Influence of the Bracket Material on the Post-Cure Degree of Conversion of Resin-Based Orthodontic Adhesive Systems

Abstract

The aim of this study was to examine the influence of the orthodontic bracket material on the short-term and long-term post-cure development of the degree of conversion (DC) of resin-based orthodontic adhesive systems. Five commercially available materials characterized by different compositions and curing modes (light-curable or dual-curable) were tested under three different light curing conditions: without brackets (control group, CO), and in the presence of metal brackets (MB group) or ceramic brackets (CB group). Fourier-transform infrared spectroscopy was used to determine the post-cure DC development, both after "short-term" periods (2, 6, and 10 min) and "long-term" periods (1, 7, and 28 days). The short-term DC values ranged from 43.9% to 76.1%, and the long-term DC values were higher and ranged from 54.3% to 85.3%. The MB group demonstrated significantly lower short-term DC values compared to the CO and the CB groups, while the CB group had statistically similar or slightly lower DC values compared to the CO group. Long-term DC values in the MB and the CB groups were statistically lower or similar compared to the CO group, which depended on the post-cure time. The results indicated that the post-cure DC development was highly material-dependent and affected by the presence of different types of bracket material.

Keywords: FTIR; degree of conversion; orthodontic adhesive systems; orthodontic brackets.

Figure 1

Study design.

Figure 2

Schematic representation of sample preparation for the short-term DC measurements.

Figure 3

Schematic representation of sample preparation for the long-term DC measurements.

Figure 4

A representative FTIR spectrum of the resin-based light-cured orthodontic adhesive system Enlight. The inset shows an enlarged part of the wavelength range of interest for DC calculations, containing the vibrational modes of aliphatic C=C (1638 cm⁻¹) and aromatic C=C (1608 cm⁻¹).

Figure 5

Degree of conversion (mean values and standard deviations) measured 2, 6, and 10 min after light curing for each system. For the comparisons among bracket types, the same uppercase, lowercase, and Greek letters indicate statistically similar values after 2, 6, and 10 min, respectively. Statistically similar values among the materials (within each combination of bracket type and time point) are marked by dots (·). CO: control group (no bracket); MB: metal bracket group; CB: ceramic bracket group.

Figure 6

Degree of conversion (mean values and standard deviations) measured after 1, 7, and 28 days. For the comparisons among bracket types, the same uppercase, lowercase, and Greek letters indicate statistically similar values after 1, 7, and 28 days, respectively. Statistically similar values within each group are connected with square brackets. The dashed bracket indicates the values that are statistically similar between the first and the last time point, but differ from the time point in between. CO: control group (no bracket); MB: metal bracket group; CB: ceramic bracket group. Statistically similar values among the materials (within each combination of bracket type and time point) are marked by dots for the MB group (·), and asterisks for the CB group (*).

Figure 7

The development of the degree of conversion across both the short-term and long-term measurements (material: Enlight). Note the break in the middle of the x-axis.