title: Comparative analysis of international standards for the fatigue testing of posterior spinal fixation systems.
abstract: BACKGROUND
  Preclinical evaluation of the long-term reliability of devices for lumbar fixation is a mandatory activity before they are put into market. The experimental setups are described in two different standards edited by the International Organization for Standardization (ISO) and the American Society for Testing Materials (ASTM), but the evaluation of the suitability of such tests to simulate the actual loading with in vivo situations has never been performed.
  
  
  OBJECTIVE
  To calculate through finite element (FE) simulations the stress in the rods of the fixator when subjected to ASTM and ISO standards. To compare the calculated stresses arising in the same fixator once it has been virtually mounted in a physiological environment and loaded with physiological forces and moments.
  
  
  METHODS
  FE simulations and validation experimental tests.
  
  
  METHODS
  FE models of the ISO and ASTM setups were created to conduct simulations of the tests prescribed by standards and calculate stresses in the rods. Validation of the simulations were performed through experimental tests; the same fixator was virtually mounted in an L2-L4 FE model of the lumbar spine and stresses in the rods were calculated when the spine was subjected to physiological forces and moments.
  
  
  RESULTS
  The comparison between FE simulations and experimental tests showed good agreement between results obtained using the two methodologies, thus confirming the suitability of the FE method to evaluate stresses in the device in different loading situations. The usage of a physiological load with ASTM standard is impossible due to the extreme severity of the ASTM configuration; in this circumstance, the presence of an anterior support is suggested. Also, ISO prescriptions, although the choice of the setup correctly simulates the mechanical contribution of the discs, seem to overstress the device as compared with a physiological loading condition. Some daily activities, other than walking, can induce a further state of stress in the device that should be taken into account in setting up new experimental procedures.
  
  
  CONCLUSIONS
  ISO standard loading prescriptions seems to be more severe than the expected physiological ones. The ASTM standard should be completed by including some anterior supporting device and declaring the value of the load to be imposed. Moreover, a further enhancement of standards would be simulating other movements representative of daily activities different from walking.
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, 20133, Milano, Italy; IRCCS Istituto Ortopedico Galeazzi, Via Galeazzi 4, 20161, Milano, Italy. Electronic address: tomaso.villa@polimi.it.
authors.forename: Tomaso
authors.initials: T
authors.lastname: Villa
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, 20133, Milano, Italy; IRCCS Istituto Ortopedico Galeazzi, Via Galeazzi 4, 20161, Milano, Italy.
authors.forename: Luigi
authors.initials: L
authors.lastname: La Barbera
authors.affiliation: IRCCS Istituto Ortopedico Galeazzi, Via Galeazzi 4, 20161, Milano, Italy; Institute of Orthopaedic Research and Biomechanics, Center of Musculoskeletal Research Ulm, University of Ulm, Helmholtzstr. 14, 89081, Ulm, Germany.
authors.forename: Fabio
authors.initials: F
authors.lastname: Galbusera
mesh_terms: D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery*; D008422:Materials Testing / Q000592:standards*; D015203:Reproducibility of Results; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical
keywords: Fatigue test; Finite element method; International standard; Spinal fixator; Validation