Generally, the bonding of one material to another is important for function, and especially in so-called ‘adhesive dentistry’. However, there are concerns about the clinical relevance and the discriminatory power of currently employed tests of bond strength.

Develop and validate a test protocol based on 4-point bending that may be used to examine the bonding of various dental materials to a range of substrates. The bonding of a resin-based composite (RBC) to dentine is taken as an example.

Slices of dentine ‘coupons’ (5.0 × 2.0 × ∼4–6 mm3) from extracted molars were prepared using a diamond saw under running water. Pairs of RBC bars (5.0 × 2.0 × ∼24 mm3) (Z250) were bonded symmetrically either side of a dentine coupon using all combinations of the following treatments: E: acid-etched (Scotchbond Universal Etchant), P: primer (Adper Scotchbond Multipurpose Adhesive); A: adhesive (Adper Scotchbond Multipurpose Primer) (all 3 M) as well as N: no treatment. Following retrieval from the mould, test pieces were immediately subjected to 4-point bending at 23 °C, cross-head speed 0.5 mm/min, until fracture. Three-way analysis of variance on log(flexural strength): etch × primer × adhesive, was applied. Scanning electron microscopy (SEM) was used to examine fracture surfaces and identify failure origins.

Flexural strengths in MPa: N: 1.38 ± 0.56; P: 9.82 ± 0.89; A: 5.12 ± 0.73; E: 9.39 ± 1.78; E + P: 21.03 ± 2.63; E + A: 12.80 ± 1.53; P + A: 17.16 ± 3.03; E + P + A: 20.84 ± 3.93. The treatment main effects (all, p < 10−12) were not additive, there being significant two-way (p < 10−5) and three-way (p = 0.037) interactions. There was no significant difference between E + P + A and E + P (p = 0.86).

With good reproducibility (low scatter), discriminatory power (clear treatment effects), economy with regard to substrates, the method has the potential to be adaptable to many systems.

Failure mode

Adhesive resin

Resin-based composite

© 2025 The Authors. Published by Elsevier Inc. on behalf of The Academy of Dental Materials.