Influence of Fixation Rigidity on Intervertebral Joints: An Experimental Comparison Between a Rigid an a Flexible Unconstrained System

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

Spinal instrumentation without fusion, used for treatment of spinal fractures and progressive pediatric spinal deformities, often fails to accomplish the desired goal due to spontaneous fusion and/or biological failure of the intervertebral joints.

PURPOSE: This study was to determine the influence of the rigidity of spinal fixation on the biological changes of the intervertebral joints.

METHODS: Twenty mini pigs in their actively growing period (age: 5.5 - 6.5 months) were subjected to posterior segmental fixation. Twelve were fixed with a rigid fixation system (Rigid fixation group; RF) using Colorado in 11 and TSRH in 1 (Sofamordanek, USA). Eight were fixed with a flexible unconstrained implant (Flexible fixation group; FF) that allowed micromotion of the rods and 6 degree of freedom between the screws and the longitudinal members (Orthobiom: Biorthex, Canada). The surgery consisted of pedicle screw instrumentation spanning 9 levels from T10 to L6 using 8 pedicle screws and two rods, and 90 degree rod rotation to create a scoliotic curve so that the adequacy of the screw could be monitored by the change in the curve magnitude. No intraarticular or extraarticular arthrodesis was attempted. The pigs were subjected to radiological examinations 2 weeks, 2 months, 6 month and every 6 months thereafter. They were euthanized 12 -18 months after the surgery for mechanical and histological examinations.

RESULTS: Analysis was performed using the data and the specimens retrieved from 12 pigs that survived more than 12 months after the surgery (6 in RF, 6 in FF). In the RF, minor fixation failure was observed in 1 animal (1 screw pullout). Initial scoliosis of 31±5°created by the rod rotation was reduced to 27±8° at the time of euthanesia but there was no statistical significance (p=0.37, paired t test) showing an acceptable maintenance of fixation. There was progressive thinning of the intervertebral discs within the instrumented extent with time. Though spontaneous bony fusion did not consistently occur, histological examination revealed severe degeneration of the intervertebral discs. Though there was no deliberate attempt of posterior fusion, all the facet joints within the instrumented extent showed spontaneous fusion with bony traberculae crossing the joint spaces. In the FF, minor fixation failures that occurred in 3 animals (2 pigs with 1 broken screw, 1 pig with 2 broken screws). However, the initial scoliotic curve of 19±4° was maintained at 17±5° showing a reasonable maintenance of fixation (p=0.21, paired t test). Disc height was well maintained within the instrument extent and the histological examination revealed viable cartilage in the posterior joints and intervertebral discs without signs of degeneration.

CONCLUSIONS: The rigidity of the spinal internal fixation influences the biological response of the intervertebral joints. Flexible unconstrained system that allows motion of the intervertebral joints may preserve the viability of intervertebral discs and the facet joint articular cartilages while maintaining a reasonably stable fixation.