title: The slippery slope: critical perspectives on in vitro research methodologies.
abstract: OBJECTIVE
  This paper attempts to provide critical perspectives on common in vitro research methodologies, including shear bond testing, wear testing, and load-to-failure tests. Origins of interest in high-quality laboratory data is reviewed, in vitro data is categorized into property and simulation protocols, and two approaches are suggested for establishing clinical validity. It is hoped that these insights will encourage further progress toward development of in vitro tests that are validated against clinical performance and/or by producing clinically validated failure or damage mechanisms.
  
  
  METHODS
  Published shear and tensile bond data (macro and micro) is examined in light of published finite element analyses (FEA). This data is subjected to a Weibull scaling analysis to ascertain whether scaling is consistent with failure from the bonded interface or not. Wear tests results are presented in light of the damage mechanism(s) operating. Quantitative wear data is re-examined as being dependent upon contact pressure. Load-to-failure test results are re-analyzed by calculating contact stresses at failure for 119 tests from 54 publications over more than 25 years.
  
  
  RESULTS
  FEA analyses and reported failure modes (adhesive, mixed, cohesive) are consistent with failure not involving interfacial "shear stresses" as calculated in published work. Weibull scaling clearly suggests failure involving external surfaces of specimens, not interfacial origins. Contact stresses (pressures) are clearly an important variable in wear testing and are not well-controlled in published work. Load-to-failure tests create damage not seen clinically due to excessively high contact stresses. Most contact stresses in the 119 tests examined were calculated to be between 1000MPa and 5000MPa, whereas clinical contact stresses at wear facets have been measured not to exceed 40MPa.
  
  
  CONCLUSIONS
  Our community can do a much better job of designing in vitro tests that more closely simulate clinical conditions, especially when contact is involved. Journals are encouraged to thoughtfully consider a ban on publishing papers using bond tests and load-to-failure methods that are seriously flawed and have no clinical relevance.
authors.affiliation: Department of Reconstructive Sciences, Center for Biomaterials, University of Connecticut Health Center, Farmington, CT 06030-1615, USA. kelly@nso1.uchc.edu
authors.forename: J Robert
authors.initials: JR
authors.lastname: Kelly
authors.forename: Paula
authors.initials: P
authors.lastname: Benetti
authors.forename: Patchanee
authors.initials: P
authors.lastname: Rungruanganunt
authors.forename: Alvaro Della
authors.initials: AD
authors.lastname: Bona
mesh_terms: D019245:Compressive Strength; D001840:Dental Bonding; D003764:Dental Materials; D019554:Dental Restoration Wear; D003799:Dental Stress Analysis / Q000379:methods*; D020342:Finite Element Analysis; D008422:Materials Testing / Q000379:methods*; D012015:Reference Standards; D015203:Reproducibility of Results; D033081:Shear Strength; D016019:Survival Analysis; D013718:Tensile Strength
chemical_list: D003764:Dental Materials