Review Article

Bioactive and inert dental glass-ceramics

Maziar Montazerian, Edgar Dutra Zanotto Department of Materials Engineering (DEMa), Center for Research, Technology and Education in Vitreous Materials (CeRTEV), Federal University of Sao~ Carlos (UFSCar), Sao~ Carlos, SP 13.565-905, Brazil

Received 10 August 2016; revised 14 September 2016; accepted 3 October 2016 Published online 31 October 2016 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/jbm.a.35923

Abstract: The global market for dental materials is predicted restorative dental glass-ceramics (RDGCs) are inert and bio- to exceed 10 billion dollars by 2020. The main drivers for this compatible and are used in the restoration and reconstruc- growth are easing the workflow of dentists and increasing tion of teeth. Bioactive dental glass-ceramics (BDGCs) display the comfort of patients. Therefore, remarkable research proj- bone-bonding ability and stimulate positive biological reac- ects have been conducted and are currently underway to tions at the material/tissue interface. BDGCs are suggested develop improved or new dental materials with enhanced for dentin hypersensitivity treatment, implant coating, bone properties or that can be processed using advanced technolo- regeneration and periodontal therapy. Throughout this paper, gies, such as CAD/CAM or 3D printing. Among these materi- we elaborate on the history, processing, properties and appli- als, zirconia, glass or polymer-infiltrated ceramics, and glass- cations of RDGCs and BDGCs. We also report on selected ceramics (GCs) are of great importance. Dental glass- papers that address promising types of dental glass- ceramics are highly attractive because they are easy to pro- ceramics. Finally, we include trends and guidance on relevant cess and have outstanding esthetics, translucency, low ther- open issues and research possibilities. VC 2016 Wiley Periodicals, mal conductivity, high strength, chemical durability, Inc. J Biomed Mater Res Part A: 105A: 619–639, 2017. biocompatibility, wear resistance, and hardness similar to that of natural teeth, and, in certain cases, these materials Key Words: glass-ceramic, dental, mechanical properties, are bioactive. In this review article, we divide dental GCs into biomedical the following two groups: restorative and bioactive. Most

How to cite this article: Montazerian M, Zanotto ED. 2017. Bioactive and inert dental glass-ceramics. J Biomed Mater Res Part A 2017:105A:619–639.

INTRODUCTION make dental materials a highly dynamic and exciting The dental materials market is composed of several seg- field.3–5 Furthermore, market research groups have pre- ments, including implants, core materials, restorative mate- dicted that the segments of restoratives, dental cements, rials, impression materials, dental cements, and bonding bonding agents, and core build-up materials will signifi- agents.1,2 The Research & Market group announced that the cantly increase in market value in the 2010–2020 decade.3–5 U.S. market value for dental materials was greater than $1 However, certain sub-segments within these categories billion as of 2013.3 According to prediction by this group, might decline, including amalgam and resin-based restora- the market is expected to grow substantially and approach tives, which undergoing replacement by more modern and $1.5 billion by 2020.3 Another report predicts that the effective materials processed by CAD/CAM, 3D printing and global dental implants and prosthetics market is expected tissue engineering.6–14 Among the most promising materials, to reach approximately $10.5 billion in 2020 and will con- zirconia (ZrO2), hybrids, glass-ceramics, and glass-infiltrated tinue to grow at an annual rate of 7.2% from 2015 to ceramics are of great importance.6–14 An analysis of glass- 2020.4 Therefore, persistent competition exists among doz- ceramic research and commercialization suggests that dental ens of companies in research and development, marketing, glass-ceramics is a potential thrust field for advanced tech- and product prices, which drives continuous technological nology.13 Furthermore, research and development in aca- progresses in dental materials.3–5 The main objectives demic communities and companies show increasing trends behind this endeavor are to ease the workflow of dentists in the number of published papers and patents in this field, and to increase the comfort of patients.3–5 These factors as summarized in Figure 1(a,b).

Correspondence to: M. Montazerian; e-mail: [email protected]

VC 2016 WILEY PERIODICALS, INC. 619 papers that have addressed promising types of dental glass- ceramics in recent years. Where deemed appropriate, we include future trends, open issues and guidance from a materials engineering perspective. This critical review article is structured as follows. A brief introduction to glass-ceramic processing, properties and the structure of teeth is given in Glass-Ceramic Process- ing and Structure and Properties of Teeth sections. Commer- cial and promising types of RDGCs are classified and reviewed in Restorative Dental Glass-Ceramics (RDGCS) sec- tion. Potential applications and promising researches on BDGCs are explained in Bioactive Dental Glass-Ceramics (BDGCs) section. Finally, open relevant issues, trends, and future research directions are highlighted in Conclusions and Trends section.

GLASS-CERAMIC PROCESSING Glass-ceramics were discovered by D. R. Stookey at Corning Inc., USA in 1953 and are polycrystalline materials produced by controlled heat treatment of certain glasses that contain one or more crystal phases embedded in a residual glass matrix.34,35 Figure 2 shows the main stages of the synthesis of glass-ceramics. First, the reactants are weighed, mixed, and melted, usually in a platinum crucible. Glass articles are FIGURE 1. (a) Number of scientific publications (articles, conference shaped by casting, pressing, blowing or any other traditional papers, books, and so forth) (Source: www.scopus.com), (b) number glass forming techniques. Finally, monolithic glass-ceramics of granted patents related to dental glass-ceramics (Source: www. are obtained after controlled heat treatment of the glass at thomsonreuters.com/derwent). The results were obtained by search- ing the keywords “dental glass-ceramic*” or “teeth glass-ceramic*” temperatures above the glass transition temperature (Tg)to or “tooth glass-ceramic*” in the article or patent title and the title, induce internal nucleation and growth of the desired crystal abstract and keywords. phase(s).35 A second, less frequently used, but interesting method is Previous review papers, book chapters15–32 and, more sintering with concurrent crystallization of a powdered recently, Gracis et al.33 have classified dental ceramics into the glass (e.g., for preparing a slurry and veneering dental mate- following three groups: (1) glass-matrix ceramics, (2) poly- rials). In this method, the melt can be quenched into water crystalline ceramics, and (3) resin-matrix ceramics. In other or air before grinding and sieving into the desired particle words, these materials are classified according to whether a sizes to obtain a “frit.” An advantage of the fritting route is glass-matrix phase is present or absent, or whether the mate- that mixtures of different compositions can be developed. rial contains an organic matrix that is highly filled with These mixtures are subsequently formed into the desired ceramic particles.15–33 Dental glass-ceramics belong to the shapes using molds (with proper binders) and densified by family of glass-matrix ceramics, and we have decided to divide them into the two subfamilies of bioactive and restorative den- tal glass-ceramics. Bioactive dental glass-ceramics (BDGCs) are materials that show bone/tooth bonding ability and stimulate a particular biological reaction at the material/tissue interface. These materials have been suggested for use in hypersensitiv- ity treatment, implant coating, bone regeneration and peri- odontal healing.15–33 In contrast, most restorative dental glass-ceramics (RDGCs) are inert and biocompatible and are considered for use in restoration and reconstruction of teeth.15–33 We recently reviewed the history and trends of bioactive glass-ceramics for use in medicine and demonstrated their enormous potential.34 We also believe that dental glass- ceramics deserve much more attention due to their attrac- tive properties, ongoing progress and significant market potential. Therefore, throughout this article, we elaborate on the processing, properties, and applications of RDGCs and FIGURE 2. Schematic diagram of the main stages of the synthesis of BDGCs. In this context, we also report on selected valuable glass-ceramics. Reproduced from Ref. 34 with permission from Wiley.

620 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

properties of both the crystals and the residual glass control the properties of glass-ceramics (including dental GCs), such as translucency, strength, fracture toughness, machinability, and chemical durability.35

STRUCTURE AND PROPERTIES OF TEETH Teeth have a complex structure that originates from special- ized cells known as ameloblasts, odontoblasts, and cemento- blasts.1,2 Figure 3 shows a cross-section of a tooth. The enamel is the hard outer layer that appears as the crown and is formed by ameloblasts. These cells accumulate on the outside layer of the tooth bud. When the tooth erupts, ame- loblasts are not able to further proliferate, and thus enamel can no longer be formed, which means that the body cannot repair the enamel. The dentine is formed by odontoblasts. These cells are located on the inner side of the tooth bud, between the enamel and the pulp. The dentine is considered FIGURE 3. Cross-section of a tooth.39 the main foundation of the tooth and supports the enamel, creates protection for the pulp, and through its covering sintering. Sintering usually proceeds concurrently with crys- below the gums, enhances the attachment by ligaments to tallization in which the free surfaces of the glass frits the surrounding bone. The pulp is the inner portion of a encourage crystallization. Sinter-crystallization is the pro- tooth, which consists of living connective tissue and odonto- cess followed in the manufacture of selected glass-ceramics blasts. The functions of the pulp are to form dentin, supply that show poor internal crystallization. Pressure is some- times applied to increase the density and minimize residual nutrients, and perceive pain. Gums surround the teeth and porosity during sintering.34,35 create a seal around them. Bone holds the teeth firmly in Finally, the sol-gel method has been used to make a new place, and nerves and blood vessels keep them alive and 1,2 generation of bioactive gel-derived glass-ceramics and allows healthy. easy tailoring of their composition to match the requirements According to the ISO 6872 standards, a dental material of specific applications.36–38 The sol-gel approach is a chemi- must have notably good chemical, mechanical, and optical cally based method for producing glass-ceramics at much properties comparable to those of natural teeth. For exam- lower temperatures than the traditional processing methods ple, the chemical durability must be higher than that of nat- 40 described above. Frequently, certain steps shown in Figure 2 ural teeth. are involved in making glasses or glass-ceramics via the sol- The mechanical properties of the three hard tissues gel method. Densification and crystallization of gel-derived mentioned above, that is, enamel, dentine and bone, are of glasses finally leads to glass-ceramics.34–38 The fundamentals great importance because stresses are applied to teeth as a of glass-ceramic development via melting or sol-gel proce- result of chewing and tooth grinding and during periods of 40 dures have been comprehensively described in numerous psychological stress. The enamel is relatively hard and textbooks.35–38 Glass-ceramics always contain a residual brittle, the dentine is much softer and more compliant, and glassy phase and one or more embedded crystal phases. The bone is even more compliant. Table I summarizes the crystal content varies between 0.5% and 99.5% but most fre- mechanical properties of enamel, dentin, and bone. There- quently lies between 30% and 70%, and the remaining con- fore, mechanical properties of dental materials such as frac- tent is the residual glass phase. Controlled crystallization ture strength (r), fracture toughness (KIC), and wear yields glass-ceramics with interesting, at times unusual, com- resistance are highly important for avoiding material dam- binations of properties such as biological, electrical, thermal, age and breakage. In terms of optical properties, these mechanical, etc.6 The types of crystals, crystal volume frac- materials must exhibit translucency, color, opalescence, and tion, distribution in the matrix, and physicochemical fluorescence similar to those of natural teeth.41

TABLE I. Mechanical properties of enamel, dentin, and bone [41] Fracture Young’s Vickers Strength (MPa) toughness (MPa m1=2) modulus (GPa) Hardness (GPa)

Enamel 260–290a 0.5–1.5 70–100 3–5 Dentin 230–305 3 15–30 0.6 Cortical bone 50–150 2–12 5–20

aIf supported by dentin.

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 621 FIGURE 4. Examples of dental restorative glass-ceramics (RDGCs): (a) dental onlay, (b) inlay, (c) crown, (d) veneer and (e) 3-unit bridge (images are from www.filippini-gray.com and www.dentopolis.blogspot.com.br).

RESTORATIVE DENTAL GLASS-CERAMICS (RDGCS) Glass-ceramics A restorative material that mimics the tooth properties can The first dental glass-ceramic was a mica-based material be used as inlays, onlays, full crowns, partial crowns, known as DicorVR that was introduced by Malament and bridges, and veneers. Figure 4 shows a selection of these Grossman45 in the mid-1980s. This castable glass-ceramic applications. was gradually modified because a high risk of fracture was observed, and the manufacturing procedure used in the dental laboratory was complicated. Later on, in the 1990s, Ceramics scientists focused on the development of stronger and more Historically, Dr. Duchateau, with the aid of dentist Dr. Chem- reliable glass-ceramics to cover all restorative aspects of ant, developed the first restorative ceramic-based material teeth.16–20,42–44 Tremendous efforts led to the introduction 42 made from porcelain in 1774 for his tooth. Approximately of various dental glass-ceramics, and, in this section, we 60 years later, Stockton Co. was the first industry to manu- explore the history, processing, properties, and applications facture a porcelain tooth in 1837. C. H. Land followed Stock- of these materials. First, a short introduction to manufactur- ton’s invention in 1889 and patented the all-porcelain ing procedures of RDGCs is presented, the RDGCs are classi- crown, which was further developed and used until the fied according to their composition and crystal phases, and 1950s. To improve mechanical strength, porcelain restora- finally, selected recent relevant research studies on each tions in the 1950s were reinforced with a metal substruc- type of glass-ceramics are reviewed. ture on which the porcelain was fused. The new restorations were known as porcelain-fused-to-metal resto- Manufacturing procedures for RDGCs rations (PFMs). Since then, these constructs have been In general, dental technicians prepare RDGCs using the fol- extensively used for improved esthetics and quality and lowing four popular methods: (1) lost-wax casting, (2) heat- have demonstrated excellent clinical performance.42,43 pressing, (3) CAD/CAM, and (4) pressureless sintering. Fig- Although PFMs have certain esthetic disadvantages due to ures 5–7 schematically show the various stages and instru- their metal frameworks (anesthetic metal rings at the tooth ments used in these techniques. margin and a grayish appearance of the gums), they are still In lost-wax casting, an accurate impression of the pre- considered the gold standard in terms of reliability. Extra pared tooth is first obtained by a dentist [Fig. 5(a)], and a efforts in this field led to the development of the first mach- cast is made from the impression [Fig. 5(b)]. Later, a model inable CAD/CAM porcelain VitablocsVR Mark I, made by Vita of the restoration that resembles the shape of the final res- Zahnfabrik Co., Germany, in 1985. This company also intro- toration is shaped on the cast using a particular type of wax duced In-CeramVR products in the 1990s, which are porous [Fig. 5(c)]. The model is invested in special refractory mate- alumina/zirconia materials infiltrated with an esthetic glass. rials [Fig. 5(d)]. Wax burnout takes place at 9008C, and the Later on, ProceraVR , a pure alumina veneered with esthetic ceramic mold is partially sintered. The material for fabrica- porcelain, was developed by Nobel Biocare, Switzerland, in tion of a glass-ceramic is supplied as a glassy ingot, which the mid-1990s. Currently, zirconia (ZrO2) and composites is located in a furnace specially designed for casting. The made of sintered ceramic with pores filled with a polymer glass ingot becomes liquid during heating, and following a material are available in the market.16–20,42–44 short time hold at 1300–15008C, the melt is forced into a

FIGURE 5. Different steps in lost-wax casting of RDGCs.46

622 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

In the heat-pressing method, the dental technician uses as-prepared glass-ceramic ingot to produce the final restora- tion. First, a mold is produced via the previously described lost-wax procedure. The mold and glass-ceramic ingot are placed in a furnace specially designed for this processing method (Fig. 6).35 Once the glass-ceramic ingot has become a very viscous liquid of approximately 1011 Pa s, it is forced

by an Al2O3 plunger (via application of a relatively low force of 200–300 N) into the hollow portion of the mold at approximately 1000–12008C. After the cylinder has FIGURE 6. Schematic of the furnace used in heat-pressing.35 adequately cooled, the investment material is removed from the glass-ceramic restoration by blasting it with silica sand, mold by centrifugal force [Fig. 5(e)]. The glass casting is aluminum oxide, glass beads, or silicon carbide grit. Gener- retrieved, excess glass is polished off, and after the final ally, heat-pressed glass-ceramics have advantages over controlled heat treatment and coloring processes, the glass- castable ones, such as the absence of shrinking during the ceramic restoration is ready for clinical use [Fig. 5(f)].35,44,46 crystallization process and no need for preparation in the The mold is cooled to room temperature, and the cast glass dental lab, a step that results in time-consuming extra work dental restoration is removed. The glass is usually transpar- flow, high risk of inhomogeneity, and poor margin fit.35 ent, and quality control can be applied to locate possible In the third method, machinable restorative glass- defects. The glassy dental restoration is placed in an oven ceramics are fabricated using a CAD/CAM (computer-aided for the controlled crystallization process. During heat treat- design and computer-aided manufacturing) system.47,48 Typ- ment, the glass is converted into a glass-ceramic. Finally, the ically, CAD/CAM dental restorations are machined from dental technician applies a colored frit to the surface of the solid blocks of partially crystallized glass-ceramics (contain- glass-ceramic to match the color of the patient’s teeth and ing a machinable crystal phase) that are subjected to further performs the final heat treatment to melt the glaze.35,44,46 heat treatment to fully develop the glass-ceramic and

FIGURE 7. Work flow of the CAD/CAM process in dentistry. (a) scanning, (b) virtual restoration (modeling), (c) machining, and (d) final product (crown and 3-unit bridge) (images are from www.dental-tribune.com, www.osovnikar.com and www.dentalcompare.com).

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 623 achieve adequate properties and color that closely match the basic shade of the restored tooth (Fig. 7). After the decayed or broken areas of the tooth are corrected by the dentist, an image (scan) is taken of the prepared tooth and the surrounding teeth [Fig. 7(a)]. This image, known as a digital impression, is used as data input into a computer. Proprietary software creates a replacement part for the missing areas of the tooth, thus creating a virtual restora- tion [Fig. 7(b)]. The software sends this virtual data to a CAD/CAM machine in which the replacement part is carved out of a solid block of glass-ceramic [Fig. 7(c)]. Stains and glazes can be fired onto the surfaces of the machined crown or bridge to correct the chromatic appearance of the resto- ration [Fig. 7(d)]. The restoration is adjusted in the patient’s 47,48 FIGURE 8. Typical microstructure of a mica glass-ceramic [Personal mouth and cemented or bonded in place. image]. In the fourth method, known as pressureless sintering,a thin layer of glass-ceramic is veneered over restorative materials such as zirconia, metals or glass-ceramics. Veneer- 26 21 ing is performed to adjust the final shade of the restoration. coefficient is 7.2 3 10 K (at 25–6008C). Shading is per- formed using colorants premixed with a glazing material. In general, a slurry containing glass-ceramic fine powders VR plus coloring agents is brushed over the surface. The arti- The applications of Dicor glass-ceramic in restorative den- fact is held in a furnace at an appropriate temperature for tistry are veneers, inlays, onlays, and dental crowns. The VR the required time to sinter and crystallize the glass-ceramic cast Dicor glass-ceramic is mostly used to fabricate com- and fuse it to the restoration.35 In the following section, we plete crowns. This glass-ceramic is not recommended for describe the various types of dental glass-ceramics. fixed partial dentures, abutments, or dental bridges because its mechanical strength is not sufficient.35 Mica-based glass-ceramics A relatively low mechanical strength and difficult proc- essing conditions are the main drawbacks of mica-based Glass-ceramics containing mica crystals (X2Y4-6Z8O20(OH,F)4 in which X 5 K, Na, or Ca; Y 5 Al, Mg; Z 5 Si or Al) were the glass-ceramics, which restrict their application and popular- first type of glass-ceramics used in the fabrication of dental ity. Therefore, numerous attempts in 1990s and early 2000s restorations. DicorVR was the first material in this family and were made to overcome the weakness of this material. Prof. was introduced by Malament and Grossman in the mid- Denry’s group at Ohio State University pioneered modifica- VR 1980s45 and commercialized by Dentsply International.35 tions of the composition of Dicor and improved its proper- ties. The Denry group replaced potassium with lithium and A typical glass composition contains (56–64)SiO2–(0–2) developed taeniolite, a lithium-containing tetrasilicic fluor- Al2O3–(15–20)MgO–(12–18)K2O–(4–9)F–(0–5)ZrO220.05CeO2 in wt %. The glass is shaped via lost-wax centrifugal casting mica (KLiMg2Si4O10F2) with improved thermal and mechani- 51 after a hold at 13708Cfor6 min. The machinable block of cal properties. By changing the glass composition, a new DicorVR MGC is also available for CAD/CAM processing.35 It glass-ceramic was developed based on the crystallization of 49 was demonstrated by Bapna and Mueller that during devit- mica (KNaMg2Si4O10F2) and K-fluorrichterite (KNaCaMg5- VR rification of Dicor , spinodal decomposition of the glass Si8O22F2) crystals, which were tougher (section Fluorrichter- 52–54 55 56 matrix occurs, followed by precipitation of fine round crys- ite Glass-Ceramics). Uno et al. and Qin et al. 1 1 21 21 tals. Further, an isothermal hold for 6 h at 10758Ctransforms substituted K or Na with Ba or Ca as the interlayer the round mica grains into plate-like mica crystals via a dis- ions of mica crystals, which resulted in the formation of 55,56 solution and reprecipitation mechanism.49 The restoration is high-strength mica glass-ceramics. Oriented mica glass- further glazed with coloring agents. The final product con- ceramics fabricated by hot-pressing or extrusion processes tains mica crystals embedded in the residual glass.35 Figure 8 had higher strength and toughness than the conventional shows the microstructure of a mica glass-ceramic. Mica has a castable mica glass-ceramics that contained randomly ori- sheet-like and inter-locked morphology amenable to machina- ented mica crystals with a house-of-cards structure.57–61 In 35 bility and with advantageous mechanical properties. The addition, these materials could be reinforced with ZrO2 crys- size and volume fraction of the mica plates are critical param- tallized from the bulk glass.62–66 It has been reported that a eters for adjusting the wear resistance and the mechanical calcium mica-based nanocomposite containing nano-size 50 properties of the respective glass-ceramic. (20–50 nm) tetragonal-ZrO2 particles exhibits notably a DicorVR is a classical example of a mica-type glass- high flexural strength (500 MPa) and fracture toughness ceramic because it is translucent, machinable, and has fair (3.2 MPa m1=2).62 The excellent mechanical performance is mechanical properties. The bending strength, fracture tough- related to crack deflection by mica plates and ZrO2 par- ness, Vickers hardness, and Young’s modulus of DicorVR are ticles.62 All of these modifications improved the properties 1=2 r 5 150 MPa, KIC 5 1.4–1.5 MPa m ,HV5 3.5 GPa, and of mica-based glass-ceramics, but they still could not com- E 5 70 GPa, respectively, and its thermal expansion pete with alternative materials such as lithium disilicate GC

624 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

TABLE 2. Prominent companies and dental glass-ceramic products

Company/country Products Technology Type

3M/USA ParadigmTM C CAD/CAM Leucite-based Aidite/CHI CameoVR CAD/CAM Lithium disilicate Den-Mat/USA LumineersVR Heat-extrusion Leucite-based Dentsply/USA DicorVR Lost-wax casting Mica-based Dicor MGCVR CAD/CAM Mica-based CeramcoVR Sintering for veneering Leucite-based Cergo KissVR Heat-pressing Leucite-based VR Celtra Duo CAD/CAM ZrO2-reinforced lithium disilicate GC Glidewell/USA ObsidianVR Heat-pressing on metal and CAD/CAM Lithium disilicate Ivoclar/LIE IPS EmpressVR Heat-pressing Leucite-based IPS EmpressVR CAD CAD/CAM Leucite-based IPS d.SIGNVR Sintering on metal frameworks Leucite and apatite IPS Empress IIVR Lost-wax casting Lithium disilicate IPS e.max PressVR Heat-pressing Lithium disilicate IPS e.max CADVR CAD/CAM Lithium disilicate VR IPS Empress Cosmo Heat-pressing on ZrO2 posts Lithium zirconium silicate IPS ErisVR Sintering on IPS Empress II Apatite-based IPS e.max CeramVR Sintering on IPS e.max CAD Apatite-based VR IPS e.max ZirPress Sintering on ZrO2 Apatite-based IPS InLineVR Heat-pressing on metal Composite of leucite GC and ceramic Noritake/JPN EX-3 PRESSVR Heat-pressing on metal Leucite-based Pentron/USA Optec OPCVR Heat-pressing Leucite-based 3GTM Heat-pressing Lithium disilicate Upcera/CHI UpSilVR press Heat-pressing Lithium disilicate UpSilVR CAD CAD/CAM Lithium disilicate Vita/GER VitablocsVR Mark II CAD/CAM Sanidine VitablocsVR TriLuxe CAD/CAM Sanidine Vitablocs RealLifeVR CAD/CAM Sanidine VR Vita Suprinity CAD/CAM ZrO2-reinforced lithium disilicate GC VitapmVR Heat-pressing for veneering Apatite-based to survive in this competitive market. DicorVR always suf- that are densified upon firing at 9508C to form a homogene- fered from relatively low strength, inability to color inter- ous glass-ceramic block. A final heat treatment at 10508C nally and a difficult manufacturing process. However, in a encourages further crystal growth. This controlled heat treat- 20-year clinical study with DicorVR GC, Malament, and Soc- ment yields glass-ceramics with flexural strength, fracture ransky67 reported an estimated success rate of >90%. toughness, Vickers hardness and Young’s modulus of 1=2 These materials are still used by dentists and technicians r 5 160 MPa, KIC 5 1.3 MPa m ,HV5 6.2 GPa, and who prefer old materials that they know well and trust. E 5 65 GPa, respectively. The leucite crystals are responsible However, these materials are undergoing gradual replace- for the high linear thermal expansion of the glass-ceramic of ment by new CAD/CAM and heat-pressed GCs. 15 3 1026 K21 to 18.25 3 1026 K21 in a temperature range of 100–5008C.35 Leucite-based glass-ceramics According to Holand€ and Beall, the leucite crystals

Leucite (KAlSi2O6) mineral was first added to feldspathic strengthen the glass-ceramic by deflecting the propagating porcelains to raise the coefficient of thermal expansion to crack.35 Furthermore, Serbena et al.68 concluded that the match the metals on which they were fused. Leucite- main toughening mechanisms in glass-ceramics (without the containing glass-ceramics were introduced in the early chance of phase transformation toughening) are caused by 1990s and were the first heat-pressed GCs.25 Leucite glass- crack bowing and trapping (for low crystallized volume ceramics are derived from the SiO2–Al2O3–K2O glass system. fractions), as well as by the greater elastic modulus and The typical glass composition is (59–63)SiO2–(19– fracture toughness of the crystal precipitates. The chemical 23.5)Al2O3–(10–14)K2O–(3.5–6.5)Na2O–(0–1)B2O3–(0–1)CeO2– durability and abrasion properties of this glass-ceramic are (0.5-3)CaO–(0–1.5)BaO–(0–0.5)TiO2 in wt %. The glass- acceptable according to the ISO standard. Glass-ceramic ceramic is produced in the form of ingots, and the dental tech- blocks for CAD/CAM processing are also available (Table 2). nicians heat-press these ingots at 11508C (with a pressure of The restorations are placed using the adhesive bonding 0.3–0.4 MPa) into a mold made using the lost-wax technique. technique. Leucite glass-ceramics are used as veneers, This temperature is held for 20 min in a specially designed inlays, onlays, and anterior and posterior crowns, but their automatic heat-pressing furnace (Fig. 6).35 Leucite crystals strength is insufficient for fixed posterior bridges. For precipitate with controlled surface crystallization. To this end, bridges, this type of GC is veneered onto a flexible, tough the crystals nucleate and grow from glass particle surfaces metallic framework.35

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 625 for the nano-leucite glass-ceramic, 76 6 7 MPa for Ceram- coVR (restorative porcelain) and 166 6 31 MPa for IPS EmpressVR (Leucite GC).72 More recently, Aurelio et al.73 observed an increased bending strength and decreased sur- face roughness for a leucite-based glass-ceramic sintered at a higher sintering temperature after machining. The crystal- line structure was not modified. However, increased sinter-

ing time and firing below Tg significantly reduced the fracture strength.73 Leucite-based glass-ceramics are also well suited for the CAD/CAM process developed by Ivoclar Vivadent AG. This product is available in multi-colored CAD/CAM blocks.74 The block (Fig. 10) allows the optical properties of natural teeth to be closely imitated. This glass-ceramic consists of a FIGURE 9. TEM micrograph of a glass-ceramic containing apatite (A) 74 and leucite (L) crystals.70 total of four to eight main and intermediate layers. In 2000–2011, novel low-wear/high-strength leucite- In the early 2000s, Holand€ and coworkers at Ivoclar based glass-ceramics were developed at Queen Mary Uni- Vivadent AG succeeded in introducing apatite-leucite glass- versity by Dr. Cattell’s team to prevent fracture and wear of ceramics as an alternative to their leucite-based product dental ceramic restorations. Dr. Cattell began his research to (Table 2).69 These researchers reached a stage at which it overcome problems related to the brittle fracture of porce- became possible to produce restorations that contain build- lain restorations and their poor survival rates and intended ing blocks in the form of needle-like apatite, similar to those to develop leucite-based glass-ceramics by heat-extrusion. of natural teeth. The needle-like apatite crystals with a hex- This novel method led to a homogenous distribution of fine agonal base (Fig. 9) positively influenced the esthetic prop- crystals and increased reliability (Weibull modulus, m 5 9.4 erties and various mechanical parameters of the material. and r 5 159 MPa) compared with commercial materials VR 75 The composition also contained fluorine, which induced the (m 5 6.1 and r 5 120 MPa, Empress ). Additionally, con- 2 formation of fluorapatite (Ca5(PO4)3F) and enhanced the trolling the leucite crystal size to 0.15 6 0.09 lm was the chemical properties of the material. Therefore, this glass- key to enhancing the properties of Dr. Cattell’s glass- 76,77 ceramic was commercialized (Table 2) as a sintered glass- ceramic. The Cattell team also focused on the funda- ceramic to replace dentin, reproduce the incisal area and mental aspects of nucleation and crystal growth of leucite create specific optical effects (e.g., opalescence over a metal- glass-ceramics and powder processing to control surface lic substrate).69,70 crystallization and produce first fine and later nano-scale Leucite-apatite glass-ceramics are applied on metallic leucite glass-ceramics.78 These studies were critical to frameworks via pressureless sintering. However, the risk of reduce the size of the leucite crystals and had enormous the glass-ceramic pulling away from the metal surface still benefits in terms of reduced enamel wear, improved exists. Therefore, sintering should be carefully performed esthetics, and increased strength. Leucite glass-ceramics within a temperature range of 550–9008C, and shrinkage were subsequently produced with significantly higher flex- must be controlled to prevent tearing. Michel et al.71 ural strength (r > 250 MPa), reliability (m 5 12), and attempted to minimize this risk by developing nano- lower enamel wear.79,80 This material could be processed coatings on leucite-fluorapatite glass-ceramic particles prior to sintering. The coating influences the rheology of the slurry and the properties of the veneer. The following two substances were chosen for the coatings: (a) a combination of inorganic chemicals (ZnCl2, AlCl3, or BCl3) and polyethyl- ene glycol (PEG) and (b) an exclusive polymer. Both groups of materials positively improved the sintering properties of the glass-ceramics and suppressed extensive tearing.71 A nano-sized leucite glass-ceramic was further developed by Theocharopoulos et al.,72 who sintered nano-sized commer- cial glass particles (CeramcoVR and IPS EmpressVR ) prepared by high-energy milling to trigger surface crystallization of leucite crystals at the nano-scale. As a result, these new glass-ceramics showed an increased leucite crystal number at the nano-scale (median crystal sizes of 0.05 lm2). These new glass-ceramics had a higher mean bending strength than the competing commercial materials. The FIGURE 10. Multicolor block of leucite-based glass-ceramic for a CAD/ mean bending strengths were, for example, 255 6 35 MPa CAM process.74

626 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

is increased to 750–8508C for approximately 2 h, lithium disilicate crystallizes. Later on, in the dental laboratory, the glass-ceramic ingot is heat-pressed at 9208C for 5–15 min to flow viscously into a mold made using the lost-wax tech- nique to form the desired restoration. The bending strength and fracture toughness of heat-pressed LS2 glass-ceramics are approximately 350–400 MPa and 2.3–2.8 MPa m1=2, respectively.31,35 Chung et al.83 have reported that repeated heat-pressing can produce a statistically significant increase in the flexural strength of lithium disilicate glass-ceramic (IPS EmpressVR II, Table 2). The interlocked lithium disilicate crystals dispersed within the glassy matrix impart desirable mechanical and optical properties to the product. One of the main toughen- FIGURE 11. Propagating crack deflected by lithium disilicate crystals ing mechanisms of brittle materials is the deflection of in the glass-ceramic.84 cracks around the crystals. This effect is illustrated in Figure 11 in which lithium disilicate crystals of 20 lm are precipi- using heat-extrusion, CAD-CAM, and 3D printing and was tated in the glass-ceramic matrix. The material was indented later commercialized by Den-Mat Holdings as an esthetic with a pyramidal tip, and the propagating cracks were devi- restorative material with the name of LumineersVR (Table 2). 81 ated from their natural path by the crystals. The fracture Fradeani et al. reported on the survival rate of leucite toughness of oxide glasses is typically between 0.50 and glass-ceramic crowns. Crowns were studied over periods 1= 0.75 MPa m 2, whereas that of the LS2 glass-ceramic ranging from 4 to 11 years. The probability of survival of 1= reaches 2.8 MPa m 2.68,84,85 125 crowns was 95.2% at 11 years (98.9% in the anterior A particular type of lithium disilicate glass-ceramic can segment and 84.4% in the posterior segment). Only six also be shaped by CAD/CAM. To meet the challenging crowns had to be replaced. Most of the 119 successful requirements of the CAD/CAM process, a particular heat crowns were rated as excellent. Guess et al.82 investigated treatment is followed, as shown in Figure 12, for example, the long-term performance of leucite glass-ceramic veneers. VR for IPS e.max CAD.86 First, a monolithic piece of glass is Twenty-five patients underwent restorations using 42 over- prepared by casting the melt in a mold [Fig. 12(a)]. Second, lap veneers (incisal/palatal butt-joint margin) and 24 full a glass-ceramic containing lithium metasilicate (Li O-SiO )is veneers (palatal rounded shoulder margin). All restorations 2 2 developed by intermediate heat treatment of the glass at were anterior veneers. The 7-year Kaplan-Meier survival 690–7108C for 10–30 min [Fig. 12(b)]. This intermediate rate was 100% for full veneers and 97.6% for overlap material (which is supplied in blue color) has been specially veneers.82 designed to facilitate the machining process in the CAD/ CAM technique [Fig. 12(b)]. After the machining process Lithium disilicate glass-ceramics [Fig. 12(c)], the material is heated at 8508C for 20–31 min In 1998, a third generation of dental glass-ceramic, lithium to form the lithium disilicate glass-ceramic that displays a disilicate (LS2) glass-ceramic was introduced for single- and characteristic tooth color [Fig. 12(d)]. The resulting dental multiple-unit frameworks. This material re-emerged in 2006 restorations can be polished and completed with glazing to as a heat-pressable ingot and a partially crystallized machi- match the color of the patient’s teeth. The glass-ceramic nable block and was successfully used to produce a crown or bridge framework with mechanical properties that were ingot containing lithium metasilicate crystals, as provided almost three times higher than those of the leucite-based by the material suppliers, has a flexural strength of glass-ceramic. Currently, lithium disilicate glass-ceramic is the most popular restorative glass-ceramic in the field of dental materials.31,35 A typical glass composition of this glass-ceramic con- tains (57–80)SiO2–(11–19)Li2O–(0.0-13)K2O–(0–11)P2O5– (0–8)ZrO2–(0–8)ZnO–(0–5)Al2O3–(0–5)MgO in wt %. Color- ing oxides are also included in the glass composition. The glass composition was designed such that it contains at least 65 wt % fine rod-like entangled lithium disilicate crys- tals (Li2O–2SiO2). The glass is molded via heat-pressing fol- lowed by a controlled heat treatment that leads to the final product. The aim of the heat treatment is to induce internal nucleation and crystallization. The initial thermal treatment FIGURE 12. Various stages of fabrication of lithium disilicate glass- is performed slightly above Tg to control the number of ceramics by CAD/CAM. IPS e.maxVR CAD from Ivoclar Vivadent AG nuclei (450–5508C for 5 min to 1 h). When the temperature was selected as an example.

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 627 130 MPa, but the strength and fracture toughness increases to 360 MPa and 2.3 MPa m1=2, respectively, after conversion of the machined glass-ceramic to the final product. The reduced strength and toughness of CAD/CAM materials compared with the heat-pressed material is due to the fact that the latter glass-ceramic contains needle-like and elon- gated crystals with higher aspect ratio than its CAD/CAM counterpart.31,35,86 An interesting study by Lien et al.87 revealed that inter- mediate heat treatments in temperature ranges below 5908C, between 590 and 7808C, and above 7808C can influ- ence the final microstructure and properties of the lithium disilicate glass-ceramic (IPS e.maxVR CAD). The finely knitted mesh of Li2O–SiO2 predominated below 5908C; spherical- FIGURE 13. Application of a lithium disilicate-based glass-ceramic like phases of Li O-SiO ,LiO-2SiO , and Li PO emerged R 2 2 2 2 3 4 (IPS e.maxV CAD). (a) Initial situation, worn gold crown. (b) Prepara- between 590 and 7808C; and, irregularly oblate crystals of tion of molar for a full contour crown. (c) Try-in of a full contour Li2O–2SiO2 arose above 7808C. At each of those three evolu- crown in its lithium metasilicate state. (d) Final clinical situation after tionary stages, the glass-ceramic formed through controlled the crystallization step (stained and glazed) and adhesive cementation of the crown. Dentist: A. Peschke (Ivoclar Vivadent AG).74 crystallization often yielded a microstructure that possessed interesting and sometimes peculiar combinations of proper- regions,91 but bridges present a higher risk of fracture than ties. Additionally, the growth of Li2O-2SiO2 crystals within the IPS e.maxVR CAD blocks was independent of the overall metal-porcelain prostheses or other more recently devel- 92,93 heating time but dependent on a minimum temperature oped ceramic materials, such as zirconia and alumina. threshold (7808C). Groups of samples heated above the min- Figure 13 shows the successful use of CAD/CAM lithium 74 imum temperature threshold (7808C) up to 8408C exhibited disilicate glass-ceramic crown in a posterior restoration. enhanced flexural strength, fracture toughness, and elastic modulus compared with those of groups that were inten- Apatite-based glass-ceramics tionally not heated above 7808C.87 To further improve the translucency and shade match and Recent research by Al Mansour et al.88 showed that to adjust the wear behavior to that of the natural tooth, lith- spark plasma sintering (SPS) can be used to refine the ium disilicate, leucite glass-ceramics, and sintered ZrO2 are microstructure of lithium disilicate glass-ceramics (IPS veneered with an appropriate apatite-containing glass- e.maxVR CAD). Densification by SPS results in textured and ceramic using a pressureless sintering process. For example, fine nano-crystalline microstructures. This group believes Ivoclar Vivadent AG developed the fluorapatite glass- VR that SPS generated glass-ceramic might have unique proper- ceramic IPS e.max Ceram, which is a low-fusing aluminosi- ties and could be useful in the production of CAD/CAM licate glass-ceramic specially designed for veneering lithium VR VR materials for dentistry.88 disilicate glass-ceramic (IPS e.max CAD and IPS e.max Press) or CAD/CAM zirconia. A typical composition of this Although P2O5 and ZnO initiate microphase separation, which induces the crystallization pathways and kinetics, it glass-ceramic is (60–65)SiO2–(8–12)Al2O3–(6–9)Na2O–(6– 8)K O–(2–3)ZnO in wt % with the addition of CaO, P O ,F appears that ZrO2 has a more beneficial effect on the crys- 2 2 5 tallization and strengthening of lithium disilicate glass- (2–6 wt %), other oxides (2–8.5 wt %), and pigments (0.1– 21,31,35 ceramics.89,90 New commercial lithium disilicate glass- 1.5 wt %). This glass-ceramic is offered in powdered ceramics for CAD/CAM are advertised as tougher and more form for the slurry layering technique and is available in all reliable due to the presence of at least 10 wt % ZrO2 dis- classical tooth shades. The material is fired at 750–7608C solved in the residual glass. These glass-ceramics (Table 2) and crystallizes in the volume, resulting in different concen- are Celtra DuoVR (Dentsply), IPS e.max CADVR (Ivoclar), and trations of nano-fluorapatite crystals with a diameter of SuprinityVR (Vita). Zirconia influences the crystallization by 100 nm and a length of <300 nm and microfluorapatite hampering crystal growth. With increasing ZrO2 content, the crystals with a diameter of 300 nm and a length of 2–5 lm. crystals become smaller. By increasing the crystallization The total adjustable amount of fluorapatite in the glass- temperature, the crystal growth increases, as expected. The ceramic is between 19 and 23 wt %. The fluorapatite crystal translucency of the glass-ceramic can be adjusted by adding (Ca5(PO4)3F) acts as a component that adjusts the optical

ZrO2. A highly translucent glass-ceramic with a contrast properties of the restoration to those of natural teeth. For ratio of 0.4 and high three-point bending strength (700– this reason, researchers have attempted to crystallize apa- 800 MPa) was developed.89 tite in the nano-sized dimension. The strength, hardness, According to Kaplan-Meier method, the cumulative sur- and thermal expansion coefficient of the final product are vival rate of lithium disilicate crowns is 94.8% after r 5 90 MPa, HV 5 5.4 GPa, and CTE 5 9.5 3 1026 K21, 8 years,91 but only 71.4% of three-unit bridges survive after respectively. The leucite-apatite glass-ceramic described in 10 years.92 Therefore, crowns made of a lithium-disilicate Leucite-Based Glass-Ceramics section is another good framework can be safely used in the anterior and posterior veneering material.74

628 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

Miscellaneous RDGCs As described previously, dental glass-ceramics are attractive materials for dental restoration because they display excel- lent esthetics, low thermal conductivity, relatively high strength, chemical durability, biocompatibility, ease of proc- essing and high wear resistance. However, compared with those of metals and ceramics, the low mechanical strength and fracture toughness of these materials restrict their application for long-term high load-bearing posterior resto- rations. Therefore, continuous attempts have been made to develop new glass-ceramics with improved mechanical properties and good clinical performance. Some of these glass-ceramics are fluorrichterite, fluorcanasite, diopside, and apatite-mullite glass-ceramics.

FIGURE 14. Back-scattered scanning electron image from dental Fluorrichterite glass-ceramics. The main characteristics of glass-ceramic containing 20 wt % ZrO heat-pressed at 10008C. Dark 2 fluorrichterite glass-ceramics are their high fracture tough- holes correspond to the Li3PO4 crystals dissolved in etchant solution 1=2 (3% HF) and ZrO2 micro/macrocrystals are brighter and elongated ness (KIC > 3MPam ), optical translucency, and high resist- (needle-like).94 ance to thermal shock.35 High-performance laboratory tableware and domestic kitchenware are manufactured from Zr-containing silicate glass-ceramics these glass-ceramics.35 In 1999, Denry and Holloway52 After the development of the ZrO2 root canal post and began to develop fluorrichterite glass-ceramic for use in implant abutment, a restorative material was required that dentistry and first investigated the role of MgO content in a can be placed on ZrO2. To fulfill this need, a lithium zirco- glass composition of 67.5SiO222Al2O3212MgO– nium silicate glass-ceramic was developed to adjust the 9CaF224Na2O–3.5K2O–1Li2O–1BaO (wt %). The hypothesis coefficient of linear thermal expansion to that of ZrO2 and was that increasing the amount of magnesium might pro- achieve a certain degree of opacity. This glass-ceramic is lay- mote the crystallization of double-chain silicate (amphibole) ered on the ZrO2 post via heat-pressing. Consequently, this crystals. Richterite is a sodium calcium magnesium silicate material offers an esthetic solution for an abutment on mineral that belongs to the double-chain silicate group. In which a leucite glass-ceramic crown (as an example) might K-fluororichterite, potassium replaces sodium and fluorine be positioned. The glass composition range is (4–15)P2O5– replaces hydroxyl. This crystal has a monoclinic (2/m) 62 (42–59)SiO2–(7–15)Li2O–(15–28)ZrO2 in wt % with the structure. The double chain (Si4O11) backbones parallel to 35,94 addition of K2O, Na2O, Al2O3, and up to 11 wt % F. The the c-axis and are bonded together by ionic bonds to inter- glass-ceramic shaped products are produced via viscous stitial cations such as Na1,K1, and Ca21. The basic struc- flow of as-prepared glass-ceramic ingots in heat-pressing tures of chain and double-chain silicates are explained in kilns. In a glass-ceramic containing 16 wt % ZrO2, crystals detail in various textbooks, such as Ref. 35. The high frac- of Li2ZrSi6O15 are crystallized at 9008C and reach a bending ture toughness of amphibole-based glass-ceramics is due to strength of 160 MPa. This material shows notably good the random orientation of the interlocked crystals, which optical properties, especially high translucency, and this gives rise to crack deflection.35 Denry and Holloway also composition can be used as an anterior prosthesis. The found that in a glass containing 18 wt % MgO, both mica glass-ceramic with 20 wt % ZrO2 is opaque white, shows and fluorrichterite are crystallized. In this material, the minor translucency, and contains rod-shaped Li3PO4, ZrO2 microstructure consists of interlocked acicular crystals of microcrystals, and ZrO2 macrocrystals after heat treatment fluorrichterite (5–10 mm) and mica, and this structure pro- at 950–10508C. Figure 14 shows a typical microstructure of moted crack deflection and arrest.52 Furthermore, this same this glass-ceramic. The material has a bending strength of research group increased the sodium amounts in a base 1=2 280 MPa and a KIC of 2.0 MPa m . The improved mechani- glass composition of 57.7SiO2223.9MgO–6CaF220Na2O– cal properties of the glass-ceramic containing 20 wt % ZrO2 8.5K2O–3Li2O–1BaO (wt %). Increasing sodium content led makes this material appropriate for use in the posterior to a decrease in all transformation temperatures, including region because the esthetics and the opacity of the glass- the onset of melting. A decrease in the viscosity of the ceramic play a less important role in this region of the glass-ceramics was observed for the glass-ceramic com- mouth. Finally, the coefficients of linear thermal expansion posed of fluorrichterite and mica and was retained after for both glass-ceramics are somewhat lower than that of the heat treatment at 10008C for 4 h.53 The glass-ceramic con-

ZrO2 post. As a result of this adjustment, a crack-free bond taining 1.9 wt % sodium had the highest mean fracture 1=2 between the glass-ceramic and the ZrO2 abutment is toughness of 2.26 6 0.15 MPa m , which was not signifi- achieved.35,94 Although these glass-ceramics were tested cantly different from that of the control material (OPCVR , under a relatively short period of clinical observation, no Pentron).54 The microstructure of this glass-ceramic (Fig. signs of failure of conventionally cemented zirconia posts 15) exhibited prismatic fluorrichterite and interlocked were observed with these materials.95,96 sheet-like mica crystals. Crystallization of fluorrichterite

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 629 crystallization in glasses with an acicular interlocked micro- structure gives rise to strength and fracture toughness of the resulting glass-ceramics.35 A number of initial studies were performed by Anusavice’s and Noort’s team at Florida and Sheffield Universities, respectively, to evaluate potential application of fluorcanasite glass-ceramics in restorative dentistry. In the period 1997–2003, van Noort et al.103 dem- onstrated that fluorcanasite glass-ceramics derived from the

base glass composition of 60SiO2210Na2O–5K2O–15CaO– 10CaF2 (wt %) show promising properties and can be fabri- cated using conventional routes.103–105 At the same time, Anusavice and Zhang106 reported that the chemical durabil- ity of fluorcanasite glass-ceramics is not adequate for dental applications, and they were not able to improve either chemical durability or mechanical strength via the addition of Al O up to 15 wt %. It was observed that increased FIGURE 15. Scanning electron micrograph of mica-fluorrichterite 2 3 glass-ceramic. The darker area represents prismatic fluorrichterite Al2O3 content significantly affected the crystal size, crystal crystals dispersed in the brighter area of mica crystals.54 shape, aspect ratio, and crystal aggregation characteristics of the fluorcanasite glass-ceramics.106,107 In an attempt to control the solubility of this glass-ceramic, systematic addi- 108 might account for the significant increase in fracture tough- tions of SiO2 and AlPO4 were tested by Bubb et al. The 2 ness compared with that of mica-based glass-ceramics (as solubility was reduced from 2359 to 624 mg/cm (according an example).54 The effect of crystallization heat treatment to ISO 6872). An initial increase was observed in biaxial on the microstructure and biaxial strength of fluorrichterite flexural strength, that is, 123–222 MPa with small additions, 97 but larger additions reduced the strength to 147 MPa. These glass-ceramics was also reported by the same authors, findings were attributed to an increased volume fraction of who observed two-fold variation in the biaxial flexural residual glassy matrix.108 Stockes et al.109 attempted to fur- strength of fluorrichterite glass-ceramics depending on the ther reduce the chemical solubility of these glass-ceramics temperature and duration of the crystallization heat treat- by investigating the mixed alkali effect due to variation in ment. This result was believed to be due to the formation of the K and Na contents and found that by changing the alkali a low-expansion surface layer composed of roedderite ratio of the base glass composition (the above composition) (K Mg Si O ). The expansion mismatch promoted the 2 5 12 30 from K/(K 1 Na) 5 0.33–0.47, it was possible to signifi- development of surface compressive stresses and efficiently cantly reduce the chemical solubility of the glass-ceramic. increased the flexural strength of the glass-ceramic. Higher This glass-ceramic exhibited a minimum chemical solubility heat treatment temperatures or longer durations likely led of 650 mg/cm2 at a composition of K/Na 5 7/8. This solu- to an increase in thickness of this layer, thereby reducing bility is acceptable for dental core ceramics, which should the intensity of the surface compressive stresses and caus- have a solubility of <2000 mg/cm2, but it is not suitable for ing a decrease in strength. In addition, these conditions direct contact with the mouth environment, which requires caused a coarsening of the microstructure that could also a solubility of <100 mg/cm2.40,109 Finally, Pollington and weaken the glass-ceramic by reducing the number of possi- van Noort110 managed to adjust the chemical solubility and ble crack-crystal interactions.97 The significance and long- mechanical properties of the glass-ceramic with ZrO2 addi- term goal of Denry and Holloway’s work was to develop a tion, and their optimum composition approximately con- dental glass-ceramic processed at low temperature, for tained 61SiO226Na2O–8K2O–11CaO–12CaF222ZrO2 (wt %). example, with heat-pressing, that retained the fluorrichterite The appropriate melting schedule for this composition was microstructure and excellent mechanical properties. Later found to be 1 h of holding and stirring at 13508C. The heat on, other scientists also attempted to crystallize different treatment schedule of 2 h nucleation and 2 h crystallization chain silicate minerals, such as diopside or wollastonite, in produced the greatest amount of the fluorcanasite phase. the vicinity of mica crystals to benefit from their toughening The glass-ceramic had an acceptable chemical solubility 98–100 101 102 ability. Almuhamadi et al. and Sinthuprasirt et al. (722 6 177 lg/cm2) and high biaxial flexural strength also prepared diopside and leucite-diopside glass-ceramics, (250 6 26 MPa), fracture toughness (4.2 6 0.3 MPa m1=2), respectively, to produce novel strong and thermally compati- and hardness (5.2 6 0.2 GPa) and had the potential for use ble veneers for zirconia restoration to overcome chipping as a core material for veneered resin-bonded ceramic resto- and failure issues. The improvements were significant, but it rations. Furthermore, this fluorcanasite glass-ceramic was appears that these materials have not yet been considered found to be machinable using standard CAD/CAM technol- for clinical applications. ogy and demonstrated a high degree of translucency.110 It has also been proved that this glass-ceramic forms a suffi-

Fluorcanasite glass-ceramics. Fluorcanasite (Ca5Na4K2- cient and durable bond when bonded with a composite 111 Si12O30F4) is another double-chain silicate mineral which its resin, without the need for acid etching with HF solution.

630 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

112 More recently, Eilaghi et al. have shown that fluorcanasite ceramics. However, 3 wt % ZrO2 and an extra amount of SiO2 glass-ceramic can be pressureless sintered at 10008Ctoan had no significant effect.122 The mechanical properties of the appropriate relative density of 91.3 6 0.1% and desirable resulting glass-ceramics after temperature changes (5–608C) in mechanical properties (r 5 137 6 7 MPa and aqueous media remained nearly unchanged for the samples con- 1=2 112 KIC 5 2.6 6 0.1 MPa m ). taining TiO2 and ZrO2, whereas a high reduction was observed with the addition of BaO and extra amounts of SiO2.Further- 113 Apatite-mullite glass-ceramics. In the mid-1990s, Hill et al. more, after immersion in hot water, the concentration of Ca21 introduced apatite-mullite glass-ceramics as potential dental or and F2 ions released from samples with BaO or with excess bioactive glass-ceramics. The optimum glass composition was amounts of SiO2 were higher than those of TiO2-andZrO2-con- 123 33.33SiO2211.11P2O5222.22Al2O3222.22CaO–11.11CaF2 in taining glass-ceramics. It is apparent that these apatite- mol %. This glass-ceramic, which was heat treated at approxi- mullite glass-ceramics are promising restorative materials, but mately 9008C, consisted of elongated fluorapatite (Ca10(PO4)6F2) their high chemical solubility still restricts their application for and mullite (3Al2O3Á2SiO2) crystals. Crystallization occurred by use in the mouth environment. Therefore, these materials must an internal nucleation mechanism that involved prior amor- be first considered for core build-up. phous phase separation. A fracture toughness value >3MPam1=2 113,114 115,116 was reported. Later, Gorman and Hill attempted to Commercial RDGCs develop a dental restoration material using a similar glass- Several companies around the world already manufacture ceramic via the heat-pressing technique by reducing the Al2O3 restorative dental glass-ceramics. Table 2 summarizes the content and envisioning that this reduction could adjust the vis- most prominent companies and their glass-ceramic prod- 115,116 cosity for heat-pressing. The conclusion of these research- ucts. These glass-ceramics compete with other types of erswasthatglasseswithvariousAl2O3 contents are easily restorative materials, mainly zirconia and composites. formed and crystallized to fluorapatite. Mullite and/or anorthite were formed as a second crystal phase. However, crystallization BIOACTIVE DENTAL GLASS-CERAMICS (BDGCS) during heat-pressing resulted in a loss of control of the process In the late sixties, Larry L. Hench invented a new generation but was not considered detrimental if the future growth of these of materials known as “bioactive glasses”30 and observed 115 6 crystals could be controlled. A fracture toughness of 2.7 that certain glasses could bond to living bone and even soft 0.4 MPa m1=2 was reported for the glass-ceramic containing tissues by a complex mechanism based on ion leaching, con- 32.6SiO 210.9P O 220.3Al O 232.6CaO–3.6CaF (mol %) that 2 2 5 2 3 2 trolled dissolution of glass and precipitation of an apatite was heat treated for 8 h at 11508C. The highest flexural strength layer on their surfaces. Bioactive glasses show both osteo- of 194 6 75 MPa was obtained by heat-pressing the same glass conduction and osteoinduction properties and can be used for 1 h at 11508C. Increasing the holding time increased the crys- in a variety of applications in dentistry and medicine, such tal size and the extent of microcracking in this glass-ceramic, as bone grafting, scaffolding, drug delivery, coatings and soft thus lowering the flexural strength. Microcracks appeared to tissue engineering. For example, some commercial bioactive increasethefracturetoughnessoftheglass-ceramics,probably glass products (PerioglasVR , NovaBoneVR , and NovaMinVR )have by a crack termination mechanism.116 However, the relatively been used clinically as ossicular reconstruction prostheses, high solubility of apatite-mullite glass-ceramics was always the endosseous ridge maintenance implants, bone grafts to main issue.117 Consequently, Fathi et al.118 evaluated the effect of restore bone loss from periodontal disease in infrabony varying the CaF2 content on the chemical solubility. They defects, bone grafts in tooth extraction sites and alveolar increased the CaF2 in the initial glass from 4 to 20 mol %. All compositions easily formed glasses and, upon heat treatment, ridge augmentation, orthopedic bone grafting in general, VR crystallized to form apatite and apatite-mullite. Increasing the and as particulate in Sensodyne toothpaste for dentinal hypersensitivity treatment. However, the major disadvan- CaF2 content led to an increase in bending strength but also increased the solubility. The chemical solubility (150–380 mg/ tages of these materials are low mechanical strength and cm2) was still higher than that of the control glass-ceramic (IPS fracture toughness. These poor mechanical characteristics EmpressVR II, 78 mg/cm2)butwasacceptableforadentalcore restrict their use to only a few applications that do not 34 ceramic.118,119 A maximum bending strength of 157 6 15 MPa demand significant loads. To improve their mechanical 119 strength, various types of glasses can be crystallized by con- was reported for a sample containing 20 mol % CaF2. These trolled heat treatments and are known as “bioactive glass- same researchers also added TiO2 and ZrO2 to control the mechanical properties and solubility,120,121 and their studies ceramics.” The history, processing, properties, and applica- demonstrated that up to 1 mol % of ZrO2 and TiO2 were effec- tions of bioactive glass-ceramics, particularly in medicine, 34 tive for controlling the solubility and mechanical properties of have been reviewed elsewhere. In spite of the fact that these apatite-mullite glass-ceramics.120 The lowest chemical sol- crystallization increases the mechanical properties, it ubility and highest bending strength were 204 6 29 lg/cm2 and retards the rate of biochemical reactivity in general. How- 120 174 6 38 MPa, respectively. However, increasing the TiO2 ever, certain highly bioactive glass-ceramics exist that can concentration to >2.5 wt % led to a significant increase in solu- compete with bioactive glasses in terms of reactivity and bility and reduced bending strength.121 Mollazadeh et al.122 offer better mechanical properties.34 These glass-ceramics showed that 3 wt % TiO2 and BaO addition increased the bend- are usually synthesized via controlled heat treatment of bio- ing strength and fracture toughness of apatite-mullite glass- active glasses and are used to coat dental implants, treat

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 631 hypersensitivity, heal periodontal diseases, and repair root glass-ceramic surface and did not modify either the nature canal/dentin. In this section, we review promising of the crystalline phases (needle-like fluorapatite) or their researches on bioactive glass-ceramics specifically devel- shape. This method has been recommended for coating of oped for dental applications. Ti-6Al-4V screws for dental applications.130 Ferraris et al.131 reported that glazing could be successfully used to coat zir- Coatings on dental implants conia dental implants with bioactive glass-ceramics. An It is well known that biologically inert implants such as Ti intermediate layer often forms between the coating and the alloys and zirconia (ZrO2) do not bond to bone by biochemi- substrate, for example, between a glass coating and a zirco- cal interaction but simply via morphological fixation. Their nia substrate, and a “composite” layer exists made of glassy inadequate adhesion to bone leads to interfacial displace- phase and zirconia particles. During thermal treatment ments and clinical failure. Moreover, metals may chemically above the liquidus temperature, the glass diffuses within degrade. Therefore, metallic and ceramic implants can be the zirconia substrate, and hence, the zirconia granules are coated with bioactive materials, such as hydroxyapatite, surrounded by a glassy matrix, leading to the formation of a glasses, and glass-ceramics, to enhance interfacial bonding “composite” layer, which assures the continuity of the ther- and protect them from corrosion. Glass-ceramic coatings for mal and mechanical properties of the zirconia substrate to biomedical applications were the subject of many studies the glass-ceramic coating. The osseointegration of bioactive 124 and have been comprehensively reviewed by Rawlings, glass-coated zirconia cylinders has been evaluated in an ani- 125 126 127 128 Cao and Hench, Kasuga, Verne, and Marghussian mal model and compared with that of uncoated cylin- 129 34 and more recently by Xuereb et al. and our group. ders.131 After 30 days, bone bonding was better than that of Almost all commercial bioactive glass-ceramics, for example, VR VR VR VR the uncoated cylinders, but after 60 days, the difference was Cerabone , Biosilicate , Bioverite , and Ceravital , and equal within the statistical uncertainty.131 Nano-structured promising ones such as fluorapatite, apatite-mullite, calcium coatings on dental implants have also been developed by pyrophosphate, and composites have been used to coat Ti Xiao et al.,132 who attempted to coat titanium substrates or ZrO dental implants.34 To this end, many methods, such 2 using the liquid precursor plasma spraying (LPPS) process. as enameling, plasma spraying, flame spraying, sputtering, Tetraethyl orthosilicate, triethyl phosphate, calcium nitrate, laser deposition, electrophoretic deposition, and so forth, and sodium nitrate solutions were mixed together to form a have been attempted for application of perfect coatings.127 sol after hydrolysis, and this liquid was used as feedstock Indeed, all coatings that are produced as amorphous or for plasma spraying of P O –Na O–CaO–SiO bioactive glass- glassy layers in the first stage of their fabrication process 2 5 2 2 ceramic. Bioactive glass-ceramic coatings with nano- and are subsequently subjected to a controlled heat treat- structures were successfully synthesized, and the coatings ment to transform them into polycrystalline materials, can showed quick formation of a nano-structured HCA layer be classified as glass-ceramic coatings.128 The results are after soaking in SBF. Overall, the results indicate that the promising, but scientists are still working to optimize/ LPPS process is an effective and simple method for synthe- improve the coating properties before translation into clini- sizing nano-structured bioactive glass-ceramic coatings with cal applications. These properties include homogeneity, ease 132 of application, interface bonding strength and good biologi- good in vitro bioactivity. cal properties. However, degradation over time, which leads One type of dental implant failure, known as peri- to detachment of the coating, is still a noticeable drawback implantitis, is derived from bacteria. Peri-implantitis is a and cannot be predicted. Clinical trials are still required to destructive inflammatory process that affects the soft and 1 produce convincing results for long-time successful out- hard tissues surrounding dental implants. Peri-implantitis comes.124–129 We do not intend to review, as others is related to accumulations of periodontal pathogens, and did,124–129 the entire body of researches that address glass- its treatment includes removal of the bacterial biofilm, ceramic coatings on dental implants, but instead, we explain debridement of the exposed surface, and surgical regenera- 1 selected promising studies in this work. tion of the peri-implant pocket. A bioactive and bactericide Verne et al.130 prepared double-layer bioactive glass- coating can minimize this problem. Therefore, researchers ceramic coatings on Ti-6Al-4V implants using a simple dip- are currently seeking such a coating. For example, Esteban- Tejeda et al.133 and Dıaz et al.134 have tried to develop a coating and firing process. A 61.1SiO2212.6CaO–10.3Na2O– 7.2MgO–6.0P2O522.8K2O in wt % based glass was used as mechanically stable antimicrobial glass-ceramic coating on the first layer in direct contact with the metallic substrate, zirconia (Y-TZP) and Ti-6Al-4V substrates. A soda-lime-based and a 26.2SiO2217.9Al2O3217.5P2O5210.5K2O–19.6CaO– glass was first deposited on implants by dip coating or 8.3F- in wt % based glass-ceramic was used as the outer flame spraying. Glass-ceramic coatings containing nepheline bioactive layer. The Verne group designed above low- (Na3KAl4Si4O16) and Na2Ca2Si3O9 crystalline phases were melting intermediate glass composition that could work coated on Y-TZP and Ti-6Al-4V substrates after heating at below the point of transformation of a-Ti to b-Ti to ensure 7508C for 1 h. Precipitation of needle-like hydroxyapatite on adhesion of the outer bioactive layer. The intermediate layer the coating took place during the artificial saliva solubility was also chosen to match the thermal expansion coefficient test, indicating the bioactive character of this layer. The of both the metal and the outer glass-ceramic. The process coatings were proven to be antimicrobial versus Escherichia applied for the coating did not affect the bioactivity of the coli, Staphylococcus aureus, and Candida krusei.133,134

632 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE

FIGURE 16. SEM images show apatite-like deposits inside the dentine tubules (a) and on the dentine surface (b) after 7 days of treatment with toothpaste filled with a bioactive glass-ceramic.140

Hypersensitivity treatment revealed that BiosilicateVR requires only 24 h to induce the Dentin hypersensitivity (DH) is one of the most common precipitation of a homogeneous HCA layer covering the clinical problems. DH is a sharp and short pain arising from entire dentin surface.137 In a clinical study using Bio- exposed dentin surfaces in response to thermal, evaporative, silicateVR , a total of 142 patients were randomized into four tactile, osmotic, chemical, or electrical stimuli. DH is caused groups that received different desensitizing treatments of when fluids within the dentinal tubules are subjected to SensodyneVR , SensiKillVR , or BiosilicateVR dispersed in a gel changes (thermal, mechanical, and osmotic), and movement suspension or in a solution with distilled water. Over a in the fluids stimulates a nerve receptor sensitive to pres- period of 6 months, a total of 232 teeth were evaluated, and sure, which leads to transmission of the stimuli. Conse- the study was performed using pain assessments. With quently, dental materials proposed to treat DH seek to respect to the global diminution of pain over the course of interrupt the pulp neural response of pain and/or to block the study, BiosilicateVR mixed with distilled water displayed the sensitive mechanisms through closure of the open denti- the greatest effect and was able to diminish the pain in the nal tubule.1,2 first periods of the experiment. This study indicated that Treatment of hypersensitivity has been clinically micron-sized BiosilicateVR could offer an immediate, effective attempted using bioactive glasses since 2004, when a very and long-lasting treatment alternative for patients who suf- fine BioglassVR 45S5 particulate known as NovaMinVR with a fer from DH.138 Another clinical study investigated the in particle size of approximately 18 mm was first used in situ influence of BiosilicateVR on whitened enamel and dentin SensodyneVR toothpaste. The common abrasive additive in surfaces after tooth bleaching. The results indicated that toothpastes is alumina particles, which can be replaced by when combined with whitening gel, this biomaterial BioglassVR 45S5. Clinical studies have shown that the Bio- improved tooth hardness and morphology, thus preventing glassVR 45S5 particles adhere to the dentine, form a hydroxy- demineralization.139 Another group in China also tested apatite layer and block the tubules, which are near 1 mmin their novel bioactive glass-ceramic for hypersensitivity treat- diameter, thus relieving pain for a long duration.135 ment. Bioactive glass-ceramic toothpaste significantly and Although BioglassVR has shown successful results for hyper- immediately lowered dentine permeability. The highest sensitivity treatment, dentists and patients do not like the reduction in dentine permeability was observed after 7 days idea of using crushed glass in the mouth, which can often of treatment.140 Figure 16 shows the formation of apatite- cut and irritate the gums. like crystals inside dentine tubules and on its surface after Certain bioactive glass-ceramics can play a similar role brushing of the dentine with toothpaste containing bioactive in this application. Highly bioactive BiosilicateVR is the most glass-ceramic.140 One interesting advantage presented by investigated glass-ceramic by our group for this function.136 glass-ceramics in this application is that because these BiosilicateVR is the designation of the particular composition materials are fully crystalline, they do not present sharp 23.75Na2O–23.75CaO–48.5SiO224P2O5 (wt %). Under con- cutting surfaces. In contrast, glasses possess conchoidal frac- trolled double-stage heat treatments, the microstructure of ture surfaces that can lead to gum irritation during brush- the material can be engineered to be composed of one or ing.136–140 Therefore, bioactive glass-ceramics deserve much two crystalline phases as follows: a sodium-calcium silicate more attention for this application. phase (Na2CaSi2O6) or both Na2CaSi2O6 and sodium-calcium 136 VR phosphate (NaCaPO4). The first study on Biosilicate for Bone and periodontal healing hypersensitivity treatment was reported by Tirapelli et al. Teeth are supported by alveolar bone, the periodontal liga- in 2010.137 The target glass-ceramic consisted of one crys- ment, and gingival connective tissue. Alveolar bone sur- talline phase, Na2CaSi2O6. The authors evaluated the effi- rounds and holds teeth firmly in their position. The ciency of powdered BiosilicateVR in occlusion of open periodontal ligament maintains the attachment of teeth to dentinal tubules using dentin disc models. The results the jaws, whereas gingival connective tissue covers the

JOURNAL OF BIOMEDICAL MATERIALS RESEARCH A | FEB 2017 VOL 105A, ISSUE 2 633 alveolar bone to protect it from the oral environment. These thermally induced phase separation, microsphere emulsifica- vital tissues might lose their functions due to trauma, dis- tion sintering, electrospinning to form nano-fibrous struc- ease or congenital abnormalities. Malfunction of teeth in tures, freeze casting, additive manufacturing technologies, aging populations reduces an individual’s quality of life.1,2 computer assisted rapid prototyping techniques, and so Therefore, in recent years, extensive researches have been forth.146–149 The results are inspiring, and it appears that in conducted on healing or engineering of dental tissues. future, biomimetic regeneration of the complex structure of Among the large number of materials chosen for these pur- teeth demands 3D porous materials, and bioactive glass- poses, bioactive glasses and glass-ceramics have also been ceramics are promising for this application. Research and considered. Our focus in this review is on bioactive glass- development in this area might lead to the first bio-tooth ceramics used in dentistry, rather than on glasses, because development.7–9 bioactive glasses have already been widely tested and reviewed.141–143 We believe that special consideration of Bioactive and bactericidal glass-ceramics for bone regen- bioactive glass-ceramics is of great importance in this field. eration or attaching fixed restorations to soft periodontal The reason for this focus and research topics on the use of tissue. Gel-derived bioactive glass-ceramics are modern and bioactive glass-ceramics in dentistry are summarized in promising biomaterials for this application. These highly three fields of interest as follows. bioactive and mesoporous materials have begun to emerge in various applications in dentistry, including dental tissue Tough bioactive glass-ceramics for dental implants. After and bone regeneration, drug delivery and hybrid materials. the development of the first bioactive glass-ceramic As good examples of research for this application, Vallet- (CeravitalVR )byBromer€ et al. in 1973, a worldwide quest for Regı’s team has demonstrated that controlled heat treat- tougher bioactive glass-ceramics ensued.34 Such glass- ment of mesoporous gel-derived glasses can lead to glass- ceramics could be used as bone substitutes or dental ceramics with improved mechanical properties without implants. However, the fracture toughness of commercial deterioration of their bioactive behavior. A gel-derived bio- bioactive glass-ceramics and some promising ones active glass-ceramic containing pseudo-wollastonite, wollas- 1=2 (KIC < 3MPam ) were below the desired standards, and tonite, tricalcium phosphate, and cristobalite was developed significant improvement was required to approach the prop- by heating a gel-glass (55SiO2241CaO–4P2O5 in mol %) at 1=2 erties of cortical bone (KIC 5 2–12 MPa m ). This topic is 13008C. This glass-ceramic shows efficient in vitro bioactive still of great interest, and an intense search is currently behavior, that is, an apatite layer covered the sample surface underway for novel compositions and microstructures after 3 days of soaking in SBF.150,151 To use this material as 1=2 designed to increase toughness to KIC > 3MPam and a substrate for bone tissue engineering, the authors synthe- also increase bioactivity.34 For examples, Saadaldin sized scaffolds with a network of designed three- et al.144,145 suggested two wollastonite and miserite glass- dimensional interconnected macropores 400–500 lmin ceramics for dental implants and reported a high fracture diameter. Further studies revealed that the resulting glass- toughness of 4.5–5.5 MPa m1=2. We believe that materials ceramic is bioactive, cytocompatible and capable of promot- that have an interlocked house-of-cards structure are good ing in vitro the differentiation of mesenchymal stem cells candidates, but the method (indentation and crack length into osteoblasts. For that reason, this material could offer a measurement) used to measure fracture toughness is not suitable matrix for bone tissue regeneration. Additionally, in reliable because it usually shows high values and should be vivo evaluation of 70SiO2230CaO, 80SiO2217P2O523CaO 144,145 revised. and 36SiO227P2O5–44CaO–13MgO (mol %) glasses and glass-ceramics in New Zealand rabbits showed potential for Glass-ceramic scaffolds for engineering bones or bone substitute and regeneration.150,151 Another example is restoring supporting dental tissues. In recent years, bioac- the gel-derived microporous bioactive glass-ceramic devel- tive glass-ceramic scaffolds have opened new avenues for oped in Boccaccini’s group in Germany,152–154 which has treatment of bone defects in orthopedic, dental and maxillo- potential application in dental restoration. These glass- facial situations. Scaffolds contain 3D interconnected pores ceramics show bioactive behavior around the margins of with diameters of at least 100 mm that can host biofactors, fixed restorations and create a bioactive surface that can support vascularization, encourage cell growth, and thus lead to periodontal tissue attachment, thus providing com- promote tissue formation. Comprehensive reviews on the plete sealing of the marginal gap between teeth and fixed state-of-the-art in bioactive glass-ceramic scaffolds have prosthesis. The sol-gel route was adapted for the develop- 34,146–149 been published by numerous scientists who believe ment of 60SiO223P2O5214Al2O326CaO–7Na2O–10K2O (wt that glass-ceramics have a combination of better properties, %) glass-ceramic and a related composite material that such as high strength and fracture toughness (due to their combines this glass-ceramic with a mesoporous 152,153 unique toughening mechanisms), for scaffold development 58SiO2233CaO–9P2O5 (wt %) bioactive glass. This compared with those of glasses.146–149 Therefore, almost all method resulted in a homogeneous microporous glass- commercial bioactive glass-ceramics and others under devel- ceramic composite that can be applied as a coating on com- opment34 have been the subject of study for scaffold devel- mercial dental ceramic substrates.152,153 The attachment opment using various fabrication techniques, including, for and proliferation of both the periodontal ligament and gingi- example, foam-replication methods, salt or sugar leaching, val fibroblast cells confirmed the bioactive behavior of the

634 MONTAZERIAN AND ZANOTTO DENTAL GLASS-CERAMICS REVIEW ARTICLE new materials and their potential ability for use in dental inorganic fillers in these composite restoratives. In at restorations for soft tissue regeneration and sealing of the least two research studies, Liu et al.166 and Mollazadeh marginal gap.153 Furthermore, an Ag-doped composite et al.167 used porous mica-fluorapatite and fluorapatite- showed pulp-cell proliferation and antibacterial properties mullite glass-ceramic fillers to reinforce dental resin- that could facilitate potential applications in tooth regenera- based composites. tion approaches.154 Interested readers are encouraged to 6. New coating technologies and the properties of coatings refer to comprehensive reviews and book chapters such on dental implants should be improved. For example, as,8,9,34,142,151,155 which address gel-derived glasses and degradation over time, which leads to detachment of glass-ceramics and their derivative products for dental coating, is a noticeable drawback. applications. 7. The development of restorative glass-ceramics or com- posites which in contact with bone and surrounding tis- CONCLUSIONS AND TRENDS sues show a cement-like behavior and facilitate Restorative dental materials are moving from metal alloy- biological surface responses for marginal attachment is containing to all-ceramic restorations, and this review dem- another challenging field of research. onstrates that glass-ceramics work well as all-ceramic resto- 8. Glass-ceramic matrix composites have been rarely inves- rations. The following 10 topics warrant further research: tigated for restorative dentistry and demand additional attention. 1. Research and development are underway to further 9. Dental tissue engineering for construction of tooth improve the fracture toughness and esthetics of dental organs is a brand new and highly interesting direction. glass-ceramics to enable them to compete with their A clear and distinct shift is occurring in regenerative current contenders (e.g., zirconia and hybrids) for pos- medicine from use of synthetic materials or tissue terior restorations. We agree with Holand€ et al.156 that grafts to a more explicit approach that applies scaffolds comprehensive knowledge of toughening mechanisms is for hosting cells and/or biological molecules to create a necessary step to open new directions for develop- functional replacement tissues in diseased or damaged ment of tough glass-ceramics. Therefore, future research dental sites. activities should be focused on gaining a better under- 10. Finally, (expensive, time-consuming) clinical tests should standing of the mechanisms of toughening, such as be encouraged to evaluate dental glass-ceramics in real transformation toughening, bridging, microcracking, and application cases. pulling out, that can be stimulated by controlled crystal- lization of different crystals with a variety of morpholo- All of these ideas and several others not reported in this gies and microstructures. Additionally, various coloring work can only be achieved by increasing interactions among agents and pigments should be deeply and thoroughly materials engineers and scientists, chemists, dentists and tested to adjust the shades and esthetics of glass- biologists. ceramics. On the other hand, the morphology of crystals ACKNOWLEDGMENTS from the nano- to microscale, which can be controlled ~ by precise adjustment of the chemical composition and The authors are grateful to the Sao Paulo Research Foundation crystallization process, might strongly influence their (FAPESP, #2013/07793-6) for financial support of this work optical properties, but the published information on and for the post-doctoral fellowship granted to Maziar Monta- these topics is scarce. zerian (#2015/13314-9). 2. New methods (e.g., meta-analyses) can be used to REFERENCES expand the range of glass-ceramic composition. For 1. Sakaguchi RL, Powers JM. Craig’s Restorative Dental Materials, instance, it was recently demonstrated that new nano- 13th ed, Netherland: Elsevier; 2011. glass-ceramics with a notably high ZrO2 content can be 2. Powers JM, Wataha JC. 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