IMPRESSION MATERIALS &

PROCEDURES FOR R.P.D

Presented byJean Michael

Final Year Part 2

INTRODUCTION• An impression is defined as a negative likeness

of the teeth and/or edentulous areas where the teeth have been removed, made in a plastic material which becomes relatively hard while in contact with these tissues

CLASSIFICATION OF IMPRESSION MATERIALS

RIGID MATERIALS

Plaster of ParisMetallic Oxide Paste

THERMOPLASTIC MATERIALS

Modeling plasticImpression Waxes & Natural Resins

ELASTIC MATERIALS

Reversible HydrocolloidsIrreversible hydrocolloidsMercaptan Rubber-base materialsPolyether Impression MaterialSilicone Impression Materials

Condensation SIMAddition SIM

IRREVERSIBLE HYDROCOLLOIDS(ALGINATE)

Indicated for diagnostic casts, orthodontic treatment casts & master casts for R.P.D.Can be used in presence of salivaHydrophilicPleasant taste and smellNontoxic, nonstaining & inexpensive

Can be disinfected with 2% GluteraldehydeShould be stored in 100% moisture & poured within 1 HourLow tear strengthSurface details - less than elastomeric impression materialsDimensional stability – less than elastomeric impression materials

POLYSULFIDE IMPRESSION MATERIALS

High tear strengthLong working and setting time (8 to 10 minutes)Can be disinfectedCast poured will have smoother texture & will be harder as they do not retard or etch the surface of the setting stoneShould have a uniform thickness that does not exceed 3mm

Medium and heavy body should not be used in case of large/multiple undercutsLong term dimensional stability is poor due to water loss after settingShould be held still during the impression making procedureAllow to rebound for 7 to 15 minutes after removal from mouth and pour immediatelyUnpleasant odor & Stains clothes

POLYETHER IMPRESSION MATERIALS

Good surface detailsHydrophilic – good wettability for easy cast formingShorter working and setting timeFlow characteristics and flow - lowest among othersStiffness – cast breakage of while removal from tray

Unpleasant tasteAbsorbs waterCannot be immersed in disinfecting solutionsPour within 2 hours for better results

CONDENSATION SILICONES

Moderate working time (5 to 7 minutes)Pleasant odorGood tear strengthExcellent recovery from deformationCan be disinfected with disinfecting solutions without any alternation in accuracyHydrophobicIdeally pored within 1 hour

ADDITION SILICONES

Most accurate among elastic impression materialsLow polymerization shrinkage & distortionFast recovery from distortionGood tear strengthWorking time – 3 to 5 minutes

Both hydrophilic & hydrophobic forms are availableAvailable in automixing devices Pouring can be delayed up to 1 weekStable in sterilizing solutionsSulfur in latex gloves – retards the setting reaction

IMPRESSION OF PARTIALLY EDENTULOUS ARCH

Elastic impression materials are used for making impression of partially edentulous archThis is due to the presence of undercuts in the partially edentulous mouth

MATERIALS AVAILABLE FOR MAKING IMPRESSION

Reversible hydrocolloids (agar-agar)Irreversible hydrocolloids (Alginate)Elastomeric impression materials

STEPS IN IMPRESSION MAKINGPosition of patient & dentistTray selectionMixing the material & loading into the trayImpression making & removalInspecting, cleaning & disinfecting the impression

POSITION OF PATIENT & DENTISTDentist should stand & patient should sit uprightOcclusal plane should be parallel to the floor

MAXILLARY IMPRESSION- dentist should stand at the right rear of the patientMANDIBULAR IMPRESSION- dentist should stand at the right front of the patient

IMPRESSION TRAY SELECTIONStock trays for dentulous & partially edentulous arches are of 3 types:

Rimlock traysPerforated metal traysPlastic disposable trays

CHECKING MAXILLARY TRAY SIZEThere should be a clearance of 5-7mm between the inner flanges of the tray & facial surface of teeth & edentulous ridgeTray should cover the desired anatomic areasToo large a tray may be difficult to insert & may interfere with the coronoid process of mandible

CHECKING MANDIBULAR TRAY SIZEThere should be a clearance of 5-7mm between the tray and tooth surface and ridgeIf the tray extends too far in the lingually, there is a tendency to trap the tongue or floor of the mouth.Tray is held in the right handLeft thumb & index fingers are used to manipulate the right corner of the mouth

As the right flange of tray is rotated toward mouth, depress the lower lip & stretch the right corner of mouth with the left thumb & index finger

EXTENDING AN IMPRESSION TRAYSome times impression tray of adequate width may not cover the desired impression areaIn such cases, the tray is lengthened using modeling wax

MIXING IMPRESSION MATERIAL

LOADING IMPRESSION TRAYPlace impression material in small amounts.Tray should be filled in level with the flangesOverfilling should be avoided

Mandibular Impression TechniqueInject some material over occlusal surface of teeth, into vestibular areas & alveolo-lingual sulcusThen tray is rotated into mouth & is carefully seatedThe patient is asked to keep the tip of tongue in contact with the upper surface of tray during gelationMaintain the position of tray by placing the forefinger of each hand on top of tray on premolar area & thumb under patient’s chin

Maxillary Impression TechniqueInject alginate into occlusal surface & vestibular areas & wipe some amount on the palateTray must be centered & properly aligned & verify the position by looking at the patient’s face from aboveIt should protrude straight from the center of the mouth.After this, the tray is seated by using fingers of both hands over the premolar areas & stabilize the tray

Removal of Impression From MouthClinically the initial set of alginate is determined by loss of surface tackinessRelease seal by retracting lips & cheekThen impression is removed by a sudden jerk

INSPECT THE IMPRESSION FOR DEFECTS

CLEAN & DISINFECT THE IMPRESSION

Preparation Of Custom Tray

Marking the outline on the cast

Wax spacer adaptation

Self Cure Acrylic

Apply self cure acrylic over wax spacer

Attaching the Handle and polishing

Wax spacer scraped and tray perforated

Secondary Impression• Same as that for diagnostic impression.• In this procedure paint or inject impression

material in critical areas:Rest preparationHard palatePeripheral extensions

SPECIAL IMPRESSION PROCEDURES

Anatomic and Functional Form of Ridge

Anatomical form of Ridge

The anatomic form is the surface contour of the ridge when it is not supporting an occlusal load

Functional form of Ridge

The functional form of the residual ridge is the surface contour of the ridge when it is supporting a functional load

SPECIAL IMPRESSION PROCEDURES1. Physiologic or functional impression technique

Functional Relining methodMc Lean’s and Hindel’s methodsFluid Wax method

2. Selected Pressure technique

Mc LEAN’S PHYSIOLOGIC IMPRESSIONProcedure

A custom impression tray is constructed over a preliminary castFunctional impression of distal extension ridge is made. Patient applies some biting force with occlusion rimsThen an Alginate impression is made with the 1st impression held in it’s functional position with finger pressure

HINDEL’S MODIFICATIONMain difference of this with Mc Lean’s is that impression of edentulous ridge is not made under pressure but is an anatomic impression made at rest with ZOE paste.As the hydrocolloid impression was being made finger pressure was applied through holes in the tray to the anatomic impression.

Disadvantages of these methodsConstantly compressed residual ridge is prone to excessive bone resorption.If the clasp do not hold the partial denture, the denture will be pushed slightly occlusally by the tissue causing premature contacts (TISSUE REBOUND)

FUNCTIONAL RELINING METHODHere a new surface is added into the inner, or tissue side of the denture baseThe partial denture is made from a cast made from impression made with alginateA space is provided by adapting a metal spacer over the ridge on the cast before processing the denture base.A functional impression of the edentulous area is made using the cast partial denture framework.

The patient must maintain the mouth in a partially opened position Border moulding is carried out.Then a low fusing modeling plastic/green stick compound is allowed to flow over the tissue side of the denture base.It is tempered in water bath & seated in patient’s mouth.

To provide space for the impression material, modeling plastic is scraped to a depth of 1mmThe modeling plastic serves a s a tray material for the secondary impression material The final impression is made with a Zinc Oxide Eugenol impression pasteIf undercuts are present, light bodied rubber based impression materials can be used

AdvantagesThe amount of soft tissue displacement is controlled by the amount of relief given to the modeling plastic before final impression is madeGreater the relief, the less will be the tissue displacement.Tissue surface of metal frame work can be relined after insertion

DisadvantageSince open mouth technique is used it is difficult to maintain the previous occlusal contact

FLUID WAX FUNCTIONAL IMPRESSION• Make an anatomic impression of the arch using

alginate• Fabricate a refractory cast using this impression• Fabricate the partial denture framework over the

refractory cast

Draw the outline of the denture baseCast is coated with separating mediumWax Spacer is adapted over the crest of the edentulous ridge

Framework is placed over the spacerAuto-polymerizing resin is mixed to dough stage and is adapted and contoured over the framework along the length of the ridgeBorders of the tray are trimmed

Impression ProcedureWax is softened at 51 to 54 � �Softened wax is painted on the tissue surface with a brushWax is painted in excess near the border to record the sulcusTray is seated and held in position It takes at least 5 minutes for the wax to set

The tray is removed and the impression is examinedThe wax surface that has contacted soft tissue appears glossy and the other areas that has not contacted the tissues will appear dullThe impression should be placed in the mouth finally for 12 minutes

SELECTIVED PRESSURE IMPRESSIONMore force are applied to areas that can absorb stress without adverse response & protect that areas that is least able to absorb forceStress bearing areas are the buccal shelf area & the lingual slopes of residual ridge stress bearing areasThe denture base made from this impression will be closely adapted to & in firm contact with the tissues in buccal shelf area

Custom Trays

The tissue surface if the tray is trimmed with burs to provide adequate relief

Impression material is loaded on the prepared special tray and inserted into the patient’s mouthImpression is made with the patient with his mouth open under finger pressureOnly the stress bearing areas will be compressed during impression making

Materials used for Secondary ImpressionZinc Oxide Eugenol impression pasteRubber base material

Altering The Master CastThis procedure is done to obtain a ‘Hybrid Cast’ which records the edentulous areas in the functional form and the dentulous areas in the anatomic form

Conclusion• An accurate impression is vital for the success

of a cast partial denture. So proper selection of material, impression technique and the skill of the dentist plays a key role in the success of the overall treatment.

REFERENCEMc Cracken’s Removable Partial ProsthodonticsClinical Removable Partial Prosthodontics

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