Influence of radiopacifier additives on calcium aluminate cement properties

Influence of radiopacifier additives on calcium aluminate cement properties

Ivone Regina de OliveiraI,^*; Talita Luana de Andrade^I; Daniele Coelho Matsuo^I; Renata Martins Parreira^I; Marcos Jacobovitz^II; Victor Carlos Pandolfelli^III

^IInstitute for Research and Development, University of Vale do Paraíba - Univap, Av. Shishima Hifumi, 2911, São José dos Campos, SP, Brazil

^IIPrivate Practitioner in Endodontics, São Carlos, SP, Brazil

^IIIMaterials Engineering Department, Federal University of São Carlos - UFSCar, Rod. Washington Luiz, km 235, São Carlos, SP, Brazil

ABSTRACT

The aim of this study was to determine the best radiopaque additive to be incorporated to calcium aluminate cement (CAC) to promote radiopacity. Measurements of optical density were carried out on white MTA and CAC with and without additives such as: polymeric dispersant, plasticizer and various radiopacifiers. The effects of the radiopacifying addition on some properties of CAC were also evaluated. The radiopacity value for CAC-Bi₂O₃ (25%) was higher than the other radiopacifying agents tested. The addition of ZnO (25%) and 15%ZnO:10%Bi₂O₃ increased the compressive strength of CAC, whereas for Bi₂O₃ (25%) containing samples the strength was lower than for the CAC. The ZnO (25%) and 15%ZnO:10%Bi₂O₃ additions also reduced the apparent porosity. CAC does not have sufficient radiopacity to be distinguished from adjacent anatomic structures, such as dental tissues and bone. The addition of 15%ZnO:10%Bi₂O₃ can be suggested as the most suitable one to obtain the best compromise between good physical and mechanical properties and ideal radiopacity for clinical purposes.

Keywords: dental cement, radiopacity, physical and mechanical properties

1. Introduction

Mineral Trioxide Aggregate (MTA) has been considered as the ideal root-end filling material because of its low solubility, low cytotoxicity, good tissue tolerance and induction of mineralized tissue formation¹. MTA is a white or grey powder which sets when there is moisture^2,3. MTA primarily consists of tricalcium silicate ( 3CaO.SiO ₂), dicalcium silicate ( 2CaO.SiO ₂)^4,5 as binder phases, and bismuth oxide to increase its radiopacity¹.

Calcium aluminate cement (CAC) has been studied as root-end filling material to overcome some MTA drawbacks such as the long setting time and its negative implications for the clinical needs^6-12. CAC has suitable physical and mechanical properties¹⁰, biocompatibility^7,12, ability of stimulating hydroxyapatite deposition in simulated body fluid solution¹³ and acts as a barrier against bacterial microleakage⁸. Calcium aluminate-based cement showed no inflammatory and less tissue reactions than MTA, and it was biocompatible when tested in rat subcutaneous tissue⁶.

A key physical property for these endodontic materials is the radiopacity. Most endodontic materials contain radiopacifying agents to visualize the root canal filling and to check the healing evolution over time¹⁴. The root-end filling material should have enough radiopacity to be distinguished from adjacent anatomical structures, such as bones and teeth^1,14,15.

The radiopacity of material is changed by adding particles containing heavy metals such as bismuth (Z=83), barium (Z=56), zirconium (Z=40), strontium (Z=38) or zinc (Z=30)^16,17.

Bismuth oxide is a well-known radiopacifying agent. However, previous studies pointed out that Bi₂O₃ affected the hydration mechanism of MTA, reducing the precipitation of calcium hydroxide in the hydrated paste¹⁸ and it was slightly leached out with the calcium hydroxide⁴.^.

The use of Bi₂O₃ with calcium silicate has been shown to be deleterious to the physical properties of the final material, affecting particularly the compressive strength¹⁹. The bismuth oxide is not inert and retards the hydration of the cement by decreasing its calcium ions release rate, changing its reparative capacity and its physicochemical properties¹⁵.

Barium sulphate and zinc oxide are applied extensively in medical devices¹⁹. However, according to Vivan et al.¹⁴, BaSO₄ itself does not provide suitable radiopacity to Portland cement. Zinc and zirconium oxides are used in dental materials for prosthetic and implant purposes and do not present toxicity¹⁵.

The extensive use of these materials indicates that they present favorable interaction with the tissues. However, adding them even at minimal amounts can affect the physical properties of the final compound. Besides its importance, there have been few investigations on the effect of radiopacifiers on the material^"s properties¹⁹.

Therefore, the aim of this study was to determine the best radiopaque additive to be incorporated to CAC to promote sufficient radiopacity for clinical purposes and the influence of this addition on some of its properties such as compressive strength, apparent porosity and setting time.

2. Experimental Procedures

White-MTA (Angelus, Londrina, PR-Brazil) and CAC (Secar 71, Kerneos, Neuilly sur Seine-France) were tested. The additives used are presented in Table 1.

The characteristics of the ZnO and Bi₂O₃ radiopacifiers, such as morphology and size distribution of particles, were analyzed by using the equipments scanning electron microscope (EVO MA10; Zeiss) and Sedigraph (5000D, Micromeritics) as showing in Figure 1. Further characterization of the materials was based on optical density, compressive strength, apparent porosity and setting time.

Aqueous suspensions of CAC (82 wt% solids) were prepared in the presence of a dispersant (D), plasticizer (P) and radiopacifying agents. Adding the dispersant and plasticizer reduced the water required to produce a workable mix of the calcium aluminate cement paste, resulting in a denser structure and, therefore, a stronger material¹⁰. Also, higher liquid to powder ratio was necessary to prepare the MTA suspension (75 wt% solids) used as control. The coagulated nature of the MTA particles does not result in fluid suspensions, unless they are diluted²⁰.

The CAC or MTA suspensions were cast into circular metallic molds (10 mm diameter × 1 mm thick). Four specimens per each studied composition were prepared. The samples were cured for 24 hours at 37ºC in a stove (MA033, Marconi) in a moisture-saturated environment (~100% RH). After that, the samples unmolded were kept at 37ºC for 24 more hours.

One sample of each composition was placed on an occlusal radiographic film (Kodak Insight, Manaus, Amazonas, Brazil) (Ref 1169143, Lot 39500204). At their side, a 99% pure aluminum step wedge with ten steps (1 to 10 mm) was also placed on the radiographic film as shown in Figure 2. The radiographs were obtained using 70 KVp, 8 mA X-ray apparatus (ION 70X, Procion, Ribeirão Preto, São Paulo, Brazil) and exposed for 0.25 seconds at a focus-film distance of 20 cm. The films were manually developed in a darkroom following the time/temperature recommendations of the manufacturer. The optical density value (OD) was measured with a photodensitometer (MRA; Indústria de Equipamentos Eletrônicos Ltda, Ribeirão Preto, São Paulo, Brazil). Optical density of the samples and each thickness of the aluminum step wedge were measured in triplicate for each film. The results of optical density represent the average of twelve measurements for each composition (3 measurements for each sample × 4 samples for each composition). The optical density data of cements tested were submitted to a polynomial regression to obtain the equivalence in radiopacity (mm Al)¹⁵.

Aqueous suspensions of CAC (82 wt% solids) were also prepared in the presence of dispersant, plasticizer and the radiopacifying agents ZnO (25%), Bi₂O₃ (25%) and 15%ZnO:10%Bi₂O₃ and cast into 16 mm diameter × 18 mm height cylinder moulds for measurements of compressive strength and apparent porosity. Eight specimens per each studied composition were prepared (five for compressive strength and three for apparent porosity). Samples were cured at 37ºC in a stove (MA033, Marconi, São Paulo, Brazil) in a moisture-saturated environment (~100% RH) for 12 hours. The samples were unmolded and placed into containers with simulated body fluid (SBF) solution at 37ºC (100% RH) for 3, 7, 15 or 30 days.

The compressive strength of the samples was measured as a function of the exposition time in SBF, whereas others were dried at 110ºC for 48 h and their apparent porosity was evaluated. The SBF solution was prepared according to the procedure described in the literature²¹.

The compressive strength was measured with the help of an EMIC Test Machine (Model DL 10.000, Curitiba, Brazil). A constant displacement rate of 0.15 mm/min was used. The compressive strength was calculated as:

where, σ_R was the rupture stress (MPa); P was the ultimate load (N); and D (mm) was the diameter of the samples.

The apparent porosity was evaluated according to the immersion test (Archimedes principle), using kerosene. This principle states that a body immersed in a fluid is buoyed up by a force equal to the weight of the displaced fluid. The buoyant force is measured by the difference, expressed in grams, between the weight of the body in air and when submerged in kerosene²².

The samples are initially weighed in dry conditions (W_d). After 1 hour of immersion in the liquid under vacuum, the sample is weighed both suspended in the immersion liquid (W_i) and humid (W_h). Thereby, the apparent porosity (A.P) of the sample is calculated considering the mass of liquid retained in its open pores:

Aqueous suspensions of CAC (82 wt% solids) were also prepared using a standard laboratory mixer (Marconi, Piracicaba, Brazil) under 2000 rpm. The setting time of CAC suspensions was evaluated in the presence of dispersant, plasticizer and radiopacifying agents ZnO (25%), Bi₂O₃ (25%) and 15%ZnO:10%Bi₂O₃. After mixing for 1 min, the suspensions were poured into a container (180 mL) where the setting time was measured with an automatic recording Vicat apparatus (Vicatronic E044, Matest, Italy). A needle is inserted in the suspension every 1 min. The time for which the penetration distance was zero indicated the setting time of the sample.

3. Results and Discussion

3.1. Radiopacity tests

The optical density of the aluminum step wedge measured for different radiographs is shown in Figure 3. The constancy of the optical density on the 4 radiographs showed the precision of the measurement. As expected, a decrease in OD was observed with increasing the step wedge thickness, as the image progressively became more radiopaque¹⁵.

The radiopacity for each tested material is expressed in millimeters of equivalent aluminum as shown in Figure 4. The radiopacity value for CAC-DP-Bi₂O₃ (25%) was higher than the other radiopacifying agents tested, including the commercial material (WMTA) which includes bismuth oxide in its composition. The CAC-DP-15%ZnO:10%Bi₂O₃ composition presented sufficient radiopacity for clinical purposes, similarly to MTA.

Bi₂O₃ is used as a radiopacifying agent for dental material such as acrylic resin and some MTAs imparting the sufficient radiopacity to them^15,19. Among all additives evaluated, Bi₂O₃ addition resulted the lowest optical density (OD).

ISO 6876:2012²³ standard establishes 3 mm Al as the minimum radiopacity for the root canal sealers. When the OD values obtained for each material were converted into radiopacity, results higher than 3 mm Al were obtained mainly for CAC-DP-Bi₂O₃ (25%). This composition presented higher radiopacity than all other radiopacifying agents tested. CAC-DP-ZnO (25%) resulted in a lower value than that required (2.2 mm Al, Figure 4) to allow radiographic differentiation among the cement and dental structures. Based on that, higher contents of ZnO were evaluated and also the mixture with Bi₂O₃ in different proportions. CAC-DP-15%ZnO:10%Bi₂O₃ compositions presented suitable radiopacity (3.5 mm Al), similarly to MTA.

Aguilar et al.¹⁵ also pointed out that Bi₂O₃ was efficient radiopacifier for cement based calcium aluminate cement, providing suitable results for all studied thicknesses. However, the present authors highlight that further research was required to evaluate the effect of the addition of this radiopacifying agent on other physicomechanical and biological properties of cement because it has been reported that bismuth is toxic and induces cell death²⁴. Other authors also affirmed that the effect that particle shape and particle size distribution of a cement-replacing material have on the properties of the resultant material still needs to be investigated¹⁹.

Therefore, the influence of ZnO (25%), Bi₂O₃ (25%) or their mixture (15%ZnO:10%Bi₂O₃) on some properties of CAC such as compressive strength, apparent porosity and setting time, was evaluated.

3.2. Compressive strength and apparent porosity tests

Results of compressive strength and apparent porosity as a function of the curing time, for samples of calcium aluminate cement containing additives (dispersant, plasticizer, and radiopacifier) are shown in Figures 5a and b, respectively.

The addition of ZnO (25%) and 15%ZnO:10%Bi₂O₃ increased the compressive strength of CAC-DP, whereas to Bi₂O₃ (25%) the values were lower than the CAC-DP. The ZnO (25%) and 15%ZnO:10%Bi₂O₃ addition also decreased the apparent porosity compared to the Bi₂O₃ (25%) ones.

An important feature that must be considered when choosing a radiopacifier additive is the mechanical strength of the resulting material. Compressive strength is an important factor to consider when the filling material is placed in a cavity that bears occlusal pressure, such as a root canal filling³, or even a restorative base. The addition of ZnO (25%) increased the mechanical strength of CAC-DP when compared to Bi₂O₃ (25%) as shown in Figure 5a. ZnO comprises finer particles than Bi₂O₃ (Figure 1) positively affecting the particle packing, resulting in lower porosity (Figure 5b) and consequently higher mechanical strength. On the other hand, Bi₂O₃ increases the porosity and decreases the mechanical strength of calcium aluminate cement as their particles that have wide size range and elongated shape. The addition of 15%ZnO:10%Bi₂O₃ mixture to CAC resulted in close values of compressive strength compared to ZnO (25%) and also lower apparent porosity than the Bi₂O₃ (25%) one. Both ZnO (25%) and 15%ZnO:10%Bi₂O₃ mixture additions also resulted in a lower decrease of the mechanical strength over time.

Camilleri and Galdolfi¹⁹ also showed that ZnO (Fischer Scientific) presents very fine particles (lower 3 µm) which are difficult to be distinguished by SEM even at high magnification. On the other hand, Bi₂O₃ (Fischer Scientific) shows elongated needle like shape particles and showed a wide particle size range (5-100 µm).

The calcium aluminate cement dissolution in contact with water promotes the release of Ca²⁺ and Al(OH)₄^- ions, which is followed by precipitation of calcium aluminate hydrate (CAH) and aluminium hydroxide (AH) due to the saturation of the solution²⁵. The type of hydrate formed determines the material properties such as mechanical strength and porosity. The time favors conversion of less stable hydrates (CAH₁₀, less dense) to a more stable one (C₃AH₆, more dense) which may explain the decrease in mechanical strength. The precipitation of AH₃ gel occurs simultaneously inducing the decrease in apparent porosity with time.

3.3. Setting time tests

Measurements of setting time using Vicat apparatus were taken for CAC suspensions with dispersant, plasticizer and the following radiopacifying agents ZnO (25%), Bi₂O₃ (25%) and 15%ZnO:10%Bi₂O₃, as shown in Figure 6. ZnO slightly retarded setting, whereas Bi₂O₃ accelerated setting.

Another important feature for application of CAC as dental material is the setting time. CAC-DP showed a reduced value (close to 20 min.), which matched to the clinical requirements. Additionally, the additives showed little influence on the setting time of CAC-DP compositions [25 wt% of ZnO (close to 29 min.), 25 wt% of Bi₂O₃ (close to 16 min.) and 15%ZnO:10%Bi₂O₃ (close to 23 min)]. The advantage of using cement with a reduced setting time is the less likelihood of constant professional procedures during the treatment. When used as a root-end or root-canal filling material the faster hardening should also reduce the risk of contamination and dislodgement after placement^20,26.

It is noteworthy that similar tests of compressive strength, apparent porosity and setting time using CAC-DP when compared with MTA were previously shown by the authors¹⁰. The CAC presented better fluidity, improved handling properties, greater mechanical strength, and reduced porosity with lower pore size when compared with MTA Angelus.

4. Conclusions

The effect of different additives not only on radiopacity but also on some physical and mechanical properties of calcium aluminate cement used in endodontic treatments was compared. The radiopacifier compound that leads to a higher benefit is Bi₂O₃ but it increases the porosity and decreases the mechanical strength of calcium aluminate cement due to their wide particle size range and elongated shape. On the other hand, the ZnO is comprised by much finer particles than Bi₂O₃ which positively affected the particle packing resulting in the increase of mechanical strength of CAC. Nevertheless, ZnO results in a lower value of radiopacity than that required by ISO 6876:2012. This work shows that it is possible to attain suitable radiopacity for clinical purposes using lower additive content besides the better compromise between physical and mechanical properties by using the mixture ZnO with Bi₂O₃, specially 15%ZnO:10%Bi₂O₃.

Acknowledgments

The authors are grateful to grant #2009/17451-0, São Paulo Research Foundation and National Council for Scientific and Technological Development - Brazil for providing financial support for this research.

Received: March 16, 2014;

Revised: July 8, 2014

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Publication Dates

• Publication in this collection
  05 Aug 2014
• Date of issue
  Oct 2014

History

• Received
  16 Mar 2014
• Accepted
  08 July 2014