Dr. Medhavi Sharma 1.pdf - mgumst

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Veneer preparation kit

Depth cuttersLVS 1-.3MMLVS 2- .5MM

Two grit diamond burs

Torpedo bur

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12 fluted tungsten carbide bur 30- micron round end tapered finishing bur

¾-inch fine garnet disc Enhance point used for final finishing

Completed preparation

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Preoperative

Postoperative

Preoperative Tooth preparation

Before polishing After polishing

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Preoperative Tooth preparation

Case 1

Case 2

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Severe defects in anterior region Gingival level adjusted with preparation

Copings on the models

FABRICATION OF CROWNS

Dentin is reduced such that mammalon structures become visible

Internal characteristics are applied

Layered with incisal material

Layered crowns on the model

Crowns bonded to the teeth using resin cement

Close up view of the inserted crowns

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• DR.MEDHAVI SHARMA

• DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS

Introduction

The dental profession has used precision-typecastings for the most part of century. Using thelost wax technique, Taggart in 1907developed a process for making castings usedin restoration of prepared teeth. Since theprocess did not take into account all thedimensional changes associated with thecasting technique, the resultant product didexhibit some problems in terms of dimensionalaccuracy.

Definition

Casting is a fabrication process whereby a totally moltenmetal is poured into a mold cavity having a desired shape ;upon solidification, the metal assumes the shape of themold.

Classification of Dental Casting Alloys

Alloy Type Total noble metal contentHigh noble metal contains≥40 wt% Au and ≥60 wt% of

the noble metal elements (Au+Ir+Os+Pt+Rh+Ru )

Noble metal Contains ≥25 wt% of the noble metal

elements

Predominantly base metal Contains <25 wt% of the noble metal elements

Journal of American Dental Association (1984)

Classification of Alloys for All Metal Restorations , Metal Ceramic Restorations, and Frameworks for Removable Partial Dentures

Alloy Type All Metal Metal Ceramic Removable Partial Dentures

High Noble Au-Ag-Cu-Pd

Metal Ceramic alloys

Au-Pt-Pd

Au-Pd-Ag(5-12 wt% Ag)

Au-Pd (no Ag)

Au-Ag-Cu-Pd

Noble Ag-Pd-Au-Cu

Ag-Pd

Metal Ceramic alloys

Pd-Au (no Ag)

Pd-Au-Ag

Pd-Ag

Pd-Cu

Pd-Co

Pd-Ga-Ag

Ag-Pd-Au-Cu

Ag-Pd

Base Metal Pure Ti

Ti-Al-V

Ni-Cr-Mo-Be

Ni-Cr-Mo

Co-Cr-W

Al bronze

Pure Ti

Ti-Al-V

Ni-Cr-Mo-Be

Ni-Cr-Mo

Co-Cr-W

Pure Ti

Ti-Al-V

Ni-Cr-Mo-Be

Ni-Cr-Mo

Co-Cr-W

Phillips’; Science of dental material ; 10th edition

Classification Of Traditional Casting Alloys

TYPE HARDNESS USE

I Soft Single Surfaces

II Medium Inlays/Onlays

III Hard Crowns/Bridges

IV Extra Bridges & partial dentures

JADA, Leinfelder ;1997;128;37-45

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Casting Shrinkage

Most metals & alloys, including gold & noble metalalloys, shrink when they change from the liquid to thesolid state.Shrinkage occurs in 3 stages:

• The thermal contraction of the liquid metal betweenthe temperature to which it is heated and the liquidustemperature.

• The contraction of metal inherent in its change fromliquid to the solid state.

• The thermal contraction of the solid metal that occursdown the room temperature.


COMPENSATION FOR SHRINKAGE

• Setting or Hygroscopic expansion of investment material: Hygroscopic low heat technique

• Thermal expansion of investment: High heat thermal expansion technique

Hygroscopic low heat technique

Obtains its compensation expansion from 3 sources:1. The 37˚C water bath expands the wax pattern

2. The warm water entering the investment mold from the top adds some hygroscopic expansion

3. The thermal expansion at 500˚C provides the needed thermal expansion.

Because of the potential for reduced venting, back pressure porosity is a greater hazard in this technique than the high heat technique, since the investments generally employed with this technique may be more dense.

High heat thermal Expansion

• Obtain its compensation expansion almost entirely by burn-out.

• Additional expansion results from the slight heating of gypsum investments on setting, thus expanding the wax pattern, & the water entering the investment from the wet liner, which adds a small amount of hygroscopic expansion to the normal setting expansion.

INVESTMENT MATERIALS

◼ Gypsum Bonded Investments

◼ Phosphate Bonded Investments

◼ Ethyl Silicate Bonded Investments


Gypsum Based Investments

The gypsum based investments have traditionally been used for casting of gold alloy inlays, onlays, crowns and fixed partial dentures.Main Constituents:- a - hemihydrate of gypsum and quartz which serves as a binder and gives strength to the investment.Gypsum shrinks at a temperature range between 200 and 400˚C, it slightly expands between 400 and 700˚C and than it undergoes significant shrinkage beyond 700˚C. Because of this property, the gypsum should not be heated beyond 700˚C. In order to compensate for this shrinkage (which could cause the pattern void to expand) the pure gypsum moulds are significantly undersized.Silica is added to provide a refractory component. If silica is added to the investment, this shrinkage can be reduced or even turned into expansion.


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Phosphate Bonded Investments

The spread of use of phosphate based investment is caused byan increase in use of metal ceramic prosthesis, which requirehigher melting temperatures than gold alloys.

The investment consists also of binders and refractory filler,which are the same as for the gypsum based investments.However, the binder in this case is magnesium oxide and amonoammonium phosphate. Carbon is also often added to theinvestment in order to help to produce a clean casting and toencourage easier divesting of the casting from the mould.Differently to the gypsum based investments, these investmentin practice do not show signs of setting shrinkage, but rather ofslight expansion.


Ethyl Silicate Bonded Investments

• Used in construction of high-fusing base metal partial denture alloys.

• Binder is Silica gel which reverts to silica on heating.

• This type of investment can be heated to 1090˚C -1180˚C & is compatible with higher

fusing alloys.

CASTING- LOST WAX PROCEDURE

The process involves producing a metal casting using a refractory mould made from a wax replica pattern.The steps involved in the process or the lost wax casting are:

· Create a wax pattern of the missing tooth / rim· Sprue the wax pattern· Invest the wax pattern· Eliminate the wax pattern by burning it (inside the furnace or

in hot water) .This will create a mould.· Force molten metal into the mould - casting.· Clean the cast.· Remove sprue from the cast· Finish and polish the casting on the die.

:SPRUE:

A sprue is the channel in a refractory investment mold through which molten metal flows.

Figure 1 – Spherical reservoir on the vertical sprue (left), indirect sprue with the horizontal reservoir bar

Sprue

Lost - Head(Reservoir)

For preventing cavities and

porosities, due to thermal contraction

LINER:Investment materials have a property of expanding on setting.If the investment was confined on the outside this could cause shrinkage of the of the mould / wax pattern.

In order to prevent this effect a flexible split ring or a rubber ring could be used.

However, the most commonly used technique is to line the casting ring with either an aluminosilicate liner or a cellulose liner.

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The molten mass

Hot zoneCrucible(melting pot)

Casting mold

CASTING THE DENTAL ALLOY

CASTING MACHINES

1. Centrifugal Casting Machine

2. Electrical Resistance-Heated Casting Machine

3. Induction Melting Machine

Melting Noble Metal Alloys:

There are several methods for melting of alloys, which are combined with the casting

methods and appropriate casting machines.:

· Torch melting,

· Electrical melting.

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Cleaning the cast:

. The dark surface of the casting can be removed by a process called pickling – heating the discoloured casting in an acid until the discoloration vanishes.

. Polishing of the casting is the final in its preparation. Rubber, rag or felt wheels impregnated with abrasivesare used in the initial phase of this stage. Final polishing is achieved using various oxides of tin and aluminium used in conjunction with a small rag or chamois buffing wheel, followed with an iron oxide rouge.

CASTING DEFECTS

Defects in casting can be classified as:

1. DISTORTION

2. SURFACE ROUGHNESS & IRREGULARITIES

3. POROSITY

4. DISCOLOURATION

5. INCOMPLETE CASTING OR MISSING DETAIL

Distortion:

Causes-• Distortion of wax pattern• Due to uneven movement of the walls of wax pattern

when investment is setting. The gingival margins are forced apart by the mold expansion, whereas the solid occlusal bar of wax resists expansion during the early stage of setting.

Solutions-• Proper manipulation of wax & handling of the

pattern.

Poor Surface finish:

Causes-• Air bubbles on the pattern• Water films causing ridges & veins on the surface.• Too rapid heating resulting in fins or spines• Underheating causing incomplete elimination of wax• Inappropriate water/powder ratio• Prolonged heating• Temperature of alloy too high• Casting pressure too high• Foreign bodies• Impact of molten alloy• Pattern position

Solutions-• Use of Vacuum Investing Technique• Vibrate before & after mixing• Use a wetting agent to reduce surface tension of wax pattern.• Air dry the wetting agent as excess water will dilute

investment, causing irregularities• The mold should be heated gradually• Heat the ring for sufficient period of time so that the

carbonaceous residue is removed• Water/powder ratio should be accurate• Gypsum bonded investments should never be heated above

700˚C

• Alloys submitted to vacuum casting showed decreased surface roughness, compared to alloys submitted to acetylene- oxygen flame casting.

Journal of Prosthetic dentistry, 2004, vol 92, 274-277

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Porosity:

Porosity of the casting can be spread within the casting and on its surface. The surface porosity adds to surface roughness, but can also be a sing of the internal porosity. The internal porosity could weaken the casting, may cause discoloration if spread to the surface and in extreme cases could lead to a leakage.

Classification of porosities (According to Phillips’):

Porosities

Solidification

DefectsTrapped gases Residual Air

Localized

ShrinkageMicroporosity

Pinhole

Porosity

Gas

Inclusions

Subsurface

Porosity


Localized Shrinkage Porosity

Causes:Generally caused by incomplete feeding of molten metal during solidification.If the sprue is not properly designed and implemented then it may solidify before the feeding is complete thus preventing a continuous supply of molten alloy. This type of defect usually occurs close to the sprue-casting junction.

Solutions:Ryge et al recommended the use of a sprue diameter larger than the thickest cross section of the casting.

Journal of dental rest.; Jan 1981; vol 60; no1; page 59-67

Phillips’; science of dental materials; 10th edition

Macroporosity:Causes-

• Primarily a shrinkage porosity• The portion of a cylindrical casting which solidifies

last is the low pressure side of the liquid metal close to the free surface of the button, therefore macroporosity always appears in this portion of casting.

Solution-• Reduce by providing a reservoir contiguous or close

to the low pressure end of the liquid metal• It may be affected by the closeness of the individual

units in a multiple unit casting.


Journal of dental restoration; Jan 1981;vol 60; no. 1;page 59-67

HOT SPOT:-

Localized shrinkage porosity may result from the formation of HOT SPOT when metal impinges on the mold surface so that here the metal remains molten while it solidifies everywhere else. This hot spot causes the local region to freeze last and result in SUCK BACK POROSITY.

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Solutions:

▪ Flare the point of sprue attachment

▪ Proper placement of sprue

▪ Do not use excessively long sprue

▪ Use a reservoir

▪ Reduce the mold-melt temperature differential (i.e. lower the casting temperature by about 30˚C).

Microporosity:

Microporosity is also caused by solidification shrinkage, but generally happens in fine grain alloys when the solidification is too rapid for the microvoids to segregate. This in turn is caused the mould or casting temperature being too low.

Trapped Gases

1. Pinhole Porosity

2. Gas Inclusion Porosity

• Both these porosities are related to the entrapment of gas during solidification.

• Both are characterized by a spherical contour.

• Gas inclusion porosities are much higher than pinhole porosities.

SUBSURFACE POROSITYCause-

Simultaneous nucleation of solid grains and gas bubbles at the first moment that the metal freezes at the mold walls

Solution-• Diminished by controlling the rate at which the molten metal

enters the mold • Ryge et al reported that subsurface & microporosities are

reduced by the use of a sufficiently long sprue & a melt temperature of 100˚C-150˚C above the melting point of alloy.

• According to Leinfelder et al reported that long sprues are preferable for thick castings while short sprues are beneficial for thin castings i.e. the sprue design should be related to the surface area-to-volume ratio of castings.


ENTRAPPED AIR POROSITY

• Referred to as Back Pressure porosity.

• Can produce large concave depression.

Causes-

Inability of the air in the mold to escape through the pores in the investment or by the pressure gradient that displaces the air pocket toward the end of the investment via the molten sprue & button.


Phillips’; science of dental materials; 10th edition

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Solution-• Proper burnout• An adequate mold & casting temperature.• A sufficiently high casting pressure• Proper liquid : powder ratio.• The thickness of investment between the tip of the pattern and

the end of the ring should not be greater than 6mm.• Srickland & Sturdevant suggested that sprue design variables

have no effect on these porosities; venting, high melt temperatures, proper positioning of the pattern in the ring, choice of investment & the use of reservoir can assist in reducing or eliminating the back pressure porosities.


• Phillips’; science of dental materials; 10th edition

INCOMPLETE CASTINGS

Causes-• Inadequate spruing ( sprue former too small)• Alloy not hot enough• Incomplete elimination of wax residues from the mold• Mold too cold• Ingate obstructed• Insufficient casting force

Solutions-

• Use proper size of sprue former

• Casting temperature should be higher than the liquidus temperature of alloy

• Ensure that no debris blocks the ingate

• Use adequate amount of force for casting

DISCOLOURATIONCauses-• Sulphur contamination of casting causing black

castings• Contamination with copper during pickling• Contamination with mercurySolution-• Avoid overheating of investment • Avoid use of torch flame as it contains sulphur• Avoid use of steel tongs to hold casting during

pickling• Castings should never be placed with amalgam dies

or kept on a table where amalgam scrap is present

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References

• Phillips’; Science of dental materials; 10th edition.

• Journal of American Dental Association, Leinfelder ;1997

• Journal of American Dental Association: 1984

• Journal of Dental Restoration; Jan 1981; vol 60

• Journal of American Dental Association; 1993

• Journal of Prosthetic Dentistry; 1989

• Journal of Prosthetic Dentistry; 2004

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CLASS—II CAVITY PREPARATION FOR

AMALGAM

.DR. MEDHAVI SHARMA

DEPT. OF CONSERVATIVE DENTISTRY & ENDODONTICS

Definition:

Cavity in the proximal surfaces of Bicuspids and Molars :

CLASS—II CAVITY PREPARATION FOR

AMALGAM

Indications and contraindications:

Incidence----When the caries incidence is high,

amalgam restoration is chosen over more

expensive gold restorations

2. Extent of proximal caries –Amalgam is the

material of choice for treatment of small proximal

lesions because cavity preparation should be

conservative, so as to have extended service

life for the restoration.

3. Age of the patient—Amalgam is most appropriate in

both young and elderly patients who are susceptible

to recurrent caries.

4 Esthetics—Since amalgam is metallic restorative

material,some patients object to the metallic

appearance especially when mesio-facial wall of the

cavity is over extended for premolars and1st

molars.

5 Economics—Amalgam is most economical.

1Facial and lingual walls

2 Axial wall

3 Gingival seat

4 Isthmus

5 Reverse curve

6 Line angles and point angles

7 Relieving contacts

8 Beveling at gingival margins

9 Roundening axio pulpal line angle

PREPARATION FEATURES:

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1. Facial and lingual walls: The walls are flared

mesio-distally as well as occlusogingivally. This

wall design provides mechanical undercuts for

retention and also provides contact clearance.

Development of walls in this manner to have

broad gingival seat preserves the marginal ridge,

resulting in narrow isthmus.

1. Axial wall : It is the internal wall that is parallel to

the long axis of the tooth. Axial wall is always

placed in the dentine for the following reasons:

a) To utilize resiliency and elasticity of dentine

[resistance form]

b) To obtain bulk for the restoration [resistance form]

c) To improve visibility and access [convenience form]

d) For the placement of retentive locks [retention form]

Axial wall has to be straight or convex or it should follow

the external contour of the tooth. But it should never be

concave.

Length of the axial wall is 0.4 to 0.6 mm, i.e. from AP line

angle to the Axiogingival line angle.

Gingival Seat :

It is the external cavity wall of proximal preparation,

which is perpendicular to the long axis of the tooth.

Gingival seat is extended either beyond the contact area

or to uncover the entire proximal carious legion

whichever is greater. This helps in clearing the gingival

margin with the next tooth.

For the amalgam restoration, gingival seat has to be placed

supragingivally, to maintain the health of interdental papilla.

Gingival seat is made flat so that masticatory forces are

distributed equally.

Width of gingival seat is 0.6 to 0.8 mm (0.6 mm into dentin)

for PM and 0.8 to 1 mm for molars.

Ideally gingival seat should consist of 2/3 of dentine & 1/3rd

of enamel . Gingival seat is prepared by using inverted cone

bur.

Isthmus :

Junction of occlusal and proximal box in a Cl-II cavity

preparation is termed as isthmus.

Ideal width for isthmus is 1/4th of the intercuspal

distance. As the width increases more, then the life of

the restoration reduces.

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Reverse curve :

Viewed occlusogingivally, the direction of mesio facial

enamel wall is parallel and the direction of enamel rods

thus creating a ‘S’ shaped curve in the proximal outline.

This is called as reverse curve.

• to provide butt joint of the cavity margins.

• to increase bulk for restoration.

• to preserve triangular ridges of adjacent cusps.

Line angles and point angles :

There are eleven line and six point angles in a Class-II cavity.

line Angles Point Angles.

Occlusal part 5 Occlusal part 2

Proximal part 5 Proximal part 2

At the junction 1/11 At the junction 2/6

Relieving Contacts :

Purpose :

• to place the proximal margins in a self cleansing areas.

(Facial,lingual and gingival)_

• to maintain a butt joint at cavosurface margin.

• to maintain access and visibility.

• to maintain bulk for restoration.

• for easy placement of matrix bond.

Beveling :

GMT is used to establish a slight cavosurface level of 20

degrees declining gingivally. This is done to match the

gingival seat at the margin with the direction enamel

rods, since enamel rods are placed in slightly apical

direction. This procedure removes unsupported enamel

rods at the gingival margins.

Roundening Axiopulpal line angles :

The same GMT is need for making the AP Line angle

rounded. This increases the bulk and therefore strength of

the restoration at the junction of occlusal and proximal

portion. Moreover this rounded junction will induce less

stress concentrations from occlusal forces thus

preventing # of the restoration at the junction.

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Cavity preparation :-

Armamentarium :-- Basic Instruments – mouth mirror, explorer,

cotton plices etc.- Hand Instruments – Excavators, enamel hatchet,

monangle or binangle, chisels, GMT.- Rotary Instruments – Burs No. ¼ , ½, 1, 2

carbide burs 55, 56, 57.- Ultra speed and conventional speed contrangle

handpiece.- Safety glasses.

Dr. Medhavi Sharma 1.pdf - mgumst

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