In earlier posts VBT has been extensively
detailed. One question that commonly is
asked during discussion of VBT with patients and caregivers is: “What are the
long-term issues with VBT?”
detailed. One question that commonly is
asked during discussion of VBT with patients and caregivers is: “What are the
long-term issues with VBT?”
The simplistic answer is: “We don’t know”.
One layer to this question is what happens to the
actual tether?
actual tether?
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If
we look at other implant systems used in the spine and other bones of the body
over the last 50+ years we can roughly sketch out some possible scenarios for
the system currently used for VBT. The
fixation in the vertebra are screws which, as a group, have a long history of
safety and efficacy. However the screw
used in VBT are designed for use in the posterior spine, and for VBT they are
placed anterior through a minimally-invasive or thoracoscopic approach. The question is will they function with the
same efficacy and safety profile. Based
on the collective experience it appears the screws have good purchase and few
issues with prominence, migration or pullout.
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The
other aspect of VBT is the tether which is made of braided polypropylene. This is the workhorse of the system, which
compresses across the convex discs and growth plates to modulate spine
growth. Since there is no fusion across
the vertebral bodies there will be constant motion on the tether. Like any non-regenerating material which is
constantly moving, the tether is subject to fatigue, which can lead to failure
or breakage of the tether. It makes
sense that the tether will eventually break, considering it is implanted in
adolescents and will be stressed for over 60+ years (or more!). Over the last year there have been reports of
segmental failure of the tether (between two screws), so it is reasonable to
assume that in the long-term the tether will likely break in multiple
locations. For the sake of the aim of
VBT to modulate growth in the immature spine, we only need it to last until the
completion of spinal growth. What is not
desired is for the tether to break prior to this time and permit the spine
deformity to get worse.
A second layer is what the tether does to the vertebral
bodies, and more importantly, to the disc between the vertebral bodies. The implications of long-term compression of
the instrumented disc and the presence of anterior instrumentation in a
non-fusion technique is unknown. Changes
to the intervertebral discs may occur and, if this happens, may cause axial thoracic
back pain or possible disc herniations in the future. Also, it is unknown if increased motion, such
as after the tether breaks, through a previously VBT-compressed motion segment is
significant. Will this cause back pain?
At the present time we just don’t know.
bodies, and more importantly, to the disc between the vertebral bodies. The implications of long-term compression of
the instrumented disc and the presence of anterior instrumentation in a
non-fusion technique is unknown. Changes
to the intervertebral discs may occur and, if this happens, may cause axial thoracic
back pain or possible disc herniations in the future. Also, it is unknown if increased motion, such
as after the tether breaks, through a previously VBT-compressed motion segment is
significant. Will this cause back pain?
At the present time we just don’t know.
More research is necessary on VBT safety, timing of
VBT placement, VBT tensioning, intervertebral disc health, and long-term
patient reported and radiographic outcomes of VBT.
VBT placement, VBT tensioning, intervertebral disc health, and long-term
patient reported and radiographic outcomes of VBT.
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References:
Website: www.zimmer.com/medical-professionals/products/spine/dynesys-dynamic-stabilization-system.html
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