title: Influence of the expansion screw height on the dental effects of the hyrax expander: a study with finite elements.
abstract: BACKGROUND
  Our objective was to evaluate the influence of the expansion screw height of a hyrax expander on the degree of dental inclination during rapid maxillary expansion by using the finite element method.
  
  
  METHODS
  The hyrax expander and the maxillary arch were modeled by using Solidworks software (Dassault Systèmes, Paris, France). Three distinct finite element method models were created by simulating different screw heights relative to the plane that intersected the center of resistance of the maxillary first molars. These 3 relative positions were 10 mm below the maxillary first molars' center of resistance, at the same level as the maxillary first molars' center of resistance, and 10 mm above the maxillary first molars' center of resistance. The initial activation of the expanders was simulated, and tooth displacements for each finite element method model were registered in the buccolingual, corono-apical, and mesiodistal directions.
  
  
  RESULTS
  The simulations tested showed that the 3 hyrax screw heights had different dental tipping tendencies. When the screw was simulated below the maxillary first molars' center of resistance, buccal tipping of the crowns and lingual tipping of the roots were registered. This tendency decreased when the screw was simulated at the same level as the maxillary first molars' center of resistance. However, when the screw was simulated above the maxillary first molars' center of resistance, the tipping tendency was inverted, with the crowns displaying lingual tipping and the roots displaying buccal tipping.
  
  
  CONCLUSIONS
  These findings might explain the importance of carefully planning the height of the hyrax expander screw, since, depending on this position, different tooth movements can be achieved. From an orthopedic perspective, the ideal screw position might be slightly above the maxillary first molars' center of resistance; this would generate less dental tipping.
authors: Pontifical Catholic University of Minas Gerais, Belo Horizonte, Brazil.
authors: Rafael Marques de Sousa
authors: RM
authors: Araugio
authors: Jánes
authors: J
authors: Landre
authors: Diana de Lourdes Almeida
authors: Dde L
authors: Silva
authors: Wellington
authors: W
authors: Pacheco
authors: Matheus Melo
authors: MM
authors: Pithon
authors: Dauro Douglas
authors: DD
authors: Oliveira
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D003198:Computer Simulation; D003799:Dental Stress Analysis; D020342:Finite Element Analysis; D006801:Humans; D021621:Imaging, Three-Dimensional; D008953:Models, Anatomic; D016382:Orthodontic Appliance Design; D010155:Palatal Expansion Technique / Q000295:instrumentation*; D014087:Tooth Movement Techniques