Impression Materials BDS 3

Impression Materials BDS 3
Dr M Slabbert Dept Prosthodontics Wits

First Impressions count
The ability to record consistently good impressions is both a science and an art. It is worth bearing in mind that the impression influences not only the quality of the subsequent restoration but also the technician’s perception of the dentist’s skill. As none of us can achieve perfection every time there is much to be said for encouraging technicians to feed back when they receive a substandard impression.” BRITISH DENTAL JOURNAL VOLUME 192 NO. 12 JUNE R. W. Wassell1 D. Barker and A. W. G. Walls: Crowns and other extra-coronal restorations: Impression materials and technique

Classification of Impressions
Preliminary impressions Taken either by the dentist or an expanded-function dental assistant. Used to make a reproduction of the teeth and surrounding tissues. Used to make (1) diagnostic models, (2) custom trays, (3) provisional coverage, (4) orthodontic appliances, and (5) pretreatment and post‑treatment records.

Classification of Impressions- cont’d
Final impressions Taken by the dentist. Used to make the most accurate reproduction of the teeth and surrounding tissues. Used to make indirect restorations, partial or full dentures, and implants.

Classification of Impressions- cont’d
Bite registrations Taken by the dentist or dental assistant. Make a reproduction of the occlusal relationship between the maxillary and mandibular teeth. Provide an accurate registration of the patient’s centric relationship between the maxillary and mandibular arches.

Study model (cast) Oral Examination Primary Impression Treatment Planning duplicate model Final Impression Mouth and tooth preparation Master model Denture Laboratory procedure investment cast refractory cast Delivery

Definition An impression is a negative record of the tissues of the oral cavity which constitutes the basal seat of the denture. An impression is made in a material which has plasticity and which hardens or sets while in contact with the tissue. What are the ideal properties of an impression material? Sulcus area What are the objectives when taking an impression ? Discussion These requirements regarding both the primary impressions “Diagnostic casts” or Definitive (Final) impressions determine the physical requirements of the impression materials used. This holds true for both Removable prosthodontics and Fixed Prosthodontics but also applies to the field of Orthognathic surgery and Orthodontics…….. Objectives for final Impression pg 128 Vol 2 Entire sulcus Existing dentition Anatomical morphology Duralay * Relining Impressions – Greenstick &ZnoEu The physical and chemical characteristics of an impression material determine both its application, handling properties and methods of disinfection as well as interactions with materials such as plasters used to create a positive replication of the oral cavity and its associated structures. In Removable Prosthodontics a Primary impression is there to provide a Negative record of entire denture bearing surface To define fully the perimeters of the sulcus and obtain certain superficial anatomical features See Primary impressions in alginate… Prof Owen Fundamentals of Removable partial dentures 2nd Ed pg Wits Pros Book Vol 2 Pg. 14 Study pages Vol 2 Primary impressions in alginate…

The Ideal Impression Material
Easy to mix and handle. Suitable working time. Suitable setting time. Compatible with die and stone Not toxic or allergenic to the patient. Dimensionally stable on setting. Accurate to record the fine details of the prepared tooth Has acceptable odor and taste. Adequate strength. Adequate shelf life. Must meet the following requirements ? What is a suitable setting time ? How does the environment affect the material hot days cold days humidity (think alginate storage) as well as where material stored

The Ideal Impression Material cont.
Economical Ready to disinfected without loss of accuracy. Fluid or plastic when inserted into the mouth. It must be an exact record of all the aspects of the prepared tooth and sufficient unprepared tooth structure immediately adjacent to margins, to allow the dentist and the technician to be certain of the location and configuration of the finish line. Other teeth and tissue surrounding the abutment tooth must be accurately reproduced to permit proper articulation of the cast and contouring of the restoration. It must be free from air bubbles especially in the finish line area. Why should it be fluid or plastic when placed into the mouth ?

High accuracy Biocompatibility
(very small contraction <0.5%) High dimensional stability Compatibility to stone High elastic recovery High tear strength Ease of use Long shelf life Hydrophilic Pleasant color &taste and Hydrophobic Proper setting time Cost

Elastic recovery The amount of rebound after a cylinder of material is strained 10% for 30 seconds. 98%

Hydrophilic VS Hydrophobic

Which one of the impression materials we choose?
What’s the job you do? primary impression final impression How accurate do you want? removable denture fixed prostheses What technique do you use?

IMPRESSION MATERIALS Key Properties
Accuracy a. Accuracy = ability to replicate the intraoral surface details. Dimensional Stability b. Dimensional Stability = ability to retain its absolute dimensional size over time. Tear Resistance c. Tear Resistance = ability to resist tearing in thin sections (such as through the feather-edged material within the gingival sulcus. Impression Impression Tray Indirect procedures in dental materials require that a cast or die be created that duplicates the geometry and dimensions of the oral tissues so that requisite laboratory work can be performed in the construction of a restoration. To capture the intraoral details, an impression material is used. These materials vary in composition and properties and are selected on the basis of relative need for (1) accuracy, (2) dimensional stability, and (3) elasticity. For example, if you are interested in establishing the occlusal relations of the maxilla and mandible, the requirement for accuracy is relatively low and alginate impressions are perfectly acceptable. However, for capturing the final details of the margin for a crown preparation, accuracy is very important and polyvinylsiloxane is normally used. Impression materials must have some strength, but generally their design is focused more on accuracy, dimensional stability, and flexibility (or tear resistance). [CLICK] When impression material comes into contact with soft and hard tissues (which are hydrophilic), it must adapt and flow over these surfaces to capture the fine details of the structure. This is referred to as “accuracy.” [CLICK] The impression must be “dimensionally stable” so that its specific dimensions are preserved until it can be poured to create a working cast. If it does distort, then the working dies will be the wrong size and/or geometry and produce an ill-fitting restoration. Only small amounts of misfit are tolerated and accommodated by the dental cement for attaching the prosthesis to tooth structure. [CLICK] [CLICK] Once an impression undergoes a hardening or setting reaction, it must be strong enough to resist tearing during removal (“tear resistance”).

What are tolerable limits for “error” in indirect procedures?
PROBLEM ANALYSIS What are tolerable limits for “error” in indirect procedures? a. Impressions = b. Casts, Dies = c. Waxing = d. Investing = e. Casting = f. Finishing, Polishing = g. Cementation = +/- 0 + 1.5%  1.5% Indirect procedures (inlays, onlays, crowns, bridges, etc.) require a number of steps in the capturing of intraoral information, fabrication of restorations in the laboratory, and cementation of the restorations in place. The steps are summarized in the figure above. The critical question is “how much error” can be tolerated in the procedure and still produce an acceptable result. The errors are summed from step-to-step and so the overall error can be significantly influenced by a large amount of error in any single step. If impressions are taken correctly and poured at the proper time then very little error is involved. [CLICK] Working casts and dies can be created with almost no error. [CLICK] Waxing is subject to large amounts of potential error due to the inherent instability with small temperature fluctuations and/or stress relaxation during cooling. [CLICK] However, this can be dramatically reduced if waxed patterns are quickly invested. [CLICK] Investment must expand to compensate for casting alloy shrinkage [CLICK] during solidification and cooling. These two steps involve the greatest amount of dimensional change overall. If it is well-matched, then together expansion and contraction will cancel each other out. [CLICK] Finishing and polishing generally involves very little production of error. [CLICK] Cementation accommodates for errors between the preparation size and the casting size. [CLICK] Generally the total error is expected to be less than 0.5%. [CLICK] If a crown preparation is assumed to be 8 mm (8000 µm) in width, then this amounts to 40 µm of error (or 20 µm per side). Prepped Tooth Width 8 mm (=8,000 m) 8,000 m x 0.5% = 40 m = 20 m/side Typical clinical error = >100 m/side

CLASSIFICATION SYSTEM
Based on Properties of Set Materials Properties: Reaction: Set: Rigid: 1. Impression Plaster Rigid Irrev (Chem) 2. Impression Compound Rigid Rev (Phys) Zinc Oxide/ Eugenol Rigid Irrev (Chem) Water-Based Gel: 4. Alginate (Irreversible Hydrocolloid) Flexible Irrev (Chem) 5. Agar-Agar (Reversible Hydrocolloid) Flexible Rev (Phys) The simplest method of classifying impression materials is by key properties: (A) rigid, [CLICK] (B) water-based, and [CLICK] (C) elastomeric. Of the rigid types, impression plaster was the first material used for both edentulous and dentulous impressions. However, since it is rigid on setting, it must be carefully fractured (at the cupids and occlusally) to produce pieces that can be removed and then glued back together to create a complete impression. Nowadays, impression plaster is only used for edentulous impressions without undercuts. Impression compound is used for single tooth impressions where there are no undercuts. Zinc oxide eugenol (ZOE) is used for edentulous impressions. Water-based systems include alginate (irreversible hydrocolloid) and agar-agar (reversible hydrocolloid). Both types of materials are inherently unstable because water is 85% of the composition. They are very easily distorted during syneresis (loss of water to the air or surrounding environment) or inbibition (absorption of water from the air). Alginate is generally wrapped in a wet paper towel to try to control these problems and poured quickly. However, it is still not accurate. It is only acceptable for low resolution applications. Elastomers are rubbery polymers that are capable of elastic deformation from undercut areas to produce a complete impression for dentate situations. There are four major types (polysulfide, silicone, polyether, and polyvinyl siloxane), BUT polyether (Pe) and polyvinyl siloxane (PVS) are most often used. Information on all four types will be reviewed but the primary focus will be on Pe and PVS. Elastomers: 6. Polysulfide (Rubber Base, Thiokol) Flexible Irrev (Chem) 7. Silicone (Conventional, Condensation) Flexible Irrev (Chem) 8. Polyether Flexible Irrev (Chem) 9. Polyvinyl Siloxane (Addition Silicone) Flexible Irrev (Chem)

Impression materials Plaster Nonelastic Zinc oxide Eugenol
Impression Compound Zinc oxide Eugenol Impression waxes Impression material Agar Reversible Impression materials are classified according to their setting behavior into Non Elastic Impression materials which include Plaster , Impression compound , ZnOxEugenol and Certain Waxes (we will discuss these individually later to include Aluminax, Mizzy Wax) Materials such as Duralay (an acrylic) can also be used to take impressions of cores and posts in Fixed prosthodontics as well as in bite registrations (however they do have their own advantages and significant disadvantages. Polysulfides Hydrocolloids Alginate Irreversible Elastic Polyethers Condensation silicone Non-aqueous elastomers Addition silicone

Nonelastic Gypsum (Plaster) Zinc oxide eugenol Impression wax
Type I Nonelastic Gypsum (Plaster) Impression Compound Zinc oxide eugenol Impression wax

Dental Plaster Dental Plaster • Type I – impression plaster
• Type II – model (laboratory) plaster (used for mounting casts) plaster is composed of the β form of calcium sulfate hemihydrate Crystals plaster is weaker than dental stone due to: 1.) porosity of the particles, requiring more water for a plaster mix 2.) irregular shapes of particles prevent them from fitting together tightly Dental Stone • Type III - dental stone (diagnostic casts) • Type IV - high strength dental stone (working models) • Type V - high-strength, high-expansion dental stone Stone is the α hemihydrate form Types of Gypsum Products What is Gypsum : Gypsum is the dihydrate form of calcium sulfate, CaSO4 . 2H2O, found in a compact mass in nature. Dental gypsum products are manufactured by driving off part of the water of the calcium sulfate dihydrate to form calcium sulfate hemihydrate. This process is referred to as calcination. • The gypsum components of these materials are identical chemically; differences in these materials is attributed to calcination. Stone is the α hemihydrate form Types of Gypsum Products dense, regularly shaped, relatively nonporous cuboidal crystal material - require less water, and are approx. 2.5 times stronger than plaster stone is widely used in making casts and moulds requiring high crushing strength and abrasion resistance high-strength stone: (Type IV) -harder cast material than dental stone -composed of modified α form of calcium hemihydrate crystals -crystals are slightly larger and more dense than those of dental stone -because of increased strength and resistance to abrasion, are used for inlay, and crown and bridge casts -may be referred to as die stones high-strength, high-expansion dental stone: (Type V) with high expansion, it is especially suited for polyether or polyvinyl impression materials Gypsum materials • Principal difference between plaster, stone, and highstrength stone is in the shape and form of the hemihydrate crystals. Crystals of dental stone and high-strength dental stone are more dense and regular in shape. This makes it possible to obtain the same consistency with less excess water with the stones than with plaster. If gypsum needs to be soaked in water (cast duplication), soaking should be done in water saturated with plaster slurry, only long enough to achieve desired degree of wetting (otherwise gypsum can dissolve). The greatest disadvantage of gypsum products is relatively poor resistance to abrasion. One way to improve abrasion resistance is by adding “gypsum hardeners” like colloidal silica or synthetic resin. Many properties are either inversely or directly related to 1.the W/P ratio: Directly proportional: manipulation and setting times Inversely related: strength and setting expansion Because of their lower water requirement, raw hemihydrate used to produce stones and die stones have a higher inherent setting expansion in normal mixes than plaster. This effect is masked by the additives used in their formulation.

Calcination Calcination H or other means
Mineral gypsum > Model plaster + Water (CaSO4 . 2H2O) Dental stone High-strength dental stone (CaSO4 . 1/2H2O) Reverse Reaction When calcium sulphate hemihydrate (dental plaster, stone, etc.) is mixed with water, the reverse reaction takes place, and the hemihydrate is converted back to the dihydrate: CaSO4 . 1/2H2O + 11/2H2O ---> CaSO4 . 2H2O cal/g mol While setting, growth and subsequent interlocking of gypsum crystals occur. Interlocking contributes to strength and dimensional change of the gypsum. Physical and mechanical properties of the gypsum mass can be influenced by manipulative procedures that influence the difference in solubility and growth of the dihydrate crystals.

Elastomeric Impression Materials
A material that is used when an extremely accurate impression is essential. The term elastomeric means having elastic or rubberlike qualities.

Elastic Agar Alginate Hydrocolloids
A material that is used when an extremely accurate impression is essential. The term elastomeric means having elastic or rubberlike qualities.

Hydrocolloids Reversible and Irreversible
Introduced by Sears 1939 First elastic Sears AW. Hydrocolloid impression technique for inlays and fixed bridges. Dent Digest 1937; 43: Lin C, Zeiber G J. Accuracy of impression materials for complete arch fixed partial dentures. J Prosthet Dent 1988; 59: Philips Science of Dental Materials 11th Ed. Part 2 Pg. 231 Hydrocolloid impression materials Hydro means water. Colloid means gelatin substance. Material used to obtain preliminary and final impressions. The two types of hydrocolloids used in dental impressions are agar and alginate. Agar is a reversible hydrocolloid because it can pass repeatedly between highly viscous gel and low viscosity sol simply through heating and cooling. However, alginate once converted to the gel form cannot be converted back into the sol, and is therefore said to be irreversible hydrocolloid material. Agar and alginate may be used independently or in combination to record crown impressions. Agar was first introduced into dentistry for recording crown impressions in 1937 by Sears1 and was the first elastic impression material available. It is not commonly used in dental practice today however, because of the need for expensive conditioning baths and water cooled trays. Alginate, unlike agar, does not require any special equipment. Being easy to use and inexpensive it is popular for less critical applications Eg. Apposing casts and study models. Alginate and agar produce impressions with reasonable surface detail. They are both relatively hydrophilic and are not displaced from wet surfaces as easily as the elastomers2. However, in respect of recording crown preparations these materials have two major disadvantages. Firstly, very poor dimensional stability because of the ready loss or imbibition of water on standing in dry or wet environments respectively. Secondly, low tear resistance which can be a real problem when attempting to record the gingival sulcus. Some work supports the use of combined reversible and irreversible hydrocolloid impression systems.3,4 These systems are used in a way similar to the putty-wash technique for silicone rubbers described later in this article, with the agar injected around the preparation to capture surface detail and the more viscous alginate in the impression tray. The advantages of this combination system compared with agar or alginate used individually is the minimisation of equipment required to record an agar impression (no water cooled tray is needed) and the fact that agar is more compatible with gypsum model materials than alginate. It is also relatively cheap in comparison to many synthetic elastomers. Lin et al.5 demonstrated that the accuracy of this combination system is better than either the reversible or irreversible materials used separately and is comparable to that of polysulphide impression materials. However, the problems of low tear resistance and poor dimensional stability still apply resulting in the need for impressions to be cast up immediately. For these reasons, most practitioners tend to reject the hydrocolloids in favour of the synthetic elastomers to produce accurate and stable impressions.

Irreversible Hydrocolloid
Material that cannot return to a solution state after it becomes a gel. Alginate is the irreversible hydrocolloid most widely used for preliminary impressions.

Makeup of Alginate Potassium alginate (Alginic Acid) (12-15%)
Comes from seaweed; is also used in foods such as ice cream as a thickening agent. Calcium sulfate (8-12%) Reacts with the potassium alginate to form the gel. Trisodium phosphate Added to slow down the reaction time for mixing.

Makeup of Alginate- cont’d
Diatomaceous earth (70%) A filler that adds bulk to the material. Controls the stiffness of the set gel Zinc oxide Adds bulk to the material. Potassium titanium fluoride (~10%) Added so as not to interfere with the setting and surface strength. Sodium Phosphate (retarder) (2%) Coloring and flavouring agents (traces)

Physical Phases of Alginate
The first phase is a sol (as in solution). In the sol phase, the material is in a liquid or semiliquid form. (sol: resembles a solution, but is made up of colloidal particles dispersed in a liquid) The second phase is a gel. In the gel phase, the material is semisolid, similar to a gelatin dessert. “gel” entangled framework of solid colloidal particles in which liquid is trapped in the interstices and held by capillary forces (Jello) There are 2 main reactions occur when powder reacts with water during setting: • First rxn. provides adequate working time: (Retarder) 2Na3PO4 + 3CaSO4 ----> Ca3(PO4)2 + 3Na2SO4 After the sodium phosphate has reacted, remaining calcium sulfate reacts with sodium alginate to form an insoluble calcium alginate, which forms a gel with the water: H2O Na alginate + CaSO > Ca alginate + Na2SO4 (powder) (gel) Normal set alginate Working time of 2 minutes and a setting time of up to 41/2 minutes after mixing. Fast set alginate Working time of 11/4 minutes and a setting time of 1 to 2 minutes. Def : Working time The time allowed for mixing the alginate, loading the tray, and positioning the tray in the patient's mouth. Setting time The time required for the chemical action to be completed.

Packaging and Storing of Alginate
Containers about the size of a coffee can are the most commonly used form of packaging. Premeasured packages are more expensive, but save time by eliminating the need for measurement of the powder. Shelf life of alginate is approximately 1 year.

Fig. 46-7 Examples of packaging for alginate.

Causes for Distortion and Dimensional Change of Alginate
If an alginate impression is stored in water or in a very wet paper towel, the alginate will absorb additional water and expand. This condition is called imbibition. If an alginate impression remains in the open air, moisture will evaporate from the material, causing it to shrink and distort. This condition is called syneresis.

ADA Specifications <3% deformation with a 10% strain

Altering the Setting Times of Alginate
Cooler water can increase the setting time if additional time is needed for the procedure. Warmer water can reduce or shorten the setting time of the procedure.

Water-to-Powder Ratio
An adult mandibular impression generally requires two scoops of powder and two measures of water. An adult maxillary impression generally requires three scoops of powder and three measures of water.

Fig. 46-8 Scoop and water measure for alginate.

Taking an Alginate Impression
Explain the procedure to the patient: The material will feel cold, there is no unpleasant taste, and the material will set quickly. Breathe deeply through your nose to help you relax and be more comfortable. Use hand signals to communicate any discomfort. Mouth Preparation Rinse and dry the patient's teeth If teeth are too dry, alginate will stick

An Acceptable Alginate Impression
The impression tray is centered over the central and lateral incisors. There is a complete "peripheral roll," which includes all of the vestibular areas. The tray is not "overseated," which would result in exposure of areas of the impression tray. The impression is free from tears or voids. There is sharp anatomic detail of all teeth and soft tissues. The retromolar area, lingual frenum, tongue space, and mylohyoid ridge are reproduced in the mandibular impression. The hard palate and tuberosities are recorded in the maxillary impression.

Trouble Shooting Inadequate working or setting time: Distortion:
temperature of the water, incomplete spatulation W/P too low improper storage of alginate powder Distortion: Tray movement during gelation or removed from mouth prematurely weight of tray compressing or distorting alginate impression not poured up immediately Tearing: removing impression from mouth before adequately set thin mixes (high W/P ratio) presence of undercuts (blocking out these areas before an impression may help) inadequate amount of impression material in tray (avoided by minimum 3 mm of impression material between tray and oral tissues) • Loss of detail: removed from mouth prematurely

• Poor stone surface (of cast)
Consistency: preset mix is too thin or thick The W/P ratio is incorrect (avoid by fluffing powder before measuring; do not overfill powder dispenser) inadequate mixing (avoided by vigorous spatulation and mixing for recommended time) using hot water: grainy and prematurely thick mix • Dimensional change: delay in pouring alginate impression stored in air: results in distorted, undersized cast due to alginate impressions losing water when stored in air Porosity: whipping air into the mix during spatulation (proper mixing: after initial wetting of powder by the water, mix alginate so as to squeeze the material between the spatula blade and the side of the rubber bowl) • Poor stone surface (of cast) set gypsum remaining in contact with the alginate for too long a period of time

Reversible Hydrocolloid
An impression material that changes its physical state from a sol to a gel and then back to a sol.

Chemical Makeup of Reversible Hydrocolloid
85% water 13% agar Agar is an organic substance derived from seaweed. Additional chemical modifiers are added to aid in the handling characteristics.

Conditioning Bath for Reversible Hydrocolloid
Three Compartments The first bath is for liquefying the semisolid material. A special water bath called a “hydrocolloid conditioner” at 212° F liquefies the material. After liquefying, the preset thermostat cools the temperature to 150° F automatically. The second bath becomes a storage bath that cools the material, readying it for the impression. At this temperature, the tubes are waiting for use. A third bath is kept at 110° F/44° C for tempering the material after it has been placed in the tray.

Types of Reversible Hydrocolloid
Tray material Packaged in plastic tubes. Each tube has enough material to fill a full arch, water-cooled tray. Syringe material Packaged in plastic or glass cartridges that fit a syringe or in preloaded syringe or preformed sticks that refill special hydrocolloid inlay syringes.

Application of Reversible Hydrocolloid Impression Material
A stock water-cooled tray is selected, making sure that the tray does not impinge on any of the teeth or soft tissue. Plastic stops are placed in the tray. Tubing is connected to the tray and to the water outlet for drainage. The material is liquefied and moved to the storage bath. The light-bodied material is placed in the syringe, and heavy-bodied material is placed in the tray. The light-bodied material is expressed around the prepared tooth, and the dentist seats the tray.

Advantages: Easily manipulated Suitable setting time Hydrophilic – can be used in contact with saliva Low Cost Disadvantages Low tear strength Not accurate – incapable to produce fine details Dimensionally unstable due to synersis and imbibition Should be poured immediately ?

Elastic Polysulfides Polyethers Non-aqueous elastomers Condensation
silicone Addition silicone

For Elastic Impression Materials
GENERAL FORMULATION For Elastic Impression Materials 1. Flexible Matrix (Continuous Phase): a. Multifunctional Pre-Polymer or Polymer b. Crosslinking Agent c. Curing Agent (Catalyst or Initiator) d. Modifiers (Accelerators, Retarders, Plasticizers, Flavoring Agents, Colorants) 2. Filler or Extender (Dispersed Phase): All “elastic impression materials” have the same general type of formulation. They all have a flexible matrix (the continuous phase) that is filled with extender or filler (the dispersed phase) to minimize the effects of polymerization shrinkage during setting on the overall accuracy and dimensional stability of the system. [CLICK] All setting reactions generate polymerization shrinkage that is approximately 1-4%. [CLICK] The addition of extensive fillers (50-70% by weight) [CLICK] decreases this to tolerable levels of about 0.2 to 0.4%. [CLICK] Most of the original liquid impression material is a high-molecular weight monomer or pre-polymer that will become reacted and crosslinked with a second moiety. Curing agents are included. Modifiers are used to accelerate the setting reaction, counteract potential side reactions, increase the flexibility, control the taste to the patient, and suitably colorize the material for identification and/or distinction from other materials. Fillers are chosen to be compatible with the resin component (hydrophilic or hydrophobic) but also be as cheap as possible since they are not required for reinforcement. Most elastomeric impression materials have a month shelf-life that is extended by storing the materials in the refrigerator. Be careful to check the expiry date so that the material does not needlessly grow too old and have to be discarded. Polymer (high shrinkage) Filled Polymer (low shrinkage)

ELASTOMERIC IMPRESSIONS
Management of Shrinkage Use 2-step techniques: (a) Light-Heavy Body, OR (b) Wash-Putty Impression Impression Tray Load tray with heavy-bodied IM Cover prep with light-bodied IM from syringe During setting shrinkage, distortion of impression is toward tray. To minimize any effects of shrinkage, almost all impression materials are supplied as a combination of (1) high viscosity and low viscosity paste-paste materials, or (2) putty-wash option. Usually they are different colors. One of the clinical tricks for minimizing the effect of polymerization shrinkage on impression distortion is to use a low viscosity version (more shrinkage due to less filler) [CLICK] to wet the surfaces of the preparation or areas of interest [CLICK] and then use high viscosity material (low shrinkage but poor flow) [CLICK] to make up most of the bulk of the rest of the impression. [CLICK] Another feature of most impressions [CLICK] is that the shrinking impression material tends to be distorted toward the impression tray. This ultimately results in an impression that is slightly oversized. [CLICK] Impression trays may be rigid (metal or custom acrylic) or flexible (plastic). Elastomeric impressions are generally used with either a custom-made acrylic resin tray or a plastic tray with some undercuts or retention holes. To insure a uniform lock of the impression material to the tray, a tray adhesive is often used. [CLICK] It behaves like rubber cement and works best in a thin well-dried layer. [CLICK] The heavy and light-bodied impression material are made of the same polymer matrix and so are well-cured together during the setting reaction. [CLICK] Shrinkage occurs toward the restrained surface (tray) and away from the unrestrained (tooth) surface. [CLICK] Impression Dies tend to be oversize and so the casting is oversize.

ELASTOMERIC MATERIALS
Mixing and Delivery Systems Polyether, Polyvinylsiloxane 2x2x2x2x2x2x2x2 = 256 folds Polysulfide Rubber, Silicone Rubber Elastomeric impression materials are 2-component chemically setting materials that are traditionally delivered as two tubes of pastes. The pastes can be mixed together with (1) mixing pads (manually), (2) auto-mixing guns (by manually expressing the two pastes through a Kenics mixing tip), or (3) mixing equipment (electrically driven pumps for proportioning of materials from larger tubes through a Kenics mixing tip). The type of mixing technology depends on the rheological behavior of the mixture. [CLICK] Materials that are Newtonian or dilatant are actually hard to mix through a mixing tip. [CLICK] Silicone and polysulfide elastomers are examples of those situations, respectively. [CLICK] Therefore, they must be mixed manually on a mixing pad. Materials that are shear-thinning (pseudo-plastic) [CLICK] benefit from high shear in a mixing tip on an auto-mix gun or auto-mix equipment. [CLICK] Polyether and polyvinyl siloxane are examples of materials that are shear-thinning. [CLICK] A typicial Kenics mixing tip is shown above dissembled to reveal a series of connected mixing paddles (alpha-helixes with alternating rotations) that split and fold the mixing stream to produce mixing. A mixing tip with 8 paddles will split and fold the stream of two components 256 times (2 x 2 x 2 x 2 x 2 x 2 x 2 x 2 = 2 to the eighth power = 256). [CLICK] The entire mixing event occurs in less than 1 second. Material that is left in the mixing tip will set and so the tip must be discarded. However, before the next mixing event, the tip functions as a cap on the dispensing gun. Mixing Options: 2 Pastes on Mixing Pad 2 Pastes in Mixing Gun 2 Pastes in Mixing Machine

Management of Distortion During Tray Removal
IMPRESSION MATERIALS Management of Distortion During Tray Removal Strain rate sensitive elastomers ! Impression Impression Tray VERY FAST (SNAP) Fast removal RAPID removal of an elastomeric impression material leads to the least chance for plastic deformation if the procedure involves rapid elastic deformation. Remove the impression with a SNAP. [CLICK] This creates a rapid loading rate. Remember that all polymers are very STRAIN RATE SENSITIVE. [CLICK] If the stress is applied quickly, then all the energy is stored elastically and the material will not undergo plastic deformation. This is particularly important in the region of the greatest deformations (e.g., the margins) [CLICK] Fast removal rates [CLICK] guaranteet that loads (see the blue dotted line) only generate elastic deformations. Slow removal rates [CLICK] would actually cause plastic deformation and distort the impression in critical areas. Slow removal

Distortion Time Related to Setting Reaction
IMPRESSION MATERIALS Distortion Time Related to Setting Reaction VPS Pe PS, Silicone 100 log TIME (minutes) CONVERSION (%) REACTION You can follow the reaction conversion (see the vertical axis) versus time or the log of time (look at the horizontal axis) of the impression material. Once the reaction is complete [CLICK] the polymerization shrinkage should be complete as well. You will recall that those materials that undergo stepwise polymerizations (polysulfide, silicone, and polyether) are relatively slow. After the impression is removed, these materials are continuing to react, shrink, and thus distort themselves. It is advantageous to pour the impression as soon as is clinically practical. [CLICK] Polyvinylsiloxane is fast because it is a chain reaction polymerization. [CLICK] Once the impression is clinically set (which corresponds to about 50% or greater reaction conversion), [CLICK] the impression tray can be removed with a SNAP. This procedure helps to force the material to behave elastically. However, some time is required for complete elastic recovery. The rule of thumb is that minutes should be permitted before pouring the impression. [CLICK] Thus, for some impression materials (polysulfide, silicone, polyether), you are balancing out the effect of time for elastic recovery and potential problems with continued shrinkage. Stick to the rule-of-thumb of waiting minutes. Polyvinylsiloxane, on the other hand, has completed its shrinkage and is stable over time.

Polysulfide Impression Material
Chemical makeup Base: Mercaptan polysulfide. Cross-linking agent: Sulfur and/or lead peroxide. Catalysts: Copper hydroxides, zinc peroxide, organic hydroperoxide. Fillers: Zinc sulfate, lithopone, or calcium sulfate dihydrate.

Manipulation and Technique Considerations for Polysulfide Material
Dispense pastes at the top of the mixing pad. Mix pastes with the tip of a spatula to incorporate the material first. Transfer the material to the fresh surface of the mixing pad. Water, saliva, and blood affect polysulfide material. Impression should be removed quickly after setting-do not rock the tray. Adhesive must be thin and dry before adding the impression material. Wait 20 to 30 minutes before pouring the impression for the stress relaxation to occur in the material. Be careful of glove powder contamination of the impression.

Polyether Impression Material
Chemical makeup Base: Polyether Cross-linking agent: Sulfate Catalysts: Glycol-based plasticizers Filler: Silica

Manipulation and Technique Considerations for Polyether Material
Material is very stiff, which makes it difficult to remove without rocking. When removing the impression, break the seal and rock slightly to prevent tearing. Water, saliva, and blood affect polyether material. Added moisture will increase the impression’s marginal discrepancy. Increased water absorption occurs if a thinning agent is used.

Silicone Impression Material
Chemical makeup Base: Poly dimethyl siloxane Cross-linking agent: Alkyl ortho silicate or organo hydrogen siloxane Catalyst: Organo tin compounds Filler: Silica

Manipulation and Technique Considerations for Silicone Material
The material has a limited shelf life. The tray requires a special tray adhesive. No syneresis or imbibition but does respond with shrinkage over time. The material is more flexible, so there is more chance for distortion during removal. Wait 20 to 30 minutes before pouring of models for stress relaxation to occur.

Polyvinyl Siloxane Impression Material
Chemical makeup Base: Silicone polym Catalyst: Chloroplatinic acid Filler: Silica

Manipulation and Technique Considerations for Polyvinyl Siloxane Material
For dimensional stability, this is the best impression material. Pouring of the model can be delayed up to 7 to 10 days. Stiffness of the material makes removal of the tray difficult. Material dispensed using auto-mixing unit and mixing tips.

Chemistry and Setting Reaction
POLYSULFIDE RUBBER Chemistry and Setting Reaction CONTINUOUS PHASE: Polymer = Mercaptan Functional Polysulfide Crosslinking Agent = Sulfur and/or Lead Peroxide Catalysts = PbO2 or Copper Hydroxides (Type I) Zinc Peroxide or Organic Hydroperoxide (Type II) DISPERSED PHASE: Fillers = TiO2 or Zinc Sulfate or Lithopone or Calcium Sulfate Dihydrate Polysulfide rubber (rubber base, mercaptan rubber, or Thiokol rubber) is the first elastomeric rubber to be developed for dentistry. Its origins were right after the second world war when natural rubber was still scarce. It is made predominantly at one plant in Mississippi that produces the material for a wide range of commercial applications, including Thiokol seals for the US Space Shuttle. Some of the noteworthy properties are that the material is brown, stains clothing easily, and produces a strong odor. While the material is cheap, it is not well-accepted by most patients. However, it has been used in dentistry for almost 50 years and still remains popular with the oldest clinicians. The general composition of polysulfide is a mercapto-functional polymer (see the chemical link within the brackets above) that is crosslinked with sulfur or oxygen from lead peroxide. [CLICK] The material is filled to reduce shrinkage using the least expensive fillers that are available at the time of production (TiO2 or zinc sulfate or lithopone or calcium sulfate dihydrate). The fillers do not matter much. Manufacturers may even change the filler from month-to-month. The reaction is shown in the figure above. Adjacent sulf-hydryl groups are condensed in the presence of oxygen to eliminate water (which stays inside of the polymer) and produces a sulf-hydryl bridge that connects the molecules. [CLICK] This is a stepwise reaction that is relatively slow, affected by heat, and exothermic. SETTING REACTION: Stepwise (relatively slow), Exothermic, Affected by temperature

Manipulation and Technique Considerations
POLYSULFIDE RUBBER Manipulation and Technique Considerations a. Two-step techniques recommended: Reduces air entrapment and surface tension effects. b. Material adversely affected by H2O, saliva, and blood. c.* Set impression should be removed quickly - do not rock tray. d. No syneresis or imbibition, but distortion due to continued reaction. e. Ideally need uniform thickness and at least 2 mm thick for accuracy. (1) Adhesive must be thin (2) Adhesive must be dry f. Paste-Paste Mixing Recommendations: (1) Dispense pastes at the top of the mixing pad (2) Mix pastes with tip of spatula only for 5 seconds (3) Transfer mass to fresh surface at center of mixing pad (4) Wipe spatula off with paper towel; Strop mass for 15s to constant color (5) Load syringe or tray (6) Use pad excess to monitor setting time Special notes on the techniques for using this material are reported above. Polysulfide is adversely affected by contact with water, saliva, and/or blood and should be carefully washed after removing the impression. It works the best in a custom acrylic impression tray utilizing a tray adhesive. Because it must be mixed manually and is difficult to mix, so a special procedure is recommended. [CLICK] The two pastes are dispensed in equal lengths [CLIKC] on a mixing pad and stirred together with the tip of a STIFF mixing spatula. The materials are collected to the center of the pad, stropped vigorously and then used to load the tray and/or light-bodied delivery syringe. [CLICK] It is important to wait minutes after tray removal for stress relaxation to occur. g. Pouring of models: (1) Wait minutes before pour for stress relaxation to occur (2) RB is non-reactive with model and die materials (3) Be careful of glove powder contamination of impression (4) RB can be electroplated

Chemistry and Setting Reaction
SILICONE RUBBER Chemistry and Setting Reaction CONTINUOUS PHASE: Polymer = Polydimethyl Siloxane Crosslinking Agent = Alkyl Orthosilicate or Organo H-Silane Catalysts = Organo Tin Compounds (e.g., tin octoate) (but not dibutyl tin dilaurate) Modifiers = Colorants, Flavorants DISPERSED PHASE: Fillers = Silica Silicone rubber was first employed as an impression material in the 1960s. At the time, clinicians were using polysulfide or reversible-hydrocolloid for most dental impressions requiring high accuracy. The polymer phase forms in a similar fashion to polysulfide rubber. Polydimethyl siloxane moieties are crosslinked by alkyl orthosilicate molecules. The reaction produces ethanol as a by-product [CLICK] but that does not seem to have detrimental effects on the properties. This type of silicone rubber is catalyzed with tin octoate. [CLICK] It is medical grade silicone rubber. Commercially available silicone rubber that you might buy in the hardware store is non-medical grade and utilizes dibutyl tin dilaurate catalyst that is extremely toxic. Several years ago there was a case of a man who used non-medical grade to repair the inside of his dishwasher and slowly poisoned he and his family as the material leached catalyst onto their dishes. Be very careful. [CLICK] Silica is the primary filler. [CLICK] This is a stepwise reaction that is relatively slow, affected by heat, and exothermic. SETTING REACTION: Stepwise (relatively slow), Exothermic, Affected by temperature H2O by-products

SILICONE RUBBER Manipulation and Technique Considerations a. Limited shelf-life: Unstable in tubes. b. Requires mechanical retention or special tray adhesives c. No syneresis or imbibition, but continued polymerization shrinkage. d. Better dimensional stability than RHC but more expensive e. Pouring of models: (1) More flexible so more chance for distortion during removal (2) Wait minutes before pour for stress relaxation to occur Special notes on the techniques for using this material are reported above. Silicone has a limited shelf-life and is best stored in the refrigerator prior to use. Silica fillers tend to settle out of the material over time. It is important to wait minutes after tray removal for stress relaxation to occur. These materials represented a substantial improvement over polysulfide rubber in terms of having no odor and generally not staining clothing. Other properties were approximately the same to polysulfide.

Chemistry and Setting Reactions
POLYETHER RUBBER Chemistry and Setting Reactions CONTINUOUS PHASE: Polymer = Amine-terminated Polyether Crosslinking Agent = Aromatic Sulfonate Catalysts = Modifiers = Colorants, Glycol Plasticizers, Flavorants DISPERSED PHASE: Fillers = Silica Polyether rubber was first produced in Europe in the late 1960s. The reaction is shown in the figure above. Amine-terminated polyether moieties are crosslinked in the presence of aromatic sulfonates. [CLICK] These materials are filled with silica to insure their stability. [CLICK] This is a stepwise reaction that is relatively slow, affected by heat, and exothermic. SETTING REACTION: Stepwise (relatively slow), Exothermic, Affected by temperature

POLYETHER RUBBER Manipulation and Technique Considerations a. Excellent impression accuracy and dimensional stability. b. Stiff and therefore difficult to remove without rocking. c. Break seal and rock slightly to prevent tearing: Low tear resistance. d. Negatively affected by H2O, saliva, and blood. (1) Since hydrophobic, moisture increases marginal discrepancy (2) Increased water absorption occurs if use thinning agents e. Can be dispensed from automated extruder and mixer (ESPE PentaMix) Special notes on the techniques for using this material are reported above. Polyether produces an excellent impression and has been very popular, especially in Europe. The material is very stiff. Typically, a plasticizer is provided as a modifier to reduce the stiffness. It is critical to release the hydrostatic seal with tissues before removal with a snap. Like polysulfide, this material is negatively affected by water, saliva, and/or blood and the impression should be quickly cleaned after removal. It is important to wait minutes after tray removal for stress relaxation to occur. Because the material is pseudoplastic, [CLICK] it can be easily mixed with a Kenics mixing tip using an automix gun or machine (like the ESPE PentaMix0.

Chemistry and Setting Reactions
POLYVINYL SILOXANE Chemistry and Setting Reactions CONTINUOUS PHASE: Polymer = Double-bond-functional Silicone Polymer Crosslinking Agent = Chloroplatinic Acid Catalysts = Modifiers = Colorants, Flavorants, Plasticizers DISPERSED PHASE: Fillers = Silica Polyvinylsiloxane (or PVS) is often called ‘viny polysiloxane’ (or VPS) as well. Since it is based on silicone chemistry, it has sometimes been referred to as an addition-silicone. [CLICK] These oligomers are double-bond-functional silicones which become polymerized by free radicals from chloroplatinic acid. The catalyst is consumed as it provides radicals and it generates hydrogen gas as a by-product. [CLICK] The mixture is filled with silica because that is the only thing with the right degree of hydrophilicity to be blended into the resin. The reaction is shown in the figure above. [CLICK] This is a chain reaction that is very fast, affected by heat, and exothermic. SETTING REACTION: Chain (very fast), Exothermic, Affected by temperature Hydrogen gas released by decomposition of crosslinking agent.

POLVINYLSILOXANE Manipulation and Technique Considerations a. BEST impression material for dimensional stability: Pouring should be delayed at least 4 hours for H2 out-gassing. Pouring can be delayed up to 7-to-10 days (or indefinitely). b. Stiffness makes removal difficult. c. Most material dispensed using auto-mixing gun and mixing tips Special notes on the techniques for using this material are reported above. The best feature of polyvinysiloxane is that the reaction is essentially complete at the time of impression removal so that it is not subject to reaction distortion. One still must wait minutes for elastic recovery to occur, but there is no distortion occurring during this waiting period. For all practical purposes, the impression does not need to be poured until it reaches the dental laboratory. Therefore, this is a favorite material for most clinicians. One of the side-effects of the reaction, is the decomposition of the chloroplatinic acid. This produces small amounts of hydrogen gas. If the impression tray is resting with the impression facing upward, then during the next 3-4 hours small gas bubbles escape from the surface. Do not pour the impression during this time or else the gas will collect at the boundary of the stone and create bubbles. Newer materials include hydrogen-getters (or scavengers) that catch the hydrogen before it can create bubbles at the surfaces.

COMMERCIAL PRODUCTS 1980-1995 Predominantly POLYSULFIDE and
SILICONE elastomers before 1995. Before 1996, all four types of elastomers were still prevalent in the US market. Costs of auto-mixing guns, mixing tips, and polymer for PVS and polyether restrained their use in dental practice. [CLICK] The largest volume of dental material used in any dental practice is impression material. This is a highly competitive market among dental manufacturers, and some companies such as ESPE have made most of their profits on this commodity. For a single procedure (e.g., crown preparation), taking one impression is equivalent in volume to about composite restorations. The cost for a single impression can vary from $12 to $18. There have even been calculations of the relative amount of material lost in the mixing tip as a consideration of the overall economics of these procedures.

COMMERCIAL PRODUCTS 1996-2004 Predominantly POLYETHER and
PVS elastomers after 1996. However, the economics shifted toward PVS and polyether systems as production costs became lower. By 1996, there was relatively rapid discontinuance of polysulfide and condensation silicone use in general practices. Clearly, PVS and Pe systems saved time and more easily produced good impressions. [CLICK] At the same time, dental manufacturers were attempting to produce hydrophilic impression materials. Additives to generate these modifications were much easier to blend with polyvinylsiloxanes. PVS (or VPS) impression material is now routinely used for just about all crown and bridge needs.

Comparison of Key Properties
IMPRESSION MATERIALS Comparison of Key Properties Poor > > Good 1. Accuracy (Reproduction of Detail): a. Wetting of Tissues: ALG < SIL, PS < PE <= RHC, PVS b. Wetting by Dental Stone: SIL, PVS < PS < PE <= ALG, RHC 2. Dimensional Stability: ALG <= RHC < PS, SIL < PE, PVS (Resistance to Distortion): a. Polymerization Shrinkage b. Thermal Shrinkage c. Loss of Components d. Stress Relaxation 3. Tear Resistance (Elasticity): ALG, RHC, PE < PVS , SIL < PS a. Elastic Deformation b. Strain Rate Sensitivity Recall the 3 principal properties of interest for impression materials – (1) accuracy, (2) dimensional stability, and (3) tear resistance. [CLICK] Associated with each of these important properties are several contributing factors that deserve special attention. Accuracy depends on not only the ability of the impression material to wet the surface of hydrophilic tissues, but also on the ability of dental stone to wet the subsequent impression. Dimensional stability is a function of several events that can increase or decrease the size of the impression – polymerization shrinkage on setting, thermal shrinkage once the impression is removed from the mouth, loss of volatile components generated by the setting reaction, and/or stress relaxation after setting. Tear resistance is improved if the elastomeric impression is more capable of elastic deformation without plastic deformation. Rankings of examples of water-based gels (RHC = reversible hydrocolloid; ALG = alginate) and elastomeric impression materials (PS = polysulfide, SIL = condensation silicone, PE = polyether, PVS = polyvinyl siloxane) are shown above. [CLICK] While PVS dominates in most categories, it is not excellent in all cases (e.g., wetting by dental stone). In a similar way, despite its relative disuse, PS is intermediate in ranking in many categories, and best in terms of tear resistance. [CLICK]

IMPRESSION IMPRESSIONS
Dimensional Stabilty PERCENT CHANGE (%) PRODUCT EXAMPLES The graph above is a summary of many different product examples in each of the 4 elastomer categories and it reports the percentage change or shrinkage of the materials [CLICK] at 30 minutes and also at 24 hours. [CLICK] Obviously, waiting longer times allows more potential shrinkage. The mean shrinkages for the 4 elastomer gropus [CLICK] is shown as well. The materials reported in this graph represent the market about 10 years ago. The actual shrinkages for polyether and polyvinylsiloxane are actually even smaller for today’s products. Everything we discussed comes down to making sure that the accuracy, dimensional stability, and tear resistance provide a good impression.

Stock tray Rim lock tray Perforated tray

Special tray (Custom tray)
Stock tray Special tray (Custom tray)

Nonelastic Plaster Zinc oxide eugenol Impression wax
Type I Nonelastic Plaster Impression Compound Zinc oxide eugenol Impression wax

Elastic Agar Alginate Polysulfides Polyethers Hydrocolloids
Non-aqueous elastomers Polyethers Condensation silicone Addition silicone

Elastic Agar Alginate Polysulfides Polyethers Hydrocolloids
Non-aqueous elastomers Condensation silicone Addition silicone

(Ideal Properties) High accuracy Biocompatibility
(very small contraction <0.5%) High dimensional stability Compatibility to stone High elastic recovery High tear strength Ease of use Long shelf life Hydrophilic Pleasant color &taste and Hydrophobic Proper setting time Cost

Impression plaster ADA type I gypsum product

Impression plaster For edentulous impression
Contraindication for undercut

Composition Potassium sulfate Reduce expansion
Calcium sulfate hemihydrate (CaSO4)2.H20 Potassium sulfate Reduce expansion Borax Reduce the rate of setting time Starch Help disintegration of the impression from plaster model

Reaction Calcium sulfate hemihydrate + water calcium sulfate dihydrate

Mechanical properties
Very low viscosity (mucostatic) Hydrophilic Good dimension stability 0.06% Great accuracy

Manipulation Easy to mix, trapping air bubbles
W/P ratio must be measured out carefully Use in special tray, thickness of mm. Setting time 2-3 minutes Must apply separating medium before pouring the impression Powder must be stored in air tight

Advantage Accuracy to soft tissue impression Good accuracy
Short setting time

Disadvantage Can not be used in undercut ridge
Able to flow to pharynges Heat due to reaction Sensation of dryness

Impression compound Type I Type II Impression compound
Green stick compound

Impression with compound
Type I Type II

Composition Natural resin 40% Waxes 7% Stearic acid 3%
Filler&inorganic pigment 50%

RESINS Amorphous organic substance which are insoluble in water
Make Thermoplastic Shellac, dammar, rosin or sandarac

WAXES Straight chain hydrocarbon CH3(CH2)nCH3
Tasteless, Odorless, Colorless Make Thermoplastic beeswax and colophany

Stearic acid Fillers Lubricant and Plasticizer Control degree of flow
Minimize shrinkage Improve rigidity Diatomaceous earth, Soapstone, talc

Thermoplastic C๐ Reversible physical process Very low thermal conductivity High thermal expansion High contraction coefficients % High viscosity

Manipulation Soften by heating over the flame or water bath
Do not heat too much volatile Take impression Room temperature water cooling Pour impression as soon as possible Warm the impression before take the cast

Alcohol lamp Alcohol torch Water bath

Advantage Can take impression again Compatible with model material

Disadvantage Very technique sensitive
High coefficient of thermal expansion => dimensional change Mucocompressive Must be poured within one hour Low detail reproduction

Troubleshooting Distortion -Material is not completely cooled
-Flexible tray -Delay in pouring

Troubleshooting Compound is too brittle or grainy
-Prolong immersion in the water bath

Zinc oxide-eugenol Impression paste

Zinc oxide-eugenol Wash impression Bite registration
For full arch edentulous impression without or minor undercut Wash impression Bite registration Temporary cementation Temporary filling

Composition (base) Zinc oxide (ZnO) Oil Hydrogenated rosin
Zinc acetate Accelerator Trace of water Initiator

Composition (reactor)
Eugenol 12~15% Oil Rosin Filler (kaolin) Non eugenol paste Carboxylic acids

Reaction ZnO + eugenol Zn eugenolate + ZnO(unreacted) (Solid)
(powder) (liquid) (Solid)

@ Very low viscosity @ Irreversible chemical change @ Low dimensional change 0.1%

Manipulation

Manipulation Mixed with stainless steel spatula in paper
Equal length of the two paste is extruded Mixed with stainless steel spatula in paper pad or glass slab Mixing time s Working time min shorten by increase Temperature&Humidity

Advantage Good dimensional stability Good surface detail reproduction
Accuracy of soft tissue impression (mucostatic) Good dimensional stability Good surface detail reproduction Inexpensive Adhere well to dental compound

Disadvantage Messiness Non elastic Time Unstable setting
May irritating to soft tissue

Troubleshooting Inadequate working or setting time
-excessive humidity and/or temperature Distortion -unstable tray

Impression wax KORECTA WAX , IOWA WAX

Manipulation Functional Impression

Disinfection of Impression Materials
Irreversible Hydrocolloids (Alginate) Diagnostic Casts: Soak 10 min in Gluteraldehyde Final Impressions: Dip in Gluteraldehyde ,rinse in sterile water, dip again let stand under damp gauze 10 min Spray with Sodium Hypochlorite rinse, spray again & let stand under damp gauze 10 min Reversible Hydrocolloid Addition –reaction Silicones In Gluteraldehyde 1 hr., Rinse sterile water Soak in fresh solution Gluteraldehyde 10 min Zinc oxide eugenol Soak in Gluteraldehyde 10 min Silicone Impressions Soak for 10 min Gluteraldehyde Polyether Impressions Fundamentals of Removable Partial Dentures 2nd Ed. CP Owen Special Considerations Ni coated trays not to be used with Gluteraldehyde Al trays not to be used with NaOCl

REFERENCES Skinner’s Science of Dental Materials
Dental Materials and Their Selection (William J. O’Brien) Introduction to dental materials (Richard Van Noort) Dr Sukontip Arwatchanakan Science of Dental Materials Phillips 11th Ed. Stephen C. Bayne Department of Operative Dentistry School of Dentistry University of North Carolina Mr. Robert Seghi Alginate Impression Materials (alginate lecture 2006.ppt)

Tray Selection Criteria
Feel comfortable to the patient. Extends slightly beyond the facial surfaces of the teeth. Extends approximately 2 to 3 mm beyond the third molar, retromolar, or tuberosity area of the arch. Is sufficiently deep to allow 2 to 3 mm of material between the tray and incisal or occlusal edges of the teeth.

Characteristics of Impression Trays
Quadrant tray Covers one half of the arch. Section tray Covers the anterior portion of the arch. Full arch tray Covers the entire arch. Perforated tray Holes in the tray create a mechanical lock to hold the material in place. Smooth tray Interior of the tray is painted or sprayed with an adhesive to hold the impression material.

Impression Trays Must be sufficiently rigid to:
Carry the impression material into the oral cavity. Hold the material in close proximity to the teeth. Avoid breaking during removal. Prevent warping of the completed impression.

Table 46-1 Types of Stock Trays

Fig. 46-3 Examples of quadrant, section, and full-arch impression trays.

Tray Adhesives VPS adhesives (blue) Rubber Base Adhesive (brown)
For polyvinyl siloxane and polyether impression materials. Rubber Base Adhesive (brown) Used with rubber base impression materials. Silicone Adhesive (orangish-pink) Used with silicone impression materials.

Characteristics or Elastomeric Impression Materials
Base Packaged as a paste in a tube, as a cartridge, or as putty in a jar. Catalyst Also known as the accelerator, is packaged as a paste in a tube, as a cartridge, or as a liquid in a bottle with a dropper top.

Forms of Elastomeric Materials
Light-bodied Also referred to as syringe type, or wash type. This material is used because of its ability to flow in and about the details of the prepared tooth. A special syringe, or extruder, is used to place the light-bodied material on and immediately around the prepared teeth.

Forms of Elastomeric Materials- cont’d
Regular and heavy-bodied Often referred to as tray‑type materials, they are much thicker. As the names imply, they are used to fill the tray. Their stiffness helps to force the light‑bodied material into close contact with the prepared teeth and surrounding tissues to ensure a more accurate impression of the details of a preparation.

Basic Impression Technique
The material selected depends upon the dentist’s preference and the type of impression required for the procedure. The dentist prepares the tooth or teeth for the impression. The light-bodied material is prepared and loaded into the syringe and transferred to the dentist. The dentist places the light-bodied material over and around the prepared teeth and onto the surrounding tissues. The heavy-bodied material is prepared and loaded into the tray and transferred to the dentist. When the impression material has reached final set, the impression is removed and inspected for accuracy. The impression is disinfected, placed in a biohazard bag, labeled, and readied for the laboratory technician.

Curing Stages of Elastomeric Materials
Initial set The first stage results in stiffening of the paste without the appearance of elastic properties. The material may be manipulated only during this first stage. Final set The second stage begins with the appearance of elasticity and proceeds through a gradual change to a solid rubberlike mass. The material must be in place in the mouth before the elastic properties of the final set begin to develop. Final cure The last stage occurs from 1 to 24 hours.

Types of Elastomeric Materials
Polysulfide Polyether Silicone Polyvinyl siloxane

Occlusal Registration
An accurate registration of the normal centric relationship of the maxillary and mandibular arches. Also commonly referred to as the bite registration.

Types of Occlusal Registration
Wax bite Useful when the diagnostic casts are trimmed. The most common technique is to use a softened baseplate wax.

Fig. 46-27 Wax bite registration.

Types of Occlusal Registration- cont’d
Polysiloxane bite registration paste: Supplied both as a paste system and also as cartridges. The material is fast setting. There is no resistance to biting forces. There is no odor or taste for the patient. It gains dimensional stability over time. It is convenient to use.

Types of Occlusal Registration- cont’d
Zinc oxide-eugenol (ZOE) bite registration paste ZOE paste has little to no resistance to bite closure and is a fast-setting material. Material is supplied in a paste system and dispensed onto a paper pad, mixed, and placed onto a gauze tray for the patient to bite into.

Impression Materials BDS 3

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