Calcium aluminate based-cements for endodontic application
Lucas da Fonseca Roberti Garcia1*
*Department of Physiology and Pathology, Araraquara School of Dentistry, Univ Estadual Paulista, Araraquara, SP, Brazil
Despite the good clinical performance of MTA cement, some of its negative features must be considered. These deficiencies justify the research and development of new materials with adequate physico-mechanical and biological properties, such as calcium aluminate-based cements. Calcium aluminate based-cements have a great potential in the dental area due to their biological compatibility, mechanical strength and good physico-chemical properties. The aim of this article was to perform a comprehensive literature review regarding the development, composition, properties and application of calcium aluminate-based cements in dental area, mainly in endodontic therapy. The use of this type of cement in dentistry has increased considerably in the last years, as the number of studies evaluating its properties.
Keywords: Mineral Trioxide Aggregate; Calcium Aluminate Cement, Physico-Mechanical Properties; Physico-Chemical; Bio- Activity
Several clinical procedures, such as root and furca perforations sealing, and retrograde filling, need a specific cement to obtain a successful treatment [1,2]. At the early 1990’s, many products, such as calcium hydroxide based-cements, glass ionomer cement and amalgam, were used for such purpose [3,4]. However, none of these materials could adequately address all the required properties of a sealing cement .
In the same period, researchers from the Loma Linda University, (California, USA), including Dr. Mahmoud Torabinejad, developed a cement called mineral trioxide aggregate (MTA), indicated for perforations treatment, root canal retrofilling, apexification and pulp capping, being its patent required in 1995; and beginning its trading under the name of ProRoot MTA (Dentsply Tulsa Dental, Tulsa, OK, USA) [6-8].
Such cement has promoted a great transformation in endodontic therapy, not only because of its excellent physicochemical and biological properties in comparison with the other materials used so far, but for possessing composition very similar to ordinary Portland cement (Type 1), widely used in building area .
Portland cement is basically classified into two sub-types: structural and non-structural [9,10]. The non-structural form has lower amount of clinker, a raw material used in the manufacture of the cement, and gypsum, at a ratio of (74-50%) relative to the structural form (100-75%) [9,10]. The clinker is responsible for a significant increase in the mechanical strength of the cement, as well as becomes it less soluble [9,10].
MTA is basically composed (% by weight) of Portland cement (75.0), plus Bi2O3 (20.0) to confer radiopacity to the cement; and dehydrated CaSO4 (5.0) . Portland cement, in turn, comprises SiO2 (21.2), CaO (68.1), Al2O3 (4.7), MgO (0.48) and Fe2O3 (1.89) . Since it is a hydraulic cement, its setting reaction begins with the addition of water, forming hydrated silica gel [13,14].
MTA cement is commercialized in two different versions, grey and white (Grey and White MTA) . The main difference between the two versions is the highest concentration of iron oxide in the Grey MTA, which, according to several studies, is the main responsible for dental tissues staining when the material is used . According to Bortoluzzi et al. , the lowest concentration of iron oxide in the white version of the cement prevents staining and gingival discoloration due to the non-diffusion of the compound into dental tissues. However, several studies have shown that both versions of the cement promote dental tissues staining [16-18].
When compared to calcium hydroxide based-cement to accidental pulp exposure treatment, the MTA cement is able to maintain a greater tissue integrity , with a thick mineralized barrier formation three times faster [20-22], presence of small inflammatory infiltrate, and a thin layer of pulp tissue necrosis [19,23-25]. It is believed that the mechanism of action of MTA cement is similar to the calcium hydroxide based-cements, promoting inflammation and necrosis of the pulp tissue adjacent to the area of exposure [26,27].
During MTA cement setting process, calcium disilicate and trisilicate react with calcium hydroxide to form calcium silicate hydrated gel, producing a highly alkaline pH . The continuous process of cement hydration leads to calcium ions release, which are diffused through the dentinal tubules, ensuring the proper reparative cement capacity . Among the other advantages of MTA in comparison with other cements used for the same purpose, its lower solubility, higher mechanical strength, better marginal adaptation and better sealing ability are outstanding .
Despite its excellent clinical applicability, some negative features of MTA cement should be taken into consideration, as its poor handling characteristics ; high cost ; high rates of solubility in moist conditions [5,33,34]; presence and release of arsenic [35-37] above safe limits proposed by the ISO 9917-1 standard , and long setting-time [6,7,39,40]. This fact justifies the development of new materials incorporating the appropriate bioactivity of MTA, however, without its undesirable properties .
Calcium aluminate-based cements
Calcium aluminates are the main components of calcium aluminate cements. Since its development in the early 1900s, calcium aluminate-based cement has been the subject of several studies, and due to its excellent properties has been used for different purposes [41-43]. Its main application is still in building, mainly in the manufacture of concrete as Portland cement, however, for extreme environments where greater resistance to heat and abrasion is required; and a faster setting reaction [41-43]. However, in the last decade, its application has been extended to other areas, such as Dentistry, as dental cements [41-43]. Just as the calcium silicate based-cements, calcium aluminate based-cements are considered hydraulic cements, since its setting reaction starts from the mixture of the powder with water . An aluminate cement is basically composed of Al2O3 (43%); CaO (19%); H2O (15%); ZrO2 (19%), besides other components, such as magnesium, silicon, iron, titanium and alkali oxides, in proportions lower than 10% .Generally, such cements are composed of three main phases responsible for the hydraulic setting process: anhydrous phase CA (CaO.Al2O3), comprising from 40 to 70% of the product; phase CA2 (CaO.2Al2O3), which is the second in proportion (>25%), and phase C12A7 (12CaO.7Al2O3), with approximately 10% [42,44].The hydration reaction of calcium aluminate based-cements has three distinct phases: ions dissolution, nucleation and precipitation of hydrated phases . When the cement particles come in contact with water, anhydrous phases of calcium aluminate are dissociated, releasing calcium and hydroxyl ions into the aqueous medium . The dissolution process continues
until the concentration of Ca and hydroxyl ions is saturated, initiating their precipitation in the form of calciumaluminate hydrates, by a mechanism known as nucleation and growth . Such precipitation decreases the Ca and hydroxyl ions levels below the saturation point, promoting the anhydrous phase formation. The process continues until most, or all of the anhydrous phase had been reacted [42,43].
In general, the chemical reaction responsible for the calcium aluminate based-cements formation can be described as follows:
CaCO3 + Al2O3 = Ca(AlO2)2 + CO2
Currently some calcium aluminate based-materials for dental application are available on the market: Ceramir Crown & Bridge (Doxa Dental AB, Uppsala, Sweden) a hybrid luting cement, composed from the mixture of a calcium aluminate based-cement and glass ionomer , Doxadent (Doxa Dental AB), a direct restorative cement , and a cement similar to MTA’s clinical applications, called EndoBinder (Binderware, São Carlos, SP, Brazil) .
EndoBinder basically consists of (% by weight) Al2O3 (≥68.0), CaO (≤31.0), SiO2 (0.3-0.8), MgO (0.4-0.5) and Fe2O3 (<0.3), and it is produced by the process of Al2O3 e CaCO3 calcination, at high temperatures ranging from 1315 to 1425°C, a viable method to produce materials with a more uniform composition . The calcium aluminate formed is cooled, and then ground up until obtaining a powder with proper particle size . At the end of the sintering process, a radiopacifying agent is added to ensure adequate radiopacity to the cement , according to the ISO 6876-7.8 standard .
The physico-mechanical properties of calcium aluminate based-cements have been described by several studies [42,50]. Authors reported important mechanical properties, such as microhardness, compressive, flexural and diametral tensile strength, superior to MTA cement, making the material a reliable option for endodontic therapy [42,50].
Garcia et al.  performed a mechanical and microstructural characterization of a novel calcium aluminate based-cement for endodontic application, by compressive and diametral tensile strength tests, Vickers microhardness test and Scanning Electron Microscopy (SEM) analysis, in comparison to both versions of MTA cement (white and grey). The calcium aluminate based-cement presented higher compressive strength than white MTA after 7 and 21 days post-setting; and higher diametral tensile strength values than grey MTA (7 and 21 days), and white MTA at 21 days post-setting. As regards Vickers microhardness, it wasn’t possible to obtain adequate values at 24 hours post-setting period, due to the low surface mechanical strength presented by the different cements in this initial period of evaluation. However, the calcium aluminate based-cement presented higher Vickers microhardness values than white MTA at 7 and 21 days, and grey MTA at 21 days post-setting.
In the same study , the SEM evaluation demonstrated several topographical accidents that formed the microstructure of the cements, such as pores and asymmetrical crystalline formation, channels and depressions. The calcium aluminate based-cement presented a crystallized structure, with aggregated particles of globular shape, homogeneous size and uniform distribution. Furthermore, the cement presented a more regular surface,with less pronounced depressions than both versions of
MTA cement (Figures 1 and 2).
Figure 1. Representative micrographs of calcium aluminate based-cement samples 24 hours post-setting. (A) Note the cement topography, with a more regular surface than MTA cement (indication). B) Globular-shaped aggregated particles with homogeneous size and uniform distribution (arrow).
Figure 2. Representative micrographs of MTA cement samples 24 hours post-setting. (A) Note the microstructure of the cement, with irregular-shaped and asymmetric particles (arrow). (B) Aggregated particles which compose the crystalline phase of the cement, with asymmetric sizes and shapes (indication).
After MTA cement handling, bismuth oxide (radiopacifyingagent) becomes part of the hydrated phase of the cement,forming a structure composed of hydrated bismuth calcium silicate, ettringite and monosulphate . Suchcompounds are released into the medium with the calcium hydroxide formed from the calcium silicate hydration process,decreasing the precipitation of hydrated calcium hydroxide, thus, compromising the physico-mechanical propertiesof MTA [52,53].
Regarding to calcium ions release, when the different cements were compared in the same period of analysis, no significant difference among them was observed. The hydration process of calcium aluminate based-cements results in calcium aluminate and aluminum hydroxide hydrates [42,44], thus, calcium ions release could beattributed to the decomposition of calcium aluminate hydrate [42,44].
Cite this article: Garcia L F R. Calcium Aluminate Based-Cements for Endodontic Application. J J Dent Res. 2014, 1(2): 008.