Part 1

Dentists must realize that, to patients, all restorative dentistry (except for the direct alleviation of pain) is essentially cosmetic dentistry.

What is the first question the patient asks as the doctor gets ready to insert a new crown? Doctor, how's the color match? The patient is really asking, "Doctor, does my new crown look like a tooth?" Does it look real in my mouth?" Sophisticated patients are expecting and demanding that the restorations we place in their mouths not only function well, but also look like real teeth.

The ultimate test, of course, is in implant dentistry, where we are expected to not only replace tooth structure, but also often lost bone and soft tissue. We are asked to restore facial profiles, provide lip support, and then allow for proper phonetics and mastication. No wonder this seems like a daunting task. Doctors who deal with implant dentistry, and, for that matter, all restorative dentistry, have to answer a few basic questions: Where do the teeth go in the face? Where do they go in relation to one another? How are they individually shaped?

Site Determination
This article will outline a prosthetic protocol in which the answer to these questions will become apparent by following a specific sequence of case analysis and development. All implant dentistry should start with a determination as to where the teeth need to be, and not with, "Where is the available bone?" Certainly, bone quantity and quality are very significant on the design of the final restoration. But, if abutment placement, based primarily on available bone, will not support teeth in the proper location, then another prosthetic solution needs to be considered. Implant dentistry is a prosthetic discipline with a surgical component -- not the other way around.

With the advent of the newer abutment systems and the trend toward using screw retained custom or prepable stock abutments to support cement retained restorations, there is no reason why successful implant dentistry shouldn't be an integral part of the general practice. When implant fixtures are placed in the correct location (whether by a surgical specialist or a well-trained generalist) the restoration becomes routine crown and bridge. The key, then, is to determine where the implants go.

Team Approach
The doctor must have the same organized approach to evaluate and treat every case, whether it involves natural teeth, implants, or a combination. Each case presents a different challenge, but the protocol to achieve a solution should always be the same. The doctor needs to know where he/she wants to go, and equally important, how to get there. The practitioner must record baseline information, have a definite plan to evaluate that information, and then follow a step-by-step sequence to achieve the final results.

The patient and the laboratory play specific roles in the process. The laboratory's input is invaluable in developing a treatment plan, including diagnostic wax-ups, provisionals, stents and indices. The patient must be included in the process by being given the opportunity to evaluate cosmetics, phonetics and the "feel" of their new teeth before the final restoration is constructed. This is accomplished by the meticulous fabrication of a provisional that will preview the final restoration for the patient and doctor. Once the patient accepts the provisional, all that remains is to duplicate it in the final restorative material. This insures patient satisfaction.

Guidelines
The protocol for a predictable implant restoration is aesthetic-driven. We must satisfy the patient's desires. Achieving this goal, lies in the analysis of tooth function. Over the last several years, an occlusal philosophy has been developed through a detailed study of anatomy and function, in which tooth and, specifically, cusp placement can be explained and determined in precise ways. Every tooth contour fulfills either an aesthetic, phonetic, or functional requirement. Following these guidelines in a logical sequence will produce a restoration that is not only aesthetically beautiful but functionally correct. The restoration is maintainable over time, because it also takes into consideration the distribution of forces generated in the oral cavity.

Hinge Axis
Analysis starts not with the teeth but with the jaw. The mandible moves on a hinge axis, through an arc of rotation, the center of which is the temperomandibular joint. Any attempt to construct a restoration must take into consideration this arc of opening/closing. To do this we need an instrument that will duplicate jaw movement at the bench [figure1]. The semi-adjustable articulator is this instrument. The articulator itself is nothing more than a hinge. However, to make this instrument effective, the correct center of rotation, the condylar hinge axis must be located on the patient and then transferred into the articulator. This is accomplished with a facebow. When the casts are accurately mounted on a semi-adjustable articulator. This is accomplished with a facebow. When the casts are accurately mounted on a semi-adjustable articulator, then mandibular movements can be duplicated. When the diagnostic casts have been facebow mounted on an articulator in this manner, the casts can be moved to reproduce the patient's jaw movements in function, and cusp tips can be placed accurately. When the patient receives the restoration, the opposing teeth will meet the way the restoration was planned, because the mandible will be moving on this same hinge axis. This is a key step. If casts are mounted by hand, on an arbitrary hinge axis, then when the restoration is tried in, the cusps will strike in different places as the mandible moves. It's for this reason, that cases fitting well on the model require time-consuming intraoral adjustments.

Articulators
Many dentists have educated on needlessly complicated and cumbersome articulators. These instruments were so intimidating that many practitioners refused to use them. Modern articulators and facebows have been developed that are simple and convenient. The Artex articulator and Rotofix facebow [figures 1 & 2] are examples.

It is impossible to analyze jaw movement and accurately construct the restoration without using facebow mounted articulated casts. Recognizing the importance of the arc of rotation and considering that the articulator is nothing more than an accurate hinge,hand mounted models make no sense!

Occlusion
The basis of all restorative treatment starts with stable temperomandibular joints. If the discs and condyles are not stable and asymptomatic in their fossas, then jaw movement is not predictable and reproducible. Stable joints are the starting points.

The foundation for all restorative dentistry is based on a specific occlusal scheme that should be applied consistently in all situations, from a single crown to a complex implant restoration. This philosophy, as proposed by Dawson, is based on an understanding of anatomy and function. All the components of the masticatory system must exist in harmony for the system to be stable. Anatomic harmony exists when all parts of the system are in a structural equilibrium between the condyle, disc, muscles, and teeth; between anterior teeth, lips and tongue; between the upper and lower teeth; and between the teeth and the face.

Functional harmony is maintained if, when the mandible moves in function, the lower teeth meet evenly with the upper teeth. There is a specific arc of opening and closing. Functional harmony also includes several precise relationships -- between the teeth and the lip closure path, which is the path the lower lip follows as it rises up to meet the upper lip (this position of the lower lip against the incisal edges of the maxillary teeth determines their buccal inclination); between the teeth and the lips in phonetics; and between the teeth and the neutral zone. The teeth must conform to the neutral zone which is the space created between the inward pressure of the muscles of the lips and cheeks and the outward pressure of the tongue.

If anatomic and functional harmony is created in our restorations, then tooth position and cusp tips will invariably be esthetic. Basic principles of stress and force distribution are the same whether the restoration is tooth or implant supported. The occlusal philosophy does not change.

Centric Relation
The first goal of stable occlusion is to have maximum intercuspation in centric relation. Centric relation is that stable jaw position. Dawson defines it as "the relationship of the mandible to the maxilla when the properly aligned mandibular condyle disk assemblies are in the most superior position against the maxillary fossa eminentia irrespective of vertical dimension or tooth position."

This jaw position of centric relation in which the condyles are fully seated is the starting point of all mandibular movement. The condyles must not be restricted to centric relation, but must be free to rotate and move in and out of centric relation and down the eminence during opening/closing, protrusive and lateral movements.

Maximum Intercuspation
Once the mandible is in centric relation, then there are certain guidelines for tooth and cusp position. In centric relation, the teeth must be able to achieve maximum intercuspation; in such as way that every tooth should have a stable centric holding contact with an opposing tooth, and all these contacts should be of equal intensity. In other words, when the teeth come together every tooth should have a contact with an opposing tooth. Ideally, these contacts should be in the long axis of the tooth with the maxillary lingual cusps contacting central fossas or marginal ridges of upper teeth. Lower incisors should be simultaneous, discrete finite points of equal intensity. The tooth position of maximum intercuspation is also called centric occlusion (centric relation is a jaw position; centric occlusion is a tooth position).

Anterior Guidance
The next occlusal consideration is how to guide the movement of teeth from maximum intercuspation into all excursive movements. The anterior teeth do this when the mandible moves into protrusive or lateral movement from maximum intercuspation. This is called anterior guidance. In protrusive, the lower incisal edges should ride along the palatal inclines of the upper teeth in one continuous, smooth movement. The posterior teeth immediately separate. In excursive movements, there must be immediate disclusion of the posterior cusp fossa relationship, so that the centric holding cusps do not engage the inclines or cusp tips of the opposing teeth as they slide. These movements are controlled by the anterior teeth only -- the anterior guidance. In straight protrusive movement, the lower incisal edges glide along the plane of the palatal surface of the maxillary incisors [figure 3]. It is desirable, but not necessary, that all four incisors guide this movement. In lateral movements, the contact between the upper and lower canines on the working side guide the movements. There are no contacts on the balancing side. This is called "canine guidance and posterior disclusion" [figure 4].

Simply put, the anterior teeth guide all the mandibular movement. This occlusal scheme is critical for the rehabilitation of the natural dentition and, along with the analysis of force vectors, is particularly appropriate for the implant restoration, considering the anterior region usually has the most available and strongest bone. The unique nature of the jaw as a lever places the least amount of force in the anterior region.

Tooth Position
There are specific guidelines for the shapes and positions of the teeth, their relationship to one another and the perioral musculature. There is a definite sequence in evaluating and organizing this information. The position of the upper incisors is most significant. The incisal edges are considered first, as they relate to the upper and lower lips, the smile line, and phonetics. The correct length of the incisal edges is checked in the mouth by observing the position of the lips in F and E sounds. In the "F" sound, the upper incisal edge should barely touch the wet/dry line of the lowerlip and then release [figure 5]. In the wide smile, or "E" sound, the line made by the tips of the upper teeth should parallel the contour of the lower lip [figure 6]. The emergence profile of the apical third should be in direct line with the buccal alveolar bone. The middle third supports the upper lip. The buccal incisal edge is tucked in to conform with the vermilion border of the lower lip. The lingual concavity and cingulum, as they relate to the leading edge of the lower incisors, determine the anterior guidance and phonetics.

Lower Incisors
The lower incisors and, specifically, the lower incisal edge, form the foundation of the entire occlusal scheme. The incisal plane should be level, establishing the leading edge of the labio incisal line angle, as the stable stop against the cingulums of the upper incisors. This will also set the platform for the anterior guidance. The incisal plane height should line up with the posterior curve of spee, with a slight anterior convexity.

Posterior Stops
The posterior teeth are considered last. Stable holding stops are built into centric relation. These centric stops should be consistent with the stable holding stops established by the incisal edges of the lower incisors and cingulum rests on the upper incisors. As mentioned, centric stops should strike simultaneously with equal intensity, and immediately disclude in all excursive movements.

No Compromise
These rules apply to the aesthetic restoration of natural teeth. The need for correct incisal edge placement, lip support, and phonetics does not change for implant supported restorations. If anything, a thorough understanding of these principles is even more important, because in many cases we have lost much of the alveolar bone that guides us. Fixture and abutment position must be determined by using other available guidelines. However, the need for appropriate force distribution is greater. The relationship of the fixture location in the available bone versus the need for proper tooth position will determine the type of final restoration. Fixtures can only go in bone that will support them, and teeth must be in their proper location. There can be no compromise. Either the bone must be augmented or the restoration changed from a fixed restoration, replacing lost tooth structure, to a detachable restoration that replaces lost bone and soft tissue.

Part II
In part two of this article these principles will be applied to a case study [figures 7,8,9]. A protocol and sequence will be presented, detailing the exact steps taken by the surgeon restorative dentist and the dental lab in the rehabilitation of a periodontally compromised maxilla with a totally implant supported fixed restoration. The case involves sinus augmentations, multiple surgeries and a series of fixed provisional restorations.

The author would like to acknowledge his teacher, Dr. Peter Dawson. The theories of occlusion presented here are expanded upon in Evaluation, Diagnosis, and treatment of Occlusal Problems, Peter E. Dawson, 1989, C. V. Mosby Co., St. Louis. The Artex articulator and Rotofix facebow are distributed by Jensen Industries, Inc.

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