J Korean Neurosurg Soc.  2015 Jul;58(1):43-49. 10.3340/jkns.2015.58.1.43.

The Change of Sagittal Alignment of the Lumbar Spine after Dynesys Stabilization and Proposal of a Refinement

• Park WM
• Kim CH
• Kim YH
• Chung CK
• Jahng TA

• KMID: 2067103
• DOI: http://doi.org/10.3340/jkns.2015.58.1.43

Abstract

OBJECTIVE
Dynesys(R) is one of the pedicle-based dynamic lumbar stabilization systems and good clinical outcome has been reported. However, the cylindrical spacer between the heads of the screws undergoes deformation during assembly of the system. The pre-strain probably change the angle of instrumented spine with time and oblique-shaped spacer may reduce the pre-strain. We analyzed patients with single-level stabilization with Dynesys(R) and simulated oblique-shaped spacer with finite element (FE) model analysis.
METHODS
Consecutive 14 patients, who underwent surgery for single-level lumbar spinal stenosis and were followed-up more than 24 months (M : F=6 : 8; age, 58.7+/-8.0 years), were analyzed. Lumbar lordosis and segmental angle at the index level were compared between preoperation and postoperative month 24. The von Mises stresses on the obliquely-cut spacer (5degrees, 10degrees, 15degrees, 20degrees, 25degrees, and 30degrees) were calculated under the compressive force of 400 N and 10 Nm of moment with validated FE model of the L4-5 spinal motion segment with segmental angle of 16degrees.
RESULTS
Lumbar lordosis was not changed, while segmental angle was changed significantly from -8.1+/-7.2degrees to -5.9+/-6.7degrees (p<0.01) at postoperative month 24. The maximum von Mises stresses were markedly decreased with increased angle of the spacer up to 20degrees. The stress on the spacer was uneven with cylindrical spacer but it became even with the 15degrees oblique spacer.
CONCLUSION
The decreased segmental lordosis may be partially related to the pre-strain of Dynesys. Further clinical and biomechanical studies are required for relevant use of the system.

Keyword

Degenerative disease; Dynamic; Finite element analysis; Lumbar; Spine; Stabilization

MeSH Terms

Animals
Finite Element Analysis
Head
Humans
Lordosis
Spinal Stenosis
Spine*

Figure

• 

  Fig. 1 Three-dimensional finite element model of the Dynesys spinal system implanted L4-5 spinal motion segment with three loading conditions. For analysis, a compressive force of 400 N along the direction of the L4 vertebra center to the L5 vertebra center (vertical thick arrow) is applied onto the L4-5 motion segment as body weight in a standing posture and 10 Nm of moment is applied onto the superior plane of the L4 vertebra in the flexion and extension positions, and for lateral bending (left) and torsion (counterclockwise), respectively. Note deformation of the spacer caused by the implantation of a cylindrical spacer between the screws with a 16° angle.

• 

  Fig. 2 The method of sloping cylindrical spacer. The inferior border of the spacer is cut with angles (a) of 0°, 5°, 10°, 15°, 20°, 25°, and 30°.

• 

  Fig. 3 Decreased segmental angle due to reaction force. A : The preoperative Cobb's angle between L4-5 was -6.8°. B : At postoperative month 1, the angle was -12.9°. C : At postoperative month 24, segmental angle was -4.6°. Note parallelized cephalic and caudal pedicle screw during follow-up.

• 

  Fig. 4 The maximum von Mises stresses on the spacer. The distribution of von Mises stresses on the spacer was depicted from A-C. Darker color represents higher stress in the figure. A : Initially, the von Mises stresses are concentrated at the dorsal caudal edge (arrow) of the spacer with a 0° cutting angle. B : The spacer has a pretty even distribution of stresses with the cutting angle of 15°. C : The stresses are converged on the ventral side (arrow) with a cutting angle of 30°.

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