title: Preclinical evaluation of posterior spine stabilization devices: can we compare in vitro and in vivo loads on the instrumentation?
abstract: To discuss whether the standard test method for preclinical evaluation of posterior spine stabilization devices with an anterior support correctly describes the effect of two short-segment posterior stabilization techniques frequently used in clinical practice for the treatment of traumatic, degenerative and iatrogenic instabilities.
  
  
  
  A finite element study compared a validated instrumented L2-L4 segment undergoing standing, upper body flexion and extension to ISO 12189 standards model under a compressive load. A bridge instrumentation, with screws only at cranial and caudal levels, and a full stabilization, using screws at every level, are considered for both conditions. The internal loads on the spinal rod and the stress values on the implant are analysed in detail.
  
  
  
  Using ISO model and a bridge stabilization construct allow to overstress the pedicle screw more than a full stabilization with respect to the corresponding L2-L4 segment undergoing upper body flexion, while the stress on the spinal rod is comparable. Choosing softer/stiffer springs would involve higher/lower loads on every component.
  
  
  
  ISO model predicts the effects of using both a full and a bridge posterior instrumentation. The study justifies the use of both conditions during in vitro reliability tests to achieve meaningful results easy to compare to clinically relevant loading modes and known in vivo failure modes.
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milan, Italy. luigi.labarbera@polimi.it.|IRCCS Galeazzi Orthopaedic Institute, via Riccardo Galeazzi 4, 20161, Milan, Italy. luigi.labarbera@polimi.it.
authors.forename: Luigi
authors.identifier: 0000-0002-9822-2116
authors.initials: L
authors.lastname: La Barbera
authors.affiliation: IRCCS Galeazzi Orthopaedic Institute, via Riccardo Galeazzi 4, 20161, Milan, Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Galbusera
authors.affiliation: Institute of Orthopaedic Research and Biomechanics, Centre of Musculoskeletal Research Ulm, Ulm University, Helmholtzstrasse 14, 89081, Ulm, Germany.
authors.forename: Hans-Joachim
authors.initials: HJ
authors.lastname: Wilke
authors.affiliation: Laboratory of Biological Structure Mechanics, Department of Chemistry, Materials and Chemical Engineering "Giulio Natta", Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milan, Italy.|IRCCS Galeazzi Orthopaedic Institute, via Riccardo Galeazzi 4, 20161, Milan, Italy.
authors.forename: Tomaso
authors.initials: T
authors.lastname: Villa
mesh_terms: D001863:Bone Screws; D020342:Finite Element Analysis; D006801:Humans; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery*; D008422:Materials Testing; D015203:Reproducibility of Results; D013123:Spinal Fusion / Q000295:instrumentation*; D016474:Weight-Bearing / Q000502:physiology*
keywords: ASTM F1717; Anterior support; Bridge instrumentation; Finite element; ISO 12189; Preclinical evaluation; Spine stabilization; Standard