According to the 'elastic bonding' concept, a thick intermediate layer of flexible resin can absorb part of the polymerization shrinkage stress and may absorb shocks during function. Objectives: To evaluate the effect of an intermediate layer of a low-viscosity resin (Protect Liner F, Kuraray) on the fatigue resistance of a hybrid composite (Clearfil Photo Posterior, Kuraray) bonded to dentin using a two-step self-etch adhesive (Protect Bond, Kuraray). The hypotheses tested were that an intermediate layer of a low-viscosity resin (1) increases the fatigue resistance to dentin, but (2) has no effect on the static bond strength. Methods: Micro-tensile bond strength (µTBS) samples were loaded until failure or inserted in a micro-rotary fatigue testing device (De Munck et al., Biomaterials, 2004). To ensure exact centric alignment throughout the test, the preparation of a round constricted interface area (about 1 mm²) and the fatigue testing itself were performed in the same lathe. Specimens were tested at 4 Hz until failure or until 10⁵ cycles were reached. The maximum loading stress of all specimens was determined by an adapted staircase method. The mean load at which 50% of the samples failed (median micro-rotary fatigue resistance, µRFR) was determined (in MPa) using logistic regression and the 2 groups were compared using multiple logistic regression. µTBS results were analyzed using a paired t test. Results:

Protect Bond	µRFR	25%-75% quartile	n	µTBS ± SD	n
No Protect Liner F	28.4a	25.4 – 31.4	24	31.2 ± 10.4A	12
With Protect Liner F	21.6b	16.8 – 26.4	23	32.5 ± 8.5A	9

Conclusion: An additional elastic intermediate layer decreased significantly (p=0.0178) the fatigue resistance (rejection of hypothesis 1), but did not alter the µTBS (p=0.590, acceptance of hypothesis 2). The decrease in µRFR must be attributed to the lower mechanical properties of the intermediary layer. jan.demunck@med.kuleuven.ac.be

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