Tip and Torque Accuracy According to the ISO 27020:2019 Norm in Currently Available Pre-Adjusted Orthodontic Brackets

Abstract

Background: The precision of bracket manufacturing is fundamental to ensure the correct expression of the inbuilt information. The objective of this study was to determine the actual tip and torque values of a pool of stainless steel brackets, pre-adjusted according to the MBT prescription values, and to compare these actual values with those stated by the manufacturers in order to test their compliance with the tolerance limits reported in the ISO 27020:2019. Methods: A sample of 360 stainless steel brackets, from 12 different providers, were evaluated. All brackets had a nominal slot size of 0.022 in., belonged to the upper right central incisor, and were manufactured with the metal injection molding technique (MIM). For each provider, three different batches of the same bracket series were tested. A single-blind design was used for bracket coding. Results: Only five systems displayed torque mean values that matched the declared values (p > 0.05). Only one system did not respect the tolerance limits established in the ISO 27020:2019 norm. The tip values were different from those declared in seven of the assessed systems; however, none exceeded the tolerance limits. The inter-batch variability in most cases was not statistically significant. Conclusions: In most of the assessed systems, there can be a difference between the actual and the declared torque values, while tip information is usually accurately incorporated into the bracket slot. Lack of precision in the manufacturing process can reduce the efficacy of the appliance and force the clinician to compensate for dimensional inaccuracy through wire bending.

El objetivo del estudio fue determinar los valores reales de torque y angulación de la ranura de una muestra de brackets metálicos pre-ajustados de prescripción MBT y compararlos con los valores teóricos declarados por los fabricantes. Además se verificó si la muestra cumplía con los límites de tolerancia descritos en la Norma ISO 27020:2019. Para ello se evaluaron 360 brackets metálicos de 12 fabricantes diferentes fueron evaluados. De cada fabricante se evaluaron 3 lotes de fabricación distintos. Solo cinco de los doce sistemas evaluados presentaron valores de torque próximos al valor teórico declarado y con diferencias que no se encontraron estadísticamente significativas (p> .05). Sin embargo, solo uno de los sistemas evaluados mostró valores promedio fuera de los límites de tolerancia descritos en la Norma ISO 27020:2019. Respecto al tip todos cumplieron con los límites de la Norma ISO y la variabilidad entre lotes no fue estadísticamente significativa. Se puede concluir que existen diferencias significativas entre los valores de la prescripción declarados por los fabricantes y los valores reales de torque y angulación mesio-distal de la ranura, lo que obliga al ortodoncista a tener que incorporar dobleces en los arcos para acabar sus casos con éxito.