title: Cyclic mechanical loading promotes bacterial penetration along composite restoration marginal gaps.
abstract: OBJECTIVE
  Secondary caries is the most common reason for composite restoration replacement and usually forms between dentin and the filling. The objective of this study was to investigate the combined effect of cyclic loading and bacterial exposure on bacterial penetration into gaps at the interface between dentin and resin composite restorative material using a novel bioreactor system and test specimen design.
  
  
  METHODS
  Human molars were machined into 3mm thick disks with 2mm deep × 5 mm diameter cavity preparations into which composite restorations were placed. A ∼ 15-30 μm (small) or ∼ 300 μm wide (large) marginal gap was introduced along half of the interface between the dentin and restoration. Streptococcus mutans UA 159 biofilms were grown on each sample prior to testing each in a bioreactor both with and without cyclic loading. Both groups of samples were tested for 2 weeks and post-test biofilm viability was confirmed with a live-dead assay. Samples were fixed, mounted and cross-sectioned to reveal the gaps and observe the depth of bacterial penetration.
  
  
  RESULTS
  It was shown that for large gap samples the bacteria easily penetrated to the full depth of the gap independent of loading or non-loading conditions. The results for all cyclically loaded small gap samples show a consistently deep bacterial penetration down 100% of the gap while the average penetration depth was only 67% for the non-loaded samples with only two of six samples reaching 100%.
  
  
  CONCLUSIONS
  A new bioreactor was developed that allows combining cyclic mechanical loading and bacterial exposure of restored teeth for bacterial biofilm and demineralization studies. Cyclic loading was shown to aid bacterial penetration into narrow marginal gaps, which could ultimately promote secondary caries formation.
authors.affiliation: Materials Science, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR, USA.
authors.forename: D
authors.initials: D
authors.lastname: Khvostenko
authors.affiliation: Department of Restorative Dentistry, School of Dentistry, Oregon Health & Science University, Portland, OR, USA.
authors.forename: S
authors.initials: S
authors.lastname: Salehi
authors.affiliation: Materials Science, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR, USA.
authors.forename: S E
authors.initials: SE
authors.lastname: Naleway
authors.affiliation: Department of Restorative Dentistry, School of Dentistry, Oregon Health & Science University, Portland, OR, USA.
authors.forename: T J
authors.initials: TJ
authors.lastname: Hilton
authors.affiliation: Department of Restorative Dentistry, School of Dentistry, Oregon Health & Science University, Portland, OR, USA.
authors.forename: J L
authors.initials: JL
authors.lastname: Ferracane
authors.affiliation: College of Dental Medicine, Midwestern University, Glendale, AZ, USA.
authors.forename: J C
authors.initials: JC
authors.lastname: Mitchell
authors.affiliation: Materials Science, School of Mechanical, Industrial, and Manufacturing Engineering, Oregon State University, Corvallis, OR, USA. Electronic address: jamie.kruzic@oregonstate.edu.
authors.forename: J J
authors.initials: JJ
authors.lastname: Kruzic
mesh_terms: D018441:Biofilms; D019149:Bioreactors; D003188:Composite Resins / Q000737:chemistry; D003731:Dental Caries / Q000382:microbiology*; D003737:Dental Cavity Preparation; D003763:Dental Leakage; D018772:Dental Marginal Adaptation; D003793:Dental Restoration, Permanent / Q000379:methods*; D003799:Dental Stress Analysis; D004867:Equipment Design; D006801:Humans; D066298:In Vitro Techniques; D008963:Molar; D013242:Sterilization; D013295:Streptococcus mutans
chemical_list: D003188:Composite Resins
keywords: Biofilm; Cyclic loading; Marginal gap; Resin composite; Secondary caries; Streptococcus mutans