title: Evaluation of varying amounts of thermal cycling on bond strength and permanent deformation of two resilient denture liners.
abstract: BACKGROUND
  Two problems found in prostheses with resilient liners are bond failure to the acrylic resin base and increased permanent deformation due to material aging.
  
  
  OBJECTIVE
  This in vitro study evaluated the effect of varying amounts of thermal cycling on bond strength and permanent deformation of 2 resilient denture liners bonded to an acrylic resin base.
  
  
  METHODS
  Plasticized acrylic resin (PermaSoft) or silicone (Softliner) resilient lining materials were processed to a heat-polymerized acrylic resin (QC-20). One hundred rectangular specimens (10 x 10-mm 2 cross-sectional area) and 100 cylindrically-shaped specimens (12.7-mm diameter x 19.0-mm height) for each liner/resin combination were used for the tensile and deformation tests, respectively. Specimen shape and liner thickness were standardized. Specimens were divided into 9 test groups (n=10) and were thermal cycled for 200, 500, 1000, 1500, 2000, 2500, 3000, 3500, and 4000 cycles. Control specimens (n=10) were stored for 24 hours in water at 37 degrees C. Mean bond strength, expressed as stress at failure (MPa), was determined with a tensile test using a universal testing machine at a crosshead speed of 5 mm/min. Analysis of failure mode, expressed as a percent (%), was recorded as either cohesive, adhesive, or both, after observation. Permanent deformation, expressed as a percent (%), was determined using ADA specification no. 18. Data from both tests were examined with a 2-way analysis of variance and a Tukey test (alpha=.05).
  
  
  RESULTS
  For the tensile test, Softliner specimens submitted to different thermal cycling regimens demonstrated no significantly different bond strength values from the control; however, there was a significant difference between the PermaSoft control group (0.47 +/- 0.09 MPa [mean +/- SD]) and the 500 cycle group (0.46 +/- 0.07 MPa) compared to the 4000 cycle group (0.70 +/- 0.20 MPa) ( P <.05). With regard to failure type, the Softliner groups presented adhesive failure (100%) regardless of specimen treatment. PermaSoft groups presented adhesive (53%), cohesive (12%), or a combined mode of failure (35%). For the deformation test, there was no significant difference among the Softliner specimens. However, a significant difference was observed between control and PermaSoft specimens after 1500 or more cycles (1.88% +/- 0.24%) ( P <.05).
  
  
  CONCLUSIONS
  This in vitro study indicated that bond strength and permanent deformation of the 2 resilient denture liners tested varied according to their chemical composition.
authors.affiliation: School of Dentistry, Paulista University, Sao Paulo, Brazil.
authors.forename: José Renato Ribeiro
authors.initials: JR
authors.lastname: Pinto
authors.forename: Marcelo Ferraz
authors.initials: MF
authors.lastname: Mesquita
authors.forename: Mauro Antônio de Arruda
authors.initials: MA
authors.lastname: Nóbilo
authors.forename: Guilherme Elias Pessanha
authors.initials: GE
authors.lastname: Henriques
mesh_terms: D000180:Acrylic Resins / Q000737:chemistry*; D000268:Adhesiveness; D000704:Analysis of Variance; D001840:Dental Bonding; D003799:Dental Stress Analysis / Q000295:instrumentation; D003822:Denture Liners; D004548:Elasticity; D006801:Humans; D008422:Materials Testing; D008689:Methacrylates / Q000737:chemistry; D019904:Polymethyl Methacrylate / Q000737:chemistry; D012826:Silicone Elastomers / Q000737:chemistry; D013314:Stress, Mechanical; D013499:Surface Properties; D013718:Tensile Strength; D013816:Thermodynamics; D014867:Water / Q000737:chemistry
chemical_list: D000180:Acrylic Resins; D008689:Methacrylates; C098877:PermaSoft denture liner; C488492:QC 20 (denture base resin); D012826:Silicone Elastomers; D014867:Water; D019904:Polymethyl Methacrylate