The Effect of Surface Treatments on the Shear Bond Strength of Acrylic Resin Denture Base With Different Repair Acrylic Resin: An In Vitro Study.

BACKGROUND: Fracture of the denture base is a common problem associated with dental prostheses. Fractured denture base surfaces treated with chemical agents and mechanical features have the potential for improved bond strength. AIM: The aim of this study was to evaluate the effect of surface treatment on the shear bond strength of heat-cured denture base with different repair acrylic resins.
MATERIALS AND METHODS: A total of 100 circular specimens (2-cm diameter × 3.3-mm thickness) were fabricated from heat-cured denture resins (DPI) according to the manufacturer's instructions. The heat-cured denture base acrylic resin specimens were divided into two groups: In group 1, auto-polymerizing acrylic resin (DPI) was used as a repair resin, and in group 2, light-cured acrylic resin (VLC) was used as the repair resin. Further, the heat-cured denture base acrylic resin specimens were subdivided into five subgroups. The shear bond strength (in megapascal) was measured in a universal testing machine at a crosshead speed of 1 mm/min. The results were subjected for statistical analysis. RESULT: Comparison of mean and standard deviation of shear bond strength between DPI and VLC group using one-way analysis of variance showed that the mean shear bond strength of DPI group is higher than that of VLC group.
CONCLUSION: From the study, heat-cured denture base specimens repaired with auto-polymerizing repair resin showed higher mean shear bond strength than the visible light cure resin material.

INTRODUCTION

Denture bases made of acrylic resin are preferred because they chemically bond to denture tooth and are easier to adjust. Unfortunately, removable prosthesis of acrylic resin will fracture if dropped or stressed beyond their fractured strength. Many techniques and materials have been used to repair fractured denture. The ultimate aim is to restore the original strength of denture and to avoid further fracture. Denture repairs involve joining two parts of fractured denture with a denture repair material.

Repair of denture base is accomplished by an auto-polymerized repair acrylic resin and grinding the surface of the denture base. Attempts to improve bond strengths of denture base to the repair acrylic resin have involved mechanical and chemical treatments. Some authors have shown increased bonding strength by grinding a diatoric recess, which is a channel placed in the denture base to serve as a mechanical feature of retention. Recent studies tested an acrylic resin denture base surface treated with airborne-particle abrasion using 50-µm aluminum oxide particles. The study showed that airborne-particle abrasion increased bond strength and a larger particle size for air abrasion further improved the bond strength.[1] Priming of the denture base surface with monomer liquid during processing yielded significantly higher bond strength values than other surface treatments.[23]

Polymethyl methacrylate (PMMA) is the most commonly used material for denture base fabrication. However, there have been reports of allergic response in patients sensitive to methyl methacrylate monomer.[4] Other organic solvents such as chloroform,[5] acetone,[6] ethylene chloride, and ethyl acetate have also been used for repair process. In some studies, organic solvents showed increase in bond strength between acrylic repair resin and denture base resin.[57]

This study evaluated different surface treatment methods for denture base repair. The bond strength between the denture base and the auto-polymerizing repair acrylic resin and light-cured acrylic resin was tested. The purpose of this study was to determine the bond strength of an acrylic resin denture base repaired with auto-polymerizing repair acrylic resin and light-cured acrylic resin. This would lead to a recommendation for repair of acrylic resin denture base, and a method that would reduce the likelihood of further fracturing of the denture base.

MATERIALS AND METHODS

A total of 100 circular specimens (2-cm diameter × 3.3-mm thickness) were fabricated from heat-cured denture resins (DPI) according to the manufacturer’s instructions. The procedures for mixing and packing the resin into die stone molds were according to the conventional laboratory procedure for denture processing. The temperature was maintained at 74°C for 90 minutes, and then it was raised to 100°C for additional 30 minutes. The flasks were cooled to room temperature and the specimens were removed from the molds. Flash was carefully removed; to standardize, excess of 0.3 mm was removed by grinding on 600-grit silicon carbide polishing paper to remove surface irregularities and compressed air was applied to remove excess material.[89] Specimens were stored in distilled water at room temperature for 48 hours before testing, to simulate the effect of saliva.

After water storage, all specimens were ultrasonically cleaned with distilled water and dried with compressed air. The heat-cured denture base acrylic resin specimens were divided into two groups: in group 1, auto-polymerizing acrylic resin (DPI) was used as a repair resin, and in group 2 light-cured acrylic resin (VLC) was used as the repair resin.

Further the heat-cured denture base acrylic resin specimens were divided into five subgroups: for auto-polymerizing acrylic resin (DPI; n = 50), group A (n = 10), group B (n = 10), group C (n = 10), group D (n = 10), and group E (n = 10), and for light-cured acrylic resin (VLC; n = 50), group F (n = 10), group G (n = 10), group H (n = 10), group I (n = 10), and group J (n = 10). These are detailed as follows:

Control group, which is untreated and no surface modification (group A [n = 10], group F [n = 10])

150-µm alumina air abrasion at a right angle to the surface from 5-mm distance for 10 seconds at an emission pressure of 0.48 MPa using a delta blaster (group B [n = 10], group G [n = 10])

Immersion in acetone (chemical etchant) for 30 seconds (group C [n = 10], group H [n = 10])

Diatoric recess on ridge lap area of the denture base. Size of the recess was standardized by the 701 straight fissure carbide bur used with low-speed micro motor fixed in the dental surveyor mandrel (group D [n = 10], group I [n = 10]).

Combination of 50-µm alumina air abrasion followed by the application of acetone for 30 seconds (group E [n = 10], group J [n = 10]).

All the specimens were mounted on flat surfaces facing up in the plastic holder (PVC pipe 25-mm diameter, 20-mm height) Figure 1. For auto-polymerizing acrylic resin (DPI), the specimens were mixed according to the manufacturer’s recommendation and placed into the PVC pipe, and excess material was removed from the top of the pipe. For light-cured acrylic resin (VLC), a brass ring (7-mm inner diameter and 2.5-mm height) Figure 2 was used to add the repair resins, and the specimens were cured in the Triad curing unit with a stable platform as recommended by the manufacturer.

Figure 1

Prepared specimens

Figure 2

Specimens with brass mould

The shear bond strength (in megapascal) was measured in a universal testing machine Figure 3 at a crosshead speed of 1 mm/min. The results were subjected for statistical analysis.

Figure 3

Shear bond strength tested in universal testing machine

RESULTS

Comparison of mean and standard deviation of shear bond strength between DPI group and VLC group using one-way analysis of variance [Table 1] showed the mean shear bond strength of DPI group is higher than that of VLC group but the increase in mean value is statistically insignificant.

Table 1

Mean shear bond strength values for acrylic repair resins and different surface treatment

Repair materials	Surface treatment	Mean ± SD	Number	Trimmed mean (5%)*
DPI–RR cold cure	Untreated	4.555 ± 1.701	10	4.575
	Air abrasive	4.428 ± 1.671	10	4.411
	Acetone	2.603 ± 0.531	10	2.561
	Diatoric recess	4.354 ± 1.578	10	4.296
	Air abrasive and acetone	3.287 ± 0.337	10	3.282
	Sum	3.845 ± 1.476	50
VLC	Untreated	1.986 ± 1.072	10	4.575
	Air abrasive	2.740 ± 1.305	10	4.411
	Acetone	1.882 ± 0.789	10	2.561
	Diatoric recess	2.909 ± 1.266	10	4.296
	Air abrasive and acetone	3.445 ± 1.33	10	3.282
	Sum	2.592 ± 1.265	50

Significant at α < 0.05.

Further analysis with Tukey’s comparison procedure for multiple testing revealed that the mean shear bond strength was significantly different between all the groups. No significant differences (P > 0.05) were detected between the control group and specimens treated with acetone, diatoric recess, and combination of air abrasive followed by acetone according to the Tukey’s multiple comparison test. The shear bond strength of the mechanically treated samples was significantly higher than the control or chemically treated groups (P < 0.05).

DISCUSSION

In clinical situation, the fracture of heat cure denture bases is a common problem in both removable and complete dentures. Attempts to repair the fractured denture bases have been generally performed using auto-polymerizing resin with variable rate success. Recently, visible light cure and microwave cure denture base materials have also been used to repair the fractured denture bases. When repairing the denture base, it is important to know whether the bond strength between the denture base and the repair material is adequate, which is mandatory for the success of the repair base.

To improve the bond strength between the fractured denture base and the repair material, several mechanical and chemical surface treatments of the denture bases were tried. In this study, the shear bond strength of heat cure denture base with auto-polymerized and visible light cure resin is compared.

Shear strength is the maximum force that a material can tolerate before shear failure. Investigation of the shear bond strength of material interfaces is important. Many studies examined the transverse strengths of repaired specimens of acrylic resin.[1011] As the number of interface failures was more than cohesive failure, it is essential to evaluate the bonding failure of repair resin to base material by measuring shear bond strength. The shear bond test applies a shear load directly to the interface between repair material and denture base resin, which allows the results to be easily compared between materials.

The repair of a denture base can be performed using several materials, such as auto-polymerizing acrylic resin, heat-cure acrylic resin, visible light-polymerized resin, and microwave-polymerized acrylic resin. The choice of material depends on the working time, the strength to be obtained with the repair material, and the degree of dimensional stability maintained during and after repair.[12] Most debonded denture teeth repairs are made using a resin, which is generally simple and quick.[13] Specimens repaired with auto-polymerizing acrylic resin have approximately 60–65% of the original strength of the denture whereas the strength of heat-polymerized acrylic resin repairs ranges from 75% to 80% of the original bulk material. Repairs using heat-cure acrylic resins are seldom performed because they require a custom split cast gypsum mold, extended treatment time, and additional laboratory fee.[14] The patient must also be without the denture during the laboratory repair procedures. To overcome the limitations of heat-polymerized and auto-polymerizing acrylic resins, a visible light-polymerized system was introduced in 1984 and has been used in several applications including relining and repair of dentures.[7]

When a denture base is repaired, the bond strength between denture base resin and the repair material should be as strong as the parent denture base resin. The success of denture repair, however, depends on the adhesion between the repair material and the denture base.

Effect of mechanical treatment

Different opinions about the appropriate shape of the joint surfaces have been reported. Several studies have indicated different edge profiles, such as a butt joint,[1516] 45° bevel joint,[141716] bevel joint rounded,[1218] and rabbet joint.

Mechanical surface treatment prior to denture base repair resulted in a significant improvement in the shear bond strength of the base materials. This finding is in agreement with a study by Minami et al.[1] He reported a significant increase in bond strength between the denture base resin and an auto-polymerizing resin; the mean bond strength of the specimen was 8.2 MPa. In addition, Jagger et al.[19] found that a rough surface increases the friction between the denture base and the repair material, requiring more debonding force at the interface. Amarnath et al.[20] recommended sandblasting the acrylic denture base prior to repair and found that the bond strength of self-cure resin with denture base was 2.90 MPa in control group and 7.56 MPa in sandblast group.

Effect of chemical treatment

Chemical treatment with etchants on the surface causes crazing as well as the formation of numerous pits up to 2 μm in diameter. Wetting the repair surfaces with methyl methacrylate monomer has been used to soften the PMMA, which changes the morphology and chemical properties of the surface promoting adhesion. Vallittu et al.[22] found that when the monomer treatment was performed for 180 seconds, favorable results were attributed by the formation of new polymer chains between the heat-polymerized acrylic resin fracture surfaces. Similarly, Olvera and DeRijk[23] observed that monomer treatment for 4 minutes was the optimum treatment time for repairing denture base specimens. Alternatively, chloroform, acetone, and methylene chloride have been used as softening agents in several situations, including repair of denture bases.

Effect of water storage

It has been demonstrated that the strength of a denture repair may be time dependent. According to Harrison et al.,[24] auto-polymerizing acrylic resin repair is relatively weak at 1 hour after the laboratory procedure is completed. Paired specimens reached optimum strength after 1 day to 1 week of water immersion. Razavi et al.[7] found that the shear bond strength of visible light-polymerized resin significantly increased after 48 hours of water immersion. From the available literature, it can be assumed that repaired materials generally do not reach their optimum properties the following day, and for this reason, repaired dentures ideally should not be returned to the patient for at least 24 hours.[1]

The type of repair material exerts a considerable effect on the bond strength. In clinical situations, fractured denture base repaired with auto-polymerizing resin provide better bond strength, reduce the chair-side appointments, and enable faster repair. This study recommends mechanical treatment on the denture base prior to repair for optimum results.

CONCLUSION

The success of PMMA denture repair depends on many variables such as combination of denture base resin and repair materials applied, repair surface design, repair surface treatments, and use of reinforcements. Few studies simulating clinical conditions of repair dentures have been performed.

From the study, heat cure denture base specimens repaired with auto-polymerizing repair resin showed higher mean shear bond strength than the visible light cure resin material. Also, mechanical surface treatment improved the bond strength than chemical surface treatment. So it is concluded that mechanically surface treated auto-polymerizing resin repair is more effective in the denture repairs. Future investigations should incorporate more closely simulated clinical conditions such as construction of denture-shaped base specimens.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.