Force analysis of self-ligating orthodontic brackets under space closure conditions

Objective: Self-ligating orthodontic brackets, which are increasing in popularity amongst clinicians, do not require ligatures to retain the arch wire. This reduces the force dissipation to friction, allowing a “sliding” motion of the wire. Research suggests that these brackets generate significantly less frictional force, and require significantly less force to produce tooth movement. The purpose of this study is to photoelastically evaluate stress magnitudes and distributions of the space closure mechanics for both conventional and self-ligating bracket designs. Methods: Two idealized mandibular models, with interdental spaces of 0.5mm, were fabricated using different bifringent materials to simulate bone and teeth. Both traditional straightwire orthodontic brackets (CS) and self-ligating brackets (SL) were bonded from second molar to second molar on their respective model. Ideal bracket placement between models was standardized using a resin-positioning stent. A round nickel-titanium wire was installed. Non-closing conditions were simulated first using individual elastics for CS, and closing the ligating mechanism for SL. Power-chain was then applied to both models to evaluate the stress of space closing conditions. The resulting stresses were observed and photographed in a circular polariscope. Results: When an arch wire using CS brackets with individual elastic ties was compared to the SL condition, both designs demonstrated similar mild stress distributions. Upon loading, both the CS and SL designs had similar force distributions localized between the simulated dentition. When identical space closure forces were applied via a closed power-chain, greater magnitude of stress was observed with SL. Conclusions: Under both passive and loaded conditions both bracket designs (CS and SL) demonstrate similar force distributions. When equal space closure force is applied, the SL appliance produces greater stress upon the interdental complex, owing to the reduced force loss to friction. With SL, less force is required to produces similar stress magnitudes on the dentoalveolar complex.