title: Comparative analysis of international standards for the fatigue testing of posterior spinal fixation systems: the importance of preload in ISO 12189.
abstract: BACKGROUND
  Preclinical evaluation of the mechanical reliability of fixation devices is a mandatory activity before their introduction into market. There are two standardized protocols for preclinical testing of spinal implants. The American Society for Testing Materials (ASTM) recommends the F1717 standard, which describes a vertebrectomy condition that is relatively simple to implement, whereas the International Organization for Standardization (ISO) suggests the 12189 standard, which describes a more complex physiological anterior support-based setup. Moreover, ASTM F1717 is nowadays well established, whereas ISO 12189 has received little attention: A few studies tried to accurately describe the ISO experimental procedure through numeric models, but these studies totally neglect the recommended precompression step.
  
  
  OBJECTIVE
  This study aimed to build up a reliable, validated numeric model capable of describing the stress on the rods of a spinal fixator assembled according to ISO 12189 standard procedure. Such a model would more adequately represent the in vitro testing condition.
  
  
  METHODS
  This study used finite element (FE) simulations and experimental validation testing.
  
  
  METHODS
  An FE model of the ISO setup was built to calculate the stress on the rods. Simulation was validated by comparison with experimental strain gauges measurements. The same fixator has been previously 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 the FE predictions and experimental measurements is in good agreement, thus confirming the suitability of the FE method to evaluate the stresses in the device. The initial precompression induces a significant extension of the assembled construct. As the applied load increases, the initial extension is gradually compensated, so that at peak load the rods are bent in flexion: The final stress value predicted is thus reduced to about 50%, if compared with the previous model where the precompression was not considered.
  
  
  CONCLUSIONS
  Neglecting the initial preload due to the assembly of the overall construct according to ISO 12189 standard could lead to an overestimation of the stress on the rods up to 50%. To correctly describe the state of stress on the posterior spinal fixator, tested according to the ISO procedure, it is important to take into account the initial preload due to the assembly of the overall construct.
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, 20161 Milano, Italy. Electronic address: luigi.labarbera@polimi.it.
authors.forename: Luigi
authors.initials: L
authors.lastname: La Barbera
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta," Politecnico di Milano, 20133 Milano, Italy.
authors.forename: Claudia
authors.initials: C
authors.lastname: Ottardi
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, 20161 Milano, Italy.
authors.forename: Tomaso
authors.initials: T
authors.lastname: Villa
mesh_terms: D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery; D008422:Materials Testing / Q000379:methods / Q000592:standards*; D019736:Prostheses and Implants / Q000009:adverse effects / Q000592:standards*; D012015:Reference Standards; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013314:Stress, Mechanical
keywords: ASTM F1717; Fatigue testing; Finite element; ISO 12189; Precompression; Preload; Standard; Strain gauge