General Session: Biomechanics
Presented by: C. Cain - View Audio/Video Presentation (Members Only)
C. Kleck(1), D. Illing(1), E. Lindley(1), V. Patel(1), C. Barton(1), T. Baldini(1), C. Cain(1), E. Burger(1)
(1) University of Colorado Anschutz Medical Campus, Denver, CO, United States
Purpose: Pseudoarthrosis, sacral stress fractures, and instrumentation failure are all complications observed in long posterior lumbar and thoraco-lumbar fixation constructs. The long lever arm of the spine construct creates stress across the lumbosacral region. Various techniques have been developed to decrease the rate of pseudoarthrosis, including iliac bolts, anterior L5-S1 interbody cages, and anterior column fixation with interbody devices. While studies have been performed to support these methods in clinical use, there are limited data comparing various fixation methods biomechanically. Thus, the purpose of this study was to evaluate the biomechanical properties of anterior and posterior spinal fusion constructs using a combination of interbody devices with or without screws, anterior fixation, and iliac bolts.
Methods: Twelve fresh frozen cadavers were instrumented from L2-S1 for biomechanical evaluation. Strain gauges were used to measure strain on the rods between L5-S1, on connectors between S1 and iliac bolts, and on S1 screws. Each specimen was sequentially tested in flexion, extension, and rotation with different construct configurations, including interbody devices and iliac bolts.
Results: Extension: Overall, the addition of anterior and posterior instrumentation significantly decreased strain on the S1 screw, L5-S1 rod, and S1-iliac bolt connector. Flexion: Compared to posterior fusion without iliac bolts, strain at the S1 screw increased with iliac bolts, cages placed by a TLIF approach, and cages placed by an anterior approach. However, the strain decreased with the placement of an anterior cage and iliac bolts or an ATB. Rotation: Compared to posterior instrumentation with iliac bolts, strain on the S1 screw significantly decreased when instrumentation was changed to a SynFix implant and iliac bolts were removed. A significant decrease at the S1 screw was seen when the ATB was used without bolts in the setting of the SynFix implant.
Conclusion: While the results only present significant findings, several other trends were identified. The results indicate that flexion increases strain at the S1 screw and L5-S1 rod. This appears to increase with the use of anteriorly placed interbody devices or iliac bolts, and can be mitigated with the combination of iliac bolts and interbody cages. It appears iliac bolts actually increase the strain in flexion greater than interbody cages alone. When evaluating extension-based activities, anteriorly placed devices appear to decrease strain when compared to iliac bolts, but a combination of techniques provides the greatest decrease. In rotation, anterior interbody constructs appear to increase S1 strain compared to iliac bolts, but again, a combination of bolts and anterior devices led to decreased strain.