Influence of a screw with a hinge on the pull-out force at the cranial end vertebra in ventral derotation spondylodesis (VDS).
BACKGROUND
Pull-out of the cranial end-vertebra screw following a correction of a scoliosis with the VDS implant is a common complication, especially in the thoracic region. It requires an extension of the fusion length or at least reduces the outcome of the correction.


METHODS
We therefore looked for a new approach to solve the problem of end-vertebra screw pull-out. It was postulated that the axial pull-out force at the end-vertebra screw is reduced by a screw with a hinge between the head and threaded part. The aim of our study was to calculate theoretically and then determine experimentally the screw pull-out force of regular and modified VDS screws with a hinge during lateral shifting of the screw heads in the frontal plane.


RESULTS
The use of a modified end-vertebra screw with a hinge on the cranial end vertebra of the correction region changes the shape of the threaded rod between the last two screws. Theoretically estimated, this leads to a reduction of the pull-out force to one-fourth of the value for the original VDS screw. Experimentally, the mean pull-out force for the modified end-vertebra screw was 110+/-8.6 N in comparison with the original screw, where the pull-out force was 214+/-7.2 N.


CONCLUSIONS
A reduction of screw pull-out can be expected when a screw with a hinge is used for the cranial end vertebra.
Department of Orthopaedics and Orthopaedic Surgery, Friedrich Alexander University, Rathsberger Strasse 57, 91054 Erlangen, Germany. constantin.kloeckner@waldkrankenhaus.de
C
C
Klöckner
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Rohlmann
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G
Bergmann
D001863:Bone Screws; D004867:Equipment Design; D006801:Humans; D013123:Spinal Fusion / Q000295:instrumentation*