Vertebral Body Tether

Vertebral Body Tethering Part 5

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?”
The simplistic answer is: “We don’t know”.
One layer to this question is what happens to the
actual tether? 
  1. 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.
  2. 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.
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.
Newton PO, Fricka KB, Lee SS, et al.  Asymmetrical flexible tethering of spine
growth in an immature bovine model.
Spine 2002;27(7):689-93.
Braun JT, Ogilvie JW, Akyuz E, et al.  Fusionless scoliosis correction using a shape
memory alloy staple in the anterior thoracic spine of the immature goat.  Spine 2004;29(18):1980-9.
Newton PO, Farnsworth CL, Faro FD, et al.  Spinal growth modulation with an
anterolateral flexible tether in an immature bovine model: disc health and
motion preservation.  Spine 2008;33(7):724-33.
Chay E, Patel A, Ungar B, et al.  Impact of unilateral corrective tethering on
the histology of the growth plate in an established porcine model for thoracic
scoliosis.  Spine 2012;37(15):E883-9.
Crawford CH 3rd, Lenke LG.  Growth modulation by means of anterior
tethering resulting in progressive correction of juvenile idiopathic scoliosis:
a case report.  J Bone Joint Surg [Am]
Samdani AF, Ames RJ, Kimball JS, et al.  Anterior vertebral body tethering for
immature adolescent idiopathic scoliosis: one-year results on the first 32
patients.  Eur Spin J 2015;24:1533-9.

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