Esthetic dentistry and occlusion

20 Esthetic Dentistry and Occlusion Dewitt C.Wilkerson, III

In 1896, American architect Louis Sullivan stated that form followed function, a principle that has been become an integral part of modern architecture and engineering design. Sullivan believed that the shape of a building or object should be primarily based on its intended function or purpose. This rule is evident in the dynamics of the masticatory system. Clinicians must understand the multiple relationships involved with occlusion to achieve results that are both esthetically pleasing and functionally stable. Restored smiles remain stable only when dental occlusion is established in precise functional harmony with the temporomandibular joints and the neuromuscular system (Fig. 20-1).

CONCEPTS OF OCCLUSION A comprehensive discussion of occlusion is beyond the scope of this textbook. However, some fundamental concepts applicable to most dental treatments are, therefore, appropriate for esthetic dental procedures. The dental literature contains numerous definitions of specific occlusion terminology that can result in conceptual misinterpretations. Therefore this chapter uses the definitions provided in the Glossary of Prosthodontic Terms published in the 2005 Journal of Prosthetic Dentistry.1 Centric occlusion. The occlusion of opposing teeth when the mandible is in centric relation. This may or may not coincide with the maximal intercuspal position. Maximal intercuspal position (also called maximal intercuspation). The complete intercuspation of the opposing teeth independent of condylar position sometimes referred to as the best fit of the teeth regardless of the condylar position. Centric relation. The maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anteriorsuperior position against the shapes of the articular eminencies. This position is independent of tooth contact. (This is a 440

partial definition. The entire definition can be found in the Glossary of Prosthodontic Terms published in the 2005 Journal of Prosthetic Dentistry.)1

Temporomandibular Joints and Surrounding Structures Predictably, stable occlusion depends on proper joint mechanics and masticatory muscle activity. The joints must be evaluated for stability before any occlusal therapy. A screening evaluation should include a review of the patient’s joint history. Questions should be asked to identify any known signs or symptoms related to pain, joint noise, joint locking, limited range of movement, or trauma to either jaw joint. Further clinical testing may include joint palpation, orthopedic load testing2 (Fig. 20-2), Doppler auscultation, and joint vibration analysis. If joints are identified with apparent intracapsular disorders, further testing through imaging may be indicated. Such joints often require stabilization therapy before initiating dental treatment. Therapy for damaged joints may include occlusal splint therapy, physical therapy, and in severe cases, joint surgery. In complex restorative cases, centric relation may be the only reference point available to the clinician. Centric relation is precise, repeatable, and stable, in healthy joints with properly aligned condyle-disk assemblies.3 Dawson’s bilateral manipulation is one of numerous methods method of reproducing mandibular centricity.

Uniform Tooth Contacts Proper occlusion refers to the simultaneous contact of both posterior and anterior teeth. This provides the following benefits: 1. It protects the joints and surrounding structures by reducing the percentage of absorbed force.4 The positioner muscles of mastication, especially the lateral pterygoids, can fully release

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Temporalis muscle

interferences, but an inconsistent quantifier of small occlusal interferences.8 Patient feedback can help to identify occlusal prematurities at the 50-mm level, half the thickness of a human hair. In certain cases the T-scan II computerized occlusal analysis (Tekscan, Boston) is helpful (Fig. 20-3). It uses a thin Mylar film containing sensors to record occlusal contacts at the micron level. This information is relayed to a computer monitor for immediate interpretation. This is the most sophisticated means of evaluating occlusal forces presently available.9

Coupled Anterior Teeth (Mutually Protected Occlusion)

Temporomandibular joint Masseter muscle FIGURE 20-1   ​A healthy masticatory system is the result of

harmonious function of the joints, muscles, and posterior and anterior teeth. (From Newman MG, Takei HH, Klokkevold PR,  et al. (eds): Carranza’s clinical periodontology, ed 10, St. Louis, 2006, Saunders.)

Anterior coupling is a term used to designate the approximation of the maxillary and mandibular anterior teeth when the posterior teeth are at maximal intercuspation.10 It is desirable for both anterior and posterior teeth to simultaneously touch in maximal intercuspation (Fig. 20-4). This creates a sharing of occlusal forces over the maximum number of teeth on closure. It also is critical to fulfill the goal of excursive occlusal contacts, namely, immediate anterior teeth guidance and immediate posterior teeth disclusion. “Several authors believe that mutually protected occlusion (disocclusion through anterior teeth) is ideal, since the canine teeth present higher proprioception, are ideally located (thus promoting immediate disocclusion of posterior teeth),

FIGURE 20-2   ​Dawson’s bilateral manipulation technique is

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one method used to guide the joints into centric relation when this positioning is desired.

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their contraction, as the elevator muscles guide the end closure. This release of the positioner muscles protects the muscles.5 2. It protects the teeth by reducing the percentage of force through each tooth.6

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Noninterfering Teeth The goal of all occlusal therapy is to position teeth so that none interfere with normal jaw function from border to border when the teeth are in occlusion. Ideally, all teeth should touch simultaneously on first contact. New restoration should not cause a shift off the arc of closure (anterior slide), the line of closure (left or right),7 or vertical hyperocclusion. It is critical that these interferences be meticulously eliminated. Articulating ribbon is a helpful aid in identifying gross

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9% 14

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6% Left 54.8% Right 45.2%

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F I G U R E 2 0 - 3  ​A, Patient undergoes T-scan II occlusal analysis.

B, Data are relayed to a computer monitor for immediate interpretation.

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FIGURE 20-4   ​Ideally anterior teeth should uniformly couple

and guide all excursive contacts.

have considerable volume and bone support and present lower electromyographic activity, i.e. they promote higher muscular relaxation, besides presenting better conditions to distribute and afford occlusal loads without harmful consequences to posterior teeth and supporting structures.”11 It is necessary for anterior teeth to couple, that is, simultaneously contact, with the posterior teeth so as to maximize physiologic health of the masticatory system. By harmoniously sharing contact exclusively on the six anterior teeth, in all excursive movements several beneficial effects are realized: 1. The posterior teeth are protected from tooth-to-toothabrasive wear, harmful lateral stresses, cervical abfraction, and the damaging effects of parafunctional habits, because they are disoccluded. 2. The EMG activity of elevator muscles is significantly reduced by the disocclusiondistoclusion of the posterior teeth.12-18 Anterior teeth have a mechanical advantage over posterior teeth because they are farther from the fulcrum. This positioning gives them better leverage to offset the closing muscles of mastication. The mechanical advantage is apparent when one tries to “bite hard” with the front teeth as compared to biting hard with the molars.19-25 This has been shown to significantly reduce chronic myofascial pain in some patients.26 3. The reduction of elevator muscle force reduces the loading forces applied at the joint level.

CLINICAL TIP Anterior teeth have the important function of immediately disoccluding the posterior teeth when the joints leave centric relation.

T, D, S, F, V Phonetics When maxillary and mandibular anterior teeth are properly positioned, they will function in harmony with normal speech. T, D, S, F, and V are the strategic sounds that relate to tooth position affecting proper phonetics (Fig. 20-5). “T” and “D” sounds are affected by the position of the lingual cingulum of the maxillary anterior teeth, from the maximal intercuspation contact to the gingival margin.27 Overcontouring or undercontouring this area can adversely affect clear phonetics. Sharp angles should be avoided. “S” sounds are used as a critical guideline in removable and fixed prosthetics. “The vertical dimension of speech should be used as the primary guide for establishing the vertical

F I G U R E 2 0 - 5  ​Phonetics must be tested in the provisional

restoration stage to verify correct occlusal relationships.

dimension of occlusion. When “s” sounds are being enunciated at conversational speed, the mandible moves to the most forward and upward (closed) position it ever assumes during speech. Because the posterior teeth must never contact during speech, the greatest vertical dimension of occlusion for any person must be 1 mm less than the vertical dimension of speech. Otherwise speech contacts will occur. Therefore the vertical dimension of speech should be located first and can be used as a protective guide to determine what the vertical dimension of occlusion should be.28 The relationship of the lower incisal edges to the maxillary incisors when pronouncing crisp “S” sounds should occur with a clearance of 1 to 1.5 mm. This can occur in a near edge-to-edge position, the “Classic S Position,” or in some Class I and II cases, sharp enunciation may occur in an “Atypical S Position,” as far apical as the gingival tissues. Using the “S” sound to assist in determining the ideal relationship between the maxillary incisors lingual contours and the mandibular incisors horizontal and vertical position is described as the “closest speaking space.” This can occur in a near edge-toedge position, the “Classic S Position,” or in some Class I and II patients, sharp enunciation may occur in an “Atypical S Position,” as far apical as the gingival tissues. “F” and “V” sounds relate directly to the position of the incisal edges of the maxillary incisor teeth. The positions in which these sounds are made are also strategic markers for the ideal length and horizontal position of the maxillary central incisors. The maxillary incisor teeth should contact the inner vermillion border of the lower lip as air is sealed to form a clear F and V sound. This should occur with repetitive unstrained effort of the facial muscles, when the incisal edges are in harmony with the lower lip and speech.

CLINICAL TIP Phonetics plays a critical role in evaluating the proper position of both maxillary and mandibular anterior teeth.

INCISOR-CANINE GUIDANCE In all mandibular excursions only the incisor and canine teeth should be in contact and the posterior teeth should immediately disocclude. Two primary factors determine the guiding angles of the maxillary anterior teeth. The first factor relates to the eruption of the teeth, into what is described as the neutral zone.

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A popular term in denture prosthetics, the neutral zone is an area between the oral musculature, where forces generated by the tongue are neutralized by the forces generated by the lips and cheeks. Denture teeth and flanges placed into this zone of equal and opposite muscle forces, are stable.29,30 Natural teeth erupt into the neutral zone. Muscle forces the position of each tooth. Strong forces from the lips will result in maxillary incisors that are steep and even potentially lingually inclined. Conversely, strong tongue forces pushing outward, may create maxillary incisors with a very shallow inclination. There are no norms or averages to create standardized anterior guidances for comfortable and stable restorative/cosmetic dentistry outcomes. The neutral zone creates the second factor that determines the guiding angles of the maxillary anterior teeth, the envelope of function.31 In natural dentition the movement of the mandibular incisor teeth when masticating, swallowing, and speaking is dictated by the position of the maxillary incisors. Neuromusculature harmony occurs when muscle memory engrams coordinate jaw movement within the available parameters, set by the maxillary incisors and canines. The steepness or shallowness of the lingual contours of the maxillary teeth dictates how vertically or horizontally the mandibular teeth function. The exquisite sensitivity of this system, through tooth mechanoreceptors, creates a very refined relationship for the protection of the teeth. The maxillary anterior teeth create a neuromusculature “electrified fence” that the mandibular teeth must function within for long-term stability. Functioning outside of these parameters results in damage to the teeth and supporting structures, as is observed with parafunctional habits. The position of the maxillary incisors and canines, as dictated by the neutral zone, creating the envelope of function, have important clinical significance in many cases. For example, when a “tight neutral zone” is observed, with steep tooth inclinations (Fig. 20-6), it is very important that new restorations not extend facially from the original position. In this case, if teeth leave the neutral zone of balanced muscle forces, they will be unstable (i.e., if maxillary anterior veneers are placed, the patient returns later complaining that “the front teeth are hitting too hard.” This occurs because the restored teeth were in violation of the positional parameters determined by the musculature. Another notable example

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F I G U R E 2 0 - 7  ​Maxillary anterior teeth must be positioned

in harmony with the patient’s neutral zone and envelope of function.

occurs when the lingual contours of maxillary anterior teeth are restored to a steeper angle than the natural teeth. This frequently creates an apparent parafunctional erasure mechanism to regain lost freedom of function, resulting in mobile maxillary anterior teeth, excessive wear of the mandibular incisal edges, and discomfort. Shallowing anterior guidance creates greater functional freedom, but steepening the guidance can create dysfunction in many cases. The patient should be instructed to be very cognizant and communicative about the comfort, phonetics, function, and esthetics of the provisional restorations before the clinician communicates to the laboratory through “approved provisional restorations.” This is absolutely vital to achieving predictable and successful outcomes (Fig. 20-7).

CLINICAL TIP A restored anterior guidance that is stable long-term, involves not only separating posterior teeth, but also verification of functional harmony with the muscles.

OCCLUSAL VERTICAL DIMENSION

FIGURE 20-6   ​A “tight neutral zone” with steeply inclined

maxillary incisors and strong lower lip pressure must be carefully managed restoratively to avoid a neutral zone violation.

Clinical experience has demonstrated that the masticatory system is very amenable to reasonable changes in occlusal vertical dimension (OVD), if the functional guidelines previously discussed are meticulously followed. In general, OVD is determined by muscle. Similar to the neutral zone concept, whereby muscles determine the horizontal position of teeth, the repetitive contraction of the elevator muscles upon swallowing will dictate the eruptive position of opposing teeth.27 When a full complement of teeth is present, even with significant tooth wear, the concept of “lost vertical” may be a misnomer. It appears that although tooth structure may be lost, alveolar bone is added as compensation, and muscle contraction length remains relatively the same. Nevertheless, the clinician is faced with a restorative challenge. There is decreased tooth structure, and lack of room to restore, at the present OVD. It is typically acceptable to open the OVD enough to restore lost tooth structure back to its original form within reason. If the OVD must be altered, it is

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prudent to make the most conservative changes possible to achieve an optimal esthetic and functional result. Research has shown that alterations of OVD, accompanied in the optimum condylar position, are well tolerated by the TMJ. When altering OVD, problems are often encountered when bilateral tooth contacts are uneven or anterior guidance is adversely managed.33 For every 1 mm, the condyles are seated superiorly; from their position in maximal intercuspation (an average of 1 mm down/forward from centric relation)34 the anterior teeth can open 2 mm without changing the contracted muscle length of the masseter muscles. Therefore when the occlusion is restored in centric relation, it is possible to gain restorative room anteriorly, without changing muscle-contracted length.35 Increasing OVD changes anterior facial height primarily by rotational condylar movement without the accompanied changes in posterior facial height or muscle length.35 An adaptive muscle response with concurrent long-term stability is anticipated as long as vertical support and anterior guidance are present.

Occlusal Evaluation for Restoring a Worn Anterior Dentition36 Armamentarium • Semiadjustable articulator • Patient study models • Appropriate occlusal relation record Clinical Technique.  ​This technique is used to establish centric occlusion (condyles in centric relation) with the teeth in maximal intercuspation. 1. Mount the casts on a semiadjustable articulator to establish centric relation. 2. Evaluate tooth-by-tooth conditions and treatment requirements. 3. Evaluate the maxillary and mandibular occlusal planes and determine any treatment requirements. 4. Determine the optimal vertical and horizontal position of the mandibular anterior incisal edges determine the optimal vertical and horizontal position of the maxillary anterior incisal edges. 5. Select the occlusal vertical dimension. 6. Wax to provide ideal centric occlusion contacts on all teeth. 7. Eliminate balancing and working posterior interferences. 8. Harmonize the anterior guidance. 9. Reevaluate the overall functional and esthetic results. Changes in OVD must be evaluated with provisional restorations for acceptable comfort, phonetics, function, and esthetics, before communicating to the laboratory through “approved provisional restorations” (Fig. 20-8).

F I G U R E 2 0 - 8  ​Severe wear cases may require some increase in

OVD, which should evaluated by the placement of provisional restorations, with an idealized occlusion.

cause of the damage should be determined. Occlusal disharmony can be a cause of structural destruction, discomfort and dysfunction.37 Another destabilizing factor is parafunction activity unrelated to mastication or speech that can be injurious to the teeth and supporting structures. It appears to have multiple causes, including malocclusion. Studies have shown that sleep bruxism, in many instances, is related to sleep arousal, with a change in cardiac and brain activity preceding jaw motor activity. Peripheral microarousals can affect sleep-wake mechanisms.38 Systemic disease, such as Parkinson’s bruxism, will naturally persist after restorative therapy is complete. Sleep apnea is a condition of airway obstruction and cessation of breathing for periods of 10 seconds or greater during sleep.39 Studies are being conducted to analyze the relationship between apnea, reduced oxygen saturation, “fight or flight” responses, increased cortisol release, accelerated heart rate, and bruxism.40 Gastric reflux caused by negative airway pressure and positive gastric pressure is also associated with sleep apnea.41 Nonfunctional factors of occlusal/dental instability are also associated with eating disorders, low pH beverages, toothpaste abrasion, and fruit mulling (Figs. 20-9 and 20-10).42 When parafunctional factors persist, after ideal occlusal therapy is completed, an oral appliance is indicated to control the parafunction and its deleterious effects.

CLINICAL TIP Occlusal vertical dimension can be conservatively altered to accommodate a worn dentition, as long as all other factors of an ideal occlusion are meticulously fulfilled. The results must be evaluated and approved by the patient using provisional restorations.

Nonfunctional Factors Before restoring a damaged dentition the cause of the damage and whether or not restoring the dentition will eliminate the

F I G U R E 2 0 - 9  ​Classic signs of nonfunctional bulimic disorder,

destroying maxillary anterior tooth structure.

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phonetics, effortless function, and natural esthetics, a service has been provided that is life altering, in the most positive way (Fig. 20-11).

CONCLUSION It is critical that clinicians understand the multiple relationships involved with occlusion in order to achieve results that are both esthetically pleasing and functionally stable. Restored smiles remain predictably stable only when dental occlusion is established in precise functional harmony with the temporomandibular joints and the neuromuscular system. FIGURE 20-10   ​Nonfunctional factor of severe root damage

caused by coarse “natural dentifrice” 15 years after dentition has been restored.

FIGURE 20-11   ​Understanding the basic principles of occlu-

sion enables clinical results that will endure the test of time.

Dietary discussions are important for patients with a low pH intake. The reduction of low pH beverages and acidic fruits is sometimes critical for long-term restorative integrity. Patients with eating disorders such as bulimia and anorexia live in a life-threatening situation physically and psychologically. Dentists are often the first health professionals to recognize the problem. It is often uncomfortable but necessary to address the issue with the patient, the family, and the physician (see Chapter 29). Patients who damage their teeth and/or gingiva by excessive toothbrushing and/or overly abrasive dentifrices should be properly advised. Functional esthetics brings a beauty to the outcome of smile enhancement that extends far beyond a youthful appearance. When beauty includes relaxed comfort, confident stability, clear

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