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

· (a - Biorthex, Inc.) Research Center, Sainte Justine Hospital, University of Montreal.
Montreal, Canada

Pedicle screw fixation is gaining increasing popularity in the pediatric spinal disorders. Yet there has been few reports on the biological response of the actively growing spine to the rigid segmental pedicle screw constructs similar to those used in the treatment of spinal deformities.

PURPOSE: to determine the biological response of the actively growing spine to long segmental pedicle screw constructs.

METHODS: Twelve mini pigs in their actively growing period (age: 5.5 -6.5 months, 32± 5 kg) were subjected to posterior segmental pedicle screw instrumentation in scoliosis construct using rigid rod-screw implants (11 Colorado, 1 TSRH: Sofamordanek, USA). The surgery consisted of pedicle screw instrumentation spanning 9 levels from T10 to L6 using 8 pedicle screws and two rods. Following the attachment of the rods to the screws, the rods were rotated 90 degrees to create a scoliotic curve so that the construct would be subject to similar mechanical forces as in a scoliosis patient. No intraarticular or extraarticular arthrodesis was attempted. The pigs were subjected to radiological examinations 2 weeks, 2, 6, 12 and 18 months after the surgery. They were euthanized at 18 months for radiological and histological examinations.

RESULTS: Analysis was performed with 6 pigs that survived the 18 months period. There was no significant fixation or mechanical failure of the implants despite the conspicuous growth of the animals during the observation period (weight at euthanesia: 63±15 kg). Initial scoliosis of 31±5°created by the rod rotation was slightly reduced to 27±8° at 18 months, but there was no statistical significance (p= 0.37, paired t test). Though there was no change in the distance between the uppermost and the lower most pedicle screws, the vertebrae within the instrumented extent showed longitudinal growth of 6±3 mm from 29±3 mm to 34±3 mm (p=0.000, paired t test). The longitudinal growth occurred at the expense of the disc spaces that progressively narrowed to eventual obliteration. On histological examination, the instrumented region was completely fused posteriorly with crossing of the osseous traberculae across the former facet joints even though there was no deliberate attempts of posterior arthrodesis. Intervertebral discs were severely atrophic in all the discs with occasional spontaneous anterior fusion with bony traberculae crossing the disc space.

CONCLUSIONS: Even in the actively growing spine, the force of growth does not overcome the restrictive force offered by segmental pedicle fixation. Longitudinal vertebral growth does occur within the confines of the instrument construct, but at the expense of the disc spaces and eventually leads to bony fusion between the vertebrae. Our results may explain the favorable outcomes in pedicle fixations in pediatric population, showing little implant failure or nonunion.