Finite Element Analysis of the Biomechanical Effect of Coflex(TM) on the Lumbar Spine

OBJECTIVE: The biomechanical properties of the Coflex(TM) (Paradigm Spine, NY, USA), a device designed to provide dynamic stabilization without lumbar fusion, have not been clearly defined. The purpose of this study was to determine the efficacy and biomechanical effect of Coflex(TM) using finite element model (FEM). METHODS: A 3D geometric model of the L3-L5 was created by integrating computerized tomography (CT) images. Based on the geometric model, a 3D FEM was created and the Coflex(TM) model was incorporated into the base model. Mechanical load dependent on the postural changes and boundary conditions, were imposed to simulate various 3D physiological states. The simulation analysis included stress and strain distributions, intervertebral disc deformation, and the range of motion of the facet joint and lumbar spinous process. RESULTS: Coflex(TM) significantly restrained displacement in extension, lateral bending and compression of joint between the L4-5 as one in the experimental group was observed -1.3% of flexion, -24.5% of extension, -44.5% of lateral bending and -37.2%. The average intradiscal pressure of the L4-5 decreased by 63% and the average facet contract force of the L4-5 decreased markedly by 34% in the experimental group. A load of 120 MPa from extension was observed at the base of spinous process in the experimental group. CONCLUSION: The Coflex(TM) can be safely used for achieving functional dynamic stabilization of the lumbar vertebral column while preserving the intactness of the other components. However, the fatigue fracture of the L4 spinous process should be carefully monitored.

Risk Factors and Surgical Treatment for Symptomatic Adjacent Segment Degeneration after Lumbar Spine Fusion

OBJECTIVE: The present study analyzed the risk factors, prevalence and clinical results following revision surgery for adjacent segment degeneration (ASD) in patients who had undergone lumbar fusion. METHODS: Over an 8-year period, we performed posterior lumbar fusion in 81 patients. Patients were followed a minimum of 2 years (mean 5.5 years). During that time, 9 patients required revision surgery due to ASD development. Four patients underwent autogenous posterolateral arthrodesis and extended transpedicle screw fixation, 4 patients underwent decompressive laminectomy and interspinous device implantation, and 1 patient underwent simple decompression. RESULTS: Of the 9 of patients with clinical ASD, 33.3% (3 of 9) of patients did not have radiographic ASD on plain radiographs. Following revision surgery, the clinical results were excellent or good in 8 patients (88.9%). Age > 50 years at primary surgery was a significant risk factor for ASD development, while number of fusion levels, initial diagnosis and type of fusion were not. CONCLUSION: The incidence of ASD development after lumbar surgery was 11.1% (9 of 81) in this study. Age greater than 50 was the statistically significant risk factor for ASD development. Similar successful clinical outcomes were observed after extended fusion with wide decompression or after interspinous device implantation. Given the latter procedure is less invasive, the findings suggest it may be considered a treatment alternative in selected cases but it needs further study.