StomatologyEduJournal1-2015 | Page 14

DENTAL MATERIALS likely cause drift of the grid during processing. Previous studies have shown that fibers or mesh without surface modification would not render the denture base any strengthening effect. Some studies even showed negative effect. Substantial improvement of strengthening was observed only when the surface of the strengthener fiber or mesh was gritblasted, coating of bonding agent or combination of the two.17 The results of this study showed basically that the metal grid alone had a statistically significant strengthening effect in flexural strength and modulus. However, the improvement of flexural strength from the grids receiving gritblasting and coating with bonding agent (Group 4) is not statistically significantly from the control (Group 1). Contrary to previous studies, the results show that there is no statistical difference among the groups comprising strengtheners. In other words, grit blasting and coating of bonding agent provided no additional benefit in enhancing flexural strength and modulus. Grit blasting is a well-recognized method of surface roughening that leads to bonding enhancement. In addition, the bonding agent used in the study is an ethanol solution known to consist of three distinct compounds; namely, silane methacrylate, phosphoric acid methacrylate and disulphide methacrylate.16 They are designed to bond to three different surfaces: silane targeting silica (not present in the test groups), a phosphate containing methacrylate focusing on metal oxides (in this case chromium oxides) and a disulfide methacrylate aiming for gold. The cause of inability of the bonding agent and grit blasting to enhance flexure strength and modulus should be clarified. When glass fibers in the form of woven, loose and continuous fiber bundles were first investigated for reinforcing PMMA denture base resin, they were either mixed discretely with the PMMA dough or by lamination with glass cloth.18 The addition of fibers did not give a substantial improvement to the tensile strength caused by the lacking of adhesion between the polymer matrix and the untreated fibers. These fibers acted as inclusion bodies in the PMMA resin mixture that actually weakened the resin instead of strengthening. Treating glass fiber with silane compounds before incorporating in the PMMA dough presented a potential of strengthening PMMA.19 If the fibers were made into cloth form instead of loose form, untreated cloth form fibers had 104 demonstrated strengthening effect whereas untreated loose glass fibers had a weakening effect.17 Using metallic inserts in reinforcing PMMA denture base resin yielded similar results, where embedding metal fibers weakened the polymer due to poor adhesion between the acrylic resin matrix and the metal fiber inserts.17 Gritblasting17, silanization of the metallic insert20, and metal bonding adhesive resin21 have been reported to improve the adhesion between the metal surface and acrylic resin. It is of interest to observe that untreated metal grid strengthened the acrylic resin but no additional improvement was realized when the metal grid had been gritblasted or coated with bonding adhesive for metal surface. This unexpected result could only be caused by the design of the grid. Metal grid used in this study is a perforated stainless steel with staggered pattern of ø 2.5mm holes and 1.25 mm between adjacent perforations. When this grid is incorporated during the processing described earlier, the resin fills the perforated spaces (Fig. 2). The result is an interpenetrating structure between the metal grid and denture resin that interlocks the resin and grid mechanically. Therefore, as the resin component is being stressed during testing, the metallic component will be subjected to the same stress simultaneously. In other words, metal grid strengthener exhibits strengthening effect without the need of bonding between the metal grid and the resin. Although the study shows that resultant bonding from gritblasting and adhesive coating would not improve the flexure strength and modulus of the denture base resin, microleakage can still occur from lack of bonding or poor bonding between the denture base resin and the cast metal framework of a removable partial denture. It can cause discoloration and staining of the margins at the metal–resin interface over time.22 Therefore, the need of a durable bonding between the metal grid and the denture resin remains critical with respect to staining over time. Should that type of bond be achieved by gritblasting, adhesive resin or combination of the two should be investigated further by additional in vitro studies with specimens after a long-term storage in appropriate solutions or through longterm clinical observation. The presence of a metal grid within a denture base resin qualifies the specimen design in this study as a composite structure and the mechanical property of the final structure will depend on those of the metal grid and the resin. STOMA.EDUJ (2015) 2 (2)