Finite Element Analysis Of The Upper Cervical Spine Injury

Background:With the development of our society and the increasing modern traffic and transportation, modern architecture and compete sports,patients with upper cervical spine injury increased year by year. Rapid development of the CT three-dimensional reconstruction technique and widespread application on clinic, more and more upper cervical injury patterns were found not only complex but also very variable. Traumatic mechanisms of the upper cervical spine are still unclear and heated discussed at present, many authors propose the possible injury mechanisms, however, identify biomechanical proofs are still not available until now. Distinctly understand the traumatic mechanisms of the upper cervical spine could greatly help and guide the clinician exactly diagnosis upper cervical injuries and determine the appropriated treatment plan. Thus, it is great necessary to study the traumatic mechanisms of the upper cervical spine.In the traditional spinal biomechanical research, cadaveric model, animal model and physical model were utilized.Biological variability,difficulty, and costs associated with numerous tests tend to be the limiting factors of the human cadaveric model.Animal models are much easier to be harvested. But their structural functions differ from humnans, which are limited in understanding the traumatic mechanisms. Physical models, because of their lack of biofidelity in terms of geometric and structural properties, are limited in the issues and questions that they can address. Three-dimensional finite element method, an advanced stress numerical analysis method, is utilized in human spinal biomechanical research at present. Compared to the traditional biomechanical research methods, they have great advantages: they can provide numerous samples according to the researchers'demand; they are also repeatable and can vary any parameter and quantify the effects secondary to the change of that particular variable on the final outcome in the parametric studies. They can depict the stress distribution in the skeletons not only in quantity but also in quantity when skeletons received various violence, which is extremely difficult achieved by the traditional biomechanical research methods. From the view of clinical application, finite element method is an important and a great valuable supplementary for human cadaveric biomechanical experimental research.Objective:1. To reconstruct and visualize the three-dimensional digital model of the upper cervical spinal motion segment (C0~3) based on continuous lamellar spiral CT images of upper cervical spine segments.2. To construct detail axis 3D finite element model and to explore the injury mechanism and types of fracture under different boundary conditions.3. To explore the influence of the ossification degree of the subdental synchondrosis to the fracture types of the dens.Methods:1. A 32-year-old, weighted 65kg, height 169.5cm, healthy male volunteer was chosen according to the average Chinese people stander. Before his cervical vertebrae CT data obtained, appropriated radiographic studies were performed, including stander anterior-posterior, open mouth, bilateral and bioblique position radiographs with the head in the neutral position, to exclude the skeletal pathologies of the cervical spine. From external auditory foramen to the first thoracic vertebra segment, 232 dicom format CT images with a slice interval of 0.699mm,pixel width of 0.355mm and a 512×512 voxel slice density were obtained using GE Light Speed VCT scanner (GE Medical Systems, America. Scan conditions: 120 kV, 297.75 mAs. Slice interval: 0.699mm). Cervical CT data was imported into MIMICS 10.01 software (Materialise, Belgium), using image segmentation edit tools and setting the segmentation CT threshold at 435~3074 HU, MIMICS software automatically generated the mask of the bones. Base on the mask, using the edit tools to edit the mask, fill the holes and remove the uncorrelated noises of the mask, then utilizing the region growing tool automatically generate each individual three-dimension bony structure of the upper cervical spine segment. Adjust the contrast of the images, according to the different contrast between the soft tissue and bony structures in CT images, utilize the edit tool of the software and anatomy knowledge to generate the disc between C2/3, facet joint articular cartilage of C1-3, articular cartilage opereulums of the median and lateral atlantoaxial joint. Ligamentum transversum atlantis, anterior longitudinal ligament, posterior longitudinal ligament, flaval ligament, interspinous ligament and supraspinous ligament were reconstructed with the same method.2. The cortical and the trabecular bone three-dimensional finite element models of the axis were very well constructed from the cervical spine CT data using MIMICS 10.01 software (Materialise, Belgium), and then imported into Ansys 10.0 software (ANSYS, Inc. Pennsylvania, America) for remenshing, material assignment and model analysis to simulate the axis injury situation. Theoretical stress distribution of the axis was calculated by applying anterior-posterior direction shearing force on the anterior articular surface of dens under three boundary conditions to explore the possible fracture types under the corresponding conditions.3. Detailed three-dimensional of the axis finite element model was reconstructed based on a normal adult volunteer's upper cervical spine continuous lamellar spiral CT images. By altering the trabecular bone's Yong's module of the cartilage matrix ossification region, anterior-posterior direction shearing force was applied on the anterior articular surface of dens under tow boundary conditions and theoretical stress distribution of the axis was calculated to explore the stress distributions and predict the possible fracture types.Results:1. Successfully reconstructed the three-dimensional digital model of the upper cervical spinal motion segment. The model included C0-3 bony structures and distinguished the cortical and cancellous bone, cartilaginous opereulums from C1 to C3, disc of C2/3 and 6 types of ligaments. The three-dimensional mode1 was built with good fidelity and geometric similarity. The initial finite analysis results of the mode1 were matched to the results of the biomechanical studies and clinical findings. This model could be used for further finite element analysis of the biomechanical studies of the upper cervical spine.2. When the axis received anterior-posterior shearing force ,①bilateral inferior articular facets nodes of the axis were completely constrainted to simulate the axis in neutral position pretrauma, high stress concentrated at bilateral pars interarticularis ;②one inferior articular facet nodes were completely constrainted to simulate the axis lateral bending pretramu, high stress was mainly distributed at one side of the pars interarticularis and the junction site of the inferior facet and laminae of the axis;③completely constrainted the inferior aspect nodes of the axis as the previous biomechanical studies, high stress concentration was located at the junction of the dens with the vertebral body.3. Under the two boundary conditions (bilateral inferior articular facets nodes of the axis were completely constrainted to simulate the axis in neutral position pretrauma; completely constrainted the inferior aspect nodes of the axis as the previous biomechanical studies), finite element analysis results demonstrated that the ossification degree of the subdental synchondrosis did not obviously influence the high stress distributions nor the maxium von Mises equivalent stress.Conclusions:1. Mimics software provided a more quick and accurate method to establish three-dimensional digital model of the upper cervical spinal motion, and facilitated further research the behaviors of the upper cervical spine.2.①Lateral bending of the cervical spine pretrauma may possibly be an very important factor to induce asymmetry Hangman's fracture;②Besides the direction of the violence, different restrictions of the cadaveric sample might directly impact the reproduction of the axis fracture types in the axis fracture biomechanical studies.3. The degree ossification of the subdental synchondrosis did not impact the fracture type of the dens.