• Luigi La Barbera, Fabio Galbusera, Hans-Joachim Wilke, and Tomaso Villa.
• 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.
• Eur Spine J. 2017 Jan 1; 26 (1): 200-209.

PurposeTo 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.MethodsA 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.ResultsUsing 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.ConclusionsISO 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.

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