ORIGINAL ARTICLES DERIVED FROM RESEARCH

EX VIVO EVALUATION OF THE RETENTION OF PROCERA^® ZIRCONIA-YTTRIA SYSTEMS WITH DIFFERENT CEMENTS¹

Richard Milton Mejía Bravo²; Carlos Bernardo Caparroso Pérez³; Xiomara Cristina Ruiz Restrepo⁴; José Fernando Espitia Mesa⁴; Jenny Alexandra Moreno Castillo⁵; Andrés Felipe Montoya Sepúlveda⁵

¹ Research project awarded with a congratulations note and Public Distinction of Merits for ranking first during the Graduate Research Day. August 11, 2011. Email address: rbravo74@hotmail.com
² Dentist, Specialist in Comprehensive Dentistry for Adults with a focus on Prosthodontics. Associate Professor, School of Dentistry, Universidad de Antioquia
³ Graduate students, Comprehensive Dentistry for Adults with a focus on Prosthodontics. School of Dentistry, Universidad de Antioquia
⁴ Graduate students Comprehensive Dentistry of Adults with emphasis in Prosthodontics, School of Dentistry, Universidad de Antioquia
⁵ Undergraduate students, School of Dentistry, Universidad de Antioquia

SUBMITTED: MARCH 13/2013-ACCEPTED: AUGUST 27/2014

Mejía RM, Caparroso CB, Ruiz XC, Espitia JF, Moreno JA, Montoya AF. Ex vivo evaluation of the retention of Procera^® zirconia- yttria systems with different cements. Rev Fac Odontol Univ Antioq 2014; 26(1): 44-61.

ABSTRACT

INTRODUCTION: since the specialized literature does not report reliable protocols for cementing zirconia-yttria restorations, the purpose of this study was to perform an ex vivo assessment of the retention by frictional strength of Procera^® zirconia-yttria systems cemented with four materials: Ketac Cem (3M) ESPE), Multilink Automix (Ivoclar Vivadent), Panavia F 2.0 (Kuraray), and RelyX U100 (3M ESPE) on natural teeth.
METHODS: We gathered 40 recently extracted third molars, which were disinfected with 0.5% NaOCl and stored in saline solution, embedded in acrylic resin, and cut at a 10° convergence angle per wall with an axial carving of 1.5 mm in depth and 4 mm in height. Impressions were taken with Aquasil Ultra^® Dentsply and cast with type IV plaster, obtaining 40 Procera AllZircon (Nobel Biocare) structures. 10 samples per material were cemented and thermocycled x 5,000 cycles of 15 s. They were then tractioned with a universal testing machine and failure types at the different interfaces were analyzed. One-way multiple-range ANOVA statistical analysis was performed.
RESULTS: average retention values in Newtons were 440 N for Ketac Cem, 698 N for Multilink Automix, 686 N for Panavia F 2.0, and 551 N for RelyX U100. Dentin adhesion failure occurred in 53.8% of cases, followed by mixed failure, with 12.82%.
CONCLUSIONS: resinous cements with acid phosphate monomers reported higher retention values in comparison with conventional acid-base cements. Adherence to dentin and cement handling can affect the final result.

Key words: dental porcelain, dental cements, denture retention.

INTRODUCTION

In order to cope with the current increase in patients' aesthetic needs, all-ceramic indirect restorations have been developed, supplied with optical and aesthetic properties which make them very similar to natural teeth.¹

Thanks to the advances in dental ceramic materials, several systems have been developed for manufacturing metal-free restorations. Techniques such as casting and injection, infiltration, and those developed with CAD/CAM technology allow designing and developing restorations with the help of computers. These systems offer diverse applications in prosthodontics and may be made with different materials, one of them being Ythrium- partially stabilized zirconium, a highly resistant material commonly used to produce fixed partial dentures (FPDs).^2-6 Currently, manufacturing this material is possible thanks to commercial systems like Procera^® AllZircon (Nobel Biocare).^{5, 6}

All-ceramic restorations improve retention in a mechanical way, in accordance with dental preparation, including micromechanics, surface modification by sandblasting, or acid/chemical etching with silanes and cementing agents, which hold together restoration and tooth. ^{2, 7} Over time, changes in dental cements' structure and chemical reactions have been implemented, improving their physical properties and going from conventional acid-base cements, such as zinc phosphate and glass ionomers, to resinous cements of the latest generation.^{2, 8-15}

Currently, zirconia restorations partially stabilized with yttria are usually cemented with conventional luting agents or adhesive systems in different commercial presentations.^16-27

Some authors point out that zirconia structures do not require adhesive cementation.²⁸ Other in vitro studies have assessed bond strength among adhesive systems with Bis-GMA and zirconia-yttria systems with surface treatment through glass particles incorporated to air pressure (sandblasting) plus silane application. After mechanically simulating aging, bonding has spontaneously failed.^29-33 Therefore, it has been concluded that resin cements with modified phosphate—such as Panavia F 2.0—are the only ones that provide zirconia restorations with long- lasting bonding. This was confirmed in a long-term study, in which samples were stored for two years using thermal cycling.³² It has been recommended to combine surface treatment with sandblasting and the application of resin cements containing the monomer 10-Methacryloyloxy-decyl dihydrogenphosphate (MDP) to obtain long-lasting bonding to zirconia-yttria.³¹ However, other studies fail to report higher retention values in zirconia structures treated with resinous cement containing MDP.^34-37

Taking into account the importance of cementing agents for restorative dentistry, and given the lack of an established protocol for cementing zirconia- yttria restorations, the purpose of this study was to perform an ex vivo assessment of retention by frictional strength of Procera^® zirconia-yttria systems cemented with four luting agents: Ketac Cem (3M ESPE), Multilink Automix (Ivoclar Vivadent), Panavia F 2.0 (Kuraray) and RelyX U100 (3M ESPE) on natural teeth. In addition, this study intended to determine the type of failure occurring at different cementing interfaces.

METHODS

This was an ex vivo experimental study on a sample of 40 recently extracted third upper and lower molars, with adequate remaining tooth structure and divergent roots. Following extraction, the samples were disinfected with 0.5% sodium hypochlorite, and throughout the process they were kept in a closed container with saline solution at room temperature. The roots were cut for retention and vertically embedded in acrylic, with the cementoenamel junction located 1 mm above the silicone mold of 1 cm wide by 2.5 cm high. The teeth were cut with troncoconical green grain diamond burs, at a convergence angle of 10° per wall, total occlusal convergence of 20° COT, axial carving of 1 to 1.5 mm and 4 mm in height. Each cut was made with a new bur and by a standardized operator. Tooth impressions were taken on 25 x 19 mm acrylic trays after applying an adhesive substance on the mold's inner surface. Polivinilsiloxane was used as impression material in both light and heavy consistency (Aquasil Ultra^®, Dentsply), using one- step technique according to the manufacturer's instructions. The impressions were emptied on type IV plaster (Elite Rock, Zhermack) (figure 1).

After emptying, the master mold was scanned on a Nobel Biocare Procera-Forte^® equipment, transferring electronic information for its production. The design of the proposed structure included occlusal thickness larger than normal, since the coronal surface was increased in the form of a 10 mm cylinder of 5 mm in thickness; the axial walls were 0.6 mm thick. This structure was waxed and both preparations and waxing were scanned. Data were processed in the Procera 2.0 software (figure 2). Structures were burred and sintered according to the manufacturer's protocol for individual Nobel Biocare crowns.

Before cementation, the teeth were cleansed with an organic solvent. Teeth and their structures were randomly distributed in groups of 10 teeth per the 4 cements under study, as follows: group 1 (control), glass ionomer (Ketac Cem 3M ESPE), group 2, Multilink (Ivoclar Vivadent) self-etching dual-curing resinous cement, group 3, Panavia F 2.0 (Kuraray) self-etching dual-curing resinous cement with MDP, and group 4, RelyX U100 (3M ESPE) self-etching dual-curing resinous cement.

Following the manufacturer's instructions, the prepared cementing agent was applied onto the structures, which were placed on teeth by applying 100 g of axial force in order to fix them; this force was measured with a digital device. An oxygen barrier was applied—if indicated by the manufacturer— and cement deposits were removed. Samples in groups 2, 3 and 4 were photopolymerized with a high intensity lamp (BluePhase, Ivoclar Vivadent) to 1,100 mW/cm2 for 40 seconds on each side and at room temperature (figura 3). They were later stored in a container with water at 34° C for 24 h and subjected to thermal cycles in water at 5 and 55° C x 5.000 cycles with intervals of 15 seconds.

Using a traction/compression testing machine (Digimess TC500) owned by the Material Testing Department of Universidad de Antioquia School of Engineering, the samples were subjected to evacuation forces along the teeth's axial axis until failure (figura 4). Failure types were observed through an optical microscope with magnification of 40 increases. Retention was measured as traction force in Newtons (N) and failure types were recorded as shown in tabla 1.

One of the researchers was previously trained in cement manipulation, following each manufacturer's instructions, through a pilot test, using two additional samples for handling protocol standardization. In addition, an intra-operator was standardized on the variables of retention and failure type, according to what had been previously established.

This information was introduced in an Excel^® database and its statistical analysis was made with version 17 of SPSS^®, owned by Universidad de Antioquia School of Dentistry, conducting one-way ANOVA homogeneity tests and Fisher's exact test.

RESULTS

The cementing agents under study, Ketac Cem, Multilink Automix, Panavia F 2.0, and RelyX U100, showed average retention force values of 440, 698, 686, and 551 N respectively (table 2 y figure 5)). The ANOVA test conducted to compare strength in the four types of cement showed that at least one of them is different from the others, with a value of p = 0.013 (table 3). Also, multiple (post-hoc) ranges were analyzed by means of the Bonferroni test in order to establish the cements in which the differences occurred. It turned out to be that only Multilink statistically differs from glass ionomer, while the resinous cements did not show statistically significant differences with each other (table 4).

The results concerning failure type characterization are presented in table 5 y en las figures 6 and 7. The predominant failure type was adhesion to tooth surface, with 53.8%, followed by cohesion to zirconia structure, with 17.9%; other failure types occur in lower percentages within the total sample. When making comparisons within each cement, adhesive failure to tooth structure occurs in 40 to 60%, and the other failure types do not occur in all the cements.

DISCUSSION

The results of this study reject both hypotheses. The alternate hypothesis had suggested that zirconia- yttria structures made with CAD/CAM Procera^® systems and cemented with Panavia F 2.0 (Kuraray) self-etching dual-curing resinous cement with MDP produced higher retention values than conventional Ketac Cem (3M ESPE) glass ionomer cement, Multilink Automix (Ivoclar Vivadent) self-etching dual-curing resinous cement, and RelyX U100 (3M ESPE) self-adhesive self-etching dual-curing resinous cement. The Panavia F 2.0 cement did not produced the highest retention values, with an average value of 686 N, compared with Multilink Automix, which showed an average value of 698 N.

The null hypothesis had suggested that zirconia- yttria structures made with CAD/CAM Procera^® systems and cemented with RelyX U100 (3 M ESPE) self-adhesive self-etching dual-cure resinous cement, Panavia F 2.0 (Kuraray) dual-cured resinous cement with MDP, Multilink Automix (Ivoclar Vivadent) self-etching dual-cure resinous cement, and Ketac Cem (3M ESPE) conventional glass ionomer cement did not show significant retention differences. These results prove that there are significant differences between the Ketac Cem (3M ESPE) and the resinous cements used in this study, especially with Multilink Automix and Panavia F 2.0, which presented a value of p = 0.013.

The findings of the present study agree in some aspects with the study by Palacios,³⁴ who reported higher retention values for RelyX Luting (3M ESPE), with 8,5 MPa, compared with Panavia F 2.0 (Kuraray) 6.9 Mpa, and RelyX Unicem (3M ESPE) 6.7 MPa, which coincides with the results obtained in this study. In both studies, Panavia F 2.0 does not show high retention values when used in cementing CAD/CAM Procera^® zirconia-yttria structures on human teeth, compared with other cements.

However, there are methodological differences between this study and that of Palacios. The latter treats zirconia structures with sandblasting of 50 mm aluminum oxide, but our study did not perform any kind of surface treatment to zirconium, the retention strength was reported in Newtons, and the method of sample size standardization consisted of measuring hard tissue thickness (enamel dentine) of proximal and occlusal surfaces using x-rays, considering the factor of distortion or magnification. In addition, measurements were made with a digital caliper before and after the preparations. As a method of sample standardization, Palacios used copies of the occlusal walls with self-curing acrylic after preparation, which were scanned and compared with the images of 3 circles of known area and perimeter to determine both perimeter and occlusal area in each specimen, using specialized software. Axial length was measured with a digital caliper. To obtain the specimens' axial area values, each sample's perimeter and axial length was multiplied, and to establish total area, the occlusal and axial areas in each specimen were totaled.³⁴

When reviewing other studies that apply similar methodologies, like that of Ernst et al,³⁵ in which zirconia structures (LAVA) on third molars were cemented using luting agents such as Ketac Cem, Panavia F and RelyX Unicem, some coincidences with our study are found in terms of the behavior of resinous cements and conventional ionomer. Ernst reports that ionomer yields one of the lowest retention values, 1.9 MPa, a result that agrees with the one in the present study. By comparing resinous cements such as Panavia F, RelyX Unicem, and other resinous cements used in the same study, their results can also be correlated with ours, since adhesive agents such as C&B SuperBond without zirconia structures surface treatment showed retention values higher than those of Panavia F, with 4.8 Mpa and 4.0 Mpa respectively. Additionally, the author reports higher retention values for RelyX Luting than for Panavia F, with values similar to those of Palacios, ³⁴ in terms of the behavior of these cementing agents.

The results of the present study also correlate with those of Shahin and Kern³⁶ concerning the behavior of conventional and resinous cements. These authors used premolars that were prepared for full crowns with an angle of convergence of 12° and 3 mm in height. They later adhered zirconia structures (VITA In ceram YZ, machined with the Cerec system) using adhesive agents such as prompt-natural zinc phosphate cement, Ketac Cem (3M ESPE) conventional glass ionomer, and Panavia 21 (Kuraray) resinous cement. The purpose of their study was to assess retention of zirconia crowns with resinous and conventional cements as well as the effect of surface treatments and aging on retention values. Their results show that Panavia 21 has better performance as a bonding agent in retaining zirconia structures without surface treatment, obtaining 5.8 MPa, compared with Ketac Cem, which reported approximately 2.8 Mpa.

Concerning Multilink Automix, which according to our study offers better retention than Panavia F 2.0, the study by Ernst,³⁷ who evaluates retention values of conventional and resinous cements on zirconia crowns, yields results similar to ours. The author uses human teeth prepared for full crowns with 10° convergence angle and 3 mm in height, and cements LAVA structures with modified FujiCem ionomer resin, Variolink II/Syntac Clasicc conventional adhesive cements, Panavia F 2.0/Multilink self- etching resinous cements, and RelyX Unicem/ Maxcem self-adhesive resinous cements. He treats surfaces with Rocatec^®, preparing only an additional group with RelyX Unicem Aplicap, and carries out the cementation protocol for each cement, according to the manufacturer's instructions.

His study reports higher average values for RelyX Unicem (7.5 Mpa), RelyX Unicem/Rocatec (7.2 MPa), Multilink/Monobond S (5.4 Mpa), and Multilink/Metal prime (5.3 MPa), compared with the 2.1 MPa value of Panavia F 2.0. These results agree with those obtained in the present study in terms of the behavior of Multilink Automix. However, in our study the statistical difference is not as significant as that of Ernst.³⁷

It is interesting to note the superior behavior of Multilink Automix according to the studies by Ernst³⁷ and our own study, which can be attributed to the presence of acid monomers in this cement. Studies by Mirmohammadia16 and Thompson41 mention the presence of new acid monomers produced by manufacturers, since the MDP molecule patent is owned by Kuraray. These new molecules are added to resinous cements formulas, which in the case of Multilink Automix is a self- etching phosphate monomer characterized by hydrolytic stability, a terminal phosphate and at least two bonding sports to the resin matrix by oxygen bridges. Concerning RelyX U100, it is a self-etching phosphorylated methacrylate designed for direct bonding to enamel and dentin, with two phosphate groups and at least two double carbon bonds, which demonstrate high adherence values to zirconia and interspersion to the resin matrix.

In addition to the chemical bonding mechanism, the Multilink Automix manufacturer specifies the use of primer as a zirconia structure conditioner prior to the application of resinous cement, which comes in the form of an automix syringe for direct application on restorations. Concerning Panavia F 2.0 and RelyX U100, plasters A and B are manually dispersed and mixed on the mixing pad, and this could alter the base/catalyst plaster proportion while applying. Studies like the one by Behr et al,⁷ which assessed changes in the physical properties of some cements, including Panavia F 2.0, showed that it is sensitive to inappropriate mixing ratio, especially when chemically polymerized, and mixtures of plaster A to a lesser proportion than plaster B cause significant decrease in flexural resistance values, compared with the 1:1 ratio defined by the manufacturer. Another aspect to consider is that manual mixing can add oxygen, resulting in polymerization inhibition of approximately 100 mm of the material exposed to oxygen, even after cementing along the restoration edges, so it is recommended to use barriers such as glycerin in agents such as Multilink Automix and Panavia F 2.0.

With respect to failure type, Palacios' study³⁴ is similar to the present study in terms of adhesive failure to tooth structure being the predominant one, since the most quantity of cement was found on the zirconia structures. However, the second type of failure turns out to be adhesion to zirconia, followed by cohesion to it, and the three types of failures occurred on Panavia in a higher percentage. In the present study, the second type of failure was cohesion to zirconia structure, especially on Panavia 21 and RelyX U100, followed by mixed failure, which occurs on Ketac Cem in a higher percentage.

Additionally, the results of this study and those reported by Shahin and Kern³⁶ are similar in terms of failure type, since Ketac Cem shows adhesive failure to tooth and to zirconia structure in a proportion of 50% per failure type, while in the case of Panavia 21 the main failure type is adhesion to tooth, with a higher percentage of resinous cement adhered to the zirconia structure, exceeding 75%, which is considered by the authors as adhesive failure of cement to dentin.

These results suggest that adhesion to zirconia structures with or without surface treatment is no longer a problem to solve. The studies show that resinous cements are bonded to the ceramics of zirconium oxide stabilized with yttria, reporting high adhesive values without surface treatment, and that these values considerably increase with the different surface treatment systems, from sandblasting with aluminum oxide particles in different sizes to the methods most recently developed by manufacturers, such as selective infiltration and acid etching at high temperatures.^{36, 41-45}

CONCLUSIONS

1. Resinous cements containing acid phosphate monomers show higher retention values to tooth surface in zirconia-yttria structures, as proven in this study (with 698 N for Multilink Automix) when compared with conventional acid-base cements.
2. In this study, the predominant failure type is adhesion to dentine in each of the cements used, in comparison with other types of failure. This suggests that dentin as an adhesion substrate remains difficult to handle, even with the manufacturers efforts to optimize their products.

ACKNOWLEDGEMENTS

To the Polymer processing Laboratory, Polymer Material Group. Universidad de Antioquia. Coordinating Engineer: Diego H. Giraldo V. Tests performed by material engineers: Nadia Henao and Rubiela Montoya.

CONFLICTS OF INTEREST

The authors declare having no conflicts of interest.

REFERENCES