33 - Assessment of shear bond strength and degree of conversion of composite resins used for creating orthodontic attachments with different light-curing methods

Abstract

Background

Fractures or debonding of attachments can delay orthodontic treatment, affecting movement and aligner tracking.

Objectives

To evaluate the shear bond strength and degree of conversion of resins used for orthodontic attachments under different photopolymerization protocols.

Methods

The sample consisted of 120 bovine lower incisors embedded in acrylic resin and rectangular attachments were bonded on the vestibular surface. The sample was divided into three groups based on the flow resins tested: nanohybrid, microhybrid, and nanoparticle. Each group was further subdivided into four subgroups (n=10) according to polymerization time: 3 seconds (3.200 mW/cm²), 6 seconds (3.200 mW/cm²), 20 seconds (1.000 mW/cm²), and 40 seconds (1.000 mW/cm²). After the attachment installation, the samples from each group were subjected to thermocycling. After this phase, the specimens were subjected to shear bond strength testing using a universal testing machine (AME5k) with a 5 kN load cell, until fracture occurred. For the degree of conversion (DC) test, 60 composite resin specimens were prepared with the same dimensions as the shear test attachments. These specimens were divided into three groups (n=20) based on resin type and further subdivided (n=5) according to polymerization time. The DC was measured 24 hours after polymerization using FT-IR infrared spectroscopy. Data were analyzed using two-way ANOVA followed by Tukey's post-test.

Results

Statistical tests showed significant differences for resin type (p<.001) and polymerization time (p<.001), with interaction between the factors (p<.001) regarding shear bond strength. For the degree of conversion, there was a significant difference for resin (p=.024) and polymerization time (p=.020), with no interaction between the factors.

Conclusions

The nanoparticle resin and longer polymerization times (20 and 40 seconds) demonstrated better mechanical performance in shear bond strength and a higher degree of conversion.