Open access peer-reviewed chapter - ONLINE FIRST
Abstract
Dental prosthesis nowadays fabricated from Poly (methyl methacrylate) (PMMA) due to its easy handling, exceptional appearance. However, this material as an ideal denture base is still restricted by a few limitations such as poor strength and radiopacity. Attempts to improve the mechanical and radiopacity properties of denture base materials through the inclusion of verity of fiber and fillers. A nano-filler modified with the silane coupling agent could improve the dispersiblity of the fillers in polymer matrix. The clinical problem of using silanes in adhesion promotion is bond degradation over time in the oral environment. This chapter presents the fillers as reinforcement agent for improving denture base properties. It reviews different types of fibers and fillers added to PMMA denture base resin and evaluates their effect on the physical and mechanical properties. Comprehensive research in review of literature were carried out included longstanding and update studies in electronic data base including PubMed, Google search, Science Direct and Research Gate. All studies were presented and their finding were discussed. The future of manufacturing applications in 3D printing and CAD/CAM technology of denture base resins with improvement in their properties for 3D printing technology and digital denture base fabrications was also presented.
Keywords
- PMMA
- denture base
- filler
- physical and mechanical properties
1. Introduction
As dental resins enter into the new era of development, the choice of suitable materials becomes more diverse as they have broadened the range of dental products in all areas of dentistry. Prosthetic dentistry has also broadened its range of many promising materials for denture fabrication. Many different types of materials were used for fabrication of denture base. Prior to 1940, vulcanite was the most widely used denture base polymer. This is a highly cross-linked nature rubber, which is difficult to pigment and tends to become unhygienic due to uptake of saliva. Various other materials have been used in denture construction, including cellulose products, phenol-formaldehyde, vinyl resin, polyamine polymers. However, they have suffered from a variety of problems [1, 2].
In the 1930s, Walter Wright and the Vernon brothers at the Rohm and Haas Company in Philadelphia developed poly (methyl methacrylate) (PMMA), a hard plastic. Although many other materials were utilized for dental prosthetics none could come close to that of PMMA, and nowadays more than 90% of dentures are fabricated from this acrylic polymer [3]. The popularity of PMMA is associated with its favorable working characteristics, processing ease, accurate fit, stability in oral environment, superior esthetics, and use with minimum and inexpensive equipment [4].
This chapter presents a comprehensive review of various fibers and fillers utilized in PMMA denture base resin and their influence on the physical and mechanical properties. The review involves relevant data and sources from scientific papers, reviews, and abstracts published in dental literature. The search for published materials was conducted using both general and specialist databases, such as Google Scholar, Research Gate, and PubMed, with the aid of specific keywords including denture base, PMMA, reinforcement, nanoparticles, fibers, and fillers.
2. The criteria for an ideal denture base material
The requirements of a denture base material can be conveniently categorized into physical, chemical, mechanical, biological, and miscellaneous properties.
2.1 Physical properties
Ideal denture base material should be capable of matching the appearance of the natural oral soft tissue. The importance of this requirement varies considerably, depending on whether the base will be visible when the patient opens his mouth. A polymer, which is used to construct a denture base, ought to have a value of glass transition temperature (Tg), which is high enough to prevent softening and distortion during service. Although the normal temperature in the mouth varies from 32 to 37°C, account must be taken of the fact that patients sometimes take hot drinks higher than this temperature, and also clean their dentures in very hot water despite being advised not to do so.
The base is supposed to have good dimensional stability in order for the shapes of the denture not to change over a period of time. In addition to distortion, which may occur due to thermal softening, other mechanisms such as relief of internal stresses, and water absorption may contribute to dimensional instability. The material should ideally have a low value of specific gravity and dentures should be as light as possible.
The denture base should have radiopaque characteristics leading to its event of detection using normal diagnostic radiographic techniques. Early radiological detection of the denture or fragment of denture is immense help in deciding the best course of treatment.
2.2 Mechanical properties
The denture base should be rigid and has high modulus of elasticity. A high value of elastic limit is required to ensure that stresses encountered during bite and mastication do not cause permanent deformation. It ought to have sufficient flexural strength to resist bending and fracture. The base material should have an adequate fatigue life and high impact strength. Denture base materials need to possess sufficient abrasion and indentation resistance to prevent excessive wear of material by abrasive denture cleaners or foodstuffs.
2.3 Chemical properties
A denture base material should be chemically inert. It should be naturally insoluble in oral fluids and should not absorb water or saliva since this may alter the mechanical properties of the material and cause unhygienic denture.
2.4 Biological properties
In the unmixed or uncured states, the denture base material should not be harmful to the technician involved in its handling. Furthermore, it has to be non-toxic and non-irritant to the patient. The base should neither promote nor sustain the growth of bacteria and fungus.
2.5 Miscellaneous properties
An ideal denture base material ought to be relatively inexpensive and has a long shelf life so that the material can be purchased in bulk and stored without deteriorating. The material should be easy to manipulate and fabricate without having to resort to expensive processing equipment. It should be easy to repair on the occasion of fractures.
2.5.1 Radiopacity of denture base material
Many attempts were tried to achieve the radiopacity into PMMA as denture base material. These efforts include the addition of finely divided metal such as powdered dental amalgam or gold powder, simple halogen-containing molecules such as tetrabromoethane or organo metallic such as triphenly bismuth. Additionally, the incorporation of insoluble inorganic heavy metal salts such as BaSO4, BaF2, BiCl3, BiBr3, or finely divided glasses containing barium or bismuth [5].
All these techniques exhibit specific disadvantages, which had precluded their use. Out of all the heavy metal inorganic salts and glasses that have been investigated, only BaSO4 has achieved conventional exploitation as an x-ray opacifying agent for both denture base and also as the opacifying medium in methacrylate.
As far as denture base is concerned the resin produced is opaque white and even when tinted, the translucent of nature gum tissue cannot be mimicked so it was never poplar [6]. Moreover, the x-ray opacity was poor and not only simply a question of adding more BaSO4 as not only did the esthetic qualities suffer further but the mechanical properties began to deteriorate as well [5, 6]. There is another radiopacifier described based on iodine-methacrylate, where the contrast agent was introduced via the liquid component and via iodine-containing copolymer [7]. Since the conversion of monomers during curing is incomplete, free monomer will always be present and as a result, there is always a risk for in situ release of iodine-containing methacrylates. Because nothing is known about the toxic effects of such a monomer, this poses an unknown and presumably unacceptable risk for the patient. Therefore, a covalently bound x-ray opacifier would eliminate many of the inherent difficulties that others have found toward the successful development of an x-ray opaque PMMA denture base material.
Lewis et al. [8] studied radiopacifying particle reinforced PMMA. Their approach for enhancing the properties of the polymer and reducing the production of wear particles and debris was directed toward the mechanical reinforcement through the radiopacifier particles. They investigated the improvement of the interface adhesion by establishing covalent chemical bonding between the inorganic fillers (oxide particles) and the PMMA matrix. This was achieved through the preliminary treatment of the fillers surface with a silane bonding agent as 3-(trimethoxysilyl) propyl methacrylate (γ-MPS), capable of later copolymerizing with the (co) monomers.
Abboud et al. [9] studied the mechanical characterization of acrylic resin prepared from γ-MPS treated alumina particles, which could act simultaneously as radiopacifying and reinforcing agents. They found that for some formulations, the compressive strength and modulus reached 150 and 3400 MPa respectively. Those formulations require high concentration of silanated alumina particles (over 35%wt) and such composites are unprocessable due to the lack of liquid monomer for simultaneously wetting the filler surface and dissolving the PMMA beads.
Mohamed [10] studied the ability of HA and alumina in acting as both radiopacifying and reinforcement agents in PMMA denture base material. The effects of the particles microstructures, surface treatment of fillers with γ- MPS were also evaluated. The author concluded that the incorporation of γ- MPS treated ceramic fillers into PMMA matrix in general, has increased the flexural modulus, tensile strength, tensile modulus, and fracture toughness of PMMA denture base material.
2.5.2 Fracture toughness
Fracture toughness is an intrinsic characteristic of a material concerning resistance to crack propagation. It is a measure of the energy required to initiate and propagate a crack in a material, which may lead to catastrophic failure. Fractures are usually classified as brittle fracture and ductile fracture. In brittle fracture, the materials behave elastically up to the point of failure. There is hardly observable deformation of the materials prior or during breakage [11].
The fracture surfaces are relatively smooth and largely perpendicular to the direction of the applied stress. The two surfaces can be fit together quite accurately. Ductile fracture implied that large permanent deformation has occurred before failure, requiring a significant greater amount of energy absorption by the part before failure. Since the result of permanent deformation, the fracture surfaces do not match, and the cross-sectional area at the location of the fracture is reduced from the original value.
The common measurement of fracture toughness is called the critical stress intensity factor (KIC). The KIC is the critical value of material that fracture occurs when an applied stress intensity factor on the material is greater than the critical value. The stress intensity factor K, is a measure of applied stress associated with crack size. When the KIC value is exceeded at the crack tip, the material will then fracture spontaneously leading to complete failure. Below the KIC value the crack can still grow slowly. Because the KIC is measured as stress intensity and not just as a stress, its units are MN/m1.5 [12].
The fracture toughness test is very efficient and other parameters can also be derived from it including modulus of elasticity and the plasticity of the material. The fracture toughness is closely related to fatigue strength [12].
Factors that contribute to stress concentration enable the initiation and propagation of cracks, thereby influencing the rate of failure. As described by Yee [13] the observed energy loss during the impact test depends on four factors; (a) the energy to bend the specimen up to the point of crack initiation, (b) the energy to propagate the crack through the specimen, (c) the kinetic energy of the fractured specimen, and (d) the vibrational or otherwise dissipated energy. As previously reported in the literature, the fracture toughness test appears to be more reliable and advantageous than impact testing when determining the influence of variation in the material composition [14].
Zappini et al. [15] determined the fracture toughness of denture base resins and compared the results with impact strength measurements. Seven heat-polymerized denture base resins were chosen for the study. They concluded that the specimen geometry and testing configuration influenced the impact strength measurements and the fracture toughness method seemed to be more suitable than impact strength measurements to demonstrate the effects of resin modifications. Moreover, the differences between conventional and so-called “high impact” denture base resins were more clearly demonstrated with fracture toughness measurements.
3. Filler reinforced denture base
Reinforcement in particulate denture base may comprise of either flexible particulate rubber, such as in high impact polystyrene (HIPS), or rigid mineral fillers, including hydroxyapatite, alumina calcium carbonate, barium titanate, and others. Numerous factors contribute to the properties of particulate-filled denture base, including the type of filler, type of matrix, filler-matrix interaction, filler volume fraction, filler-filler interaction, voids, and defects. It has been observed by Harper et al. that smaller particle size yields better properties enhancement than larger particle size. Geometrically balanced particles, such as spherical glass beads, produce isotropic composites, while platelet and angular-shaped particles not only introduce anisotropy to the mechanical properties but also increase the viscosity of the melt [16].
The composite function employs a continuous phase matrix that serves the purpose of retaining the particulate filler while also providing it with a protective enclosure against both mechanical and chemical damage. Additionally, the matrix acts as a medium for overall stress distribution, allowing for applied loads to be passed on to the filler. It is crucial that the matrix possess the ability to distribute the particulate filler evenly in order to prevent agglomeration.
The mechanical and chemical properties of the composites, as well as their suitability for high temperature environments, are largely determined by the type of matrix utilized. The interaction between the filler and matrix occurs at the interface, which is a small region located between the contact surfaces of the two materials. This interaction plays a critical role in determining the extent to which the load is shared between the matrix and the filler. An effective interface is essential for the transfer of stress from the matrix to the filler, which possesses superior rigidity and strength. This mechanism allows for the reinforcement of the overall composites and thus enables them to endure higher levels of applied stress, resulting in improved properties. The interaction between filler and matrix, particularly in the case of mineral fillers such as hydroxyapatite and alumina with non-polar organic matrices, can be further improved with the use of coupling agents, typically silane coupling agents [17].
The silane coupling agent is comprised of two distinct components, each of which exhibits a strong affinity toward either the filler or the matrix. As a result, it functions as a bridge of sorts that enhances the interaction between the filler and matrix through the formation of a chemical bond between the two phases, as opposed to relying solely on a mechanical interlock [18].
Debnath et al. [19] investigated methacrylic resin-based dental composites treated with silane coupling agent to provide the interfacial phase that holds together the organic polymer matrix with the reinforcing inorganic phase. In this study, fiber pullout tests were used to measure the interfacial bond strength at the fiber-matrix interface. Glass fibers (approximately 30 μm diameter, 8 cm length, MoSci) were silanated using various concentrations (1, 5, and 10%) of either 3-methacryloxypropyl-trimethoxysilane (MPS) or glycidoxypropyl trimethoxy silane (GPS) in acetone (99.8%). Rubber (poly (butadiene/acrylonitrile), amine terminated, (Mw 5500) molecules were also attached to the fiber surface via GPS molecules. A positive correlation was found between the amount of silane on the filler surface and the property loss after soaking. Rubber treatment provided improvement in interfacial strength. 5% MPS samples had the highest strength both in soaked as well as unsoaked samples.
The filler volume fraction, also known as the filler content, plays a crucial role in determining the properties of a composite material. Typically, as the amount of filler increases, the modulus also increases, but this is accompanied by a greater brittleness in the material. The reason for this is that as more filler is added, the polymer content decreases [20]. In order to achieve the maximum loading of filler while maintaining the desired tensile strength and toughness of the polymer, it is essential that the filler is evenly distributed and that the interface between the filler and matrix is of high quality. Furthermore, as the amount of filler increases, it becomes increasingly important to consider the interactions between the individual filler particles [21].
The filler-filler interaction deals with the affinity of fillers toward each other and toward composite matrix. When the filler-filler interaction is stronger than the filler-matrix interaction, it leads to the clustering of fillers in a particular region. This, in turn, acts as a site of stress concentration which can result in premature failure or cracking. To mitigate this issue, the use of dispersing agents and effective mechanical mixing can be employed to decrease the filler-filler interaction. Additionally, voids and defects caused by the presence of moisture-containing fillers or during processing also have a significant impact on the properties of composite materials. In fact, they exhibit an effect that is equivalent to filler agglomeration, serving as a source of stress concentration and a site for crack initiation, ultimately leading to premature failure, as noted by Atkins and Mai [22].
3.1 Fibers
The filled of acrylic resin with fibers has been documented as a means of enhancing the flexural and impact strength, also to the fatigue resistance of the resin [23]. Numerous investigations have been carried out utilizing various kinds of fibers, including but not limited to nylon, polyethylene, polyamide fiber, and glass fiber, which are widely used owing to its biocompatibility and exceptional esthetic and mechanical characters [24].
3.1.1 Glass fiber
The utilization of glass fiber as a reinforcing agent has been discovered to produce a significant enhancement in the flexural strength, impact strength, fracture toughness, and Vickers hardness of acrylic resin, as evidenced by previous studies [25]. Moreover, it has been observed that deformation of the denture base is reduced to less than 1%. Researchers have found that the manner in which the glass fiber interacts with the denture base affects its properties. In particular, it has been reported that positioning the glass fiber in close proximity to the surface of the denture base leads to an improvement in flexural strength, fracture toughness, and flexural modulus. Conversely, placement of the glass fiber in neutral stress areas only enhances flexural toughness, while placement in the compressive side results in an increase in surface flexural modulus [26].
On the other hand, the findings of a study indicate that the impregnation of glass fiber into acrylic resin does not have an impact on its linear dimensional stability [27]. Additionally, the use of preimpregnated and silane-treated glass fiber, specifically with 3-(Trimethoxysilyl) propyl methacrylate (TMSPM), has been shown to increase both flexural and impact strength of the acrylic resin [28]. Furthermore, the introduction of silanized glass fiber to heat-cured and light-cured resins has been deemed biocompatible. Notably, the utilization of fiber-reinforced nanopigmented PMMA has been found to reduce porosity and
3.1.2 Polyamide fiber
Polyamide fibers, including both Nylon and Aramid fibers, were used to reinforce denture base. Research has shown that Aramid fiber reinforcement enhances the biocompatibility of the resin, while simultaneously increasing its flexural strength and modulus [30]. Nevertheless, it is worth noting that an increase in fiber concentration can lead to a decrease in the resin’s hardness, which is considered a disadvantage. Additionally, the yellow color of the Aramid fiber is also regarded as a drawback [31]. On the other hand, Nylon has been found to improve the fracture resistance of PMMA due to its high resistance to continual stress. As a result, incorporating Nylon fiber into PMMA has been shown to increase its structural elasticity [32].
3.1.3 Polyethylene and polypropylene fibers
The incorporation of polyethylene fiber into PMMA resulted in a significant increase in impact strength, with even greater improvements observed through the application of fiber surface treatment, as previously noted in studies [33]. In contrast, while woven polyethylene fiber reinforcement has been shown to enhance the elastic modulus and toughness of PMMA, the practicality of fiber etching, preparation, and positioning has been reported to be difficult [34]. The use of polypropylene fiber, with surface treatment leading to further enhancements in impact strength [35]. In fact, plasma-treated polypropylene fibers have been found to provide the highest impact strength, offering a viable option for strengthening acrylic resin and reducing the likelihood of fracturing. Incorporating silanized polypropylene fiber into heat-cured PMMA resin has also been shown to significantly improve transverse, tensile, and impact strengths, although it should be noted that wear resistance may be negatively impacted [36].
3.1.4 Natural fibers
Two natural fibers, namely oil palm empty fruit bunch (OPEFB) and ramie fiber, have been utilized to reinforce denture base resins. The incorporation of OPEFB has led to a significant increase in the flexural strength and flexural modulus of acrylic resin, as evidenced by previous research [37]. Short ramie fiber, on the other hand, has been observed to increase the flexural modulus of acrylic resin relative to conventional PMMA. However, the flexural strength has decreased due to weak interfacial bonding between the filler and matrix. It should be noted that the long form of ramie fiber poses a disadvantage, as it necessitates additional work, such as cutting and preparation [38].
3.1.5 Hybrid reinforcement
The idea to reinforce PMMA with a variety of fibers was initially introduced by Vallittu [39]. This combination could include varied types of fibers, metal oxides, ceramics, and fibers with metal oxides or ceramic materials. The integration of hybrid fiber reinforcement has effectively increased the flexural strength and toughness of the reinforced acrylic resin. Similar results were noted through the inclusion of metal oxides and ceramics, specifically NPs, in PMMA. Moreover, this method provides improved surface roughness, tensile strength, flexural modulus, hardness, thermal conductivity, radiopacity, and reduced shrinkage. Additionally, it exhibits antibacterial properties without any cytotoxicity. A combination of fibers and other fillers has also resulted in an elevation of impact strength, hardness, surface roughness, and thermal conductivity, as well as compressive and fatigue strengths [40, 41, 42, 43].
3.2 Fillers
Numerous investigations have demonstrated that the utilization of fillers amplifies the potency of denture base resin and markedly ameliorates its characteristics. The addition of nanofillers has been recommended as a means to augment the properties of PMMA. Nanofillers, owing to their elevated surface area, small dimensions, and uniform distribution, have been shown to enhance the thermal properties of PMMA and heighten its thermal stability in comparison to pure PMMA. The characteristics of the resin, which is reinforced by nanofillers, are contingent upon the magnitude, morphology, category, and concentration of the supplementary particles [44, 45, 46, 47].
3.2.1 Metal oxides
Several investigations have indicated that the augmentation of PMMA with metal oxides has led to enhancements in both the physical and mechanical properties of the material, as well as the tactile sensations experienced by patients in response to hot and cold stimuli [48]. Consequently, the addition of metal fillers to denture base resin was expected to result in improved food sensation and healthier oral mucosa. Nevertheless, certain researchers have noted that the incorporation of metal oxides into acrylic resin has been observed to have a deleterious impact on the strength of the denture base, owing to stress concentration in the vicinity of the embedded metal particles and their weak adhesion to the polymer. Several techniques, including sandblasting, silanization, and metal adhesive resins, have been recommended to enhance the bond between the acrylic resin and metal surface [49, 50].
3.2.2 Alumina (Al2O3)
The incorporation of alumina particles into acrylic resin has been found to have a positive impact on the properties of denture base, as reported previously [51]. Furthermore, the addition of alumina powder to acrylic resin has been shown to enhance its thermal conductivity and mechanical properties, according to previous research [52]. Additionally, the reinforcement of PMMA with Al2O3 has been found to increase the flexural strength, impact strength, tensile strength, compressive strength, and surface hardness of the resin. Moreover, the inclusion of aluminum in PMMA has been found to significantly reduce warpage. However, some studies have reported that the addition of aluminum decreases both the impact and tensile strength of PMMA. It has also been observed that the flexural properties of acrylic resin can be improved by treating Al2O3 particles with a coupling agent. Similarly, the treatment of aluminum particles with silane has been found to significantly increase the compressive, tensile, and flexural strength as well as the wear resistance of reinforced denture base resin, as reported in several studies [53, 54, 55].
The impact of alumina reinforced denture resin on surface roughness and water sorption has been investigated. One study observed no significant changes in these properties. However, another study found that the addition of Al2O3 resulted in a decrease in water sorption and solubility, while yet another study reported an increase in water sorption [56]. Additionally, the thermal stability of PMMA was found to increase upon the addition of Al2O3 nanoparticles. The thermal properties and flexural strength of acrylic resin were also shown to improve with the addition of silanized Al2O3 NPs, while water sorption and solubility decreased. Finally, biocompatibility was demonstrated when alumina NPs were added to both microwave-treated and untreated PMMA powder in a different study [57, 58].
The utilization of alumina-reinforced PMMA is hindered by the occurrence of resin discoloration, thereby restricting its application to non-visible areas. While the introduction of Al2O3 into PMMA resulted in a considerable enhancement of thermal conductivity, the flexural strength values of PMMA remained unaltered to a significant extent [59, 60].
3.2.3 Zirconia (ZrO2)
The inclusion of zirconia (ZrO2) fillers into PMMA resulted in a notable increase in its flexural strength, as reported by various studies [61, 62]. However, it was also observed that there was a slight reduction in the flexural strength, which could be attributed to the clustering of particles within the resin, leading to material weakness. Furthermore, the incorporation of ZrO2 into PMMA led to a significant improvement in the impact strength, fracture toughness, and hardness, as documented in one study. Nonetheless, one study found that the impact strength and surface hardness of zirconia-reinforced resin did not increase significantly compared to unreinforced PMMA. In fact, a decrease in both impact strength and surface hardness was reported in some cases. It is important to note that the addition of ZrO2 had a significant impact on the thermal conductivity of PMMA, which increased considerably. However, studies have yielded varying results with respect to the effect of ZrO2 on the water sorption and solubility of PMMA. While some studies suggest that adding ZrO2 led to a significant decrease in the water sorption and solubility of PMMA, other studies found an insignificant difference in water solubility and an increase in water sorption within the limit of ADA specifications [63, 64].
The inclusion of zirconia nanoparticles has been proposed as a means to enhance the mechanical properties of PMMA. The addition of zirconia nanoparticles to PMMA has been shown to increase its impact strength, flexural strength, compressive strength, fatigue strength, fracture toughness, and hardness. Furthermore, it has been suggested that zirconia nanoparticles may have antifungal properties and could potentially serve as a preventive measure for patients who are susceptible to fungal infections. However, one study has reported a negligible increase in the hardness of nano-ZrO2/PMMA, and no significant alteration in its surface roughness [65, 66, 67].
The impact of the inclusion of ZrO2 nanoparticles (NPs) on the color properties of PMMA was not found to possess any significant color alterations. To enhance the bond strength between ZrO2 NPs and PMMA, a silane coupling agent was employed, resulting in an increase in the acrylic resin’s flexural strength and impact strength; however, its tensile strength was not enhanced. Nonetheless, a study discovered that the incorporation of silanized ZrO2NPs improved the tensile strength and fatigue strength of PMMA. Furthermore, the addition of silanized ZrO2 NPs to acrylic resin resulted in a significant increase in hardness and a slight increase in surface roughness, while apparent porosity, water sorption, and solubility decreased. Additionally, zirconia nanotubes were found to exhibit a superior reinforcing effect compared to zirconia NPs. However, surface treatment would decrease the reinforcing effect of ZrO2 nanotubes compared to ZrO2 NPs. Specifically, flexural strength was optimized when 2 wt% untreated ZrO2 nanotubes were incorporated into PMMA [68, 69, 70].
3.2.4 Hydroxyapatite (HA)
The utilization of bio-ceramic systems based in HA has proven to be a significant category of bioactive materials that can effectively promote bone regeneration and consequently facilitate a robust interface fixation between host tissues and medical or dental devices. The incorporation of HA can substantially improve the properties of denture base materials, particularly in terms of their radiopaque nature. Research has demonstrated that the addition of 5 and 10% HA that has been treated with γ-MPS significantly enhances the flexural, flexural toughness, tensile strength, and hardness of PMMA denture base resin. Furthermore, it has been observed that the radiopacity of denture base material is also elevated [9, 10, 16, 17].
The introduction of HA fillers to PMMA results in superior mechanical properties, including an increase in flexural strength and flexural modulus of PMMA. This is primarily due to the enhanced interfacial interaction between the HA filler and the PMMA matrix that is brought about by the treatment with γ-MPS. However, the immersion of PMMA in water has been found to cause a reduction in the flexural properties due to water’s plasticizing effect, which weakens the bonding between the HA filler and the PMMA matrix. In contrast, the addition of HA NPs has been shown to increase both the fatigue and compression strength of PMMA resin in comparison with pure PMMA, in addition to inducing a significant increase in thermal conductivity [11, 12].
3.2.5 Titanium (TiO2)
Numerous investigations have been conducted to examine the impact of the inclusion of TiO2 on the attributes of PMMA. It has been determined that the introduction of TiO2 particles may enhance the flexural strength, fracture toughness, and hardness of PMMA as well as its thermal conductivity. Furthermore, the incorporation of TiO2 into PMMA has been shown to cause a noteworthy increase in impact strength, while simultaneously leading to a significant reduction in water sorption and solubility [71].
In contrast, certain investigations have indicated that the incorporation of TiO2 into PMMA does not enhance its flexure strength due to the clustering of particles within the resin, thereby compromising its structural integrity. However, research has demonstrated that the introduction of TiO2 nanoparticles into PMMA can influence its thermal properties (such as a reduction in thermal expansion coefficient and contraction) and mechanical stability (with a decrease in E-modulus), although it may result in a reduction in flexural strength and toughness.
To optimize the properties of PMMA composite, it is crucial to ensure strong adhesion between the resin matrix and filler particles, which can be achieved by employing a titanium coupling agent to reinforce titanium-reinforced PMMA. Furthermore, the addition of silanized TiO2 nanoparticles to PMMA has been observed to improve its impact strength, transverse strength, and surface hardness, while reducing water sorption and solubility. However, it also increased surface roughness after adding 3 wt% of silanized TiO2 nanoparticles to acrylic resin [11, 72]. Additionally, the incorporation of apatite-coated titanium dioxide and fluoridated apatite-coated titanium dioxide into PMMA, followed by ultraviolet irradiation, has been found to effectively prevent candida adhesion due to their antifungal properties, thereby promoting appropriate denture hygiene. However, the addition of BaTiO3 as a radiopacifier to PMMA resulted in a slight decline in fracture toughness properties. Although PMMA/BaTiO3 composite material has demonstrated thermal stability, its increased density could compromise denture retention [11].
3.2.6 Silicon dioxide (SiO2)
Numerous investigations have been carried out to examine the impact of the addition of SiO2 on the properties of PMMA. It has been determined that the mechanical and thermal properties of PMMA can be improved by incorporating SiO2 nanoparticles (NPs). The introduction of SiO2 NPs has led to an enhancement in both the transverse and impact strength of PMMA. Additionally, surface hardness has been observed to increase with a higher concentration of SiO2 NPs. However, it has been discovered that a low concentration of SiO2 NPs can lead to an improvement in both hardness and fracture toughness. Conversely, an increase in SiO2 NPs content results in agglomeration and crack propagation, which can reduce both hardness and fracture toughness [73, 74].
The inclusion of surface-treated SiO2 has been found to enhance the flexural strength of PMMA, albeit it does not affect the hardness. In contrast, a recent study has revealed that silica NPs have an adverse effect on the flexural strength of PMMA. The reinforcement of acrylic resin with glass flakes, which are silica-based fillers, has been found to improve its fracture toughness. Furthermore, the use of silane coupling has resulted in further enhancement of the resin’s properties. Micas, a group of lamellar silicate minerals, have also been proposed as a means of improving the properties of resin. These minerals are characterized by their high aspect ratio and have been observed to enhance the mechanical, thermal, and dimensional properties of PMMA. The incorporation of mica has been found to increase the hardness of acrylic resin, while its flexural strength has been reduced due to the weak bond between mica and the acrylic resin [75, 76, 77].
The incorporation of fluoride glass fillers into PMMA has been observed to reduce microbial adhesion, albeit with a slight increase in the surface roughness of the denture base resin. This was documented in Refs [78]. Additionally, the utilization of nanoclay as an additive in composite and acrylic polymers has been noted to enhance their properties. It was found that the introduction of nanoclay particles into PMMA led to an improvement in its thermal conductivity, albeit at the expense of its flexural strength, as reported in Ref. Furthermore, the placement of silicon carbide filler powders in the palatal region of dentures has been shown to enhance the thermal conductivity of PMMA without compromising its strength or increasing its weight. This was detailed in Ref. [79].
3.2.7 Polyetheretherketone (PEEK)
Polyetheretherketone (PEEK) is a semi-crystalline engineering plastic that exhibits remarkable mechanical and thermal properties. With its impressive advantages, including its lightweight nature, non-toxicity, corrosion resistance, and low modulus closely resembling that of natural bone, PEEK has emerged as a highly promising clinical implant for orthopedic applications [80, 81].
Recently, PEEK has been introduced to enhance the general mechanical performance of the PMMA resin base. The PMMA resin has been combined with TiO2 and PEEK, leading to a significant improvement in both the average bending strength and the flexural modulus [82]. In the realm of dental implant applications, Chen et al. [83] have found PEEK compounds to be promising restorative materials. Moreover, a recent study Schwitalla et al. [84] has demonstrated that the mechanical properties of PEEK were unaffected by artificial saliva solution with different pH values over a period of 30 days at 25°C. There are some studies that suggest PEEK as a potential restorative material in the oral cavity [85, 86].
Chen et al. [83] developed a practical and convenient protocol for light-curing resin utilized in the 3D industry. This protocol enhances the antibacterial and mechanical properties of PMMA resin through the combination of nanofillers of surface-modified TiO2 and micro-fillers of PEEK. The study’s findings demonstrated that PMMA composite resins reinforced with TiO2–1%-PEEK-1% exhibited the most optimized properties. The researchers concluded that the incorporation of 1% of TiO2 would be an effective amount to enhance both the mechanical and antibacterial properties of PMMA composite resin. In addition, the PMMA (TiO2–1%-PEEK-1%) composite resin’s printed model presented a smooth surface and precise resolution. This indicates that the functional dental restoration material would be a suitable light-curing resin in the 3D industry [83].
3.2.8 Silver (Ag)
Several studies have reported that the inclusion of silver nanoparticles (AgNPs) in denture base acrylic resin exhibits antifungal properties. This effect is particularly noticeable at high concentrations and has been shown to act as a latent antifungal material, with low-releasing Ag+ [84, 85, 86, 87]. Conversely, Wady et al. [88] found that the incorporation of silver NPs in PMMA did not impact the adhesion of
Incorporating silane-treated Ag particles significantly increased the compressive strength of PMMA. Also, addition of 10 and 20 wt% silane-treated silver fillers enhanced the tensile and flexural strength of PMMA. The inclusion of silane-treated silver particles demonstrated a noteworthy increase in the compressive strength of PMMA. Furthermore, when 10 and 20 wt% silane-treated silver fillers were added, both the tensile and flexural strength of PMMA were enhanced. The introduction of silver powder to PMMA also led to a significant increase in thermal conductivity, while the flexural strength values of PMMA remained unaltered. The physical and mechanical properties of PMMA were improved by the incorporation of silver nanoparticles (NPs), including increased thermal conductivity and compressive strength [90].
Hence, the application of Ag NPs is recommended in the palatal region of maxillary acrylic resin dentures. Additionally, it has been determined that PMMA-silver NPs do not exhibit cytotoxicity. However, the tensile strength of PMMA did not undergo significant changes after the inclusion of 0.2% Ag NPs in comparison with unmodified PMMA. Nevertheless, a significant decrease in tensile strength was observed after the incorporation of 2% Ag NPs. Moreover, PMMA-Ag NPs have been reported to exhibit poor color stability. The addition of AgNPs to acrylic denture base material can enhance its viscoelastic properties [91].
The potential benefits of incorporating nano-gold (Au), platinum (Pt), and palladium (Pd) into PMMA denture base have been suggested in recent studies. However, the available literature on the effects of adding nano-gold to PMMA remains limited. Encouragingly, it has been observed that the inclusion of Au NPs can significantly enhance the flexural strength and thermal conductivity of PMMA, resulting in nearly double the value of pure PMMA and potentially increasing patient satisfaction. Additionally, the addition of Pt NPs may improve the mechanical properties of PMMA and provide an antimicrobial effect [92].
The results of the investigation revealed that the utilization of Pt led to a notable increase in the bending deflection of PMMA. Furthermore, it was observed that palladium had a positive impact on the bending strength of the material, in contrast to silver and gold, which both exhibited the lowest level of bending strength. Additionally, the incorporation of gold and palladium proved to enhance the Vickers hardness of PMMA, whereas the introduction of platinum was shown to have a decreasing effect on this property [93].
The halloysite nanotube, a naturally occurring silica-based mineral, was initially introduced by Abdallah [94] in 2016 as a means to enhance the properties of PMMA. It was observed that the addition of halloysite nanotube in small proportions led to an increase in the hardness of PMMA, however, there was no significant improvement in the flexural strength and Young’s modulus. The use of carbon family fillers, specifically carbon fillers, to reinforce PMMA, is not a common practice due to issues such as biological complications, suboptimal esthetics, and challenges in handling and polishing. However, nano-carbon has emerged as a prominent sector of nanotechnology in recent times [92].
3.2.9 Nano-carbon
The introduction of carbon nanotubes at a rate of 1% into PMMA led to a significant increase in both the impact strength and flexural strength of the resin, albeit at the cost of decreased hardness, according to one study. Another study found that the addition of 1.5% single-walled carbon nanotubes had a significant effect on the impact and transverse strength of PMMA, yet led to a noticeable reduction in surface hardness. In contrast, a separate study revealed that the addition of single-walled carbon nanotubes had an insignificant effect on the flexural strength of PMMA. Furthermore, the incorporation of 0.5 and 1% multiple-wall carbon nanotubes (MWCNTs) into PMMA resulted in an improvement in both the flexural strength and resilience of the resin, but with higher concentrations of MWCNTs, the fatigue resistance was seen to decrease [93].
3.2.10 Nano-diamonds
The exceptional characteristics of nano-diamonds, specifically their high level of hardness and thermal conductivity, have led researchers to explore their potential for enhancing the mechanical properties of PMMA. The incorporation of NDs into PMMA resulted in a significant increase in impact strength, as well as an improvement in fracture toughness, albeit only at the lowest concentration of NDs. Additionally, the scratch resistance of PMMA was shown to be enhanced through the use of heat-treated NDs. However, the agglomeration of the nanoparticles was identified as a primary drawback, as it could potentially serve as a site for stress concentration [95].
4. Advanced perspective dental prosthesis
The employment of digital dentistry technology has emerged as the prevailing trend in dental prosthetics across the globe in contemporary times. The advent of digital three-dimensional (3D) printing technology has significantly enhanced the diagnostic rate and demonstrated remarkable potential in the realm of personalized medicine. In comparison to the conventional manufacturing process, 3D-printed prostheses boast shorter production cycles and higher precision, thereby optimizing the comfort of denture patients. Presently, approximately 75% of 3D printing dental applications utilize light-curing technology, wherein light-curing resins are extensively used as fillers and restorative materials in stomatology. PMMA is a widely used commercial light-curing resin in the 3D printing industry owing to its low odor, low irritancy, good flexibility, and cost-effectiveness. However, the inherent limitations of PMMA, including substantial shrinkage rate during light-curing, brittleness, poor mechanical properties, and low antibacterial activity, among others, have restricted its widespread clinical application. Hence, further research and development are imperative to enhance the existing properties of 3D printing denture base materials [96, 97].
5. Conclusion
Although PMMA has been utilized for constructing denture bases and has become the preferred material for such fabrication, it exhibits low fracture resistance, specifically under fatigue failure within the oral cavity and impact failure outside of it. Consequently, various reinforcement agents, such as fibers or filler particles in micro and nano scales, have been investigated to enhance the properties of denture base materials. This has resulted in the expanded use of dental composites in numerous applications, which in turn has spurred the demand for continued research and improvement to enhance their properties and performance. One promising approach involves employing a nanofiller modified with a silane coupling agent to improve the dispersibility of fillers in a polymer, thereby enhancing the mechanical properties of the composite resin. The future of manufacturing applications in three-dimensional (3D) printing and CAD/CAM technology of denture base resins with improved properties will necessitate the development of materials suitable for 3D printing technology and denture base applications.
References
- 1.
Anusavice KJ, Shen C, Rawls R. Ch. 19. In: Phillips’ Science of Dental Materials. Library of Congress Cataloging-in-Publication Data or Control Number Mosby. 12th ed. Printed in China; 2013. pp. 465-466 - 2.
Ferracane JL. Ch. 2. In: Material in Dentistry Principles and Applications. 2nd ed. Philadelphia, USA: J. B. Lippincott Williams & Wilkins; 2001. pp. 10-17 - 3.
John J, Gandahar SA, Shah I. Flexural strength of heat-polymerized poly methyl methyl methacrylate denture resin reinforced with glass, aramid, or nylon fibers. The Journal of Prosthetic Dentistry. 2001; 86 :424-427 - 4.
Rudigier J, Dreaner K, Grunert A, Ritter G. Effects of adding X-ray contrast materials to bone cements. J. Acts Traumat. 1977; 7 :35-38 - 5.
Davy KWM, Anseau MR, Berry C. Iodinated methacrylate copolymers as X-ray opaque denture base acrylics. Journal of Dentistry. 1997; 25 :499-505 - 6.
Deb SV, BT. The effect of cross-linking agents on acrylic bone cements containing radiopacifiers. Journal of Biomaterials. 2001; 22 :2177-2181 - 7.
Van Hooy-Corstjens CSJ, Govaert LE, Spoelstra AB, et al. Mechanical properties of a new acrylic radiopaque iodine-containing bone cement. Journal of Biomaterials. 2004; 25 :2657-2666 - 8.
Lewis GA. Properties of acrylic bone cement; state of the art review. Journal of Biomedical Materials Research Part B: Applied Biomaterials. 1997; 38 :155-182 - 9.
Abboud M, Vol S, Duguet E, Fontanille M. PMMA-based composite materials with reactive ceramic filler, part III. Radiopacifying particle- reinforced bone cements. Journal of Materials Science. Materials in Medicine. 2000; 11 :295-300 - 10.
Mohamed SH. Physical, Mechanical and Biological Properties of PMMA Denture Base Material Filled with Ceramic Filler [Thesis]. Penang, Malaysia: USM; Polymeric Engineer, University Science Malaysia; 2006 - 11.
Elshereksi NW, Mohamed SH, Arifin Z, Mohd Ishak ZA. Effect of filler incorporation on the fracture toughness properties of denture base poly (methyl methacrylate). Journal of Physical Science. 2009; 20 (2):1-12 - 12.
Mohamed SH, Arifin Z, Mohd Ishak ZA. The effect of physical mixing on mechanical properties of hydroxyapatite filled poly (methyl methacrylate). Journal of the Institute of Materials Malaysia (JIMM). 2003: 4 (2):10-18 - 13.
Yee A. Impact resistance.In. In: Mark HF, editor. Encyclopedia of Polymer Science and Technology. 2nd ed. Vol. 8. New York: Wiley; 1987. pp. 36-68 - 14.
Cho K, Yang JHP, CE. The effect of rubber particle size on toughening behaviour of rubber-modified poly (methyl methacrylate) with different test methods. Journal of Polymer. 1998; 14 :3073-3081 - 15.
Zappini G, Kammann A, Wachter W. Comparison of fracture tests of denture base materials. The Journal of Prosthetic Dentistry. 2003; 90 :578-585 - 16.
Harper EJ, Braden M, Bonfield W. Mechanical properties of hydroxyapatite reinforced poly (ethylmethacrylate) bone cement after immersion in a physiological solution: Influence of a silane coupling agent. Journal of Materials Science: Materials in Medicine. 2000; 11 :491-497 - 17.
Wang M, Bonfield W. Chemically coupled hydroxyapatite-polyethylene composites; structure and properties. Journal of Biomaterials. 2001; 22 :1311-1320 - 18.
Fu Q , Wang G, Liu C. Polyethylene toughened by CaCO3 particles, the interface behaviour and fracture mechanism in high density polyethylene/CaCO3 blends. Journal of Polymer. 1995; 36 :2387-2401 - 19.
Debnath S, Wunder SL, JI MC, Baran GR. Silane treatment effects on glass/resin interfacial shear strengths. International Journal of Dental Materials. 2003; 19 :441-448 - 20.
Silva ALN, Rocha MCG, Moraes MAR, CAR V, Coutinho FMB. Mechanical and rheological properties of composites based on polyolefin and mineral additives. Journal of Polymer Testing. 2002; 21 :57-60 - 21.
Rusu M, Sofian N, Rusu D. Mechanical and thermal properties of zinc powder filled high density polyethylene composites. Journal of Polymer testing. 2001; 20 :409-417 - 22.
Atkins AG, Mai YW. Elastic and Plastic Fracture Metals, Polymers Ceramics, Biological Materials. Chichester: Ellis Horwood Limited; 1985 - 23.
Alla R, Sajjan S, Alluri V, Ginjupalli K, Padhya NU. Influence of fiber reinforcement on the properties of denture base resins. Journal of Biomaterials and Nanobiotechnology. 2013; 4 :91-97 - 24.
Tacir IH, Kama JD, Zortuk M, Eskimez S. Flexural properties of glass fiber reinforced acrylic resin polymers. Australian Dental Journal. 2006; 51 :52-56 - 25.
Moreno MV, Acosta-Torres LS, Barcel Osantana FH, Vanegas-Lanc RD, Plata-Rodrı'guez ME, Casta VM. Fiber-reinforced nano-pigmented poly (methyl methacrylate) as improved denture base. Journal of Applied Polymer Science. 2012; 126 :289-296 - 26.
Sang-Hui YU, Yoon LE, Seunghan OH, Hye-Won CH, Yutaka OD, Ji-Myung BA. Reinforcing effects of different fibers on denture base resin based on the fiber type, concentration, and combination. Dental Materials Journal. 2012; 3 :1039-1046 - 27.
Farina AP, Cecchin D, Soares RG, et al. Evaluation of vickers hardness of different types of acrylic denture base resins with and without glass fiber reinforcement. Gerodontology. 2012; 29 :155-160 - 28.
Hamouda IM, Beyari MM. Addition of glass fibers and titanium dioxide nanoparticles to the acrylic resin denture base material: Comparative study with the conventional and high impact types. Oral Health and Dental Management. 2014; 13 :107-112 - 29.
Singh K, Sharma SK, Negi P, Kumar M, Rajpurohit D, Khobre P. Comparative evaluation of flexural strength of heat polymerized denture base resins after reinforcement with glass fibers and nylon fibers: An in vitro study. Advances in Human Biology. 2016; 6 :91-94 - 30.
Agha H, Flinton R, Vaidyanathan T. Optimization of fracture resistance and stiffness of heat-polymerized high impact acrylic resin with localized e-glass fiber reinforcement at different stress points. Journal of Prosthodontics. 2016; 25 (8):647-655 - 31.
Fonseca RB, Favarão IN, Kasuya VB, Abrão M, Da Luz NFM, Naves LZ. Influence of glass fiber wt% and silanization on mechanical flexural strength of reinforced acrylics. Journal of Materials Science and Chemical Engineering. 2014; 2 :11-15 - 32.
Yu SH, Ahn DH, Park JS, et al. Comparison of denture base resin reinforced with polyaromatic polyamide fibers of different orientations. Dental Materials Journal. 2013; 32 :332-340 - 33.
Chen SY, Liang WM, Yen PS. Reinforcement of acrylic denture base resin by incorporation of various fibers. Journal of Biomedical Materials Research. 2001; 58 :203-208 - 34.
Soygun K, Bolayir G, Boztug A. Mechanical and thermal properties of polyamide versus reinforced PMMA denture base materials. The Journal of Advanced Prosthodontics. 2013; 5 :153-160 - 35.
Uzun G, Hersek N, Tinçer T. Effect of five woven fiber reinforcements on the impact and transverse strength of a denture base resin. The Journal of Prosthetic Dentistry. 1999; 81 :616-620 - 36.
Ismaeel IJ, Alalwan HA, Mustafa MJ. The effect of the addition of silanated polypropylene fiber to polymethylmethacrylate denture base material on some of its mechanical properties. Journal of Baghdad College of Dentistry. 2015; 27 :40-47 - 37.
John J, Ann Mani S, Palaniswamy K, Ramanathan A, Razak AA. Flexural properties of poly (methyl methacrylate) resin reinforced with oil palm empty fruit bunch fibers: A preliminary finding. Journal of Prosthodontics. 2015; 24 :233-238 - 38.
Xu J, Li Y, Yu T, Cong L. Reinforcement of denture base resin with short vegetable fiber. Dental Materials. 2013; 29 :1273-1279 - 39.
Vallittu PK, Narva K. Impact strength of a modified continuous glass fiber polymethylmethacrylate. The International Journal of Prosthodontics. 1997; 10 :142-148 - 40.
Safarabadia M, Khansarib NM, Rezaeic A. An experimental investigation of HA/AL2 O3 nanoparticles on mechanical properties of restoration materials. Engineering Solid Mechanics. 2014; 2 :173-182 - 41.
Han Z, Zhu B, Chen R, Huang Z, Zhu C, Zhang X. Effect of silver supported materials on the mechanical and antibacterial properties of reinforced acrylic resin composites. Materials and Design. 2015; 65 :1245-1252 - 42.
Aljafery MA, Basima MA. Effect of addition ZrO2-Al2 O3 nanoparticles mixture on some properties and denture base adaptation of heat cured acrylic resin denture base material. Journal of Baghdad College of Dentistry. 2015; 27 :15-21 - 43.
Alhareb AO, Akil HM, Ahmad ZA. Mechanical properties of PMMA Denture Base reinforced by nitrile rubber particles with Al2O3/YSZ fillers. Procedia Manufacturing. 2015; 2 :301-306 - 44.
Muklif OR, Ismail IJ. Studying the effect of addition a composite of silanized Nano-Al2 O3 and plasma treated polypropylene fibers on some physical and mechanical properties of heat cured PMMA denture base material. Journal of Baghdad College of Dentistry. 2015; 27 :22-27 - 45.
Alhareb AO, Ahmad ZA. Effect of Al2 O3/ZrO2 reinforcement on the mechanical properties of PMMA denture base. Journal of Reinforced Plastics and Composites. 2011; 30 :1-8 - 46.
Zhang XY, Zhang XJ, Huang ZL, Zhu BS, Chen RR. Hybrid effects of zirconia nanoparticles with aluminum borate whiskers on mechanical properties of denture base resin PMMA. Dental Materials Journal. 2014; 33 :141-146 - 47.
Salman TA, Khalaf HA. The influence of adding of modified ZrO2 - TiO2 nanoparticles on certain physical and mechanical properties of heat polymerized acrylic resin. Journal of Baghdad College of Dentistry. 2015; 27 :33-39 - 48.
Oei JD, Zhao WW, Chu L, et al. Antimicrobial acrylic materials with in situ generated silver nanoparticles. Journal of Biomedical Materials Research. 2012; 100 :409-415 - 49.
Yadav P, Mittal R, Sood VK, Garg R. Effect of incorporation of silane treated silver and aluminum microparticles on strength and thermal conductivity of PMMA. Journal of Prosthodontics. 2012; 21 :546-551 - 50.
Asar NV, Albayrak H, Korkmaz T, Turkyilmaz I. Influence of various metal oxides on mechanical and physical properties of heat-cured polymethylmethacrylate denture base resins. The Journal of Advanced Prosthodontics. 2013; 5 :241-247 - 51.
Sehajpal SB. Effect of metal fillers on some physical properties of acrylic resin. The Journal of Prosthetic Dentistry. 1989; 61 :746-751 - 52.
Safi IN. Evaluation the effect of nano-fillers (TiO2, AL2 O3, SiO2) addition on glass transition temperature, E-Moudulus and coefficient of thermal expansion of acrylic denture base material. Journal of Baghdad College of Dentistry. 2014; 26 :37-41 - 53.
Arora P, Singh SP, Arora V. Effect of alumina addition on properties of polymethylmethacrylate – A comprehensive review. International Journal of Biotech Trends and Technology. 2015; 9 :1-7 - 54.
Ellakwa AE, Morsy MA, El-Sheikh AM. Effect of aluminum oxide addition on the flexural strength and thermal diffusivity of heat-polymerized acrylic resin. Journal of Prosthodontics. 2008; 17 :439-444 - 55.
Vojdani M, Bagheri R, Khaledi AR. Effects of aluminum oxide addition on the flexural strength, surface hardness, and roughness of heat-polymerized acrylic resin. Journal of Dental Sciences. 2012; 7 :238-244 - 56.
Yadav NS, Elkawash H. Flexural strength of denture base resin reinforced with aluminum oxide and processed by different processing techniques. Journal of Advanced Medical and Dental Sciences Research. 2011; 2 :33-36 - 57.
Chaijareenont P, Takahashi H, Nishiyama N, Arksornnukit M. Effect of different amounts of 3-methacryloxypropyltrimethoxy silane on the flexural properties and wear resistance of alumina reinforced PMMA. Dental Materials Journal. 2012; 31 :623-628 - 58.
Arora N, Jain V, Chawla A, Mathur VP. Effect of addition of sapphire (aluminum oxide) or silver fillers on the flexural strength, thermal diffusivity and water sorption of heat-polymerized acrylic resins. International Journal of Prosthodontics and Restorative Dentistry. 2011; 1 :21-27 - 59.
Jasim BS, Ismail IJ. The effect of silanized alumina nano-fillers addition on some physical and mechanical properties of heat cured polymethylmethacrylate denture base material. Journal of Baghdad College of Dentistry. 2014; 26 :18-23 - 60.
Abdulkareem MM, Hatim NA. Evaluation the biological effect of adding aluminum oxide, silver nanoparticles into microwave treated PMMA powder. International Journal of Enhanced Research In Science Technology & Engineering. 2015; 4 :172-178 - 61.
Asopa V, Suresh S, Khandelwal M, Sharma V, Asopa SS, Kairal LS. A comparative evaluation of properties of zirconia reinforced high impact acrylic resin with that of high impact acrylic resin. The Saudi Journal for Dental Research. 2015; 6 :146-151 - 62.
Ayad NM, Badawi MF, Fatah AA. Effect of reinforcement of high impact acrylic resin with zirconia on some physical and mechanical properties. Rev Clín Pesq Odontol. 2008; 4 :145-151 - 63.
Gad MM, Rahoma A, Al-Thobity AM, ArRejaie AS. Influence of incorporation of ZrO2 nanoparticles on the repair strength of PMMAdenture bases. International Journal of Nanomedicine. 2016; 11 :5633-5643 - 64.
Zhang XJ, Zhang XY, Zhu BS, Qian C. Effect of nano ZrO2 on flexural strength and surface hardness of polymethylmethacrylate. Shanghai Kou Qiang Yi Xue. 2011; 20 :358-363 - 65.
Gad M, ArRejaie AS, Abdel-Halim MS, Rahoma A. The reinforcement effect of nano-zirconia on the transverse strength of repaired acrylic denture base. International Journal of Dentistry. 2016; 2016 :7094056 - 66.
Ahmed MA, Ebrahim MI. Effect of zirconium oxide nano-fillers addition on the flexural strength, fracture toughness, and hardness of heat-polymerized acrylic resin. World Journal of Nano Science and Engineering. 2014; 4 :50-57 - 67.
Cierech M, Kolenda A, Grudniak AM, et al. Significance of polymathyl methacrylate (PMMA) modification by zinc oxide nanoparticles for fungal biofilm formation. International Journal of Pharmaceutics. 2016; 510 (1):323-335 - 68.
Safi IN, Hassanen KA, Ali NA. Assessment of zirconium oxide nano fillers incorporation and silanation on impact, tensile strength and color alteration of heat polymerized acrylic resin. Journal of Baghdad College of Dentistry. 2012; 24 :36-42 - 69.
Hameed HK, Abdul RH. The effect of addition nano particle ZrO2 on some properties of autoclave processed heat cures acrylic denture base material. Journal of Baghdad College of Dentistry. 2015; 27 :32-39 - 70.
Mohammed D, Mudhaffar M. Effect of modified zirconium oxide nano-fillers addition on some properties of heat cure acrylic denture base material. Journal of Baghdad College of Dentistry. 2012; 24 :1-7 - 71.
Wei Y, Wang X, Tang Q , Guo M, Zhao J. Reinforcement of denture base PMMA with ZrO2 nanotubes. Journal of the Mechanical Behavior of Biomedical Materials. 2014; 32 :192-197 - 72.
Alwan SA, Alameer SS. The effect of the addition of silanized Nano titania fillers on some physical and mechanical properties of heat cured acrylic denture base materials. Journal of Baghdad College of Dentistry. 2015; 27 :86-91 - 73.
Elshereksi NW, Mohamed SM, Arifin Z, Mohd Ishak ZA. Thermal characterization of poly (methyl methacrylate) filled with barium titanate as denture base material. Journal of Physical Science. 2014; 25 :15-27 - 74.
Alnamel HA, Mudhaffer M. The effect of silicon dioxide Nano-fillers reinforcement on some properties of heat-cure polymethylmethacrylate denture base material. Journal of Baghdad College of Dentistry. 2014; 26 :32-36 - 75.
Balos S, Pilic B, Markovic D, Pavlicevic J, Luzanin O. PMMAnanocomposites with low silica addition. The Journal of Prosthetic Dentistry. 2014; 111 :327-334 - 76.
Franklin P, Wood DJ, Bubb NL. Reinforcement of poly (methyl methacrylate) denture base with glass flake. Dental Materials. 2005; 21 :365-370 - 77.
Ghaffari T, Barzegar A, Hamedi Rad F, Moslehifard E. Effect of nanoclay on thermal conductivity and flexural strength of PMMAacrylic resin. Journal of Dentistry (Shiraz). 2016; 17 (2):121-127 - 78.
Cevik P, Yildirim-Bicer AZ. The effect of silica and prepolymer nanoparticles on the mechanical properties of denture base acrylic resin. Journal of Prosthodontics. 2018; 27 (8):763-770 Epub 2016 Nov 29 - 79.
Al-Bakri IA, Harty D, Al-Omari WM, Swain MV, Chrzanowski W, Ellakwa A. Surface characteristics and microbial adherence ability of modified polymethylmethacrylate by fluoridated glass fillers. Australian Dental Journal. 2014; 59 (4):482-489 - 80.
Kul E, Aladağ Lİ, Yeşildal R. Evaluation of thermal conductivity and flexural strength properties of poly (methyl methacrylate denture) base material reinforced with different fillers. The Journal of Prosthetic Dentistry. 2016; 116 :803-810 - 81.
Brockett C, Carbone S, Abdelgaied A, Fisher J, Jennings L. Influence of contact pressure, cross-shear and counterface material on the wear of PEEK and CFR-PEEK for orthopaedic applications. Journal of the Mechanical Behavior of Biomedical Materials. 2016; 63 :10-16 - 82.
Trindade R, Albrektsson T, Galli S, Prgomet Z, Tengvall P, Wennerberg A. Bone immune response to materials, part I: Titanium, PEEK and copper in comparison to sham at 10 days in rabbit tibia. Journal of Clinical Medicine. 2018; 7 :526 - 83.
Chen SG, Yang JY, Jia YG, Lu B, Ren L. TiO2 and PEEK reinforced 3D printing PMMA composite resin for dental Denture Base applications. Nanomaterials. 2019; 9 :1049. DOI: 10.3390/nano9071049 - 84.
Schwitalla AD, Zimmermann T, Spintig T, Kallage I, Müller W-D. Fatigue limits of different PEEK materials for dental implants. Journal of the Mechanical Behavior of Biomedical Materials. 2017; 69 :163-168 - 85.
Gao S, Gao S, Xu B, Yu H. Effects of different pH-values on the Nanomechanical surface properties of PEEK and CFR-PEEK compared to dental resin-based materials. Materials. 2015; 8 :4751-4767 - 86.
Stawarczyk B, Beuer F, Wimmer T, Jahn D, Sener B, Roos M, et al. Polyetheretherketone—A suitable material for fixed dental prostheses? Journal of Biomedical Materials Research Part B: Applied Biomaterials. 2014; 101 :1209-1216 - 87.
Acosta-Torres LS, Mendieta I, Nuñez-Anita RE, Cajero-Juárez M, Castaño VM. Cytocompatible antifungal acrylic resin containing silver nanoparticles for dentures. International Journal of Nanomedicine. 2012; 7 :1-10 - 88.
Wady AF, Machado AL, Zucolotto V, Zamperini CA, Berni E, Vergani CE. Evaluation of Candida albicans adhesion and biofilm formation on a denture base acrylic resin containing silver nanoparticles. Journal of Applied Microbiology. 2012; 112 :1163-1172 - 89.
Ghafari T, Hamedi-Rad F, Ezzati B. Does addition of silver nanoparticles to denture base resin increase its thermal conductivity? The Journal of Dental Sciences. 2014; 32 :139-144 - 90.
Ghaffari T, Hamedi-rad F, Ezzati B. In vitro comparison of compressive and tensile strengths of acrylic resins reinforced by silver nanoparticles at 2% and 0.2% concentrations. Journal of Dental Research Dental Clinics Dental Prospects. 2014; 8 :204-209 - 91.
Morsy MA, Al-Daous MA. Gold nanoparticles-PMMA composite for denture base: Synthesis, mechanical and thermal characteristics. AKU Journal of Science and Engineering. 2014; 14 :369-374 - 92.
Wang R, Kayacan R, Küçükeşmen C. Nanotubes/PMMAcomposite resins as denture base materials. In: Zhang M, Naik RR, Dai L, editors. Carbon Nanomaterials for Biomedical Applications. 1st ed. Berlin: Springer International Publishing; 2015. pp. 227-240 - 93.
Mahmood WS. The effect of incorporating carbon nanotubes on impact, transverse strength, hardness, and roughness to high impact denture base material. Journal of Baghdad College of Dentistry. 2015; 27 :96-99 - 94.
Abdallah RM. Evaluation of PMMA resin mechanical properties with incorporated halloysite nanotubes. The Journal of Advanced Prosthodontics. 2016; 8 :167-171 - 95.
Jamróz W, Szafraniec J, Kurek M, Jachowicz R. 3D printing in pharmaceutical and medical applications. Recent Achievements and Challenges in Pharmaceutical Research. 2018; 35 :176 - 96.
Hammesfahr PD, O’Connor MT, Wang X. Light-curing technology: Past, present, and future. The Compendium of Continuing Education in Dentistry. 2002; 23 :18-24 - 97.
Totu EE, Nechifor AC, Nechifor G, Aboul-Enein HY, Cristache CM. Poly(methyl methacrylate) with TiO2 nanoparticles inclusion for stereolitographic complete denture manufacturing—The future in dental care for elderly edentulous patients? Journal of Dentistry. 2017; 59 :68-77