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    History of Isola-VSP Fatigue Testing Results with Correlation to Clinical Implant Failures

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    The objective was to compare the history of Isola-VSP in vitro fatigue testing results with clinical implant failures from a five-center retrospective survey of 2499 cases to determine if the appropriate types of tests had been performed. To determine the effect of bending iron marks, bends, and connectors on 1/4" rod fatigue, 4-point bend fatigue tests were conducted. To characterize bone anchor-connector-rod assemblies, unilateral construct flexion fatigue tests were conducted. Clinically 111 components failed: 41 screws, 57 rods, nine transverse connectors, two interbody graft/cages, one extended slotted connector, and one at unreported location. The screw, rod, and connector clinical data correlate to the lower to higher relative fatigue strength respectively of original integral nut screws; rods at bending iron marks, connectors and lordotic bends; original slotted connector, current slotted connector, and straight rods with unblemished surface. In vitro and clinical failure locations also correlated. The transverse connector cross member failed near the longitudinal rod in 8/9 instances. This implies a lateral bending profile similar to that produced by the H construct used to test them in reversed lateral bending. Recommendations relative to ASTM standards/guides include: incorporation of an H construct to test transverse connectors in lateral bending, replacement of fixed-fixed end with fixed-free end assembly in F 1798-97, and replacement of constrained fixtures in F 1717-01 with unconstrained.


    Retrospective clinical survey, implant failure, in vitro test history, relative fatigue strength, bent/straight rod, bending iron marks, rod, connector, pedicle screw

    Author Information:

    Carson, WL
    Professor Emeritus Mechanical and Aerospace Engineering, University of Missouri-Columbia, Columbia, Missouri

    Asher, MA
    The University of Kansas Medical Center, Kansas City, Kansas

    Boachie-Adjei, O
    The Hospital for Special Surgery, New York, New York

    Akbarnia, B
    San Diego Center for Spinal Disorders, La Jolla, California

    Dzioba, R
    The University of Arizona Health Sciences Center, Tucson, Arizona

    Lebwohl, NH
    University of Miami, Miami, Florida

    Committee/Subcommittee: F04.25

    DOI: 10.1520/STP11126S