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

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.

Biomechanical Efficacy of Various Anterior Spinal Fixation in Treatment of Thoraco-lumbar Spine Fracture

PURPOSE: To evaluate the biomechanical results according to various anterior spinal fixation methodology in the treatment of thoracolumbar spine fracture. MATERIALS AND METHODS: The comparative analysis of fixation method was evaluated by three dimensional finite element model using the 1 mm reconstruction image of CT. Authors evaluated the flexion, extension, lateral bending, torsional stresses with 12 fixation methods for the compression and burst fracture. RESULTS: In biomechanical analysis, stiffness of body-fixation device was more stable in two-rod system in compression fracture and was stable in one-rod, two-rod system in burst fracture, but two-rod system was showed over-increase of stiffness. CONCLUSION: Authors recommend the usage of two-rod system in anterior fixation only and anterior one-rod system in anterior-posterior fixation.

Finite Element Model of A-P Instrimentation in Thoracolumbar Burst Fracture / 대한척추외과학회지

STUDY DESIGN: Finite element models of the thoracolumbar spine with various techniques used in spinal fractures were developed to investigate the effects of fixation techniques on spinal stiffness. OBJECTIVES: To develop finite element models of the thoracolumbar spine with various fixation techniques to compare their spinal stiffness characteristics. SUMMARY OF LITERATURE REVIEW: Various anterior and posterior instrumentation options have been applied to stabilize unstable burst fractures of the thoracolumbar spines. The biomechanical effects of different instrumentation options on spinal stability are still unknown. MATERIALS AND METHODS: The 3-D finite element model of the human thoracolumbar spine (T12-L2) was reconstructed from CT images. Various anterior and posterior instrumentation techniques, 1-rod and 2-rod anterior fixations, anterior fixations with posterior fixation, and posterior fixation only, were virtually performed in the developed model with a long cage after corpectomy. Five loading cases, axial compression, flexion, extension, lateral bending, and torsion, were applied up to 1000 N and 10 Nm, respectively. The axial displacement and the rotations of T12 with respect to L2 were measured to analyze the stiffness of the spinal segments. RESULTS: The posterior fixation technique increased the stiffness of the spine the most. The addition of an anterior rod from 1 to 2 increased the stiffness significantly without posterior fixation, but little effect was found with posterior fixation. Among all fixation techniques, the inter-segmental stiffnesses were similar to those of the intact model in torsion cases. In the other loading cases, the inter-segmental stiffnesses were much greater than those of the intact models. CONCLUSIONS: Finite element models of the thoracolumbar spine were developed with various fixation methods. The intact models were validated with in-vitro experimental tests. The posterior fixation technique had a more significant effect on spine stability than did anterior fixation. And anteroposterior fixation techniques provided increased spinal stiffness