The Journal of Bone and Joint Surgery (American). 2008;90:2695-2706.
doi:10.2106/JBJS.G.01424
© 2008 The Journal of Bone and Joint Surgery, Inc.
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Spinal Growth Modulation with Use of a Tether in an Immature Porcine Model

Peter O. Newton, MD1, Vidyadhar V. Upasani, MD2, Christine L. Farnsworth, MS1, Richard Oka, MS1, Reid C. Chambers, BA1, Jerry Dwek, MD1, Jung Ryul Kim, MD3, Andrew Perry, MD2 and Andrew T. Mahar, MS1

1 Rady Children's Hospital San Diego, 3030 Children's Way, Suite 410, San Diego, CA 92123. E-mail address for P.O. Newton: pnewton{at}rchsd.org
2 University of California at San Diego, 9500 Gilman Drive, San Diego, CA 92093
3 Chonbuk National University, 634-18 Geumam-dong, Jeonju 561712, Republic of Korea

Investigation performed at the University of California at San Diego and Rady Children's Hospital and Health Center, San Diego, California

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from DePuy Spine, Inc. and the Rady Children's Hospital Orthopedic Research and Education Fund. In addition, one or more of the authors or a member of his or her immediate family received, in any one year, payments or other benefits in excess of $10,000 or a commitment or agreement to provide such benefits from a commercial entity (DePuy Spine, Inc.). Also, a commercial entity (DePuy Spine, Inc.) paid or directed in any one year, or agreed to pay or direct, benefits in excess of $10,000 to a research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which one or more of the authors, or a member of his or her immediate family, is affiliated or associated.

Background: Spinal growth modulation by tethering the anterolateral aspect of the spine, as previously demonstrated in a nonscoliotic calf model, may be a viable fusionless treatment method for idiopathic scoliosis. The purpose of the present study was to evaluate the radiographic, histologic, and biomechanical results after six and twelve months of spinal growth modulation in a porcine model with a growth rate similar to that of adolescent patients.

Methods: Twelve seven-month-old mini-pigs underwent instrumentation with a vertebral staple-screw construct connected by a polyethylene tether over four consecutive thoracic vertebrae. The spines were harvested after six (n = 6) or twelve months (n = 6) of growth. Monthly radiographs, computed tomography and magnetic resonance imaging scans (made after the spines were harvested), histologic findings, and biomechanical findings were evaluated. Analysis of variance was used to compare preoperative, six-month postoperative, and twelve-month postoperative data.

Results: Radiographs demonstrated 14° ± 4° of coronal deformity after six months and 30° ± 13° after twelve months of growth. Coronal vertebral wedging was observed in all four tethered vertebrae and progressed throughout each animal's survival period. Disc wedging was also created; however, in contrast to the findings associated with vertebral wedging, the tethered side was taller than the untethered side. Magnetic resonance images revealed no evidence of disc degeneration; however, the nucleus pulposus had shifted toward the side of the tethering. Midcoronal undecalcified histologic sections showed intact bone-screw interfaces with no evidence of implant failure or loosening. With the tether cut, stiffness decreased and range of motion increased in lateral bending away from the tether at both time-points (p < 0.05).

Conclusions: In this porcine model, mechanical tethering during growth altered spinal morphology in the coronal and sagittal planes, leading to vertebral and disc wedging proportional to the duration of tethering. The resulting concave thickening of the disc in response to the tether was not anticipated and may suggest a capacity for the nucleus pulposus to respond to the compressive loads created by growth against the tether.

Clinical Relevance: Multilevel flexible tether placement may provide a growth-modulating mechanism for scoliosis correction while maintaining disc health. Understanding the response of the growing vertebrae and discs in this proposed method of fusionless scoliosis treatment remains paramount.


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