323 - Does Bilateral Facet Screws Provide Similar Stability as a Pedicle Scr...

#323 Does Bilateral Facet Screws Provide Similar Stability as a Pedicle Screw System? A Finite Element Study

Basic Sciences-Research

Poster Presented by: V. Goel

Author(s):

A. Agarwal (1)
A.K. Agarwal (1)
V.K. Goel (1)

(1) University of Toledo, Bioengineering, Toledo, OH, USA

Abstract

Introduction: Pedicle screw fixation system has been the gold standard for stabilization of spine for fusion, since late 1980s. But this fixation system has several disadvantages like excessive muscle disruption (especially multifidus) and blood loss, high post-operative pain due to retained hardware even after fusion, and cumbersome surgical technique. Also the question of safety arises from potential neurological and vascular injury as reported in the literature. Therefore, a low profile and less invasive system capable of providing similar stability has been an issue of consistent debate among spine biomechanists and surgeons.

In this finite element study we compare a bilateral facet screw system with a pedicle screw system in terms of stability provided by each.

Methods: An experimentally validated finite element model of the L4-5 functional spinal unit (FSU) [1,2] was modified to simulate two posterior fixation instrumentations, both made up of titanium: bilateral facet screws and pedicle screw-rod system, with and without anterior lumbar interbody fusion (ALIF) carbon-fiber reinforced cage. 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.

Results:

Case I: without ALIF cage

For bilateral facet screw fixation, flexion, extension, lateral bending, and axial rotation range of motion (ROM) decreased by 62%, 98%, 98%, and 63% respectively compared to an intact FSU. For pedicle screw system, flexion, extension, lateral bending, and axial rotation ROM decreased by 77%, 95%, 85%, and 69% respectively.

For bilateral facet screw fixation, intradiscal pressure (IDP) during flexion and extension decreased by 17% and 83% respectively. For pedicle screw fixation, IDP during flexion and extension decreased by 33% and 75% respectively. For both the fixation system the decrease in IDP was 56% and 30% during lateral bending and axial rotation.

Case II: with ALIF cage

For bilateral facet screw fixation, flexion, extension, lateral bending, and axial rotation ROM decreased by 88%, 94%, 87%, and 91% respectively compared to an intact FSU. For pedicle screw system, flexion, extension, lateral bending, and axial rotation ROM decreased by 88%, 91%, 89%, and 86% respectively.

Discussion and Conclusions: In case I, except for flexion, bilateral facet screw was able to provide similar or higher stability compared to pedicle screw system. The difference between two fixation methods in flexion was of about 15% relative to intact. The decrease in ROM in both fixation systems concurs with their respective decrease in IDPs.

In case II, both the fixation system was able to provide similar stability as can be seen from the percentage decrease of ROM. Therefore, in presence of an interbody fusion cage, both bilateral facet screw and pedicle screw system provide similar stability.