Abstract
Objective
Zirconium oxide can be added to dental materials rendering them sufficiently radiopaque. It can thus be used to replace the bismuth oxide in mineral trioxide aggregate (MTA). Replacement of Portland cement with 30% zirconium oxide mixed at a water/cement ratio of 0.3 resulted in a material with adequate physical properties. This study aimed at investigating the microstructure, pH and leaching in physiological solution of Portland cement replaced zirconium oxide at either water–powder or water–cement ratios of 0.3 for use as a root-end filling material. The hydration characteristics of the materials which exhibited optimal behavior were evaluated.
Methods
Portland cement replaced by zirconium oxide in varying amounts ranging from 0 to 50% in increments of 10 was prepared and divided into two sets. One set was prepared at a constant water/cement ratio while the other set at a constant water/powder ratio of 0.3. Portland cement and MTA were used as controls. The materials were analyzed under the scanning electron microscope (SEM) and the hydration products were determined. X-ray energy dispersive analysis (EDX) was used to analyze the elemental composition of the hydration products. The pH and the amount of leachate in Hank’s balanced salt solution (HBSS) were evaluated. A material that had optimal properties that satisfied set criteria and could replace MTA was selected. The microstructure of the prototype material and Portland cement used as a control was assessed after 30 days using SEM and atomic ratio diagrams of Al/Ca versus Si/Ca and S/Ca versus Al/Ca were plotted.
Results
The hydration products of Portland cement replaced with 30% zirconium oxide mixed at water/cement ratio of 0.3 were calcium silicate hydrate, calcium hydroxide and minimal amounts of ettringite and monosulphate. The calcium hydroxide leached in HBSS solution resulted in an increase in the pH value. The zirconium oxide acted as inert filler and exhibited no reaction with the hydration by-products of Portland cement.
Significance
A prototype dental material composed of Portland cement replaced with 30% zirconium oxide as radiopacifier leached calcium ions on hydration which reacted with phosphates present in simulated tissue fluids. This resulted in bioactive cement that could prospectively be used as a root-end filling material. The zirconium oxide acted as inert filler and did not participate in the hydration reaction of the Portland cement.
1
Introduction
Mineral trioxide aggregate (MTA) was introduced as a root-end filling material due to its hydraulic characteristics. MTA is composed of a mixture of Portland cement and radiopacifier namely bismuth oxide . The bismuth oxide is added as Portland cement has a radiopacity lower than the recommended 3 mm thickness of aluminum as suggested by ISO 6876; 2002 . The addition of 20% bismuth oxide to Portland cement increases the radiopacity value to 6–8 mm Al .
The addition of bismuth oxide to Portland cement resulted in the bismuth taking silicon lattice sites in the calcium silicate hydrate structure . After 28 days of hydration only 8% of the original bismuth oxide added to the cement remained in the oxide form. The rest was either incorporated in the cement matrix or leached out in solution . Other radiopacifying materials have been suggested in the literature. These include gold powder, silver/tin alloy , barium sulfate , iodoform, zirconium oxide , zinc oxide , lead oxide, bismuth subnitrate, bismuth carbonate and calcium tungstate . A number of these alternative radiopacifiers have been investigated and it was demonstrated that the replacement of 20–25% of Portland cement resulted in a material which was sufficiently radiopaque without any deleterious effects on the physical properties . Addition of silver/tin alloy although resulting in a cement of adequate physical properties leached high levels of tin as the tin was not stable in alkaline solution . Barium sulfate replacement and was more inert. However it exhibited a lower radiopacity value than MTA. Gold was stable and no leaching of gold was reported. Despite its good chemical behavior, gold is relatively expensive and imparts a yellowish color to the material .
The addition of various percentage replacements of Portland cement with zirconium oxide mixed at either water/cement or water/powder ratio of 0.3 was also investigated. All percentage replacements resulted in radiopacity values higher than 3 mm Al. The addition of zirconium oxide did not affect the physical properties of the material . Using the digital logic method it was concluded that 30% replacement of Portland cement with zirconium oxide mixed at a water/cement ratio of 0.3 exhibited optimal properties .
This study aimed at investigating the microstructure, pH and leaching in physiological solution of Portland cement replaced zirconium oxide at either water–powder or water–cement ratios of 0.3 for use as a root-end filling material. The hydration characteristics of the materials which exhibited optimal behavior were evaluated.
2
Methods
The materials used in this study included white Portland cement (PC; CEM 1, 52.5N; LaFarge Cement, Birmingham, UK) and zirconium oxide (ZrO 2 ; Sigma–Aldrich, Buchs, Switzerland). ProRoot MTA (Dentsply Tulsa Dental, Johnson City, TN, USA) lot number: 08003394 was used as control. The Portland cement was replaced by zirconium oxide in varying amounts ranging from 0 to 50% in increments of 10. The Portland cement/zirconium oxide mixtures were mixed with water either at a water/powder (WP) ratio or at a water/cement (WC) ratio of 0.3 ( Table 1 ). ProRoot MTA was mixed at a water/cement ratio of 0.3 as recommended by the manufacturer.
| Sample name | PC (wt.%) | ZrO 2 (wt.%) | Water/powder ratio | Water/cement ratio |
|---|---|---|---|---|
| WP/WC 0 | 100 | 0 | 0.3 | 0.3 |
| WP 10 | 90 | 10 | 0.3 | 0.333 |
| WP 20 | 80 | 20 | 0.3 | 0.375 |
| WP 30 | 70 | 30 | 0.3 | 0.429 |
| WP 40 | 60 | 40 | 0.3 | 0.5 |
| WP 50 | 50 | 50 | 0.3 | 0.6 |
| WC 10 | 90 | 10 | 0.27 | 0.3 |
| WC 20 | 80 | 20 | 0.24 | 0.3 |
| WC 30 | 70 | 30 | 0.21 | 0.3 |
| WC 40 | 60 | 40 | 0.18 | 0.3 |
| WC 50 | 50 | 50 | 0.15 | 0.3 |
2.1
Microstructural examination
The morphology of both un-reacted powders and hydrated cements was evaluated using a scanning electron microscope (SEM Leo 1430 Oxford, Cambridge, UK) with both secondary electron and backscattering modes. Elemental analysis was also performed using X-ray energy dispersive analysis (EDX). The Portland cement, zirconium oxide and ProRoot MTA powders were sprinkled on the top side of carbon double-sided tape attached to an aluminum stub.
Cubes measuring 7 mm × 7 mm × 7 mm ± 1 mm were prepared for each cement mixture and were allowed to cure for 28 days in distilled water at 37 ± 1 °C. Longitudinal cross sections of the cube specimens were prepared using a circular micro-cutter. The cut surface of the cross-sections was left unpolished. The specimens were attached to aluminum stubs using carbon double-sided tape and carbon coated. SEM/EDX analysis was carried out to analyze the microstructure and the elemental composition of selected regions within each specimen. Particle size determination was performed from the micrographs of the powders.
2.2
Evaluation of pH
Six disks 15 ± 1 mm diameter with a thickness of 1 ± 0.1 mm of each cement type were prepared. These were stored in an incubator at 37 ± 1°C for 24 h and then were removed from the molds, and immersed upright in 10 mL Hanks balanced salt solution (HBSS; H6648, Sigma–Aldrich, St. Louis, MO, USA). The composition of the HBSS was (g/L) 0.4 KCl, 0.06 KH 2 PO 4 anhydrous, 0.35 NaHCO 3 , 8.0 NaCl, 0.05 Na 2 HPO 4 anhydrous and 1.0 d -glucose. The pH readings of the storage solution were taken using a pH meter (Hanna HI 9811, Hanna Instruments, Woonsocket, RI, USA) prior to immersion and after 1, 7, 14 and 28 days.
2.3
Evaluation of leaching
Determination of leaching of cements in physiological solution was performed using inductively coupled plasma atomic emission spectroscopy (ICP-AES). Specimens 10 ± 1 mm in diameter with a thickness of 1 ± 0.1 mm were prepared. These were allowed to cure for 24 h in an incubator at 37 ± 1 °C and a relative humidity of not less than 95%. The cements were weighed and placed in sealed plastic containers filled with 5 mL HBSS. A blank HBSS in a plastic container was also prepared. The specimens were placed in an incubator at 37 ± 1 °C for 28 days. Following this period, the HBSS solutions were analyzed using ICP-AES. Leaching in solution was calculated in μg/g using the following formula:
Amount of leachate per weight of cement = Amount of leachate per litre × 0.005 Weight of cement specimen × 1000
2.4
Assessment of hydration characteristics
Portland cement replaced with 30% zirconium oxide was mixed with water at a water/cement ratio of 0.3. Portland cement mixed at the same water/cement ratio was used as control. The pastes were compacted in a cylindrical mold (30 mm in diameter) using a stainless steel plugger. The specimens were cured in sealed plastic (polythene) containers at 37 °C for 30 days using a thermostatically controlled water bath (MGW Lauda M 20, Leica Microsystemes SA, Rueil-Malmaison, France). They were immersed in acetone for four days to remove any remaining water, and then dried in a vacuum desiccator for 8 h. The dried specimens were mounted in epoxy resin using vacuum impregnation. The hardened resin block was sawn (Labcut 1010, Agar scientific, Stansted, UK) and ground under copious water irrigation using progressively finer grits of abrasive paper to produce a flat surface. A thin conductive carbon coating was applied to the sections prior to examination in the SEM (ISI SS40; ISI, Tokyo, Japan). Elemental analysis was performed using energy-dispersive X-ray analysis (SAMx Numerix, Levens, France). Quantitative chemical analyzes (verb)/analyses (noun) were carried out using X-ray standards obtained from minerals for each element. Oxygen was calculated by stoichiometry . The examination procedure consisted of collecting backscattered electron images to show the general microstructure, and a series of X-ray spectra. For each specimen, between 50 and 60 spectra were collected. The X-ray spectra were quantified using suitable mineral standards and the data plotted in a standard format showing atomic ratios of Si/Ca versus Al/Ca and Al/Ca versus S/Ca. These atomic ratio plots are used in SEM/EDX studies of cementitious materials and enable the principal hydration products to be differentiated.
2
Methods
The materials used in this study included white Portland cement (PC; CEM 1, 52.5N; LaFarge Cement, Birmingham, UK) and zirconium oxide (ZrO 2 ; Sigma–Aldrich, Buchs, Switzerland). ProRoot MTA (Dentsply Tulsa Dental, Johnson City, TN, USA) lot number: 08003394 was used as control. The Portland cement was replaced by zirconium oxide in varying amounts ranging from 0 to 50% in increments of 10. The Portland cement/zirconium oxide mixtures were mixed with water either at a water/powder (WP) ratio or at a water/cement (WC) ratio of 0.3 ( Table 1 ). ProRoot MTA was mixed at a water/cement ratio of 0.3 as recommended by the manufacturer.
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