Vertebral Body Tethering (Part 5 and the last one on this topic)

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.

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References:

Website: www.zimmer.com/medical-professionals/products/spine/dynesys-dynamic-stabilization-system.html

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 3^rd, 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] 2010;92(1):202-9.

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.