#330 Comparison of Bilateral Facet Screw with Interspinous Fixation and Stand-alone Interspinous Fixation? A Biomechanical Finite Element Study
Poster Presented by: V. Goel
A.K. Agarwal (1)
V.K. Goel (1)
A. Agarwal (1)
(1) University of Toledo, Bioengineering, Toledo, OH, USA
Introduction: There are many approaches to lumbar spinal fusion surgery, and all involve adding bone graft to an area of the spine to set up a biological response. Recently, interspinous (ISS) fixation has become popular. Our belief is that ISS fixation alone is not sufficient. Supplemental fixation like the facet screw fixation is essential for the construct to become stable. In the present study, we have compared the stability of an interspinous fixation with and without facet screws.
Methods: An experimentally validated finite element model of the L4-5 functional spinal unit (FSU) [1,2] was modified to simulate two posterior fixations: interspinous fixation with and without facet screws. All the implant were given a material property of titanium (E=115GPa, v=0.3). The models were fixed at the inferior-most surface of L5, and subjected to 400 N follower load and 7.5 Nm of flexion, extension, lateral bending and axial rotation moments.
Case I: stand-alone interspinous fixation
For stand-alone interspinous fixation, ROM during flexion, extension, lateral bending, and axial rotation decreased by 68%, 96%, 63%, and 26% respectively, compared to an intact FSU. The corresponding decreases in IDP during flexion, extension, lateral bending, and axial rotation decreased were 23%, 73%, 48% and 14%, respectively.
The maximum Von Mises stress developed in the interspinous plate, between the two adjacent spinous processes, was 12.2MPa, 51MPa, 83.7MPa and 44.5MPa during flexion, extension, lateral bending and axial rotation, respectively.
Case II: bilateral facet screws with interspinous fixation
The range of motion (ROM) during flexion, extension, lateral bending, and axial rotation decreased by 67%, 99%, 83%, and 65% respectively, compared to an intact FSU. Intradiscal pressure (IDP) during flexion, extension, lateral bending, and axial rotation decreased by 25% , 85%, 58% and 33% respectively.
The above mentioned maximum Von Mises stress decreased by 42%, 64%, 75% and 59% during flexion, extension, lateral bending and axial rotation, respectively.
Discussion and Conclusions: In the sagittal plane, both the posterior fixation system provides similar stability. But for lateral bending and axial rotation, the stability provided by the bilateral facet screw with ISS fixation is higher compared to stand-alone interspinous fixation system. The bilateral facet screw with interspinous fixation system provides an additional decrease of 20% and 39% in ROM during lateral bending and axial rotation respectively. Also, the decrease in ROM in both posterior fixation systems concurs with their respective decrease in IDPs. The predicted decrease in maximum Von Mises stress in the interspinous plate when used with facet screw suggest that plates can be fabricated much thinner.
Our data shows that biomechanically there is a need for additional fixation (such as facet fixation) for the posterior stabilization with interspinous fixation device to achieve greater stability in all planes and with reduced stresses in the ISS plates.
1. Goel VK et al.: JBJS (Am), 88:103-9, 2006, (Suppl 2).
2. Grauer J et al: The Spine Journal (2006). 6:659-666.