Shear Bond Strength 114

A major factor to be taken into consideration when assessing the intraoral performance of a composite material is bonding. The shear bond strength is a measure of the adhesion of a material to the tooth structure. It is important to assess the shear bond strength as it determines how much force is necessary in order to break the bond between material and tooth and it can be a predictor of the performance of the material when forces are applied in the oral cavity.

The initial outcomes of the shear bond strength presented with great variation, high standard error and very low shear bond strength results for all the different formulations that where tested apart from Fuji II & Fuji IX, which had mean shear

bond strength of 6.8 Mpa and 5.7 Mpa respectively. Fuji II- LC sets initially through resin polymerization and Fuji IX via an acid-base reaction creating a chemical bond to the dentine. As a result, these materials don’t need the prior application of etch or bond. However, literature has shown that Fuji II and Fuji IX lack strength and wear off with time, so they cannot be used as permanent restorations in areas of the mouth that the forces are increased (Xie et al, 2000).

The storage medium of teeth was also reviewed during this project. The initial protocol included disinfection of the teeth in thymol solution, which was subsequently changed to Chloramine T 1%. Studies have shown that both these storage media have a minimal effect on the dentine and on the bond strength (Mobarak et al, 2010; Haller et al, 1993).

Another factor that can explain the low initial shear bond strength results is the difficulty to overcome some limitations of the human tooth, which are thoroughly describe in the next part. These include the presence of dentinal tubules and fluid and the presence of pulp. In this project some of the human teeth used for shear bond tests would not keep any material in place. When the tests were repeated using a tooth ID and the same tooth was used for each material, better consistency was obtained in the results, while specific teeth could have higher or lower shear bond strength measurements for all the different materials tested, coming in agreement with the previous observations and giving some indication that the results can be tooth- related.

The use of etch only appeared to have a negative impact on the bond strength and the formulation had hardly any bond into the tooth surface for all the teeth that were used apart from one that showed 5.34 MPa shear bond strength. This can be explained by the fact that A6 has self-etching properties. As a result, further etching could lead to the breakdown of the dentine collagen fibrils and would not enhance the composite penetration. This finding comes in agreement with studies testing the bond strength and micro-leakage of a commercial self-adhesive (Vertise Flow, Kerr) (Munck et al, 2004; Rengo et al, 2012). However, one study testing the same self-adhesive material has shown beneficial effect of pre-etching the dentin with phosphoric acid (Leung et al, 2005; Poitevin et al, 2013). According to a more recent systematic review, it seems that etching in universal adhesives has a positive impact on the enamel and improves the bond strength, but this is not the case for dentine (Rosa et al, 2015).

The formic acid pre-treatment of the dentine had a negative impact on the shear bond strength in this project. This could be because of the collapse of collagen fibrils due to the formic acid, leading to material inability to penetrate the dentine. A recent study though that was assessing one-step self-etch adhesives found that the prior conditioning of the dentine with NaOCl can deproteinize the dentine smear layer and lead to a significant improvement in the bond strength of all adhesives tested (Thanatvarakorn et al, 2018).

The over-drying of the dentine (60s air) had a negative effect on the shear bond strength when the A6 and C2 formulations were tested. This result could be explained by the hydrophilic nature of the two experimental formulations, which need some moisture in order to have reaction. On the other hand, the Z250 with dry and etched dentin followed by the bonding agent gave results that were significantly higher as Z250 has hydrophobic nature, thus over-drying the dentine was positive for the shear bod strength. On the contrary, the experimental composites are hydrophilic materials and presence of moisture could benefit the bond strength.

The ivory dentine resulted in more consistent outcomes of the shear bond strength, something that would be expected, given the fact that it comes as a single bulk, without differences in density of the dentinal tubules and without the presence of dentinal fluid and pulp as it happens with human dentine. The ivory dentine was tested in two different temperatures and the results showed that the temperature could affect the shear bond strength of the C2 formulation when tested in ivory dentine. The mean shear bond strength for the C2 in the ivory blocks kept in the 37oC incubator appears to be lower compared to the mean shear bond strength when the formulations were tested in room temperature and the 37o group also presented less consistency in the results.

Overall, the C2 (10% MCPM, 2% PLS, PLR 3) and A6 (5% MCPM, 5% PLS, PLR 3) formulations have shown higher results compared to the Z250 when the shear bond strength was assessed without the use of etch and bond. Specifically, a statistically significant difference was found between the results of A6 without etch and bond and those of Z250 without etch and bond with the latter being significantly lower. However, these results are not yet conclusive, as further research is being done in order -first of all- to create a standardised protocol of testing the shear bond strength in human dentine and be able to achieve repeatable results, given the fact that the