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sushantpatel_doc's picture
Joined: 30 Nov 2009

When preparing a single-unit crown, or multiple-unit bridge, dentists inevitably follow a similar routine. The prepared tooth/teeth are impressed in a good quality impression material such as a polyether, or the most widely used impression material, an addition silicone vinyl polysiloxane (VPS),1 which accounts for 95% of the impressions sent to the dental laboratory.2 The impressions are taken in a full-arch stock or custom tray, the bite registration is taken after the preparations are completed, and an opposing model is fabricated from alginate to allow mounting of the case in the laboratory. Once the prosthesis is returned to the dental practitioner and inserted; if it is found to be high in occlusion, the dental technician is often blamed for an error in technique.3 In reality, it is the built-in variability of the above technique sequence and material selection, which is still routinely taught in many dental faculties, that leads to clinical frustration and valuable time wasted in trying to make the unit/units "fit." Why is it that a 12 times greater accuracy in the maximal intercuspal position is found with the dual-arch cast?4
This article looks at the "normal" sequence described above, identifies the variables, and describes how to minimize them.

Why is it that as dental students we are taught to take the bite registration after tooth preparation and after the patient has been anesthetized? This approach certainly makes sense for extensive restorations, or when involving terminal teeth in the arch as abutments for a multiple-unit restoration. However, if the clinician is preparing a single-unit restoration, which represents the majority of the crown and bridge impressions at dental laboratories,5 why not take the bite registration before the patient is anaesthetized and still has proprioception? In this way, there is an increased likelihood that the casts will be mounted in the patient's acquired centric. In addition, if the dentition is intact, the working stone model of the single preparation can be easily mounted more accurately using this bite registration.
Bite registration or interocclusal records are taken with many different registration materials in different ways; can the dental laboratory technician actually use them to relate the models in their proper orientation? Laboratories still receive wax bite registrations which are unreliable due to dimensional changes when cooling.6 Furthermore, they are easily distorted on removal from the mouth, in transit, or with temperature changes (Figures 1 and 2).7 The use of resin copings to record centric relation has been described by Anselm Wiskott and Nicholls,8 and a comparison between using impression plaster, wax, and Duralay acrylic resin showed that hand articulation was the most accurate method of relating casts to maximum intercuspation.9 The use of polyether bite registration materials has been shown to result in vertical discrepancies in the interocclusal relationships of casts.10 Elastomeric materials may deform11 or distort when pressure is applied during mounting of a case (elastics are often used to hold the casts together), resulting in faulty restorations.12 Of course, VPS impression materials, designed to flex when withdrawing a full tray impression from the mouth, cannot be used! It is critical not only for these bite registration materials to be dimensionally accurate but to be very stiff to resist distortion (such as Affinity QuickBite [CLINICIAN'S CHOICE] that has a durmeter of more than 90). When looking at the VPSs; Imprint Bite (3M ESPE), Silagum Automix Bite (DMG America), O-Bite (DMG America), Blu-Mousse Classic (Parkell), Exabite II (GC America); one polyether, Ramitec (3M ESPE); and one dimethacrylate base material Luxabite (DMG America); Chun, et al13 found that these materials presented significantly different polymerization shrinkage kinetics and showed dimensional changes even after the setting time indicated by the respective manufacturers. However, a study by Millstein and Hsu14 looking at Coe Bite Crème (GC America), Blu-Mousse (Parkell), Correct Bite (Pentron Clinical Technologies), Blue Velvet (J. Morita), Memosil D.D. (Heraeus Kulzer), and Ramitec (3M ESPE) showed that all brands were highly accurate and dimensionally stable.

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