Influence of Prolonged Dental Bleaching on the Adhesive Bond Strength to Enamel Surfaces

Abstract

The objective of this in vitro study was to assess the influence of prolonged bleaching pre- and postrestoration on the bond strength (microshear) to enamel using 4% hydrogen peroxide (PH4). In the postrestorative bleached specimens evaluation, the composite cylinders were assembled after bleaching, while in the prebleached specimens, the cylinders were assembled before. Therefore, in the postbleached specimens, 60 bovine teeth were randomly assigned as follows: G1: control; G2: 14 days bleaching before bond strength (BS) testing; G3: 21 days; and G4: 28 days. In prebleached specimens, 180 bovine teeth were randomly assigned as follows: G1: control; G5: 14 days bleaching, storage in artificial saliva (AS) for 24 h before BS testing; G6: 14 days beaching, AS storage for 7 days before BS testing; G7: 21 days bleaching, AS storage for 24 h before BS testing; G8: 21 days bleaching, AS storage for 7 days before BS testing; G9: 28 days bleaching, AS storage for 24 hours before BS testing; and G10 : 28 days bleaching, AS storage for 7 days before BS testing. The results were submitted to ANOVA one-way (postrestoration bleaching) and two-way (prerestoration bleaching) and Tukey's post hoc test (p ≤ 0.05). In the postrestoration bleaching, no statistical difference between times was found. However, when bleached groups were compared to the control (G1), an expressive difference was detected (p ≤ 0.0001). For prerestoration bleaching, all experimental groups were statistically different from G1 (p ≤ 0.05), except G6 (p ≥ 0.01), and for G5 and G6, statistical differences were found (p ≤ 0.01). There were no significant differences between G7 and G8 and between G9 and G10, regardless of the AS storage times (p ≥ 0.05). It was concluded that prolonged bleaching with PH4 decreased adhesion resistance regardless of the moment of the bleaching (post- and prerestoration bleaching).

Figure 1

A and B: acid-resistant double-sided tape perforated by rubber sheet punch (0.8  mm diameter), delimiting the adhesive bond area to the enamel. C: conditioning with 35% phosphoric acid (Adper Scotchbond 3M ESPE, Sumaré, SP, Brazil) for 30 seconds; gel removal with an air–water spray for 30 seconds; drying with air jets. D: application of the adhesive system according to the manufacturer (Adper Single Band 2, 3M ESPE, Sumaré, SP, Brazil). E: photoactivation for 20 seconds with an LED-type photoactivation device (Ultrablue D-2000. DMC, São Carlos, SP, Brazil), (900  mW/cm²). F, G, and H: acid-resistant tape first layer removal; Tygon® pipes placement, 0.8  mm internal diameter and 0.5  mm high, coinciding with tape demarcations. Pipes filled with Filtek Z350 XT composite resin (3M ESPE, Sumaré, SP, Brazil) and photoactivation for 40 seconds. Two cylinders of composite resin were made in each tooth. After 24 h of storage in distilled water (37°C), removal of Tygon® pipes and the second layer of tape with the aid of scalpel blade n 12. I and J: 4% hydrogen peroxide with calcium (White Class with 4% calcium; FGM, Santa Catarina (SC), Brazil), using a 0.1 mL proportion of bleaching gel for 0.05 mL of artificial saliva, and conditioned during the bleaching process (2 (h), in a biological stove (37°C). Application delimitation of bleaching agent in 2 mm distance around the adhesive interfaces using a gingival barrier (TOP DAM, FGM, SC, Brazil). After bleaching gel was applied, specimens were washed using an air–water spray at approximately 5 cm from enamel surface, for 1 minute, and the gingival barrier was removed. Between sessions, specimens were stored in AS and conditioned in a biological stove (37°C). The gel was applied daily, according to the described duration for each group (Table 1). K: at the end of the dental bleaching systems for each group, samples were attached to a universal testing machine for the microshear test (Kratos Equipment's LTDA, Cotia, SP, Brazil).

Figure 2

A: 4% hydrogen peroxide with calcium (White Class with 4% calcium; FGM, SC, Brazil) (PH4), using a 0.1 mL proportion of bleaching gel for 0.05  mL of AS, and sample packaging, during the bleaching process 2 h in a biological stove (37°C). B: placement of the customized acetate trays proportioned with PH4 and AS. C and D : acid-resistant double-sided tape perforated with rubber sheet punch (0.8 mm diameter), delimiting the enamel adhesive bond area. E: conditioning with 35% phosphoric acid (Adper Scotchbond 3M ESPE, Sumaré, SP, Brazil) for 30 seconds; gel removal with an air–water spray for 30 seconds, drying with air jets. F: application of adhesive system according to manufacturer's recommendations (Adper Single Bond 2, 3M ESPE, Sumaré, SP, Brazil). G: photoactivation using an LED-type photoactivation device with 900 mW/cm² (Ultra Blue D-2000. DMC, SP, Brazil). H, I, and J: acid-resistant tape first layer removal; Tygon® pipes placement, 0.8 mm diameter and 0.5 mm high, coinciding to tape demarcations. Filling of the pipes with Filtek Z350 XT composite resin (3M ESPE, Sumaré, SP, Brazil) and photoactivation for 40 seconds. Two cylinders of composite resin were made in each tooth. After 24 h of storage in distilled water (37°C), removal of Tygon® pipes, and the second layer of tape with the aid of scalpel blade n°12. The resin cylinders were examined with a stereoscopic microscope (40x). K: at the end of the dental bleaching systems in each group, samples were attached to a universal testing machine for the micro-shear test (Kratos Equipment's LTDA, Cotia, SP, Brazil).

Figure 3

(a) Mixed fracture pattern in G1; (b) mixed fracture pattern in G2; (c) mixed fracture pattern in G3; (d) mixed fracture pattern in G4.

Figure 4

Mixed fracture pattern in (a) G1; (b) G2; (c) G3; (d) G4; (e) G5; (f) G6; (g) G7.