Copia - PRESENTACIÓN BÁSICA

Unit 3.

Lidia Pons.

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Mixing and pouring dental stone.

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Pouring dental models.

The next task after taking the impression, is pouring up the model. There are many variables involved in achieving an ideal model.

Pouring dental models: gypsum materials.

The first step is choosing the proper stone, based on the reason for the impression. This chart shows just some of the most common used stones and the proper water/powder ratio.

Pouring dental models: gypsum materials.

Gypsum (calcium sulphate dihydrate) is a naturally occurring mineral used in dentistry to fabricate models , casts and dies. Calcination is the process of heating the gypsum to dehydrate it (partially or completely) to form calcium sulphate hemihydrate. Plaster and stone are products of the dehydration process. The differences in the types of gypsum are related to the amount of water removed, resulting in varying densities and particle sizes of the material. It is desirable that all gypsum products are strong, compatible with impression materials and waxes and fluid at the time of pouring into the impression; they should also have good dimensional stability. Powder and water ratios of gypsum products are important. The less water used in the mixture, the stronger the model. Excess water in a mixture of gypsum increases the setting time and reduces the strength and hardness of the final product.

Type V: dental stone, high strength, high expansion

Pouring dental models: gypsum materials.

We therefore need to distinguish between different types in accordance with the ISO standard mentioned above: Type I gypsums are dental plasters for impressions. This type is also referred to as plaster of Paris and is used in impression making. Type II gypsums are dental plasters for mounting and models. These products are suitable for making study models and models for mounting in an articulator. Type III gypsums are dental stones and intended for the production of high-strength models. Type IV gypsums are high-strength dental stones and are used to make models that must preserve the details of an impression for an extended period of time. They are therefore used mainly to make models for fixed and combined prostheses.

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Bite registration is not important just for recording a specific patient’s occlusal scheme. Indeed, in various clinical situations, dentists may need to alter a patient’s occlusal pattern with regard to both the vertical and sagittal dimension. Using the bite registration of the modified pattern, the dental technician can articulate the situation the dentist expects in the chair and fabricate the prosthetic restorations or orthodontic appliances on the basis of the information provided by the dentist. Bite registration is important when we need to transfer the maxillomandibular relationship from our patient’s mouth to the articulator; correct transfer of the intermaxillary relationships onto the articulator eliminates the need for occlusal adjustments on the final prosthesis, which require greater time and generate higher costs. Correct bite registration therefore allows the dental technician to correctly articulate the models, and to study and set-up the restorations in accordance with the in vivo occlusal scheme of the patient or the occlusal scheme according to which the dentist intends to rehabilitate the patient.

BITE REGISTRATION

MATERIALS FOR BITE REGISTRATION

The first material used for bite registration was plaster of Paris or impression plaster, a type 1 plaster. Plaster registrations are very accurate and extremely rigid. They resist distortion and offer good dimensional stability. Unfortunately, plaster is difficult to use. Thermoplastic modelling compounds were therefore added to overcome these problems. Unfortunately, in turn, these presented a further two insurmountable problems: high initial viscosity and extreme final hardness, leading to possible abrasion of the stone model. Waxes were introduced at a later date. These versatile materials are commonly used for bite registration purposes. Their success is explained by their versatility, ease of use and low cost. Unfortunately, they do not offer sufficient dimensional stability and suffer from a very high coefficient of thermal expansion. This can lead to significant distortion, spoiling the accuracy of the registration. All materials have advantages and disadvantages, of course, and zinc oxide eugenol paste suffers from particularly long setting times, fragility and the formation of byproducts that cause dimensional variation. Acrylic resins. They are very commonly used for single interocclusal jigs. Acrylic resins are both accurate and rigid after curing. Their main disadvantage is that they contract on curing, potentially causing distortion. Once cured, they are also very hard and can damage the stone model. Elastomers possess the best dimensional stability of all materials, are easy to use and offer virtually no resistance to mandibular closure. They are also easy to trim when cured and guarantee excellent detail reproduction. Addition silicones are dimensionally stable. One of the main disadvantages with addition silicones is that, when the stone model is being positioned, force can inadvertently be applied to the silicone, causing distortion. Registrations should therefore be trimmed.

BITE REGISTRATION

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BITE REGISTRATION

BITE REGISTRATION

Wax Bite Registration

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Trimming a dental model.

In some cases, only minimal trimming is required just to make a quick diagnostic model, but nothing really display worthy. Sometimes though, presentation quality is important and that is where certain trimming techniques can make a big difference in the visual aspect of the model.

Trimming a dental model.

The model trimmer is the workhorse in the lab and clinical setting for establishing a well-formed cast, once the impressions are poured in the gypsum of choice. When the casts are recovered from the impression, you are ready to trim the model.

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How to use a Facebow

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