Influence of addition of calcium oxide on physicochemical properties of Portland cement with zirconium or niobium oxide.

CONTEXT: Calcium oxide (CaO) may be added to mineral trioxide aggregate (MTA) or Portland cement (PC) to improve physicochemical and biological properties. AIMS: To evaluate the physicochemical properties of PC associated with radiopacifiers and CaO.
MATERIALS AND METHODS: MTA Angelus, PC + 30% zirconium oxide (Zr), or 30% niobium oxide (Nb) associated with 10 or 20% of CaO were evaluated. Gilmore needles were used to evaluate initial and final setting time. Compressive strength was evaluated after the periods of 24 hours and 21 days. pH was analyzed after 3, 12, 24 hours, 7, 14, 21 days. Solubility and flow tests were performed based on the ISO 6876. The data obtained were submitted to analysis of variance and Tukey tests (P ≤ 0.05).
RESULTS: The associations with 10% CaO showed greater strength that the associations with 20% CaO. The shortest initial setting time was observed for the association PC + Zr + 20% CaO and MTA. All the cements presented alkaline pH. The flow of all cements was similar. The highest solubility was found in the associations with 20% CaO.
CONCLUSION: The addition of CaO to PC favored the alkaline property and the PC + Zr + 20% CaO presented setting time similar to MTA.

INTRODUCTION

Mineral Trioxide Aggregate (MTA) is a calcium silicate cement indicated to seal communication between pulp cavity and external dental surface.[1] This material has biocompatibility, antibacterial activity, sealing ability, and bioactivity.[23] The chemical similarity between MTA and Portland cement (PC) is known, since MTA is composed of PC as its main component.[4] Bismuth oxide is added to MTA to promote radiopacity.[5]

MTA has some disadvantages, especially its long setting time, and the presence of bismuth oxide, which alters physicochemical properties.[6] Zirconium oxide (Zr) promotes adequate radiopacity[7] and biocompatibility when associated with PC.[8] Tanomaru-Filho et al., (2012)[9] observed that the association of PC with Zr has proper physicochemical properties. Niobium oxide (Nb) may also be used as a radiopacifying agent, improving biologic properties of materials.[10]

In addition to the main components of MTA, such as calcium silicates, CaO may be included in its formulation.[11] CaO plays an important role in the biological response due its conversion into calcium hydroxide.[11] Estrela et al., (2012)[12] and Camilleri et al., (2012)[13] observed that PC has more CaO in its composition than MTA Angelus. CaO reacts with water and plays an important role in the hydration reaction during setting of the cement.[14]

The aim of this study was to evaluate the effect of the addition of CaO on the physicochemical and mechanical properties of PC associated with Zr or Nb as radiopacifying agents.

MATERIALS AND METHODS

The materials evaluated and proportions for manipulation were:

MTA Angelus (MTA, Angelus, Londrina, Brazil): 1g: 0.3 mL distilled water.

60% white PC (PCB-40, Votorantin, Pedro Leopoldo, MG, Brazil) + 30% Zr (Sigma-Aldrich, São Paulo, SP, Brazil) + 10% calcium oxide (Sigma-Aldrich, São Paulo, SP, Brazil) (PC + ZrO2 + 10% CaO): 1g: 0.27 mL distilled water.

60% White PC + 30% Nb (Sigma-Aldrich, São Paulo, SP, Brazil) + 10% CaO (PC + Nb2O5 + 10% CaO): 1g: 0.40 mL distilled water.

50% White PC + 30% Zr + 20% CaO (PC + ZrO2 + 20% CaO): 1g: 0.27 mL distilled water.

50% White PC + 30% Nb + 20% CaO (PC + Nb2O5 + 20% CaO): 1g: 0.40 mL distilled water.

Compressive strength

The cements were placed in a cylinder mold measuring 6-mm diameter and a thickness of 12 mm, and maintained at 37°C and 95% humidity. After 24 hours and 21 days, they were submitted to compressive test (Emic DL 2000, São José dos Pinhais, PR, Brazil) using a 5KN load cell and 0.5 mm/minute speed. The maximum stress was expressed in MPa.

Setting time

Setting time test was evaluated in accordance with the ISO 6876 Standard (2002).[15] Metal rings measuring 10-mm diameter and 1-mm thickness were used (n = 6). The specimens were maintained at 37°C, 95% humidity. For the initial setting time, a Gilmore needle with 100 ±0.5 g and tip diameter of 2 ±0.1 mm was used. For the final setting time, a Gilmore needle with 456 ±0.5 g and tip diameter of 1 ±0.1 mm was used.

pH

For the pH tests, polyethylene tubes measuring 10-mm length and 1-mm diameter were filled with each material (n = 10). Each tube was immersed in 10 mL of distilled water and maintained at 37°C during the experimental periods. At each period, tubes were removed from the flasks and put into new flasks containing 10 mL of distilled water. The experimental time intervals were 3, 12, and 24 hours, 7, 14, and 21 days. The pH of the solutions was measured at each experimental periodo, using a previously calibrated Ultrabasic pH meter (Denver Instrument Company, Arvada, Colorado, USA).

Flow

Flow test was evaluated in accordance with ISO 6876 Standard (2002).[15] After material manipulation, 0.05 mL was placed in the center of a glass plate measuring 4 × 5 cm. Three minutes after manipulation, another glass plate measuring 4 × 5 cm and mass of 20 g was placed. A weight of 100 g was placed on the top, for 7 minutes. Measurements of flow were taken using two methods. In the first technique, largest and smallest diameter of each specimen was measured. Specimens with difference between largest and smallest diameter less than 1 millimeter were used as recommended by the ISO 6876 (2002).[15] A second evaluation method was used. Specimens were photographed, and the area of cement was measured in mm2 using a software (UTHSCSA Image Tool for Windows Version 3.00), as described by Tanomaru-Filho et al., (2007),[16] analyzing gutta-percha flow.

Solubility

Specimens measuring 1.5 mm thickness and 7.75 mm in internal diameter were fabricated with a nylon thread embedded in the cement mass. The materials were kept at 37°C and 95% humidity, for a period longer than three times the setting time, and placed in a dehumidifier for 7 days. For mass measurement, a precision balance HM-200 (A & D Engineering, Inc., Bradford, MA, USA) was used. The samples were suspended by the nylon thread inside the plastic containers containing 7.5 ml of distilled water. The containers remained in at 37°C for 7 days, when the specimens were removed from the flasks, dried with absorbent paper, and placed in a dehumidifier, until stabilization of mass, when the final mass of each sample was obtained. The solubility value (n = 6) was determined by percentage of mass loss after immersion in water.

Statistical analysis

The data obtained were submitted to analysis of variance and Tukey test, with significance level set at 5%, using Prism 5.0 (GraphPad Software, San Diego, CA, USA).

RESULTS

The results showed that MTA has higher compressive strength than other cements. It was observed an increase in the compressive strength for all materials. After 21 days, the associations PC + ZrO2 + 10% CaO and PC + Nb2O5 + 10% CaO showed greater strength that the associations with 20% CaO, as shown in Table 1.

Table 1

Mean values and standard deviation of the compressive strength test after 24 hours and 21 days, the initial and final setting time test, the diameter and area after the flow test, and percentage of loss of mass after the solubility test

The shortest initial and final setting time was observed for the association PC + ZrO2 + 20% CaO and MTA (P < 0.05).

All the cements showed alkaline pH [Table 2]. In the three initial periods, 3, 12, and 24 hours, all experimental specimens presented a higher pH value than MTA. In the time periods of 7 and 14 days, all the cements presented a pH similar to MTA. Except association PC + ZrO2 + 10% CaO, all cements presented a pH higher than MTA after 21 days.

Table 2

Mean pH values and standard deviation observed for the experimental materials and different periods

For mean diameter analysis, the lowest value was observed for the association PC + Nb2O5 + 10% CaO, which was similar to the association of PC + Nb2O5 + 20% CaO. In the analysis of area, all cements presented a similar flow (P ≥ 0.05), as observed in Table 1.

MTA, and PC + 10% CaO with both the radiopacifying agents were statistically similar and presented lower solubility than the association with 20% CaO [Table 1].

DISCUSSION

CaO plays an important role in biologic response during the mineralization process.[12] Zr associated with PC resulted in a cement with an adequate radiopacity,[7] above the minimum value recommended by Specification 57 of ANSI/ADA (ANSI/ADA, 2000).[17] The association of Zr with PC was shown not to be cytotoxic.[8] Another chemical element that has been studied for improving the biological properties of materials is Nb, due to its biocompatibility.[10]

Biomaterials containing CaO have been developed, such as Biodentine. Biodentine is composed of tricalcium silicate, dicalcium silicate, calcium carbonate, CaO, and Zr.[13]

In this study, the final setting time for MTA (137,8 min) was similar to observed in previous study.[18] The shortest initial and final setting time was observed for the association PC + ZrO2 + 20% CaO, which was similar to the MTA initial and final setting time. The shortest setting time of PC + ZrO2 when CaO was added, may be related to the hydration of the cement in the presence of CaO, which leads to calcium hydroxide formation, as demonstrated by Camilleri et al., (2013).[12] A longer setting time was observed when Nb was used as radiopacifying agent, which may be related to the larger quantity of water used for material manipulation.[19] Tanomaru-Filho et al., (2012)[9] observed that the addition of ZrO2 to white PC presented an initial setting time similar to MTA, and reduction of the final setting time in comparison to pure PC.

An increase in the compressive strength was observed for all materials. The incorporation of additives to MTA has been shown to decrease its strength.[20] In the present study, MTA showed a higher compressive strength than other MTA associations, which may be explained by the larger quantity of CaO in the cements.[1213] Camilleri et al., (2013)[11] observed that CaO has large particle size and this component may not reacts during the hydration process.

The obtained results in the present study showed that cements associated with 20% CaO have higher solubility. Húngaro Duarte et al., (2012)[21] demonstrated that white MTA presents a higher solubility than white PC or the association PC with ZrO2. The cements MTA and PC + 10% CaO associated with ZrO2 and Nb2O5 presented lower solubility than the association with 20% CaO. The results in the present study showed that the MTA and PC + 10% CaO cements associated with ZrO2 or Nb2O5 are in accordance with ANSI/ADA No. 57 (2008),[17] which established that the solubility must be lower than 3%.

All the cements presented an alkaline pH. Analyses in the periods of 3, 12, 24 hours, and 21 days, showed that experimental cements presented a higher pH than MTA, except the association PC + ZrO2 + 10% CaO after 21 days, which was similar to MTA. This alkalinity is a result of the calcium hydroxide produced during hydration. Calcium hydroxide releases calcium and hydroxyl ions.

The flow of retrofilling materials may favor retrograde filling and sealing. Using area analysis, all cements presented similar flow. In the analysis of mean diameter, a greater flow was observed for the associations with 20% CaO, and the association of 10% CaO and ZrO2. The flow observed for MTA Angelus was in agreement with Camilleri (2009)[22] for white Pro Root MTA.

It was concluded that the addition of CaO to PC favored the alkaline propertyl and the association PC + ZrO2 + CaO presented potential for clinical use. Additional studies on the biocompatibility and bioactivity of the material must be conducted.