the teeth to penetrate into the wax establishing vertical dimension. The registration is taken by injecting fast set registration material anterior and posterior to the wax.
A second means to achieve physiologic occlusion is through transcutaneous electrical neural stimulation or TENS (Fig. 2). Benefits of TENS include muscle relaxation, improved circulation and lymphatic drainage in addition to being an aid for mandibular positioning. Application of TENS is accomplished by placing a surface electrode over the skin lateral to the coronoid notches. Ultra-low frequency current is passed through the skin to the underlying 5th and 7th cranial nerve branches (Fig. 3). This in turn causes contractual stimulation and relaxation of the muscles of mastication and facial expression. The rhythmic contraction causes elevation of the mandible from physiologic rest position along a muscularly induced trajectory. A registration medium is introduced and the superior position of this trajectory is recorded via the registration.
The third and most optimal method of taking a physiologic bite registration uses a combination of ultra-low frequency TENS, surface electromyography (EMG) and a computer enhanced jaw tracking device (EGN). Surface electromyography allows for verification of muscle relaxation (Fig. 4) while computerized jaw tracking allows the three dimensional position of the mandible to be displayed on a computer screen (Fig. 5). This enhances the ability of the clinician to monitor mandibular movements during the bite registration procedure. The physiologic rest position is confirmed in three dimensions and elevation of the mandible through freeway space to a pre-determined target vertically, laterally and most importantly antero-posteriorly is monitored on the computer screen. In my opinion, this method assures the most accurate positioning of the mandible relative to the physiologic demands of the stomatognathic system.
BENEFITS OF PHYSIOLOGIC OCCLUSION
Optimum physiologic maxillo-mandibular positioning has at least three primary benefits: 1) enhance and maintain a symptom free environment, 2) enhance intra- and extra-oral esthetics and 3) enhance longevity of newly placed dental restorations.
Anytime dental restorations are performed, the pre-treatment and post-treatment symptomatic condition of the patient should be evaluated. Since the goal of physiologic dentistry is to find the position of the mandible vertically, antero-posteriorly and laterally when the muscles of mastication are in their most relaxed and efficient state, resolution of TMJ symptoms and maintaining that symptom free state continues to be a primary benefit.
Second, proper intra- and extra-oral proportions can be created. When a patient’s dentition is worn down, recreating proper height to width ratios can be difficult or impossible without changing the vertical dimension of the mandibular relationship. When the patient is overclosed, new restorations may have to be “squeezed” in order to fit the environment they have. But when the patient is treated through physiologic occlusal positioning, proper tooth dimensions can be achieved and the result is improved intra- and extra-oral esthetics.
Finally, physiologic occlusion can create a harmonious relationship helping to prevent the fracture of esthetic restorations. Certainly, all esthetically oriented dentists have at one time or another had one of their patients present with a broken restoration. Unfortunately, placement of blame is many times one of the first thoughts that enter the minds of both the patient and the doctor. But at the same time improvements in material systems, adhesive strengths and our ability to provide totally bonded restorations gives us restorative choices that until recently have not existed. So why do these fractures occur?
It is important to recognize the stomatognathic system is dynamic and therefore forces placed on natural teeth can cause wear, breakage and chipping. And if the patient’s natural dentition can do this, why is it we seem to want to declare war on those forces, trying to override them, by placing new and stronger restorations? Through my twenty years of clinical observations I have noted that most patients with broken, chipped and worn anterior teeth are vertically overclosed. As the mandible continue
s to move superiorly, the proprioceptive influence of the anterior teeth can distalize the mandible causing the anterior teeth to be subjected to repetitive glancing blows. These repetitive glancing blows can have resultant damage as evidenced by wear and fracture. Thus, it is my opinion these changes in the dental condition are strongly related to the cranio-mandibular relationship at centric occlusion. Physiologic occlusion will reduce the distalizing effect on the mandible and restoration longevity will be improved.
CONCLUSION
Physiologic occlusion can have an important role in any dental restorative procedure. It is the unstrained occlusal state that helps to keep the muscles of mastication from their destructive tendencies. When the treating dentist takes into consideration the physiology of the entire TMJ (Teeth, Muscles and Joints) they will be able to develop a mandibular position that is healthy and stable. Our patients will look better and the durability and longevity of the restorations provided will be enhanced. Observation and respect for the physiology of the entire masticatory system will benefit both the patient and the dentist.
Dr. Stevens is recognized as an international speaker on the subjects of smile enhancement, principles of occlusion, full mouth restoration and diagnosis and treatment of temporomandibular disorders (TMJ). Since 1989, he has been instructing doctors and their staff on the use of computer enhanced electro-diagnostic equipment in dentistry.
Oral Health welcomes this original article.
REFERENCES
1.Jankelson RR: Neuromuscular Dental Diagnosis and Treatment. St Louis: Ishiyaku EuroAmerica, Inc., 1990.
2.Carlson JE: Orthocranial Occlusion and The Acculiner System. Blue Pine Unlimited, 2000.
