John W. McLean

The Science and Art
of Dental Ceramics

Volume II:
Bridge Design and Laboratory Procedures in Dental Ceramics

The Science and Art
of Dental Ceramics

Volume II: Bridge Design and Laboratory
Procedures in Dental Ceramics

John W. McLean, O. B. E.
D.Sc., M.D.S. (University of London)
L.D.S. R.C.S. (England)

Consulting Professor
in Fixed Prosthodontics and Biomaterials

Louisiana State University Medical Centre School
of Dentistry

In collaboration with
John R. Hubbard and Michael I. Kedge

Quintessence Publishing Co., Inc. 1980
Chicago, Berlin, Rio de Janeiro, Tokyo

Second Printing, 1982

© 1980 by Quintessence Publishing Co., Inc., Chicago, Illinois.
All rights reserved.

This book or any part thereof must not be reproduced by any means or in any form without the written permission of the publisher.

Lithography: Industrie- und Presseklischee, Berlin.
Printing: Universal Printing Company, St. Louis
Binding: Becktold Company, St. Louis
Printed in the U.S.A.

ISBN 0 93138611X

Dr. John McLean has received wide recognition for his contribution to the practice of aesthetic dentistry. In 1963 he initiated a programme of research with the Warren Spring Government Laboratory in England to develop a stronger dental porcelain without sacrificing aesthetics. This programme led to the development of the aluminous porcelains, which have largely supplanted regular dental porcelain for the production of jacket crowns. For his services to dental research, he was made an Officer of the Order of the British Empire and awarded in 1977 the Schweitzer Award for research by the Greater New York Academy of Prosthetic Dentistry.

The second volume of “The Science and Art of Dental Ceramics” is a continuing effort to present in detail the finer points of ceramic art in dentistry. It is our intention at Louisiana State University School of Dentistry to use this work as a basis for a continuing education programme in dental ceramics which we hope will encourage younger men to specialize in this rapidly expanding field.

Jack Rayson, Dean

“Half the joy in life really consists
in the fight, not in the consequent success.”

Sir Barnes Wallis, C.B.E., F.R.S.
(1887-1979)

The first volume of “The Science and Art of Dental Ceramics” was written principally for the serious student or researcher in dental ceramics and attempted to penetrate in some depth the newer research in dental porcelains and metal-ceramics. This volume also covered the clinical use of dental porcelains both in relation to tooth preparation and its effect on stresses occurring in the ceramic restoration. The principles of colour in relation to porcelain veneer crowns was also given prominence and this section of the book is of particular value to the dental technician.

The second volume, on laboratory procedures in dental ceramics, although principally orientated towards the dental technician, also covers a number of fundamental aspects relating to bridge design which are of value to the practising dentist. An understanding of the building of occlusion in dental porcelain is also of equal importance to both dentist and technician. Although many dentists do not construct their own ceramic work, a sound working knowledge of the building of colour in depth in dental porcelain must enhance his rapport with the dental technician.

The building of ceramics with a brush and methods of obtaining colour and translucency are dealt with in detail. The importance of obtaining a natural enamel effect cannot be over-emphasised since much of the ceramic work today lacks the natural depth of translucency of human teeth. Ceramic crowns should be submerged in a sea of translucency if they are to be undetectable from their human counterparts, and colour must be seen in depth and not created by surface staining.

Metal-ceramic crowns are deservedly the most widely-used restorations in fixed prosthodontics. However, their increasing use has brought with it the problem of maintaining the health of the periodontium. The development of the correct contour in porcelain veneer crowns is given great prominence in this book together with methods of reducing the reflectivity of metal-opaque backgrounds. Only by careful study of tooth anatomy, design factors and colour control in metal-ceramic restorations can the technician and dentist avoid the artificiality of many porcelain veneer restorations. The “metal-ceramic smile” is now so frequently seen that it has become accepted by many of us without question. The high aesthetic standards set by the porcelain jacket crown have been forgotten and many laboratories have lost the facility for laying down a platinum matrix, a simple operation performed quickly by a trained apprentice. It is for this reason that a chapter is devoted to the complete porcelain veneer crown so that the technician has a standard from which to work. Only then can he recognise deficiencies in the metal-ceramic crown and improve his art.

The techniques described in this volume have been evolved over a period of eighteen years in our laboratory and have been found to be the most successful both from an aesthetic and economic point of view. Alternative systems which we do not use are only described briefly but it should be made clear that there are several roads to Rome and other ceramists may find alternative pathways to be better. For example, there are more expensive systems of constructing working casts and dies and ceramists may argue the case for the use of a spatula for building porcelain. The base-metal alloys have yet to become accepted as viable alternatives to the noble alloys, but are worthy of further investigation. Scientific data is still lacking in this area and most private laboratories in England continue to use the gold-platinum-palladium alloys.

The porcelains described in this volume are manufactured in Europe. There are comparable materials available in North America and the only reason for not mentioning them is that we have confined ourselves to describing materials which have been used by us over long periods and with which we are completely familiar. No one system or brand of porcelain can solve all our problems and it is for this reason that emphasis is placed on selecting a specific type of porcelain for each clinical case. It is hoped that the colour photographs show what we are trying to achieve in aesthetics and physiological contour.

Throughout the text, the new Système International d’Unités (SI) is used and, for convenience, a conversion table is inserted at the end of this volume.

The Harvard System of references has been chosen quite deliberately since it requires the insertion in the text of the name of the author quoted as a reference. The reader is then able to identify the person quickly and realise who had developed the original thought.

John W. McLean
38 Devonshire Street
London, W1N 1LD

The production of this book is, in part, the history of the development of alumina reinforced ceramics. During this period, the technicians I have worked with performed much of the pioneer work in developing the aluminous porcelains and the platinum bonded alumina crown. Without their help the profession would not have had access to these materials. In addition to Mick Kedge and John Hubbard, who have collaborated in producing this book, I am indebted to Richard Gale, John Seviour, Michael Kempton, Cliff Quince, Clive Everingham, Chris Newell, David Frost, Paul Smith and Gerry Litman. Their willing assistance has improved our understanding of the ceramic restoration.

I am greatly indebted to Dr. Jack Rayson, Dean of the School of Dentistry, Louisiana State University, for the assistance given to me in the School during the production of this book. In particular, I would like to thank Dr. Edmund Jeansonne and Dr. Howard Bruggers for their work on the platinum bonded alumina crown, and my partner, Mr. Denzil Little, for continued support in the clinical work.

My two colleagues, Mr. John Bunyan and Mr. Fred Harty, have given me considerable editorial help which has been invaluable.

Mrs. Kathleen Wilder has prepared all my manuscripts in her usual indefatigable style. Without her accuracy and speed this work would not have been possible.

My daughter, Diana, was responsible for all the line drawings and colour illustrations, and to her must go a special vote of thanks in interpreting my ideas.

Finally, I would like to thank Mr. Haase for agreeing to give me unlimited scope in publishing “The Science and Art of Dental Ceramics” in colour, and Mr. Peter Sielaff and the staff of the Quintessence Publishing Company for their meticulous work, particularly with the colour illustrations.

Cover

Table of Contents

Foreword

Preface

Acknowledgements

Structure

Reproducing Natural Teeth in Dental Porcelain

Colour and Translucency

Incisal Third

Middle Third

Cervical Third

Texture

Types of Porcelain

Types of Crowns

The Complete Porcelain Veneer Crown

1. The Felspathic Crown

2. The Aluminous Porcelain Crown

Metal-Reinforced Porcelain Crowns

The Cast Metal-Ceramic Crown

Enamels

Physical and Chemical Characteristics of Alloys Used for Ceramic Bonding

Methods of Attaching Porcelain to Metal

Molecular Bonding

Mechanical Bonding

Base-Metal Alloys

Electro-deposition of Metals

The Bonded Alumina Crown

Strength of the Porcelain Veneer Crowns

Selecting the Type of Porcelain Veneer Crown

Contra-indications for the Bonded Alumina Crown

References

Technical Considerations

Methods of Building and Condensing Porcelain

Types of Binders

Building Porcelain

Instruments

Additional Equipment

The Incremental Brush Technique

Recommended Method for Condensing Dentine and Enamel Porcelains

Vibratory Brush Technique

Condensing Opaque Porcelains

Condensing Aluminous Core Porcelain

Firing Dental Porcelain

Classification of Stages in Maturity

Aluminous Porcelain Special Precautions

Glazing and Devitrification

Thermal Shock

Lighting

Lighting for the Ceramic Furnace

The Furnace

Types of Furnace

Furnace Calibration

Dental Casts and Dies

Types of Die Material

Recommended Die Materials

Stone Dies

Metals

Recommended Use for Die Materials

Working Casts and Dies

The Working Cast with Removable Die

Dowel Pin System

Silver-plating Technique

Compatibility of Impression Materials

Compatibility of Tray Materials

Laboratory Procedures

The Artificial Stone Cast

Stone Die (first pour)

Pouring the Stone Dies

Pouring the Stone Base

Sectioning the Dies

Trimming the Dies

References

Reproducing Tooth Anatomy in Dental Porcelain

The Aluminous Porcelain Crown

Construction and Shade Matching

The Platinum Matrix

The Tinner’s Joint

Forming the Matrix

Construction Using Approximal Tinner’s Joint

Building the Core Porcelain for a Maxillary Central Incisor

Technique

Cervical Ditching

Vitadur N Core Porcelain – First Firing

Vitadur N Core Porcelain – Second Firing

Summary of the Firing Requirements for Core Porcelains

Application of Vitadur N Gingival and Body Porcelains Shade VitaA2

Application of Vitadur N Enamel Porcelain Shade Vita A2

Special Effects and Characterisation of the Enamel

Surface Finishing of the “Green” Porcelain

Firing the Aluminous Veneer Porcelains Vitadur N

Finishing

Sintered Diamond Finishing Tools

Surface Characterisation

Surface Characterisation of Teeth Showing Little Surface Abrasion

Surface Characterisation of Teeth Showing Heavy Abrasion

Staining for Aluminous Porcelain Crown Shade Vita A2

Glazing for Vitadur N Crown

Avoiding Thermal Shock and Slump of Porcelain

Removal of Platinum Foil

Finishing of the Porcelain Margins

Lack of Fit – Common Faults in Construction

High Alumina Reinforcements

The High Alumina Bite Pad

Technique

The Platinum Bonded Alumina Crown

Objectives of the Technique

Construction

Technique for a Maxillary Central Incisor

Preparation of Outer Foil for Tin-Plating

Sandblasting

Degreasing the Foil

Operating Ceramiplater

Electroplating the Outer Foil

Oxidising the Outer Foil for Porcelain Bonding

Building the Core Porcelain

Porcelains

Burnishing the Twin Foils

Core Porcelain Application

Second Core Porcelain Application

Building the Dentine and Enamel Porcelain Vitadur N for Multiple Crowns

Applying Enamel Porcelain

Characterising the Enamel

Firing the Platinum Bonded Alumina Crown Vitadur N

Finishing and Staining

Glazing

Removal of Platinum Foil

Strength of Platinum Bonded Alumina Crown

Platinum Bonded Crown with Lingual Metal Collar

Construction

Construction of De Trey NBK 1000 Bonded Alumina Crown with Platinum Collar for Maxillary First Premolar, Matching Biodent Shade 32

First Firing of Core Porcelain
Biodent Systomat-M Furnace

Second Firing of Core Porcelain

First Firing of Veneer Porcelains Biodent Systomat-M Furnace

Finishing the Lingual Platinum Collar

Glazing in Biodent Systomat-M

Clinical Use of Vita-Pt Crowns and Cast Metal-Ceramic Crowns

References

Noble-Metal Alloy Systems

Base-Metal Alloys

Factors in the Design of the Preparation and Metal Coping

Reduction of Stress in the Porcelain

Minimising Creep of the Metal and Maintaining Marginal Fit During Firing of the Porcelain

The Shoulder Versus Bevel

Aesthetics of the Gold Bevel

The Gold Collar

Optimising the Aesthetics of the Facial or Buccal Porcelain and Providing Adequate Strength in the Metal to Avoid Porcelain Fracture

Recommended Designs for Preparation

Labial Shoulder

Recommended Designs for the Preparations and Metal Coping

Metal-Substructure-Design

Design for Lingual Metal Coverage in Anterior Teeth

Design of Coping for Posterior Teeth

Design of the Approximal Shoulder Area

Occlusal Support

Summary of Requirements for Design of Metal Copings

References

Construction of Gold Alloy Coping

Preparation of Wax Pattern

Dual Wax Technique

Die Spacers

Forming the Pattern

Sheet Wax Technique for Maxillary Central Incisor Preparation with Labial Shoulder and Lingual Chamfer

Finishing the Cervical Margin

The Wax Dip Technique. Mandibular First Molar with Labial and Lingual Chamfer with Gold Collar

Full Wax Contour Technique. Maxillary First Bicuspid with Labial and Lingual Chamfers and Occlusal Gold Coverage

Forming the Veneering Area

Full Wax Contour Technique-Mandibular Teeth

Partial Occlusal Coverage with Metal

Selection and Attachment of Sprues

Venting

Technique for Attaching Sprues

Anterior Single Coping

Withdrawal

Positioning of Patterns and Sprues in Casting Ring and their Effect on the Shrinkage of the Casting

The Thermal Zone

Attachment of Sprues to Crucible Former

Posterior Coping

Investing the Pattern

Wetting Agents

Investments

Phosphate Bonded Investments

Investing

Casting

Centrifugal Casting

Melting Procedures

Vacuum-Pressure Casting

Technique for Standard Centrifugal Casting

Melting Alloy

Torch

Melting Range (1085°C to 1215°C) for Gold-Platinum-Palladium Alloys

Finishing of Metal Castings

Surface Preparation of Casting

Recommended Stones

Objectives of Finishing

Double Oxidation Finish

Second Oxidation

Electrolytic Degreasing

Preparing the Solution

Steam Cleaning

Oxidation Cycle

Oxidation and Degassing-Discussion

Factors Affecting the Fit of Cast Metal Copings

Cement Film Thickness and Final Fit

Porcelain Application Metal-Ceramic Crown

Opaque Porcelain

Applying Body and Incisal Opaque Porcelains

Firing First Opaque Layer Vita-Vacumat “S” Furnace

Second Application of Opaque Porcelain

Firing the Second Opaque Layer

Applying the Veneer Porcelains

Labial Surface

Lingual Surface

Applying Enamel Porcelain

First Biscuit Firing Vita Vacumat “S” Furnace

Finishing

Approximal Contacts

Second Biscuit Bake

Firing Vita VMK 68 Porcelain in the Biodent Systomat Furnace

Occlusal Adjustment

Finishing the Facial and Lingual Surfaces

Surface Texture

Final Smoothing and Polishing

Staining

Glazing

Vita Vacumat “S” Furnace

Glazing in the Biodent Systomat Furnace

Finishing and Polishing the Gold Surfaces

Heat Treatment

Construction of a Maxillary First Molar Metal-Ceramic Crown Using De Trey’s Biodent Universal Porcelain

Applying the Opaque Porcelain

First Firing of Opaque in the Biodent Systomat-M

Second Application of Opaque Porcelain

Second Opaque Porcelain Firing Firing

Applying the Dentine Porcelain

The Occlusal Surface

First Firing in the Biodent Systomat-M Furnace

Additions of Porcelain

Finishing

Staining and Glazing

Glaze Firing in the Biodent Systomat-M

Anatomy of Posterior Teeth in Porcelain

The Base-Metal Alloy Crown

Methods of Processing

Melting Nickel-Chromium Alloys

Finishing

Porcelain Application

Applying Opaque and Veneer Porcelains

Soldering

White Gold Alloys

References

Types of Special Effect

Neck or Gingival Effects

Gingival Effect Using Vitadur-N Effect Powders

Vita Shade A.1 (no root exposed)

Neck Effect Using Body Dentine Porcelain (Vitadur-N)

Vita Shade A.3 (with root exposed)

The Dentine Effect

Changing Shade in Small Areas

Changing Shade over Labiocervical Surface

Dentine Mamelons

Reproducing a Discoloured Filling

Incisal Effects

Incisal Orange Hue

Simulating Blue Translucency of Approximal Enamel

Simulating Enamel Check Lines – The Wedge Technique

Hypocalcification

The Fixed Stain Technique

Surface Staining

Dry Technique

Cervical Staining

Lingual Staining

Incisal Staining

Wet Technique

Approximal Staining

Altering Colour and Translucency

Reducing Translucency

Altering Colour

Mixing Porcelain Powders

Changing Hue by Surface Staining

Converting Vita Shade A.3 to A.4

Converting Vita Shade A.4 to A.3

Converting Vita Shade B.2 to B.3

Converting Vita Shade A.2 to B.3

Multi-blended Opaques

De Trey Biodent Universal Opaques for Metal Bonding

Vita VMK 68 Opaques for Metal Bonding

Vita Vitadur-N Aluminous Core Opaques

Use of Opaques and Core Porcelains in Shade Control

Characterisation of Opaques in Limited Areas

Clinical Applications

Neck or Gingival Effect

Dentine Mamelons

Enamel Defects

Enamel Cracks

Incisal Orange Effect

Discoloured Fillings

Decreasing Value

Multi-coloured Opaques

Incorrect Shape and Surface Characterisation

Porcelain Butt Fit for the Metal-Ceramic Crown

Porcelain Butt Fit using Direct Porcelain Application with Alumina/Opaque Core Porcelains

Shade Prescription and Communication with the Laboratory Technician

Shade Prescription

References

Design of Framework

Depth

Width

Length

The Abutment Crowns

Stresses on the Anterior Bridge

Design for the Anterior Fixed Bridge

The Posterior Fixed Bridge

Dimensions of Connectors

Design of Pontics

Types of Pontic

Hygienic Pontic

Tissue Contact

Full Porcelain Coverage

Full Metal Coverage

Faults in Design

Methods of Waxing Framework

Construction of a Three-Unit Metal-Ceramic Bridge

Waxing Framework

Spruing

Compensating for Wax Distortion on Removal and Casting

Split Wax Pontic Technique

Types of Solder

Soldering

High Temperature Pre-soldering

Assembly of Castings

Investing

Procedure

Pre-heating and Burn-out of Duralay

Fluxing and Soldering

Finishing the Casting

Try-in of Castings in the Mouth

Surface Preparation

Building the Veneer Porcelain with Vita VMK 68 Porcelain

Applying the Opaque Porcelains

Applying the Dentine Porcelains

Applying the Enamel Porcelains

First Bake

Characterising the Enamel Using the Fixed Stain Technique

Staining and Glazing

Case Before and After Treatment

References

Attachments Used in Metal-Ceramics

Construction of Metaux Precieux III Attachment in Novostil

Incorrect Position of Matrix

Correct Position of Matrix

Inserting Matrix into Wax Pattern

Carving the Coping

Spruing

Investing

Casting

Finishing

Waxing the Patrix

Investing and Casting

Finishing

Clinical Application Using Multiple Splinting of Teeth with Interlock Attachments

Preparation

Determining the Aesthetics and Position of Splinted Crowns

Metal Framework

Posterior Bridges

Try-in of Metal Framework

Occlusion and Porcelain Butt Fit

Summary of Requirements for Fixed Splinting

The Metal-Ceramic Pontic for Removeable Prosthesis (The Denture Pontic)

Denture Pontic Using C. M.22-07-5 Snap Attachment in Ceramicor Metal

Denture Pontic Using Regulex Extracoronal Adjustable Slide Attachment 23-08-5

Removeable Prosthesis Using Regulex Attachment Combined with Metal-Ceramic Bridgework

Dalbo Extra-Coronal Attachment

Description

Indications for Use

Attachment of Patrix

Dalbo Denture Pontic

Post-soldering of Regular Gold Alloys to Metal-Ceramic Bridgework

Soldering

References

Periodontal Health

Fracture Resistance

Positioning the Cusps

Size of Cusp Tip and Fossa

Centric Relation

Methods of Building Porcelain Cusps

Full Wax-Up with Plaster or Silicone Rubber Key

Brush Additive Technique

Enamel and Dentine Application

Full Porcelain Occlusion

Brush Additive Technique with Articulator Pin Set at Correct Vertical Height

Building the Mandibular Porcelain Cusps

Grinding-in the Occlusion

Porcelain Cone Technique with Diagnostic Wax-Up

Building the Porcelain Cones

Air Abrasive Technique for Carving Porcelain

Building the Maxillary Cusps

Evaluation of Methods of Building Occlusion

Full Mouth Rehabilitation Using Alumina and Metal Reinforced Ceramics

Major Alterations to Occlusion

Treatment Plan

Construction of Porcelain Rehabilitation

Ridge Relationship for Anterior Pontics and Occlusal Design in Metal-Ceramic Restorations

Treatment Plan

Preparation and Construction

References

Crowns for the Adolescent Patient

Bonded Alumina Crown with Cervical Platinum Foil Reinforcement

The Alumina Tube Pontic

Effect of Light Transmission on Cervical Foil Reinforced Crowns

The Platinum Bonded Alumina Bridge

Construction

Joining the Aluminous Porcelain Copings

References

In 1971, the Federation Dentaire Internationale, by vote of the members of its Assembly (all members being representatives of their national dental organizations), adopted an all-number, computer-compatible, easily understood tooth numbering system. This system, illustrated below, is used throughout the text whenever the teeth are not otherwise identified.

F.D.I. Tooth Numbering System
Permanent teeth

Maxillary right 18 17 16 15 14 13 12 11	Maxillary left 21 22 23 24 25 26 27 28
48 47 46 45 44 43 42 41 Mandibular right	31 32 33 34 35 36 37 38 Mandibular left

For example, the maxillary left lateral incisor and the mandibular right second premolar are designated 22 and 45, respectively.

Structure

Before attempting to construct ceramic restorations, it is essential to have a thorough understanding of the anatomy and structure of teeth. In addition, the effect of reflection, transmission and refraction of light in the tooth has a very important part to play in obtaining natural textures and colour effects. The influence of metal and opaque backgrounds on colour and translucency can be dramatic, and for this reason the ceramist must have a basic understanding of the properties of light in relation to dental ceramic work. This information has already been given (Sproull, 1973, 1977; Lemire and Burk, 1975; McLean, 1979, Monograph III; Preston, 1977) but the critical aspects of colour control will be repeated in this book.

Reproducing Natural Teeth in Dental Porcelain

The main features in a natural tooth that must be reproduced in dental porcelain are colour, translucency and texture.

Colour and Translucency

The total colour effect in a natural tooth is derived from a combination of light directly reflected from the tooth surface combined with light that has been reflected from the dentine and which has already undergone some internal reflection and refraction. The dentine is the prime source of colour and the reflected rays of light which are emitted through the enamel are modified by the thickness and degree of translucency of the enamel. Human enamel contains approximately 97 per cent by weight mineral matter, mostly in the form of hydroxyapatite. Enamel is very translucent and may transmit up to 70 per cent light through a 1 mm thick section. By contrast, dentine only contains about 70 per cent by weight of hydroxyapatite and the rest is collagen matrix. Dentine is still translucent but will generally not transmit much more than 30 per cent light on a 1 mm thick section.

The reflection and transmission of light producing colour and translucency in a tooth are illustrated in Figure 1-1 and compared with a section through a natural tooth (Fig. 1-2). The tooth can be divided into three areas.

Incisal Third

Enamel constitutes the major part of this area and as the thinner incisal edge is approached it can assume almost a glass-like clarity (Fig. 1-1). The enamel also extends around the approximal areas and will highlight the approximal space. High translucency in this area is one of the reasons why human teeth do not have a stark appearance; the edges are softened by light transmission (Fig. 1-3a and b). Failure to capture this effect in ceramics is a common fault resulting in a solidity and starkness that is totally unnatural. The metal-ceramic crowns in Figure 1-4a have been replaced with more translucent aluminous porcelain crowns and higher light transmission has softened their appearance (Fig. 1-4b).

Middle Third

The middle third of the tooth contains a major quantity of dentine and is less translucent. Colour of the enamel will be influenced by the dentine hue and its natural blue-greyness will have overtones of yellow, orange and brown (Fig. 1-1).

Cervical Third

As the enamel approaches the cervical line of the tooth crown, it thins down to a chisel or chamfer edge. The influence of the underlying dentine on the colour of the tooth is quite marked. The neck of the tooth will assume a deeper hue which will vary in colour from orange-yellow to a distinct brown, depending on the age and degree of calcification of the dentine. In addition, the pink gum will have some effect on hue (Fig. 1-1). Tooth manufacturers have recognised this gradation in colour and therefore produce porcelain to match each area (Fig. 1-5):

Incisal enamels for high translucency.

Overlay enamels for general coverage.

Body dentines for bulk build-up.

Gingival dentines for increasing colour intensity.

In addition, opaque porcelain for masking metal or cement surfaces and concentrated colours for duplicating internal and surface stains are supplied.

Texture

A natural tooth in its unworn state does not present an absolutely smooth surface. In general, it may be seen as a gentle, undulating surface, traversed horizontally with very fine grooves. These are the perikymata, and accurate simulation of these surface irregularities is as important as the matching of shade and shape. If light should reflect off a restoration in a different way from the neighbouring teeth, even if shade and form matching are accurate, it will give the effect of being artificial. Glazing of a ceramic crown is therefore a critical procedure. In addition to the effect on texture, if a crown is over-glazed, it will produce an homogenous glassy surface which obliterates all the “prismatic” effect of dental porcelain. A correctly glazed enamel will only melt to a depth of about 25 µm (McLean, 1979) and the rest of the porcelain should only be fused or sintered at its grain boundaries, i.e. the surfaces of the glassy grains of porcelain should remain almost intact (Fig. 1-6). When the grain boundaries are fused together and not obliterated a prismatic effect is created which can closely simulate the hydroxyapatite structure of human tooth enamel (Fig. 1-2b).

A perfectly matched ceramic crown should submerge itself in the mouth. It must have depth of translucency, with colour built in to the various layers of porcelain. Ridge and point angles should blend with the surrounding teeth and facial and lingual planes must harmonise with the dental arch. In order to achieve these objectives it is clear that only complete porcelain veneer crowns can be made with all these desirable characteristics since once a metal coping is used to reinforce the porcelain, most of the light transmission is blocked in the cervical two-thirds of the crown.

A natural tooth always allows diffuse transmission of light (Figs. 1-1 and 1-2) and the metal-ceramic crown violates this property. This point is well illustrated in Figure 1-7 in which a metal-ceramic crown has been illuminated with a fibre-optic light to show the dark shadow cast by the metal. By contrast, a complete aluminous porcelain veneer crown shown in Figure 1-8 is transmitting light throughout the entire body. Because of this property these crowns can be made with greater depth of translucency and they exhibit less specular reflection from the facial surfaces.

By contrast the metal-ceramic crown will only permit diffuse reflection and specular reflection of light in the body area and resultantly tend to look brighter in the mouth. This brightness is due to high reflection from paint-on opaques and is difficult to eliminate unless there is at least 1.5 mm of porcelain covering the metal.

The greatest problem in making metal-ceramic crowns is to reduce specular reflection from the facial surfaces. The ceramist is always fighting for space to avoid high spots. Methods of creating the illusion of translucency will be shown but it must be clearly recognised that ultimate perfection still lies in duplicating the structure of natural teeth. This can only be done where there is depth of porcelain, and translucent veneer porcelains can be used to advantage. Surface colourants can never provide the answer since colour must be seen in depth. Surface colourants behave like opaque porcelains and produce highly reflective surfaces (Fig. 1-9). Surface stain should be confined to its correct place, to simulate surface stains or defects that are naturally occurring in human teeth. In very special circumstances it may be necessary to use a wash of stain to reduce a bright spot over the opaque but this is an undesirable compromise.

The metal-ceramic technique will be given great prominence since it is the most useful and widely used system available in dental ceramics. However, an awareness of the aesthetic problems of these materials can only help both the dentist and the technician to raise their standards even further.

There is a tendency today, often because of commercial pressures, to use only one brand or system of porcelain. No single system can answer all the clinical problems presented, and for this reason, both metal-ceramics and aluminous porcelains are used in our laboratory to meet the specific requirements of each case. In many instances the two are combined in one rehabilitation with alumina reinforced crowns being used on the incisors and cast metal-ceramic crowns on the posterior teeth. In view of the versatility of current porcelains, it is essential that technicians become familiar with all systems and types of porcelain available.

Types of Porcelain

There are three main types of porcelain:

1. Regular felspathic porcelain.
2. Aluminous porcelain.
3. Metal bonding porcelains.

For convenience, all the above porcelains may be classified in their temperature ranges (British Standard No. 5612).

Types of Crowns

There are two main types of crown. The complete porcelain veneer and the metal-reinforced porcelain crown. These will now be considered in detail.

The Complete Porcelain Veneer Crown

1. The Felspathic Crown

Porcelains made for this technique have been in existence the longest and continue to set a high aesthetic standard. They are generally in the medium maturing range (1050°C–1200°C) and made for vacuum firing. The types of felspathic porcelain used in making a jacket crown are illustrated in Figure 1-10.

The opaque – a felspathic glass loaded with opacifiers such as zirconium or titanium dioxide.

Body dentine – coloured felspathic glasses with high translucency.

Gingival dentine – coloured felspathic glasses with reduced translucency.

Overlay enamel – highly translucent felspathic glasses often containing sub-micron opacifiers or crystalline material to create special colour effects.

Incisal enamel– colourless glasses to simulate thin incisal edges.

Concentrated stains and modifiers.

A typical medium maturing felspathic porcelain for vacuum firing would have a formula as follows:

Maturing temperature 1060°C–1080°C

2. The Aluminous Porcelain Crown

The aluminous porcelains were developed in England by McLean and Hughes (1965) with the object of improving the strength of the porcelain jacket crown without sacrificing aesthetics.

The porcelains used in this technique are illustrated in Figure 1-11.

Core porcelain – the highest strength porcelains containing up to 50 per cent by weight fused alumina crystals. The fused alumina crystals (specific surface area of approximately 3500 cm²/gram) are mixed or fritted with a glass powder of matching thermal expansion. The transverse strength of these alumina-glass composites is 130 to 150 N/mm² (dental porcelain strength 70 to 85 N/mm²). Their low expansion and refractory nature also makes them very resistant to thermal shock.

Dentine or body porcelain – made from a borosilicate glass, containing a high dissolved alumina content or, alternatively, a completely dissolved (fritted) felspar glass flux containing additional quantities of alumina both in the free state and dissolved in the glass. The free alumina crystal content of these glasses is restricted to about 5 to 10 percent. Details of their composition are given in Volume I, page 99 (McLean, 1979).

Enamel porcelains – made from the same frit as the dentine porcelain except that their free alumina crystal content is reduced to improve translucency.

Metal-Reinforced Porcelain Crowns

The Cast Metal-Ceramic Crown

This crown is the most widely-used restoration in dental ceramics and comprises a cast metal-alloy coping onto which a porcelain veneer is fired. The process is similar to the enamelling of metals.

The porcelains covering the metal are of high expansion (ca. 13–14 x 10⁻⁶°C) and are made from felspathic glasses similar in composition to the regular felspathic porcelain but with higher alkali content to obtain expansion. The metal-ceramic crown is illustrated in Figure 1-12.

Opaque porcelain – The new paint-on opaques have very high covering power and may be applied in thicknesses as little as 100 µm. However, they can be highly reflective and must be covered with at least 1 mm of veneer porcelain if reasonable aesthetics are to be achieved.

Enamels

Body dentine porcelain – felspathic glasses with high colour saturation used for main build-up of gingival and body areas of tooth crown.

Enamel porcelain – felspathic glasses for covering the body porcelain to produce translucency and incisal characteristics. Firing temperatures for all the porcelains used in metal bonding techniques 900°C to 960°C.

Physical and Chemical Characteristics of Alloys Used for Ceramic Bonding

If a good bond has been achieved between porcelain and metal, the breaking stress should approximate to the tensile strength of the opaque porcelain and any fracture should occur through the porcelain (Nally, 1969; McLean and Sced, 1973; O’Brien, 1977).

The development of metal-ceramic systems in dentistry imposes severe disciplines and good aesthetics are obviously a high priority. If a strong bond between metal and porcelain is to be achieved which also satisfies aesthetic demands, then a metal-ceramic system should meet the following requirements:

1. Good wetting of the metal or metal oxide by the porcelain is essential.
2. The oxide should be soluble in the porcelain.
3. The oxides formed on the surface of the metal should not discolour the porcelain or interfere with glass formation.
4. The metal or metal oxides should not react in any way so as to reduce the strength of the porcelain or introduce high internal stresses, e.g. by raising or lowering the thermal expansion of the interfacial porcelain.
5. The metal or metal oxide should not corrode or produce toxic effects in surrounding tissue.
6. The metal should be clinically acceptable both in regard to castability, accuracy of fit and appearance.
7. The metal should exhibit minimal creep during firing of the porcelain and possess adequate mechanical strength for multiple splinting and bridgework.
8. The casting should be fine grained to prevent high temperature fracture.
9. The metal should be readily post-soldered or pre-soldered.

Methods of Attaching Porcelain to Metal

The mechanisms for attaching porcelain to metal are now much better understood. The ceramo-metallic bond is probably derived from three main components – molecular, mechanical and compression bonding (Fig. 1-13; Vickery and Badinelli, 1968; McLean, 1979; Cascone, 1977).

Molecular Bonding

Two possible mechanisms for the bonding process can be envisaged. The oxide formed on the surface of the metal could act as a permanent component of the bond like a sandwich structure, in which it is separately bonded to both metal substrate and porcelain. An alternative approach is, however, to envisage the coating oxide as only a means to an end whose function, by dissolution into the glass phase of the porcelain is to bring the porcelain into atomic contact with the metal surface (Fig. 1-13). Wetting of the metallic substrate by the glassy porcelain is thus very effective and direct bonding of porcelain to metal by interatomic forces is facilitated (McLean and Sced, 1976).

There are several ways of providing a surface oxide on the metal for porcelain bonding:

1. By introducing traces of base metals into precious metal alloys which, on heating, will produce thin oxide films, e.g. gold-platinum alloys for ceramic bonding.
2. By direct oxide production via the constituents of the alloy, e.g. nickel-chromium, cobalt-chromium alloys.
3. By surface coating of metals with oxidisable metal films such as indium or tin, e.g. electro-deposition of tin on platinum.

In the first case, small quantities of iron, indium, zinc or tin can be incorporated into a gold-platinum alloy and during heating of the metal and baking of the porcelain, the base metals migrate to the gold alloy surface and form an oxide film which can provide the medium for molecular bonding (McLean and Sced, 1973; Nally et al., 1968; von Radnoth and Lautenschlager, 1969; Cascone 1977; Lugassy, 1977). In the case of base metals such as the nickel-chromium alloys, these will oxidise very readily, and the control of this oxide production is one of their major problems (McLean and Sced, 1973). More recently, it has been shown that electro-deposition of base metals such as tin or indium onto precious metals or gold alloys will also form oxide films on heating and provide a mechanism for porcelain bonding (McLean and Sced, 1976).

Mechanical Bonding

In addition to the provision by the oxide of an easily-wettable surface on the metal, there is evidence that surface roughness produced by sandblasting can provide mechanical “keying” and increase the surface area for porcelain attachment (McLean and Sced, 1976; Lugassy, 1977). However, it should be noted that with increasing surface roughness a situation could be reached where stress concentrations would be introduced, resulting in premature failure of the porcelain (Kelly et al., 1969).

Base-Metal Alloys

With the rising cost of precious metal and the demand for stronger alloys with high temperature creep resistance when the porcelain is applied, the interest in non-precious alloys is increasing. The term “non-precious” is not accurate and all the current nickel-chromium alloys should be termed base-metal alloys. These alloys are being introduced onto the market in ever-increasing quantities and with claims for their reliability often unsupported by any scientific evidence. It is hardly surprising that both dentist and technician find it hard to keep up with all the brand names. Words such as “semi-precious” are being used to describe metals that often only contain one percent of a noble metal, and in many cases “non-precious alloys” are presented in such a way that the technician does not even realize that they contain only base-metals such as nickel and chromium.

The most popular alloys for ceramic bonding are of the nickel-chromium type. Chromium is a very necessary constituent of all ceramic type base-metal alloys since without it the corrosion resistance of the alloy is unacceptable under mouth conditions. Unfortunately, metals such as chromium or nickel present considerable problems during ceramic bonding and these may be explained as follows. When the porcelain is fired onto the metal, a layer of nickel and chromium oxide rapidly builds up at the ceramo-metallic interface. Depending on the preparation of the metal and the length of firing, this oxide layer will vary in thickness, long firing periods generally increasing the thickness of the layer. A thick oxide layer is very undesirable since it can act as a sandwich between the porcelain and the metal and failure can then occur in the oxide layer (Fig. 1-14).

In addition, it was found that even when oxidation of the base-metal alloy was suppressed by atmosphere control (e.g. firing in non-oxidising atmosphere) direct chemical reduction of some constituents of the porcelain by nickel and chromium is still likely, with nickel or chromium oxide as one of the products (Fig. 1-15; McLean and Sced, 1973). This reduction process may easily occur in dental practice, since once the porcelain matures, the bond interface is effectively shielded from the furnace atmosphere. When these oxides are combined in the dental porcelain, they can reduce the expansion coefficient of the porcelain and produce a high degree of residual stress at the bond which can result in failure such as “pop-off” of buccal or lingual surfaces (Figs. 1-16 and 1-17). Nickel and chromium have been detected in the porcelain after firing, confirming the above evidence (McLean and Sced, 1973; Lugassy, 1977).

Because of the possibility of high degree of residual stress at the bond, the bond strength of both cobalt and nickel-chromium alloys may appear to be higher than the gold alloys when fired in oxidising atmosphere and the mean breaking stress can be above that at which tensile failure of the porcelain occurs with gold alloys. However, such results can be misleading since the high degree of residual stress created at the bond by the lowering of the thermal expansion by chromium or nickel oxide must be accounted for. If this residual stress was acting in opposition to the applied stress it would be necessary to overcome it before a positive stress was applied and the indicated load at failure could be misleadingly high.

It is apparent that the base-metal alloy systems are technique sensitive and the question is ‘How do we control oxide production so that consistent bonding of porcelain may be obtained?’ Successful bonding of porcelain to base-metal alloys is very dependent upon both casting and firing techniques. A clean and homogeneous casting is the basic requirement for successful work. Sprues of 3.5 diameter should be attached in such a manner that they fan out from a central reservoir and allow uninterrupted flow of the metal (Weiss, 1977). The casting should be thoroughly sandblasted with 30 µm Al₂O₃ grit and cleaned with a solvent such as acetone or alcohol before applying the opaque. After the opaque porcelain is applied, the cardinal point to bear in mind is that vitrification must be rapid and complete before a thick oxide layer can form and interfere with bonding. The use of metal firing trays to improve the thermal conduction, and furnace muffles with even heat distribution will assist the technician. Short firing cycles of one or two minutes are also recommended and opaque porcelains which can vitrify in this time are now available.

Work is now in progress to try and eliminate excessive oxide formation, or reduction of the porcelain by base-metal constituents, by using various coating agents. One such material^a utilizes aluminium powder blended with a ceramic frit as the coating agent. Presumably it is intended that the aluminium competes with the nickel and chromium for oxygen, thus preventing rapid oxidation of the base-metal alloy. In addition, the formation of aluminium oxide in the frit allows the bonding agent to take part in glass formation. The exact role of aluminium has yet to be established and further evidence is still required of its effect on the glass interface.

Other methods which may also be of some interest rely on the electro-deposition of a precious metal, such as gold or rhodium on the surface of the base-metal casting. In this case it is hoped that the precious metal surface will act as a barrier to any oxide formation. Bonding to the porcelain would then be achieved by electro-deposition of a thin coating of tin on the surface of the precious metal which is subsequently oxidised during porcelain application (McLean and Sced, 1976).

Despite all these improvements in technique and surface bonding, a further criticism must be levelled at the base-metal alloy systems. The casting fit of the current materials still remains in doubt and Nitkin and Asgar (1976) concluded that in terms of fit, base-metal castings were inferior to castings made from high or low gold content alloys. However, they rightly concluded that base-metal alloys show potential despite the poor results obtained and they considered that two areas of technology needed improving. Firstly, improved casting techniques such as induction casting and, secondly, the development of a different type of investment with higher expansion.