Using an Orthodontic Force Tester to Simulate Clinical Environment for Space Closure and Measuring the Applied Three-Dimensional Load System

Abstract

Applied orthodontic load systems (forces and moments) cause teeth to move from their existing position in the dental arch. The types of tooth movement can be classified as tipping, rotation and translation in three-dimension. If the desired tooth movement is pure translation, a force should be applied directly at the center of resistance. Since the center of resistance of teeth cannot be identified or accessed easily and reliably, and orthodontic brackets are applied most practically on the buccal surfaces of the tooth crowns, applying a force at the center of resistance is not realistic. Therefore, the applied force should be accompanied by a moment to moderate tipping. The control of the movement relies on the ability to quantify and manipulate the orthodontic load system, specifically the moment-to-force ratio (M:F). The inability to control the orthodontic load system can result in undesirable tooth movement as well as a decrease in the efficiency of overall treatment. The importance of the three-dimensional (3-D) load system is well established although it has never been satisfactorily measured. The purpose of this study was to measure forces and moments generated by a commercially available T-loop closing loop archwire in three axes simultaneously at two different locations utilizing the orthodontic force tester (OFT) and a custom-made dentoform that simulates a typical space closure clinical case. The parameters in the design of a closing archwire that influence the 3-D orthodontic load system were tested to analyze the effects of these variations. The five parameters that were investigated include activation, loop location, gable direction, gable angle, and gable type. The overall null hypothesis was that the variations in the design of a closing archwire would not influence the 3-D orthodontic load system (p>0.05). A full factorial analysis of variance (ANOVA) model was used to model the absolute value of the forces (Fx, Fy, Fz ) and moments (Mx, My, Mz) in each plane separately. Additionally the ratios of the moment in the x-plane (Mx) to the force in the y-plane (Fy) and of the moment in the y-plane (My) to the force in the x-plane (Fx) were calculated for each experimental run. Separate ANOVA models were run for each sensor type (lateral incisor and canine). In lieu of multiple pair-wise comparisons, Tukey's minimum significant difference was estimated assuming a significance level of alpha = 0.05. Along with estimates of the means and standard deviations of the forces and moments, appropriate 95% confidence intervals were estimated for each mean. Statistical significant interactions were found for the variations that were tested, therefore the Null Hypothesis was rejected. The various directions of Fy and its overall low magnitude at the lateral incisor bracket challenged the accepted notion that the lateral incisor moved distally during space closure. A resultant force may indeed be in the direction toward the center of the arch rather than the center of the space. It was noted that the intrusive/extrusive, the buccal/lingual root moments forces and the mesial/distal root moments were influenced more by the Second Order Gable Bends than the First Order Gable Bends. It could be concluded that 10,10 First Order Gable Bends and 10,10 or 20,0 Second Order Gable Bends should be used for most clinical space closure needs at anterior or middle T-loop spring positions with 1 mm or 2 mm activations. Future studies investigating self-ligating brackets, different closing loop designs, modifications, and materials are necessary to understand the 3D orthodontic force system further and design the ideal system that would allow clinical space closure as desired.