The Effect of Segmental Pedicle Screw Instrumentation on Actively Growing Spine: A Long-term Experimental Study

Kim, Won Joong; Lee, Sang Ho; Shin, Song Woo; Rivard, Charles H; Coillard, Christine; Rhalmi, Souad

Abstract

OBJECTIVE
Pedicle screw is gaining popularity in pediatric deformities. However, biological response of actively growing spine to rigid pedicle screw fixation remains unclear. The objective of this study is to determine the biological response of growing spine to rigid segmental fixation. METHODS: Twelve mini pigs in actively growing period were subjected to posterior segmental screw -rod instrumentation spanning nine levels and creation of experimental scoliosis. There was no attempt of posterior arthrodesis. The pigs were subjected to periodic radiological examinations and were euthanized at 18 months for analysis. RESULTS: There was no significant fixation failure despite conspicuous growth of the animals. Initial scoliosis of 31+/-5degrees was reduced to 27+/-8degrees at 18 months, but there was no statistical significance (p=0.37). Though there was no change in length of the implant construct, the vertebrae within the instrumented section showed mean longitudinal growth of 6+/-3 mm (p=0.000). The growth occurred at expense of the disc spaces that progressively narrowed with time. On necropsy, the instrumented region was completely fused posteriorly with crossing of the osseous traberculae across the former facet joints. Intervertebral discs were severely atrophic in all the discs with occasional spontaneous fusion. CONCLUSION: Even in the actively growing spine, the force of growth does not overcome the fixation offered by segmental pedicle screws. Longitudinal growth occurs at the expense of the joint spaces and leads to spontaneous intervertebral fusion. Our results may explain the favorable outcomes in pedicle fixations in pediatric population, showing little implant failure or nonunion.