CAN WE STOP THE BREAKAGE OF OUR RESTORATIONS?

Clayton A. Chan, DDS, FICCMO

Anterior breakage, bruxism, disclusion, no disclusion, function vs. parafunctional are concerns that we dentists are faced with in our everyday practices, especially when dealing with aesthetic concerns.

I pose the questions even to myself, why the breakage, why the strains? Dental material companies have done a marvelous job at researching and improving the materials and choices we have in this day and age of dentistry. Adhesive strengths, bonding capabilities, types of materials used, etc. have given us aesthetic oriented dentists choices beyond our wildest dreams. The manufacturers of dental materials continue to work on technology to keep pace with the demands and requirements of durability, longevity and aesthetics.

AN UNDERLYING CAUSE

The human masticatory system is a dynamic system. If the natural dentition also breaks down, chips, fractures and wears out its own original enamel through normal or even abnormal demands, then why would we want to continue building man made structures on teeth to withstand against forces that sometimes are more powerful than just the excursive or protrusive interfering forces. Eichner (1964), found that the maximum biting forces can be six times as great as the chewing forces which are applied when food is being masticated. The highest forces occurred in the intercuspal positions. Attempting to keep anterior forces and contacts to a minimum is good, yet it sometimes impacts the aesthetic look of ideal lengths of anterior incisors. We all have learned that restoring the anteriors with even the strongest dental materials still takes its toll over time with chips, fractures, breakage, fatigue and abnormal wear, when we do not address the underlying cause of optimal physiologic mandibular positioning. Check with nature! When the masticatory muscles are calm and function at its optimal resting length, as I have written in the past, "HAPPY MUSCLES DON'T BREAK TEETH". I must stress the importance that harmonizing the occlusion to an optimally measured physiologic mandibular position and movements will lessen and prevent tooth breakage. Again, if the muscles are "happy" the patient will NOT brux!

PHYSIOLOGY OF MASTICATION AND SWALLOWING

The physiology of mastication and swallowing has always promoted a great deal of interest in dentistry. The result has been extensive research in mandibular movements, mandibular posturing, chewing, chewing forces, chewing efficiency, muscle activity and tooth contact patterns. As a result many authorities believe that almost all the research has shown that tooth contact rarely occurs in a border movement position, but rather during chewing and swallowing. Most of the contacts have been shown to occur in an occlusal position slightly anterior to centric occlusion, the so-called maximum intercuspation (habitual occlusion). When observing our patients natural dentition and looking at the anterior incisal wear either on the lower incisal edges or the upper anteriors, we clinicians should ask ourselves where did the mandible have to position itself for that to occur? How many thousands of times did the mandible move to that position to wear away the natural hard enamel during habitual functioning moments of posturing, swallowing, chewing, speaking, even repose. But, then suddenly it had to move to another position (an avoidance position) due to the malocclusions and discrepancies in intercuspation for maximum bracing. Skids occurred, rubs, strains, forces were exerted, crowding, tooth rotations, fatigue and eventual breakage of tooth structures and/ or restorations occurred in the regions of the mouth, we could only see after obvious damage was done. The muscles of mastication naturally posture the mandible within the confines of their resting length. They are most comfortable and least resistant during resting, speaking and swallowing when functioning within the "physiologic zone" typically anterior and inferior to the strained (hypertonic) habitual CO position.

It is the acquired/ habitual occlusal schemes that dictate the habitual intercuspating of teeth resulting in further rubs, wear and tear on tooth structure. When the muscles of mastication want to posture the mandible to its physiologic position while the teeth, the dominating structures, force the bite and mandible into another position, disharmony, stresses and strains occur.

It is important to recognize that the functioning forces applied during chewing and swallowing are relatively small compared with the maximum biting forces, which can be applied as a result of isometric muscle contraction.

FUNCTIONAL MOVEMENTS OF THE MANDIBLE

Dental literature comments much on the studies of masticatory movements, chewing forces, tooth contact patterns and swallowing. Masticatory movements have been studied by Luce (1889), Hildebrand (1931), Sheppard (1959) Koivumaa (1963), Hickey et al, (1963), Byron (1964), and Ahlgren (1966, 1976). These studies showed that masticating is a highly complex neuromuscular activity that varies from individual to individual, each possessing their own characteristic chewing patterns developed from early childhood in the mixed dentition stage. The masticatory muscles develop accommodations and compensations to the developing vertical dimensions that have formed as each tooth develops and grows into the dental arches. A mandibular position is then accommodated to those acquired occluding positions that allows the best fit of the teeth, yet compensations are continually made by the surrounding musculature as well as the joints to meet the dominating factors of hard structures, the intercuspating teeth. Posselt, 1952 as well as many others as Ingervall, 1964; Cohen, 1965; Hodge and Mahan, 1976; Reider, 1978, all reported that this anterior posterior discrepancy existed in 90% of the population with a distance between the retruded and the intercuspal position of 0.25 mm - 2.25 mm on average. It is no wonder that clinical dentist's are able to see anterior wear and recommend treatment for those worn and broken down areas. What we all are trying to determine is how best to minimize the potential breakage factors which impacts the time spent in redoes, our patients feelings of unhappiness and perceptions of what we have done, the effort we dentist stress ourselves over in wondering why our bonding materials and restorative materials are not holding up.

"The discrepancy between centric relation and maximum intercuspation is a reality which every clinician performing occlusal therapy must accept." (Pameijer, 1970.) He wrote: "The question - 'at which mandibular relation maximum intercuspation should occur in order to obtain a harmonious entity with the neuromuscular system'? then needs to be answered."

He further wrote: "The chewing process consists of a series of acquired reflex movements, modified by the size and consistency of the bolus of food. The system itself is protected by neuromuscular reflexes, called nociceptive reflexes, which prevent damage by traumatic functional forces." Studies have shown that almost every chewing cycle ends with tooth contact; most of these contacts occur with the teeth in maximum intercuspation in normal healthy schemes; those with non-healthy occlusal scheme, unhealthy condyles and discs, lacking adequate support to the masticatory system during chewing and swallowing cycles, will result in destructive tooth contacts. "Tooth contacts in centric relation (where ever that is defined) rarely occurs in chewing and swallowing", since chewing and swallowing are functional movements that do not confine themselves to border movements.

MANDIBULAR POSITIONING IMPACTS INTERCUSPATION

Rather than continue to concentrate on stronger materials, better bond strengths, stronger better reinforced materials, which have proven to be better than ever, the treating clinician should now seriously consider the physiology of muscles and their impact it has on mandibular position which effects occlusal and incisal position. The mandible with comfortable rested muscles that function in a normal unstrained manner are not destructive to there supporting dentition. Breakage, incisal wear, porcelain, composite and tooth fracturing, are a result of a pathological phenomenon which directly leads to possible lack of a proper mandibular positioning, lack of proper vertical dimension, lack of an optimal AP position, increased muscle hypertonicity and activity, resulting in further parafunctional habits of bruxism. Healthy stable dentitions are ones that present with an optimal mandibular to cranium relationship having minimal to no dental or periodontal problems with musculature that harmonizes and supports the hard structures optimally.

There should be harmony between the masticatory muscles, the teeth and the jaw joints. Whenever these three anatomical factors are not harmonizing one with another further wear and tear are exerted to the stomatognathic system resulting in breakdown and clinical frustration.

BACK TO THE BASICS

From my personal experience, I have observed that most anterior incisal wear cases are typically overclosed vertically with mandibles posteriorly positioned. Many of us clinicians have been taught to believe that opening the bites is detrimental to the patient. We have also been informed to believe that bringing the condyles down and forward are questionable. Those that have been treating young and adult patients from a functional orthodontic/ orthopedic perspective for years in this country and abroad have realized this myth to be untrue. In my opinion, there has been more detriment to dentition by not having an adequate vertical support of the occlusal table in the posterior region, which has been well documented scientifically, published in literature and observed by many dentists that is undeniable. Those cases, which present with insufficient vertical typically have compromised swallowing patterns and often times have many undetected musculoskeletal occlusal signs and symptoms that have been confirmed by kineseographic studies. A lack of proper posterior molar and bicuspid occlusal support, vertically, will naturally result in an accentuated anterior overlapping of incisal edges. With this overlapping of incisal edges and contouring, which we aesthetic conscientious clinicians strive to develop, bringing length back to the worn dentition, we then naturally risk these works of art to be placed in harms way by pathologic mandibular movements both protrusively and excursively. When these incisal edges are not within the confines of the functioning mandibular path of movement of normal muscle physiology, wear and tear occur.

Why work against nature? Each patient's case is unique. For all the more reason we should seriously consider the following:

Develop a proper maxillo-mandibular physiologic position vertically, sagittally and frontal/ laterally to prevent anterior breakage and establish rested masticatory musculature.
Establish proper tongue space for proper swallowing patterns to occur rather than the tongue posturing between the teeth when swallowing (natures compromised splint, indicative of a deficient vertical or insufficient freeway space). When swallowing, teeth should brace against themselves, not the tongue bracing (shimming) and posturing the mandible. (One swallows normally about 2000-3000 times daily).
Establish a physiologically positioned mandible in the anterior-posterior position (sagittally), especially if doing full mouth rehabilitation, thus avoiding the need for long centric.
Establish a proper arch width horizontally, giving additional room for tongue swallowing patterns as well as facial aesthetics in the smile (fuller smile) with buccal corridors fuller.
Constricted airway breathing patterns should be minimized by developing the dental arches horizontally resulting in a lowering of steep upper palates and infringement of the turbinates with those patients with high vaulted palates (improved oxygen intake, less mouth breathing problems, diminished aberrant tongue swallowing patterns and stable occlusion. A stable occlusion, improved aesthetics, enhanced facial features, calmer comfortable muscles of the face, head and neck regions will result when addressing the stomatognathic system in its entirety.

Those patients that present with severe bruxism problems even though one may have a perfect equilibrated/ coronoplastied bite, with minimized lateral interferences and meets all the so-called gnathological requirements for good occlusion, will still be faced with issues when the mandible is not placed at a physiologic position and the muscles are not calmed. The question is: "Where is this position and how does one obtain it?" Educated guesses may work a certain percent of the time, but very unpredictable. A consistent, time efficient and reliable method available today is using neuromuscular instrumentation that allows one to "measure" muscle activity and visualize muscle activity at rest and in function. Deprogramming and calming the hypertonic hyperactive muscles before obtaining an optimal interocclusal/ bite registration/ position is best and rapidly achieved via TENSing the masticatory muscles. EMG data can be used to verify rested muscle activity. A calm rested mandibular position can also be verified via jaw tracking instrumentation (computer mandibular scanning- CMS). It allows one to visualize the hyperactive and resting muscles and the effects that various mandibular positions (vertically, AP, laterally, frontally, sagittally) can have on muscle activity. With this scientific objective data, one can determine with confidence where the mandible can optimally function thus reducing breakage, wear and tear, strains and torques.

Clayton A. Chan, DDS. FICCMO

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