| Operation is often necessary to atlantoaxial instability patients for it is easy to cause the compression and irritation of spinal cord, vertebral artery and nerve root. The posterior atlantoaxial fusion is a good choice to treat C1-C2 instability, but many cases, which complicated with destructively posterior arch of atlas or irreducible dislocation in posterior approach, have to treated by anterior technique. Anterolateral retropharyngeal approach offers several advantages. The surgical approach is far less traumatic and deceases risk of vertebral artery. It also permits release of locked C1-C2 joints and decortication of the articular cartilages at the atlantoaxial joints to enable fusion. Recently, endoscopic surgery,which has been applied to the craniovertebral junction, allows for more application of anterolateral retropharyngeal approach. Some patients with a diagnosis of irreducible atlantoaxial dislocation (IAAD) who were treated by the technique of endoscopically assisted anterolateral retropharyngeal release have a satisfied clinical outcomes. A novel device for anterior atlantoaxial fixation called anterior atlantoaxial transarticular locking plate system is presented by CAI Xian-hua et al. The aim of this study is to evaluate the biomechanical properties of anterior atlantoaxial fixation with finite element method and provide theoretical guidance to system improvements. Part One Development and validation of a finite element model of the atlantoaxial instabilityObjective:The finite element model of the occipito-atlantoaxial complex was developed and validated in physiologic and instable condition.Method:There was a male healthy volunteer with age of 21 years old. Three-dimensional finite element model of upper cervical spine was established by MIMICS 13.0, Freeform Plus and ANSYS 9.0. By deleting the unit of alar ligament, the model for atlantoaxial instability was developed. The weight of the skull was stimulated by applying vertical load of 50 N on the facet, and the inferior surface of the C2 vertebral body was fully constrained. Pure moment loading of 1.5Nm was applied incrementally to stimulate the various movements of the head and cervical spine under flexion, extension, axial rotation and lateral bending configurations. The range of motion (ROM) under different loading configurations were analyzed and compared against the in vitro experimental data.Results:1.The finite element model of the occipito-atlantoaxial complex which composed of 86050 node points and 55371 elments was geometrically. It contains tansverse ligament, anterior longitudinal ligament, posterior longitudinal ligament, supraspinous ligament, inter spinous ligament, capsular ligament, alar ligaments, nuchal ligament,and apical ligament.2.1n the physiologic occipito-atlantoaxial complex model, the ROM of atlantoaxial joint was 11.7°(flexion),9.5°(extension),38.7°(lateral bending),4.1°(axial rotation) and the ROM of occipitoatlantal joint was 3.1°(flexion),20.5°(extension),7.6°(lateral bending),5.1°(axial rotation).3. In the Atlantoaxial Instability model, the ROM of atlantoaxial joint was 23.6°(flexion),12.7°(extension),49.1°(lateral bending),6.2°(axial rotation) and the ROM of occipitoatlantal joint was 3.3°(flexion),21.2°(extension),9.1°(lateral bending),5.8°(axial rotation).Conclusion:The finite element model of the occipito-atlantoaxial complex was geometrically. Compared with ROM data in vitro model from Panjabi and Ding, the finite elment model can stimulate the natural and instable condition of upper cervical spine and facilitate the further biomechanical research.Part Two Development and analysis of anterior atlantoaxial transarticular locking plate system with finite element methodObjective:To develop the finite element model of anterior atlantoaxial transarticular locking plate system and to evaluate the biomechanical properties with finite element method.Method:Three-dimensional finite element model of the anterior atlantoaxial transarticular locking plate system was established by Freeform Plus and ANSYS 9.0. The weight of the skull was stimulated by applying vertical load of 50 N on the Cl facet, and the inferior surface of the C2 vertebral body was fully constrained. Pure moment loading of 1.5Nm was applied incrementally to stimulate the various movements of the head and cervical spine under flexion, extension, axial rotation and lateral bending configurations. The range of motion (ROM) and von mises stress under different loading configurations were analyzed.Results:1. The finite element model of the anterior atlantoaxial transarticular locking plate system which composed of 10622 node points and 5440 elments was geometrically resemble.2. The displacement of anterior atlantoaxial transarticular locking plate system was small in different forces and the maximum value of transarticular screws was under extension. The number under flexion, extension, axial rotation and lateral bending configurations were 7.411×10-5m,11.89×10-5m,3.227×10-5m,2.501×10-5m.3. Most of stress concentrated on the nail holes and transarticular area. The number under flexion, extension, axial rotation and lateral bending configurations were 5.213×107Pa,1.029×107Pa,1.164×107Pa,3.213×107Pa。Conclusion:The finite element model of the anterior atlantoaxial transarticular locking plate system was geometrically resemble. The displacement were small and most of stress concentrated on the nail holes and transarticular area in different forces.Part Three Contrast analysis between anterior atlantoaxial transarticular locking plate system and anterior transarticular screws with finite element methodObjective:To develop the finite element model of anterior transarticular screws and compare the biomechanical properties of the two different fixation.Method:By deleting the unit of plate and stimulating the bone units, the model of anterior transarticular screws fixation for atlantoaxial instability was established. The weight of the skull was stimulated by applying vertical load of 50 N on the C1 facet, and the inferior surface of the C2 vertebral body was fully constrained. Pure moment loading of 1.5Nm was applied incrementally to stimulate the various movements of the head and cervical spine under flexion, extension, axial rotation and lateral bending configurations. Two kind of fixation devices (anterior transarticular screws and anterior atlantoaxial transarticular locking plate system) were compared for the displacement, the maximum von mises stress of the transarticular screws and maximum von mises between the screws and vertebra.Results:1.The finite element model of the anterior transarticular screws which composed of 3649 node points and 1904 elments was geometrically resemble.2. The displacement of anterior transarticular screws was small in different forces and the maximum value was under extension. The number under flexion, extension, axial rotation and lateral bending configurations were 9.232×10-5m,13.86×10-5m,3.756×10-5m,3.421×10-5m. The displacement of transarticular screws in anterior transarticular locking plate instrument decreased to a level of about 24.57%,16.57%,16.39% and 36.79%.3. Most of stress concentrated on the transarticular area in anterior transarticular screws. The number under flexion, extension, axial rotation and lateral bending configurations were 5.441×107Pa,1.059×107Pa,1.292×107Pa,3.641×107Pa. The stress of screws in anterior transarticular locking plate instrument decreased a little.Conclusion:The biomechanical properties of anterior atlantoaxial transarticular locking plate system is superior than the anterior transarticular screws. |
PDF Full Text Request