Impression materials, introduction, classification and requirementsASST PROFESSOR

Dr Mumtaz ul Islam B.Sc. BDS. MHR. M.Phil.

Impression

Negative likeness or copy in reverse of the surface or object Dental impression

An imprint of the teeth and adjacent structures for use in dentistry

Impression material Any substance or combination of substances used for making an impression

Impression technique A method and manner used in making a impression

Why impression

Procedures can not be performed in oral cavity (mouth) Model

A positive replica of any object

Dental model Accuracy of an appliance depends on model Accuracy of a model depends on its impression

How to take dental impression

Impression material transferred to patients mouth Material is soft initially it is loaded (carried) in to a tray Impression tray holds the soft material in position After setting of the material impression tray removed from the mouth Model prepared by dental plaster or stone

Classifications

Chemical type Alginates, silicones, polysulphides, etc

Properties of the materials either before or after setting Before setting property involved is viscosity

Tissue compression or displacement (mucocompressive)

Mucostatic initially very fluid

Classifications cont,

According to set material Rigidity & elasticity

Depends mainly on recording capability of an undercut

Any surface which restricts the insertion or removal of a material

Flexible to withdraw elastic in recovery

Elastic and non elastic

Classification according to chemical type and elastic properties

Stress

Plays important role in viscosity

Increase in stress produces fluidity in material (pseudoplastic)

Space also puts more pressure on the tissues and displace it

A close fitting impression tray

Classification according to viscosity

Requirements

Factors which effect the Accuracy

Dimensional stability

Manipulative variables Additional factors

Cost taste colour etc

Accuracy

Low viscosity Interaction with saliva and other liquids

Hydrophobic

Hydrophillic

Blow holes

Wetting

Contact angel and setting reaction

Angel of contact of an aqueous solution with any surface Low the angel more will be its wettability

Resulting model lacks fine details

Blow holes

During setting contraction occurs contraction towards tray

Results an expansion of impression and more space

overs sized die loose fitting crown

Over sized die

Setting expansion

During setting expansion occours Expansion away from tray (material expands)

Results an contraction of impression and less space

Under sized die

Fitting will be difficult and impossible

Temperature

Temperature of the mouth is 32° - 37° C Room temperature is 23° C Thermal contraction occurs Depending on the coefficient of thermal expansion (CTE) Impression material and tray both shows different directions and values Kept minimized through materials of same or less difference in CTE

Contraction of tray and impression

Adhesion of impression material with tray

Good adhesion is necessary Firm attachment if not present detachment occurs Only a partial detachment from the tray causes gross defect Defective impression ultimately gives a defective cast or die Resultant restoration or appliance will be ill or non fitting Tray adhesive where applicable is necessary otherwise perforated trays used

Tray adhesive

Perforated trays for complete dentures

Perforated trays for partial dentures

What is this assignment

Recording of undercut area

That area which restricts any object from withdrawal or insertion To record that area material must have

Good elasticity and tear resistance and

low rigidity which facilitates easy recovery

Compressive and tensile stresses are also produced during removal Different materials shows different results at recovery

Removal from undercut area

Recovery with different materials

Distortion

Degree of undercut Thickness of impression material Time elapsed in compressed state Viscoelastic properties of impression material

Ideal impression material for undercut

Elastic enough For viscoelastic material distortion can be minimized by early recovery Early recovery should be soon after setting Material should show a good tear strength to compensate tensile stress Should not tear even if in the form of thin sections

Impression of an undercut area

Dimension stability

Accuracy of impression material During insertion into the patient’s mouth, setting, and withdrawal

After taking the impression delay in model or cast making

Now accuracy depends upon its dimension stability

Impression should retain its original shape indefinitely

Factors affecting dimensional stability

Continuation of setting reaction beyond setting time This happens in viscoelastic and elastic materials

Allow sufficient time for full recovery

Delay before pouring the gypsum cast

Internal stresses Thermo plastic Impression materials like compound and waxes

Material tends to relieve internal stresses

Distortion occours

Factors affecting dimensional stability contd, Effects of volatile substances Materials may contain volatile substances as a main ingredient After setting it produces a volatile substance These easily evaporates and distort the impression Mainly distortion resulted by shrinkage For most of the materials except elastic and viscoelastics gypsum cast should be poured immediately

Manipulative variables

Dispensation Mixing

Mixing through proportioning Streak free mix

Contrast color material mixed on paper pads or glass slab

Powder liquid and paste liquid systems are difficult

Shows no setting at certain areas due to incomplete mixing

Setting starts as the material started to mix retarders delay setting

Manipulative variables Contd,

Non mixing Non mixing are thermoset material

Require heat for softening and re-hardens above mouth temperature

No induction time period immediately use

Working time Start of mixing till material becomes not manageable to use at room temperature

Setting time After mixing to initial setting

Manipulative variables Contd,

Setting often goes beyond its prescribed time Wiser approach is to leave impression in the mouth for some extra time

Especially for elastic materials

Convenient material for patient and operator shows Long working time

Short setting time

Chemically setting impression materials are convenient

Viscosity and time

Clinical considerations

Analysis of the problem, consideration of requirements, available material and choice Undercut area present elastomeric material

Rigid materials can be used at soft tissue undercut areas

No undercut area rigid material

Impression plaster is exception it can be used for undercut area Due to its reassembling after fracture property

Both materials are available with accuracy for both conditions Selection depends on cast and accuracy more accuracy increase the cast

Impression trays

Impression tray give rigid support to the impression material Facilitate introduction into the mouth Custom made

Made on casts obtained from patients impression with low accuracy material in stock tray

Rigid enough with a uniform space well extended to record the impression

Stock Metallic and plastic

Available in variety of shapes to provide best fit

Under or over extended which can be corrected by addition of thermoplastic material or cutting

Impression trays Contd,

Perforated, non perforated and rimlock, rimlock trays may include in both Perforated by means of holes, wires interlocks at the base Non perforated through contact adhesives needs drying before use Trays with water cooling system for reversible hydrocollides

Stock trays can also be used

Plastic trays when used with high viscosity material may be problematic

Tissue management

A complete impression not only contain the teeth but the soft tissue around it For removable appliances achieved by careful impression or syringing

Produces a complete impression which is impossible on the other hand

Fixed appliances requires fine details of gingival margins or beneath Impression under the gingival margin area requires gingival displacement

Three means are available for displacement

Troughing

Gingival displacement through any mean Packing a retraction card Electrosurgical cauterization Soft tissue Laser

Retraction cord

Suture material, braided, knitted cords custom made thin copper filament Wide range of diameter impregnated with hemostatic agents

Epinephrine, aluminum and ferric chloride etc.

Epinephrine containing cords should be used with great care contraindicated for cardiac patients

Acidic hemostatic agents can cause dissolution of enamel induce sensitivity

Packing of retraction cord

Main object is to displace the gingival tissue laterally not apically Flat thin plastic instrument or a custom designed cord packer Cord must be packed laterally

The controversy

How many cords should be used single or multiple At base hemostatic cord and above expanding cord is used Outer cord is removed the fine one left in crevice Not more than 20 minutes a cord should be packed On removal dry cord should not be removed Cords may induce gingival recession

Electro cauterization and soft laser

Widen the gingival crevice also produce hemostasis Careful technique is essential for both If properly performed no recession occurs For cauterization a fine single wire ensures less damage Marginal deficiencies and overgrowths can be corrected

Technique for impression

Variable for each material Moisture control and cleanliness is essential Salivary control usually attained through cotton rolls and absorbent pads (dry guards) Rubber dam can also be used for isolation of teeth

Contra indicated with addition silicones

Plasticizer reacts with platinum catalyst and acts as retarder

Placement of material

Material should be placed on occlusal surface of teeth if present Smeared or syringed around teeth and loaded in the tray This will prevent air entrapment on occlusal surface of teeth Entrapped air produces bubbles on the occlusal surface of the tooth Cast will give occlusal inaccuracy This problem exists in teeth having steep cuspal angel and tortuous fissures

Consistency of material

Material inserted in the tray and mouth in a flow able state Retained till setting Tray should be kept still as possible Prevent distortion of the impression Manufacturers recommendations should be followed Elastomeric materials if removed before setting results in distorted impression

Points to remember

Material in close contact with lips, tongue and mucosa sets rapidly Recovery of impression must be delayed to avoid differential setting A thorough examination of teeth and impression is necessary Adhesion of material with tray must be checked especially in the molar area Extended material beyond the distal ends of tray should be trimmed

Cross infection control

Disinfection is a mandatory protocol for all procedures Impression sent to lab or received from lab must be disinfected Impression must be washed with running water to remove debris blood and saliva Immersion in 1:5 or 1:10 house hold bleach solution for 10 minutes

Ratios of 5 and 10 depends upon the concentration of original solution

Inactivate HIV and HBV without any distortion

Disinfectants

Aerosol disinfectants not reliable Gluteraldehydes and sodium dichloroisocyanates toxic and needs to replace daily Sodiumperoxymonosulphate 2% for 10 minutes except in reversible hydrocolloides Impressions of patients with contagious diseases like AIDS, HCV and HBV

Needs sterilization rather disinfection

Immersion in 2% gluteraldehyde solution for 10 hours or in phenolic buffer 6.75 hours Silicone rubbers are the only impression material kept immersed

Guidelines

For patients with contagious diseases Multiple models should be prepared

Especially where multiple insertions of appliance required

Where applicable autoclave sterilization is the safest method Gypsum casts can not be sterilized so multiple casts should be prepared Updated guidelines on cross infection control must be followed