Dawson Vistas October 2009 : Page 5
established, a trained laboratory assistant performs much of this process. Failure in any step is magnified and com- promises the diagnostic and restorative outcomes. It is unfortunate if the treatment is based on an inaccurate set of mounted diagnostic models or a pretreatment diagnos- tic wax-up has the incorrect occlusal scheme. It is worse if the restorative try-in of a prosthetic case does not fit due to inaccuracies from the office’s records process. The author’s father, Dr. Everett Cobb, taught both den- tal materials and operative dentistry courses at George- town University’s dental school. He would repeatedly challenge his students with this question: “What is the most important step to any procedure?” The answer: “The one you’re on.”Nowhere in the records process is this truer than in the creation of face-bowmounted, cen- tric relation-articulated diagnostic models. Attention to detail will help assure success. The process involves six key steps to assure accuracy: 1. Diagnostic impressions 2. Materials 3. Face-bow 4. Centric relation record 5. Mounting the models 6. Articulator settings—condylar path DIAGNOSTIC IMPRESSIONS AND MODELS Essentially, the diagnostic models need to be a re-creation of the patient’s maxillary and mandibular arches, with an accurate representation of the teeth and supporting tissues. This starts with precise impressions that capture these structures. The impressions should include the teeth, gin- giva, palate (on the maxillary impression), frenum, and extension of the impression into the vestibule. The teeth should be free from voids and occlusal artifacts (Figure 2). Such artifacts could result in an unstable fit to a face-bow or centric relation record, leading to a mismounted model. The gingival tissues need to be well reproduced, especially because macro- and microesthetic decisions pertaining to smile design may be partially based on the models. An “unreadable”model will render these decisions difficult. The supporting structures such as frenum attachments, palate, and alveolar process also need to be recorded. A “U-shaped” maxillary impression without the palate is not accurate. The supporting structures are needed for proper evaluation and possible stabilization of guides and stents to be used in the restorative phase. Envision a case in which most of the maxillary anterior teeth are to be prepared and accurate provisional restorations based on a pretreatment wax-up stent is to be used. The palate is needed as a stop to the provisional stent.Without it, the provisional stent cannot be properly seated, leading to inaccurate provisional restorations. MATERIALS Dental materials is one of the first courses taught in den- tal school. Often, dentists may not spend adequate time to stay informed of the recent developments in this ever evolving area.Many of the most important advancements in dentistry would not have occurred if not for the im- provements in materials. Some of these newer materials can help improve the accuracy of impressions and models. For years, dentists have used one material to create study models and another, more accurate impression ma- terial for final crown-and-bridge working models. If the goal is accuracy,why would dentists not want to use a more accurate material? The more closely a patient’s dentitions can be reproduced, the more precise dentistry will be. For years, irreversible hydrocolloids (alginate) have been the standard impression material for creating diag- nostic models.When used correctly, it can create a very accurate impression. However, by today’s standards and by dental professionals’ requirement for the most accu- rate impression possible, alginate has some disadvan- tages.Alginate is not dimensionally accurate for the long term and can distort within minutes. It is affected by humidity and temperature and can be poured only once. In addition, alginate is affected by some surface disinfec- tants and is subject to inaccuracies in water-to-powder ratios and mixing techniques.3 Alternatives to irreversible hydrocolloids include poly- vinyl siloxanes (PVS), which improve upon many of algi- nate’s shortcomings. It is dimensionally more accurate and stable and can be sent to a laboratory. It can be poured multiple times and is not affected by surface disinfectants. Disadvantages include a slightly longer set time, which can be an issue for some patients (Figure 2). In reality, both materials are needed.Most prelimi- nary impressions can be taken with PVS for the improved accuracy.When alginate is used, dentists can try to limit the variables as much possible. Precisely following the manufacturer’s instructions for water-to-powder ratios, using an auto mixer for thorough mixing, and pouring the model immediately after the impression is taken all will improve the accuracy of an alginate impression. Once the impression is perfected, a model should be based on that impression.Attention to detail and follow- ing the manufacturer’s directions for the material is es- sential.When comparing dental stones commonly used for models, a less accurate stone,with a greater degree of dimensional instability, is often the choice for diagnostic models and a more precise stone, with less dimensional VISTAS: Complete & Predictable Dentistry 5
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