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Effect of graded facetectomy on biomechanics of Dynesys dynamic stabilization system
Ali Kiapour1, Divya Ambati, Robert W Hoy
1Engineering Center for Orthopaedic Research Excellence, University of Toledo, Toledo, OH 43606, USA.
Study Design:
Finite element (FE) method was used to compare the biomechanics of L3-S1 lumbar spine with graded facetectomy before and after placement of Dynesys.
Objective:
To evaluate the biomechanics of Dynesys as a function of graded bilateral facetectomies.
Summary Of Background Data:
Spinal fusion or posterior dynamic stabilization systems are used to restore stability after facetectomies.
Methods:
The intact FE spine was modified to simulate decompression at L4-L5 with 50% and 75% and total facetectomy with/without dynamic stabilization with Dynesys. Biomechanics of the implanted level was investigated under different physiological loadings.
Results:
Total facetectomy increased the motion in extension (8.7° vs. 2.7° for intact) and axial rotation (8.4° vs. 2.4° for intact). However the decrease in motion in the Dynesys model ranged from 65% in axial rotation to 80% in flexion for all facetectomies, except in the total facetectomy axial rotation case (motion higher than intact). The center of rotation of dynamic stabilized segment moved inferior/posterior in partial facetectomy and superior/posterior in total facetectomy with respect to the intact and destabilized cases. The Dynesys screws observed peak stresses up to 28% higher than those of a rigid fixation system in certain loadings, such as lateral bending and extension. The critical loosening torque applied to the screws in total facetectomy case was 6 times the partial facetectomy case in axial rotation.
Conclusion:
Partial facetectomy had a minimal effect on range of motion on the Dynesys-implanted segment. However, in the case of total facetectomy the motion increased by almost 40% in flexion and by 200% in axial rotation. The higher stresses applied to the screws in Dynesys in specific loadings may lead to higher risk of screw failure in Dynesys than in a generic rigid fixation construct.
