Compressive strength of calcium silicate-based cement

Summary Introduction The aim of this study was to compare compressive strength (Cs) of new nanostructural calcium silicate based cement (nCS) with commercial calcium silicate cement and conventional GIC. Methods Four nanostructural materials were tested: nanostructural calcium silicate based cement (nCS) (Jokanović et al.), MTA Plus (Cerkamed, Poland), Fuji IX (GC Corporation, Japan) and Ketac Universal Aplicap (3M ESPE, USA). Five samples of each material were mixed in accordance with manifecturer’s guidelines and positioned in metal moulds (ϕ4mm and 6mm). Compressive strength (Cs) expressed in MPa was determined after 24 hours, 7 days and 28 days respectively. Measurements were performed on universal testing equipment (Tinius Olsen, USA) at a crosshead speed of 1mm/min. For processing the results one-way ANOVA and post-hoc test were used. Results The highest values of compressive strength after 24h was found in conventional GIC Fuji IX (mean 38.56±13.31) and Ketac Universal (mean 40.77±7.96). Calcium silicate cements after 24h showed low values of compressive strength (MTA Plus 5.91±0.28 MPa, nCS 1.35±0.36 MPa). After 7 days, FUJI IX 47.42±9.33 MPa and Ketac Universal 35.25±10.60 MPa showed higher value of compressive strength than MTA Plus (15.09±2.77 MPa) and nCS (11.06±0.88 MPa). After 28 days the Cs value for conventional GIC Fuji IX was 48.03±7.82 MPa and Ketac Universal 36.65±11.13 MPa while for calcium silicate cements it was 16.47±1.89 MPa and nCS 14.39±1.63 MPa. There was statistically significant difference (p<0.05) in Cs between conventional GIC and CS cements after 24h, 7 and 28 days. Conclusions Calcium silicate cements initially showed lower values of compressive strength than conventional GIC that increased over time.


INTRODUCTION
Ideal material for root reparation should be able to close communication between the root canal and surrounding tissue, is biocompatible, dimensionally stable and insoluble when in contact with tissue fluids. The material is often placed in the root with an acidic environment, frequently with bacterial inflammation; therefore low pH level is an important factor that adds to the hardness and other properties of the cement [1]. In the past, materials such as calcium hydroxide, zinc oxide eugenol cements, resin composites, glass ionomer cements have been used for root canal perforation treatment but not all of them meet criteria of an ideal material [2].
GIC are developed by combining two different cements: silicate and zinc polycarboxyilate cements [3]. Conventional GIC are made by an acid-base reaction of glass ions with a water solution of polyacrylic acid. They are considered potential biomaterials for orthopedic application because of their ability to adhere to bone and metals and good stability in wet environment. However, lack of bioactive potential and poor mechanical characteristics are some of the issues of this cement.
In the past few years, biocompatible calcium silicate hydraulic cements have been introduced in endodontic therapy. Mineral trioxide aggregate (MTA) is usually used as biomaterial for root and functional perforation reparation, as well as in other indications [4]. MTA is a bioactive material that forms an apatite layer on its surface when in contact with phosphates from tissue liquids but it also forms hybrid layer between dentin and calcium silicate materials [5]. It also releases some of its components in phosphate saliva puffers that encourage biomineralization processes [6]. There are number of calcium silicate cements on the market with the goal to surpass the deficiencies of the original formulation. MTA Plus is a nanostructural MTA released in 2012, with shorter binding time and lower concentration of heavy metals (up to 90%) in its formulation.
Nanoparticles allow uniform and homogenous structure, as well as lower temperature release while hydrating the cement (source: manufacturer). The use of nanoparticles has become an important research aspect in dentistry, with the focus on improving mechanical characteristics and antibacterial effect of the particles. The size of nanomaterial particles (<100nm) that is similar to the size of biological molecules and structures (proteins, DNA, water) indicates possible uses in biomedical researches.
Newly synthesized nanostructural material used in our study uses tri-calcium and di-calcium silicates as a base. This calcium silicate system is produced with hydrothermal sol-gel method and self-expanding burning reaction [7], which secures its high activity and short bonding time. The smallest parts of this system are about 19.9 nm and show notable system activity [8,9].
Compressive strength tests are used in dentistry for simulations of masticatory forces that clinically affect restoration or materials for covering or replacing tissue. The majority of masticatory forces are of compressive nature and their exact value is hard to determine.
The aim of this study was to test the compressive strength of a newly synthesized nanostructural CS cement and compare it to the commercial MTA Plus and conventional GIC that are used in functional or crown perforation reparations. The null hypothesis was that there was no difference in compressive strength between conventional and calcium silicate cements.

MATERIAL AND METHODS
Four cements were used in the research: nanostructural calcium silicate system (nCS) (Jokanovic et al.) where 60% of the total mass were β-C 2 S i C 3 S phases, 20% calcium carbonate and 20% BaSO 4 (Merck, Germany) as X-ray contrast. Water/powder mixing ratio was 1:2; MTA Plus (Cerkamed, Poland) hand mixed in the ratio 0,34g distilled water and 1gMTA powder; conventional GIC Fuji IX (GC Corporation, Japan), by the product instructions mechanically mixed in capsules for 10 seconds in an amalgamator at 4500 rpm; and self-adhesive and self-bonding GIC Ketak Universal Aplicap (3M ESP, USA), with glass oxides and liquid component that consisted of copolymer of acrylic and maleic acid and tartaric acid. As per manufacturer's instruction the capsules were mechanically mixed for 10 seconds in an amalgamator at 4500 rpm.

Sample preparation
After mixing all materials were placed in two parted metal moulds 4mm in diameter and 6mm tall and they were condensed with a hand plunger, 5 samples for each material. The samples were kept on 37°C in a steam bath for 24h, 7 and 28 days. All cylindrical samples were polished with the finest abrasive paper and minimal pressure and visually checked. Samples with visible structural damage were eliminated from the study.
Compressive strength testing was done according to international standard ISO 9917-1:2007 (Dentistry-waterbased cements-Part 1: powder/liquid acid-base cements) using a universal test machine (Tinius Olsen, USA; 5KN) at the speed of 1 mm/min along the longer axis of cylindrical samples [10]. The force needed to break the sample was noted and compressive strength in MPa was calculated with the formula Cs= 4P/πd where P was the maximum force needed to break the sample measured in N, and d was the diameter in mm.
For processing the results one-way ANOVA and posthoc test was used. The level of significance was set at p<0.05.

RESULTS
The results are showed in the Tables 1-2 and Figure 1.
The highest values of compressive strength after 24h were shown by conventional GIC Fuji IX (mean 38.56±13.31) and Ketac Universal (mean 40.77±7.96), but without sta-   There was no statistically significant difference between them. After four weeks, an increase in Cs value was noticed in calcium silicate cements, MTA Plus 16.47±1.89 MPa, and nCS 14.39±1.63 MPa but without statistically significant difference between them. Between the conventional GIC and CS cements there was a statistically significant difference (p<0.05).

DISCUSSION
Compressive strength is an indirect measure of bonding and strength of the material [11,12]. It is an important property that may affect clinical performance [13]. This factor plays an important role in the treatment of functional perforations where cements are directly exposed to occlusal forces [14].
In the literature, significant variations in measured comprehensive strength have been reported as numerous factors can affect it. The cylindrical shape of the samples is convenient but sample surface perfection and intimate contact between samples and testing machine is hard to achieve [8]. Also, the size and shape of the samples, the preparation of the samples and hydration time, water/ powder ratio, mixing technique, pressure while compacting, as well as the moisture and temperature of the room affect results [15,16].
Conventional GIC are in wide use in clinical practice as cements or restorative materials. Many researches have been done with the goal to enhance mechanical and biological properties of GIC with incorporation of bioactive ceramic particles, glass powder and similar. Adding Zn has shown to have a stimulating effect on bone formation in in vitro and in vivo conditions, as well as antibacterial activity, similar to silver. Adding MgO increased cell proliferation [3]. Titanium oxide is added because it is chemically stable, biocompatible and has antibacterial properties, and has shown significant activity against Streotococcus mutans in nano-formulation [8].
It has been confirmed in our research, as well as by other researchers, that cement reaction continues after one day, because crosswise bonds are established in the cement matrix [3]. Shiowaza [17] pointed out that cement maturation (acid-base reaction) is continued in the first week that can be seen as an increase in compressive strength and it is then stable for the next 12 months. These results are consistent with the results of our research, where there was no further compressive strength value increase between 7 and 28 days.
Compressive strength is considered as one of the most important physical properties of hydraulic cements and it is in correlation with the degree of hydration [2], where hydration reaction is the key for hardening of hydraulic silicate cements.
Compressive strength of calcium silicate cements is initially, after 24h low. Bonding and hardening of the hydraulic cements depends on the formation of the CSH gel and ettringate (hydrated calcium sulfoaluminate) on the nucleation points of calcium hydroxyl crystals [18]. The presence or absence of these crystal formations (ettringate crystals) in different formulations of calcium silicate cements is probable reason for different values of compressive strengths between them [2]. In the ISO standard compressive strengths are still not defined for pulp covering or perforation materials, there is only a suggestion that materials are to be compared to the value of stress that occurs during amalgam condensation [19]. After seven days, the value of compressive strength of calcium silicate cements increases, where Cs of nCS further increases even after 28 days due to cement hydration.
The difference in compressive strength values between materials with similar or even the same composition can be explained by the size of the particles [11,19], as well as experimental conditions. That is how Akbari et al. [6] found that the Cs of White MTA (Angelus, Brazil) was 1.16 MPa after 24h and 2.19 MPa after 7 days, while Natale et al. [20] found Cs to be 18 MPa after 7 days. Noh et al. [21] found that WMTA (ProRoot MTA) after 24h had an average value of 19.41 and after 7 days 46.18 MPa, while Basturk et al. [16] showed results as high as 84.17 MPa after 4 days for ProRoot MTA. The microstructure and homogenicity of the cement affect its strength because finer particles have greater ability to absorb moisture.
Hand mixing of materials can result in inadequate hydration due to the limited formation of micropores inside the material that compromise water penetration in the material. Mitchell and Douglas pointed out that hand mixed cements have lower comprehensive strength due to trapped air, while capsulated cements mixed in a centrifuge have higher Cs [22,16].
Nanostructural materials have particles that are not over 100 nm in size (most often between 5 and 50 nm), but therefore have up to ten times bigger surface area, which stimulates greater ettringate crystal formation [23]. Nanostructures strive to solve one of the key problems of endodontic cements like bonding time. Experiments indicate that in almost all nano-powders kinetic absorption and desorption can be improved simply by reducing the particle size [14].
Perfecting materials that can be used as biological bone "substitutes" is currently one of the most valuable and most active fields of biomaterial research. Biocompatibility and bioactivity of these materials secure the interaction with biological systems. Bioactive materials like calcium silicate cements, especially with nanostruc-ture, stimulate regeneration of damaged tissue, therefore renewing the function of damaged tissue or organs [7].

CONCLUSION
The null hypothesis that there is no difference in Cs between conventional and calcium silicate cements is rejected. The compressive strength of conventional glass ionomer cements was significantly higher after 24 h, increased after 7 days and remained the same after 28 days. MTA Plus showed higher compressive strength after 24 h and 7 days than newly synthesized nanostructural calcium silicate cement (nCS) but the values were similar after 28 days. Compressive strength of calcium silicate cement grows with time and cement hydration.