title: The effect of mechanical undercuts on the strength of composite repair.
abstract: BACKGROUND
  The authors conducted an in vitro study to test the hypothesis that undercuts prepared in old composite restorations could improve the strength between the restoration and a flowable composite as repair material.
  
  
  METHODS
  The authors used three composites to fabricate cylinders as repair substrates. The etched-only group was ground, etched, dried and built up with a flowable composite. For the undercut group, the authors introduced arrays of fissures on the surfaces before preparing the specimens for subsequent buildup in the manner described for the etched-only group. They made nonrepaired cylinders for baseline measures of strength. They sliced all finished cylinders into slender bars with a diamond saw. Flexure strength values were determined by a three-point-bending test.
  
  
  RESULTS
  Nonrepaired bars exhibited statistically significantly higher flexure strength values than did repaired bars, as determined by Wilcoxon rank sum test. Two-way general linear model showed that both material (P < .0001) and undercut (P = .0207) exhibited a statistically significant influence on the repaired flexure strength. Repair substrate with elastic modulus close to that of repair material exhibited a greater percentage of recovery of the respective cohesive strength. Compared with the etched-only group, the undercut group yielded a higher mean flexure strength with one composite but a lower mean flexure strength with the other two. Examination of the fractured surfaces showed that a significant number of undercuts were filled only partially.
  
  
  CONCLUSIONS
  Flexure strengths of repaired specimens always were lower than the cohesive flexure strengths of the materials being repaired. Undercuts did not generally improve repair strength.
  
  
  CONCLUSIONS
  Small undercuts on the surface of composite often are difficult to fill completely, resulting in areas of stress concentration that result in no improvement in the repair strength.
authors.affiliation: Department of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville 32610-0446, USA. cshen@dental.ufl.edu
authors.forename: Chiayi
authors.initials: C
authors.lastname: Shen
authors.forename: Eduardo
authors.initials: E
authors.lastname: Mondragon
authors.forename: Valeria V
authors.initials: VV
authors.lastname: Gordan
authors.forename: Ivar A
authors.initials: IA
authors.lastname: Mjör
mesh_terms: D000704:Analysis of Variance; D017438:Bisphenol A-Glycidyl Methacrylate; D003188:Composite Resins; D001840:Dental Bonding; D003737:Dental Cavity Preparation / Q000379:methods*; D017269:Dental Prosthesis Repair; D003793:Dental Restoration, Permanent / Q000379:methods*; D003799:Dental Stress Analysis; D004548:Elasticity; D006801:Humans; D016014:Linear Models; D008422:Materials Testing; D018583:Pliability; D019233:Retreatment; D018709:Statistics, Nonparametric; D013718:Tensile Strength
chemical_list: C470433:Beautifil composite resin; D003188:Composite Resins; C444701:Filtek Z250; C470818:Heliomolar Flow; D017438:Bisphenol A-Glycidyl Methacrylate; C039734:Durafill