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PROCERA® ALUMINUM OXIDE CROWN

Finite Element Analysis: Determining Bond Strength and
Mode of Failure of 3 Luting Agents Used with Procera Crowns

Rui-Feng Wang, BS
Brien R. Lang, DDS, MS

OBJECTIVES: Bond strength between an all-ceramic crown and cement has been measured using mechanical testing procedures based on load to fracture data. Finite element analysis has also been suggested as a method to measure these bond strengths. Therefore, a study was initiated with the purpose of demonstrate the advantages of the finite element method (FEM) to measure bond strength when a Procera® AllCeram coping is cemented onto dentin using three different cements.
Methods: In this study, mechanical bond strength data for three luting agents and the Procera® AllCeram crown were used to design finite element models with “spring” elements positioned between the luting agent and the crown ceramic surface. The higher the spring value (N/mm) the less the potential for movement at the luting/ceramic interface.
Results: In the cement thickness of 60 µm FEA model, a spring value of 1165 N/mm was needed between the Zinc Phosphate and the ceramic elements at load to fracture of 1099 N. For Fuji Plus the spring value was 511 N/mm at a load to fracture of 999 N, and for Panavia 21 a spring value of 1120 N/mm at load to fracture of 1091 N. The spring stiffness between cement and teeth were 1165, 511 and 1120 N/mm for the above cements, respectively. For the thickness 120 µm FEA model, the spring stiffness of Fuji Plus and Panavia 21 were unchanged, but for Zinc Phosphate the spring stiffness was different than the 60 µm FEA model in order for the FEA to simulate the mechanical test results.
Conclusions: The spring value for Zinc Phosphate was extremely high indicating little or no movement at the interface. The spring value fort Fuji Plus and Panavia 21 were much lower indicating movement during loading. Panavia 21 demonstrated more movement than the other two luting agents which may be an indication of bonding.

INTRODUCTION

In general, the bond strength between an all-ceramic crown and cement has been measured using in-vitro simulation models and a mechanical test referred to as the “crush a crown test.” These studies report the strength of the copings based on load to fracture data and the mode of fracture of the cements on microscopic evidence of cohesive or adhesive bonding. In testing, the mode of fracture of Zinc Phosphate cement is different when compared to a resin reinforced glass ionomer cement (Fuji Plus®) or resin cement (Panavia 21®).[1] In general, Zinc Phosphate cement will experience an adhesive bond or more of a surface attachment of the cement to the tooth or the cement to the ceramic coping. The fracture mode is observed in testing as separation of the cement from the surfaces of the two different interfaces (tooth and coping). In testing, the mode of fracture for Fuji Plus or Panavia 21 is usually cohesive, where the attachment of the cement enters the microscopic surface of the two interfaces. The fracture mode is demonstrated in testing as separation within the body of the cement. Another method suggested to examine load to fracture of a ceramic coping, and the bond strength and mode of fracture of the cement, is finite element analysis (FEA). Therefore, a study was initiated with the purpose to demonstrate the advantages of the finite element method (FEM) to measure bond strength and mode of fracture of three materials used to cement the Procera® AllCeram Aluminum Oxide coping onto dentin.

MATERIALS AND METHODS

A finite element model was created using the software program HyperWorks® 4.0 (Altair Corporation, Troy, MI). The model specifications are presented in Figure 1. The model illustrated in Figure 2 consisted of a die (tooth) representing a maxillary first molar prepared in accordance with the requirements established for the Procera® AllCeram crown (Nobel Biocare, AB). The thickness of the coping was a uniform 0.600 mm with a cement space of 0.060 mm and 0.120 mm were positioned between the surface of the tooth and the inner surface of the coping. A loading stylus was also modeled for the FEM. When the master model was completed, it was replicated to create three models for finite element analysis of the three cement agents. Properties data were entered into the preprocessing file of the finite element program ABAQUS® 6.2-1 (Hibbitt, Karlsson & Sorensen, Inc., Pawtucket, RI) for the cements, dentin, and the aluminum oxide material used in manufacturing the coping (See Table 1). The finite element analysis was performed under a load applied in the center of the coping in N (Newtons) until fracture of the coping occurred.

Fig. 1. The FEM Model Dimensions with Cement Space Equal to 0.060 mm

Fig. 2. The Finite Element Analysis (FEA) Model

The mechanical test load to fracture data for the Procera copings with a cement thickness of either 0.060 mm or 0.120 mm for the three cements is presented in Table 2. No significant differences were found when comparing the load to fracture data of the copings for all three cements at either the 0.060 mm or 0.120 mm cement space thicknesses. No significant differences in the copings load to fracture data were observed when comparing Fuji Plus and Panavia 21 with themselves at the 0.060 mm and 0.120 mm cement spaces. Significant differences were observed in the copings load to fracture data when comparing Zinc Phosphate at 0.060 mm and 0.120 mm cement space thickness.

The load to fracture data for the copings using finite element analysis are present in Table 3. The stiffness of the spring between the cement and the coping was equal to the stiffness of the spring between the cement and the tooth for the adhesive mode of fracture (Zinc Phosphate). But, the stiffness of the above two springs for the cohesive mode of fracture (Fuji Plus and Panavia 21) were unequal. For Zinc Phosphate the spring values for the cement spaces of 0.060 mm (1165) and 0.120 mm (354) were significantly different. For Fuji Plus and Panavia 21 the spring stiffness values for each cement space were the same. However, the stiffness values between the cement and the coping vs. between the cement and the tooth were different (511 vs. 950 and 1120 vs. 1140).

In this finite element study, the boundary conditions between the tooth and the cement and between the cement and the coping were designed with “spring” elements (Figure 3). Spring elements allow for a difference in the characteristics of the attachment between the cement agents and the surfaces of the tooth and the coping. The higher the spring value the more rigid the attachment and the lower the spring value the more flexible (less rigid) the cement attachment.

In the finite element analysis, the copings were subjected to the same load to fracture values for their respective cements as reported in an earlier study. The only variable adjusted was the spring element values in the three models (Table 3). For Zinc Phosphate at the 0.060 mm cement space thickness, the interfaces between the cement and both the tooth and coping were designed with a very high spring stiffness value (1165). This spring value would create an interface that would be stiff and more subject to separation at the interface when the coping was subjected to a load. For Fuji Plus and Panavia 21 the interfaces between the cement and both the tooth and coping were less rigid and were designed with lower spring values (Fuji Plus = 511 and 950, Panavia 21 = 1120 and 1140), respectively. These spring values would create an interface that would be more flexible and permit movement between the tooth and cement and the cement and coping across the interface.

From the FEA, that amount of cement space (poor fit) had a greater influence on the load to fracture data of the copings than the strength of the cements. The lower spring value (354) for the Zinc Phosphate cement when the cement space was 0.120 mm would indicate that the interface between the cement and the tooth and the cement and coping was less stiff. From the results it would appear that fit of the copings becomes a significant factor when using Zinc Phosphate as compared to Fuji Plus and Panavia 21.

Fig. 3. The Stress Distributions from the Finite Element Analysis (FEA)

CONCLUSIONS

Within the limitations of this study, the following can be concluded:
1. To simulate the mechanical test results using finite element analysis, the boundary conditions for the model must use spring elements to create differences in the characteristics of the attachment between the cement agent and the surfaces of the tooth and the coping.
2. The higher the spring value the more rigid the cement attachment and the lower the spring value the more flexible (less rigid) the attachment.
3. The stiffness of the spring between the cement and the coping was equal to the stiffness of the spring between the cement and the tooth for the adhesive mode of fracture (Zinc Phosphate).
4. The stiffness of the spring between the cement and the coping and the cement and the tooth were unequal for the cohesive mode of fracture (Fuji Plus and Panavia 21).
5. For Zinc Phosphate the spring values for the two interfaces for the cement spaces of 0.060 mm and 0.120 mm were significantly different.
6. For Fuji Plus and Panavia 21 the spring values for the two interfaces for the cement spaces of 0.060 mm and 0.120 mm were the same.
7. Although the load to fracture data at both cement spaces of 0.060 mm and 0.120 mm exceed the clinical requirements for strength of the coping for all three cements tested, a poor fit (excessive cement space) reduces the load to fracture value when using a cement that fails by the adhesive mode (Zinc Phosphate) as compared to cements that fail by the cohesive mode (Fuji Plus and Panavia 21).

REFERENCES

1. El-Ebrashi, SK. The effect of coping/die fit of Procera® aluminum oxide copings cemented with different cements. Horace H. Rackham School of Graduate Studies, University of Michigan, Master of Science Thesis, 1998.

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