NANO-WHITE MTA: A REVIEW.

stabilising component of NWMTA. the effect of attack the

Due to the widespread clinical application of MTA, it should possess sufficient radiopacity to be distinct from the adjacent anatomic structures such as bone and tooth. Without a radiopacifier, tricalcium silicate cements have intrinsic radiopacity values ranging from 0.86-2.02 mm aluminium (Al), whereas according to the international standards for dental root canal sealing materials ISO 6876, values lower than 3 mm Al are not recommended. Therefore, a radiopacifier has to be added to tricalcium silicate-based materials. Bismuth oxide is a radiopacifier agent added to MTA in a 1:4 (wt%) ratio, which provides a radiopacity higher than 3 mm Al as suggested by ISO 6876. ProRoot White MTA (WMTA) (Dentsply Tulsa Dental Specialties, Tulsa, OK) has been reported to have a radiopacity ranging from 5.34-6.92 mm Al (Saghiri M.A et al, 2015a).
Currently, MTA is marketed in 2 forms: gray (GMTA) and white (WMTA). MTA was introduced in gray, but because of the discoloration potential of GMTA, WMTA was developed and marketed since 2002. Investigations showed that lower amounts of iron, aluminium, and magnesium are present in WMTA than in GMTA. In endodontics, WMTA (ProRoot, Dentsply Tulsa Dental, Tulsa, OK, USA) has several advantages over other materials used for root repair, including biocompatibility, good sealing ability and antibacterial properties (Saghiri M.A. et al, 2012). GMTA consists of dicalcium and tricalcium silicate and bismuth oxide, while WMTA is primarily composed of tricalcium silicate and bismuth oxide. The WMTA lacks the aluminoferrite phase that imparts the grey colour to GMTA (Parirokh M et al, 2010a). A qualitative surface analysis of WMTA and GMTA showed that the crystal size of GMTA is approximately 8 times larger than that of WMTA. Map images show that oxygen is distributed throughout both crystalline and amorphous phases of GMTA and WMTA, and therefore all of the elements are present in their oxide form (Parirokh M et al, 2010a).
Along with the advantages of biocompatibility and good sealing ability, the drawbacks of MTA include discoloration potential presence of toxic elements in the material composition, difficult handling characteristics, long setting time, high material cost, the absence of a known solvent for this material and the difficulty of its removal after curing.
Malamed reported that in most clinical cases MTA is applied in an inflamed area, which is considered to be an acidic environment. This acidic environment can cause acid corrosion in which CH, C-S-H, and the calcium sulfoaluminate phases decompose and produce porosities. Moreover, MTA becomes considerably porous after its setting time at the MTA-dental interfaces, which results in or causes microleakage (Saghiri M.A. et al, 2014).
Inspite of having a better understanding of the material chemistry and recent improvements in physical properties, no material has yet been found that is ideal for any dental application. Attempts have been made to improve the properties of MTA by incorporating materials into its structure by modifying its composition by adding 1% methylcellulose and 2%, 3%, or 5% calcium chloride (CaCl 2 ) to improve the handling and to lower the setting time of MTA. However, the physical and chemical properties are often affected adversely. Attempts were tried to reduce the particle size of this cement to nano-scales, and adding additives to this composition resulting in a new version of WMTA called the nano-WMTA (NWMTA) (Saghiri M.A et al, 2015b).

Nano-WMTA (NWMTA):-
Nanotechnology is the science of manipulating matter measured in the billionths of meters or nanometer, roughly the size of 2 or 3 atoms 10 − 9 m = 1 nm NWMTA is calcium silicate-based cement composed of nano-sized particles ranging between 40 and 100 nm that provides a surface area about 4 times higher than that of WMTA. An increase in the surface area of the material obtained by nano-sizing the powder particles would (Saghiri M.A et al,2015b) 1. promote the rise in pH 2. increase the calcium ion release in solution.
3. enhanced the hydration phase of cement particles. 4. better interlocking of powder particles to create a cement structure with better integrity.

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It is seen that in mixing and working phase as more particles are involved in reaction with the hydration phase a less porous set material is formed. The difference between the constituent elements of NWMTA and WMTA was related to the presence of strontium with a uniform distribution on the surface. In NWMTA, a constant and uniform nonporous grey image was observed, which might be ascribed to a more proper hydration of products and good interlocking of the crystal compounds of WMTA (Saghiri M.A et al, 2015b). Komabayashi and Spangberg showed that MTA's particle size has a great impact on the extent of particles penetrating the dentinal tubules (Komabayeshi T. et al 2008).   Strontium carbonate:-By preventing particle agglomeration, it helps uniform distribution of ingredients. It also reduces setting time and increases the bioactivity of the cement (Saghiri et al, 2012). An energy-dispersive spectroscopy dot map shows that WMTA doesn't have strontium in it's composition. Studies have shown that as a biodegradable material, strontiumdoped calcium polyphosphate (SCPP) can release Ca 2+ and Sr 2+ as the main degradation products. The release of calcium ion can be important in calcium-mediated signalling during angiogenesis. Strontium salts improve the bioactivity of bone substitute materials ( Tricalcium aluminate in Portland cement causes an -interstitial phase‖ which is beneficial due to facilitating the formation of the desired silicate phases. It reacts most strongly with water of all the calcium aluminates, and it is also the most reactive component of the Portland clinker phases. Its hydration to phases of the form Ca 2 AlO 3 (OH).nH 2 O leads to the phenomenon of -flash set‖ (instantaneous set). Due to this advantage, it was reported that tricalcium aluminate is associated with adverse effects that can reduce the durability of concrete by following effects: Heat release: A large amount of heat is generated during the phenomenon of -flash set‖ which can cause spontaneous overheating in large masses of concrete. In order to overcome this negative effect first, tricalcium aluminate levels are reduced and secondly small amount of calcium sulfate (typically 4-8%) is added to the cement. Sulfate ions in solution form an insoluble layer of ettringite (3CaO.Al 2 O 3 .3CaSO 4 .32H 2 O) on the surface of the aluminate crystals, passivize them and slightly contribute to the strength of the cement.

Sulfate attack:
The elimination of the sulfate corrosion may be ensured by using low tricalcium sulfate content cement or by adding zeolite to the cement. Zeolite is composed of crystalline hydrated aluminosilicate of alkaline metals and metals of alkaline soils (Ca, K, Na, Mg). Adding zeolite could decrease the volume of tricalcium aluminate in the cement. The amount of zeolite in NWMTA is 2% to prevent its adverse effect on the compressive strength of cement (Saghiri et al, 2014). The bismuth oxide particles can affect the hydration process of MTA by their distribution inside the hydrated cement material. Hence, the nano-size bismuth oxide particles, because of their higher surface area, can affect the hydrated structure of cement in a way that the orientation of ettringite crystals can become more uniform, resulting in a dense and solid cement structure. Therefore, nano bismuth oxide enhanced the physical properties of NWMTA.
A study revealed that NWMTA had radiopacity values greater than 3-mm thickness of Al. This implies that they can be candidates for root-end filling materials in terms of their radiopacity. To achieve a material with enhanced physical properties and acceptable radiopacity the ideal percentage for the addition of nano-size bismuth oxide was at least 20% (wt%) (Saghiri M.A. et al, 2015a).

Properties of NWMTA:-Setting Time:-
Many additives such as CaCl 2 , polymers, plasticizers or other materials have been incorporated to improve setting time. However, they increase toxicity and compromise physical properties. Allen T. confirmed that the surface area of powder is related directly to the setting time of the cement base material (Allen T, 1997).
Final setting time of WMTA is more than 3 h while the initial setting time has been reported to be approximately 40 min (Islam I. et al, 2006) which is not desirable when WMTA is used as a root-end filling material. The initial setting time of NWMTA was approximately 6 min. The difference in initial setting time can be due to the increased surface area of NWMTA than WMTA thereby, NWMTA reacts more rapidly with water and prevents washout of the cement plug before final setting ( Hydrated MTA has a weaker microstructure than Portland cement. Additives such as bismuth oxide do not contribute to hydration reactions and ultimately increase the porosity of the cement and decrease the compressive strength of the cement (Coomaraswamy K.S. et al, 2007). In the same manner, tricalcium silicate cement (CSC), with properties similar to those of MTA, showed higher physical properties than CSC with 10% bismuth oxide. It was pointed out that dicalcium silicate in comparison with tricalcium silicate needs more time for hydration which makes MTA require a wet environment in order to gain its optimal physical properties. The effective barrier thickness of this cement is 4mm (Saghiri M.A et al, 2015a). pH:-Calcium ion content in NWMTA is 80% which is higher than that in WMTA (50-75%). On the other hand, the increase in total surface of NWMTA and improvement in hydration of cement particles leads to an increase in calcium hydroxide formation. Both these phenomenon enhance calcium release from the material and elevate the pH value in the peripheral environment of NWMTA (Saghiri et al 2015 b).
Saghiri et al confirmed that NWMTA containing zeolite had better acidic resistance in comparison with BA or WMTA. Two studies done by Saghiri et al reported that WMTA showed more defects, especially in low pH environments, while NWMTA demonstrated better nucleation of calcium-silicate-hydrated needles (Saghiri M.A. et al, 2013c). Therefore, NWMTA had less porosity, better hydration and good interlocking crystals than WMTA even in an acidic pH. NWMTA kept its high strength in acidic environment compared to WMTA and BA. An acidic environment has an adverse effect on the microhardness of WMTA cement (Namazikhah M.S. et al, 2008). WMTA is affected to a large extent than NWMTA, due to the greater porosity of WMTA compared to NWMTA. The greater porosity accelerates acid penetration into the surface texture and decrease surface microhardness. Greater porosity can lead to increased crack propagation. NWMTA has two times less porosity due 1569 to it's nanosized particles, leading to reduced crack propagation and acid penetration into the surface texture compared with WMTA (Saghiri M.A. et al, 2012).

Microhardness:-
Microhardness has an inverse relationship with porosity. WMTA with a higher microhardness value has less porosity. For an impermeable root end filling material low porosity is highly desirable. ( At a pH value of 7.4 the microhardness of NWMTA and WMTA were 81.53 ± 2.62 and 51.31 ± 1.14 respectively and in the acidic environment pH= 4.4 the microhardness of both cements decreased to 54.59 ± 1.08 for NWMTA and 16.10 ± 2.84 for WMTA. In acidic environments NWMTA has some cement loss but it has higher microhardness in comparison to WMTA (Saghiri M. A. et al, 2012). Microhardness, initial setting time and surface porosity comparison of both WMTA and NWMTA revealed significant differences. This might be due to the influence of specific surface area of powders on the exothermic reaction of the cement during hydration, which may affect physical properties of calcium silicate cements to some extent ( Therefore, NWMTA showed better compressive strength value in acidic environments compared to WMTA and BA due to less porosity, better hydration, and good interlocking crystals. Therefore, pH alterations can jeopardize the structure of WMTA that may increase/decrease compressive strength (Hashem A.A.R. et al, 2012). Soluability:-NWMTA had less solubility than WMTA in low pH. This is due to the characteristics of NWMTA particle form, size, and different composition of this cement. The uniform distribution of particles, and the increased surface area observed in NWMTA, results in less porosity and better interlocking of particles in the set NWMTA and reduced solubility of the cement for a longer time period (Saghiri M.A. et al, 2014). Zeolite, beside anticorrosive action against sulfate, can be regarded as a stabilizing agent for cements. Zeolitic cement can replace Portland cement in many applications with the advantage of higher resistance to acidic and sulfate attack, which could reduce the solubility of the applied cement (Juengsuwattananon K. et al, 2010).
Thermocycling:-Thermocycling process has been introduced as an artificial aging methodology which is utilized for evaluating the influence of thermal stresses on the bond strength of dental materials. By the means of these thermal cycles in in vitro situation, the impact of different coefficients of thermal expansion for these restorative and repair materials can be investigated (Saghiri M.A. et al, 2013b). Investigators have shown that thermocycling regimen comprising a minimum of 500 cycles in water between 5ºC and 55ºC is an appropriate artificial aging test (The ISO TR1994). ). This type of bond failure can be attributed to the nanoscale particles which are responsible for intimate contact of powder particles that resulted in interlocked pattern and more possibility for cohesive mode of failure. Therefore, thermocycling can adversely affect the push-out bond strength of root-end filling materials. According to SEM images the structural changes made by the thermal stresses from thermocycling process can produce undesirable damages that can decline the dislodgement resistance of these materials. (Saghiri M.A., 2013b).
Biocompactibility:-Bioactivity is highly desirable for an endodontic root-end filling materials. Recent studies have confirmed that incorporation of a small amount of strontium to bone cements can create or increase bioactivity and bioconductivity properties. (Peng et al, 2010, Yang et al, 2011. Therefore, adding small amounts of strontium (<5%) to WMTA did not adversely affect its physical properties. NWMTA is very biocompactible. As compared to WMTA, NWMTA had better inflammatory response and bone regeneration properties. However, a better bone healing response was seen when nano-tricalcium aluminate was added (Saghiri M.A, et al 2012).

Conclusion:-
Reducing the size of particles and their uniform distribution will better accommodate and fill small gap spaces within the product is reported to play an important role in shortening the setting time and increasing the microhardness even at low pH. NWMTA exhibits higher compressive strength in all pH conditions and can be advocated as an excellent root end filling material, especially when the applied material might be exposed to acidic environments (Saghiri M.A. et al 2013a). Thermal changes can significantly increase the cohesive failure of NWMTA and WMTA which might have negative impact on the sealing characteristics of these cements, especially in the presence of dislodgement forces such as occlusal forces in clinical situation (Saghiri M.A, 2013b).