Clay has been used for many years. On firing it can produce what we know as "china", and it contains a material called Kaolin (hydrated aluminium silicate). However, dental porcelains, although very similar in their process don't contain kaolin due to its opaque qualities.
Ceramics are inorganic, non-metallic solids which can vary in structure from amorphous to crystalline. Porcelains are at the glass end of the spectrum with an amorphous structure and what ceramics are more crystalline in structure.
How the ceramic is made: The raw material is powdered and mixed. Heating is carried out, till fusion of the powder occurs. It is then cooled rapidly causing high stress which propagates through the mass, resulting in cracks and crazing, producing fritz. These are milled into a fine powder. This is the powder the lab used to mix with water, ready for the ceramic to be layered and sintered, creating the porcelain restoration.
Sintering: The coalescing of the powder into a mass by means of heating
Feldspathic/ Leucite = Potassium Aluminium Silicate
Leucite concentration can vary to provide more strath to the restoration
These are very aesthetic and can be used when veneering tech, or metal copings. Due to the glass matrix they can be etched and bonded to the tooth.
Note: Firing Temperature can define the type of porcelain ranging from ultra low fusion temperatures to higher temperatures
You may have heard the term Emax being used (brand name not endorsed by the site). This material is stronger than the traditional leucite porcelains as it has a greater crystalline structure. The material can be veered with sintered nanofluorapatite, to increase its aesthetics. The presence of a glass matrix means it can be etched and bonded to the tooth.
Alumina
With the addition of alumina, high strength cores can be provided. in addition glass (lanthanum) can be infiltrated, improving bond strength. With the alumina dispersed throughout the ceramic, it provides crack stopping properties, strengthening the restoration.
Zirconia:
Very strong. Zirconia has a property called transformation toughness. This means that if a crack or void was to occur, the material has the ability to expand around the fault and expands. On expansion the void shrinks. However, if a ceramic veneer is added to a zirconia core this expansion may increase the likelihood of the superficial ceramic to crack.
Zirconia has different allotropic forms. In its tetragonal (undistorted form) it is at its strongest, and weaker in its rectangular form; it also has a cubic form.
Zirconia is difficult to bond to, both the ceramic veneering surface and to the tooth. Therefore, retentive preps are required. Glass infiltrations are being used to try and enhance the bond to the ceramic veneer (surface application for aesthetics.
This is a highly skilled task which takes time and care. Sintering can incorporate errors into the material, increasing the risk of voids leading to propagation of crazing/ cracks. As the field of dental materials grows, different methods can be used in the attempt to decrease these errors. In addition, the glassy matrix can be etched; therefore, takes advantage of the advancements in adhesive dentistry, allowing thinner preps to be used (veneers) and provides strength on bonding to the restoration.
This method is similar to when metals get casted. The material is injected into the hollow space within the investment material.
Alumina commonly used
For example when milled from a ceramic block
As a general principle for all core materials (metal and polycrystalline), these should be kept as anatomical as possible. This helps to obtain and even distribution of veneered ceramic, allowing optimal support throughout the restoration.
Metal ceramics have also been discussed in the previous lesson, but it doesn't do any harm to recap the principles again and look at the restorations from a ceramic point of view. Metal can be altered and polished easier than ceramics and, if only selected surfaces have porcelain, the prep can be minimised. Having metal on the bite surface is indicated in bruxists, as ceramics would wear heavily on the opposing dentition.
The alloys need to be able to retain the ceramic, and this is achieved by:
Compression fit: As we now know, ceramics shrink during their construction. It is important that the alloy and ceramic have similar CTE because, if the ceramic was to contract too much compared to the underlying metal, the material would become under tension, developing stress throughout and increase the risk of crack propagation.
Micromechanical retention: sandblasting the alloy surface produces irregularities for the glass to flow into and hold on cooling.
Chemical: We know from the previous lesson, the alloys (in particular the base metals) produce an oxide surface layer.
Tooth preparations: In most cases, more tooth needs to be sacrificed to account for the minimum thickness of ceramic and the metal coping (substructure). In areas of load, the thickness of the full restoration (metal + ceramic) will be between 1.7mm - 2.0mm. In younger teeth, with larger pulp chambers, this is a greater risk of exposure. If, inadequate space is provided to the lab, aesthetics will be compromised: either with thin ceramic areas increasing the risk of fracture; or the tooth could be made over bulked and seating high in the bite, requiring adjustment and sent back to the lab to be reglazed. It is important that the metal ceramic junction isn't in the area of contact. This junctional margin should ideally be greater than 1mm away from the contact area.
The opacifying layers, although blocking out the metal shine through, don't refract light and therefore the final restoration can still look lifeless compared to other all ceramic methods used and the natural dentition.
Can a direct method be used?
Although not discussed in this lesson, composite veneers could save valuable tooth structure and if skilled, lead to great aesthetic results.
This list is not exhaustive, but hopefully helps prompt some thoughts/ questions when treatment planning.
NOTE: Below are common preparations used when supplying indirect restorations. The measurements used are a range or averaged from the various references listed at the end of the lesson. Please communicate with your labs for specific manufacturer guidelines, as the use of different materials could alter these measurements.
The below preparation incorporates all the various elements of tooth reduction, but the application of each will depend on which veneer design has been chosen.
Computer aided design and milling is an area of dentistry which is continuing to develop and may become common place in our future years as dentists.
Some systems (brands) are closed and others are open which means they are compatible with more than one manufacture.
Some systems are subtractive = milled from a block, but as 3D printing advances, additive systems are available = build (print) the restoration.
Process overview:
Acquire data - Process date - Machine the restoration.
Intra oral scanners produce digital images which can be sent to an in-practice machine or lab. Blocks of the material of choice are milled to the dimensions scanned. If the milling processes are carried out "in-house" and the patient doesn't mind the wait; their restoration can be made in a single appointment.
Some limiting factors which are being continuously developed, are aesthetics; but the blocks are incorporating layered staining and the freedom of prep design must match the machines ability to mill. This could lead to a more destructive prep in a situation which could have been more conservative.
I hope you found this lesson useful. Please use the references and third party readings to expand your knowledge.
These resources were used throughout to help produce this lesson:
Bonsor SJ, Pearson G. A clinical guide to applied dental materials. Elsevier Health Sciences; 2012 Dec 5.
Von Fraunhofer JA. Dental materials at a glance. John Wiley & Sons; 2013 May 31.
Weinstein GM, Zientz MT, editors. The Dental Reference Manual: A Daily Guide for Students and Practitioners. Springer; 2016 Nov 29.
Banerjee A, Watson TF. Pickard's Manual of Operative Dentistry. Oxford: Oxford University Press; 2011.
Summitt JB, Robbins JW, Hilton TJ, Schwartz RS. Fundamentals of Operative Dentistry:. Berlin: Quintessenz Verlag; 2006.
Ibbetson R, Jones IR. All-Ceramic inlays and onlays for posterior teeth. Dental Update. 2019 Jul 2;46(7):610-24.
Bhakta S, Deakin K, Joshi R. A CAD/CAM designed, semi-fixed, high strength, all-ceramic prosthesis for maxillary rehabilitation–a case report. Dental update. 2014 Jan 2;41(1):62-7.
Palin WM, Burke FJ. Trends in indirect dentistry: 8. CAD/CAM technology. Dental update. 2005 Dec 2;32(10):566-72.
BDJ
Mizrahi B. The anterior all-ceramic crown: a rationale for the choice of ceramic and cement. British dental journal. 2008 Sep;205(5):251.
