Rigid Impression Materials Dental Material Notes

Rigid Impression Materials Impression Compound And ZOE Paste

A dental impression is a negative record of the tissues of the mouth. It is used to reproduce the form of the teeth and surrounding tissues.

The negative reproduction of the tissues given by the impression material is filled up with dental stone or other model materials to get a positive cast. The positive reproduction of a single tooth is described as a ‘die’, and when several teeth or a whole arch is reproduced, it is called a ‘cast’ or ‘model’.

Read And Learn More: Basic Dental Materials Notes

The application of dental impression compound has also decreased with the increased use of rubber impression materials, which can also be electroformed to produce metal dies. However, impression compound is useful for checking cavity preparations for undercuts and for making impressions of full crown.

Advantages Of Using A Cast Or Model

• Models provide a three-dimensional view of the oral structures, thus aiding in diagnosis and treatment planning.
• Many restorations or appliances are best constructed on casts. It may be inconvenient to both dentist and patient if these have to be made directly in the patient’s mouth.
• Models can be used to educate the patient.
• They serve as treatment records.
• By using casts, technical work can be passed on to technicians, saving valuable clinical time.

Desirable Properties Of An Impression Material

• Should be nontoxic and nonirritant to dentist and patient.
• Acceptable to the patient.
  • Have a pleasant taste, odor, consistency and color.
  • Should set quickly once placed in the mouth.
• Should be accurate.
  • Accurate surface detail.
  • Elastic properties with freedom from permanent deformation after strain.
  • Dimensionally stable.
• Have adequate shelf life for storage and distribution.
• Be economical.
• Handling properties.
  • Sufficient working time.
  • Set quickly in mouth (saves chairside time).
  • Be easy to use with the minimum equipment.
  • Satisfactory consistency and texture.
• Have adequate strength so that it will not break or tear while removing from the mouth.
• Should be compatible with the die and cast materials.
• Should be able to be electroplated.

Classification Of Impression Materials

There are several ways of classifying impression materials.

• According to mode of setting and elasticity.
• According to tissue displacement during impression procedure.
• According to their uses in dentistry.

According to mode of setting and elasticity

• The terms thermoset, thermoplastic, rigid and elastic are used to describe these materials.

According to tissue displacement

Depending on whether tissues are displaced while making impressions a material may be

• Mucostatic
• Mucocompressive (Mucodisplacive)

Mucostatic materials produce minimal displacement of the tissue during impression, e.g. plaster, zinc oxide eugenol, low viscosity alginates, low viscosity elastomeric materials, etc.

Mucocompressive materials are more viscous and displace the tissues while recording them, e.g. compound, high viscosity alginates, high viscosity elastomers, etc.

According to their uses in dentistry

• Impression materials used for complete denture prosthesis Impression plaster, impression compound and impression paste set to a hard rigid mass, and hence cannot be removed from undercuts without the impression being fractured or distorted. Therefore these materials are best suited for edentulous mouth.
• Impression materials used for dentulous mouths On the other hand alginates and rubber base impressions are sufficiently elastic to be withdrawn from undercut areas. Such elastic impression materials are suitable for impressions for fabrication of removable and fixed partial denture prostheses, where the impressions of the ridge and teeth are required.

Rigid Impression Materials

As mentioned earlier the rigid impression materials are

• Impression plaster
• Impression compound
• Zinc oxide eugenol impression paste
• Impression waxes

(Impression plaster is described in the chapter on Gypsum Products).

Impression Compound

• Impression compound is one of the oldest of the dental impression materials. It can be described as a rigid, reversible impression material which sets by physical change. On applying heat, it softens and on cooling it hardens. It is mainly used for making impressions of edentulous ridges. A more viscous variety of compound called tray compound is used to form a tray in which a second more fluid material is placed to make a more detailed impression compound.
• Synonyms Modeling compound or modeling plastic.

Impression Compound Classification

Type 1 – Impression compound

Type 2 – Tray compound

• Type 2 Tray compound is used to prepare a tray for making an impression. A second material is then carried in it in order to make an impression of oral tissues. Since reproduction of the fine details is not essential, it is generally stiffer and has less flow than regular impression compound. The use of dental tray compound decreased with the increased substitution of acrylic tray materials.

Impression Compound Supplied As

• Supplied as sheets, sticks, cakes and cones in a variety of colors.

Impression Compound Applications

• For making a preliminary impression in an edentulous mouth (mouth without teeth).
• For impressions of full crown preparations where gingival tissues must be displaced.
• Peripheral tracing or border molding.
• To check undercuts in inlay preparation.
• To make a special tray.

Single tooth impression

• In conservative dentistry, an impression is made of a single tooth in which a cavity is prepared. The compound is softened and carried in a copper band. The filled band is pressed over the tooth and the compound flows into the prepared cavity. It is referred to as a tube impression. Tube impressions were also used to make electroformed dies.

Complete denture impressions

• In complete denture fabrication, it is common to make two sets of impressions—the preliminary and the final impression. The preliminary impression is made in a stock tray. A study cast made from this is used to construct a custom tray or special tray. The custom tray is used to make the final impression. The technique of making a preliminary and final impression greatly improves the accuracy of the complete denture.

Requirements Of Impression Compound

An ideal impression material should

• Harden at or little above mouth temperature.
• Be plastic at a temperature not injurious or harmful to oral tissues.
• Not contain irritating or toxic ingredients.
• Harden uniformly when cooled without distortion.
• Have a consistency when softened which will allow it to reproduce fine details.
• Be cohesive but not adhesive.
• Not undergo permanent deformation or fracture while withdrawing the impression from the mouth.
• Be dimensionally stable after removal from the mouth and during storage.
• Exhibit a smooth glossy surface after flaming.
• Withstand trimming with sharp knife without flaking or chipping after hardening.
• Should not boil and lose volatile components on flaming.
• Should remain stable without losing soluble plasticizers when immersed in water for long periods.

Composition

In general impression compound is a mixture of waxes, thermoplastic resins, organic acids, fillers and coloring agents.

• Plasticizers Compounds, such as shellac, stearic acid and gutta-percha are added to improve plasticity and workability. These substances are referred to as plasticizers. Synthetic resins are being used in increasing amounts. Waxes and resin give the material its characteristic thermoplastic properties.
• Fillers These are small particles of inert materials which strengthen or improve the physical properties of many materials. Fillers are chemically different from the principal ingredient. In such a case the filler particles are sometimes referred to as the core and the surrounding ingredients as the matrix. For example, the waxes and resins in impression compound impart high flow and low strength. Consequently, a filler such as talc is added to reduce the plasticity and increase strength of the matrix material. Other fillers used are diatomaceous earth, soap stone and French chalk.

Properties Of Impression Compound

Fusion Temperature

• When impression compound is heated in a hot water bath the material starts to soften at approximately 39 °C. However at this stage, it is still not plastic or soft enough for making an impression. This temperature at which the material looses its hardness or brittleness on heating or forms a rigid mass upon cooling is referred to as fusion temperature. Impression compound exhibits a fusion temperature range rather than a fixed point.
• On continued heating above 43.5 °C, the material continues to soften and flow to a plastic mass that can be manipulated. Thus all impressions with compound should be made above this temperature. Below this temperature an accurate impression cannot be expected.

Thermal Properties

Thermal conductivity

Impression compound has very low thermal conductivity, i.e. they are poor conductors of heat.

Significance

• During softening of the material, the outside will soften first and the inside last. So to ensure uniform softening the material should be kept immersed for a sufficient period in a water bath. Kneading of the material ensures further uniform softening.
• The low thermal conductivity affects the cooling rate. The layer adjacent to the oral tissues cools faster than the inside. Removal of the impression at this stage can cause serious distortion. Thus it is important to wait for the compound to cool thoroughly before removing it from the mouth.

Coefficient of thermal expansion (CTE)

The CTE of compound is comparatively high due to the presence of resins and waxes. The linear contraction from mouth temperature to room temperature is 0.3%.

Errors from thermal distortion can be reduced

• By obtaining an impression and then passing the impression over a flame until the surface is softened and then obtaining a second impression. During the second impression, the shrinkage is relatively lower, since only the surface layer has been softened.
• Another way of reducing the thermal contraction is by spraying cold water on the metal tray just before it is inserted in the mouth. Thus the material adjacent to the tray will be hardened, while the surface layer is still soft. In both techniques, the impression is likely to be stressed considerably and so the stone cast should be constructed at the earliest.

Flow

• Good flow is desirable during impression making. The softened material should flow into all the details of the tissue contour. Once the compound hardens, it should have minimum flow, otherwise it will get distorted.

Dimensional stability

• Since the release of strains is unavoidable, the safest way to prevent distortion is to pour the cast immediately or at least within the hour. Another cause of warpage is removal of the impression too early from the mouth before complete hardening.

Detail reproduction

• Surface detail reproduction is comparatively less because of its high viscosity and low flow. Because of the viscosity, pressure has to be used during impression, which compresses or distorts the tissues. Thus the tissues are recorded in a distorted state.

Manipulation

Sticks

• Small amounts of compound (stick compound) can be softened over a flame. When a direct flame is used, the compound should not be allowed to boil or ignite, otherwise, the plasticizers are volatilized.

Cakes

• Larger amounts of compound are softened in warm water in a thermostatically controlled water bath usually in the range of 65 to 75 °C. After the compound is removed from the water bath, it is usually kneaded with the fingers in order to obtain uniform plasticity throughout the mass.

Loading The Tray

• A slightly oversized tray is selected. The softened material is loaded onto the tray and quickly seated on to the tissues to be recorded. Any delay can cause the impression to harden prematurely. If the compound is too hot, it may be tempered by briefly immersing in slightly cooler water. The lips are manipulated to mold the borders of the impression while it is still soft.

Precautions

• Prolonged immersion in a water bath causes the compound to become brittle and grainy because some of the ingredients may be leached out.
• Overheating in water makes the compound sticky and difficult to handle.
• Avoid incorporating water while kneading.

Removal Of Impression From The Mouth

The impression is removed from the mouth only after it has completely cooled and hardened.

Disinfection

The recommended disinfectant is 2% glutaraldehyde.

Pouring The Cast And Cast Seperation

• The cast should be poured without delay. The cast is separated from the impression by immersing it in warm water until it is soft enough. Excessively hot water is avoided as it can make the material sticky and difficult to remove from the cast.

Advantages And Disadvantages

Advantages

• The material can be reused a number of times (for the same patient only) in case of errors.
• Inaccurate portions can be remade without having to remake the entire impression.
• Accuracy can be improved by flaming the surface.
• The material has sufficient body to support itself especially in the peripheral portions. It does not collapse completely if unsupported by the tray.

Disadvantages

• Records less detail because of its high viscosity.
• Compresses soft tissues during impression.
• Distortion due to its poor dimensional stability.
• Difficult to remove if there are severe undercuts.
• There is always the possibility of overextension especially in the peripheries.

Zinc Oxide Eugenol Impression Paste

Zinc oxide and eugenol based products are widely used in dentistry.

• Cementing and insulating medium.
• Temporary filling material.
• Root canal filling material.
• Surgical pack in periodontal surgical procedures.
• Bite registration paste.
• Temporary relining material for dentures.
• Impressions for edentulous patients.

In dentistry, zinc oxide eugenol is popular as an impression material for making impressions of edentulous arches for the construction of complete dentures. It is classified as a rigid, irreversible impression material. It cannot be used for recording impressions of dentate arches and in areas of severe undercuts.

Classification

ADA specification No. 16.

• Type 1 or Hard
• Type 2 or Soft

Available As

In paste form in two tubes

• Base paste (white in color)
• Accelerator or reactor or catalyst paste (red in color)

Composition

Zinc oxide should be finely divided and should contain a slight amount of water.

Fixed vegetable or mineral oil acts as a plasticizer and also aids in masking the action of eugenol as an irritant.

The oil of cloves contains 70–85% eugenol. It is sometimes used in preference to eugenol because it reduces burning sensation.

Gum or polymerized rosin speeds the reaction and improves homogeneity.

Canada and Peru balsam improves flow and mixing properties.

Calcium chloride acts as an accelerator for setting reactions.

Other accelerators are

• Zinc acetate
• Primary alcohols
• Glacial acetic acid

Setting Reaction

• The setting reaction is a typical acid-base reaction to form a chelate. This reaction is also known as chelation and the product is called zinc eugenolate.

\(\mathrm{ZnO}+\mathrm{H}_2 \mathrm{O} \longrightarrow \mathrm{Zn}(\mathrm{OH})_2\)

Microstructure

• The chelate (zinc eugenolate) forms a matrix surrounding a core of zinc oxide particles. The chelate is thought to form as an amorphous gel that tends to crystallize giving strength to the set mass. The formation of crystalline zinc eugenol is greatly enhanced by zinc acetate dehydrate (accelerator) which is more soluble than Zn(OH)2 and can supply zinc ions more rapidly.

Setting Time

Working time

• There should be sufficient time for mixing, loading onto the tray and seating the impression into the mouth.

Setting time

• Once the material is in place, it should set fast.

Why should an impression material set quickly in the mouth?

Any material which takes a long time to set in the mouth.

• Would be uncomfortable to the patient.
• Movement is bound to occur, resulting in stresses and errors in the impression.
• Result in a waste of time for the dentist. In a busy practice, this is unacceptable.

Initial setting time is the period from the beginning of the mixing until the material ceases to pull away or string out when its surface is touched with a metal rod of specified dimensions. The impressions should be seated in the mouth before the initial set.

The final set occurs when a needle of a specified dimension fails to penetrate the surface of the specimen more than 0.2 mm under a load of 50 gm.

Factors controlling setting time

• The particle size of zinc oxide powder If the particle size is small and if it is acid coated, the setting time is less.
• By varying the lengths of the two pastes (not recommended).
• Setting time can be decreased by adding zinc acetate or a drop of water or acetic acid (acetic acid is more effective than water. It increases the speed of formation of the zinc hydroxide).
• The longer the mixing time, the shorter the setting time.
• High atmospheric temperature and humidity accelerate the setting.
• Setting can be delayed by cooling the mixing slab, or spatula or adding small amounts of retarder oils, or waxes.

Properties

Consistency and flow

These are clinically important properties. A paste of thick consistency can compress the tissues. A thin free-flowing material copies the tissues without distorting them.

According to ADA specification No. 16, the spread is

• Type 1 pastes — 30 to 50 mm
• Type 2 pastes — 20 to 45 mm

Clinically, these materials have a very good flow. Poor quality impression pastes, thicken unduly, and have a poor flow.

Detail reproduction

It registers surface details quite accurately due to the good flow.

Rigidity and strength

The impression should resist distortion and fracture when removed from the mouth after setting. The compressive strength of hardened ZOE is approximately 7 MPa two hours after mixing.

Dimensional stability

The dimensional stability is quite satisfactory. A negligible shrinkage (less than 0.1%) may occur during hardening.

Biological considerations

Some patients experience a burning sensation in the mouth due to eugenol. It can also cause tissue irritation. Non-eugenol pastes can be substituted.

Manipulation

• The mixing is done on an oil-impervious paper or glass slab. Two ropes of paste of the same length and width, one from each tube are squeezed onto the mixing slab. A flexible stainless steel spatula is used. The two ropes are collected with the spatula and mixed until a uniform color is observed.
• Mixing time 1 minute.
• Mechanical mixing A rotary mixing device can also be used. Special circular mixing pads are attached to the circular table of the device. After dispensing the material, the machine is switched on. As the table rotates, the operator first collects the material using the sides of the spatula.
• He then spreads the material by flattening the spatula. The process of collecting and flattening is repeated alternately until a uniform mix is obtained. Mechanical mixing gives a faster, uniform mix with fewer voids and bubbles.

Impression Tray

• A custom impression tray made of stable resin is recommended for zinc oxide eugenol. The material adheres to the tray so no special adhesive is required.
• A primary compound impression can also be used as a tray. The material is loaded into the tray by swiping onto the sides of the tray and then spread in a smooth uniform motion. Loading and spreading through a patting motion can trap air.

Disinfection

• The impression is rinsed and placed in a disinfectant solution. Rinsing removes saliva and other contaminants. The recommended disinfectant solution is 2% glutaraldehyde solution. Glutaraldehyde is an organic compound with the formula CH2(CH2CHO)2. A pungent colorless oily liquid, glutaraldehyde is used to sterilize medical and dental equipment.

Pouring The Impression

• As with most impression materials the pouring of the cast should not be delayed for too long. After setting, the impression is removed from the cast after softening it through immersion in hot water.

Advantages And Disadvantages

Advantages

It has a sufficient body to make up for any minor under extensions in the tray itself during impression making.

• It has enough working time to complete border molding.
• It can be checked in the mouth repeatedly without deforming.
• It registers accurate surface details.
• It is dimensionally stable.
• Does not require separating media since it does not stick to the cast material.
• Minor defects can be corrected locally without discarding a good impression.

Disadvantages

• It requires a special tray for impression-making.
• It is sticky and adheres to tissues.
• Eugenol can cause a burning sensation and tissue irritation.
• It cannot be used for making an impression of teeth and undercut areas as it is inelastic.

Other Zinc Oxide Pastes

Surgical Pastes (Periodontal Packs)

• After certain periodontal surgeries (e.g. gingivectomy, i.e. surgical removal of diseased gingival tissues) where sutures cannot be placed, a zinc oxide-based surgical paste may be placed over the wound to aid in the retention of the medicament, to protect the wound and to promote healing (also known as periodontal pack).
• Earlier pastes were eugenol-based and have been around since 1923 (Ward’s Wondrpak). Current surgical pastes avoid eugenol because of the potential for tissue irritation. These are called noneugenol pastes.

Noneugenol Impression And Surgical Pastes

• The chief disadvantage of zinc oxide eugenol paste is the burning sensation caused by eugenol. Some patients find the taste of eugenol disagreeable and in cases where the surgical pack is worn for several weeks chronic gastric disturbance may result. Hence, non eugenol pastes were developed.

• Noneugenol pastes consist of a base and reactor paste. The base paste contains ZnO, gum and lorothidol (fungicide). The reactor pastes contain coconut fatty acids, rosin (thickening), chlorophyll (bacteriostatic), etc. Antibiotics like tetracycline may be incorporated at the time of mixing.

Setting reaction

Zinc oxide is reacted with a carboxylic acid.

\(\mathrm{ZnO}+2 \mathrm{RCOOH} \longrightarrow(\mathrm{RCOO})_2 \mathrm{Zn}+\mathrm{H}_2 \mathrm{O}\)

• The reaction is not greatly affected by temperature or humidity.
• Compared to impression pastes the surgical pastes are less brittle and weaker after hardening. The setting time is longer (around 15 minutes). They are available as a 2 paste system. The paste is mixed and formed into a rope that is packed over the gingival wounds (using wet fingers) and into the interproximal spaces to provide retention.
• The final product after setting should be sufficiently strong so that it is not readily displaced during mastication. Light-cured periodontal dressing materials and single-component pastes (that are set by heat and moisture in the mouth are also available). An auto-mixing cartridge version of Coe-Pak is also available.

Bite Registration Pastes

These are materials used for recording the occlusal relationship between two occluding surfaces, e.g. teeth, occlusion rims, etc. ZOE pastes used for this purpose have slightly different properties

• Shorter setting time to prevent distortion.
• More plasticizers to prevent it from sticking to the teeth or occlusion rims.

Other bite registration materials include wax and silicones. ZOE registrations are more rigid than registrations made in wax or silicones. They are more stable, and offer less resistance to the closing of the jaw than wax. Resistance-free closure is often indicated in complete dentures where denture base movement or tissue displacement occurring from closure is not desired