Manufacturing Materials For Dental Amalgam: Properties And Comparison

Description of Dental Amalgam

Discuss about the Manufacturing Materials for Dental Amalgam.

Dental amalgam is a component, used in dental filling. It is a mixture of four metal components; silver, mercury, tin and copper; these kinds of mixture of metals are known as metal alloy. Mercury is the chief component used in preparing dental amalgam, which makes about 50 % of the product and it is used for binding metals together, and thereby providing a strong, durable and hard filling. It is used for filling dental cavities, caused due to tooth decay. Within the alloy, mercury is 50% along with 22-32 % silver, approximately 14 % tin and around 8 % copper (Kopperud et al. 2012). The component has a 150 year proven track record as being one of the safest, least expensive a durable component used for filling a dental cavity. There are a number of alternatives of dental amalgam, but these are expensive enough to be afforded by middle class people. The dentists use amalgam as it is easier to work with amalgams to fill the teeth cavity. There are two main product types of amalgam, known as low and high copper amalgams (Correa et al. 2012). High copper amalgams are used for improving mechanical properties, corrosion, resistance and marginal integrity of the product. The high copper alloy also has 2 types, admixed alloy and single composition alloy. Amalgam is tolerant to a wide range of clinical placement conditions and moderately tolerant to the moisture presence at the time of placement. Mercury can react with other metals to form a plastic mass, packed into a prepared cavity in a tooth. It becomes hard and stronger than any other dental cement. Thus, it is a better choice, compared to the other alternatives.

Dental amalgam is used in dental filling, as a permanent filling material. Dental cavity is classified into 6 classes, including Class I to class VI.  Based on the type of dental cavity, the amalgam application properties are determined. Dental amalgam is used as a permanent filling material, for class I, where “carious lesions is on the occlusal areas or buccal areas or lingua pits on the tooth surface”; and class II dental cavities, where “carious lesion is on the posterior occlusal and inter-proximal surfaces of the tooth”. It can also be applied in class V cavities, where esthetics is not important (Peng et al. 2012).

The application properties requirements of the dental amalgam include microleakage, which occurs due to penetration of fluids or debris surrounding the margins, leading to secondary caries. As amalgam has a self-sealing property, it is applied for preventing microleakage. Creep is defined as “time dependent plastic deformation under constant stress”. ADA specification depicts that best creep should be less than 3 %. In high Cu-amalgam, creep is 04- 1%, whereas, low Cu-amalgam is 0.8- 8% (Park and Zheng 2012). Amalgam is the strongest product in compression and weaker product in shear and tension, thus, the cavity design should be prepared in such a way that can maximize the compression forces and minimize the shear forces or tension. Another property requirement for its application is dimensional change that includes contraction and expansion. Based on the ADA specification, the component should not contract or expand more than 20 u/cm between 5 minutes to 24 hours after, the triturating starts. Modern amalgam shows contraction, whereas older amalgams show expansion. Contraction results in micro leakage and secondary caries (Syversen and Kaurv 2012). On the other hand, expansion enhances creep, microleakae, restoration out of cavity and corrosion.

Property Requirements for Application of Dental Amalgam

The assessment of a wide range of candidate materials is as follows:

Type*	Ag	Sn	Cu	An	Other
TL	70.9	25.8	2.4	1.0	___
TS	72.0	26.0	1.5	0.5	___
HCS	41.0-61.0	24.0-30.5	13.0-28.3	0-0.5	In 3.4
HCAd	62.0-69.7	15.1-18.6	12.0-22.7	0-0.9	In 10
HCL	43.0	29.0	25.0	0.3	Hg 2.7

  *TL = traditional lathe cut; TS = traditional spherical; HCS = high-copper spherical;

HCAd = high copper admixed; HCL = high-copper lathe cut; GA = alloy for gallium amalgam. 

The given table represents the compositions of amalgam alloys according Weight percentage (Bahari et al., 2016).

Metal	Limits prior to 1986 (‘conventional’ alloys)	Current limits
Silver	65 (min)	40 (min)
Tin	29 (max)	32 (max)
Copper	6 (max)	30 (max)
Zinc	2 (max)	2 (max)
Mercury	3 (max)	3 (max)

The given table represents the compositional limits of the alloys of dental amalgam as specified in 1SO 1559 (Fuks 2015).

Property	Required value
Dimensional change (%) Compressive strength (MPa)	−0.1 to +0.2
at 1 hour	50 (minimum)
at 24 hours	300 (minimum)
Creep (%)	3.0 (maximum)

The given table represents the physical and mechanical properties of dental amalgam as specified in ISO 1559 (Ülker et al. 2016).  

The comparison will be made between high copper alloy and low copper alloy. In high copper alloy, for improving the mechanical properties, marginal integrity and resistivity towards corrosion, the addition of high copper is done (Jaber 2014).  It contains two kinds of alloy namely single-composition alloy and admixed alloy (Bundy and Gettleman 2013). In single-composition alloy, the addition of only one one power is carried out for mixing it with mercury (Ülker et al. 2016). On the other hand, in case of admixed alloy, the addition of two powders is carried out with the different contents are blended with mercury (Fuks 2015). The low-copper dental amalgam were utilized in the past and in the present time, they have been completely substituted by the high-copper dental amalgams due to the reason that a number of unique properties has been exhibited by high-copper dental amalgam such as less creep, less corrosion and as well as discolouration, high strength and in addition to all of these, a minimum sensitivity to handle the variables which leads to the production of clinical results of a long term (Rathore, Singh and Pant 2012). In comparison to the low-copper dental amalgam, the restorations of high copper amalgam show smaller number of prevalence of marginal failure (Ülker et al. 2016). It consists 40-60% silver, 13-30% copper, 27-30% tin and only 1% zinc. It also includes palladium and Indium. Expansion is enabled by silver and it also leads to the enhancement in strength as well as resistivity towards corrosion (Jaber 2014).  Contraction is enables by tin whereas the improvement of strength is carried out by Copper and in addition it also reduces corrosion as well as tarnishing and minimizes creep (Ülker et al. 2016). The role of zinc is to minimize the oxidation of additional alloys that are present in the metal (Rathore, Singh and Pant 2012). There is strong evidence that the amalgams that contain zinc possess a longer life in comparison to the amalgams that do not contain zinc. The reduction of creep as well as increase in the strength is carried out by Indium whereas the reduction in the rate of corrosion as well as discolouration is carried out by palladium (Ülker et al. 2016).

Comparison of Narrowed Selection of Candidate Materials

High-copper dental amalgam is the mostly preferred because of its number of exceptional characteristics such is it is less corrosive and reduces creep (Black and Hastings 2016). The durability of amalgam restoration is increased is high copper dental amalgam (Fuks 2015).  There are different types of high copper amalgams such as:

Single-composition spherical- examples: Sybraloy, Tytin and valiant.

Single-composition Lathe-cut- examples: Jentalloy and Epoque. 

Admixture of Lathe cut with Single- Composition Spherical – example: Valiant Phd.

Admixture of Lathe-cut with spherical silver-copper eutectic particles- examples: contour, original D, Dispersalloy, Indisperse.

As compared to the low-copper amalgams, high copper amalgam’s restoration exhibit lesser occurrence of marginal failure (Ülker et al. 2016).  Higher copper is added in the alloy in order to improve the marginal integrity, mechanical properties and resistance towards oxidization (Bundy and Gettleman 2013). Now days, it is widely because of its unique properties and clinical performance. In several clinical studies, a few of the high-copper dental amalgams having a single composition, exhibit the maximum clinical durability (Bundy and Gettleman 2013).

For preparing dental amalgams, liquid mercury is mixed with powdered alloy, consisting silver and tin. The milling or lathe cutting of cast ingot of the silver-tin alloy is done for getting the powdered alloy. For this, an annealed ingot of silver-tin alloy is put into the milling machine or a lathe and then the product is put in a cutting tool. In this way, the alloy particles are shaped irregularly (Brownawell et al. 2005). However, in an alternative way, through the mechanical or hand condensation, the liquid alloy can be condensed, which can give spherical particles in the alloy. The mixture o spherical and lathe-cut particles are used for amalgam preparation. In the next step, the alloy is mixed with mercury in the process known as trituration. Nowadays, vibratory mixers are used for preparing the unmixed amalgam, in two chambers of tiny capsule. In this capsule, the thin membrane, that separates the alloy power and liquid mercury is destroyed. Then, the capsule is placed in mechanical mixer arm and then the mixer is vibrated for specific time for thorough mixing of the liquid and the powder. From the capsule, the mixed amalgam, having a plastic consistency is done into the cavity. At the time of trituration process, the surface layer of the silver-tin alloy suspends in the liquid mercury, causing a new phase to be formed (Agarwal et al. 2012). As a result of new solid phase formation, the plastic amalgam paste solidifies. The final product is supplied as bulk product, alloy and mercury in disposal capsule and preweighed alloy in a form of tablet/pellet along with sachet of mercury.

Selection of Preferred Material with Explanation

While considering the environmental impact of using dental amalgam in dental filling, two aspects should be considered. If the mercury released by the use of amalgams impose risk to the environment and what are those impacts. In dental amalgam, the elemental mercury is being used. Mercury is used in different industrial practices including dental amalgam. Mercury from the dental amalgam ends up in atmosphere, soil, water surfaces and ground water via several routes including emission to soil and air, waste water discharge from dental practices, and cremation of burial of people, who had dental amalgams (Bundy and Gettleman 2013). The wastewater discharged from the dental clinics increases the inorganic mercury concentration in water bodies, which imposes a risk for the aquatic organisms. However, the key concern with mmercury emission in water is associated with the potential of methyl mercury, an organic form of mercury, which gets accumulated in organisms, which is known as biomagnifications. Methylmercury is taken in a faster rate by the acquatic animals, like fishes, thereby increasing the high risk for the fish-eating animals including humans (Agarwal et al. 2012).

The time for conversion to methylmercury is dependent upon the ecosystem. In addition, mercury poising from dental amalgam is also a major concern of using dental amalgams in dentistry. Dental amalgam contains mercury as the main component, which is a potent neurotoxin and it is getting biomoluculated in human body as well as in environment. It has been revealed that dentists are the third highest user of mercury. It cannot be filtered thoroughly from wastewater plants and thus it contaminates fresh water supply and expresses its toxic effects. It has been reviewed that the health of huge number of people has been compromised by the toxic effects of mercury containing dental amalgam. However, it has been claimed by Agarwal et al. (2012) that the mercury from dental amalgam is causing negative dental impact due to inefficient or poor management of the mercury related wastes. Proper collection and disposal mercury containing solid waste can prevent the retention of mercury in wastewater and release of mercury vapor during combustion. Additionally, amalgam separating devices can be used for reducing the amount of amalgam-contaminated water, which is released from dental clinics.

The microstructure of commercial dental amalgam is studies by X-ray diffraction, scanning electron microscopy, optical metallography and X-ray dispersive spectroscopy. In the mixture of alloy powder and mercury, during trituration, silver-mercury and tin-mercury compounds are prepared. The silver-tin compound is known as gamma phase and the silver-mercury compound is known as gamma one phase, whereas the tin-mercury phase is known as gamma two phase. The set amalgam includes non-reacted gamma particles, that is surrounded by the products including gamma one and gamma two particles (Ülker et al. 2016). In case of high copper alloys, copper contain is more than 6%. The high copper alloys has better marginal integrity, corrosion resistant and consist of improved mechanical properties, as the weakest gamma two phase is removed from the high copper amalgam alloy mixture. In case of set amalgam, the Cu6Sn5 remains as a ‘halo’, which surrounds the Ag-Cu particles and the final set material consists of two parts, a core, which includes the unreacted gamma phase and unreacted Ag-Cu surrounded by the ‘halo’ of Cu6Sn5; and a matrix containing the gamma one phase (Fuks 2015)..

Manufacturing Process Route

The dental amalgam contains the mixture of mercury and alloy mix, containing silver and tin. The strength and effectiveness of the amalgam is dependent upon its composition and the amount of each material in the alloy. The key part in the dental amalgam component is mercury, which consist of 50% weightage of the product (Rathore, Singh and Pant 2012). The product is produced by mixing the allow powder or alloy mix with liquid mercury in specific mixer machine. The high content of copper enhances the strength of the alloy mix. The manufacturing process is done in automated machines, where in two chambers of a capsule is filled with the alloy mix and mercury and the vibration of the machine causes the reaction of mercury with the alloy particles and conversion of the reactant particles into products. Living in room temperature hardens the mixture, giving a solid structure.

Reference List

Agarwal, B., Singh, S., Bhansali, S. and Agarwal, S., 2012. Waste management in dental office. Indian Journal of Community Medicine, 37(3), p.201.

Bahari, M., Oskoee, P.A., Oskoee, S.S., Pouralibaba, F. and Ahari, A.M., 2016. Mercury release of amalgams with various silver contents after exposure to bleaching agent. Journal of Dental Research, Dental Clinics, Dental Prospects, 10(2), p.118.

Black, J. and Hastings, G., 2016. d Dental Restoration Materials. InHandbook of Biomaterial Properties (pp. 191-203). Springer New York.

Brownawell, A.M., Berent, S., Brent, R.L., Bruckner, J.V., Doull, J., Gershwin, E.M., Hood, R.D., Matanoski, G.M., Rubin, R., Weiss, B. and Karol, M.H., 2005. The potential adverse health effects of dental amalgam. Toxicological reviews, 24(1), pp.1-10.

Bundy, K.J. and Gettleman, L., 2013, October. AC Electrochemical Impedance Studies of the Corrosion Behavior of Dental Amalgam. InBiomedical Engineering IV: Recent Developments: Proceeding of the Fourth Southern Biomedical Engineering Conference (p. 112). Elsevier.

Correa, M.B., Peres, M.A., Peres, K.G., Horta, B.L., Barros, A.D. and Demarco, F.F., 2012. Amalgam or composite resin? Factors influencing the choice of restorative material. Journal of Dentistry, 40(9), pp.703-710.

Fuks, A.B., 2015. The Use of Amalgam in Pediatric Dentistry: New Insights and Reappraising the Tradition. Pediatric dentistry, 37(2), pp.125-132.

Jaber, H.H., 2014. The Effect of Addmixed Ti on Corrosion Resistance of High Copper Dental Amalgam. J. of Babylon University, Engineering Sciences, 22(2), pp.413-421.

Kopperud, S.E., Tveit, A.B., Gaarden, T., Sandvik, L. and Espelid, I., 2012. Longevity of posterior dental restorations and reasons for failure. European journal of oral sciences, 120(6), pp.539-548.

Park, J.D. and Zheng, W., 2012. Human exposure and health effects of inorganic and elemental mercury. Journal of Preventive Medicine and Public Health, 45(6), pp.344-352.

Peng, J.Y., Botelho, M.G. and Matinlinna, J.P., 2012. Silver compounds used in dentistry for caries management: a review. Journal of dentistry, 40(7), pp.531-541.

Rathore, M., Singh, A. and Pant, V.A., 2012. The dental amalgam toxicity fear: a myth or actuality. Toxicology international, 19(2), p.81.

Syversen, T. and Kaur, P., 2012. The toxicology of mercury and its compounds. Journal of Trace Elements in Medicine and Biology, 26(4), pp.215-226.

Ülker, M., Malkoç, S., Ülker, H.E., Yalçin, M. and Malkoç, M., 2016. Orthodontic bonding to high-copper amalgam with different adhesive cements. Journal of Restorative Dentistry, 4(1), p.7.