Current Issue

A strong and stable bond between denture base resin and removable partial denture (RPD) metal framework requires for the esthetic appearance and clinical longevity of a restoration, which should endure a combination between mechanical, chemical and thermal stresses (1, 2). Therefore, it would be advantageous to find an effective method of achieving a strong and stable bond between the denture base resin and metal framework.

Conventional methods of attaching denture base resin to metal framework of a RPD include macromechanical retention. Previous studies (3-5) have reported the chemical etching methods to create an understructure of micromechanical retention to improve resin bond strength to alloy. Other studies (6-8) have described the sandblasting treatment of the alloy surface to enhance the bond strength between resin and alloy. However, their surface treatment method has depended on mechanical bond of resin and alloy surface, which was more affected by thermal changes than chemical bond systems (8, 9). In vitro studies have also indicated that mechanical bond systems could form a relative weak metal-resin bond. Although these are currently available for chemical bond between resin and metal frameworks, they have lower bond strength when compared with porcelain (10).

The use of silane coupling agents in enhancing the bond of organic matter to inorganic matter is well accepted in the dental literature (11). In the bonding system of resin-porcelain using a silane coupling agent, reliable bonds were achieved (12). To change the filler surface and bond the fillerto the resin matrix of the composite, silanes are also applied as surface treatment agents (13). In these silanes, organic functional groups can co-polymerize with the resin matrix. The silanes also have three alkoxy ready to hydrolyze and then to react with surface hydroxyl groups on an inorganic substrate such as metals/alloys.

Chemically suitable hydroxyl groups are found on the surfaces of titanium, chromium and molybdenum, which form a thin, passive oxide layer. They all contain hydroxyl groups on their outermost surface under standard temperature and alkali treatment (14). Hydroxyl groups on the surface alloy surface play an important role in alloy–resin bond. This is the reason why active hydroxyl groups are the initiation sites for chemical reactions during silane bonds with alloy. Other studies on hydroxyl groups (15) also showed the effects of surface oxidation on the resin bonding to titanium. Therefore, it can be concluded that hydroxyl groups are suitable for the bonding between metal and resins. Extending these concepts, our work was undertaken to provide chemical bonding between examine shear bond strength between cobalt-chromium (Co-Cr) RPD alloys and PMMA resin respectively, using silane and alkali treatments.

The results of the surface roughness for the all groups are presented in Table 2. Two-way ANOVA showed that there were no significant differences of the surface roughness (Ra) values for all groups (p>0.05). The contact angle results are summarized in Table 3. No significant differences were found between surface treatment groups within each Co-Cr alloy.

Table 4 shows the shear bonding strength results. One-way ANOVA showed that there were statistically significant differences between surface treatment groups within each RPD alloys (p<0.001). No significant difference was found in the shear bond strength between ST group and AST group, within each Co-Cr alloys. The shear bond strengths of the UT group and AT group were lowest among the surface treatment groups in each RPD alloy.

In UT groups and AT groups the mode of fracture observed by SEM (Figure 2(a) and Figure 2(c)) were complete adhesive fracture between the alloys surface and the resin. The fracture mode in ST groups and AST groups revealed that in all specimens a combination (adhesive/cohesive fracture) type of fracture had occurred (Figure 2(b) and Figure 2(d)). The effect of silane is to change the fracture mode from adhesive to combination type.

Figure 2. 
SEM images of the alloy surfaces after debonding: (a) UT, (b) ST, (c) AT, and (d) AST groups.

Figure 3 shows the higher magnification SEM images of the fracture surfaces and the EDS of specific sites. The discriminated areas of a combination fracture specimens, revealed the presence of PMMA resin (sites A and B in Figure 3). For EDS analysis selected sites in SEM image are debonded PMMA resin surfaces (area marked as A, the upper part; area marked as B, the lower part) and RPD alloys surface (area marked as C). EDS area analysis indicate that, areas A and B contain higher amounts of carbon and oxygen than the metallic C area. Among them, the A area showed a higher carbon and oxygen amounts than the B area. However, no differences of silicon amounts were found between the areas A and B. The SEM and EDS analyses showed consistent results.

Figure 3. 
SEM images of the alloys of the AST group after debonding and corresponding EDS results: (a) Biosil-F and (b) Vera PDS-Hard.

The current study evaluated the effect of chemical surface treatments on the shear bond strengths between Co-Cr RPD alloys and PMMA resin. No significant differences in surface roughness were found between untreated group and surface treatment groups. The surface roughness for all the groups were very low. In previous studies (3-8, 16), mechanical roughening of metal surface increased the metal-resin bond strength. In our work, all the treatments provided on chemical bonding and there is no mechanical bonding. And so, the bond strengths were generally low. If the surface becomes rougher, it may be possible that bond strengths are higher than the present values.

In restorative dentistry, silanes are used either as a composite filler surface modification agent or as a coupling agent to bond the composite resin with ceramics/metals (17). A schematic image of the chemical reaction of MPS with the surface oxide layer is shown in Figure 4. The alkoxy groups react with acid in aqueous alcohol solution to form intermediate acidic silanol groups (–Si–OH) at the first stage (Figure 4(b)). In the next fast step (Figure 4(c)), silane molecules are adsorbed onto the metal surface through hydrogen bonds formed between silanol groups and OH groups of the metal hydroxides. After condensation upon drying (Figure 4(d)), both Si–O–M (M = metal) and Si–O–Si covalent bonds are formed at the interface, giving an excellent adhesion to the metal substrate, as well as a hydrophobic interface (18).

Figure 4. 
Schematic images of the chemical reactions of organotrialkoxysilane (R; Functional group, R′ Alkoxy group).

The surface modifications by silanes create a silicified layer on the metal. Therefore, a silica-coated surface will be silanized, typically with an MPS solution, and a dimethacrylate veneering resin or other prosthetic acrylic materials are cured onto it (19). However, the procedures are often complicated and basically require a custom apparatus. There is also the possibility of third type, where, a primer is applied on the metal (20). In chair-side clinical applications when silanes are used, the curing is expected to happen at room temperature in the space of a few minutes. Therefore, the primer type is simple and can be easily used in clinical practice. However, metal primers are not always available for all types of dental metal alloys. And because application time of metal primer is short, stable bond between dental metal alloy and dental resin may not be achieved (21). If dental prostheses are made in the dental laboratory, prolonged application times can be used.

In this study, the RPD alloys were treated in silane solution (in ST and AST groups) for long hours to form stable bond between RPD alloys surface and silane. In addition alkali treatment created a lot of hydroxyl groups on the surface of RPD alloys. However, this effect was not seen the Co-Cr alloys. Co-Cr alloys showed strong corrosion resistance in alkaline solution and did not form sufficient bonding hydroxyl groups and so, the bonding and wettability was similar in both ST and AST conditions. Therefore, it seems that there was no effect of alkali pre-treatment over silane treatment in the Co-Cr alloys.

When the concentration of hydroxyl groups increased, the numbers of MPS molecular units bonding with hydroxyl groups are found to increase (22). Therefore, the shear bond strength of metal/polymer interface increased with increase in the concentration of hydroxyl groups on the surface oxide film (22). If drying temperature or time is not high or long enough after silane treatment, stable covalent bond may not form (Figure 3 (c) and (d)). If sufficient covalent bonds are not formed, bond strength of the silane treated surface will be low. Therefore, further research should be conducted in order to improve the bond durability between RPD alloys and PMMA resin, in the AST condition.

SEM images showed that, when untreated groups and alkali treated groups were employed in bonding RPD alloys to PMMA resin the mode of fracture of RPD alloys surface was adhesive fracture (Figure 2(a) and Figure 2(c)) with relative low bond strength values. And combination failure (Figure 2(b) and Figure 2(d)) indicates that, a stronger bond between RPD alloys and PMMA resin was established through the application of silane. Therefore, there is interaction between shear bond strength values and silane treatment. Using area analysis with the aid of EDS, silicon of silane was found on each fractured resin (Figure 3, sites A and B) surface after shear bonding test. Confirmation of fractured resin is derived from the almost exclusive presence of carbon and oxygen in this area. The elemental analysis performed at the RPD alloys surface (Figure 3, sites C) revealed decreased concentration of carbon and oxygen and increased concentration of each alloys composition, signifying the absence of silane on the alloy surfaces (Si element of Cr-Co alloys surface (Figure 3(c) and Figure 3(d)) is the trace element and not from the silane).However, on the fractured resin, EDS analysis revealed complete presence of silane, supported by the amounts of silicon element. This means that fracture of bond happens from bond layer of silane and resin. Therefore, in addition to the bond between silane and metal being important factor to improve the overall bond strength, the binding between silane and resin is also an important contributor this.

Within the limitations of the present study, the following conclusions are drawn.

• 1. The untreated groups (UT) and alkali treated (AT) groups did not show any significant effect of on the RPD alloys-PMMA resin bond strength.
• 2. The silane treated (ST) groups and alkali followed by silane treatment (AST) groups showed similar shear bond strengths.
• 4. For silane treated (ST) groups and alkali followed by silane treatment (AST) groups, the mode of fracture were combination fracture between the PMMA resin and the RPD alloy surface.
• 5. With respect to bond strength, the use of any of the silane treatment in this study apparently would be acceptable for achieving clinically durable bonds of RPD alloys to PMMA resin.