Influence of the hyrax expander screw position on displacement and stress distribution in teeth: A study with finite elements.

INTRODUCTION: This study aimed to simulate the different positions of the hyrax appliance expander screw and evaluate tooth displacement and the stress distribution standard on the periodontal ligament using the finite element method. METHODS: Part of the maxilla with anchorage teeth, periodontal ligament, midpalatal suture, and the hyrax appliance was modeled, and finite element method models were created to simulate 6 different screw positions. There were 2 vertical positions at distances of 20 mm and 15 mm from the occlusal plane. Another position was anteroposterior, the center of the screw placed between and equidistant from the mesial face of the first molar and the distal face of the first premolar, aligned to the center of the crown of the first molar, with the anterior edge of the screw aligned to the distal face of the first molar. A 1 mm activation of the expander screw was simulated. The displacement (total, vertical, and buccolingual) and the stress distribution on the periodontal ligament of supporting teeth in each model were registered. RESULTS: The model simulating the expander screw in a more occlusal and anterior position presented higher displacement values and higher stress concentration, followed by the model with the screw in a more posterior but same vertical position. With the exception of the first premolar, the teeth presented cervical-apical displacement in the vestibular face and apical-cervical displacement in palatal faces. This displacement is compatible with the vestibular inclination associated with the activation of the expander screw. The first premolar presented an atypical tendency for the mesial and lingual displacement of the vestibular surface and counterclockwise rotation. CONCLUSIONS: The supporting teeth presented a tendency for vestibular crown displacement and lingual root displacement associated with compression areas in the vestibular-cervical region and tensile strength in the linguoapical region. Placing the expander screw in a more occlusal and anterior position generated more mechanical stress transfer, resulting in greater dental displacement.

Influence of the hyrax expander screw position on stress distribution in the maxilla: A study with finite elements.

INTRODUCTION: Our objective was to evaluate the stress and deformation distribution patterns on the maxillary bone structure using the finite element method by simulation of different vertical and anteroposterior positions of the expansion screw on the hyrax expander appliance. METHODS: Part of the maxilla with anchorage teeth, midpalatal suture, and the hyrax appliance were modeled, and 6 distinct finite element method models were created to simulate different positions of the expansion screw. There were 2 vertical positions at distances of 20 and 15 mm from the occlusal plane. Another 3 positions were anteroposterior, with the center of the screw placed between and equidistant from the mesial face of the first molar and the distal face of the first premolar, aligned to the center of the crown of the first molar, and the anterior edge of the screw aligned to the distal face of the first molar. The initial activations of the expanders were simulated, and the stress distributions on the maxilla in each model were registered. RESULTS: The stress was concentrated in the anterior region of the models, close to the incisive foramen, dissipating through the palate in the posterior and lateral orientations, in the direction of the pterygoid pillar, diverting from the midpalatal suture region. When the expander screw was simulated closer to the occlusal plane and in a more anterior position, more stress was located around the incisive foramen and distributed through the midpalatal suture to its posterior portion. More posterior positions resulted in concentrated stress around the pterygoid pillars. At all simulations, the midpalatal suture showed a V-shaped expansion, with the vertex superior in the coronal view and posterior in the axial view. CONCLUSIONS: Different positions of the expander screw interfered with stress intensity and distribution patterns. When the expansion screw was simulated in a more occlusal and anterior position, it was more efficient to transfer the mechanical effects from the appliance to the bone structures.