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The purpose of this literature review is to discuss relevant scholarly articles and research data concerning the use of metal ceramic restorations in modern dental practice. In the last two decades tremendous advancements in ceramic technologies have resulted in all ceramic restorative systems. This literature review aims to identify the continuing need for metal ceramic restorative systems, despite the alternative options currently available to the modern dental practitioner. Metal ceramic restorations are definitely not a thing of the past! The academic journals of note are referenced throughout the body of the text in a numeric referencing system in which: the numbers run in order throughout the text except where a reference is made to a work already mentioned and then the same number is used again. Illustrations contained in the body of the text of the literature review are sourced from reliable textbooks and web journals.
A metal ceramic restoration is that which utilizes an alloy of metal, to add strength and fracture resistance to the ceramic portion of that indirect restoration. Indirect restorations include inlays, onlays, crowns, bridges, and veneers. Metal ceramic restorations in dentistry include the porcelain jacket crown, the porcelain fused to metal crown, the porcelain fused to metal fixed bridge and cast post and core crowns. (1)(2) It may seem ironic that metal and aesthetics are mentioned together in the context of modern restorative dentistry. The all ceramic metal free crown is marketed around the misconception that superior aesthetics can only be achieved with metal-free indirect restorations. (4) This is quite simply a falsehood, and one which will be dispelled throughout the body of this literature review.
Figure 1-Aluminous Porcelain Jacket Crown
2.2 The Natural Tooth
To understand the forces at play in the selection of a fixed restoration we must first consider the structure which it mimics (i.e. the natural tooth). Enamel- Enamel is the whitest, hardest, outermost layer of the tooth seen as the clinical crown. It is hard and brittle. Ameloblasts produce enamel and these specialized enamel producing cells are lost on tooth eruption. This is vital as teeth damaged by caries or toothwear cannot replace missing enamel, once lost it can only be replaced by synthetic means. (5) Dentine- Dentine is the softer yellower crystalline collagenous substance located beneath the enamel in the clinical crown. It is formed by odontoblasts which lie at the periphery of the pulp chamber. Dentine is produced throughout life at the expense of the size of the pulp chamber (it is laid down in an inward fashion) Dentine is the foundation of the tooth; it supports enamel, protects the pulp, and covers below the gum to give rise to the periodontal ligament which attaches the tooth to surrounding bone. (5) Considering the composition of the natural tooth we can deduce:
Teeth are non uniform in colour
Teeth have complex internal and external features
Teeth are semi- translucent in areas
Teeth have a degree of fluorescence
Teeth change shape and darken with age
2.3 Fixed Restorations- Aesthetic Control
The fundamental in obtaining a successful aesthetic crown or bridge is assessment of the patient's expectations from treatment. The dentist must ensure that expectations are realistic, that the restoration indicated is in the patient's best interest, and that both the dentist and laboratory technician possess the necessary skill to follow through with the proposed treatment. (6)The dentist can modify the patient's expectation by a variety of clinical tools, which demonstrate the proposed fixed restoration:
Diagnostic wax up of proposed restoration
Uncured composite to demonstrate the restoration
Software via computer to demonstrate the restoration
Photographs of previous cases can re-enforce what is possible as well as demonstrate the limitations. (6)
Figure 2-Diagnostic Wax up from canine to canine
2.4 Aesthetic Fixed Restorations- The Role of the Dentist
Colour selection by the dental practitioner is vital in achieving a good aesthetic outcome for the patient. This is the case for resin bonded all ceramic indirect restorations as well as metal ceramic indirect restorations. (1)(2) The colour match is performed by the dentist in the clinic using shade guides. Colour is a function of three components:
Hue- is the distinguishment of one colour from another e.g. red vs. yellow
Chroma-is the colour saturation within a colour e.g. pink is a less saturated member of the red colour family
Value- is the single most important component in colour selection. (1)(2)(3) Value is the amount of darkness or brightness in a Hue.
The shade guide used by the dentist will be based on these 3 components of colour. The 3 D Vitapan Shade guide is the most objective as it gives equal importance to Hue and Value (6)Good shade matches can vary in different light sources, and is a process that should be carried out in at least 2 light sources clinically to avoid metamerism. All colour distorters should be removed during shade matching: lipsticks and circum-oral make-up, large colourful earrings, intraoral removable prostheses e.g., dentures. (1)(2)(3)Once a correct shade has been selected the fixed restoration should blend in with the natural tooth shade, eliminating the problems with fixed restorations such as crowns in a study in the UK in 1988 which showed that "Dissatisfaction with dental appearance was likely to be found in patients with 1-2 crowns as opposed to none or many" (6)
Figure 3-Mismatched crowns with poor aesthetic appearance
Crown preparations concerning fixed restorations especially metal-ceramic restorations, are vital in achieving a favourable aesthetic outcome. The main concerns of metal ceramic restorations are:
The opaque metal oxide layers created by conventional metal- ceramic alloys, these can show through the porcelain creating a dark shadow.
The corrosion and investing of oxide layers in metal-ceramic restorations can infiltrate the surrounding gingiva creating a gingival tattoo.
The abrasiveness of conventional high fusing feldspathic porcelains on the opposing natural teeth is also of concern in aesthetic context.
All tooth preparations carry a risk of pulp damage if carried out carelessly or overzealously. The conservative crown preparations of all ceramic crowns and bridges (1.0 mm shoulder preparation) is a justified argument for their use as opposed to the metal-ceramic crown e.g. PFM. However there is poor clinical evidence of the longevity of these all ceramic fixed restorations. This is quite simply because metal-ceramic restorations were patented in 1962 by McLean and have stood the test of time.
Concerning conventional metal-ceramic restorations, a sufficient labial reduction following the contours of the natural tooth, must be present in preparation. This is to allow sufficient thickness of porcelain to be present for maximum aesthetics. The labial reduction is no less than 1.5 mm, which carries a risk of pulpal damage, especially in young teeth. Indirect restorations are always a balance between removal of tooth tissue and aesthetics. (7) However crown preparations in newer metal-ceramic systems such as Captek (using a high gold composite coping of metal) require only 1.2 mm labial reduction, which approaches the figure of 1.0mm of the all ceramic systems. The reason for less reduction in Captek is due to the improvement in depth perception provided by the high gold oxide free coping supporting a low fused lucite crystalline porcelain. (4)
Figure 4-A vertical section through the Captek Central Incisor Crown (top left)
There is a sound argument that all metal-ceramic crowns and bridges should have metal only margins because this produces the best marginal seal. (6) However even conventional metal-ceramic restorations, using a porcelain butt finish in favour of a metal collar finish, can produce good aesthetics in critical areas. The porcelain butt finish line allows more light to pass through the porcelain from the gingival aspect, giving a realistic appearance to the crown or bridge.
Tooth preparation in metal-ceramic crowns and bridges is a balance between conservation of tooth structure and the health of the pulp, with aesthetic concerns of the finished restoration. With careful treatment planning and the use of impression aids to gauge reduction amount during tooth preparation, a favourable preparation is the inevitable result. (7)
2.5 Aesthetic Fixed Restorations- The Role of the Laboratory Technician
2.5.1 Fabricating the Crown
The dental laboratory technician is an artist, and truly gifted ceramists in this field can produce aesthetic crowns and bridges, even with the conventional metal-ceramic specifications. The ceramist has a variety of choices to make based on the information he/she receives from the dentist. Communication is not always clear, and in a busy laboratory receiving dental requests from a variety of dental practices in the surrounding area, it is inevitable that some of the major decisions concerning a metal-ceramic restoration become the burden of the laboratory technician. These may be any or all of the below choices:
Coping Alloy-The constitution of the metal alloy that forms the coping for the metal-ceramic crown can be made of many different metals: high gold alloy (in 0.5 mm sections), gold-palladium alloy (costly), high palladium alloy (contraindicated for long span bridges), palladium silver alloy (discolouration concerns), nickel chromium molybdenum (difficult to cast and easily distorted giving a poor fit) (contraindicated in nickel allergy),commercially pure titanium (poor bond strength to the overlying porcelain).(3)
Porcelain Powder- The porcelain employed to mimic dentine and enamel layers is built up incrementally by the laboratory ceramist. The powders are carefully chosen based on the shade selection indicated by the dentist.(e.g. copper alloy particles give porcelain a green hue, iron oxide a reddish brown hue, and cobalt a blue hue)(3)
Opaque Layer- is the first incremental layer used to hide the metal alloy of the coping. (3)
Porcelain Fusion- Use of high fusing or low fusing porcelains are a balance between aesthetics( better aesthetics appear to lie with the lower fusing porcelains with lucite crystalline phase) or traditional high fusing porcelains (feldspathic) which result in a stronger more fracture resistant crown or bridge restoration. (8 )
As well as the above choices the laboratory technician can incorporate a number of realistic inclusions to match the existing dentition of a patient. This is a combination of artistic skill and knowledge of optical illusion by the ceramist. The ceramist can place developmental lobes, perikymata, fluorosis opacities, or staining on the crown or bridge. The ceramist also alters the surface texture of the restoration during the final increment to match that of the existing dentition. Some features such as fracture lines or opacities can be incorporated after incremental build-up but they wear away with time, and are best done during the incremental porcelain applications. Lustre is a feature of polishing and glazing of the restoration, and is carefully carried out by the ceramist, as too high a lustre raises the value of the tooth beyond that of the natural dentition. (8)
However gifted the dental ceramist, they can only achieve optimum aesthetic results if the groundwork has been carried out to a high standard by the dentist. The dentist must provide adequate space during tooth preparation, for incremental porcelain build-up, insufficient space results in inevitably poor restorations. (6)
Figure 5-Incremental Porcelain Build-Up by the Laboratory Ceramist
3 Metal-Ceramic Fixed Restorations
Fixed restorations have existed since Ancient Egypt when prosthetic teeth were made of wood and ox-bone. The modern dental practitioner is concerned with placing a fixed restoration to alleviate problems such as: aesthetic concerns, caries, gum disease, and trauma. One of the options available to the dentist is the metal-ceramic fixed restoration (9) Metal collars give the best marginal seal and fit of any indirect fixed restoration. (9) In recent times with the advent of all ceramic systems, use of metal- ceramics have been coupled with catch phrases such as poor aesthetics and excessive removal of tooth structure. This is simply not the case for newer metal- ceramics which possess excellent ability to mimic the natural dentition. Furthermore metal and porcelain are a biocompatible solution for both dentist and patient and show a long history of favourable gingival response.
3.1 The Porcelain Jacket Crown
The alumina reinforced feldspathic porcelain jacket crown was introduced in the early 1960's by Hughes and McLean. (3)The feldspathic porcelain glass contains 40-50% alumina and this strengthens the jacket crown against cracks under stress loading by 200%. The aluminous core is used as the opaque layer in incremental build-up, and conventional dentine and enamel feldspathic porcelain powders mask the greyness of the aluminous layer, to achieve a translucent, bright, and pleasing crown. The PJC is for use in the anterior restoration only; it does not possess the strength for use in bridges or in posterior restorations. (1)(2)(3)
Figure 6-Gum grafts and porcelain jacket crowns to mask mild anterior crowding
The Porcelain Fused to Metal Crown
The porcelain fused to metal crown was first described in 1956 and patented in 1962.(8) The PFM has provided dentists with a biocompatible, relatively aesthetic, fracture resistant means for restoration of posteriorly and anteriorly compromised teeth for 5 decades. (4) The reduction specifications for the PFM are 1.5 mm labially/bucally, 0.5 mm palatally/ lingually, and 1.5 mm mesially and distally. Concerns regarding the amount of tooth tissue required for the PFM tooth preparation are well founded, and have led to the popularity gain of the resin bonded all ceramic crown in the last 2 decades, where traditionally previous practice would have indicated the use of a PFM. However the all ceramic crown is neither proven in its longevity, nor is it an option for anything more than a 3 unit bridge. This limits its usefulness as a total replacement for even the conventional metal-ceramic PFM. Long span bridges are out of the question with ACC systems, metal-ceramics are the only option. (3) Separate concerns regarding the PFM and aesthetics are also justifiable, especially concerning the traditional PFM style with a full metal collar preparation. However newer metal-ceramics for the construction of highly aesthetic crowns and bridges, rival the resin bonded all ceramic, not only in appearance, but also tooth reduction, and all the while maintain the superior fracture resistance of the traditional PFM and PFM fixed bridge.
Captek- A Modern Metal Ceramic
The Captek system uses a metal-composite coping as opposed to the all metal coping of the traditional PFM. The coping in a Captek PFM is infused with molten gold 250 um for anterior teeth and 350 um for posterior teeth, and the coping can also be thinned to as little as 50 um to form an all-metal micro collar for superior fit and aesthetics, or used with a porcelain butt finish for even greater aesthetic emphasis. There is a reflection of the gold coping through the porcelain, which aesthetically warms the porcelain, as opposed to the grey oxide layer of the traditional PFM alloys. With Captek the removal of tooth tissue is 1.2 mm labiallly/bucally; approaching the 1.0 mm reduction required for the more conservative resin bonded all ceramic crowns. Marginal fit for Captek crowns have received excellent reports in studies over the last 10 years from the University of Boston and the sheer bond strength of the porcelain to the Captek coping is twice as high as that of a traditional PFM made with high quality palladium alloy as a coping. Captek can be used for long span bridges, anterior and posterior restorations. (8) (4)
Figure 7-Crystalline distribution in a traditional PFM vs. Captek PFM
Figure 8-Captek used to replace traditional PFM preparations
3.2.2 The Golden Gate Metal- Ceramic
Low fusing hydrothermal porcelains can be fused to Type IV gold metal copings to create what is known as "the golden gate" system of metal-ceramic restoration. Control of the oxide layer is heavily controlled to improve aesthetics and to avoid porcelain de-bonding. This system limits the use of multiple alloys in a patient's mouth to type IV gold. Aesthetics are excellent due to the warmth of the gold shining through the overlying porcelain layers. The main distributors of the golden gate crown and bridge system are Duceram and Duceragold. (8)
Figure 9- The Golden Gate Bridge!
The Sunrise Crown
The Sunrise Crown was described by McLean in 1976, and uses the techniques applied to the formation of the porcelain jacket crown, and modifies it to form a gold-platinum foil coping to produce a PFM called the Sunrise Crown. Platinum foil can be used with aluminous porcelains. The Sunrise Crown uses foil 50 um thick which contains gold, platinum, and oxidising elements designed to facilitate porcelain bonding. The gold in the foil prevents use of aluminous porcelains, and traditional feldspathic porcelains are used to build the outer layers of the Sunrise Crown. Compressive strengths of the Sunrise Crown are not as good as Duceragold Golden Gate crowns or Captek crowns and bridges. Clinical trials have proven that the Sunrise Crown is only for use in anterior restorations, it is not for use in posterior areas. (8)(4)
The Porcelain Fused to Metal Fixed Bridge
As previously mentioned, studies of all ceramic bridges have indicated their use is limited to a 3 unit bridge. (10) Metal- ceramic fixed bridges are used for their proven longevity and biocompatibility, and must be used in cases where a patient requires a bridge of 4 units or greater. Studies have indicated that a 3 unit metal- ceramic bridge has a survival rate of 93% in a 10 year period compared to a 6.7 % failure rate of a 3 unit all ceramic bridge in the 1st year alone. (Olsson 2003)
Figure 10- The superiority of the 3 unit metal-ceramic fixed bridge
Metal-Ceramics Here to Stay
In the context of this literature review we have come across several examples where the metal ceramic restoration is the obvious choice of indirect restoration. The metal-ceramic restoration must be used in the following clinical situations:
Replacement of an existing PFM
Replacement of an existing PFM Bridge
Long Span Bridges of 4 units or greater
Compromised tooth structure that requires re-enforced porcelain
We have also dispelled the misconception that superior aesthetics can only be found in non- metal ceramic restorations. (1)(2)(3)(4)(8)
4.1 Metal-Ceramics - A Timeline Summary
In the 1960's the discovery of metal-bonded ceramics gave dentists the option to discontinue the use of all metal restorations as the retainers for fixed partial dentures (bridges). (11) Porcelain ceramic re-enforced by metal gave patients the possibility of retainers and crowns which conformed to the natural tooth appearance anatomically and aesthetically. Fine porcelain powders that could be fired in a vacuum were discovered in the 1950's. Combining this fine porcelain firing technique with a metal coping fabricated a new kind of metal ceramic, and led to the opalescent porcelain crowns and bridges of unrivalled aesthetics. (11) Later in the 1960's the alumina re-enforced jacket crown and aluminous porcelains gave dentists a further option besides the traditional metal bonded ceramic for the restoration of anterior teeth. In following years experimentation with coping alloys and porcelains have led to very favourable metal-bonded ceramics such as Captek, The Sunrise Foil PFM, and The Golden Gate Crown and Bridge. The newer metal-ceramics rival the resin bonded all- ceramic systems, and preserve the superior fracture resistance associated with the traditional PFM and PFM fixed bridge. (11)
4.2 Metal-Ceramics- Concluding Statements
Placement of an indirect restoration must meet the patient's expectations aesthetically as well as functionally.(8) The choice of metal-ceramic will remain a balance between a conservative approach to tooth preparation, and the need for good aesthetics. The research continues to produce high strength metal-ceramics which mimic the natural semi-translucent nature of the natural tooth. At present metal-ceramics occupy an obvious place in dentistry, and given their history are likely to evolve further. Metal- ceramics are therefore not a thing of the past, but the future! (11)