title: The effect of design modifications on the torsional and compressive rigidity of U-shaped palatal major connectors.
abstract: BACKGROUND
  Rigidity is a requirement of removable partial denture frameworks. Although commonly used by dentists, the U-shaped (horseshoe) maxillary major connector lacks the rigidity of other maxillary major connector designs.
  
  
  OBJECTIVE
  The purpose of this study was to determine the effects of changing width, thickness, and shape on the rigidity of U-shaped maxillary major connectors.
  
  
  METHODS
  Five nickel-chrome alloy frameworks were fabricated from the same master cast for each of 4 different U-shape removable partial denture designs. The designs were an 8-mm wide U-shaped strap with a 6-mm posterior strap (A-P strap), a 13-mm-wide U-shape strap (Wide), an 8-mm-wide U-shape strap that widened to 13 mm at the midline (Notch), and an 8-mm-wide U-shaped strap that was twice the thickness of the other straps (Thick). A fifth group of 5 frameworks was made by removing the posterior strap from the A-P strap frameworks (A strap). Two testing points were marked on each framework corresponding to the first premolar and second molar positions. The frameworks were mounted in a universal testing machine, and vertical (torsional) and horizontal (compressive) loads were applied using a 10-kN load cell at a cross-head speed of 2 mm/min until a deflection of 1 mm occurred. A force-deflection curve was generated for each test, and slope of the curves (N/mm) was compared with analysis of variance and Scheffe's F test (alpha=0.05).
  
  
  RESULTS
  The Thick group was found to be significantly more rigid (P<.05) than the other frameworks when torsional loads were applied to both the premolar (22.42 N/mm) and molar (10.88 N/mm) areas, and when a compressive load was applied to the premolar (232.85 N/mm) area. The A-P strap group was significantly more rigid (P< or =.05) than the other designs when a compressive load was applied to the molar (69.56 N/mm) area. Both the Thick and A-P strap groups were significantly more rigid (P<.05) than the Notch and A strap groups in all tests.
  
  
  CONCLUSIONS
  Doubling the thickness of the anterior strap of a U-shaped maxillary major connector improved the rigidity of the framework to torsional loads. A posterior strap became more effective in maintaining framework rigidity to compressive forces as the length of the arch increased.
authors.affiliation: Love Dental Clinic, US Army Dental Activity, Fort Benning, Ga 31905, USA. Lawrence.Green@se.amedd.army.mil
authors.forename: Lawrence K
authors.initials: LK
authors.lastname: Green
authors.forename: Steven O
authors.initials: SO
authors.lastname: Hondrum
mesh_terms: D000704:Analysis of Variance; D001641:Bicuspid; D002858:Chromium Alloys / Q000737:chemistry*; D019245:Compressive Strength; D003799:Dental Stress Analysis / Q000295:instrumentation; D003779:Denture Design; D003832:Denture, Partial, Removable; D004548:Elasticity; D006801:Humans; D008963:Molar; D010159:Palate; D013223:Statistics as Topic; D013499:Surface Properties; D014102:Torsion Abnormality
chemical_list: D002858:Chromium Alloys