Dental amalgam

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Dental amalgam

Nasira Haque1, Safiyya Yousaf2, Touraj Nejatian3, Mansour Youseffi2, Masoud Mozafari4,5 and Farshid Sefat6,7 1 Department of Biomedical and Electronics Engineering, School of Engineering, University of Bradford, Bradford, United Kingdom, 2Medical Engineering Department, Faculty of Engineering and Informatics, University of Bradford, Bradford, United Kingdom, 3 Eastman Dental Institute, University College of London, London, United Kingdom, 4 Bioengineering Research Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Centre (MERC), Tehran, Iran, 5Department of Tissue Engineering & Regenerative Medicine, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences (IUMS), Tehran, Iran, 6Interdisciplinary Research Centre in Polymer Science & Technology (IRC Polymer), University of Bradford, Bradford, United Kingdom, 7Biomedical and Electrical Engineering Department, School of Engineering, University of Bradford, Bradford, United Kingdom

Chapter Outline 6.1 Introduction 106 6.2 Dental filling biomaterials

107

6.2.1 Gold fillings 107 6.2.2 Dental composites 108 6.2.3 Amalgam 109

6.3 History of amalgam 111 6.4 Composition of amalgam 112 6.4.1 Low-copper dental amalgam 112 6.4.2 High-copper dental amalgam 112

6.5 Amalgam bonding

113

6.5.1 Nonbonded amalgam restorations 113 6.5.2 Bonded amalgam restorations 114 6.5.3 Nonbonded versus adhesively bonded amalgam restorations 114

6.6 Material properties of amalgam

114

6.6.1 Compressive and tensile strength 114 6.6.2 Creep 115 6.6.3 Tarnish and corrosion 116

6.7 6.8 6.9 6.10 6.11

Dimensional change 117 Hardness 117 Young’s modulus 118 Failure mode 118 Biocompatibility 118 6.11.1 Toxicology of mercury 118

6.12 Conclusion

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Advanced Dental Biomaterials. DOI: https://doi.org/10.1016/B978-0-08-102476-8.00006-2 Copyright © 2019 Elsevier Ltd. All rights reserved.

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References 121 Further reading 123

6.1

Introduction

Dentistry is the branch of medicine which deals with oral health. Services in this field very much depend on the biomaterials, which have developed remarkably over the years. The biomaterials are either directly prepared and placed in a tooth cavity or indirectly made in a laboratory and cemented in the cavity. Various types of biomaterials such as polymer composites, glass ionomer cement, resin cement, gold, and dental amalgams have been used as directed restorative materials. Dental amalgam as a metallic direct restorative material is the primary focus of this chapter. Dental amalgam is a popular biomaterial which has been used successfully in the dental industry for decades. It is composed of a mixture of metal alloy and liquid mercury. Amalgam is mainly used for dental restorations. The metal alloy portion of this biomaterial has a composition made up of silver, tin, copper, and traces of other metals. The unique composition of amalgam gives rise to many useful properties such as excellent durability and strength. These are some of the many factors behind the long service life of amalgam with an average of 8
10 years (Delta Dental, 2012a,b). The main application of dental amalgam is the restoration of tooth cavities. After removing infected tooth tissue, the cavity is modified to the required shape to receive the amalgam filling. The constituents of amalgam are then mixed to form an amalgam paste, which is then placed in the cavity and left to complete the chemical reaction and become hard (FDA, 2017). Amalgam stays soft for a short period after it is mixed, enough to condense and shape onto the prepared tooth (Gay et al., 1979; Bates, 2006). This biomaterial is relatively cost-effective and widely used. Despite the benefits of amalgam fillings, the safety of them has been a very controversial subject over the years. This is due to the inclusion of elemental mercury within the composition of amalgam. Analytical chemistry techniques were used to investigate dental amalgam, and it has been established that mercury is released continuously (Gay et al., 1979; Bates, 2006). Mechanical friction from the teeth onto the amalgam and mercury being dissolved in saliva, aid the release of mercury. An epidemiological assessment found that there was little to no evidence linking this biomaterial to any effect mortality rates or chronic diseases (Gay et al., 1979; Bates, 2006). A global treaty known as the Minamata Convention on Mercury has been put into place by UNEP (United Nations Environment Programme) to decrease and where possible eliminate the use of mercury. This is to ensure the adverse effects that mercury has on the environment and the human population are decreased. Under this treaty dental amalgam is undergoing a “phase down” of usage. The reason behind this decision is to reduce the adverse effects of using this element, yet to still provide safe healthcare (FDI World Dental Federation, 2017). This book chapter aims to investigate the application of amalgam as a dental biomaterial as well as the composition, biocompatibility, bonding, and material properties of amalgam.

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Dental filling biomaterials

6.2.1 Gold fillings Gold has been used for over 4000 years as a dental restorative. During the early use of gold in dentistry, a higher importance was given to the aesthetics it had to offer, rather than its contribution to mastication. Gold fillings can be used to restore the function of a tooth affected by dental caries. It is thought that alloyed gold is an excellent choice for fillings, as it has excellent biocompatibility, is easy to manufacture, and lasts over an extended period (Knosp et al., 2003). Pure gold is usually only used for the direct filling of cavities on the tooth surface. Only small cavities may have gold inserted onto them as gold fillings are unable to withstand forces expelled during mastication. Pure gold is soft. Hence it has nearly 50% elongation and a low-stress enduring capacity. These properties allow the pure gold to be cold worked, which is necessary to place it into the cavity easily. Alloyed gold may also be used as a restorative material, and gold is commonly alloyed with base metals (such as indium and copper) and noble metals (Knosp et al., 2003). Cavities can also be filled with an investment cast gold alloy. This material is cemented and used in both dental onlays and inlays (as shown in Fig. 6.1). Alloys with 65%
75% gold are commonly used. In addition, gold can be electroformed and covered with porcelain, after which it may be cemented into the designated cavity. Restorative dentistry involves dental bridges and crowns, and involves applying electroformed gold onto porcelain veneers. Electroforming is advantageous because it prevents gold from changing dimensions when it is fired with porcelain, which then minimizes the number of steps required to achieve the desired product. The electroformed gold produced high hardness which is suitable for this application (Knosp et al., 2003). Gold is a noble metal that has a high market value. Gold fillings are costly and therefore unavailable to patients from all socioeconomic backgrounds. Gold casting requires at least two appointments since the cavity must be prepared without undercuts and then the filling must be inserted, which adds to the expense. In comparison to the longevity of a gold filling, the cost is not very significant. The process of insertion is technique sensitive and requires skill from the dentist. In addition, since gold is not tooth colored, it is, therefore, seldom used for anterior tooth restorations (Donovan et al., 2008). The gold casting is created in a laboratory where a replica of the opposing teeth is present, which allows the filling to be shaped, so it aids mastication. It is easier to achieve a smooth finish on a gold restoration, rather than other restorative materials since it is fabricated in the laboratory and is not affected by the oral environment. The smooth finish is advantageous as it is comfortable for the patient and reduces plaque build-up (Donovan et al., 2008). Gold is a “permanent” filling, in contrast to amalgam and resin composites, this is because when it is properly prepared it will not marginally wear or fracture. The gold filling protects the enamel at the edges of the restorations since it can be placed with precision to support the enamel. The coefficient of expansion of gold is

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Figure 6.1 Schematic drawing of restorative prostheses: (A) inlay, (B) onlay, (C) overlay, and (D) pinlay.

close to a tooth. This property allows both tooth and restoration to undergo contraction and expansion in a complimentary way; this is essential as the temperature within the oral cavity varies (Donovan et al., 2008).

6.2.2 Dental composites Dental composites are used as fillings to restore cavities in a tooth due to decay. Dental composites are usually made of a polymeric resin matrix and a glass filler. The bis-GMA is commonly the main monomer for the resin matrix. However, TEGDMA may be added to make the resin less viscous and, therefore easier to handle. Photopolymerization is used to harden the resin matrix, and the bis-GMA has an essential role in reducing the volumetric shrinkage caused by this process. The size of the glass filler (e.g., fibrillar silicate) can also affect the mechanical properties of the composite. Nanosized fillers aid properties such as Young’s modulus and flexural strength of the composite. The composite filling is desirable for individuals

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since it has a tooth color which is excellent for aesthetic purposes (Fong and Little, 2010). In a study where the capacity of preventing bacterial microleakage was compared between different dental restoratives, the dental composites were one of the best biomaterials which have been used (Murray et al., 2002). Microleakage causes hypersensitivity within dentin due to the irregular fluid movement within its tubules. Uncontrolled microleakage can also cause discomfort to the individual and infection which may lead to pulp inflammation (Cox, 1994). Ormocers are another form of dental composites and are formed via solution and gelation processes (Sivakumar and Valiathan, 2006). They are composed of polysiloxane and have a matrix which has organic
inorganic elements. Glass and ceramic make up the inorganic portion of the matrix. Silane molecules connect the organic and inorganic molecules. The properties of ormocers are highly dependent on the proportion of the components that it is made of. Polysiloxanes are influencing factors for the elasticity of this dental biomaterial. The hardness and polarity of an ormocer are determined by the organic polymers, whereas the inorganic polymers impact chemical stability (Zimmerli et al., 2010). This biomaterial was created to minimize volumetric shrinkage, which is caused by the likeness between the thermal expansion coefficient of other composites and the human tooth (Sivakumar and Valiathan, 2006).

6.2.3 Amalgam Dental amalgam is made into a paste and manipulated into the cavity shape onto a carious tooth. This procedure must be carried out before the paste sets and hardens. The mercury inclusion, within the composition of this biomaterial, has been a controversial matter for many years. However, it is not recommended to remove amalgam fillings that have previously been inserted without good reason. Unnecessary loss of a healthy tooth segment and mercury vapor exposure may occur because of the removal of the amalgam filling. Some individuals may be unable to receive amalgam fillings due to sensitivity or allergy to the constituents that make up dental amalgam (FDA, 2017). Dental amalgam will be focused on in greater detail within later sections, as it is the primary focus of this chapter. An amalgam is made up of mercury alloyed to one or more other metals. Mercury does not alloy with certain metals such as iron, platinum, and tungsten. Amalgamation is the process by which an amalgam is processed and is commonly an exothermic reaction (Helmenstine, 2017). There are several examples of amalgams, such as the silver and gold amalgams. In this case, the mercury amalgamates with each of the metals and is used to separate it from its ore. Amalgam extraction is uncommon currently because of its adverse environmental impact. There is a concern with regards to gold mining releasing toxic mercury vapor. Furthermore, another example is the thallium amalgam which has a lower freezing point in comparison to pure mercury. This property makes it useful to use in low-temperature thermometers. Finally, sodium amalgam can act as a reducing agent (Helmenstine, 2017) which is a species that donates electrons to another species.

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Dental amalgam is commonly known as “silver fillings” since it has a gray appearance. The main application of dental amalgam is, within dentistry, as a restoration material. The scientific research that has been reviewed by the FDA displays no link between health issues and amalgam fillings. The FDA has deemed dental amalgam fillings safe for use on both adults and children (aged 6 or above). Furthermore, it has been established from limited evidence that this filling does not have an adverse effect on the fetus during pregnancy. Also the levels of mercury in breast milk, due to the mother having amalgam fillings, are acceptable since it is lower than the value considered safe (FDA, 2017). The study that supports this statement (Lygre et al., 2016) included many volunteers and found no substantial evidence to support prenatal amalgam exposure caused health issues for the child. Pereira (2016) conducted a study which compared dental amalgam to glass ionomers, composites, and resin ionomers. When compared to the other direct placement restorations, it was established that amalgam requires a more significant amount of healthy tooth structure removed during cavity preparation. It was seen that the other methods did not require this since they all included an adhesive bonding mechanism. Though this was the case, amalgam is still widely used in clinical conditions since it has a broader tolerance range than the other methods. In an age where aesthetics are becoming more vital, dental amalgam is inadequate since it has a nontooth color. This limitation usually leads to the use of amalgam fillings on mainly posterior teeth. However, in contrast to other direct restoration methods, the cost of amalgam is significantly lower (Pereira, 2016). This is beneficial in poorer countries where people are likely to choose dental restorations, with costs being a more crucial factor in aesthetics. Failure rates of amalgam are low; therefore they require replacement less often than other restoration methods (Pereira, 2016). The durability of amalgam ranges from “good to excellent,” and this factor adds to the longevity of the dental restoration. Mastication expends many forces onto the restorations, and therefore this is an advantageous property. Amalgam also has a superior lifetime of approximately 10 years, which is a longer life span than the glass ionomer, composite, and resin ionomer. Although amalgam has the highest wear resistance in contrast to the other restoration methods, it is also the only one of a brittle nature, since it fragments around the edges (Pereira, 2016). Direct dental restorations must be placed into the cavity by a dental professional. Amalgam filling placement is not technique sensitive. Therefore extensive experience is not be required by the dentist. Amalgam can be easily manipulated, and it is possible to rectify mistakes carried out by the dentist. In addition, as opposed to composites, the placement time of amalgam is short. These factors combined help to make the dentist more comfortable and confident with amalgam usage (Pereira, 2016). To see if there is a trend of amalgam usage worldwide, Burke (2004) reviewed government guidelines on amalgam. Results displayed that there is a decrease in usage worldwide, but the rate of depreciation cannot be identified due to insufficiently available publications. The use of amalgam has declined more slowly in the United Kingdom, in comparison to the United States and Australia. One of the

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main factors influencing this decline in use is the release of mercury. Limited studies have linked the toxic mercury release to Alzheimer’s disease and multiple sclerosis (Bates, 2006). In extremely rare cases, individuals may have an allergic reaction to amalgam fillings. Metal constituents of the filling, such as the elemental mercury, can be the cause of this reaction. Individuals, who have a family history of this issue, are the most susceptible to this condition. Symptoms of this allergic reaction can include skin rashes (Oral Health Centre, 2016b). Amalgam restorations can also lead to amalgam tattoos. These tattoos can occur during the insertion of the amalgam restorations if any of the constituents are accidentally implanted onto neighboring palatal, buccal, gingival, or lingual mucosa. The accumulation of these constituents may leave a gray/black oral lesion behind. Though they are benign, they may resemble oral lesions which are caused by melanoma or Kaposi’s sarcoma. These causes are more severe, but they can be quickly ruled out by biopsy (Dubach and Caversaccio, 2011).

6.3

History of amalgam

The exact origin of dental amalgam is unknown, but in 1826 it was recorded that the Frenchman Monsieur Travaux introduced this material. During this period, dental amalgam consisted of mercury combined with finely ground silver coins. In the United States during 1895, a dental amalgam alloy which had a silver
tin composition was founded by Dr. G.V. Black. The silver
tin alloy particles were combined with mercury. Once the alloy particles and mercury reacted, the initial plastic behavior allowed it to be shaped onto the tooth and then harden into place (Hollenback, 1969; Pereira, 2016). During the early 19th century, amalgam was also used for dental restorations in Europe. During this period there were other metallic restorations available which included hammered lead and gold. In contrast to amalgam, the other restorations had an absence of toxic mercury and had an intricate insertion technique. These factors made the usage of amalgam questionable within the dental field (Greener, 1979; Nicholson, 2002). The controversy over amalgam usage led to the “amalgam wars” in the United States. Dentists during this period had a divided opinion about the safety of amalgam fillings. Some dentists ceased amalgam usage since the mercury had a toxic nature and it was seen to be unethical to put patients at unnecessary risk. The American Society of Dental Surgeons, a professional body at the time, supported the prevention of amalgam usage. They grouped together dentists who did not use amalgam against the dentists who were still using amalgam. Specifications which included thorough testing of amalgam were released in 1929, by the American Dental Association. This aided in making more desirable amalgams, that not only carried out their function but also put patients at a lower risk (Greener, 1979; Nicholson, 2002).

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Many figures had an impact on dental amalgam advancements. Notably in 1896, Dr. G.V. Black established the formulae of a balanced dental amalgam composition. This composition has a likeness to the modern-day dental amalgam. The formulae considered the level of contraction and expansion that the amalgam would go through during its lifetime. The cavity preparation techniques which he established made the insertion of amalgam simpler and still act as guidance to dentists today (Singh, 2015).

6.4

Composition of amalgam

The alloy segment of dental amalgam can be formed using techniques such as lathe cutting and gas atomization which produces lathe cut or spherical particles, respectively. Various criteria are used for classification of dental amalgams. Among those, the classification based on copper content is probably the most popular one due to the significant effect that copper has on properties of dental amalgam. The traditional amalgam which has a copper particle concentration lower than 6 wt.% is classified as low-copper amalgam, whereas when the alloy portion of the amalgam contains greater than 8
10 wt.% copper particles, it is classified as high-copper amalgam (Okabe and Cahn, 1990; Hooghan et al., 1996).

6.4.1 Low-copper dental amalgam Low-copper amalgam was initially used for fillings and has a composition of approximately 24.3
27.6 wt.% tin, 66.7
72.5 wt.% silver, 1.2
5.5 wt.% of copper, and occasionally other metals were included (Okabe and Cahn, 1990; Hooghan et al., 1996). High-copper amalgam has now replaced this compound because it has better suited qualities to function as a dental filling (Dental Science, 2016). Hooghan et al. (1996) used transmission electron microscopy to investigate the microstructure of low-copper amalgam. The analysis displayed that the unreacted γ (Ag3Sn) phase was surrounded by the γ1 (Ag2Hg3) and γ 2 (HgSn7) phases. It was also noticed that the reaction layer between γ and γ1 phase included a mixture of β1 (Ag
Hg
Sn) and some of the Cu6Sn5 (η) phase. The γ1 phase holds the amalgam together and has a similar strength and corrosion resistance to the γ phase, though it has a brittle nature. The γ2 phase is the weakest structure, and if this phase is large it may lead to corrosion (Dental Science, 2016). The amalgamation reaction of low-copper amalgams can be summarized into the following equation (Dental Science, 2016): Excess γ-Ag3 Sn 1 Hg ! Unreacted γ-Ag3 Sn 1 γ1 -Ag2 Hg3 1 γ2 -Sn7 Hg

6.4.2 High-copper dental amalgam High-copper amalgam alloy consists of 17.0
30.2 wt.% tin, 39.9
70.1 wt.% silver, 9.5
29.9 wt.% copper, and may include 0.01
2.0 wt.% zinc (Okabe and Cahn, 1990;

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Hooghan et al., 1996). Admixed or single-composition alloys are used to prepare this type of amalgam. An admixed composition consists of both lathe cut and spherical particles, whereas single-composition alloys contain the same shape of particles throughout (Dental Science, 2016). The γ2 phase (Sn7Hg) is found in the majority of silver
tin alloys (Sarkar and Eyer, 1987; Yap et al., 2004). These alloys were created to reduce the weak γ2 phase (Sn7Hg) by forming more of the (Cu6Sn5) η phase (Beech, 1982). The highcopper amalgam has superior corrosion and creep resistance properties when compared to its predecessor (Sarkar and Eyer, 1987; Yap et al., 2004). Acciari et al. (2005) have suggested that another beneficial quality of the high-copper amalgam is that it decreases toxicity due to mercury vapor emission. During the amalgamation of high-copper dental amalgam, mercury reacts with a powdered amalgam alloy and forms a metallurgical structure. The amalgam alloy is usually made up of copper, silver, tin, and other metals. The solubility of the elements involved is different. Therefore this gives rise to the dissolving of compounds. The mercury defuses into silver
tin particles and dissolves them partially. Silver
copper particles, however, are less reactive with mercury (Craig, 1985; Acciari et al., 2005). The γ1 (Ag2Hg3) and γ2 (Sn7Hg) phases are formulated when silver
tin particles are partially dissolved in the mercury. The γ2 phase surrounding the silver
tin particles and the γ1 phase surrounding the silver
copper particles react to form the η phase (Cu6Sn5). During this process, some silver
tin particles (γ phase) are left unreacted (Craig, 1985; Acciari et al., 2005). In other words, high-copper converts the weak and corrosive γ2 phase to a stronger and less corrosive γ1 phase. A structure with up to six phases is produced, and the following equations display the amalgamation process (Craig, 1985; Acciari et al., 2005): γ-Ag3 Sn 1 AgCu 1 Hg ! γ1 -Ag2 Hg3 1 γ2 -Sn7 Hg 1 γ-Ag3 Sn 1 AgCu γ2 -Sn7 Hg 1 AgCu ! η-Cu6 Sn5 1 γ1 -Ag2 Hg3 Although high-copper amalgam is preferred today, the increased content of copper does not always mean an improved clinical performance. When comparing a modified low-copper amalgam to a high-copper amalgam, it is possible that they can show similar clinical performance (Beech, 1982).

6.5

Amalgam bonding

6.5.1 Nonbonded amalgam restorations Dental amalgam does not naturally bond to the tooth surface. The amalgam must be mechanically retained to the tooth via cavity preparation. Retention methods of amalgam may include mechanical devices to aid adherence. Generally, the process of cavity preparation for nonbonded amalgam is less conservative and can lead to loss of healthy tooth structure, which is the main disadvantage of this method.

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This leads to the remaining tooth structure becoming weakened; therefore fracture may occur (Dean, 2016).

6.5.2 Bonded amalgam restorations Bonded amalgam restorations are made via an adhesive lining material that is placed under the amalgam. This dental lining has dentin bonding ability and amalgam is placed over it before the adhesive sets. A mechanical bond forms between amalgam and the bonding as the two materials intermix (Dean, 2016). Bonded amalgam restorations reduce the need for cavity preparation to retain the amalgam restoration. This is highly beneficial because the healthy remains of the tooth structure can be salvaged (Bonsor, 2011). Microleakage is the passage of bacteria, molecules, and fluids between the restorative material and tooth (Muliyar et al., 2014). It is beneficial as the restoration will have a low possibility of microleakage and bacterial invasion through the tooth
material interface. This decreases the risk of sensitivity after restoration and recurrent caries in the long term. Bonding can help with maintaining the integrity of tooth structure which in turn reduces the probability of tooth fracture (Bonsor, 2011).

6.5.3 Nonbonded versus adhesively bonded amalgam restorations A study reviewed articles investigating the bonding of amalgam, and their main findings included an investigation on 31 patients, who in total received 113 amalgam restorations. These restorations were made of either adhesively bonded or nonbonded amalgam. After 2 years there was a follow-up examination, which found no significant difference between the two groups, in marginal change or sensitivity after insertion. Although a small sample size was used and the study was carried out in a university dental clinic, the data may not be representable. This may show that there is limited evidence to show nonbonded amalgam restorations perform worse than adhesively bonded amalgam. The review also suggested dentists may have to consider the additional costs for the adhesively bonded amalgam (Agnihotry et al., 2016). Vanishree et al. (2015) found that bonded amalgam has less probability of microleakage when compared to composite resins and nonbonded amalgam restorations. Also, the study showed that the samples restored by bonded amalgam and composite resins had an inconsistent fracture resistance in comparison to nonbonded amalgam.

6.6

Material properties of amalgam

6.6.1 Compressive and tensile strength An amalgam restoration must be able to withstand biting forces acting during mastication. Therefore it is essential that it has a high compressive strength. The

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compressive strength of the high-copper amalgam is 414 MPa, whereas low-copper amalgam is considerably lower, 380 MPa (Thomas et al., 2013). Jayanthi and Vinod (2013) carried out a study to compare the compressive and flexural strength of dental materials used as core build-up procedures. This study included amalgam (DPI), Fluorocore composite (DENTSPLY, Caulk), Vitremer glass ionomer (3 M), and nanocomposite Filtek (3 M). The flexural strength of amalgam was found to be the lowest of all materials being tested. It had also been established that the compressive strength of amalgam as core build-up material was higher than Vitremer but lower than that of the composites tested. Dental amalgam is unable to withstand high tensile stresses, and therefore to avoid fracture the positioning of it within the cavity must be carefully thought out. This means the tooth cavity should be prepared in a way that the restoration is subjected to the least tensile and shear forces possible. Low-copper amalgam can withstand 60 MPa of a shear force or tension (Manappallil, 2016). During the initial placement of amalgam, since it has a low strength, patients are advised to not bite down hard on it for the first 8 hours. During the setting process when the amalgam begins to solidify, the strength of the amalgam also begins to increase. After these initial 8 hours, the amalgam has nearly reached its complete strength. It is essential for the amalgam to meet the ISO specifications of having a specified compressive strength after 1 hour (100 MPa) and 24 hours (350 MPa). Certain amalgams continue increasing in strength even after 6 months, and this may suggest a continuous reaction between the alloy particles and matrix. The composition is also a factor which affects strength. High-copper single-composition amalgams have the advantage of achieving a high strength (262 MPa) within the first hour of placement (Manappallil, 2016), which will reduce the risk of immature failure of the restoration and patients’ discomfort. Hasheminezhad et al. (2012) investigated whether the compressive strength of amalgam is affected by copper content. They found that high-copper content in amalgam leads to higher compressive strength, due to the elimination of the γ2 phase. There is a positive correlation between strain rate and fracture stress for a copper amalgam. In addition, amalgam alloy particles may change shape from lathe cut into spherical during the processing due to the increase of copper within their composition.

6.6.2 Creep Amalgam suffers from creep when oral forces cause stress and gradually changes its shape. This can occur under constant or intermittent stress (Manappallil, 2016). A high creep rate occurs when there is a gradual failure at the margins of the amalgam filling (Powers and Wataha, 2017). The amalgam begins to flow, which results in parts of it slightly detaching from the cavity and protruding. The protruded edges are unsupported and may fracture under the occlusal forces leaving marginal ditches. The unsupported edges and ditches may also trap food and lead to decay (Thomas et al., 2013). Creep is affected by microstructure in low-copper amalgam. Larger γ1 (Ag2Hg3) phase particles display a lower creep rate, whereas higher creep rates are linked to

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the γ2 (Sn7Hg) phase. Low-copper lathe cut amalgam can have up to 6% creep which exceeds the allowable limit. Due to the lack of γ2 phase, single-composition high-copper amalgams, in contrast, have a lower creep rate. Another creep preventative factor in high-copper amalgam is the (η phase) Cu6Sn5 rods stopping the γ1 phase from breaking down. To produce an amalgam with a low creep rate and high strength, it is essential to keep the mercury to alloy ratio low (Manappallil, 2016). The creep values of currently used amalgams are very low (Powers and Wataha, 2017).

6.6.3 Tarnish and corrosion Amalgam restorations that are placed into tooth cavities are susceptible to tarnish and corrosion (Manappallil, 2016). Tarnishing of the amalgam occurs at the surface level; it can be recognized by discoloration from chemical reactions with food. Whereas corrosion can chemically degrade both the surface and the bulk of a tooth, internal corrosion can decrease the strength and affect the shape of the amalgam, which inevitably can lead to failure of the filling (Powers and Wataha, 2017). Lowcopper amalgam restorations are more likely to undergo corrosion since they have a larger γ2 phase. The finding of a study (Amin, 2007) supported low-copper amalgam having a lower corrosion resistance when compared to high-copper amalgam. The study also found that corrosion resistance improved for both types of amalgam when they were left to age in an artificial oral environment. A factor which can increase corrosion levels are patients who are on high sulfur diets, which encourages the formation of black silver sulfide on the surface of the filling. High residual mercury and scratched surface texture of the restoration are the other contributing factors. After the amalgam restoration has been placed into the cavity, polishing the restoration slows down its corrosion. In addition to this, it is crucial to mix the correct proportions of mercury to alloy, since it is possible that excess corrosion may otherwise be caused (Manappallil, 2016).

6.6.3.1 Marginal sealing Upon the initial placement of amalgam, there is a slight gap between the amalgam and cavity walls (Mahler et al., 2009). This gap contains fluid containing molecules, ions, and bacteria (Ben-Amar et al., 1995). Amalgam has a unique ability to seal its marginal gap. This occurs since the margins of amalgam fillings naturally corrode while in service and this leads to corrosive deposits forming. Microleakage is decreased, as the corrosion products build up to fill the gap around the restoration, and seal it. Sealing can reduce chances of bacterial invasion. It is difficult to judge, solely from the appearance of the amalgam’s margins, whether the amalgam filling is sealed and if there is a chance of secondary caries. This is because despite having worn down margins, it is possible to have a well-sealed amalgam filling (Dental Science, 2016). High-copper amalgam has a higher corrosion resistance when compared to lowcopper amalgam. A study investigated whether it would take high-copper amalgam longer to create a seal, due to a slower formation of corrosive deposits. The results

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suggested that it is difficult to anticipate the sealing behavior of amalgam based on corrosion resistance alone. Factors such as the gap size and inclusion of zinc within the amalgams composition, also play a vital role. It was found that it is possible for high-copper amalgam to seal at a similar speed as its low-copper counterpart if the initial gap between the cavity wall and amalgam was small (Mahler et al., 2009). In addition, sealing via corrosive deposits can occur more rapidly if the amalgam is adapted to fit the cavity walls when placed (Ben-Amar et al., 1995).

6.7

Dimensional change

Amalgam undergoes dimensional changes during the setting period (Espevik, 1977). Powers and Wataha (2017) defined the overall expansion or contraction of amalgam during its initial setting reaction to be dimensional change. During the initial setting reaction, commonly expansion occurs due to matrix formation, whereas contraction is the result of a reaction between the amalgam alloy particles and mercury. Factors such as the proportion of amalgam alloy to mercury and trituration/ condensation procedures can affect dimensional change. Improper mixing force/ time can adversely affect this factor. Expansion during the setting reaction of amalgam is sought after (Powers and Wataha, 2017). Commonly trituration time of the amalgam alloy can be modified to favor the expansion of amalgam, although it is not advised since other vital properties may become compromised (Espevik, 1977). Once an ideal amalgam is placed into the prepared cavity, it would not expand or contract (Powers and Wataha, 2017). Therefore it is preferred for amalgam to have little dimensional change upon insertion. This is mainly because postoperative sensitivity can be the result of both excessive expansion and contraction. Leakage and decay could also occur if the amalgam underwent excessive contraction (Dental Science, 2016), because this would create a gap between the amalgam and cavity wall. Excessive expansion can also cause the amalgam to move out of the prepared cavity and bulge (Powers and Wataha, 2017). Each type of amalgam behaves differently. However, adherence to the manufacturer’s instructions will positively impact dimensional change (Dental Science, 2016). The requirement for dimensional change, ruled by the American Dental Association (ADA) is 20 μm/cm or less. Most modern amalgams have a dimensional change of zero (Powers and Wataha, 2017). The size of the amalgam restoration can affect the life expectancy. Smaller restorations usually undergo less stress, and therefore their life expectancy increases. Class I amalgam can last between 15 and 18 years, whereas Class II amalgam has a life expectancy of 12
15 years. The patient’s oral hygiene and diet largely impact the life expectancy of amalgam restorations (Dental Science, 2016).

6.8

Hardness

Optimal hardness in dental amalgam also contributes to the success of amalgam restorations. Although hardness is defined as the resistance of the materials against

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scratches, hardness tests actually measure the resistance of a material against indentation from a harder material onto its surface. A study was carried out to see how the addition of zinc oxide (ZnO) and aluminum oxide (Al2O3) nanoparticles affected the hardness of dental amalgam. It was found that when the 40% ZnO filler (the highest proportion) was added to the composition, the hardness increased the greatest, to a value of 0.95 GPa on the Vickers scale. It was observed that different annealing temperatures of ZnO affected the hardness of the dental amalgam. More excellent grain size is created when the ZnO nanoparticles undergo a lower annealing temperature. It was observed that a finer grain size gave better material hardness. In addition, it was observed that adding a more significant proportion of Al2O3 filler produced larger increments in hardness than increasing the ZnO filler (Yahya et al., 2013).

6.9

Young’s modulus

Beatty and Pidapartil (1993) established that amalgam has almost 3.5 times greater elastic modulus in tension in comparison to compression. This is greater than the composite resin tested, which had double the elastic modulus in tension in contrast to its compression. This factor was thought to decrease the tensile stress upon the dental amalgam bending. A study carried out by Kumar and Shivrayan (2015) found that high-copper amalgam (named Hi-Aristaloy) possessed the highest value for elastic modulus (17.28 GPa) when compared to other direct core dental biomaterials. In addition, it was found that the γ (Ag3Sn), γ1 (Ag2Hg3), and γ2 (Sn8Hg) phases in amalgam have different values of elastic modulus, where γ was seen to have the highest value (Davies et al., 2010).

6.10

Failure mode

Wang and Darvell (2007) conducted a study on the failure mode of dental biomaterials, including amalgam. When the amalgam was subjected to Hertzian indentation, it failed primarily from plastic deformation as the thickness of amalgam became larger or radial cracking originated from its underside. An amalgam of a range of different thicknesses failed. Also, it was reported that a correlation was found between failure load and thickness of amalgam, excluding extremities.

6.11

Biocompatibility

6.11.1 Toxicology of mercury The composition of dental amalgam includes approximately 50% mercury. This inclusion is very controversial since mercury has been linked to many diseases

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(Bates, 2006). Mercury is a naturally occurring element in the environment, and it can also be found in the Earth’s crust. Though amalgam fillings are a source of mercury exposure (Rathore et al., 2012), Clarkson et al. (2003) reported that the other main exposure sources are fish consumption and vaccines. Fish consumption is a source of methylmercury, whereas vaccines can cause exposure to ethyl mercury. These are both organic forms of mercury. Mercury exposure mainly occurs during the removal or insertion of the dental amalgam filling. After the filling has hardened, the mercury vapor release decreases to a value that is below the acceptable threshold. An individual may undergo delayed hypersensitivity because of the insertion of the amalgam filling, the chance of this occurring can be reduced if mercury hygiene procedures are performed (Rathore et al., 2012). The mercury vapor produced is mostly inhaled, and it may also be absorbed into the blood. Dentists are occupationally exposed while they are placing the restorations (Clarkson et al., 2003). Ucar and Brantley (2011) conducted a study to review the literature regarding the toxicology of mercury from dental amalgam. They concluded that the termination of dental amalgam use could not be justified from the literature. Mercury is continuously released from the inserted amalgam filling (Bates, 2006). Ucar and Brantley (2011) reported that factors such as age, amalgam composition, and individual mastication style affect the release of mercury into the oral cavity. Once the mercury has been released, it is dissolved in saliva and passes into the individual. This can occur if intraoral air containing mercury vapor is inhaled and if the filling particles that wear away during mastication are ingested. In addition, swallowing saliva, which has environmental mercury alongside dissolved particles, can pass mercury into an individual. Table 6.1 displays significant forms of mercury exposure, the path the mercury takes within the body, and treatment options.

Table 6.1 Major forms of mercury exposure, the path the mercury takes within the body, and treatment options. Variable

Mercury vapor

Inorganic divalent mercury

Methylmercury

Ethylmercury

Route of exposure

Inhalation

Oral

Oral (from fish consumption)

Target organ

Central nervous system, peripheral nervous system, kidney

Kidney

Central nervous system

Parenteral (through vaccines) Central nervous system, kidney

(Continued)

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Table 6.1 (Continued) Variable

Mercury vapor

Inorganic divalent mercury

Methylmercury

Ethylmercury

Local clinical signs Lungs

Gastrointestinal tract

Bronchial irritation, pneumonitis ( . 1000 μg/m3 of air) Metallic taste, stomatitis, gingivitis, increased salivation ( . 1000 μg/ m3 of air)

Skin

Metallic taste, stomatitis, gastroenteritis Urticarial, vesication

Systemic clinical signs Kidney

Peripheral nervous system Central nervous system

Appropriate half-life (whole body) (days) Treatment

6.12

Proteinuria ( . 500 μg/m3 Proteinuria, of air) tubular necrosis Acrodynia Peripheral neuropathy ( . 500 μg/m3 of air) Erethism ( . 500 μg/m3 of air)

60

40

Meso-2dimercaptosuccinic acid

Meso-2-3dimercaptosuccinic acid

Tubular necrosis Acrodynia

Paresthesia, ataxia visual and hearing loss

Paresthesia, ataxia visual and hearing loss ( . 200 μg/L of air) 70

20

Chelators not effective

Chelators not effective

Conclusion

Amalgam is a popular biomaterial that has been used successfully for over 150 years, despite the controversial matter of its mercury content. Dental amalgam is mainly used as a direct restorative material. Placement of amalgam restorations is less technique sensitive than resin composites. Therefore dentists find it easy to use with a more predictable outcome. Also, its ability to gain marginal seal after corrosion and to achieve reasonably high compressive strength contributed to its good clinical performance. Despite the risk from mercury exposure, it has kept its popularity in most countries due to its low cost.

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References Acciari, H.A., Guastaldi, A.C., Brett, C.M.A., 2005. Corrosion of the component phases presents in high copper dental amalgams. Application of electrochemical impedance spectroscopy and electrochemical noise analysis. Corros. Sci. 47 (3), 635
647. Agnihotry, A., Fedorowicz, Z., Nasser, M., 2016. Adhesively bonded versus non-bonded amalgam restorations for dental caries. Cochrane Database Syst. Rev. (3), CD007517. Amin, W.M., 2007. Comparative electrochemical investigation of the effect of aging on corrosion of dental amalgam. Quintessence Int. 38 (7), e417
e424. Bates, M.N., 2006. Mercury amalgam dental fillings: an epidemiologic assessment. Int. J. Hyg. Environ. Health 209 (4), 309
316. Beatty, M.W., Pidapartil, R.M.V., 1993. Elastic and fracture properties of dental direct filling materials. Biomaterials 14 (13). Available from: https://www.sciencedirect.com/science/ article/pii/0142961293901925 (accessed 9.03.18.). Beech, D.R., 1982. High copper alloys for dental amalgam. Int. Dent. J. 32 (3), 240
251. Ben-Amar, A., Cardash, H.S., Judes, H., 1995. The sealing of the tooth/amalgam interface by corrosion products. J. Oral Rehabil. 22 (2), 101
104. Bonsor, S.J., 2011. Bonded amalgams and their use in clinical practice. Dent. Update 38 (4), 222
224. 226-8, 230. Burke, F.J.T., 2004. Amalgam to tooth-coloured materials—implications for clinical practice and dental education: governmental restrictions and amalgam-usage survey results. J. Dent. 32 (5), 343
350. Available from: https://www-sciencedirect-com.brad.idm.oclc. org/science/article/pii/S0300571204000430?via%3Dihub. Clarkson, T.W., Magos, L., Myers, G.J., 2003. The toxicology of mercury—current exposures and clinical manifestations. N. Engl. J. Med. 349 (18), 1731
1737. Cox, C.F., 1994. Evaluation and treatment of bacterial microleakage. Am. J. Dent. 7 (5), 293
295. Available from: https://www.ncbi.nlm.nih.gov/pubmed/7986455. Craig, R.G., 1985. Restorative Dental Materials, seventh ed. Mosby. Davies, R.A., Ardalan, S., Mu, W., Tian, K., Farsaikiya, F., Darvell, B.W., et al., 2010. Geometric, electronic and elastic properties of dental silver amalgam γ-(Ag3Sn), γ1(Ag2Hg3), γ2-(Sn8Hg) phases, comparison of experiment and theory. Intermetallics 18 (5), 756
760. Available from: https://doi.org/10.1016/j.intermet.2009.12.004 (accessed 10.03.18.). Dean, J.A., 2016. McDonald and Avery’s Dentistry for the Child and Adolescent, 10th ed. Elsevier. Delta Dental, 2012a. Dental amalgam or resin composite fillings?. ,https://www.deltadentalins.com/oral_health/amalgam.html.. Delta Dental, 2012b. What is plaque?. ,https://www.deltadentalins.com/oral_health/plaque. html.. Dental Science, 2016. Amalgam: properties, uses, toxicity and advantages. ,http://www.dental-science.com/amalgam/.. Donovan, T., Simonsen, R.J., Guertin, G., Tucker, R.V., 2008. Why gold castings are excellent restorations. J. Oper. Dent. 33 (2), 113
115. Available from: https://www.jopdent. com/journal/editorial/openFile.php?takeFile 5 Volume_33__Issue_2.txt. Dubach, P., Caversaccio, M., 2011. Amalgam tattoo. N. Engl. J. Med. 364, e29. Available from: https://doi.org/10.1056/NEJMicm1011669. Espevik, S., 1977. Effect of trituration on dimensional changes of dental amalgam. Acta Odontol. Scand. 35 (5), 251
255.

122

Advanced Dental Biomaterials

FDA, 2017. About dental amalgam fillings. ,https://www.fda.gov/MedicalDevices/ ProductsandMedicalProcedures/DentalProducts/DentalAmalgam/ucm171094.htm#1.. FDI World Dental Federation, 2017. Minamata convention on mercury comes into force in august affecting use of dental amalgam. ,https://www.fdiworlddental.org/news/ 20170606/minamata-convention-on-mercury-comes-into-force-in-august-affecting-useof-dental.. Fong, H., Little, M.J., 2010. Dental Composites with Nano-Scaled Fillers. Nova Science Publishers, Inc. Available from: https://ebookcentral-proquest-com.brad.idm.oclc.org/lib/ bradford/detail.action?docID 5 3018712. Gay, D.D., Cox, R.D., Reinhardt, J.W., 1979. Chewing releases mercury from fillings. Lancet 1 (8123), 985
986. Greener, E.H., 1979. Amalgam—yesterday, today, and tomorrow. Oper. Dent. 4 (1), 24
35. Hasheminezhad, A., Zebarjad, S.M., Sajjadi, S.A., Rahanjam, L., 2012. Effect of copper content on compressive strength and microstructure of dental amalgams. Sci. Res. 4, 155
159. Available from: https://doi.org/10.4236/eng.2012.43020. Helmenstine, A.M., 2017. Amalgam Definition and Uses. ThoughtCo. Available from: https://www.thoughtco.com/amalgam-definition-4142083. Hollenback, G.M., 1969. A consideration of amalgam. J. Calif. Dent. Assoc. 45, 15
18. Hooghan, T.K., Pinizzotto, R.F., Watkins, J.H., Okabe, T., 1996. A study of a low copper dental amalgam by analytical transmission electron microscopy. J. Mater. Res. 11 (10), 2474
2485. Jayanthi, N., Vinod, V., 2013. Comparative evaluation of compressive strength and flexural strength of conventional core materials with nanohybrid composite resin core material an in vitro study. J. Indian Prosthodontic Soc. 13 (3), 281
289. Knosp, H., Holliday, R.J., Corti, C.W., 2003. Gold in dentistry: alloys, uses and performance. Gold Bull. 36 (3), 93
102. Available from: https://link.springer.com/content/pdf/ 10.1007/bf03215496.pdf (accessed 23.02.18.). Lygre, G.B., Haug, K., Skjærven, R., Bjo¨rkman, L., 2016. Prenatal exposure to dental amalgam and pregnancy outcome. Community Dent. Oral Epidemiol. 44 (5), 442
449. Available from: https://doi.org/10.1111/cdoe.12233. Mahler, D.B., Pham, B.V., Adey, J.D., 2009. Corrosion sealing of amalgam restorations in vitro. Oper. Dent. 34 (3), 312
320. Available from: https://doi.org/10.2341/08-94. Manappallil, J.J., 2016. Basic Dental Materials., fourth ed. Jaypee Brothers Medical Publishers (P) Ltd.. Muliyar, S., Shameem, K.A., Thankachan, R.P., Francis, P.G., Jayapalan, C.S., Hafiz, K.A. A., 2014. Microleakage in endodontics. J. Int. Oral Health 6 (6), 99
104. Murray, P.E., Hafez, A.A., Smith, A.J., Cox, C.F., 2002. Bacterial microleakage and pulp inflammation associated with various restorative materials. Dent. Mater. 18 (6), 470
478. Available from: https://www.ncbi.nlm.nih.gov/pubmed/12098576. Nicholson, J.W., 2002. The Chemistry of Medical and Dental Materials, Royal Society of Chemistry.. Royal Society of Chemistry. Available from: https://ebookcentral-proquestcom.brad.idm.oclc.org/lib/bradford/reader.action?docID 5 1185355&query. Okabe, T., Cahn, R.W., 1990. Encyclopedia of Materials Science and Engineering. Pergamon Press, Oxford. Oral Health Centre, 2016b. Problems After Dental Fillings. Boots Web MD.. Available from: https://www.webmd.boots.com/oral-health/guide/problems-dental-fillings. Pereira, T., 2016. Silver amalgam: a clinician’s perspective. J. Dent. Res. 4 (2), 25
30. Available from: http://www.jresdent.org/article.asp?issn 5 2321-4619;year 5 2016; volume 5 4;issue 5 2;spage 5 25;epage 5 30;aulast 5 Pereira.

Dental amalgam

123

Powers, J.M., Wataha, J.C., 2017. Dental Materials, Foundations and Applications., 11th ed. Elsevier. Rathore, M., Singh, A., Pant, V.A., 2012. The dental amalgam toxicity fear: a myth or actuality. Toxicol. Int. 19 (2), 81
88. Available from: https://doi.org/10.4103/09716580.97191. Sarkar, N.K., Eyer, C.S., 1987. The microstructural basis of creep of c1 in dental amalgam. J. Oral Rehabil. 14 (27). Shivrayan, A., Kumar, G., 2015. Comparative study of mechanical properties of direct core build-up materials. Contemp. Clin. Dent. 6 (1), 16. Singh, H., 2015. Remembering Sir G.V. Black. Indian J. Dent. 6 (3), 147
148. Available from: https://doi.org/10.4103/0975-962X.163047. Sivakumar, A., Valiathan, A., 2006. Dental ceramics and ormocer technology—navigating the future. Trends Biomater. Artif. Organs 20 (1), 40
43. Thomas, G.P., 2013. Amalgam-chemical composition, mechanical properties and common applications. AZO Materials. Available from: https://www.azom.com/article.aspx? ArticleID 5 8081. (accessed 19.01.18.). Ucar, Y., Brantley, W.A., 2011. Review article, biocompatibility of dental amalgams. Int. J. Dent. 2011, 1
7. Available from: https://doi.org/10.1155/2011/981595. Vanishree, H.S., Shanthala, B.M., Bobby, W., 2015. The comparative evaluation of fracture resistance and microleakage in bonded amalgam, amalgam, and composite resins in primary molars. Indian J. Dent. Res. 26 (5), 446
450. Wang, Y., Darvell, B., 2007. Failure mode of dental restorative materials under Hertzian indentation. Dent. Mater. 23 (10), 1236
1244. Yahya, N., Puspitasari, P., Latiff, N.R.A., 2013. Hardness improvement of dental amalgam using zinc oxide and aluminium oxide nanoparticles. In: Ochsner, A., da Silva, L., Altenbach, H. (Eds.), Characterization and Development of Biosystems and Biomaterials. Advanced Structured Materials, vol. 29. Springer, pp. 9
32. Yap, A.U.J., Ng, B.L., Blackwood, D.J., 2004. Corrosion behaviour of high copper dental amalgams. J. Oral Rehabil. 31, 595
599. Zimmerli, B., Strub, M., Jeger, F., Stadler, O., Lussi, A., 2010. Composite materials: composition, properties and clinical applications, a literature review. Schweiz Monatsschr Zahnmed 120, 972
979.

Further reading Ajlan, S.A., Aldahmash, A.M., Ashri, N.Y., 2015. Dental pulp stem cells, biology and use for periodontal tissue engineering. Saudi Med. J. 36 (12), 1391
1399. Amalgam-chemical composition, mechanical properties and common applications. AZO Mater. ,https://www.azom.com/article.aspx?ArticleID 5 8081.. American Dental Association, 2012a. Primary tooth development [Image]. ,http://www. mouthhealthy.org/en/az-topics/e/eruption-charts.. American Dental Association, 2012b. Permanent tooth development [Image]. ,http://www. mouthhealthy.org/en/az-topics/e/eruption-charts.. Beek, G.C.V., 1983. Dental Morphology, an Illustrated Guide, second ed. Billing & sons Ltd, Worcester, pp. 45
47. Berkovitz, B.K.B., Holland, G.R., Moxham, B.J., 2018. Oral Anatomy, Histology and Embryology., fifth ed. Elsevier.

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British Orthodontic Society, 2014. Patient information leaflet, tooth transplants. https://www. bos.org.uk/Portals/0/Public/docs/PILs/toothtransplantsmarch14.pdf. Chano, L., Clokie, C.M.L., Yau, D.M., 2001. Autogenous tooth transplantation: an alternative to dental implant placement. J. Can. Dent. Assoc. 67 (2), 92
96. Cohen, A.S., Pogrel, M.A., Shen, T.C., 1995. Transplanting teeth successfully: autografts and allografts that work. J. Am. Dent. Assoc. 126 (4), 481
485. Davenport, T., 2017. What are the most common dental problems?. ,https://www.verywell. com/top-common-dental-problems-1059461.. Dentagama, 2016. Image of the restorative prothesis: inlay, onlay, overlay, pinlay. ,https:// dentagama.com/news/what-is-the-difference-between-inlay-onlay-overlay-and-pinlay. (accessed 25.02.18.). D’Aguino, R., Graziano, A., Laino, G., Papaccio, G., 2008. Dental pulp stem cells: a promising tool for bone regeneration. Stem Cells Rev. 4 (1), 21
26. Harris, E.F., 2016. Odontogenesis. In: Irish, J.D., Scott, G.R. (Eds.), A Companion to Dental Anthropology. John Wiley & Sons, pp. 145
149. Healthline, 2015. Incisors. ,https://www.healthline.com/human-body-maps/incisor.. Hoffman, M., 2009. Picture of the Teeth. WebMD [Image] . Available from: https://www. webmd.com/oral-health/picture-of-the-teeth#1. Hu, J.C.-C., Simmer, J.P., 2001. Dental enamel formation and its impact on clinical dentistry. J. Dent. Educ. 65 (9), 896
905. Huang, G.T.J., 2009. Pulp and dentin tissue engineering and regeneration: current progress. Regener. Med. 4 (5), 697
707. Available from: https://doi.org/10.2217/rme.09.45. Jethmalani, Y.D., Potdar, P.D., 2015. Human dental pulp stem cells: applications in future regenerative medicine. World J. Stem Cells 7 (5), 839
851. Keels and The American Academy of Pediatrics Section on Oral Health, 2014. Management of dental trauma in a primary care setting, the section on oral health. Pediatrics 133 (2), e468. Available from: http://pediatrics.aappublications.org/content/133/2/e466. Kim, N., Cho, S.G., 2013. Clinical applications of mesenchymal stem cells. Korean J. Intern. Med. 28 (4), 387
402. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/ PMC3712145/. Konigsberg, S., Northway, W.M., 1980. Autogenic tooth transplantation: the “state of the art”. Am. J. Orthod. Dentofacial Orthop. 77 (2), 146
162. MacConaill, M.A., 2017. Joint. Encyclopaedia Britannica, Inc. Available from: https://www. britannica.com/science/joint-skeleton/Fibrous-joints#ref470986. Marrelli, M., Tatullo, M., Shakesheff, K.,M., White, L.J., 2015. Dental pulp stem cells: function, isolation and applications in regenerative medicine. J. Tissue Eng. Regener. Med. 9 (11), 1205
1216. Matalova, E., Antonarakis, G.S., Sharpe, P.T., Tucker, A.S., 2005. Cell lineage of primary and secondary enamel knots. Dev. Dyn. 233 (3), 754
759. Mayo Clinic, 2017a. TMJ Disorders. Mayo Clinic. Available from: https://www.mayoclinic. org/diseases-conditions/tmj/symptoms-causes/syc-20350941. Mayo Clinic, 2017b. Mouth cancer. ,https://www.mayoclinic.org/diseases-conditions/ mouth-cancer/symptoms-causes/syc-20350997.. Mayo Clinic, 2017c. Periodontitis. ,https://www.mayoclinic.org/diseases-conditions/periodontitis/symptoms-causes/syc-20354473.. NHS Choices, 2016. Tooth decay. ,https://www.nhs.uk/conditions/tooth-decay/.. National Institute of Dental and Craniofacial Research, 2015. Oral cancer: causes and symptoms & the oral cancer exam. ,https://www.nidcr.nih.gov/OralHealth/Topics/ OralCancer/AfricanAmericanMen/CausesSymptoms.htm..

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Newman, T., 2016. Bad breath (halitosis): causes, diagnosis and treatment. Medical News Today . Available from: https://www.medicalnewstoday.com/articles/166636.php. Norris, J., 2010. Tooth Enamel: Nature’s Crowning Achievement. University of California San Francisco. Available from: https://www.ucsf.edu/news/2010/08/6000/tooth-enamelresearch-combines-science-and-engineering. Open Stax, 2016. Anatomy & physiology. Open Stax CNX. Available from: http://cnx.org/ contents/[email protected]. Oral Health Centre, 2016a. Tooth Enamel Erosion and Restoration. Boots Web MD. Available from: https://www.webmd.boots.com/oral-health/guide/tooth-enamel-erosionrestoration. Oral Health Centre, 2017. Temporomandibular Joint Disorders (TMJ and TMD). Boots Web MD. Available from: https://www.webmd.boots.com/oral-health/guide/temporomandibular-joint-disorders-tmj-tmd?page 5 3. Science Learning Hub, 2011. Rate of digestion. ,https://www.sciencelearn.org.nz/resources/ 1834-rate-of-digestion.. Skaria, S., 2016. Fibrous Joint—Gomphosis [Image]. ,https://www.slideshare.net/ SajanScaria/artrology.. Slootweg, P.J., 2007. Dental Pathology: A Practical Introduction.. Springer. Stay, F., 2014. How your teeth affect your digestive system. ,http://www.totalhealthmagazine.com/Dental-Health/How-Your-Teeth-Affect-Your-Digestive-System.html.. Swindler, D.R., 2002. Primate Dentition: An Introduction to the Teeth of Non-human Primates.. Cambridge University Press, Cambridge. The editors of Encyclopaedia Britannica, 2007. Cementum. Encyclopaedia Britannica, Inc. Available from: https://www.britannica.com/science/cementum. The editors of Encyclopaedia Britannica, 2014. Dentin. Encyclopaedia Britannica, Inc. Available from: https://www.britannica.com/science/dentin. Watson, S., 2017. What is the Function of Tooth Pulp. Very Well. Available from: https:// www.verywell.com/tooth-pulp-dental-terms-1059180. Yau, E.C.K., 2009. Tooth autotransplantation as a treatment option. Dent. Bull. 14 (6), 21
24. Available from: http://www.fmshk.org/database/articles/03db05_2.pdf. Zimmermann, K.A., 2016. Digestive System: Facts, Function & Diseases. Live Science. Available from: https://www.livescience.com/22367-digestive-system.html.