The Influence of Fixation Rigidity on Intervertebral Joints - An Experimental Comparison between a Rigid and a Flexible System

OBJECTIVE: Spinal instrumentation without fusion often fails due to biological failure of intervertebral joints (spontaneous fusion, degeneration, etc). The purpose of this study is to investigate the influence of fixation rigidity on viability of intervertebral joints. METHODS: Twenty pigs in growing period were subjected to posterior segmental fixation. Twelve were fixed with a rigid fixation system(RF) while eight were fixed with a flexible unconstrained implant(FF). At the time of the surgery, a scoliosis was created to monitor fixation adequacy. The pigs were subjected to periodic radiological examinations and 12pigs (six in RF, six in FF) were euthanized at 12-18months postoperatively for analysis. RESULTS: The initial scoliotic curve was reduced from 31+/-5degrees to 27+/-8degrees in RF group (p=0.37) and from 19+/-4degrees to 17+/-5degrees in FF group (p=0.21). Although severe disc degeneration and spontaneous fusion of facet joints were observed in RF group, disc heights of FF group were well maintained without major signs of degeneration. CONCLUSION: The viability of the intervertebral joints depends on motion spinal fixation. Systems allowing intervertebral micromotion may preserve the viability of intervertebral discs and the facet joint articular cartilages while maintaining a reasonably stable fixation.

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

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.