Methods: MDP was solvated in a 1:1-water:ethanol solution. MDP was mixed with 0, 1, 2, 3 or 4 ppm of calcium in presence or absence of zinc. Remaining free-calcium (calcium not chemically bonded to MDP) was measured by atomic absorption spectroscopy (AAS). Further AAS was realized with zinc+MDP following a similar protocol in absence of calcium to evaluate the potential formation of MDP-zinc salt. Three experimental solutions were prepared and analysed for each mixture (n=3). MDP-containing self-etch adhesives Clearfil SE Bond (CSE, primer solution) and Clearfil S3 (S3) (Kuraray Medical, Japan) were doped with 5wt% of zinc. Zinc-doped and zinc-free adhesives were applied following manufacturer’s recommendations to flat dentine surfaces of extracted human third molars and restorations were built up. After 24h immersion in distilled water at 37ºC, the bonded teeth were cut into ~1mm² sticks and submitted to µTBS testing. Data were analysed with ANOVA, paired t-test and Tukey’s test (p<0.05).
Results: µTBS showed mean bond strength (MPa) of 39.8±7.3 for CSE, 19.3±4.9 for zinc-doped CSE and 38.3±8.6 for S3. Zinc-doped S3 was not tested because all sticks failed prematurely. AAS showed reduced free-calcium by adding MDP (p<0.001) and an increase in free calcium when adding zinc (p=0.009). Formation of monomer-zinc salt occurred as a reduction of free-zinc by adding MDP (p<0.001).
Conclusions: These outcomes showed that zinc may jeopardise the bond strength of MDP-containing adhesives and MDP-calcium salts formation. Addition of zinc should be avoided in this type of self-etch adhesive.