Prosthodontics dental materials: From conventional to unconventional

Materials Science & Engineering C 106 (2020) 110167

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Materials Science & Engineering C journal homepage: www.elsevier.com/locate/msec

Review

Prosthodontics dental materials: From conventional to unconventional a

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Fatima Saeed , Nawshad Muhammad , Abdul Samad Khan , Faiza Sharif , Abdur Rahim , Pervaiz Ahmadc, Masooma Irfand

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Interdisciplinary Research Centre in Biomedical Materials (IRCBM) COMSATS University Islamabad, Lahore Campus, Lahore 54600, Pakistan Department of Restorative Dental Sciences, College of Dentistry, Imam Abdulrahman Bin Faisal University, Dammam 31441, Saudi Arabia Department of Physics, University of Azad Jammu and Kashmir, 13100 Muzaffarabad, Pakistan d Department of Chemistry, COMSATS University Islamabad, Lahore Campus, Lahore 54600, Pakistan b c

ARTICLE INFO

ABSTRACT

Keywords: Prosthodontics materials Denture base materials Fixed prosthesis Artificial teeth materials Impression materials Ingenious alternatives Prosthetic advances

New inventions and innovations in the field of dentistry have potential applications to satisfy the patient's demand. In prosthodontics, a dental prosthesis plays a major role in improving the quality of oral health care. Currently, the trends have shifted towards the implants and implant-supported prosthesis for the replacement of missing teeth. Conventional dentures are patient's preference mainly due to financial constraints. In an attempt to find solutions to current problems, we have come across new materials zirconium, titanium and new inventions like flexible dentures, fenestrated dentures, and CAD/CAM fabricated dentures. Using the progress of past five years in the field of prosthodontics, this comprehensive review focuses on denture base materials, denture liners, removable partial dentures, fixed prosthesis such as crown and bridge materials, implant-supported a fixed denture, artificial teeth materials, impression materials, and ingenious alternatives to conventional dentures. This article also sheds some light on the current promising researches and gives insight into the problems that can be the focus of future researches.

1. Introduction Each prosthodontist makes a subconscious effort to evaluate the current status of dentistry throughtheir clinical practice. They consider themselves successful when the patient is satisfied. Apart from clinical practice, the evaluation of the current status of prosthodontics depends on dentists' personal research. This evaluation is subjected to flaws on the basis of errors in dentist personal evaluation of treatment, patient's lack of knowledge and lack of compliance [1–3]. Assessing the current status of research on prosthodontics materials is rather difficult. This specialty lacks well-allocated boundaries. Moreover, the assessment concerning prosthodontics research is rather subjective [1,2,4]. Teeth have been considered an integral part of beauty. Missing teeth not only cause the functional and structural problem but it also influences persons' psychology and social interactions [5]. History goes way back to the 18th century when first artificial teeth were made [6]. Materials used to make artificial teeth were natural teeth (both human and animal, curved to desired shape and size) ivory and porcelain [7,8]. Feldspathic porcelain was adapted from European white wares made by clay quartz feldspar. After that fine translucent porcelain was

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manufactured in 1720 s. First successful porcelain denture was made at Gerhard porcelain factory with the collaboration of Alexis Duchateau and Persian Dentist Nicholas Dubious de clement in 1774. Enameling of metal denture bases was introduced in 1773 [9]. Major drawback that porcelain had, was wear of opposing natural teeth. After several experimental trials Decor glass ceramic and fine microstructure porcelain were discovered [10]. Fine microstructure porcelain showed less wear of opposing tooth as compared to porcelain [9,11]. The glass-ceramics have optical properties similar to enamel and dentin. After that filler particle was added to glass-ceramic to improve mechanical properties and regulation of optical properties such as color and opacity. Other ceramic currently in use include: in Ceram, IPS-express, Optec, Opalescent porcelain and Porcelain fused to metal ceramics produced by different researchers [1,12]. Earlier reviews on this topic either published formerly that's why it is needed to overview the new findings, or the earlier reviews were selective in reviewing a type of prosthodontic materials [13–15]. With this article, we have provided aid to the evaluation of the present level of research on all possible prosthodontics. This article tried to shed some light on the current status of research on prosthodontics materials i.e., denture bases material and artificial tooth materials and processing

Corresponding author. E-mail address: [email protected] (N. Muhammad).

https://doi.org/10.1016/j.msec.2019.110167 Received 17 October 2018; Received in revised form 30 August 2019; Accepted 5 September 2019 Available online 07 September 2019 0928-4931/ © 2019 Published by Elsevier B.V.

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technology. This is done by reviewing past literature on prosthodontics from Scopus, PubMed, Web of Science and etc. This research will focus on the quality and quantity of research on prosthodontics for the past 5 years and recent advancement in the field of prosthodontics. Remarkable advancements concerning prosthodontics over the past five years are highlighted below:

suggested to improve the mechanical properties of PMMA [28]. In addition, it may have an antifungal effect and may play a preventive role in patients susceptible to fungal infections. Safi et al. studied the effect of zirconia NP addition on the color properties of PMMA and did not find any noticeable color changes. Poor wettability between fillers and the resin matrix reduced the physical and mechanical properties of PMMA. Therefore, a silane coupling agent was used to improve the bond strength between zirconia NPs and PMMA. It resulted in increased flexural strength and impact strength of acrylic resin, but its tensile strength was not improved. However, one study found that salinized zirconia NPs improved the tensile strength and fatigue strength of PMMA. Moreover, hardness increased significantly, and surface roughness increased slightly when salinized zirconia NPs were incorporated in acrylic resin, while apparent porosity, water sorption, and solubility decreased. SEM Analysis of zirconia reinforced acrylic resin has shown less porosity as compared to the acrylic resin without zirconia. This can be illustrated by Fig. 2. Mechanical properties of heat-cured acrylic resin i.e., impact strength, transverse strength, and surface hardness are also significantly increased by the addition of nanoparticles of salinized ZrO2. In addition to this, it discourages water sorption and solubility [34].

1.1. Denture base Search for denture base material started a long time ago, starting from wood, bone ivory, vulcanite, ceramics, wax, metal and finally polymethyl methacrylate (PMMA) polymer. PMMA was introduced in 1937, since then it has become an essential part of prosthodontics practice [16,17]. PMMA has all the properties essential for ideal denture base material except mechanical properties [18]. PMMA is not only material of choice for denture bases but also for denture reliners, rebases, maxillofacial prosthesis, orthodontic prosthesis, temporary crown, splints for surgical procedures. Characteristics of denture base materials accounting for its popularity are, remarkable esthetics, welldefined fabrication, and processing methods, ease of repair, easy availability and low cost [19]. Moreover, its coral pink color resembles the natural color of mucosa as shown in Fig. 1. Despite all positive aspects, PMMA is not devoid of shortcomings. Poor strength, accounting for a large number of dentures repair each year, is a major shortcoming [20–23]. Allergic reaction to acrylic resin is also a common problem [24]. This problem has been addressed by modifications of resin denture bases [25].

1.1.1.3. Glass fiber reinforced acrylic. Addition of glass fibers to acrylic denture base material is an effective way of increasing flexural strength, impact strength and toughness [36,37]. The strength of the fiberreinforced acrylic framework is not comparable to that of a metal framework (Fig. 3). However, it is also found to increase the Vickers hardness number of PMMA. Resistance to deformation is also remarkable i.e., less than 1%. The position of glass fiber with respect to the surface of denture base resin has a significant impact. If placed in the stress-free neutral zone, then there was no effect on the toughness and impact strength, but only flexural toughness was enhanced. If placed near the surface (compressive stress zone), then overall mechanical properties including surface flexural modulus were enhanced [37–40].

1.1.1. Enhancement of Mechanical Properties of PMMA denture bases Mechanical strength of PMMA had been a problem faced by prosthodontists. In order to solve this problem researches have been conducted by using several materials. Reinforcement of PMMA has resulted in improved flexural strength, impact strength and increased fatigue resistance (Table 1) [20,23,27]. 1.1.1.1. Alumina air abrasives. Surface treatment of the polymethyl methacrylate with alumina abrasives leads to the improved flexural strength of the polymethylmethacrylate. This treatment has been found to yield promising results with all types of PMMA i.e., self-cured, heatcured, and light-cured. Surface treatment with alumina air abrasive enhances the flexural strength of PMMA to about 60% as compared to the one with no treatment [17].

1.1.1.4. Titanium oxide nanoparticles. Effect of titanium oxide nanoparticles on the acrylic denture base is solely dependent on its added amount. Its addition in large concentrations has adverse effects on flexural strength of acrylic, however; its less concentration about 1% wt. has positive effects on impact strength. About 5% TiO2 increases the microhardness of acrylic resin [38,42]. 1.1.1.5. Silanated propyl propylene. Silanated propyl propylene has been added to heat-cured acrylic resin (Table.1) and it has proven to improve the mechanical strength of heat-cured acrylic denture base resin [40,43].

1.1.1.2. Salinized zirconium oxide. Reinforcement of acrylic denture base material with zirconia (ZrO2) has a significant impact on mechanical properties of denture base material [28]. Several studies found that incorporating stabilized zirconia filler in PMMA, alone or in combination, significantly increased its flexural strength [22,29–33]. However, a slight decrease in flexural strength was also noticed [28]. It may result from the clustering of the particles within the resin, which weakened the material. Also, the addition of ZrO2 significantly increased the thermal conductivity of PMMA. Variable results were obtained regarding the effect of ZrO2 on the water sorption and solubility of PMMA. Adding zirconia nanoparticles (NP) was

1.1.1.6. Polyamides: (aramid and nylon). Aramid reinforced acrylic denture bases have been found to have a positive impact on flexural and impact strength and are biocompatible with denture base resin. It has a negative impact on hardness. Decreased water sorption due to less porosity and remarkably decreased microbial colonization due to less microbial adherence is noticed with fiber-reinforced PMMA [38]. 1.1.2. Advancements to prevent microbial colonization in denture bases Polymethyl methacrylate is susceptible to microbial colonization which eventually leads to denture-related diseases like stomatitis. PMMA has inner rough surface that predisposes to microbial colonization. In recent years, research is more focused on the modification of polymethylmethacrylate to improve its surface properties and resistance to bacterial and fungal infections [44]. Immunodeficiency of a patient along with the susceptibility of PMMA to microbial colonization accounts for an increased prevalence of denture stomatitis In order to make a denture base resistant to microbial infections several types of research have been conducted to

Fig. 1. Sample of acrylic resin used for the construction of denture bases [26]. 2

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Zirconia

Alumina

Fibers

Materials

Flexure strength and impact strength. Biocompatible and reduced porosity and C. albicans adherence. Biocompatible, flexural strengths and flexural modification Fracture resistance and structural elasticity. Impact strength, elastic modulus, and toughness [53,54]. Impact strength [22,54]. Transverse, tensile, and impact strengths [55]. Flexural strength and flexural modulus. Flexural modulus.

Silanized glass fiber

Polypropylene

Silanated polypropylene OPEFB

Vegetable fiber

Compressive, tensile, flexural strength, wear resistance, water sorption, and solubility. Thermal stability, flexural strength, water sorption, solubility, and biocompatibility. Flexural strength. Impact strength, fracture toughness, and hardness. Water sorption and solubility [57]. Impact strength, flexural strength, and radio-opacity. Compressive strength, fatigue strength, fracture toughness, and hardness, as well as color properties [28,57–59]. Flexural strength, impact strength, and hardness. Surface roughness slightly increased, and porosity decreased. Tensile and fatigue strength and decreased water sorption and solubility. Flexural strength.

Silane-treated Al2O3

ZrO2 NPs

Zirconia nanotubes

Silanized zirconia NPs

ZrO2

Al2O3 NPs

Thermal conductivity, flexural strength, impact strength, tensile strength, and surface hardness of the resin.

Al2O3

Nylon Polyethylene

Aramid

Flexural strength, impact strength, toughness and hardness, and reduced deformation of the denture base [37].

Glass fiber

Increased effect

Reinforcements

Table 1 Summary of modifications/reinforcements of denture base material (PMMA) and their results [20,22,27].

Decreased candida adhesion to denture bases [35].

No effect (surface roughness and water sorption).

Impact and tensile strength of PMMA.

Flexural strength.

Wear resistance.

Hardness, yellow color.

No effect (linear dimensional stability).

Decreased/no effect

Observable improvement in denture base properties with zirconia NPs incorporation. Silanized zirconia NPs resulted in superior mechanical properties and adequate surface properties of PMMA denture base resin.

Alumina filler mainly used to improve thermal conductivity, and the silanized type improves the physical and mechanical properties of denture base resin.

The most common reinforcement repair material under research is the silanated glass fiber. It highly improves the physical properties of denture base resin in addition to its biocompatibility. Also, its ease of application renders its priority for use [37].

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MM Gad et al. [22,27]

MM Gad et al. [22] Abdulrazzaq Naji S et al. [27] MM Gad et al. [22] Abdulrazzaq Naji S et al. [27] MM Gad et al. [22] Abdulrazzaq Naji S et al. [27]

Takahashi T et al. [51] Takahashi T et al. [51] Mundra et al. [54] Akinici et al. [53] MM Gad et al. [22] Takahashi T et al. [51] Mundra et al. [54] MM Gad et al. [22] John, Jacob et al. [56] MM Gad et al. [22] MM Gad et al. [22] Abdulrazzaq Naji S et al. [27] MM Gad et al. [22] Abdulrazzaq Naji S et al. [27]

He, Xinye et al. [52]

Takahashi T et al. [51] MM Gad et al. [22]

Hamouda et al. [37] Takahashi T et al. [51] MM Gad et al. [22]

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4

Hybrid reinforcements Flexural strength and toughness, surface roughness, tensile modulus, hardness, and thermal conductivity, reduced shrinkage, and have antibacterial properties without showing cytotoxicity. Impact strength, hardness, surface roughness, thermal conductivity, compressive, and fatigue strengths.

Metal oxides and ceramics

Fiber and other fillers i.e., nitrile butadiene rubber particles and ceramic fillers (Al2O3, SiO2 and YSZ).

Flexural strength and toughness.

Impact, transverse strength, and hardness. Flexural strength.

SiO2 Surface-treated silica

Hybrid fibers

Fracture toughness. Thermal dimensional properties and hardness.

Nanoparticles

Glass flake Mica

Hardness and thermal conductivity, impact strength, fracture toughness, and scratch resistance.

Nano‑carbon SWCNT'S MWCNTs

Carbon Family

Silica-based

Flexural strength and flexural modulus. Fatigue and compression strengths. Impact strength and flexural strength. Impact and transverse strength. Flexural strength and resilience.

Silane-treated HA filler HA NPs

HA

Transverse strength, hardness, water sorption, and solubility. Radio-pacifier, thermally stable.

Flexural and fatigue strength, thermal diffusivity. Water sorption, water solubility. Tensile and flexural strengths [60]. Antifungal properties, thermal conductivity, and compressive strength. Not cytotoxic. Viscoelastic properties. Flexure strength, fracture toughness, and hardness. Impact strength, water sorption, and solubility. Thermal stability, E-Modulus.

Flexural strength and thermal conductivity.

BaTiO3

Titanate-coupling agents

TiO2 NPs

TiO2

Silver NP

Silver

Increased effect

Reinforcements

Nano‑gold

Titanium

Silver

Materials

Table 1 (continued)

No effect (hardness)

Flexural strength.

Fatigue resistance.

Flexural properties on water storage. Hardness. Hardness. No effect (flexural strength).

Fracture toughness High density.

Flexural strength and toughness. Surface roughness.

No effect (impact and transverse strength, hardness, and surface roughness). Tensile strength. Poor color stability. No effect (C. Albicans adherence) Flexure strength.

Decreased/no effect

Although a few types of research have been done on hybrid Reinforcement, they revealed superior surface properties, mechanical properties, thermal conductivity, and biocompatibility.

Carbon NPs and nanotubes enhance denture base strength. Meanwhile silanized NPs improved the properties of denture base resin but it was decreased with silanized nanotubes. Although a few types of research have been done on ND, it showed improvement in physical and mechanical properties, as well as the thermal conductivity of denture base resin. Different forms of silica were used. The siliconized and fluoridated one improved the mechanical properties and maintained surface properties of denture base resin, as well as improved denture hygiene.

Noticed improvement but need further investigations.

TiO2 NPs addition improves the mechanical and surface properties of denture base resin as well as thermal conductivity. An extra improvement was noticed with titanate-coupling of agents.

It also improved thermal conductivity, and it is biocompatible.

Silver mainly used as an antimicrobial agent, it was effective in reducing Candida adhesion.

Comments

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MM Gad et al. [22] Abdulrazzaq Naji S et al. [27]

References

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Fig. 2. Shows SEM images of self-cure resin with or without ZrO2. In this figure, A represents self-cured acrylic resin without ZrO2 nanoparticles, B shows ZrO2 reinforced acrylic resin, C and D are highly magnified images of A and B respectively [35].

modify the denture base [45–49].

placing the zinc powder, the reaction temperature was maintained for 30 min. Afterward, the furnace was cooled to room temperature under an argon atmosphere, and then final ZnO nanoparticles were collected after the completion of the reaction. This method ensures precise control over the size of particles [50]. These particles have been found to render denture bases resistant to fungal infection due to decrease adhesion of microorganisms on denture bases. ZnO-NPs have high hydrophobicity, hardness, and absorbability that prevents adhesion of microorganisms to denture bases (Table 1) [46].

1.1.2.1. ZnO nanoparticles. ZnO nanoparticles modification of denture base has yield promising results. ZnO-NPs are synthesized by microwave solvothermal method. In this method, an appropriate amount of zinc in powder form is positioned in the middle of the quartz boat. When the temperature in the horizontal tube furnace reaches 800 °C, the above-mentioned assembly is placed within the quartz tube and it is heated at a rate of 10 °C/min. Before inserting the quartz tube, the furnace was purged under the mixture of argon and oxygen with the flow rate kept at 50 and 10sccm respectively. After

Fig. 3. A is showing metal framework, B represents a fiber-reinforced framework on the cast [41].

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1.1.2.2. Ag Nanoparticles. It has been observed through studies that the Ag-NP acrylic base has more antifungal properties than acrylic bases. Biomass adherence of Candida albicans on denture bases have significantly decreased. Ag-NP not only improves the antimicrobial and antifungal activities but also improves the viscoelastic properties (Table 1) [47–49,51,52].

patient, nevertheless, its prevalence is quite low (0.6%). In contrast, risk of titanium allergy is high for a patient with a history of an allergic reaction [62]. 1.1.3.2. Porcelain and zirconia-based denture framework. It has only been a decade since the introduction of zirconia restoratives and its popularity and success is significantly increased. This is owing to its excellent compatibility, mechanical characteristics and considerably simple manufacturing procedure i.e., CAD/CAM. An additional advantage of zirconia is conservative preparation of tooth because lack of need for veneer porcelain interface space. Dental applications of zirconia utilize 3 mol% yttria for stabilizing zirconia. This zirconia has excellent mechanical properties i.e., high flexural properties which is due to stress-induced t-m transformation in which volume increase of 4.5% is also seen. Zirconia has three phases, monoclinic at room temperature, tetragonal at 1170 °C and cubic above 2370 °C. Zirconia exhibits the highest flexural strength and toughness of all dental ceramics. However, other glass-ceramics are monolithic i.e., singlephase [73,74]. Titanium and zirconia are rising as alternatives to conventional fixed prosthesis metal framework. Zirconia restorations, made by zirconia-based ceramics, have been introduced into dental practice to fabricate fixed dental prosthesis by dental computer-aided design and manufacturing(CAD/CAM) [75,76]. Studies have proven that zirconia and titanium-based fixed dentures manufactured by CAD/CAM have high accuracy and precision [77]. Initially, there were problems associated with the chipping of veneers which were later found to be solved with the adoption of slow cooling and heating rates [78]. The problem of veneer chipping can also be solved by the use of ceramics comprising of leucite [79]. Tetragonal zirconia polycrystals (Y-TZP) usually partially stabilized by yttrium has been recently introduced for its superior mechanical properties i.e., toughness, abrasion resistance, strength, and biocompatibility. Y-TZP being white in color, it has been shaded by varying concentration of added colorants to match the respective tooth color [80]. Even though it's a desirable material but it has many issues to be applied clinically for restorations. Miura et al. [13] highlighted the chipping of porcelain used in the zirconia-based all-ceramic restorations in a large number of clinical cases [81]. Usually, in veneered zirconia, there is chipping off a large portion of porcelain which shows left overstress in zirconia porcelain interface, is responsible for failure. This failure is found to be due to low thermal diffusivity of zirconia in comparison to metals and alumina [82]. Clinical study of five years using three- to five-unit zirconia framework fixed partial dentures established almost 15% failures are from chipping of the porcelain veneer layer but failures of metal framework are not observed [83]. Similarly, it was noted that the short-term cohesive failure of porcelain to the zirconia-based all-ceramic was higher as compared to the porcelain fused to metal restoration. This cohesive failure is considered to be due to difference in coefficient of thermal expansion between the core and veneering porcelain. Altering the processing technology significantly influences the failure rate of zirconia crown [84]. In addition, it was very difficult to handle the fracturing of porcelain fired onto the zirconia-based all-ceramic restorations as both have different thermal expansion [85]. These problems could be overcome by adjusting the thickness of the porcelain as well as the cooling effect while fusing the porcelain on zirconia-based all-ceramic [13,86]. The basic problem of zirconia is its opacity when compared with glass-ceramics. This inferior optical characteristic has been addressed by veneering the core material with porcelain or by making monolithic prosthesis with somewhat more translucent ZrO2 materials [80]. In spite of the fact that the translucency of monolithic zirconia has been improved drastically but still there is a long way to go for it to be considered as a substitute to enamel or dentin [80]. Monolithic zirconia, completely stabilized materials (CZF and PA) have been found to have more translucency than partially stabilized zirconia (ZT) [87].

1.1.3. Alternative materials for denture bases 1.1.3.1. Titanium as denture base framework material. In the late 20th century, titanium and titanium alloy have been preferred as a denture base framework material [62]. Titanium has superior biocompatibility, low density, superior mechanical properties, and corrosion resistance. These properties of titanium are equivalent to gold alloys. Titanium is not only used in prosthodontics, but it is also found useful in other fields of dentistry. Studies have been directed to titanium alloying, casting techniques, wear résistance, bonding to ceramics, denture base material, and prosthetic composites. This is to ensure successful clinical application of titanium in different fields like implant dentistry, prosthetic dentistry, orthodontics i.e., in orthodontic wires (NieTi, Beta titanium) [62–65]. Recently, Titanium crowns are manufactured by Procera system, a system initially introduced by Dr. Anderason in 1987 and later on advanced by Nobelpharma Sweden, inc [66]. Titanium crowns developed by this system is considered useful for clinical usage under particular modifications of tooth preparation and space length at the shoulder and occlusal surface. Usually casting investment results in accurate reproduction of shapes of castings but this process is problematic in case of titanium alloy. The major problem with the use of titanium alloys is its high reactivity and melting point of titanium melts. A corrosion-resistant refractory material is needed that can withstand a high melting point [67]. Properties of titanium are affected by casting procedures. Research has enlightened us that the degree of the contamination zone, yield strength and percentage elongation of titanium casting is drastically affected by casting procedures. The extent of damage is dependent upon the type of investment material [67,68]. Research has been directed towards the development of new material for titanium alloy casting. The casting of a highly reactive alloy has experimented with ceramic corundum molds on hydrolyzed ethyl silicate solution. It has insufficient inertness to reactive metal alloys. This insufficient inertness is accounted for the presence of free silica, potent oxidant of alloys like aluminum and titanium under vacuum. This problem was addressed by using aluminum borophosphate concentrate, cured chemically by periclase, as silica free binder of ceramic corundum molds. It not only helped in improving chemical inertness but eased the operating process of mold formation. It has added the benefit of being cheap [69,70]. The efficiency of titanium cast parts is dependent on the dimensional stability and surface quality of investment. Ceramic molds are commonly produced by dip coating. Some studies suggested that cohesive homogenous coating of Calcium zirconate (CaZrO3) by replica technique is also a promising technology for the production of effective coating for titanium casts. [71]. CaZrO3 is recently introduced as a new ceramic refractory material. It is stable in the highly reducible environment and when in contact with titanium alloy, it usually melts and forms Ti6Al4V. In addition to refractory material; binder also holds significance in case of titanium alloy casting because conventionally used silica binders adversely affect the corrosion resistance of titanium alloy [67]. Another concern with the use of titanium alloys was the release of titanium. As we know that in the case of NiTi alloy release of nickel is associated with allergic reactions in susceptible people. This concern leads the researchers to conduct researches or studies on the release of titanium from titanium alloys. Studies concluded that there is a minimal release of titanium from titanium alloy and moreover, titanium is biocompatible [62,72]. Titanium allergy has been seen in implant 6

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major functions of liners is to reduce the age changes in the denture bearing area. For denture liners to function properly it is essential that a resilient and strong bond is formed between the denture base and liner [97–99,101]. Denture liners are more accurately called as soft denture liners because the basic difference between the denture base and denture liner is their softness (ease of deformation) and resilience (ability to return to original shape) [103]. Soft liner or resilient liners are plasticized acrylic resin and silicone liners. Both are auto polymerizing or light-cured [101]. Plasticized acrylic resins comprise of powder and liquid. The powder contains an acrylic polymer, copolymer whereas liquid contains acrylic monomer. A plasticizer is added to maintain the softness of the material. Silicone elastomeric liner material containing dimethylsiloxane retains its elastomeric properties for a long period of time. They are helpful in improving masticatory efficiency and reducing the discomfort of the patient. Soft tissue liners or temporary liners are given immediately after surgery. Temporary liners are devoid of monomers. They contain polymethylmethacrylate powder, aromatic esters, and alcohol. Long term liners are either plasticized acrylic resin or silicones. Acrylic liners are used up to 6 months. Long term liners used up to 12 months; it contains vinyl polysiloxane similar to silicone impression materials. These materials have been facing problems regarding bonding to denture bases. These problems have solved by the addition of a suitable solvent-based primer or other mechanical maneuvers. Tissue conditioners: As the name suggests, they are used for tissue conditioning. These materials are also called shock absorber because they absorb some of the energy through the mechanical chewing impact. They are usually recommended for the patients having chronic soreness, discomfort, and pain due to prolonged exposure to rigid surfaces of the denture to reduce discomfort [101]. It is used for conditioning and treatment of patients with implant surgery, immediate surgical splints, relining of cleft palate speech aids, as a functional impression material, on a provisional and diagnostic basis. The tissue conditioners comprise of plasticized vinyl polymer, silicone rubber, copolymer, polyphosphazene, fluoroethylene, addition silicones. Liquid of these materials contains ethyl alcohol and dibutyl phthalate (aromatic amine). It is used as a plasticizer to reduce the transition temperature of the polymer [101]. The tissue liners have a very short life as the plasticizer evaporates from tissue liner in short period of time causing the material to harden really quickly. They are often used to take a functional impression. In cases where they are used as a shock absorber, they need to be replaced every 2 to 3 days. Improvement of the fitting surface of the denture on a temporary basis is often accomplished with tissue conditioners. After 2 to 3 days, this material hardens leading to greatly increased risk of trauma. Drawbacks of denture lining materials include rough texture, color changes, porosity, insufficient tear strength, microbial colonization esp. Candida albicans and poor bonding strength [104]. These problems are predominantly seen in long term liners. Currently, several attempts have been made to overcome these obstacles. A most common cause of failure of denture baseliners is the poor bond strength. The possible causes of failure of various denture liners are given in the following Table 3.

Nowadays new ceramic products have been developed in quest of ceramic with superior esthetics and remarkable mechanical properties. These are briefly mentioned below with added benefits. 1.1.4. Acrylic denture base allergies Owing to its vast use, acrylic allergies are a common finding in dental practice. Although acrylic resins are considered biologically safe, it has the potential to cause contact dermatitis in sensitized individuals [88]. Residual monomer in unpolymerized MMA leaches out and causes mucosal irritation and contact dermatitis not only in patients but also in the dentist and dental technicians [24,88,89]. MMA products are also known to cause asthma [88,90]. 1.1.4.1. Prevention of acrylic resin allergy by titanium dioxide coating. Coating of titanium dioxide on denture base acrylic resin prevents all sort of mucosal, subcutaneous and cutaneous irritation. It not only prevents discomfort but also allergy/sensitization of mucosa to acrylic, addressing a significant problem of prosthodontics. Hence coating with titanium dioxide renders acrylic resins more biocompatible [91]. Titanium oxide coating also prevents adherence of microbes to acrylic resin. Hence preventing common infections like candidiasis in old immunocompromised patients [92]. Acrylic with nanoparticles of silver also displays antifungal properties due to slow releases Ag. Some problems like color instability need to be addressed [93,94]. 1.1.5. Occupational hazards of denture base resin Researchers have affirmed acrylic resin to be biologically less risky material. However, recently there are rising concerns about the safe use of acrylic resins in clinical practice. Incompletely polymerized acrylic resin is hazardous because of released toxic chemical products i.e., methyl monomer, formaldehyde, methyl methacrylate, dibutyl phthalate, phenyl benzoate and salicylate [25]. Researchers have reported signs and symptoms of acrylics cytotoxicity which includes, localized mucosal inflammation, ulceration, and burning sensation on exposed site in the oral cavity. Respiratory irritation is also clinically visible. Exposure sites usually presented with necrosis, histiocytosis, fibrosis, and excessive keratinization. Unpolymerized acrylic resins have been reported to cause oxidative burst resulting in free radical production which in turn causes DNA damage. Cytotoxic changes in hepatocytes, delay in the cell cycle, decreased antioxidants and induction of apoptosis and necrosis. These changes are due to the presence of residual monomer in acrylic resins [25]. Most commonly used denture cleansing method i.e., coagulant method in which denture is immersed in chemicals has proven to be toxic. Disinfectants remain usually impregnated in the dentures even after rinsing [95,96]. 1.2. Denture liners Owing to the continuous residual ridge resorption in an edentulous patient, often a well-fitting denture can become loose after a short period of time. This problem is usually solved either by relining or rebasing depending on the condition of a denture [97–99]. Relining or rebasing of denture has a positive and significant influence on the patient quality of life [99,100]. These materials are employed for the replacement of a fitting surface of the denture or to provide a soft tissue cushion for protection against trauma due to ill-fitting denture [98]. Denture liner composition and types are given in Table 2. They are used for stress absorption, uniform distribution of stress over the denture bearing areas and providing a soft barrier covering the injured and painful areas due to bone resorption e.g., sharp bone edges, deep undercuts, atrophic and sensitive mucosa. These soft liners can be used for ridges with deep undercuts (Fig. 4). It relieves discomfort due to pressure points. One of the

1.2.1. Methods to enhance the bond strength of denture liners Major and most common setback encountered with the use of denture liners is insufficient bond strength. Lack of reliable bond between the denture base and liner usually results in early detachment of denture liner and promotes colonization of microbes. It also leads to decreased durability and efficiency of denture liners. The bond strength usually depends on surface design, methods of etching, bonding, the type of denture bonding agents and thickness of liner used (Table 4). 7

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Table 2 Denture liners and their composition [97]. Brand

Material type

Composition

Manufacturer

Vertex Rapid Simplified (VRS)

Heat-polymerized denture base acrylic resin Auto polymerized denture base acrylic resin CAD-CAM denture base acrylic resin Auto polymerized resilient denture liner Autopolymerized resilient denture liner Heat-polymerized resilient denture liner

PMMA

Vertex-Dental B.V. (Carl Zeist AG)

Vertex Self-Curing (VSC) Ivo Base CAD (IBC) Ufi Gel SC (UGS) Silagum-Comfort (SLC) Vertex Soft (VTS)

PMMA PMMA Addition polymerized silicone

Ivoclar Vivadent AG VOCO GmbH

Addition polymerized vinyl silicone, hydrogen polysiloxane, fillers, pigments, additives, platinum catalyst Methyl methacrylate, ethylene glycol dimethacrylate, acetyl tributyl citrate (plasticizer), polymethyl methacrylate

DMG Dental

1.2.1.1. Mechanical methods. Attempts to create a rough acrylic interface with alumina abrasion and lasers i.e. -YAG, Nd-YAG, HoYAG have been made to improve the bond strength [101,104–107]. Another surface roughening method, oxygen plasma treatment has yield promising results [99]. It is found to increases the tensile strength of the bond between the denture base and denture liner. However, further studies are required to subject above-mentioned methods through scrutiny [101,106].

Vertex-Dental B.V.

CAD/CAM denture bases with denture liners. Autopolymerising denture base resin is found to have the highest binding capacity with all kinds of denture liners. This is well illustrated through above-mentioned Fig. 5 [104,108]. In clinical practice, Minimum bond strength of 0.44 MPa is essential for denture liners to function effectively. In accord with this parameter, liner-based materials i.e., UGS-IBC, VTS-IBC, would not be suitable clinically. Polymerized vinyl silicone liners (SLC) have the highest bond strength in case of all three denture base resins. Owing to the difference in the chemical composition of denture baseliners and denture base resins (PMMA), the chemical bond between SLC liners and denture base resin is not possible. The adhesive is needed for bonding with denture bases [97].

1.2.1.2. Chemical methods. Attempts to chemically improve the bond strength between the denture base and liner included. Treatment of liner surface with acetone, methyl methacrylate (MMA), methylene chloride and by altering the acrylic liner surface with 36% phosphoric acid improves bond strength [104]. However, methylene oxide is found to be toxic hence it can be replaced by ethyl acetate [17]. Moreover, reinforcement of acrylic liners surface with net woven fibers of glass have been found successful in improving the bond strength [101]. The bond strength between denture base material and denture liners also varies with the use of different denture base materials. Variation in bond strength with the use of different denture base materials can be explained with the help of the following chart. Though all the above-mentioned materials have the same chemical composition, PMMA synthesized by different methods have different influence on tensile strength of the bond between the denture base and denture liners. According to a study, least bond strength is noticed with

1.2.2. Modifications to prevent microbial colonization in denture liners Denture lining materials primarily PMMA based denture liners are susceptible to microbial adherence, infiltration, and growth. The inner surface of PMMA based denture liners is rough; hence a contributing factor to denture stomatitis. Other factors like porosity, poor oral hygiene, Immunocompromising conditions like HIV. Denture liner accelerates the growth of bacterial and fungal species. In order to prevent the colonization of denture liners with microbes, several attempts are made through modifications of acrylic resin-based denture liners [109]. Silver nanoparticles have been added in a soft lining material

Fig. 4. (a) Mandibular ridge with severe anterior undercuts. (b) Silicone adhesive applied to the tissue surface of the mandibular denture. (c) Separating media applied to the master cast. (d) Silicone soft liner placed over tissue surface of the denture. (e) Trial closure with cellophane sheet. (f) Permanent silicone soft liner relined denture. (g) Definitive prosthesis [102]. 8

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and clotrimazole incorporation in acrylic resin has yield positive results. Incorporation of miconazole in the form of powder or gel has inhibitory influence against candida infection [101].

Table 3 Failure modes of each resilient denture liner bonded to each denture base resin material [97]. A comparative evaluation of properties of zirconia reinforced high impact acrylic resin with that of high impact acrylic resin. Materials

Thermocycler

Vertex Rapid Simplified Ufi Gel SC Adhesive Silagum Comfort Predominantly adhesive, some mixed Vertex soft Adhesive Vertex self-curing Ufi Gel SC Silagum comfort

Non-thermocycler

1.2.2.1. Organically modified silicate (ORMOSIL) denture liners. Organically modified silicate consists of siloxane backbone with numerous functional groups attached to it (QAMS). Rubbery ORMOSIL is dissolved in methyl methacrylate; QAMS can be incorporated in PMMA by copolymerization or covalently incorporated in the pressure-processed acrylic resin. It has shown kill on contact property on streptococcus mutans and actinomyces species. It also inhibits adhesion to the acrylic resin surface. QAMS-containing acrylic resins demonstrated reduced water wettability and upgraded toughness, without unfavorably affecting the flexural modulus, strength, water sorption, and solubility, in comparison with QAMSfree acrylic resin. The acrylic resin with enhanced characteristics has the potential to concurrently prevent oral infections during prosthesis wear and improved fracture resistance of prosthesis [111]. A quaternary ammonium silane-functionalized methacrylate (QAMS) is a macromonomer which possesses the antimicrobial characteristic. It is manufactured using techniques of sol-gel chemistry. One of unique characteristic possessed by this macromonomer is flexible SiO-Si bonds. This property has added further advantages to QAMS including the ability to repair itself when damaged due to mechanical stresses and water sorption and enhanced polymerization. It is a resin preparation which is formed by copolymerization of QAMS with bisGMA. It prevents the fractures of resin-based filling due to caries recurrence by its unique kill on contact properties [112].

Adhesive Adhesive Adhesive

Vertex soft

Adhesive Predominantly mixed, some cohesive, few adhesive Adhesive

Adhesive Predominantly mixed, few cohesive, few adhesive Adhesive

IvoBase CAD Ufi Gel SC Silagum comfort

Adhesive Adhesive

Vertex soft

Adhesive

Adhesive Predominantly adhesive, few mixed Adhesive

Table 4 Materials suitable for the fabrication of flexible denture [125]. Materials

Manufacturers

1. 2. 3. 4.

Valplast Int.Corp.USA [125] Bredent Germany [125] Valplast Int.Corp.USA

Valplast Flexiplast Lucitone FRS Flexite, Flexite plus, Flexite M.P, Sun flex,Pro flex

1.2.3. Color instability of denture liners Discoloration of denture liners has been seen by an accumulation of staining pigments, degradation, and dissolution of intrinsic components and growth and colonization of chromogenic bacteria. Beverages like tea and coffee that contains flavonoids, methyl xanthene, nicotine, and caffeine are likely to cause discoloration of denture liners. Smoking causes discoloration not only of denture liners but also teeth [101].

because of its antibacterial and antifungal activities. Several types of research have proven enhanced antibacterial and antifungal effects with Ag-NPs reinforced soft lining material [110]. These antimicrobial effects of Ag-NP reinforced liners are quite unexplained. It is considered to be either because of the release of Ag or due to direct interaction between the material and microbes. Studies have suggested remarkably reduced candida infection with Ag-NP reinforced PMMA as compare to PMMA [110]. Another modality to prevent fungal infections is to incorporate antifungal properties in denture liners by addition of antifungal agents in denture liners. In this manner, the antifungal agents will have sustained release from tissue conditioners thereby preventing the fungal infections i.e., Candida infection. Chlorhexidine, nystatin, fluconazole

1.2.4. Porosity of denture liner The porosity of denture liners leads to small water pockets within the material. Water absorption accounts for the dimensional instability of denture liners. Types of filler used, and its bonding characteristics have a great influence on the dimensional instability of denture liner. Heat polymerized silicone have denser cross-linking and better bond strength that leads to denser and less porous material that has less water sorption [101].

Fig. 5. Comparison of tensile strengths of heat polymerized, auto polymerized and CAD/CAM denture base acrylic resins [97]. 9

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1.3. A removable partial denture (RFD)

Crown when used as retainers it helps in effective distribution of masticatory forces along the long axis of abutments Moreover, it is involved in the provision of protection, support, and resistance against dislodging forces. These doubles crowns are used more effectively to support implants rather than remaining loose teeth to construct the implant-supported prosthesis [123]. CAD/CAM is a new technique for metal processing and is frequently used for crown fabrication. Fabrication of RPD frameworks has not been very frequently attempted. With this technique will bring accuracy, ease, and efficacy in prosthodontics practice. There are new advances in this technique like sintering, milling, and laser melting. Milling results in crown similar in marginal fit to that of cast crown [119,123]. Double crown retained RPD having both CAD/CAM fabricated crowns along with the milled framework of CoeCr denture has recently been attempted. Considering the accuracy and precise fabrication of CAD/CAM fabricated crown and frameworks, it is found to yield promising results. More evidence-based on clinical trials in this respect is needed [123].

With the advancement in dental science, technology, advancing dental awareness and advanced dental health care services, there are more partially edentulous patients rather than completely edentulous patients. Partially edentulous patients are usually entertained with removable or fixed/implant-supported prosthesis [113]. Although various treatment options are available for addressing the problem of partial edentulous patients including implants but removable partial denture (RFD) are regularly used because they offer us with a conservative, cheap approach to replacement of lost teeth. It also preserves the oral health by allowing effective plaque control. Removable partial dentures are preferred over other available options under certain circumstances i.e. poor oral tissue conditions, and when other available options like implants are not applicable i.e., deep undercuts, inadequate bone density, smoking habit, general health issues like diabetes [113]. The difference of toughness i.e., resilience between periodontal tissue of abutment and residual ridge mucosa are generally recognized as the main problem with this type of treatment. The transferred stress to abutments depends on the clasp (direct retainers), rest, and connector‘s rigidity, design, location, magnitude, direction, and denture base design and extension. Another component important for the proper functioning of RPD is indirect retainers which prevent rotation of denture base around the axis of a denture [113].

1.3.2. Flexible removable partial dentures Acrylic is used as denture fabricating material since a very long time. It is still in use due to patient preferences. The cause of this patient inclination and preference ranges from cost-effectiveness to preservation of mental health. Dentures fabricated with PMMA usually have difficulty fitting in deep undercuts. Hence in order to overcome this problem, flexible partial dentures are introduced [124]. Flexible dentures are fabricated from thermoplastic resins. These are the materials which achieve moldable state above a certain temperature and achieve solid-state on cooling. Thermoplastic resin used to make flexible denture are as follow: thermoplastic, acetal, thermoplastic polycarbonate, thermoplastic acrylic, thermoplastic nylon. Thermoplastic nylon has a mechanical characteristic of being flexible hence used for flexible tissue supported the partial denture. Nylon is formed by condensation polymerization of diamine and a dicarboxylic acid [125]. Flexible partial dentures not only serve as an alternative to conventional dentures but also cater to the needs of patients with complicated dentition. Flexible partial dentures are used in cases where there is, bilateral undercuts (Fig. 6) tilted tooth forming an undercut, patient allergic to acrylic resin denture bases or alloy component i.e., nickel, esthetic issues due to clasp, financial restraints, and in order to overcome the problem caused by cast partial denture. Other clinical indications include bony tuberosities, exostoses with deep undercuts, syndromes associated with microstomia, local and systemic ailments like scleroderma, oral submucous fibrosis with limited mouth opening [125]. Many problems faced by clinicians in use of cast partial dentures (CPD) are eliminated with the use of a flexible denture. It is cost-effective, easy to use as well as lighter as compare to CPD. For Clinicians, it offers less time taking and less complex technique and easily adapted thermoplastic nylon have superior esthetics due to its translucency. Moreover, it does not compromise esthetic in cases where clasp has to be given on canines [124]. A major drawback of thermoplastic nylon flexible denture is the requirement of different processing and insertion technique hence fabrication needs special training. There is a lack of mechanical bonding with denture teeth. Special equipment and instruments need to be purchased in order to adjust the flexible denture. Hence, it's not costeffective. It undergoes discoloration (staining) due to color instability. Modulus of elasticity of flexible denture is less than that of PMMA dentures. As the name suggests flexible denture fails to maintain the vertical dimension under occlusal forces due to flexibility. It is a bad conductor hence fails to offer the patient with natural sensations of hot and cold. Relining and rebasing of flexible denture is not possible. More studies need to be done in order to improve the above-mentioned

1.3.1. Currently used metals for RPD framework Recently the trend has shifted towards the use of expensive yet functionally and esthetically pleasing alternatives to RPD i.e., implants. Although metals are still used for fabricating removable partial dentures framework researcher have explored to find the best material available for fabricating removable partial and complete denture. These researches have suggested CoeCr alloy and titanium as most compatible and suited for fabrication of RPD framework [114–116]. Pros of using metal as framework outweigh the cons i.e., high strength and stiffness in thin sections, sensation of hot and cold just like natural dentition, fabrication with least gingival covering in thin sections, corrosion resistance, and repassivation [114–116]. Owing to its inertness and biocompatibility, titanium is preferred material for RPD framework even in large partial dentures. However, few cases of inflammation are reported with titanium denture frameworks [113]. CoeCr alloys have superior physical and mechanical properties, but they are far from being ideal. Several studies have suggested that metallic dentures after a certain period of time suffer distortion i.e., the metallic clasp does not fit the abutment after some time [113]. Problems with the use of metal include metal display causing esthetic concerns, galvanism, bone loss of abutment teeth and plaque deposition. Initially, biofilm develops on the metallic denture that eventually harbors bacteria on the surface leading to plaque deposition. In such cases, in order to maintain healthy gingival regular disinfection of prosthesis is needed. Some studies have found the maximum time period of functioning of the CoeCr denture framework to be a decade or so. Another study has tested the physical properties of CoeCr RPD through studies of stress-strain patterns of CoeCr alloys and has concluded that this material has mechanical limitations for use as a retentive clasp [117]. Further researches to overcome these encountered obstacles needs to be conducted [118]. From an international point of view, Removable partial denture retained by removable partial denture is a rather unique option for prosthodontics patients. A primary crown usually made up of CoeCr and noble alloys are chemically bonded to abutment teeth whereas the secondary crown is attached to the prosthesis. Combination of materials that have proven to be successful for this technique is as following i.e., ceramics for primary crowns and galvanic gold for the secondary crown. It is more suitable to make a framework with composites [119–123]. 10

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Fig. 6. Flexible denture (A) Partially edentulous oral view without flexible denture (B) Flexible denture tissue surface, (C) A view with a flexible denture, and (D) Denture with metallic rest and acrylic clasp [126].

applications of ceramic in dentistry include artificial teeth, crown, orthodontics brackets, bridges, implants posts, abutments, and veneer, particularly over metalcore. Porcelain is prepared by using kaolin, quartz, feldspar in a specific ratio and heating it at high temperature. It is usually glazed at high temperature to give translucent, white material. In dentistry, it is used to make artificial teeth with superior properties [130,131]. Dental ceramic is the most commonly used material for crown fabrication due to its extraordinary characteristics of superior esthetics, resistance to chemical degradation and least plaque adherence. It has superior biocompatibility as compare to metal fused to the porcelain crown. A major cause of their limited use is poor mechanical properties i.e., brittleness, strength affected by moisture, and low tensile strength [132]. In metal fused to porcelain, usually metal provides strength and porcelain provides esthetics. Metal ceramic crowns have proven to have more overall survival rate than a ceramic crown [133]. Due to the opaque nature of ceramics, there is a significant impact on the translucency of the crown. Current researches have been directed towards the development of ceramics with better esthetics and strength. Previously fixed prosthesis was considered to be only of two types: All-ceramic or porcelain fused to metal types. All-ceramic restorations have clinical applications limited to anterior teeth or single tooth due to poor mechanical strength. Recently researches have been directed to enhance the mechanical properties of ceramic while retaining the esthetics. The modifications of ceramic with promising results are listed below: lithium disilicate reinforced ceramic or oxide ceramics i.e., Alumina or zirconia. Owing to their improved mechanical strength, the restorations incorporating these modifications were not only used for posterior teeth rehabilitation but also for multiple teeth [134]. Recent studies have indicated that the modified and advanced ceramics are much better than the previously available first-generation ceramics specifically in terms of mechanical properties i.e., strength and esthetics. Moreover, these advancements have placed modified ceramics i.e., ceramic oxides or lithium silicates reinforced ceramics at the same level as that of porcelain fused to metal ceramics [134]. Despite all that advancement, disastrous fracture or damage to these ceramics has been reported over time especially in posterior regions and multiple tooth restorations. Feldspathic porcelain has low mechanical strength and hence only used in areas of low mechanical and functional load i.e., anterior teeth.

problems with a flexible denture [124]. 1.3.3. Implant-supported and assisted removable partial denture The implant-supported prosthesis has provided a new paradigm to prosthetic restorations. The advantages of implant-supported restorations are the preservation of ridge height and reduction in bone resorption. The implant-supported prosthesis includes overlays, overdenture, and hybrid, all ceramics, and porcelain fused to metal restoration. The age, psychological, esthetic preferences, hygiene requirements, financial constraints, anatomic limitations, ridge height, interdental space, and height dictates the treatment planning outline. Conventional dentures i.e., distal extension denture presents us with many problems like instability, lack of retention, metal clasp, and other esthetic concern. The major setback with the use of implants is the cost of treatment. This problem can be solved by using the implant-supported prosthesis which provides cheaper but effective means of prosthetic restorations [127,128]. More studies have revealed that stable occlusal contacts are achieved by using implants along with the denture for prosthetic rehabilitation. Analysis of occlusal area, contact area, and masticatory movements by T-scan and tracking device respectively revealed that prosthesis with an implant has more surface area and force and provides the patients with comfort, effective mastication due to better retention and stability. In edentulous patient, where the stressbearing area is limited (mandibular region), an implant-supported complete denture is more suitable because it provides a better masticatory function [128,129]. 1.4. 4. Fixed prosthesis: crown and bridge materials Ceramic is the most commonly used material for crown and bridge formation. Over the years many types of research have been conducted for the improvement of ceramic material. 1.4.1. Ceramics Ceramic word is derived from Keramos meaning burnt substance. Ceramic is called so because it is prepared by heating at high temperature [130]. Ceramics are defined by American ceramic society as crystalline inorganic substances formed by the union of metals and nonmetals (alumina, C, oxygen) at high temperature. Ceramics have a wide range of applications in multiple fields including dentistry. The 11

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1.4.1.1. Sintered ceramics. Ceramic is sintered by heating at a high temperature. This is done to consolidate its particles. It reduces porosities and volume of ceramic. Surface porosities produce a rough surface on ceramics. This roughness causes wear of opposite dentition. Amount of porosities is directly related to the time and temperature of sintering. Dispersion strengthening is a process of reinforcement of ceramics with magnesia and alumina [130,138,139].. Process of zirconia reinforcement of ceramics also called transformation toughening was found to increase strength. Stabilized zirconia has translucency, low fusion temperature, high fracture strength. Moreover, all ceramics with zirconium reinforcement are radio-opaque and need capping for better esthetics [114].

Table 5 Some recent modification of ceramics to get high strength [136]. Fabrication methods

Ceramic utilized

Crystallization stages

Sintered ceramics

Leucite reinforced feldspathic porcelain Zirconia containing porcelain Magnesia containing core Aluminous

Castable ceramics

Mica based porcelain Hydroxyapatite type porcelain Lithia containing porcelain Leucite containingporcelain Lithium containing porcelain cerestore Cerec system Celay system Porcera system CAD/CAM based Leva CAD/CAM

Sanidine Mirage II Forsterite Alumina lightly sintered Densely sintered Tetrasilicic fluromica Oxyapatite Lithium disilicate Leucite Lithium disilicate Spinel Tetrasilicic flourmica Sanidine Sanidine Alumina Alumin Leucite Lithium disilicate [137] Y-TZP

Heat pressured ceramics Machinable ceramics

1.4.1.2. Glass-ceramic. Restoration with glass-ceramics is formed in a non-crystalline state that is later converted into a crystalline form. This process is known as devitrification. The first step in the process of fabrication involves the formation of the wax pattern using phosphate bonded investment. Firing of ceramics is done following which molten metal is cast into a pattern at 1380 °C then the whole assembly is heated at a temperature of 1075 °C for the time period of 6 h after removal of sprue, which causes crystallization of tetra-silicic fluromica. Crystal nucleation and growth also are known as ceramming occurs. These crystals are responsible for the increase in toughness, strength, and resistance to abrasion and increased durability of ceramics [136].

Porcelain fused to metal restoration was deemed the gold standard for use as fixed restoration in a posterior region or for multiple teeth. But this is no longer the case, recently new ceramics have given us new horizons to ponder [134]. The common problems encountered with all-ceramic crowns are mainly ceramic chipping and framework fracture [74]. However, framework fracture is not encountered with the use of a metal-ceramic crown. Mechanical stability was a problem encountered mainly by early silica-based/feldspathic ceramics. For lithium silicate/reinforced or zirconia-based restoration, framework fracture is 2.3% and 0.4% respectively. Loss of retention and veneer chipping are the problems encountered mainly in the use of zirconia-based crowns [131,134]. Uniform porcelain veneering of zirconia has showed increased fracture load resistance [131]. However, zirconia restorations are advantageous to porcelain restorations as they cause less enamel wear as compared to porcelain fused to metal restorations [73]. A most common cause of failure of metal fused to ceramics is dental caries. Better hygiene practice is essential to ensure a high survival rate [133]. No ceramic system is deemed perfect for particular clinical use until now. Their use depends on the clinician individual preference. In order to improve the mechanical properties of existing ceramics, newer ceramics have been prepared by reinforcement with following crystalline materials involving new techniques over the last decade (Table 5) [131,135]. Survival rates of few crowns have been given in Table 6. Clinical trials and follow up on all-ceramic crowns have been considered insufficient to reach any conclusion. Moreover, studies on all-ceramic crowns are rather limited. In order to draw a conclusion, we need more studies with longer duration [134].

1.4.1.3. Zirconia containing lithium silicate ceramics. This technique consists of the addition of 10 wt% zirconium oxide to lithium silicate glass compositions. Zirconia acts as a nucleating agent but remains in solution in the glassy matrix, with two main consequences: A dual microstructure consisting of very fine lithium metasilicate(Li2SiO3) and lithium disilicate (Li2Si2O5) crystals are obtained, with a glassy matrix containing zirconium oxide in solution [140]. The new IPS castable glass-ceramics primarily contains lithium disilicate glass-ceramics. They are of two types i.e., IPS express 2 and IPS e max ceramics. Both of them contain microscopic particles of lithium disilicate in a matrix. In between, they are the submicroscopic lithium orthophosphate. Crown shape and shade are created by veneering IPS express 2 cores with fluorapatite crystals in aluminosilicate glass. Evaluation of IPS express restorations over a long period of time has given successful and promising results i.e., 100% for the crown and 50% for partial fixed dentures over the period of 2 years. IPS Emax is more translucent and moreover, it allows ceramic crowns to be fabricated without the need for veneering directly on anterior and posterior teeth. IPS Emax has superior mechanical properties i.e., résistance to cracking [137]. Due to its superior strength, its use is suggested in the posterior region i.e., stress-bearing area [133]. 1.4.1.4. Ceramics using CAD/CAM. Introduction of CAD/CAM for the fabrication of inlays, on lays, crowns, bridges has proved to be a turning point. It has led to the development of a new group of machinable ceramics. On one hand, this technique is safe, less time consuming and on the other hand, it is expensive, utilizes complicated equipment and needs expertise. Crown made by this technique is delivered chairside in a single appointment. These crowns have adequate fracture resistance [136].

Table 6 Different types of crown and their survival rates [134].

1. 2. 3. 4. 5. 6. 7. a

Types of crown

5-year survival rates (95% Cl)a

Metal ceramic crown Feldspathic/silica-based ceramics crown Lithium disilicate based ceramics crown Glass-reinforced ceramics Densely sintered alumina Densely sintered zirconia Composite crown

96.6% 94.6% 96.6% 96.4% 96.7% 98.5% 83.4%

1.5. Implant-supported fixed denture: complete (ISFCD) and partial dentures (ISFPD) At first, dental implants were used for edentulous patients to support a complete denture [141]. Later on, multiple implant-supported prostheses were designed that did not touch the dental ridges. In the past few years, the use of ISFCD is increased remarkably. Owing to the advantage of longevity, it has drastically increased popularity [142]. Despite all its benefit, ISFCD is not free from disadvantages i.e., fracture

Based on Poisson regression. 12

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of veneers and material fracture. Moreover, the repair process is expensive and time-consuming. In order to provide the temporary replacement studies were conducted. Some studies suggested the use of metal reinforced interim prosthesis for several days [142]. Several studies support successful results of the implant-supported prosthesis. Osteointegration is essential for normal functioning of an implant in the oral cavity until recently we knew osteointegration (primary) as the process of a structural and functional connection between the living bone and non-living implant but studies have shown their interaction at a molecular level (secondary osteointegration). The success of implant therapy depends on a deeper understanding of biomechanical features that affect interfacial tissue [143,144]. The bone density, quality, implant shape, design and surface properties have a significant effect on implant success. Some researchers have pointed out the need for the more detailed hypothesis regarding the relationship between interfacial mechanics and bone physiology i.e., bone healing and remodeling. These advances in implant technology will have a great impact on the future of implant-supported prosthesis [145]. An implant-supported prosthesis is associated with a high success rate on a long term basis [146]. Future researches should be directed towards making the implant therapy cost effective.

Gottfried purman. In the eighteenth century, there were reports of an impressive technique that involved pressing a piece of bone or ivory on the oral tissues that were painted with a coloring material and then carving out the fitting surface at the chairside.1 Philip Pfaff in 1756 was the first to make an impression of an edentulous jaw with 2 pieces of wax and then join them and making a cast using plaster of paris [149]. Other impression materials used were zinc oxide eugenol impression paste and compound, although their applications were limited by their inability to surpass undercuts without distorting or fracturing. Reversible hydrocolloids were introduced in 1925, followed by the irreversible hydrocolloids becoming available in 1941 [150]. The disadvantage of the hydrocolloids is shrinkage caused by the loss of water, leading to inaccuracy. In 1953, polysulfide was used as an impression material along with condensation reaction silicones but they both show significant shrinkage over a period of several hours, mainly because of the evaporation of low-molecular-weight by-products. In the late 1960s, polyether was proposed as an alternative polymer because of its improved mechanical properties and low shrinkage. In the 1970s, polyvinyl siloxane (PVS) appeared on the market and became very popular, in part because of its high dimensional stability [151]. Impression, negative replica of intraoral structures, is taken to transfer the record of intraoral structures on to the gypsum cast. The gypsum cast is used as a template to produce orthodontic appliances, dentures, crowns, and other devices. To achieve precise and detailed reproduction of intraoral structures, impression materials should have sufficient hydrophobicity i.e., non-soluble in oral fluids. It also needs to be biocompatible, resilient, and dimensionally stable. It should have an adequate setting and working time [151,152].

1.5.1. Resin-bonded fixed partial denture Prosthetic appliances constructed to replace one or multiple teeth with a pontic system. This attached partial denture is usually supported by abutment tooth. It basically has an advantage of minimum invasiveness. In case of other fixed partial denture, abutment needs to have certain occlusal preparation. Disadvantages, however, include limited longevity. Restoration failure causes less damage to abutments in comparison with other fixed partial denture or bridges [147].

1.7.1. Irreversible hydrocolloid impression materials Owing to their low cost, ease of use and sufficient wettability, alginate impression materials have been frequently used to fabricate diagnostic casts. These casts are further used to fabricate partial and complete dentures. Interim/temporary cast can also be fabricated with the help of alginate. The flexibility of alginate makes its removal from deep undercuts easy. It has sufficient mixing time and setting time. Its hydrophilic nature is the cause of its dimensional instability due to imbibition and syneresis. It causes the loss of surface details just after 10 min. Low tear strength and distortion limits alginate use, hence, can only be poured once. It has hydrophilic nature which is responsible for accurate reproduction of surface details [153].

1.6. Artificial teeth materials Artificial teeth have been classified according to many parameters including shape, material. According to the shape they are classified into three types i.e., anatomical teeth, non-anatomical teeth, and functional teeth with 200 cusp angles. Classification on the basis of the type of material gives us two types of teeth: porcelain, resin, and hard resin teeth. 1.6.1. Porcelain as artificial teeth Porcelain has superior properties i.e., more abrasion resistance, biocompatibility, less plaque accumulation and discoloration making it suitable for use as denture teeth material. It has low impact resistance, low bond strength and produces clicking sounds on contact [148]. It is no longer in use due to extensive abrasion of opposing teeth [136].

1.7.2. Polyether In the 1960s, Polyether, a hydrophilic material was introduced. This material has remarkable wettability, hence, can work in a moist environment. This property helps in the easier formation of the gypsum cast. The setting of these materials takes place through cationic polymerization using the reactive ethylene imines terminals to bind the molecules. This reaction has no by-product. Advancement of polyether has led to the development of new products which are comparably more flexible than the previous product with their property of retraction from the undercuts. Absorption of water leads to distortion of material hence cannot be stored in a humid environment. These materials are available in the market in variable viscosities. This material has the advantage of being prepared by both single-phase and with a syringe-and-tray technique. Dispensing in motorized mixing unit is the most famous method used [153].

1.6.2. Acrylic resin as artificial teeth Acrylic resin is essential for artificial teeth and denture base material. Acrylic resin artificial teeth are preferred over porcelain because it can better withstand the occlusal forces, better adhesion to denture bases, easily repaired (occlusal adjustments) but color instability and least abrasion resistance are problems that need to be addressed. Continuous exposure to different beverages, tea, and coffee, cyclic chewing forces, brushing and cleaning solutions makes artificial teeth susceptible to discoloration, surface abrasion, and roughness. PH, duration of exposure polarity and presence of specific pigments affects the affectivity of dentures. This poses an esthetic and functional problem for denture users. It decreases the efficiency period of denture i.e., life span. It makes denture wearing experience unpleasant. In order to improve these problems, future steps need to be taken.

1.7.3. Addition silicones (POLYVINYL SILOXANES (PVS)) Most popular material among impression materials is addition silicone. Reason for its popularity is its availability in various viscosities and amazing properties including reproduction of surface details, high elastic recovery, easy retrieval from undercuts, multiple pouring. The chemical reaction consists of a molecule with hydrosilane terminal (base paste) reacting with a vinyl terminal of siloxane oligomers (accelerator paste) in the presence of a catalyst(platinum). There is no

1.7. Impression materials In the mid-seventeenth century, there was the first evidence of wax impression of jaw and teeth reported to be made by German surgeon, 13

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byproduct of the reaction, but secondary reaction often causes the release of hydrogen with the source being hydroxyl group of siloxane molecule released by oligomerization reaction. Hence the impression should not be poured immediately, wait at least 60 min before pouring. Addition silicones polymerization reaction is usually inhibited by sulfur-containing substances, hence avoid contact with latex gloves, rubber dam, and gingival retraction cords [153].

1.7.8. Digital dental impressions Digital scanner for digital intraoral impressions was first introduced in the 1980s and now the advancement in technology has led to the development of in-office scanners. These scanners are easy to use and often results in the production of an accurate and precisely fitting prosthesis. Digital impression technique is a radical step in the history of science and technology. Digital impressions have been found to have accuracy and precision that is not possible with the use of elastomeric impression materials [155]. Introduction of 3D digital impression technology is causing the elastomeric impressions to become obsolete. It offers accuracy, efficiency, and stress-free procedure with the additional advantage of less time consumption; hence it makes the job of a dentist relatively easy. Owing to these advantages, this technology will soon become an essential part of the dental clinic. With the introduction of this technology, there is a significant reduction in returns and repair of the prosthesis. Furthermore, accurate appliances usually require less time for insertion hence providing the patient with positive dental experience [156–158].

1.7.4. Vinyl siloxanether (VINYL POLYETHER SILOXANE) Vinyl siloxanether is an impression material that is made by combining the properties of polyether and addition silicone, vinyl siloxanether or vinyl polyether siloxane. It was introduced in 2009. In addition to the hydrophilicity of polyether, it also provides the high elastic recovery of addition silicone. These features make this material promising for tough situations like that of moisture control i.e., narrow gingival crevice. However, literature provides little insight into the accuracy of this material [153]. 1.7.5. Hydrophilic polyvinyl siloxane Conventional polyvinyl siloxane was hydrophobic in nature and hence proper moisture control is required to obtain an acceptable impression. Many new materials have been developed to improve this property. This new material has been advertised as hydrophilic material, which means that they can work in a humid environment like that of the oral cavity. This is due to the presence of intrinsic surfactants that improve the wettability and facilitate fabrication of gypsum materials. However, this hydrophilic material is hydrophobic in the liquid state hence it can't function efficiently when wet. As a consequence, their surface details reproduction is quite inconsistent in the absence of moisture control [153].

1.8. Ingenious alternatives to conventional dentures 1.8.1. Computer-aided complete dentures CAD/CAM was first developed in 1971 by Duret and first dental restoration fabricated with CAD/CAM in 1983. Computer-aided design and manufacturing are constantly advancing technology. Advancement in this technology allows us to fabricate denture directly from data obtained from the patient's mouth. Its applications range from replacement of missing teeth to fabrication of maxillofacial and implant prosthesis. It can be used for the fabrication of denture in complicated cases like cavitated dentition. It is constantly employed for research purposes [66]. The technology of computer-aided complete denture is rapidly evolving [159]. Some studies demonstrated the suitability of design and fabrication of complete denture using the trial version of 3Shape Dental System 2013, the CAM of WIELAND V2.0.049, and WIELAND ZENOTEC T1 milling machine [160–164]. Moreover, inlays, on lays, crown, implant abutments, FPD, bridges, and other prosthesis are also manufactured by CAD/CAM [165].

1.7.6. Fast-Set elastomeric materials In a few dental patients the gag reflex is so significant that impression taking becomes a headache with conventional impression material. For the reduction of chair time and the provision of fast means of impression fabrication, new fast setting material is introduced. There is a significant limitation to literature when it comes to the accuracy of the material. One study has suggested that fast setting polyether has shown more positive results than addition silicone. Both of them have been found to produce a clinically acceptable impression with satisfactory reproduction of details. Studies have also suggested that the fast set PVS needs pouring in high expansion dental stone and die relief to limit the amendments of tissue surface and attain a better marginal fit. This is not required for fast set polyether [153]. Thiol-ene functionalized siloxane are prepared and evaluated for use as a rapid set elastomeric impression material. Redox initiated polymerization method is used to form crosslinking of thiol-ene siloxane. Mechanical properties are modified by plasticizers and kaolin fillers. It records the surface details at both gross and fine level. It is anticipated that thiolene siloxane will possess the faster setting time and will give detailed reproduction of fine details of the tissue surface [152,154].

1.8.2. Fenestrated dentures A debilitating condition that is prevalent in elder adults is edentulism. Owing to its debilitating and deleterious impact on physical and mental health, health care workers should work towards the protection of physiologic dentition through dental education, the practice of preventive dentistry, and provision of dental healthcare [166–171]. Tooth loss is usually followed by the residual ridge resorption and reduction in denture bearing areas. Initiatives should be taken to preserve the remaining teeth in a partially edentulous patient. Many studies have suggested that edentulous patients are constantly faced with problems of denture retention and stability [172,173]. Retention problems are more in mandibular denture than in maxillary dentures. Soft denture liner use in edentulous patients is known to improve the masticatory efficacy and relieve discomfort in point pressure areas [174]. In order to improve masticatory function, retention, and quality of life, Attempt has been made to form soft liner associated fenestrated denture. It is a conservative attempt to preserve the remaining teeth and decrease residual ridge resorption. It also prolongs the life of the remaining teeth by reducing the concentration of stresses on them [103].

1.7.7. Alginate substitute materials In the 1980s, new material named as alginate substitute material is introduced in the dental market as low cost, medium body polyvinyl siloxane materials. Their mechanical properties are exceptionally more than that of conventional alginates. These materials have a price ranging between the traditional PVS and alginate. They provide well-defined consistency and setting time, facilitating the impression procedure. Better detail reproduction, tear strength, and dimensional stability are also additional benefits of alginate substitute material. Their immediate need for pouring and no chance of the second trial are major disadvantages of this material. The impression of this material can also be sent to the laboratory and digitized to create a virtual cast [153].

1.8.3. 3D dentures Dentistry has embraced the digital age, and this is evident by its incorporation into the earliest dental treatment i.e., dentures.3D printing is introduced by Professor Emanuel M Sachs at MIT. It is a remarkable process that has the flexibility to form a 3D version of any shape. This process can form products out of ceramics, metal, polymer 14

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and other composites. This process occurs by deposition of layers of powder on the top of the fabrication chamber. This layer deposition continues until the product is formed followed immediately by the heat treatment to remove the surplus amount of powder [161,175]. This also has been used to study the complete denture by CAD/CAM i.e., computer-aided design and manufacturing. Over the last two decades, rapid prototyping has been used for fabrication of the 3D removable partial dentures [176]. Present studies suggest that the dentures can be fabricated with the help of a CAD/CAM system and 3D printing technique. This method paves way for the easier, efficient and unconventional fabrication of dentures in the future with the simplification of laboratory procedure and minimal chair time [176,177]. 3D dentures provide us with precisely manufactured dentures with superior esthetics and efficiency. Another milestone for 3D denture fabrication availability will be using imaging technique for the development of face digital simulator while ensuring low radiation doses [177].

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1.9. Conclusion and future perspectives Since prosthodontics material science lacks well-allocated boundaries, the assessment concerning prosthodontics research is rather subjective. For the past five years considerable research has been done in this field which has led to the introduction of new materials like titanium and titanium alloys, zirconia as material for denture framework, and implant, Zirconia containing lithium silicate ceramics, zirconia as implant abutment, enhancement of mechanical properties of acrylic resins and prevention of acrylic resin allergies by TiO2 coating. It is concluded that the mechanical properties of denture base material can be enhanced by reinforcement with different materials. Among all those materials the most promising ones are glass fibers and nanoparticles. The hybrid system, new reinforcement system i.e., hybrid fibers, hybrid fillers or both can yield a significant improvement in mechanical properties of PMMA denture base materials. Most of the research work is more focused on in vitro studies. This focus must shift towards in vivo clinical trials. According to reviewed literature on denture liners, soft resilient liners hold significance to improve the masticatory functions and comfort of denture wearers especially silicone-based liners when used for a long time. More efficient studies on denture liner materials that allow us to reach conclusions and improve the clinical significance of denture liners are needed. Despite their high cost, Implant use for tooth replacement has proven significant due to the provision of efficient mastication, and bone height preservation and durability. In order to make implant costeffective use of implant abutment for dentures have been tried and new materials for dental implants needs to be explored. Over the years the impression techniques have evolved, and digital impression technique is paving way for less time consuming, the easy, comfortable and precise way for taking an impression. More research in the field of dentistry will result in better materials and viable options for making prosthodontics more comfortable and more esthetically acceptable to the patients. Acknowledgment This work has been supported by Interdisciplinary Research Centre in Biomedical Materials (IRCBM) COMSATS University Islamabad, Lahore Campus, Pakistan and HEC Pakistan under grant NO NRPU 4146. References [1] M. Kaku, Journal of prosthodontic research 60 (2016) 143–144. [2] G. Carlsson, R. Omar, J. Oral Rehabil. 37 (2010) 143–156. [3] K. Baba, Journal of prosthodontic research 58 (2014) 1–2.

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