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.

The Clinical and Radiological Effect of Abnormal Axis after Cervical Arthroplasty

OBJECTIVE: The clinical outcomes according to the radiological results after cervical total disc replacement (TDR) are not well established. Here, the authors reviewed the clinical results according to the asymmetry in radiographs. METHODS: This retrospective analysis included patients after TDR (Mobi-C(R) disc) with at least 12 months follow up, and the clinical and radiological data were obtained preoperatively and postoperatively for 12 months. Clinical outcome measures numerical rating scale (NRS) score for neck pain, visual analog scale (VAS) for arm pain, and the Oswestry disability index (ODI) value. The asymmetries of TDRs were evaluated on the anterior-posterior (AP) and the lateral radiographs, and the radiographic adjacent segment degenerations were evaluated for 12 months. RESULTS: A total of 24 patients (one level cervical TDR; 10 male and 14 female; aged 41.50+/-8.35 years) were included in this study. The clinical results including NRS for neck pain, VAS for arm pain, and ODIs were similar between the normal and asymmetrized TDRs in AP and lateral radiographs. The radiographic adjacent segment degenerations were significantly increased in deviated TDRs (AP>10 mm asymmetry and lateral>10 mm asymmetry). CONCLUSION: Asymmetrical location of TDR is not related to the clinical outcomes, but related to the risk of radiographic adjacent disc segment degeneration.

The Biomechanical Effect of Stem Design and Methods of Fixation in Revision Total Knee Arthroplasty / 대한정형외과학회잡지

PURPOSE: This study examined the effect of the stem design and method of fixation on biomechanical features around a stem tip in revision total knee arthroplasty using finite analysis. MATERIALS AND METHODS: A 3D Finite element model was reconstructed for a CT scan of the normal tibia from a 26 year old male and the CAD model of total knee arthroplasty revision was developed. The design change in the stem such as the length, diameter, slot, press fit and coefficient of friction was performed. The contact pressure, von-Meises stress around the stem and the micromotion were evaluated. RESULTS: A longer length and larger diameter press fit stem significantly increased the contact pressure and stress at the end of stem. The distal slot reduces the contact pressure and stress at the end of stem. Less displacement between the tibial component and bone was noted in the increased coefficient of friction. CONCLUSION: A stem with shorter length sufficient to engage proximal diaphysis, a closer diameter of the proximal canal and a minimal press fit can be used to reduce the contact pressure and stress if the patient's surgical anatomy such as bone loss and quality is tolerable in revision total knee arthroplasty.

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