Introduction

Acrylic and porcelain tooth are the two commonly available materials to rehabilitate edentulism. But acrylic resin teeth are generally preferred to porcelain teeth as a functional unit with acrylic resin denture base as they are bonded by chemical attachment resulting in an overall stronger one unit denture [1].

The failure rate of acrylic resin dentures due to fractures is excessively high [2] and the most prevalent type of failure encountered is the teeth fracture or debonding [3]. The most prevalent type of denture failure occurs between an acrylic resin denture base and acrylic resin tooth, with a prevalence of about 33% [4, 5]. According to studies, 26-33% of denture repairs were due to debonded teeth, commonly leading to cost and distress for patients [6-8]. This detachment is probably due to the lower ridge lap surface area available for bonding and the direction of the stresses that occur during the functions. The bond failure between denture base denture and teeth was mostly cohesive or adhesive [9-12]. An adhesive failure occurred when no trace of denture base material was observed on the ridge laps of the teeth after a fracture. Besides, when there were remnants of the denture base material on the ridge lap of the teeth after fracture bond, the failure was considered cohesion failure [9]. The adhesive failure may be affected by changes in bond interfaces due to laboratory errors the type of resin base material used, the presence of impurities on the tooth surface in close contact with the denture base, and its chemical and physical characteristics [13-16].

Studies report that surface modification on the ridge lap portion of the acrylic tooth will drastically enhance the bonding nature of the tooth. And many studies had been advocated different surface modifications of the tooth. On the other hand, the type of polymerization technique plays an unavoidable role in the strength of polymers. Various studies reports that the injection molding polymerization technique seems to be more advantageous than conventional and microwave polymerization techniques in terms of dimensional stability, accuracy, and strength. But then, no studies had been employed to measure the bond strength of surface modified acrylic teeth polymerized under injection molding technique.

Materials and Methods

A total of 51 polymethyl methacrylate (PMMA) maxillary right central incisor (SR IVOSTAR Small-bold 41of Ivoclorvivadent, USA) with measurement of 9.6mm inciso cervically and 7.7mm mesiodistally were selected. A black line was scribed around the cervical area, 1mm above the bottom of the tooth (ridge lap area) using a vernier caliper. Then the samples were grouped as,

Group A – 17 samples - No surface treatment (Control)

Group B – 17 samples – Sandblasting on the ridge lap surface

Group C – 17 samples – Diatoric cavity on the ridge lap surface

Surface treatment

The samples of Group A acts as control which will not have any surface treatment over the ridge lap surface. A putty index (Aquasil soft putty, Dentsply, India) of 3×3cm was made to held the tooth in position while doing surface treatment for Group B and C samples. The ridge lap area of Group B samples was treated with 50 μm aluminum oxide particles (Aluminox, Delta, Chennai, India) at a pressure of 4psi which was kept at a distance of 1 cm for 10 seconds. Group C samples were modified with a diatoric cavity (2.3 mm diameter x2 mm deep) by using round bur no:8 (Midwest, Dentsply, India) at a speed of about 40,000 rpm in a milling machine (Amann Girrbachaf 350, Austria) (Figure 1). To maintain the standardization of procedure, a total of 11 samples were surface treated by a single observer in a day.

Mold preparation

A square metal mold containing 8 slots of diameter 7.5mm, height 7.5mm with a flat base was constructed for the fabrication of specimens (Figure 2).

Specimen preparation

The mold was filled with modeling wax (Hindustan modeling wax No:2, India) up to the height of mold, on which the PMMA teeth (control and surface treated) were immersed up to the scribed cervical line, which forms 45° angulation with the flat base of wax. The obtained wax patterns of samples were packed in a single flask (Ivoclar BPS processing flask, USA) using type 2 dental stone (Gyprok, Australia). To standardize the procedure, single flasks were used to fabricate about 8 specimens in a day in the presence of one observer. All 8 specimens in each flask were interconnected with wax rolls through which the polymer material was injected. Then, the counterpart was assembled.

Dewaxing was done by immersing the flask in a water bath at100 °C for about 10 mins. The flask was then retrieved and cleaned to remove any excess residual wax. A coat of separating medium (Separating fluid, Ivoclar Vivadent, USA) was applied all over the mold. The two parts of the flask were assembled back. Heat cure acrylic resin (SR Ivocap, Ivoclar Vivadent, USA) available in pre-measured capsules were selected for the acrylization. The monomer and polymer capsules were mixed in a vibrator (Cap vibrator, Ivoclar Vivadent, USA) was attached to the flask unit.

The assembly was fixed to a polymerizing machine through which the resin material enters the mold cavity with the help of 6pa pressure. Later the assembly was immersed into a hot water bath which rises from 37°C to 100°C in about 45 mins for the polymerization to occur. The flask was retrieved and kept for bench cooling for about 30 mins. Then the samples were retrieved from the mold, followed by finishing and polishing done by using abrasive & polishing agents (Figure 3). To prevent any distortion, distilled water was used to store the fabricated samples at room temperature until testings were done.

Measuring shear bond strength

The samples are then loaded in a Universal testing machine (Autograph- Shimadzu, Japan) to assess the strength of the shear bond. The acrylic cylindrical parts of the specimen were fixed to the UTM machine at an angulation of 45°. The load was applied using a cylindrical pin on the incisal part from the lingual aspect with 0.5 mm/min cross-head speed. The load was applied until the tooth fractures and the digital values of bond strength were recorded. The obtained values were presented by inNewton’s (N). Since due to the irregular morphology and complexity of teeth ridge lap surface, the available surface area was not calculated. Hence, the measured failure load inNewton’s was reformed into Kgf. The following formula was used to calculate the bond strength

Where, A – surface area (mm²)

             F – Load at fracture (N)

             B – Bond strength (MPa)

The results obtained in Megapascals (MPa) were analyzed statistically using post hoc test and one-way ANOVA with SPSS v.17.

Results and Discussion

The significant differences in the strength of shear bonding between acrylic resin and surface treated and untreated PMMAteeth were evaluated. The bond strength of all groups measured in the Universal testing machine was analyzed statistically using post hoc test and one-way ANOVA.

Table 1 describes the Mean and SD of shear bond strength of surface treated and untreated PMMA teeth with acrylic denture base. The mean shear bond strength of Groups A, B, and C was 101.03±17.2, 105.88±24.5, and 121.07±13.8 Kgf, respectively with a p-value of 0.010. The mean shear bond strength was in a clinically acceptable range. Statistically, a significant difference was observed between Groups C & B, as well as Groups C & A.

Table 1. Mean and SD of shear bond strength of Group A, B, and C

As shown in Table 2, Group C had a significantly higher value of about 121.071 Kgf than Groups A & B, which were 101.035 & 105.882kgf, respectively.

Table 2. Oneway ANOVA for Groups A, B, & C

In Table 3 the Post Hottest indicating a significant difference between the Group C and Group A samples having the mean difference of 20.035 with a 0.01 p-value. Whereas Group B samples do not show much difference when compared with Group A and Group C.

Table 3. Intergroup Comparison of shear bond strength of Groups A, B, & C using POST HOC test (Bonferroni Test)