April 1, 2003
by Frederick M. McIntyre, DDS, MS
Our patients’ demands for beautiful smiles have created an aesthetic revolution in the dental materials industry today. Never before has the dental profession experienced such rapid development in dental materials; all related to the demand for aesthetic dentistry. New composites and porcelains have been developed which are virtually undetectable from the natural dentition. However, these new materials have been less forgiving than the traditional porcelain-fused-to-metal restoration. They have placed a greater demand on the restorative dentist to create functional harmony within the masticatory system. The dentist has created the perfect smile in some cases for their patients, only to experience catastrophic failure because he or she did not account for the envelope of function.
The envelope of function defines the relationship of the mandibular incisors to the maxillary incisors during mandibular movement within the patient’s function zone. To ensure predictable longevity when restoring the anterior dentition, the restorative dentist must recognize signs of instability within the masticatory system, satisfy the requirements of anterior guidance, and distribute occlusal loading and muscle activity as it relates to the individual patient. A patient’s anterior guidance and occlusal requirements must be developed in relationship to the location and degree of instability found within the masticatory system.
Instability can manifest itself through signs of hypermobility, fremitus, hypersensitivity, TMJ disturbances, hypermuscle activity, loss of supportive bone, and excessive wear of the dentition1 (Fig. 1). The restorative dentist must guard against being fooled by the proprioceptor engram system because of a patients apparent lack of pain.1 When instability is present, thought must be given to the envelope of function and the position of the anterior teeth. The existing anterior guidance, or lack of will need to be evaluated for each patient and treated according to the patient’s needs. The restorative dentist will need to communicate the necessary information to the laboratory so that definitive restorations can be fabricated with proper function and aesthetics for predictable longevity.2
Often, anterior guidance does not receive the detailed evaluation that is necessary to provide functional harmony. The anterior guidance defines the anterior border of the functional zone related to the position of the incisors. Posteriorly, the functional zone is defined by the most superior position of the condyle against the eminence, while the lateral borders are defined by the maxillary and mandibular buccal cusp tips.2 Anterior guidance provides the aesthetics of the smile, phonetics, lip path closure and the neutral zone.5
The functions of anterior guidance include the disclusion of the posterior teeth to improve the shearing function of the anterior teeth, and “proprioceptive inhibition” within the masticatory system by limiting muscle force to the anterior fibers of the temporalis and the lateral pterygoids during protrusive movement of the mandible. It functions to protect the temporomandibular joint and the posterior teeth from excessive occlusal loads and muscle activity. The restorative dentist can balance occlusal loads and muscle activity within the masticatory system by providing the proper disclusion for the patient related to instability present. The restorative dentist will design anterior guidance with either a group function or cuspid disclusion as determined appropriate for the patient.
Anterior guidance requires stable posterior contacts at the proper VDO. It requires correct lingual contours to redirect and redistribute occlusal forces during protrusive disclusion. In 1976, DiPietro studied the relationship of the Frankfort mandibular plane angle to anterior disclusion.4 Individuals with a low FMA with strong muscular fare better with canine disclusion, whereas individuals with a high FMA have a tendency towards group function disclusion. The status of the TMJ, condylar guidance and lateral border movements must be evaluated from centric relation. Head position can effect the arc of closure which can require modification of the centric contacts on the lingual surfaces of the maxillary anterior teeth.3 The anterior teeth may need to be corrected to provide for long centric when necessary. Along with the functional requirements of anterior guidance, aesthetic issues related to the anterior teeth must be addressed. Tooth position should be evaluated for gingival symmetry, biologic width and other related soft tissue issues. In more complex cases, the orthodontist and the maxillofacial surgeon may have major roles in the development of anatomic and functional harmony related to anterior guidance.
Diagnosis is paramount to designing anterior restorations for function and aesthetics. The information gathered will direct the restorative dentist’s decisions pertaining to preparation design, functional and the aesthetic designs of the definitive restorations.
Diagnosis begins with an analysis of the face. The face influences both the functional and aesthetic design of the definitive restorations. Functionally, the face may provide clues to muscle hyperactivity, irregular condylar growth, facial type, and rheumatoid related TMJ disturbances. These conditions may manifest as enlarged masseter muscles, tenderness to palpation within the elevator muscles, slanted occlusal plane, deviated chin, disproportional facial thirds, short face syndrome, “long face” syndrome, and redden patches of skin overlying the TMJ’s.
Aesthetically, the face will provide clues to the midline, location of the occlusal plane, tooth size and proportions. The midline can be evaluated by comparing the position and cant of the maxillary incisors to a line formed through the points “globella” and “cupid’s bow” of the maxillary lip. The occlusal plane can be evaluated by comparing the anterior curve of the plane with the curvature of the lower lip when the patient smiles. Tooth size and proportion can be evaluated with the use of a tooth size indicator (Dentsply) which relates the shape and proportions of the face to the maxillary central incisors. The golden proportion for the maxillary anteriors can be then obtained from a denture tooth mould guide. The width of the nose and nares can also be used as a guideline to tooth position and size.
The status of the temporomandibular joint can then be evaluated using clinical history, load testing, chin point loading, range of motion, anterior loading, Doppler and, in some cases, radiographic evaluation. The joint can be classified using Piper’s classification.1 A compromised joint will require alteration of the occlusal scheme to balance loading and muscle activity in favor of the joint. Alteration of the occlusal scheme may effect preparation design and the design of the definitive restorations.
The anterior teeth are then analyzed in relation to function and aesthetics. Functionally, the anterior teeth are evaluated in respect to the neutral zone, lip path closure, phonetics, anterior guidance, envelope of function, occlusal plane, maxillary incisal edge position and wear patterns. All of these parameters are important to determine the proper tooth position for the maxillary anterior teeth. To develop the proper envelope of function when the maxillary central incisors are idealized for length and aesthetics, the lingual surfaces of the maxillary teeth may need to be altered in contour, the position of the mandibular incisors may need to be changed, a change in VDO may be required, or a combination of changes may be necessary to provide the proper anterior guidance and envelope of function in harmony with the lip closure path and neutral zone.
In cases of severely worn dentition or patients who lack anterior guidance, the condylar guidance will need to be determined to evaluate its role in disclusion of the posterior teeth. In patients with worn dentitions, the wear patterns need to be analyzed as to character of wear,
e.g. attrition, abrasion, erosion or abfraction, the direction of wear patterns and the rate of wear. In cases of attrition, it should be determined whether it is parafunctional or bruxism. A horizontal bruxer is more difficult to treat than a vertical bruxer.2 The vertical bruxer may need more overjet, overbite, while the horizontal bruxer may require alteration of the lower anterior tooth height, change in VDO, and group function to distribute occlusal load. Detailed health histories, physical exam, and trial splints are useful to develop the proper occlusal schemes and definitive restorations.
Cephalometrics can be of great value to the restorative dentist when restoring the anterior dentition to proper function (Fig. 2). The patient’s anterior tooth positions can be analyzed. The plane of occlusion, skeletal classification and facial type can be determined. The restorative dentist must be aware that there are several analyses, and cephalometric norms vary with racial types.
The anterior teeth must also be evaluated for aesthetics. The size, shape, facial contour, emergence form, interproximal contacts and incisal edge position are important to the design of the definitive restorations. Evaluation of the midline and diastemas that are present are important to develop proportionally for the definitive restorations. Tissue symmetry and biologic width are important for margin placement, proper gingival heights and correct emergence form for the definitive restorations.
Information gathered from the physical diagnosis, and radiographic evaluation should be used in conjunction with properly articulated diagnostic casts to develop preparation design and provisionals that will become the blueprint for the definitive restorations. The posterior occlusion can be evaluated on the articulator to determine if selective occlusal grinding will be necessary to develop stable posterior contacts. The maxillary incisal edge position can be transferred from the mouth to the articulator using composite that has been added to the incisors in the mouth and contoured to the proper incisal edge position in relationship to the lower lip during fricative sounds (Fig. 3). The wax up of the anterior teeth is finalized according to functional and aesthetic needs of the patient. Preparation stents, and provisional stents are then fabricated to develop the proper preparation design, and provisionals to evaluate function and aesthetics before the definitive restorations are fabricated.
Material manufacturers provide guidelines for preparation design related to the requirements of the material. These guidelines are important, however, they are only one parameter which will influence the preparation design. Tooth position within the arch, the envelope of function, occlusal plane, and the proper occlusal loading and muscle activity are the other parameters that must be accounted for in preparation design. They may dictate which surface may need to be prepared first when repositioning the tooth within the arch. Functioning surfaces may require additional reduction to change angles of disclusion. In other cases, less reduction may be necessary on facial surfaces because the incisal edge position has been altered. The restorative dentist must evaluate all the information available before preparation of the teeth so that preparation design can account for material requirements, and the functional and aesthetics needs of the patient.
Provisional restorations are developed from the diagnostic wax up. The diagnostic wax up should be fabricated to represent the design of the definitive restorations, which will provide the necessary functional and aesthetic needs of the patient. After preparation of the teeth, provisionals are fabricated using stents that were processed from the diagnostic wax up. The provisionals become the blueprint to “test” the function and aesthetics of the definitive restorations. The patient will function for a period of time with the provisional restorations to determine if changes will be necessary to accommodate function or aesthetics. If changes are made to the provisionals, an impression is made and a cast of the altered provisionals is fabricated. The cast is cross-mounted with the opposing cast and the master cast of the preparations. The cast of the provisionals is used to fabricate an incisal guide table to communicate information to the laboratory about the lingual contour of the maxillary teeth and the anterior guidance. A silicone matrix is made on the provisional cast that can be used by the laboratory to reproduce the incisal edge position as it relates to lip path closure and neutral zone. The maxillary casts must be mounted with a facebow to develop the proper arch of closure. The interocclusal record must establish the proper VDO in order to maintain the correct envelope of function.
For predictable longevity when restoring anterior teeth to proper function and aesthetics, the restorative dentist must control occlusal loading and muscle activity within the patient’s function zone as defined by the envelope of function. To achieve muscle harmony, the restorative dentist must understand the requirements of anterior guidance. Functional and aesthetic harmony are developed through examination, radiographic interpretation, mounted study casts, and a diagnostic wax up for preparation design and provisional restorations that blueprint the definitive anterior restorations (Fig. 4).
Frederick M. McIntyre, DDS, MS, Clinical Professor of Restorative Dentistry at the University at Buffalo, School of Dental Medicine; Diplomate, American Board of Prosthodontics. He is Director of the Aesthetic Dentistry Education Center, Past Director of Postgraduate Prosthodontics and Clinical Coordinator of Continuing Dental Education Programs in Aesthetic Dentistry at the University at Buffalo, School of Dental Medicine. In addition, he is a consultant to the Veteran’s Administration Medical Center in Buffalo, NY. Dr. McIntyre maintains a private practice within the University Dental Associates at the University at Buffalo, School of Dental Medicine.
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2.McNeil C: Science and practice of occlusion. 1st ed. Chicago: Quintessence: 1997.
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5.Broderson SP: Anterior guidance — the key to successful occlusal treatment. J Prosthet Dent;39:396-399, 1978.