Dawson Vistas October 2009 : Page 6
change, for working models.Why would dentists want to be inaccurate with diagnostic models from the very be- ginning? The closer the models represent patients, the more accurate diagnostic and clinical treatments will be. To create the most accurate model, dentists want to use the most accurate materials and to standardize techniques. Generally speaking, die stone is one of the most accurate stones and generally has a thermal expansion percentage of less than 0.10%. In creating the most accurate model, a stone with a degree of thermal expansion closer to die stone is used.Many options are available and easily identified once dentists take time to investigate. Orthodontic stone tends to be more accurate and can be found with an expan- sion of less than 0.10%. The stone also has to be mixed per the manufacturer’s instructions, which includes proper water-to-powder ratios (that is,weighing the stone and measuring the water). Standardizations can be created in the office to simplify this step. In addition,a vacuum mixer is invaluable for helping eliminate air bubbles. To create a set of models that most relate to patients, dentists need to start with the most accurate materials for both impression and dental stones (Figure 3).Manu- facturers’ protocols need to be followed. Abbreviating steps will result in bigger problems. Properly trained staff members are essential to this process and often may per- formthe task better than the dentist. FACE-BOW The face-bow is essential for establishing the incisal/ occlusal plane of the maxilla and recording the condylar axis.Thus the proper arch of closure is recorded and relates the maxillary model to the articulator mirroring the same relationship as is in patients’ skulls.4 Face-bow mounting of the maxillary model is the first step in the mounting process. Many types of face-bows are available and relate to the various articulators. One of the most common is an earbow, which uses the ear as a reference point.With the patient upright, the bow of the face-bow is related to the ears and eyes. In most circumstances, the bow is parallel to the eyes, ensuring that the mounted maxillary model has an occlusal plane aligned with the bench top. This allows the dental tech- nician to create anterior restorations with an accurate incisal plane and proper central incisor embrasure. Inaccuracies in the face-bowcan lead toa mounting that is set at an angle, which, in the case of anterior restorations, can lead to the creation of a cant or occlusal plane discrepancies. Under most circumstances, the interpupillary line is correct, however, sometimes, both the ears and eyes may have discrepancies. In such cases, a simple level attached to the bow may aid in establishing the proper incisal plane (Figure 4). This step is critical when anterior restorations are planned. In addition, the face-bow photograph is a valuable laboratory communication tool when facial dis- crepancies are apparent (Figure 5). CENTRIC RELATION RECORDS Muchhas been written about occlusal bite records and meth- ods for obtaining accurate and repeatable centric relation bite records.A study byMcKee showed obtaining centric rela- tion records was learnable and repeatable.5 One of the main FIGURE 3 Stone models reproducing detail from the PVC impressions.Note the occlusal surface and soft tissue detail. 6 VISTAS: Complete & Predictable Dentistry
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