Effect of different 10-Methacryloyloxydecyl Dihydrogen Phosphate concentrations on the shear bond strength between zirconia and composite resin

Zirconia ceramics are widely used in restorative dentistry for their mechanical strength, but their lack of a glassy phase makes them resistant to conventional acid etching. While 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP) is utilized to facilitate chemical bonding, the influence of monomer concentration on molecular coordination at the interface remains unclear. This study evaluated the effect of varying 10-MDP concentrations on shear bond strength (SBS) and investigated the underlying chemical bonding mechanisms. Specimens were randomly divided into nine groups ( n = 6): seven experimental groups treated with varying concentrations of experimental MDP solutions (2, 4, 6, 8, 10, 12, and 14% v/v in ethanol), a commercial MDP primer positive control (CMP; Z-Prime™ Plus), and a non-MDP primer-treated negative control (NMP). Zirconia surfaces were treated with these MDP solutions prior to bonding with flowable composite resin. Shear bond strength was measured using a universal testing machine. Chemical coordination modes were analyzed using 31 P Nuclear Magnetic Resonance (NMR) spectroscopy and Energy-Dispersive X-ray Spectroscopy (EDS) to correlate molecular configurations with SBS data. Shear bond strength was significantly influenced by MDP concentration, increasing from 2% (7.1 ± 0.4 MPa) to a peak at 10% (11.3 ± 0.8 MPa), followed by a significant decline at 12% (7.6 ± 0.7 MPa) and 14% (7.7 ± 0.8 MPa). 31 P NMR analysis revealed that these variations correspond to five distinct bonding configurations (S1–S5). The superior performance of the 10% concentration was associated with the highest proportion of the S2 configuration, representing ionically bonded bidentate complexes. In contrast, the reduction in SBS at higher concentrations coincided with a decrease in S2 intensity and the emergence of S3 (bridging) and S4/S5 (phosphate oligomers). Bond strength is dictated by the distribution of MDP coordination modes rather than total presence. A 10% concentration is the ideal threshold for maximizing strong S2 ionic bonding. Exceeding this limit promotes non-productive phosphate multilayers that compromise interface stability.