November 13, 2018
by Ashkan Ebrahimpour; Omar El-Mowafy
Aesthetic tooth-colored restorations for both anterior and posterior teeth have been in the highest demand in recent years. This increasing demand motivated the dental industries to enhance the aesthetic and mechanical properties of restorative materials. These efforts lead to the introduction of contemporary ceramics such as lithium disilicate glass-ceramic (also known as IPS e.max which is a brand name for the material) and yttria-stabilized tetragonal zirconia polycrystalline (zirconia) that present higher aesthetics and mechanical properties compared with their predecessors.
These mechanical and aesthetic properties of the new materials necessitate restorations of various thicknesses that consequently requires different amount of tooth structure reduction. There are several other factors affecting the dentist’s decision-making for material selection such as quantity and quality of the tooth structure, shade of tooth pre and post preparation, position of tooth in the arch, condition of neighboring and opposing dentition, available restorative space, occlusal scheme, malocclusion, patient’s parafunctional habits and patient’s expectations.
Despite all the diagnostic elements to be considered, restoring teeth at minimal biologic cost remains a fundamental principle of tooth preparation. It affects the postoperative vitality and sensitivity as well as long-term prognosis of the restored tooth as well as the restoration.
The objective of this article is to review the suggested preparation protocols for three different restorative materials of lithium disilicate glass-ceramic (IPS e.max), yttria-stabilized tetragonal zirconia polycrystalline (zirconia) and porcelain-fused-to-metal (PFM) and also compare the amount of tooth structure reduction needed for these three materials.
Ceramics in Dentistry, Classification and Indications
Considering the durability and somewhat aesthetics, PFM crowns have been the material of choice for single crowns and bridges for many years. That being said, new ceramics such as lithium disilicate glass-ceramic and zirconia are rapidly replacing PFM.
Ceramics can be classified based on different properties. Three main categories of ceramics based on glass-content are presented below:
a) Predominantly glass ceramics (e.g., feldspathic, lithium disilicate glass-ceramic)
b) Particle filled glass ceramics (e.g., alumina)
c) Polycrystalline ceramics (e.g., zirconia)
The flexural strength represents the highest stress experienced within the material at its moment of yield. It is a measure showing the strength of a material and it is measured by mega pascal (MPa).
Historically, predominantly glass ceramics had the lowest flexural strength of around 60 to 70 MPa. By adding leucite, the flexural strengths of leucite-reinforced ceramics were increased to around 120 to 150 MPa. The new glass ceramics such as Ivoclar Vivadent lithium disilicate glass-ceramic (IPS e.max) have much higher flexural strength (@500 MPA).
Monolithic polycrystalline ceramics specifically zirconia, has the highest flexural strength. Currently, there are three main zirconia products in the market of North America with different aesthetics and subsequent different flexural strengths. The most aesthetic monolithic zirconia has a flexural strength of slightly less than 800 MPa. The monolithic zirconia material with the highest flexural strength is extremely opacious with the flexural strength of about 1400 MPa. There is another zirconia product with 1100 MPa flexural strength that is less aesthetic but stronger than the former group and apparently less strong but more aesthetic than the latter group. Indications of the above-mentioned materials are summarized in Table 1.
Amount of Reduction Required for Different Ceramics
A review of Prosthodontics reference books and the manufacturers’ guidelines performed for providing preparation guidelines for three materials of PFM, lithium disilicate glass ceramic and zirconia.
The classic guideline for PFM preparation is a uniform axial reduction of 1.2 mm on the entire facial surface. To provide the best aesthetics, incisal reduction of 2 mm is recommended. In posterior teeth, the suggested amount of reduction is 2 mm for functional cusps and 1.5 mm for non-functional cusps. It is advocated to place a shoulder facial margin to accommodate the thickness of both metal substructure and the porcelain. The thickness provides strength at the marginal area as well as aesthetics. For the palatal margin, reduction of 0.5 (for chamfer metal margin) is proposed. In posterior clinical scenarios were more strength and less aesthetics are needed or where the clinical crown height is minimal, a metal occlusal restoration could be made. The suggested amount of occlusal reduction for this restoration is 1.5 mm uniformly for both functional and non-functional cusps.
For e.max material, an axial reduction of 1 mm followed by an occlusal/incisal reduction of 1.5 mm should be performed. A shoulder or chamfer of uniform width (approximately 1 mm) is advocated as a gingival finish line to provide a seat that resist forces directed from the incisal.
As discussed, there are different zirconia products with different properties available in the market that apparently affects the amount of tooth structure reduction. Although some clinicians report less amount of reduction for high flexural strength monolithic zirconia, the current guidelines for the amount of reduction needed for monolithic zirconia are suggesting axial reduction of 0.5 to 1 mm with incisal or occlusal (for both functional and non-functional cusps) reduction of 1.5 mm.
Amount of reduction for the materials are summarized in Table 2.
Comparison of the Amount of Tooth-Structure Removal for Different Materials
To find the amount of reduction needed for the above-mentioned three materials, groups of resin teeth of different morphologies (Upper left central incisor i.e. 21, first premolar i.e. 24 and molar i.e. 26) were individually weighed to high precision, then prepared following the reviewed preparation guidelines for three different materials of PFM (with and without metal occlusal for molars), lithium disilicate glass ceramic and zirconia. Then the teeth were re-weighed after preparation and the amount of structural reduction was calculated.
A total of 50 teeth of three different morphologies were prepared for the study. Teeth 21 and 24 consisted of three groups of PFM, lithium disilicate glass-ceramic and zirconia and each group included five samples (i.e. total of 2 (tooth morphology) x 3 (types of preparation for different materials) x 5 (number of teeth in each group) = 30.
Teeth 26 also consisted of three above mentioned groups plus one more group for PFM with metal occlusal preparation. Again, each group included five samples (i.e. 4 (types of preparation for different materials) x 5 (number of teeth in each group) = 20. This produced a whole sample of 50 teeth (30+20) for the study.
The results of the statistical analyses revealed that the amount of tooth reduction for zirconia crown preparations was the lowest and statistically different compared with the other two materials. No statistical significance was found between the amount of reduction for porcelain-fused-to-metal and lithium disilicate glass-ceramic crowns.
Therefore, solely considering saving the amount of tooth structure when preparing teeth for three materials of PFM, lithium disilicate glass ceramic and zirconia with current guidelines, it does not make a difference to prepare a tooth for PFM or lithium disilicate glass ceramic. The only material that can potentially save more tooth structure is zirconia.
As the properties of the dental materials change, there will be new preparation guidelines that will alter the existing conclusion. OH
Oral Health welcomes this original article.
About the Authors
Dr. Ashkan Ebrahimpour received his DDS and specialty certificate in Prosthodontics in Europe. He continued his dental education after moving to Canada, and obtained a Master of Science Degree in Dentistry followed by a Master of Science in Prosthodontics at the Faculty of Dentistry, University of Toronto. In addition to private practice, Dr. Ebrahimpour works at the University of Toronto Faculty of Dentistry as a part-time clinical instructor.
Dr. Omar El-Mowafy if a Full-professor with tenure status at the University of Toronto, Faculty of Dentistry. He is also a full-member of the School of Graduate Studies at the University of Toronto. He maintains a private dental practice in the city of Mississauga.