Calcium silicate materials in endodontics

Modifications to decrease setting time and improve handling properties

The setting time of MTA, at 2 hours and 45 minutes, is longer than acceptable for most restorative clinical procedures.28 In order to decrease setting time, several strategies have been used for accelerating the hydration reaction of the cement. One of the earliest reports used 10% and 15% CaCl₂ added to an accelerated Portland cement (Rugby cement, Rugby, UK) that helped decrease setting time and was found to be non-toxic.29

MTA Angelus (Angelus Soluções Odontológicas, Londrina, Brazil) was introduced in 2001 and is composed of 80% Portland cement and 20% bismuth oxide.30 Calcium sulphate, a retardant for hydration, was eliminated from its composition to help decrease the setting time. Initially, MTA Angelus was grey in colour and was later changed to a white coloured material which was named MTA Branco.31

The use of various additives and mixing liquids has been another strategy to help improve the setting time of MTA. Several compounds have been tested, some of which also help improve the handling characteristics of the cement. Calcium formate reduced the setting time by 90%, KY Jelly and 5% CaCl₂ decreased this by 50–64%, 3% CaCl₂ did not alter setting times, saline and 2% lidocaine with 1:100,000 epinephrine delayed the setting time by 80% and 140%, respectively, and MTA did not set with 2% chlorhexidine gluconate gel.26^,32^,33 Even though 3% NaOCl gel decreased setting times, the reported reduction varied from 30–60%.26^,30ProRoot MTA (white) mixed with 15% sodium phosphate dibasic (pH 9.5) set at 26 min and with 15% monobasic sodium phosphate (pH 4.4), within 33 min, as compared with distilled water at 150 min.34 0.1% citric acid has been found to be similar to 10% CaCl₂ as an accelerant but reduced the compressive strength of ProRoot MTA from 39 MPa to 27.23 MPa.35

A Nano white MTA (NWMTA) has been described with small amounts of Sr to improve bioacitivity and a reduced particle size that resulted in an increased surface area (7.8 m²g ^-1) as compared with white ProRoot MTA (WMTA) (1. 8 m²g ^-1).36 This helped reduce the intial setting time from 43 min (WMTA) to 6 min (NWMTA) and also increased the microhardness of NWMTA.

The use of an admix of 1–3% methyl cellulose (improved cohesiveness) and 2% calcium chloride (accelerant) with grey ProRoot MTA resulted in a mix that handled in a similar way to a reinforced zinc oxide eugenol cement and set three times faster (57±3 min) than an unmodified MTA.37 23.1% calcium lactate gluconate (CLG), as a dual functional additive reduced setting times to 12 min, with better handling properties than ProRoot MTA mixed with water or CaCl₂. CLG. It also helped rapid initial set by supplying Ca ²⁺ and reducing the free water in the cement, with subsequent precipitation of amorphous CLG that behaved as a plasticizer. Even though CaCl₂acts as an effective accelerant, because of its crystalline nature, it tended to precipitate as sand-like particles and offered less enhancement of cement cohesiveness.38 However, it reduced compressive strength of the material significantly.35

Modifications for radio-opacifiers

The original radio-opacifier used in MTA was bismuth oxide. However, authors have reported that increasing the concentrations of bismuth oxide affected material properties adversely by increasing porosity (from 15–31%). This may lead to greater solubility and degradation of the material.39 Even though there are studies that have shown a lack of tissue response to Portland cements containing bismuth oxide,20^,40 it has also been shown to induce cytotoxicity in dental pulp cells.41 The ISO specification for dental root canal sealing materials states that radio-opacity should be greater than a 3 mm thickness of aluminium (Al).42 Even though the radio-opacity of Portland cement with bismuth oxide was found to be the best (5.88 mm Al), both zirconium oxide (3.87 mm Al) and iodoform (3.5 mm Al) also surpassed the ISO standard. Barium sulphate in Portland cement displayed values below those recommended by the standard (2.35 mm Al). Another white Portland cement has been modified by the addition of ZrO₂ or calcium tungstate but has significantly increased the final settings times (more than 300 min).43

Modification for application as root canal sealers

Several sealers based on the calcium silicate chemistry have become available recently and include:

One of the first calcium silicate sealers to be reported in the literature was the Endo CPM ™ sealer (Egeo SRL).44 This sealer consists of 50% MTA (SiO₂, K₂O, Al₂O₃, SO₃, CaO and Bi₂O₃); 7% SiO₂, 10% CaCO₃, 10% Bi₂O₃, 10% BaSO₄, 1% propylene glycol alginate, 1% propylene glycol, 1% sodium citrate and 10% CaCl₂ (as per manufacturer).45

iRoot SP (Innovative BioCeramix Inc) was first reported as a root canal sealer in 2009. The material has also been retailed as SmartPaste Bio (SmartSeal DRFP Ltd, Stamford, UK). According to the manufacturer's description, the sealer is a convenient, premixed, ready-to-use injectible white hydraulic cement paste developed for permanent root canal filling and sealing applications. The sealer is also supplied with root canal obturation points termed ProPoints. ProPoint (also made available as C point, LLC, Shrewsbury, MA, USA), has a radio-opaque polyamide core coated with a hydrophilic polymer coating. The sealer is an insoluble, radio-opaque, aluminium-free material based on a calcium silicate composition, which requires the presence of water to set and harden.46 This material has also been licensed as EndoSequence BC Sealer (Brassler, Savannah, GA, USA) and Hyseal-bio (LLC, Shrewsbury, MA, USA). Brassler has also made available a Root Repair Material in the form of a mouldable putty and a preloaded syringe with delivery tips. The manufacturer has stated that it is composed of calcium silicates, zirconium oxide, tantalum oxide, calcium phosphate monobasic and filler agents. It has a working time of 30+ min and final set is achieved within approximately 4 hours.

The Root Repair Material is bioactive.47 The material is nanofilled, allowing good adaptation to the root canal and flow into open dentinal tubules.

Other calcium silicate materials

New Endodontic Cement (NEC) consists of different calcium compounds (calcium oxide, calcium phosphate, calcium carbonate, calcium silicate, calcium sulphate, calcium hydroxide and calcium chloride).48 This was later termed the Calcium Enriched Mixture (USPTO number: 7,942,961). The setting time was significantly shorter than ProRoot MTA (50 min as against 70 min). It has been shown to have good handling, provide an effective seal49 and bioactivity.50 Electron probe microanalysis revealed that there were compositional differences in NEC and ProRoot MTA.

In 2009, the first study on Bioaggregate (DiaRoot® Bioaggregate, Innovative BioCeramix Inc, Vancouver, BC, Canada) was published regarding its antibacterial effects.51 It has been described as a bioceramic material intended for perforation repair and root-end filling. It is a laboratory-synthesized, water-based cement which is claimed to be produced under controlled conditions, resulting in a pure and fine white hydraulic cement-like powder composed of contamination-free biocompatible ceramic nanoparticles.52 Its composition has been described as being aluminium free.51 The components of DiaRoot® Bioaggregate are tricalcium silicate (41%), dicalcium silicate (24%), tantalum pentoxide (25%), calcium phosphate monobasic (6%) and amorphous silicon oxide (4%).52 The liquid supplied with the material consists of 100% deionized water. The hydrated cement has grains surrounded by a matrix of calcium silicate hydrate and calcium hydroxide with a phosphate-containing phase.53 Since the studies by Zhang et al and Park et al,51^,54 several biological studies for Bioaggregate have been performed. The material has the ability to form surface apatite crystals that help contribute to bioactivity.47 It has a long setting time (1260 min), low washout and mechanical properties (compressive strength 16.34 MPa, Vickers hardness 10.7) and high fluid uptake and water sorption.55

Biodentine ™ (Septodont, Saint Maur des Fossés, France) is a calcium silicate based material introduced in 2010 with several clinical applications. The material is available as a unit dose in a disposable capsule. The liquid is provided separately and needs to be added to the capsule which can then be closed and vibrated in a mechanical mixer to obtain a homogeneous mix (Figure 9). The powder is composed of tri-calcium silicate (main core material), di-calcium silicate (second core material), calcium carbonate and oxide (fillers), iron oxide (shade) and zirconium oxide as the radio-opacifier.56 The liquid contains calcium chloride as the accelerator and a hydrosoluble polymer as the water reducing agent, also known as superplasticizers. The setting time claimed by the manufacturer is within 9–12 minutes. Compressive strength values were 241 MPa as against 7.5MPa for ProRoot MTA at 24 hours. Clinical applications include:

While setting, the material has hydrating cement grains surrounded by a matrix of calcium silicate hydrate and calcium hydroxide along with the presence of calcium carbonate.53 The uptake of calcium and silicon by dentine from Biodentine ™ is more than white ProRoot MTA in the presence of phosphate buffered saline.57 An interfacial interaction via a dynamic mineral interaction zone has been reported between Biodentine ™ and dentine.58 The material has a high washout, low fluid uptake and sorption, low setting time (45 min) and superior mechanical properties (compressive strength 67.18 MPa and Vickers hardness 48.4).55

Tech Biosealer (Isasan SRL, Rovello Porro, CO, Italy) is a material with different powders designed for varied applications. It is composed of Portland cement white CEM 1, calcium sulphate hemihydrate, calcium chloride, bismuth oxide, montmorillonite and sodium fluoride. Montmorillonite is a phyllosilicate mineral characterized by high and irreversible swelling capacity owing to water adsorption. Chemically, it is hydrated sodium calcium aluminium magnesium silicate hydroxide (Na,Ca)_0.33(Al,Mg)₂(Si₄O₁₀) (OH)₂. nH₂O. The cement has a unique mixing liquid composed of Dulbecco's phosphate buffered saline.59 There are four formulations for the material according to their application: for use as a sealer (Tech Biosealer endo), apexification (Tech Biosealer apex), root-end filling (Tech Biosealer root end) and pulp capping (Tech Biosealer capping).60 The hydration of the cement, kinetics of the reaction and nanostructure formation have been studied using time-domain nuclear magnetic resonance and this revealed a faster set for the material than ProRoot white MTA.61 The material is bioactive with the formation of apatite on its surface, biocompatible and with good apical seal.62^,63^,64

MTA Plus (manufactured by Prevest-Denpro, Jammu City, India for Avalon Biomed, Bradenton, FL, USA) is a powder-liquid system with the option of mixing the powder (tricalcium and dicalcium silicate, bismuth oxide, calcium sulphate and silica) with either water or an anti-washout gel (AWG), the latter accelerating the set of the material, as per manufacturer's claim, to one hour.65^,66 The anti-washout property helps prevent disintegration of the cement when exposed to fluids, which may happen when the surgical site is rinsed with an irrigant. The AWG is a water soluble polymer composed of approximately 97.8% water mass and traces of SiO₂, K₂O, Cl and CaO.66 The AWG used with MTA Plus reduces the material washout and is similar to amalgam and IRM as compared with MTA Plus with water or MTA Angelus.67 The hydration reaction of the powder mixed with the AWG is different from water, with larger reaction rims around the unhydrated cement particles seen with the former.66 With AWG, calcium ion released was significantly less than with water at 14, 21 and 28 days, the setting time was reduced by 65 minutes to 100 minutes and the compressive strength was significantly higher, both when dry and immersed in HBSS.66 A zone of collagen degradation has been identified from the surface of dentine exposed to MTA Plus that was associated with minimal amounts (picograms) of dentinal loss.68

Hybrid calcium silicate cements

The adaptation of calcium silicate material chemistry by incorporating it with a completely different material has led to the development of a hybrid category of materials. A self-setting, pulp-capping cement composed of calcium phosphate/calcium silicate and bismutite (Bi₂O₂CO₃), named calcium phosphate/calcium silicate/bismutite cement (CPCSBi) was designed to maximize the advantages from each constituent.69 The dissoluble dentine matrix components extracted from CPCSBi exposed to dentine powder demonstrated increased expression of dentine sialophosphoprotein (DSPP) and sotecalcin in human pulp cells which indicated its suitability as a pulp-capping agent.

A calcium silicate cement, mainly based on dicalcium silicate and tricalcium silicate, was prepared using 5% calcium chloride additive and further mixed with alpha-tricalcium phosphate to help apatite formation.70 Dulbeco's phosphate buffered saline was used as the mixing liquid. The cement displayed high bioactivity and human marrow stromal cell proliferation.

A novel root-end filling material, termed NRC, has been developed from bioactive powders and a hydroxylethylmethacrylate (HEMA)-based resin monomer.71 The powder consists of 33.4% calcium oxide, 33.3% calcium silicate and 33.3% triphenylbismuth carbonate. The liquid is a mixture of hydroxyethylmethacrylate (10 ml), benzoyl peroxide (0.03 g), N-N-dimethyl-para-toluidine (0.02 ml) and sodium para-toluenesulphinate (0.01 g). NRC sets in 12.5 minutes, with compressive strength and pH similar to white MTA. It was not found to be cytotoxic.

The first light-cured Mineral Trioxide Aggregate material was introduced in 2008.72 Light-cured MTA presented a moderate chronic inflammatory that was more intense than with Angelus MTA and without dystrophic calcifications. The light-cured MTA consisted of AeroSil (fumed silica) 8%, biocompatible hydrophilic resin 42.5% (which in turn was composed of BisGMA 20%, proprietary biocompatible resin [FDA] 77.25%, modifying agent 2.4%, initiating agent 0.32%, stabilizer for the initiating agent 0.032%), active ingredients in MTA 44.5% and barium sulphate 5%.72 The light-cured MTA displayed the least solubility values and calcium release.

Another light-cured MTA product has been reported recently.73 The powder is composed of di-and tri-calcium silicates, tricalcium aluminate, barium sulphate, calcium sulphate and calcium chloride. The liquid phase contained 2-hydroxyethyl methacrylate, triethyleneglycol dimethacrylate (TEGDMA), camphoroquinone and ethyl-4-(dimethylamino) benzoate. The cement sets in 2 minutes with a high calcium ion release (150–200 ppm) and alkalinity (pH 10–12). Formation of bone-like apatite spherulites was observed after 1 day and the cement had good marginal adaptation. The cement also allowed Saos-2 cell viability and growth and no toxicity was observed. The presence of HEMA-TEGDMA hydrophilic resins helped create a polymeric network that was able to stabilize the outer surface of the cement and also the hydrophilic matrix that was capable of water sorption. Rapid nucleation for apatite formation occurred via the hydration of the silicates and the chelation of calcium to oxygen-containing groups of the resin.

TheraCal LC (Bisco Inc, Schaumburg, IL, USA) is a light-cured, resin-modified, calcium silicate filled liner used for direct and indirect pulp capping as well as under different restorative materials. The material is composed of Portland cement Type III (20–60%), polyethylene glycol dimethacrylate (10–50%), bisphenol A diglycidyl methacrylate (5–20%) and barium zirconate (1–10%).74 The cytotoxicity of the material has been tested.75TheraCal demonstrated a reduction in cell metabolism ranging from 31.5% to 45.9% which was lower than for Vitrebond (73–77%) and Ultrabend Plus (64–71%). It has higher calcium-releasing ability and lower solubility than ProRoot MTA, does not meet the ISO 6876 guideline regarding radio-opacity, and may be cured to a depth of 1.7 mm to prevent dissolution.76

A study has also examined the possibility of using 4-methacryloxyethyl trimellitate anhydride/methylmethacrylate-tri-n-butyl borane (4-META/MMA-TBB) with Mineral Trioxide Aggregate to overcome the shortcomings of the latter.77 The authors mixed MTA powder with the liquid of 4-META/MMA-TBB resin and found the cement to set faster (11 vs 318 minutes for MTA) with less leakage. The pH for MTA was higher during the entire period of the study, though the differences between the two cements were significant only for the first 48 hours. Both cements did not reveal any cytotoxicity.

Calcium-aluminosilicate or fluoride-containing calcium-aluminosilicate Portland-derived mineral powders have been mixed with methacrylate HEMA/TEGDMA/polyacrylic acid-based resin to prepare experimental composites. These are ion-leachable composites that possessed the ability to remineralize human apatite-depleted dentine surfaces.78

Light-cured and chemically-cured composites have been developed using MTA Plus or MTA Plus mixed with water.79 The resin itself did not chemically react with the cement powder, which remained unhydrated within the composite. More apatite formation was seen on the surface for the MTA Plus mixed with water, as was a higher alkalinity (12.7) and calcium ion concentration. The chemically cured composite had the lowest values.

Summary of modification to composition

Several modifications have been made to the original MTA formulation, some of which have resulted in the development of new products with a decreased setting time and improved handling characteristics. Several experimental variants of Portland cement with additives have also been investigated. Calcium silicate materials have also been mixed with other materials to enhance their properties. Based on the variety of materials available and under investigation, calcium silicate cements (for use in dentistry) may be defined as those that are composed (at least in part), of either di-/tri-/tetra-calcium silicate phases with a hydration process that is the sole or contributory mechanism for the setting reaction which results in the formation of leachate and crystalline phases that promote bioactivity. Based on their chemistry, a classification for calcium silicate materials is proposed (Table 2).