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Bilevel spinal implant constructs are 3 dimensional with 6 degrees-of-freedom of superior relative to inferior vertebra motion. ASTM F 1717-01 pinned fixtures constrain 3 degrees-of-freedom, which for posterior constructs are: lateral translation, PA axis rotation, and axial rotation to be about axis through the center of each vertebral body mounting pin. Also, F 1717-01 only illustrates testing of rectangular constructs that are symmetrical about a mid plane. Clinical examples, unconstrained finite element models, and hand held-loaded models are used to illustrate that in general some of the primary components of construct displacement and modes of failure are those constrained by the fixtures in F 1717-01; with one exception being the axial loading of rectangular constructs that remain symmetrical in geometry and biomechanical characteristics of components and interconnections. This raises two questions: the possibility of some clinical modes of failure being obscured by F 1717-01, and the clinical relevance of some numerical test results. Gimbal-gimbal or pushrod-gimbal fixtures for unconstrained axial and torsional load, static and fatigue testing, unsymmetrical as well as symmetrical constructs is proposed as a replacement for the current pinned fixtures.
Construct, constrained, unconstrained, axial, torsion, F 1717-01
Professor Emeritus Mechanical and Aerospace Engineering, University of Missouri-Columbia, Columbia, Missouri