Impression materials
Introduction :
The imprint presides over the development of the prosthetic construction, and its value in the search for optimal adaptation precision.
The choice of impression material is one of the fundamental steps in the prosthetic chain. It must make it possible to obtain a working model whose fidelity reflects its capacity to record an established clinical situation.
Despite all the efforts made in recent years by CAD/CAM (computer-aided design and manufacturing) to introduce optoelectronic imprinting into a scientific manufacturing design, the traditional “chemomanual” imprint is still relevant and most imprint materials have seen their mechanical and surface properties constantly improve, in the service of dimensional precision and the definition of surface conditions.
1. Definition:
An imprint is defined as a hollow or raised mark obtained by pressure (Le petit Larousse illustré, 2007).
In clinical prosthetic procedures, impression materials are used to accurately record and reproduce dental and tissue structures.
2. Classification:
For O’Brien, two broad classes of imprinted materials can be distinguished after their reaction: materials exhibiting elastic behavior, and those exhibiting inelastic, or rigid, behavior.
- Elastic imprint materials are classified into two families:
- elastomers (polysulfides, silicones and polyethers),
- irreversible hydrocolloids of the alginate type and reversible hydrocolloids of the agar-agar type.
- Inelastic or rigid imprint materials include:
- waxes and thermoplastic compositions,
- zinc oxide-eugenol pastes,
- the plaster.
3. General characteristics:
The general desired properties of an elastic imprint material are:
- pleasant smell and taste;
- non-toxic, non-irritant;
- use with minimal equipment;
- consistency compatible with their clinical use;
- athermal setting reaction;
- working time of at least 3 minutes;
- viscoelastic characteristics allowing:
- significant elastic deformation;
- low permanent deformation;
- high loyalty;
- dimensional stability for at least 24 hours, under normal hygrometric and thermal conditions corresponding to those prevailing in a dental office and a prosthetics laboratory;
- decontamination not affecting accuracy;
- compatible with replica materials;
- without reaction gas release;
- ergonomic packaging;
- cost price in accordance with the desired result.
4. Specific characteristics:
Among the objectives sought for an imprint material , precision is the term most often put forward. It reflects the material’s ability to reproduce the shapes (volumetric or dimensional precision) and microsurfaces of a volume (the precision of reproduction of details called fidelity).
A number of parameters determine the ability of a material to meet these different objectives.
4.1. Wettability:
Wettability refers to the ability of a fluid to spread on the surface of a solid. Low wettability of an impression material reduces its ability to spread and consequently to record oral surfaces.
Generally speaking, it depends on the viscosity , thixotropy, and hydrophilicity of the material.
4.2. Hydrophilicity:
The concept of hydrophilicity refers to the measurement of the contact angle between a material and a liquid, water. If the angle is less than 90°, the material is hydrophilic.
It should be noted that for the same liquid/solid pair, the contact angle varies depending on the roughness, the nature of the substrate and the time.
4.3. Viscosity:
Viscosity, which is defined as resistance to flow, is linked to intermolecular interactions and the rate of charges present in the material. Even if viscosity does not directly affect the wetting ability of a material, it influences its flow kinetics and thus determines a fundamental clinical factor, the degree of compression of the tissues during the impression. It also conditions its ease of mixing, its possibility of injection using a syringe, and its ability to flow.
4.4. Reproduction of details:
Detail reproduction is optimized by the wettability of the impression material, its low viscosity and its compatibility with the replica material. It is measured in microns.
4.5. Dimensional stability:
While dimensional accuracy is an immediate parameter, stability is a longer term one. Dimensional stability measures the degree of conformity of the impression with the original situation. Factors affecting dimensional stability are temperature variations through the coefficient of thermal expansion, contraction due to the setting phenomenon and the elimination of volatile by-products.
The dimensional stability of materials depends on their support: the impression tray. Space of 2 to 3 mm thanks to sheets of wax or tin, the individual impression tray ensures an inevitable, but regular contraction of the material.
The storage environment is important to ensure this dimensional stability. Storage time increases the risk of the properties of imprinted products being altered.
4.6. Mechanical properties:
The elastic properties of impression products depend on the crosslinking rate, the quantity of fillers and plasticizers. Possible deformations of the materials result from intrinsic qualities that are insufficient to withstand the constraints (shrinkage of the mouth, casting of the plaster, etc.).
Only compliance with sufficient setting time and a homogeneous mixture guarantees the expected properties.
Premature removal of a material that has not completely finished setting is one of the most common causes of deformation.
4.7. Disinfection:
There is no standard protocol due to the large number of materials and disinfectants.
Only immersion ensures good disinfection of the imprint, unlike spraying, the result of which is more random. Immersion is recommended for all materials except hydrocolloids, provided that a sufficient but not excessive immersion time is respected (less than 1 hour). A long immersion of the materials changes their dimensions.
4.8. Toxicity:
Naturally derived impression materials such as plaster and hydrocolloids are non-toxic. The biocompatibility of synthetic impression materials is often related to their chemical instability.
Conclusion :
Impressions are a crucial step in prosthetic treatments. The choice of material and impression technique contributes greatly to their success.
Only knowledge of the properties of these materials and a rigorous clinical examination are able to respond to the clinical circumstances encountered.
The optimization of imprinting techniques is today more linked to the improvement of material properties than to the evolution of implementation techniques.
Impression materials
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