This article, as part of a 3-part series, is designed to give dental students an overview into the World of Crowns. Useful as a revision and reference tool, this article is intended to be easy to read for anyone unfamiliar with Crown provision, with tips and guidance from an existing dental student. A PDF download version will be available of the entire series in Part 3 of this great revision resource.
With Special thanks to:
Professor Callum Youngson (Head of Liverpool University Dental School)
Dr Sophie Desmons (Clinical tutor at Liverpool University Dental School)
Leona Yip (Editor)
Part 1 – An Introduction to Crowns
A Crown is a type of dental restoration that covers a dental implant, or a tooth that has undergone moderate to severe destruction by caries or wear. It is essentially a “cap” that covers a tapered tooth preparation, to restore function and aesthetics. Crowns are fabricated outside of the mouth (Indirect) and can be made from a number of materials, with choice depending on the clinical situation and patient requirements. The indirect method of producing a Crown is more expensive than direct methods of restoration because of the extra time and resources required in production (often by a dental lab), however it allows for the production of an overall better restoration in terms of both strength and form.
This revision article aims to give a basic overall understanding of the sequences involved in producing a Crown, beginning with a pre-operative assessment, to final cementation.
All images, other than those explicitly stated, have been provided courtesy of Liverpool University Dental Hospital.
Crown preparations involve the removal of a large amount of coronal tooth structure and should only be considered after less destructive alternatives have considered, but are too un-retentive, un-aesthetic or lacking resistance. It is a given, that in Crown preparation, a small amount of sound tooth structure is removed, however this is done with the intention of saving the tooth from subsequent loss of larger quantities of tooth. A full coverage Crown can be indicated if all the axial walls of a tooth have been affected by caries or wear, and a Three Quarter Crown is indicated if one of the axial walls of the tooth remains sound.
The decision to restore a tooth with a Crown is further influenced by a number of factors, including patient expectations, the patient’s oral hygiene and periodontal status, occlusion, and the dentist’s ability to perform the procedure.
The following factors all need to be factored in before a decision can be made on whether a Crown is suitable or not.
It is very important to gauge an individual’s expectations from the Crown, and compare to one’s own clinical judgement on what can be achieved realistically. The response will also influence selection of material, with often a compromise between function and aesthetics.
For example, for an anterior tooth, a Gold Shell Crown would be very retentive and require least removal of tooth tissue compared with an All Ceramic Crown; however few patients would accept this aesthetically.
Treatment tolerance and maintenance
-If the patient is unable to open their mouth sufficiently or for long periods of time, this would impinge on a dentist’s ability to carry out the preparation and take impressions.
-If a patient suffers from parafunction such as bruxism, a Crown lifespan would be significantly reduced.
-A patient must be able to maintain good oral hygiene, for the longevity of the Crown, and periodontium.
Justification of tooth removal
-The inevitable removal of sound tooth in a Crown preparation, and therefore weakening of tooth structure, must be justified by the provision of a Crown that will serve to protect the tooth from further loss of tooth structure, and gain of function.
-Studies have shown that 1-15% of vital teeth become devitalized after Crown preparation, due to exposure of thousands of dentinal tubules, and this needs to be kept in mind when considering Crown preparation, as this can lead to periapical pathology later on.
-The oral environment in terms of plaque control, parafunction, caries risk and occlusion need to be considered for the success of a Crown. Parafunction can be difficult to control, however the other factors are manageable and once ideal, are key to the success of Crown work.
-Plaque is a primary cause of caries and periodontal disease, and it is important that a patient’s oral hygiene is stabilized before the provision of any Crown work.
-High plaque levels in a patient with Crown restorations increase the risk of caries progression and tooth loss, especially where there are marginal deficiencies between the tooth and Crown. Periodontally, plaque can lead to loss of attachment, and gum recession, which would lead to mobility and an unaesthetic appearance around the margins of the tooth.
-It is the dentist’s responsibility to communicate the importance of good oral hygiene and the direct influence on the Crown’s success. This information should be followed by oral hygiene instruction including, tooth brushing advice, flossing techniques, mouthwash advice etc. A tell-show-do method has proven to be successful in teaching patients how to brush and floss.
-Dietary advice is also important in reducing caries risk. The patient should be taught the link between sugar attacks and acids with the risk of caries and to limit such exposures. The UK has an aging population, and root caries within this cohort is likely to be an increasing threat. Root caries is difficult to deal with, and will cause the failure of even the best Crown restoration.
-An accurate assessment of occlusion in ICP and the guiding teeth in both lateral and protrusive movements is required in order to understand the stresses that the Crown will undergo. This can be observed visually, using articulating paper and on study models.
-An ideal occlusion would be canine guided on lateral excursion, in the absence of non-working side interferences
-Nearly all anterior teeth are involved in guidance on protrusion.
-Posterior teeth undergo the greatest amount of occlusal stress
-Decision of the material of the Crown is influenced directly by the occlusion
-Teeth with attachment loss can be Crowned, and it is only in cases where the disease is unstable/uncontrolled that a Crown be contraindicated.
-A Crown with poor margins can compromise the health of the periodontal tissues, especially if meticulous oral hygiene isn’t maintained.
-Ideally, all Crown margins should be placed supragingivally, to avoid the problems associated with gingival recession, however, in cases where subgingival preparations are indicated, it is important the margins are as smooth as possible.
-Subgingival preparations should lie within the depth of the gingival sulcus, and should never encroach onto the biologic width of the periodontium. The biologic width is an approximately 2mm of distance established by the supracrestal connective tissue and the junctional epithelium. If a Crown margin encroaches on the biologic width, it causes inflammation, which may lead to attachment loss, apical gingival migration and pocket formation.
-In a vital tooth with no pulpal involvement, a Crown can be planned without endodontic treatment, but a note kept in mind that there is a risk of devitalisation after tooth preparation.
-In a Non-vital tooth, or pulpally involved tooth, endodontic treatment should be carried, to remove infection, prior to Crown placement.
-In a tooth that already contains a root filling, where there is still pathology, a decision needs to be made on whether to re-endo or not. The success rate of re-endo treatment is low, however, there is an increased chance of success if referred to a specialist.
-Caries extent and existing restorations should be assessed, because the Crown needs to rest on a sound margin of tooth and over a strong enough preparation or core.
-Frequently, the tooth to be Crowned has an existing restoration. All previously placed materials should be removed, unless it has been recently placed and you are sure it is retained to sound tooth. If >50% of coronal tooth structure remains after caries and restoration removal, and no more increase in strength is required, then a bonded compomer or resin ionomer base may be used to restore the tooth to the required preparation form. If <50% of coronal tooth structure remains, and there is not a minimum of 2mm sound tooth circumferentially and gingivally to the preparation, a high-strength core build-up is needed to increase tooth strength and provide retention and resistance form.
-Adequate abutment height is critical for the retention of Crowns.
-If insufficient abutment height remains, and gingival tissue is appropriate, Crown lengthening surgery can be undertaken.
-Retention can be increased by grooves.
-A minimum of 2mm of sound tooth structure is required in order to create a “ferrule”, which is essential to distribute lateral forces, and this does not include the core build up material.
Ferrule- “…360° collar of the Crown surrounding the parallel walls of the dentine extending coronal to the margin of the preparation”- Sorensen & Engelman 1990
-Sufficient occlusal space is required for the provision of a Crown, and this is deficient in cases of moderate to severe toothwear. In such cases, a Dahl appliance or grinding of an opposed tooth may be required to create such space, however neither are to be taken lightly and will require a full occlusal assessment.
Own experience and skills
-It is important to evaluate your own skills against the complexity of a particular case, and refer where treatment is beyond one’s scope of experience, knowledge and skill.
-Even after the above considerations, a degree of flexibility should be kept within one’s mindset, because of complications or other needs that may arise, for example, the need for endodontic treatment, a lack of improvement in oral hygiene, caries that extends deeper than initially thought etc. The patient should also be aware of such possibilities.
Types of Crown
Full coverage Crowns
- Full Metal Crowns
- Metal Ceramic Crowns
- All Ceramic Crowns
- Composite Crowns
-Superior retention and resistance compared to Three Quarter Crowns and Veneers
-Good cosmetics achieved with MCC’s, ACC’s and Composite Crowns
-Should only be considered once less destructive alternatives have been considered
-Used where all axial surfaces have caries or have been previously restored
-“Tie” together tooth surfaces, for strength and support
Full Metal Crowns
-These are Crowns cast entirely in metal, and can be made from a number of different alloys (mixtures of metal). They can be made in thin sections, whilst still maintaining their properties in strength, and therefore tooth preparations are less destructive than MCC’s and ACC’s.
-FMC’s can have a hardness similar to enamel, and are used in situations where occlusal loading is high, for example posterior teeth
-Occlusal and interproximal tooth contacts can be achieved easier with FMC’s and so it’s use is indicated in cases where this would be difficult to achieve with other materials
-Using dissimilar metals adjacent to each other can cause adverse reactions, and so if a patient has existing successful metal FMC’s then it could be wise to use the same again.
-Aesthetically, FMC’s do not match the cosmetics of a normal tooth, and patient preference could be a major factor in the decision to provide one or not.
Metal Ceramic Crowns
-These are Crowns consisting of a Metal “coping/cap” with a Ceramic layer fused over it
-Combines the strength of a metal substructure, with the better aesthetic properties of porcelain
-Very destructive preparation, to accommodate for the thickness of the Metal AND overlying Ceramic, however less so than an ACC.
-MCC’s can be made with entire Porcelain coverage or partial coverage: Metal occlusally and lingual/palatally. The advantage of the latter is that a less destructive tooth preparation is required, with retention and resistance form maximised. Metal occlusal contacts are also easier create and adjust, and cause less opposing tooth damage compared to Porcelain.
-Used on posterior teeth where aesthetics are important, and FMC’s are contraindicated for any other reason
-If there is insufficient space anteriorly for an ACC, due to the thickness of the Ceramic, an MCC could be indicated
-If a visible ACC repeatedly fails due to occlusal stress, an MCC could be indicated
-If there is a risk of excessive opposing tooth wear. In such cases, the opposing tooth could be ground down and provided with a composite, or the patient could be provided with a night time occlusal splint.
-In a young patient, where risk of pulpal damage during preparation is high. In such cases, a Dentine Bonded Ceramic Crown may be a viable option.
All Ceramic Crowns
-An All Ceramic Crown is, as the name suggests, made entirely from ceramic, and can produce the excellent aesthetic results compared to other Crowns.
-ACC’s are relatively weak restorations, being brittle in thin sections, so therefore are usually restricted to anterior restorations where occlusal forces are usually lower and aesthetics are important.
-Apart from the Dentine Bonded kind, preparation’s for ACC’s are the most destructive compared to other Crown preparations, so alternatives should be considered first.
-Due to the aesthetic nature of ACC’s they can be used to mask severely discoloured anterior teeth and existing post and core substructures. In existing Post and Core restorations where there is a risk of trauma, an ACC is preferred over an MCC, because stresses are more likely fracture the Porcelain, rather than being transferred to the Post Core leading to root fracture.
-Edge to edge occlusion, due to risk of fracture under occlusal loading.
-Where opposing teeth occlude in the cervical fifth of the palatal surface.
-Where ideal preparation form cannot be achieved to support the porcelain.
Resin Bonded Porcelain Crown
-Although these can classified under the ACC group of restorations, it is worth a separate entity, because of the differences in properties and tooth preparation.
-RBPC’s are a comparatively recent addition to a dentist’s armamentarium, and have been described as a full-coverage ceramic restoration, which is bonded to the underlying tooth using a resin composite based material. The bond interface lies between a micromechanically retentive ceramic fitting surface and a dentine bonding system.
-RBPC preparation is less destructive than other All Ceramic preparations, which causes less pulpal irritation and adheres to the concept of being a conservative as possible. With this however, comes the difficulty in production by the lab, and in fabricating a temporary restoration, which is important for pulpal protection.
-As with other ACC’s the manufactured RBPC’s are weak until bonded to the underlying tooth, especially because DBC’s are thinner in section.
-Good fracture resistance vs other ACC’s
-Can facilitate for situations of large preparation taper, because retention lies to a larger extent in the bond strength
-Luting material is insoluble in liquid
-No marginal gaps, as these are filled with the luting material
-Fluid isolation is essential with the dentine bonding systems, and this can be difficult to achieve, especially where margins are subgingival
-The luting procedure is more time consuming, and highly technique sensitive
-There is a lack of longitudinal data on effectiveness
-These are Crowns made from composite systems with reinforced fibres
-Currently, not widely used, however the potential for future use is considerable due to decreased lab costs, good aesthetics and less wear to opposing teeth
-Do not chip as easily as Porcelains
-Greater wear rate and future staining due to abrasion removing protective surface.
Post and Core Crowns
- -These are Crowns placed on a core with an attached pre-fabricated or custom Post that goes into the pulp chamber and root canal of an endodontically filled tooth for the purposes of increased retention where it cannot be achieved by other means.
-Previous thought, was that the inclusion of a post would strengthen a root filled tooth, however studies have failed to prove this, and rather there is a risk of root fracture as occlusal forces would be directed down the long axis of the post.
-Three quarter Crowns are used where the one wall of a tooth remains intact and healthy, hence
-A conservative option, however technically challenging.
-Before we carry on, it is important to get a grasp with some basic principles and terminology related to occlusion:
ICP (aka Centric Occlusion and Maximum Intercuspation) – The position of the mandible, when the maxillary and mandibular teeth are at their most interdigitated.
-Generally this is the position in which restorations are made
RCP (aka Centric Relation) – The reproducible position of the mandible independent of tooth-tooth contact(s) when the mandible is closing in terminal hinge axis
-There are a few situations in which a Crown will need to be adjusted to conform in RCP too and these include:
-When altering a patient’s OVD
-When ICP is not stable
-Where you need to habituate the mandible distally
-Where deflective contacts in RCP exist and need to be removed. A deflective contact is any contact that diverts the mandible from the normal path of closure into ICP
-When restoring anterior teeth, and movement into RCP results in heavy anterior forces against teeth to be prepared
-If RCP is a considerable factor, casts mounted in RCP and mounted on a semi-adjustable articulator can be used to further assess occlusion and allow for trial adjusting
Terminal Hinge Axis (aka Retruded Axis)- The most retruded position of the mandible, determined by the TMJ, not tooth contact. The mandible moves in a purely rotational movement in this position.
Working Side (aka Rotating/ Functional side)- The side to which the mandible moves during a functional movement, i.e. if the mandible moves to the right, the right side is the working side, and the left is the non-working side.
Non- working Side (aka Balancing side)- The side opposite to the working side during lateral excursion.
Functioning Cusps- These are the Palatal Cusps of the maxillary teeth and Buccal cusps of the mandibular teeth which occlude with opposing fossae, marginal ridges and cusp slopes.
Non-functioning/Balancing Cusps- These are the Buccal Cusps of the maxillary teeth and Lingual cusps of the mandibular teeth.
Guidance- The contacting teeth when a patient slides their mandible slides laterally or antero-posteriorly from ICP are guiding teeth. The path that the mandible takes is determined partly by the teeth in contact, and hence they provide “Guidance”.
-When a patient slides their mandible to the side they are about to chew on, this side becomes the “working side” and the opposite side becomes the “non-working side”
-“Canine guidance” is when only the upper and lower canines on the working side are in contact during lateral excursion, causing all of the posterior teeth to disclude. For restorative purpose canine guidance is the ideal.
-“Group function” is when several pairs of teeth are in contact on lateral movement to the working side, usually being premolars or premolars and canines
-Incisors and canines usually provide guidance on protrusive movement
-Guidance teeth undergo non-axial (lateral or antero-posterior) forces on excursion and if a guiding tooth is to be Crowned, there is an increased risk of fracture or decementation, particularly if the loads are heavy. Considering the guiding teeth before preparation is essential, and will also influence your choice of material. If the guiding teeth needing to be Crowned are strong enough, then the same guidance pattern can be re-established, however if it is felt that occlusal stresses are too high, then guidance can be moved onto other teeth. Changing guidance can be achieved by altering the occlusion slightly, for example if Crowning a guiding canine tooth; the Crown can be adjusted out of occlusion transferring the role to the premolars instead.
Interference- Interferences are any teeth that hinder smooth guidance of the mandible into ICP. A “Working side interference” is an interfering tooth/teeth on the same side that the mandible is moving to. A “Non-Working side interference” involves a tooth on the side that the mandible is moving away from in this sense. Generally it is best to remove working and non-working side interferences before a tooth is prepared.
Now that the terminology and basic principles have been outlined, we can further discuss and look into the clinical applications
Initial Occlusal examination
-An initial examination with regards to ICP, RCP and tooth relation in guidance is essential before any tooth preparation.
ICP- to assess reproduceability and contacts to be re-established
RCP- to assess if any deflective contacts are present
Guidance- if the Crown’s to be prepared are guiding teeth, this will affect choice of material and whether guidance needs to be altered. Mark teeth with different coloured articulating paper under different excursions for an extra visual guide.
-TMJ dysfunction – Palpate the muscles of mastication for tenderness
-Feel for any clicking or crepitus
-Assess mandibular movements, keeping an eye for any deviation
-Individual teeth – assess mobility, wear, caries
-A more detailed initial examination may be needed if there are specific occlusal problems or a history of Temporomandibular Dysfunction
-It is useful at this stage to take impressions for upper and low hand held study casts, for further examination where there is an unimpeded view of occlusion. Study casts can also confirm whether there is a stable ICP, and if there is not, the possibility of inter-occlusal records would need to be considered. Study cast impressions can be done in Alginate with stock trays, with an emphasis on impression quality in the occlusal areas rather than the peripheries and depths of the sulci. It is important to note that simple Study Casts do not provide any information of excursion or RCP, and if this is required, Articulate Study Casts are indicated.
Articulated Study Casts (Semi-adjustable articulator)
-Where you need to ensure correct guidance against your restoration, especially where multiple Crowns are being prepared.
-If OVD is being increased
-Where Working and Non-Working Occlusal interferences are being removed
-Where jaw position is being stabilized by an Occlusal splint
-Where an Occlusal splint is being prescribed to protect the Crown/Crowns from bruxism
-To accurately position the casts to a patient’s anatomy in terms of condylar hinge axis and jaw relation, a Facebow record can be taken alongside an Inter-Occlusal record. There are a number of systems on the market to create a Facebow record and they all work to allow the technician to mount Study Casts and mimic excursions around to hinge axis as close as possible. Inter-Occlusal records are achieved by getting a patient to bite on silicone or wax, leaving a record of intercuspation. It has been shown that Inter-Occlusal records, in many cases, reduce the accuracy of mounting of the casts. Inter-Occlusal records should not be used in single tooth restoration cases where a patient has stable ICP, but rather where casts are unstable and teeth to be Crowned will be key for support. A good Inter-occlusal record captures the tooth cusp tips, and not fissures or soft tissue.
Replicating tooth guidance
-Where teeth to be prepared are involved in guidance, in both protrusive and lateral excursions, a record of tooth guidance may be needed. This is so the technician can replicate the existing form of the tooth/teeth, in order to re-establish occlusion after preparations.
-Making a record of these excursions is particularly important where the tooth or teeth to be prepared are alone in guiding excursions.
-Where numerous teeth are to be Crowned, guidance can be lost altogether, if a record beforehand is not taken.
-The two most effective methods to overcome this technical difficulties are:
-The “Crown about method”. Where alternate teeth are prepared, hence maintaining some tooth surface for guidance
-The Custom Incisal Guidance Table – Copies protrusion and lateral excursion by placing a mound of putty on the Incisal Guidance Table of an Articulator, and moving the STUDY casts (and pin) in the full range of protrusion and lateral excursions. Once the putty has set, a permanent record of excursion is made, which is later used for guiding the WORKING casts.
-Aesthetics is the branch of philosophy, which deals with the nature of art, beauty and taste, with the creation and appreciation of such subjects. When dealing with shade, shape and form of restorations, the decision on whether they are visually pleasing or not, is very subjective to a patient’s own opinion. For this reason, it is important to gauge a patient’s demands before the outset of any treatment. The ideals of patient then need to be weighed up against tooth removal and potential damage to the pulp and periodontium, with consideration to a dentist’s scope of skills. Where demands are unrealistic, it is important to communicate this with the patient beforehand, making a decision based on existing knowledge and experience. Due to the subjective nature of aesthetics, clinical experience will play a major role in being able to judge visual outcome, and it is better to undervalue on the scale of realistic possibility, rather than promise over-realistic outcomes.
-Currently, the most aesthetically pleasing Crowns, which have been studied over an extensive period of time, are those containing Porcelain. In most cases, this comes at the cost of tooth tissue, because a thicker Crown shell needs to be made for strength, in particular Metal Ceramic Crowns, which have the most destructive preparations. As stated previously, a more destructive preparation is more likely to cause pulpal damage, and a decision needs to be made on whether to sacrifice long-term pulp health for aesthetics, or lose aesthetic quality for pulp health.
-Here is a table of what is generally accepted as aesthetically pleasing types of Crown:
-After a judgement has been made as to what is achievable in relation to patient demands, there are ways to help a patient visualise what the restorations may look like. Time spent showing the patient aesthetic possibilities at this stage is invaluable in saving future disappointment.
- Diagnostic Wax Ups- Created by the lab, these are study casts, with the potential restoration “waxed up” to get an idea of form
- Composite additions- Composite can be added to the patients teeth, without etching or application of a bonding agent to show the patient form and shade
- Black ink- Can be applied on teeth to mimic the effect of tooth reductions
- Computer generated restorations- Software can be used to generate computer images for a patient to see, and can be edited to patients preferences whilst in consultation
- Photographs of previous restorations- Can show a patient the possibilities and limitation if similar cases are used
- Temporary restorations- Gives the patient a feel of the form and shape of the restoration in relation to their other teeth and facial features before a final restoration is placed
-The margins of the restoration can supra or sub-gingival (up to 1mm).
-Where the tooth margin cannot be seen, there is good reason to place the margins supra-gingivally. Finishing and maintenance is easier with supra-gingival preparations, and there is lower risk of damaging periodontal health.
-Where Crown margins will cause an aesthetic problem, preparations can be placed up to 1mm sub-gingivally. However, caution must be taken where there are prominent roots or thin gingival coverage, because there is a risk of recession.
-If preparations need to go further than 1mm subgingivally to because of insufficient preparation height (and therefore retention), Crown lengthening is an option to avoid encroaching into the biological width of the periodontium.
-In terms of metal or porcelain at the margins, there are arguments stating that metal produces the most predictable marginal seal. However, in certain areas, where aesthetics are of utmost importance, a porcelain margin on a shoulder finish line will produce the best results. It is also worth noting that even a metal margin placed subgingivally can show through the gingival tissue, affecting aesthetics, especially where tissue is thin.
-Shade matching is another aspect of aesthetics that is very subjective, and a good example where patient involvement is important. Because of the subjective nature, there are a number of systems on the market to help make a decision on the shade, with the most common being shade guides, where a patient’s teeth are compared to common shades used in Crown manufacture.
-Difficulties arise in shade matching, because teeth are non-uniform in colour, have defects, unique features, are semi-translucent and appear different shades in different lighting conditions.
-Colour can be described by Hue, Value and Chroma. Hue is the name of the colour, for example red, blue, green. Value is the lightness or darkness of a colour, a high value indicating something is light and a low value, dark. Chroma is the amount of saturation of a particular hue, for example red with a high chroma would a deeper more intense red, than red with a low value chroma.
-Vita 3D Master and Ivoclar Chromoscope are two examples of shade guides based on Hue, with Value and Chroma subdivisions.
-Electronic devices have been created to judge shades of teeth, however their effectiveness has not been fully evaluated, therefore it is useful to use these as reference alongside your own judgement
-Devices with magnification and colour corrected lighting are useful tools in assessing surface detail as well as shade
-The following is a scheme for shade determination from the BDJ 2002; 192: 443-450 – Crowns and other extra-coronal restorations: Aesthetic control:
-The above scheme provides good guidance in a methodical approach to shade determination, however, the results of the restoration will only be as good as your communication with the dental laboratory. It is important that all the information obtained is communicated clearly, and clarity can be emphasised by the use of diagrams, photos, two-way communication methods and even visits to the dental lab if necessary.
-In cases where shade matching and detail has been a difficulty, trial placement of restorations can be done, and this would be done, before the final surface glaze has been implemented. Cementation during this trial period would involve the use of modified Zinc oxide and eugenol based cements, because removal could be a great difficult otherwise. Pigmented luting cements to alter porcelain shades slightly can also be used in such cases.
The references will be in a precise and numbered system in the third part of the series, which will include a PDF download of the article.
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Walmsley, et al. Restorative dentistry. 2 ed2007.
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Herbert T. Shillingburg et al. Fundamentals of Fixed Prosthodontics. 3 ed1997.
Wassell RW, Walls AWG and Steele JG. Crowns and extra-coronal restorations: Materials Selection. British Dental Journal 2002; 192: 199-211.
Steele JG, Nohl FSA, and Wassell RW. Crowns and extra-coronal restorations: Materials Selection. British Dental Journal 2002; 192: 377-387.
Mitchell L, Mitchell D. Oxford Handbook of Clinical Dentistry. 5 ed2009.
Wise D. Occlusion and restorative dentistry for the general practitioner. Part 2- Examination of the occlusion and fabrication of study casts. Br Dent J 1982; 152: 160-165
Nohl FSA, Steele JG and Wassell RW. Crowns and extra-coronal restorations: Aesthetic Control. British Dental Journal 2002; 192: 443-450.
Sproull R C. Color matching in dentistry. Part II: Practical applications of the organization of color. J Prosthet Dent 1973; 29: 556-566
Bishop K, Briggs P, Kelleher M. Margin design for porcelain fused to metal restorations which extend onto the root. Br Dent J 1996; 180: 177-184
Sorensen J A, Torres T J. Improved color matching of metal-ceramic restorations. Part I: A systematic method for shade determination. J Prosthet Dent 1987; 58: 133-139