The use of Portland cement and its modified forms as a dental core build-up material

Abstract

The use of mineral trioxide aggregate (MTA) for various applications in dentistry is difficult due to its long setting time and poor compressive strength. The aim of this research project was to study the properties of MT A, and modify the material in an attempt to make it suitable as a core build-up material. Materials and Methods The chemical constituents of MT A (Dentsply, Tulsa Dental Products, Tulsa UK, USA), Portland cement (CEM 1 45,5N; Italcementi spa, Bergamo, Italy) and other cement prototypes wos determined hy performing both phase and elemental analysis on both un-reacted powders and hydrated cements. The cement properties were improved by addition of calcium aluminate cement to Portland cement, by exclusion of gypsum from the end stage of the manufacturing process and also by using calcium fluoroaluminate (CF A) cement. Fast setting proprietary brands (3) were also tested. In addition, various fillers (3), and micro-silica were added in conjunction with the use of a superplasticizing admixture. DSP (densified with small particles) mortars and composite materials were thus produced. Compressive strength testing was performed using two methods of testing cements namely using cylinders 6 mm in diameter and 12 mm high and cubes of side 70.7 mm. Both methods of testing were compared. The method of testing compressive strength was also modified and flexural strength of the composite materials was also measured. Biocompatibility was assessed using a cell culture technique and cell growth and proliferation was evaluated under the scanning electron microscope. The method of testing biocompatibility was also improved upon and assessment of cell activity by a vital dye that measures mitotic dehydrogenase activity was also used. The assessment of cell activity using a vital dye was preferred to scanning electron microscopic assessment of cell proliferation as it avoided material contamination during processing. The cements were tested by placing the cells directly on the cement and cells were also grown on an elution. Other tests performed included pH evaluation, marginal adaptation tested using a fluorescent dye and a confocal microscope and

field emission scanning electron microscope, and acid resistance tests performed to check resistance of material to acid attack evaluated using the polarized light microscope, the scanning electron microscope and the confocal microscope. The different testing methods were compared. Results The chemical constituents of MTA (Dentsply, Tulsa Dental Products, Tulsa OK, USA) showed the material to be similar to Portland cement except for the bismuth oxide which is present in MTA. Both MTA and Portland cement were composed of 1ri ami rli cakilll11 ~iliootc, which on hydration prodl](~cd eakilll11 silicate hydr11te Klnd calcium hydroxide. The materials were susceptible to contamination by inorganic compounds and other chemicals routinely used to evaluate cell viability under the scanning electron microscope. Calcium hydroxide was not always produced during hydration. Emaco Ultra-rapid, and both calcium sulpho-aluminate (CSA) and calcium fluoro-aluminate (CF A) did not show any release of soluble calcium ions. Combinations of Portland cement (OPC) and high alumina cement (HAC), various proprietary brands of cement, the use of cement clinker interground without the gypsum during the manufacturing process and the use of CSA and CF A cement resulted in a reduced setting time. The setting time was less than 9 minutes for all the cement prototypes tested. Compressive strength testing showed that one fast setting proprietary brand (Emaco Ultra-rapid), and both CSA and CF A had compressive strength values comparable to that reported for glass ionomer cement. The CSA cement was stronger than the CFA cement both in compression and in flexure at all curing times (p < 0.001) except for flexural strength at 1 day where no difference between the two cements was demonstrated (p > 0.05). Addition of granite to the CSA cement resulted in an increase in flexural strength at 28 and 56 days but a reduction in the compressive strength at all ages (p < 0.001). Addition of granite to CF A increased the flexural strength up to 28 days (p < 0.01) but the flexural strengths of the cement and the cement composite were the same at 56 days (p > 0.05). The mixtures of OPCIHAC and the other proprietary brands showed very low strengths. The cement clinker had low initial strengths. Addition of bismuth oxide affected the material adversely with marked reduction in compressive strengths. The fast-setting DSP mortars had a lower compressive strength at 1 day (p < 0.005), but there was no difference between the cements and DSP mortars at later curing times. Vacuum mixing did not improve the compressive strength of the prototype cements at any age. Wet curing improved the compressive strength of Portland cement at all ages (p < 0.05) in comparison to the prototype cement where compressive strength at 1 day (35.98 Nmm-2

, p = 0.011) and 7 days (44.08 Nmm-2

, p = 0.025) was reduced by immersion in water. The filler replaced cement prototypes were more stable and less susscebtable to changes in less compressive strength by varifying the curing method (p >0.05). The compressive strength of Portland cement was different between the two methods namely the testing of cubes and that of cylinders (p < 0.001). All the fast-setting cements tested showed no difference in compressive strength regardless of the method of testing at 1 and 7 days (p > 0.05), but the cylinders showed a lower compressive strength at 28 days (p < 0.05). Thus, specimen size and shape seemed to affect the compressive strength testing results. The pH was alkaline for all the cements tested. Storage solutions also demonstrated a high pH. Prototype materials took up more water than glass ionomer cement. Curing at 100% humidity resulted in a net loss of weight for all the materials tested. Glass ionomer cement restorations showed marginal leakage along their walls and along the restoration floor. The changes caused by acid contact were different for the CSA and CF A cements compared with glass ionomer cement. These cements exhibited changes in their internal chemistry with no changes in surface characteristics while the glass ionomer cement showed erosion of the cement matrix early after acid contact. No changes were observed in the cement composite based on calcium sulpho-aluminate.

Biocompatibility testing of the cements showed that cell proliferation was enhanced only in cements which produced soluble calcium ions as a by-product of cement hydration (MTA, cement clinker and Portland cement). Indirect studies of the eluants showed an increase in cell activity after 24 hours compared with the control in culture medium (p < 0.05). Direct cell contact with the cements resulted in a fall in cell viability for all time points studied (p < 0.001). Emaco Ultra-rapid, and both CSA and CF A cements did not encourage cell growth similar to glass ionomer cement. The addition of bismuth oxide did not interfere with the biocompatibility of the cements. Conclusions MTA (Dentsply, Tulsa Dental Products, Tulsa OK, USA) and Portland cement (CEM 1 45,5N; Italcementi spa, Bergamo, Italy) were composed primarily of tricalcium silicate and calcium hydroxide was produced as a reaction by-product on hydration. The setting time of MTA could be reduced by addition of calcium aluminate cement. In excess of gypsum the cement produced showed both a reduction in setting time and good compressive strengths. Similar results were obtained for CF A and a brand of fast setting cement. Exclusion of gypsum from the cement at the end of the manufacturing process reduced the setting time but initial strengths were low. The compressive strength of these prototype materials was enhanced by the use of a superplasticizing admixture that allowed the reduction of the water/cement r(ltio at the same rheology. Addition of an inert material to the cement increased the flexural strength. Most standard testing procedures used to test dental cements did not seem suitable for testing cements based on Portland cement. Processing for scanning electron microscopy reacted with the cement hydration by-products; the cements were even susceptible to changes in size of specimen when tested in compression and use of acidic dyes which are used routinely for evaluation of marginal adaptation of materials affected the cement. Materials which had calcium hydroxide produced during hydration encouraged cell proliferation. In fact cells grew preferentially on the material elution rather than directly on the cement surface.