title: ASTM F1717 standard for the preclinical evaluation of posterior spinal fixators: can we improve it?
abstract: Preclinical evaluation of spinal implants is a necessary step to ensure their reliability and safety before implantation. The American Society for Testing and Materials reapproved F1717 standard for the assessment of mechanical properties of posterior spinal fixators, which simulates a vertebrectomy model and recommends mimicking vertebral bodies using polyethylene blocks. This set-up should represent the clinical use, but available data in the literature are few. Anatomical parameters depending on the spinal level were compared to published data or measurements on biplanar stereoradiography on 13 patients. Other mechanical variables, describing implant design were considered, and all parameters were investigated using a numerical parametric finite element model. Stress values were calculated by considering either the combination of the average values for each parameter or their worst-case combination depending on the spinal level. The standard set-up represents quite well the anatomy of an instrumented average thoracolumbar segment. The stress on the pedicular screw is significantly influenced by the lever arm of the applied load, the unsupported screw length, the position of the centre of rotation of the functional spine unit and the pedicular inclination with respect to the sagittal plane. The worst-case combination of parameters demonstrates that devices implanted below T5 could potentially undergo higher stresses than those described in the standard suggestions (maximum increase of 22.2% at L1). We propose to revise F1717 in order to describe the anatomical worst case condition we found at L1 level: this will guarantee higher safety of the implant for a wider population of patients.
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', Politecnico di Milano, Milano, Italy IRCCS Istituto Ortopedico Galeazzi, Milano, Italy luigi.labarbera@polimi.it.
authors.forename: Luigi
authors.initials: L
authors.lastname: La Barbera
authors.affiliation: IRCCS Istituto Ortopedico Galeazzi, Milano, Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Galbusera
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering 'Giulio Natta', Politecnico di Milano, Milano, Italy IRCCS Istituto Ortopedico Galeazzi, Milano, Italy.
authors.forename: Tomaso
authors.initials: T
authors.lastname: Villa
authors.affiliation: Department of Neurosurgery, Humanitas Clinical and Research Center, Milano, Italy.
authors.forename: Francesco
authors.initials: F
authors.lastname: Costa
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Centre of Musculoskeletal Research Ulm (ZMFU), Ulm University, Ulm, Germany.
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Wilke
mesh_terms: D001858:Bone Nails / Q000592:standards*; D001863:Bone Screws / Q000592:standards*; D003198:Computer Simulation; D019544:Equipment Failure Analysis / Q000592:standards*; D005593:Fracture Fixation, Internal / Q000295:instrumentation* / Q000592:standards*; D006801:Humans; D008954:Models, Biological; D012015:Reference Standards; D016103:Spinal Fractures / Q000503:physiopathology* / Q000601:surgery; D013131:Spine / Q000503:physiopathology* / Q000601:surgery; D014481:United States
keywords: ASTM F1717; ISO 12189; fatigue testing; finite element model; parametric study; pedicle anatomy; pedicle screw; pedicular inclination; standard