Spinal Growth Modulation with Use of a Tether in an Immature Porcine Model

• Published in: Journal of bone and joint surgery. American volume 2008-12, Vol.90 (12), p.2695-2706
• Main Authors: Newton, Peter O, Upasani, Vidyadhar V, Farnsworth, Christine L, Oka, Richard, Chambers, Reid C, Dwek, Jerry, Kim, Jung Ryul, Perry, Andrew, Mahar, Andrew T
• Format: Article
• Language: English
• Subjects: Animals; Biological and medical sciences; Bone Development - physiology; Disease Models, Animal; Diseases of the osteoarticular system; Equipment Design; Internal Fixators; Kyphosis - surgery; Medical sciences; Orthopedic surgery; Range of Motion, Articular; Scoliosis - surgery; Spinal Fusion - instrumentation; Surgery (general aspects). Transplantations, organ and tissue grafts. Graft diseases; Swine; Swine, Miniature; Thoracic Vertebrae - growth & development; Time Factors; Weight-Bearing
• ISSN: 0021-9355; 1535-1386
• DOI: 10.2106/JBJS.G.01424

BackgroundSpinal 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.MethodsTwelve 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.ResultsRadiographs 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 animalʼs 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).ConclusionsIn 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 RelevanceMultilevel 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