EFFECT OF DIFFERENCE 10-METHACRYLOYLOXYDECYL DIHYDROGEN PHOSPHATE CONCENTRATIONS ON THE SHEAR BOND STRENGTH BETWEEN ZIRCONIA AND COMPOSITE RESIN

Abstract

Zirconia ceramics are favored in restorative dentistry for their superior mechanical strength, biocompatibility, and aesthetic properties. However, achieving a durable adhesive bond remains a significant clinical challenge because zirconia lacks a glassy phase and exhibits high chemical stability, which results in resistance to conventional acid etching and silanization. To overcome this, functional phosphate monomers such as 10- methacryloyloxydecyl dihydrogen phosphate (10-MDP) have been utilized to facilitate chemical bonding. 10-MDP promotes adhesion by forming chemical interactions between the zirconia surface and the resin-based cement. Despite its widespread use, the influence of varying 10-MDP concentrations on bond strength and the specific molecular coordination at the interface requires further investigation. This study therefore evaluated the effect of different 10-MDP concentrations on the shear bond strength between zirconia and composite resin while investigating the underlying chemical bonding mechanisms through spectroscopic analysis. Methods: 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 controlB (CMP; Z-Prime™ Plus), and a non-MDP treated primer 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 31P Nuclear Magnetic Resonance (NMR) spectroscopy and Energy-Dispersive X-ray Spectroscopy (EDS) to correlate molecular configurations with SBS data. Results: 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).31P 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). Conclusions: 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