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Impression Materials for Removable & Complete Dentures

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Impression Materials for Removable & Complete Dentures. Rola Shadid , BDS, MSc. Rigid Impression Materials. Are those that set to a rigid consistency It is capable of recording tooth and tissue details accurately , they cannot be removed from the mouth without fracture and reassembly . - PowerPoint PPT Presentation

Text of Impression Materials for Removable & Complete Dentures

IMPRESSION MATERIALS AND PROCEDURES FOR REMOVABLE PARTIAL DENTURES

Impression Materials for Removable & Complete DenturesRola Shadid, BDS, MSc

Rigid Impression MaterialsAre those that set to a rigid consistency

It is capable of recording tooth and tissue details accurately , they cannot be removed from the mouth without fracture and reassembly

Plaster of Paris Plaster of paris was once the only material available for partial denture impression, but now elastic materials have replaced them

used by many dentists to record maxillomandibular relationship

Metallic Oxide PasteMetallic oxide paste is some form of a zinc oxide eugenol combination

Not used as primary impression materials , and not used for impressions that include remaining natural teeth

Not used in stock trayMetallic Oxide PasteBorder molding with metallic oxide impression pastes is not advisable because wrinkles will occur if movement is permitted at the time of setting

All metallic oxides have a setting time during which they should not be disturbed and after which no further border molding is effective

Metallic Oxide PasteMixing time 30 to 45 secsSetting time 3 to 5 minsBrittle, messy, sometimes irritates patient due to eugenol contentMetallic Oxide Paste UsesSecondary impression for complete denture

Altered cast technique

Relining distal extension denture bases

Metallic Oxide Paste

Thermoplastic Impression MaterialAre those that become plastic at higher temperature and resume their original form when cooled

Modeling plasticIt is manufactured in several different colors, each color being an indication of the temperature range at which the material is plastic and workableIf modeling plastic is softened at a temperature above that intended by the manufacturer , the material becomes brittle and unpredictable , also there is the danger of burning patient's mouth

Modeling plastic Forms1. Red brown material in cake form 2. Red, gray, green modeling plastics in stick form

Modeling plastic Used in border molding of impression tray

The modeling plastic are used as a secondary impression material to record edentulous ridges in partial denture construction (building up the underside of the denture before recording the tissue with secondary impression material)

Red brown material in cake formThe most commonly used modeling plastic for corrected impressions of extension base areas for Kennedy Class I and II partial denture basesIt should be dipped and kneaded until soft and subjected to no more heat than necessary before loading the tray and positioning it in the mouth, then it should be chilled in cold water after each removal from the mouth

Red, gray, green modeling plastics in stick formUsed in border molding of impression tray

Red and gray sticks have a higher and broader working range than do the cake of like color

The modeling plastic are used as a secondary impression material to record edentulous ridges in partial denture construction, when this is done it is generally used only as a mean of building up the underside of the denture before recording the tissue with secondary impression material

Modeling Plastic (Compound)

Impression waxes and natural resinsThey are mouth temperature waxes

Iowa may be used as a secondary impression material or as impression material for relining

Since mouth temperature waxes have the ability to flow as long as they are inside mouth they permit equalization of pressure and prevent over displacement of tissuesImpression waxes and natural resinsImpression waxes may be used to correct borders of impressions made of more rigid materials (useful for correction of small imperfection in ZnOE impression)Other type of waxes have more resinous base , they are used to record tissues under an occlusal loadResinous waxes are not ordinarily used in partial denture impression technique except for secondary impression for recording partial lower free end

18Elastic materialsAre those that remain in an elastic or flexible state after they have set and have been removed from the mouth

19Hydrocolloids Reversible vs. irreversible hydrocolloidsAll hydrocolloids are dimensionally stable only during a brief period of time

They are hydrophilic materials so can gain water and expand (imbibition), so must not be immersed in water

They lose water rapidly and shrink if exposed to air

HydrocolloidsThe principal difference between reversible and irreversible hydrocolloids is that reversible hydrocolloid convert from the gel form to sol by application of heat , it may be revert to gel form while irreversible hydrocolloids become a gel via a chemical reaction and this change is irreversible

HydrocolloidsThey should be poured immediately no more than 15 minutes delay and stored during this period in a saturated atmosphere ( wrapping impression in a damp paper towel )

HydrocolloidsHydrocolloids exhibit a phenomenon known as syneresis , which is associated with the giving off a mucinous exudates that will affect the gypsum material and produce soft or chalky cast surface

This can be prevented by pouring the cast immediately and using some chemical accelerators as potassium sulfate *

All modern irreversible hydrocolloid impression materials have an accelerator incorporated into the powder and no longer need to be treated with a fixing solution.23Irreversible hydrocolloids (Alginate) *Used for making diagnostic casts, orthodontic treatment casts, master casts for removable partial denture

Mixing time: 1 minSetting time: 2 -4.5 minsFast set: 1-2 mins

Do not alter powder liquid ratio to change setting time

24Advantages of AlginatePour well with stone

Pleasant taste , odor , nontoxic , nonstaining and inexpensive

Disadvantages of AlginateLow tear strength

Provide less surface details than other materials

Not dimensionally stable

Disadvantages of AlginateGelation reaction takes place next to the tissues and any movement of the tray during setting will result in internal stresses that will cause impression distortion

It must be introduced to the mouth at approximately 70 F which result in an immediate increase in the viscosity and surface tension , air bubbles are harder to dispel and so more air will be trapped than in a reversible impression.Precautions to be Observed in theHandling of Hydrocolloid ImpressionsShould not be exposed to airShould not be immersed in water Mucinous exudate has a retarding effect on the chemical reaction of gypsum products *Should be protected from dehydration by placing it in a humid atmosphere or wrapping in a damp paper towelThis can be prevented by pouring the cast immediately or by first immersing the impression in a solution of accelerator, if an accelerator is not included in the formula.28Mercaptan rubber base impression materials (Polysulfide, Thiokol)It can be used for RPD impressions especially for altered cast technique or secondary impression

To be accurate the impression must have a uniform thickness that does not exceed 3mm

Non-Aqueous Elastomers:PolysulfidesCondensation SiliconesAddition Curing SiliconesPolyethers

29Mercaptan rubber baseMaterials that are highly cross_ linked (medium and heavy body ) do not recover well from deformation and should not be used in large multiple undercuts

The long term dimensional stability is poor because of water loss

It should be left for 7 to 15 minutes to rebound from deformation after it is removed from mouth

Mercaptan rubber baseHigh tear strength

Long working , setting time (8 to 10 minutes)

Because of long setting time they lend themselves better to border molding in adequate supporting trays more than hydrocolloids

Offensive odor, Water accelerates the setting reactionHeat accelerates the setting reaction

31Mercaptan rubber base

Mercaptan rubber base

Polyether impression materialsExcellent accuracy and wettingGood dimensional stabilityVery good shelf life (~ 5years, less in warm conditions)Thixotropic *

*Thixotropic (flow readily when stressed when the tray is seated) and cease to flow thereafter; so produces good surface detail of hard and soft tissue without drawing back into the throat and make them useful for border molding procedures

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Polyether impression materialsLow to moderate tear strengthFlexibility is the lowest of any of the elastic impression materials, this limits its use in RPD impressions Shorter working time than PVS siliconesStiff material may lead to breakage of cast on removal of cast from impressionUnpleasant taste Higher permanent deformation than polyvinyl siloxanes

Polyether impression materialsPolyether impression materials are not compatible with polyvinyl siloxane, so should not be used with polyvinyl siloxane custom tray

The material absorb water, must not be stored in water

Polyether impression materialsThe material should be poured within 2 hours; but if they are kept dry, acceptable casts can be poured within 7 days

Polyether impression materials

Silicone impression materialsThere are two types of silicone impression material :

Condensation silicones ( polysiloxanes ) Addition reaction silicones (polyvinyl siloxanes)

Condensation silicones ( polysiloxanes )

5 to 7 minutes working time that can be altered by adjusting the amount of acceleratorPleasant odorHigh tear strengthExcellent recovery from deformationShould be poured within 1-2 hours

Addition reaction silicones ( polyvinyl siloxanes ) Can be poured within up to 1 weekThixotropicSulfur in latex gloves and in f

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