Bis[2-(Methacryloyloxy) Ethyl] Phosphate as a Primer for Enamel and Dentine

Abstract

Dental resin composites are commonly used in the restorative management of teeth via adhesive bonding, which has evolved significantly over the past few decades. Although current self-etch bonding systems decrease the number of clinical steps, the acidic functional monomers employed exhibit a limited extent of demineralization of enamel in comparison to phosphoric acid etchants, and the resultant superficial ionic interactions are prone to hydrolysis. This study evaluates the etching of primers constituted with bis[2-(methacryloyloxy) ethyl] phosphate (BMEP) of dental hard tissue, interfacial characteristics, and inhibition of endogenous enzymes. We examine the incorporation of 2 concentrations of BMEP in the formulation of experimental primers used with a hydrophobic adhesive to constitute a 2-step self-etching bonding system and compare to a commercial 10-methacryloyloxydecyl dihydrogen phosphate (10-MDP)-containing system. The interaction of the primer with enamel and dentine was characterized using scanning electron, confocal laser scanning, and Raman microscopy while the polymerization reaction between the BMEP primers and hydroxyapatite was evaluated by Fourier-transform infrared spectroscopy. The inhibitory effect against matrix metalloproteinase (MMP) enzymes of these primers was studied and percentage of inhibition analyzed using 1-way analysis of variance and Tukey's post hoc test (P < 0.05). Results of the scanning electron microscopy micrographs demonstrated potent etching of both enamel and dentine with the formation of longer resin tags with BMEP primers compared to the 10-MDP-based system. The BMEP polymerized on interaction with pure hydroxyapatite in the dark, while the 10-MDP primer exhibited the formation of salts. Furthermore, BMEP primers were able to inhibit MMP activity in a dose-dependent manner. BMEP could be used as a self-etching primer on enamel and dentine, and the high degree of polymerization in the presence of hydroxyapatite can contribute to an increased quality of the resin polymer network, prompting resistance to gelatinolytic and collagenolytic degradation.

Keywords: adhesives; dental bonding; dental etching; matrix metalloproteinases; phosphoric acid esters; polymerization.

Figure 1.

The chemical structure of the phosphoric acid esters used in this study and the etch patterns produced on enamel and dentine. (A) The structure of (a) 10-methacryloyloxydecyl dihydrogenphosphate (10-MDP) and (b) bis[2-(methacryloyloxy) ethyl] phosphate (BMEP). (B) Scanning electron microscopy images of the self-etch primers on enamel (left column) and dentine (right column) using (a, b) CFSE, (c, d) BMEP15, and (e, f) BMEP40. A distinct etch pattern was obtained with BMEP40 on enamel (e), exposing the enamel prisms. A decrease in pH of the primers (right column, top to bottom) also increased the extent of demineralization, and the dentine tubules were enlarged (f).

Figure 2.

Confocal laser scanning microscopy images showing the hybrid layer formed using the dentine bonding systems. (A–A′′) CFSE, (B–B′′) BMEP15, and (C–C′′) BMEP40. (A–C) Images of the resin-dentine interface where the adhesives were labeled with fluorescein (green). (a′–c′) Images of the resin-dentine interface where the primers were labeled with rhodamine B (red). (a′′–c′′) Composite images demonstrating an orange color, which corresponds to the mixture between the primer and adhesive components, indicating the ability of the dentine bonding systems to diffuse into the etched dentine tubules, creating a gap-free interface and distinct hybrid layer. The images clearly demonstrate the deeper etch pattern created by BMEP15 and BMEP40 primers compared to CFSE, but the corresponding adhesives were able to penetrate the full depth of the etched dentine. C, composite; HL, hybrid layer; RT, resin tags.

Figure 3.

The Raman and FTIR spectra of the primers evaluated in this study as well as the structure of the primers allowed to self-polymerize. (A) Raman spectra in the region of 700 to 2,000 cm⁻¹ starting from 8 µm into the dentine and ending 8 µm beyond the resin-dentine interface for (a) CFSE, (b) BMEP15, and (c) BMEP40. The characteristic phosphate bands at 960 and 1,118 cm⁻¹ are more intense toward the dentine but can still be seen beyond the resin-dentine interface owing to the functional phosphate groups in all bonding systems. The characteristic adhesive bands at 1,458, 1,608, 1,637, and 1,715 cm⁻¹ can be detected up to 8 µm into dentine for (b) BMEP15 and (c) BMEP40 groups but not for (a) CFSE, indicating their deeper etching ability. Peak assignments: 1,637 cm⁻¹, aliphatic C=C; 1,608 cm⁻¹, aromatic C=C; 1,458 cm⁻¹, CH₂; 1,118 cm⁻¹, PO₄ (υ₃); 960 cm⁻¹, PO₄ (υ₁). (B) Fourier-transform infrared spectroscopy spectra of the primers CFSE, BMEP15, and BMEP40 without hydroxyapatite (HA) following light polymerization (left column), with 5% HA following light polymerization (middle column), and with 5% HA and allowed to self-polymerize in the dark for 24 h (right column). The primer BMEP40 demonstrates complete interaction with HA as demonstrated by the absence of the peak at 1,637 cm⁻¹. Peak assignment: 1,637 cm⁻¹, aliphatic C=C. (C) The structure of the primers left in the dark to self-polymerize following solvent evaporation. CFSE demonstrates the formation of a calcium salt characteristic of 10–methacryloyloxydecyl dihydrogen phosphate (10-MDP)–based systems, BMEP15 demonstrates the formation of a thick consistency, and BMEP40 demonstrates the formation of a solid cross-linked structure.

Figure 4.

The MMP inhibitory results of the investigated primers, neat BMEP monomer and controls. (A) The inhibitory percentage of rhMMP-2 and rhMMP-8 (means and standard deviation) by the test compounds and inhibitor control (BB-94). Similar lowercase and uppercase letters indicate no statistical differences in the percentage of inhibition of rhMMP-2 and rhMMP-8, respectively. (B) The dose-dependent inhibitory percentage of rhMMP-2 and rhMMP-8 by the test compounds. (C) The mode of inhibition assessed by addition of 0.01% (w/v) trypsin to all wells. Note only partial further cleavage of the gelatin and collagen substrates by trypsin in the BMEP-containing wells, indicating that the substrates were protected by the presence of BMEP.