The aim of this study was to compare the mechanical strength of different joints made by conventional brazing, TIG and laser welding with and without filling material. Five standardized joining configurations of orthodontic wire in spring hard quality were used: round, cross, 3 mm length, 9 mm length and 7 mm to orthodontic band. The joints were made by five different methods: brazing, tungsten inert gas (TIG) and laser welding with and without filling material. For the original orthodontic wire and for each kind of joint configuration or connecting method 10 specimens were carefully produced, totalizing 240. The fracture strengths were measured with a universal testing machine (Zwick 005). Data were analyzed by ANOVA (p=0.05) and Bonferroni post hoc test (p=0.05). In all cases, brazing joints were ruptured on a low level of fracture strength (186-407 N). Significant differences between brazing and TIG or laser welding (p<0.05, Bonferroni post hoc test) were found in each joint configuration. The highest fracture strength means were observed for laser welding with filling material and 3 mm joint length (998 N). Using filling materials, there was a clear tendency to higher mean values of fracture strength in TIG and laser welding. However, statistically significant differences were found only in the 9-mm long joints (p<0.05, Bonferroni post hoc test). In conclusion, the fracture strength of welded joints was positively influenced by the additional use of filling material. TIG welding was comparable to laser welding except for the impossibility of joining orthodontic wire with orthodontic band.

Este estudo avaliou a influência da aplicação de uma pressão definida antes da polimerização sobre a resistência à tração de dois sistemas adesivos à dentina molhada e perfundida. Os sítios de adesão foram analisados em microscopia eletrônica de varredura (MEV) a fim de determinar a ocorrência de alterações morfológicas. Discos de dentina de 120 terceiros molares humanos foram preparados de tal forma a simular a pressão intrapulpar. Os espécimes foram alocados aleatoriamente em 6 grupos (n=20), de acordo com as diferentes condições experimentais propostas. Os sistemas adesivos (Syntac® e ScotchbondTM Multi-Purpose) foram aplicados sobre a dentina molhada ou perfundida. Nesta última, compressão (7 N) foi aplicada antes da polimerização do sistema adesivo. A resistência à tração foi registrada em máquina universal de ensaios. A análise qualitativa dos sítios de adesão foi realizada em MEV após dissolução da dentina em 50% de ácido nítrico. A aplicação da pressão antes da polimerização do sistema adesivo resultou em diminuição significativa da resistência à tração do Syntac®, ao passo que para o ScotchbondTM MP nenhuma alteração significativa foi observada. A análise em MEV não mostrou aumento considerável do tamanho dos tags de resina após a aplicação da pressão. Esses achados sugerem que a aplicação de sistemas adesivos à dentina sob pressão previamente à polimerização não teve efeito positivo sobre a resistência à tração.

This study evaluated the influence of defined pressure application before light-curing on the tensile bond strength (TBS) of two adhesive systems to wet and perfused dentin. Bonding sites were analyzed by scanning electron microscopy (SEM) to assess morphological changes. Dentin discs from 120 human third molars were prepared in such a way to allow simulation of intrapulpal pressure. The specimens were randomly assigned to 6 groups (n=20) according to different experimental conditions. The adhesive systems (ScotchbondTM Multi-Purpose and Syntac®) were applied either to wet or perfused dentin. In the latter, compression was applied before adhesive light-curing . TBS was recorded using an universal testing machine. Qualitative examination of the bonding sites was performed by SEM after dissolving the dentin with 50% nitric acid. Pressure application prior to light-curing resulted in a remarkable decrease of TBS for Syntac®, while no significant change was observed for ScotchbondTM MP. SEM analysis showed no considerable lengthening of the resin tags after pressure application. These findings suggest that application of adhesive systems to dentin under pressure before light-curing had no positive effect on TBS.