Evaluation Of The Film Thickness Of New Adhesive Luting Resins

Recently, several composite resins have been reformulated as adhesive luting agents. The film thickness of some resinous cements has been reported in the literature, however the results were inconsistent. The purpose of this investigation was to compare the film thickness of five relatively new adhesive luting resins with that of zinc phosphate cement. The method used was generally in compliance with the American Dental Association (ADA) specification No. 8 for zinc phosphate cement. Tested materials were manipulated as described by the manufacturer. The film thickness of each cement was measured 10 times. The results, presented in an increas­ing order of film thickness were: Mirage FLC, Panavia Ex, Voco F21, Flecks zinc phosphate, C & B Metabond, and finally All-Bond. The film thickness of the adhesive luting resins tested were comparable to or thinner than zinc phosphate cement except for All-Bond luting resin. Sugges­tions are made regarding the selection of an adhesive luting resin. 

When Bowen¹ first introduced resins, based on aromatic dimethacrylates of the Bis-GMA type, they exhibited improved properties over resins based on methyl methacrylate. This was especially true regarding viscosity, wettability, lower polymerization shrinkage, and a strong bond to tooth structure. A variety of Bis-GMA based restorative resins are used as luting agents.² A com- monly reported disadvantage of these luting agents was their high film thickness, a situation that may prevent complete seating of the restoration, and could lead to decreased retention and increased solubility.³'⁴²⁵⁶ Reformulated adhesive luting resins are marketed and used more routinely for the cementation of conventional fixed partial denture.⁷ Although the film thickness of certain available brands has been reported in the literature,⁸¹¹ the results were inconsistent.

The purpose of this study was to determine the film thickness of five new adhesive luting resins and compare it with that of zinc phosphate cement, since it is the most "tried and proven" material for permanent conventional cementation.³ However, the chemical components, filler particle size, viscosities, diluting monomers, and setting reactions were reformulated to result in a lower film thickness of the various new adhesive luting resins. 
 

Five adhesive luting resins plus zinc phosphate cement were evaluated in this study (Table 1). The total was six groups, each group included 10 sam- ples. The technique for measuring the film thickness was generally in compliance with ADA specification No. 8 for Zinc phosphate cement. Tests were con- ducted at room temperature of 23°C ± 2°C and a relative humidity of 50% ±  10%. Materials were manipulated according to the manufacturer's instructions. Immediately after mixing each cement, the materials were placed between two identical flat plastic discs*, 3/4 inch in diameter and of uniform thickness and smoothness, then sustained under 15 kg static load for 10 minutes with a custom made loading device [Fig 1]. Following loading, the com- posite of 2 plastic discs with the cement film in between was sectioned at three levels [Fig.2] to facilitate direct measurement of the resin film at dif- ferent localities under a magnifying lens of a travel- ling microscope equipped with a filar unit measuring eyepiece with an accuracy of 0.5 micrometer**. The lowest reading was registered as film thickness for each specimen. The mean film thickness for each group and it's standard deviation was then calcu- lated. The resulting data were subjected to one way analysis of variance (ANOVA) to determine whether differences exist among the groups, and Tukeys' multiple range test to determine which groups are statistically similar.

The mean film thickness and standard deviations for all the groups are presented in Figure 3. ANOVA revealed significant differences among groups at the p < 0.05 level. Table 2 shows a sum- mary of Tukeys' multiple range test.

Luting resins with the least film thicknesses were: Mirage FLC, Panavia Ex and Voco F21 with no statistical differences among them. Zinc phosphate cement and C & B Metabond luting resin had statis- tically significant higher film thickness than Mirage FLC, but not when compared to Panavia Ex and Voco F21. All-Bond luting resin exhibited the thic- kest film among the six cements tested in this study with highly significant differences when compared to the other groups.
 

The standard test for film thickness outlined in ADA specification No. 8 for zinc phosphate cement¹² requires loading of the cement between two glass discs. The film thickness was then deter- mined by subtracting the initial thickness of the two glass discs before loading the cement from their thickness and after loading using a micrometer. In this study, a modification of the ADA testing technique was used. The glass discs were replaced by plastic discs. This modification allows multiple sectioning of the discs in order to make direct mea- surement of the film thickness at different localities. Interestingly, a variation in the film thickness was observed at various localities between the two discs. The authors attributed the differences of film thickness at various localities on each specimen to the viscosity and unequal distribution of the mate- rial during loading. This observation confirms the findings of Jorgensen¹³ and Windeler¹⁴ that the ADA film thickness testing method, and the method used in this study determines the viscosity and not the grain size of the cement. However, having sectioned the plastic discs, we were able to determine the minimum film thickness of the tested cement sandwiched between the two plastic discs. The results of this study indicated that the film thickness of Panavia Ex and All-Bond luting resins were considerably lower than the findings reported by White.¹⁰ The differences in the results of the two studies could be attributed to replacing the glass discs with plastic discs. Since multiple sectioning of the discs was possible, we were able to measure the minimum film thickness rather than the maximum. However, the film thickness of Panavia Ex in this study was in general agreement with those recorded by Sadig¹⁵ and Tjan⁹ confirming the lower film thickness of Panavia Ex luting resins when compared to zinc phosphate cement. The agreement is attributed to the fact that both previ- ous studies used the tapered die system, i.e. natural teeth and castings. The determination of film thick- ness using the tapered-die system always demonstrated a lower film thickness when com- pared to the ADA technique.¹³¹⁴⁹ theorized the improved seating of crowns cemented to natu- ral teeth with resin cement to the thixotropic phenomenon and the good flow of the cement and the lubricating effect, thereby eliminating or reduc- ing the coefficient of friction between the casting and the prepared tooth.

According to ADA specification No. 8, type I cements with film thickness less than 25 /xm are designed for the accurate seating of precision appliances. ADA type II cements can have maximum film thickness of 40 /xm which had been recommended for all uses, except the cementing of precision appliances.¹² All materials tested in this study can generally be categorized under type I cement excluding the All-Bond luting resin. This exception is because the mean film thickness of the All-Bond cement was quite high, around 60 /xm.

While the ADA standard test for film thickness registers the maximum film thickness, the section- ing method employed in this study allows for the determination of the minimum film thickness sandwiched between the two plastic discs. Since the reported film thickness of cements per ADA glass discs technique is higher than what had been observed in this study, the authors are of the opin- ion that the standard ADA film thickness test might not represent detailed experimental determination, and the classification of cements as type I or II might not be clinically significant. This conclusion is sup- ported by the fact that cements with film thickness exceeding the ADA recommendations are used routinely and successfully in cementing cast resto- rations with acceptable marginal gap integrity and clinical longevity.⁷'¹¹¹³¹⁶ reported that a film thickness of 25 ^tm provide maximum resis- tance to dissolution. However, when Mesu¹⁷¹⁸ compared the solubility rates of different classes of luting agents, they found that the solubil- ity rate of resinous cements was almost nil in addi- tion to their acceptable wear characteristic. Hence, Christensens¹⁹ stated that the significance of the film thickness of luting resins is questionable, since the cement is insoluble. Further, Burke⁷ reported that because of the high bond strength of the adhe- sive luting resins to both tooth structure and cast- ings, these materials are used routinely for the cementation of conventional fixed partial dentures even when the film thickness exceeds the ADA specification. One particular study,²⁰ showed that the strongest bond strength of resinous cements occurred when the film thickness was 80 /xm, which was later confirmed by Dixon et al.²¹ It is also worthwhile to note the findings of Jorgan²²/xm is ideal but 50 micrometer does not present a noticeable biological risk. In addition to this, several inves- tigators had considered marginal gaps, less than 100 m, as clinically acceptable.²⁴²⁵ Tjan Fusayama and Plum that a restoration with a film thickness of 25 Therefore, the authors believed that the film thickness is just one factor to be considered when selecting an adhesive luting resin. Other factors of important considera- tion include compressive and tensile strengths, handling characteristics, shelf life, ability to release fluoride, esthetics and economics.

From the results of this study, it is concluded that: