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Since the American Dental Association (ADA) changed how it defines the term "ceramic" to include 3D printed materials that are predominantly composed of a ceramic or glass filler, interest in using these materials for permanent crowns has skyrocketed. 3D printing has been used in other industries for many years; however, its use and awareness regarding how it can be used is still limited in the dental community. According to the results of a recent survey, only 31% of dentists and 56% of dental technicians have undergone some form of training (including attending a lecture) on 3D printing.1 As awareness and adoption of 3D printing technologies improve, it is important to critically evaluate these new materials and applications relative to current milled alternatives and historical data on composite resin crown materials.
Machining Versus Printing
Broadly, we need to evaluate 3D printing versus machining with respect to materials and equipment. What are the benefits and disadvantages of each relative to current and future developments? We have more than 30 years of data to support the success of milled ceramics. Machining has been proven to produce accurate, tough, and overall well-fitting restorations. Machined restorations are primarily milled from traditional ceramics, such as feldspathic porcelain; glass-ceramics; or zirconia materials. Dense, full-contour milling blocks can provide reliable, nearly pore-free restorations.2 Although zirconia is the most widely used indirect fixed restorative material, it has become a commodity with wide-ranging properties for blocks that purportedly have the same composition.3 In addition, the strength, toughness, and accuracy of zirconia restorations may all be compromised due to poor processing techniques by the manufacturer, inaccurate firing techniques, and finishing errors.4-6
Beyond inconsistencies in the blocks and the potential for errors during processing, machining also wastes a lot of material. Furthermore, the accuracy of milled restorations may be compromised due to over milling, which can be related to minimum bur diameter and tooth preparation techniques.7 Alternatively, the fit of printed polymer restorations has been shown to be well within acceptable guidelines and sometimes even superior to the fit of machined restorations.8 The accuracy of the occlusal surfaces of printed restorations may also be superior to those of machined restorations.9
3D printing is rapidly evolving as an alternative to machining and other traditional fabrication techniques.10 Most 3D printers utilize some type of monomer-based material that is cured layer by layer. The different 3D printing technologies available include SLA, DLP, LCD, and CLIP. Among other benefits, a huge advantage of 3D printing is the conservation of the materials, which may be reused. The printing strategy, material type, and post curing process used all affect the accuracy and mechanical properties of 3D printed objects.11 Until recently, most 3D printing materials were in the form of unfilled monomers, such as those used for denture bases and provisional restorations. Some newly formulated printing resins are filled with glasses and ceramics to improve their mechanical and physical properties and permit the printing of permanent restorations. The advantages of 3D printing materials for permanent crowns include wear kindness, stress absorption or resilience, decreased gap sizes, and cost.
Physical Properties
With any new material, we need to understand the properties and establish criteria for clinical selection. In Brave New World, Aldous Huxley wrote, "One believes things because one has been conditioned to believe them." As a result of the conditioning of the dental community, it often focuses on strength as the ultimate determinate for the clinical use of a material. However, if we evaluated natural tooth structure with respect to strength only, it would seem to be completely unacceptable as a crown material. The three-point flexural strength of the combined enamel-dentin complex is only about 45 MPa to 125 MPa.12 If we judged the use of restorative materials solely based on strength, we would be extracting all of our teeth and replacing them with implant-supported zirconia restorations.13
Zirconia, which demonstrates flexural strength of 900 MPa to 1200 MPa, is widely regarded as being at the pinnacle of strength. Interestingly, in an ADA survey of clinicians about zirconia use, 52% reported issues with debonding, 31% reported wear of the opposing teeth, and 23% reported fractures.14 Therefore, it appears that strength alone will not solve all problems. Ceramics offer several advantages, such as color stability, wear resistance, and good stiffness, but also present drawbacks, such as brittleness that can result in sudden failure, potential enamel wear, and chipping, especially during machining. Alternatively, composite resins tend to be wear kind, to result in minimal chipping, to be easier to adjust, and to demonstrate excellent resilience. However, they may wear, change color, or be too flexible. Load bearing capacity varies with material type and thickness; therefore, minimum tooth reduction requirements are established in order to achieve a minimum restoration thickness with adequate load bearing capacity. Polymer-based restorations may demonstrate better load bearing capacity due to their resilient nature, which may make them ideal for applications in the area of implant-supported prostheses, particularly where there is concern about stress transfer to the bone because these materials might act somewhat as a shock absorber. The minimum thickness needed for polymer-based materials may be significantly less than that needed for corresponding traditional ceramic materials.15 In addition, polymer-based restorations may achieve improved overall esthetic results due to their higher translucency and better ability to match the shade of the surrounding dentition.16
The materials for printing and machining that are now classified as a "hybrid ceramic" are composite resins that are modified with separate glass or ceramic fillers. A high filler content is required for a material to be classified as such, and the composition of the filler content is of paramount importance to the success of polymer-based restorations. The filler content of hybrid ceramic materials may influence all of the mechanical and physical properties, including strength, toughness, elastic modulus, wear resistance, color stability, polishability, gloss retention, and bond strength.17 Filler content maybe be measured by the percentage of its weight or volume. It is easier to achieve a high weight percentage due to the high molecular weight of the fillers relative to the polymers. Materials with lower filler content generally demonstrate less desirable properties. 3D Printing materials for single unit restorations should demonstrate properties that at least match those of highly filled direct composite resins and machinable composite resins.
Lessons Learned
In The Life of Reason: Reason in Common Sense, philosopher George Santayana suggests that "Those who cannot remember the past are condemned to repeat it." Astute dentists with many years of experience have seen new materials that suddenly appear, fail in clinical use, and then disappear. We have learned important lessons from composite resin materials that were touted as the new wonder materials and alternatives to traditional ceramics but then found to be less filled composite resins with poor properties. In a Clinician's Report study that examined various composite resin materials for crowns, a high unresolved sensitivity rate of 17% to 50% was shown, which may be related to the high flexibility of composite resins with low amounts of filler material.18 One of the problems with materials with low filler content is that they possess a low modulus of elasticity. Elastic modulus is essentially a measurement of a material's ability to resist deformation under stress. Using a material with a value that is too low can result in deformation under occlusal forces and clinical situations that include sensitivity as well as rapid wear, discoloration, and debonding of the restoration.19 Although the ideal value for the elastic modulus of restorative materials is not agreed upon, research has shown that values of approximately 4 GPa or less may lead to clinical failure. Ceramics have elastic modulus values that range from approximately 35 GPa all the way up to 125 GPa, which is for zirconia.20 The elastic modulus of dentin and enamel are approximately 18 GPa and 100 GPa, respectively.21 Although stiffer materials with a high elastic modulus are stronger, they do not bend much, if at all, before they fracture. Therefore, in order to produce materials with resiliency, we need a find a happy medium between the very low values of unfilled polymers and the very high values of traditional ceramics.
Evidence for Printed Materials
There is limited data available for many of the new materials for 3D printing. The mechanical properties of a variety of these materials, which were obtained from manufacturer self-reported data and independent research, are summarized in Table 1.22-25 In looking at the literature, the filler content values of many of these materials are reported by the manufacturers as a range of 30% to 50%. Assuming that this is weight%, many of these materials may not actually meet the ADA criteria. Other properties that need to be taken into consideration in the evaluation of new 3D printed crown materials include surface finish and rate of wear. The surface finish of a restoration may affect bacterial adhesion and color stability. More specifically, a rough composite resin surface may lead to plaque accumulation and recurrent decay.26 Staining is also increased if a highly polished and smooth surface cannot be achieved. Color stability is partially related to the surface finish of hybrid ceramic materials, but it can also be greatly influenced by the chemical bond between the filler and the surrounding polymer. If the bond is not stable, over time, cracks can open between polymer matrix and the filler, allowing the percolation of fluids into the material that may result in discoloration and excessive wear as well as fracture of the restoration.27 There is very limited data on wear rate of hybrid ceramic materials. One study that performed 400,000 wear cycles using a University of Alabama at Birmingham type in vitro wear tester determined that the volumetric wear of hybrid ceramic materials ranged from 0.012 mm3 to 0.019 mm3, depending on their composition.24 Wear rates of these materials, which were obtained in the laboratory of the author of this article using the same type of device, are presented in Table 2.
Conclusion
Manufacturers are intent on developing new products that are beneficial for our patients, but many new materials demonstrate significant variability in their composition and properties. Polymer based, hybrid ceramic materials present numerous advantages for the fabrication of permanent restorations, from single units to all-on-X applications. However, it is incumbent upon clinicians to properly evaluate these new materials in order to make informed decisions regarding how, where, or even if they should be used clinically.
About the Author
Russell A. Giordano II, DMD, DMSc
Professor of Restorative Sciences & Biomaterials
Director of Biomaterials
Assistant Dean of Biomaterials & Biomaterials Research
Henry M. Goldman School of Dental Medicine
Boston University
Boston, Massachusetts
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