Published in:
01-10-2003 | Original Article
Influence of a screw with a hinge on the pull-out force at the cranial end vertebra in ventral derotation spondylodesis (VDS)
Published in: Archives of Orthopaedic and Trauma Surgery | Issue 8/2003
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Introduction
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.
Materials and 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.
Conclusion
A reduction of screw pull-out can be expected when a screw with a hinge is used for the cranial end vertebra.