Clinical Study And Three-dimensional Finite Element Analysis On The Mechanism Of Cervical Hyperextension Injury

Cervical spine hyperextension injury caused by rapidly hyperextensive force is one of the most common type of the cervical spine injuries.The damage structures of cervical spine hyperextension injury include spinal cord and disco-ligamentous complex（DLC）,as well as paraspinal structures such as blood vessels and soft tissue.Among them,DLC injury and spinal cord injury are most deserved attention.They are directly related to the determination of the spinal cord decompression and the spinal stability reconstruction segments.Meanwhile patients with hyperextension injuries often have varying degrees of degenerative changes in cervical spine.DLC injury level is not completely consistent with the level of spinal cord injury,which is related to the degenerative state of cervical spine.At present,there is a lack of research on the mechanism of DLC injury in hyperextension injury under the degenerative state of cervical spine.Finite element（FE）analysis is a computational technology,which uses the method of mathematical approximation to simulate the real physical system.It uses simple and interactive elements to approach the infinite unknown real system through a finite number of unknown quantities.It is used to simulate and predict how materials will react when subject to different forces.It offers the opportunity to replace experimental models,such as cadaveric and animal testing,as a powerful,noninvasive alternative.As technology continues to improve,so too does the applicability of FE analysis and modeling.This technology is more and more widely applicable in the field of orthopedics.Objective:The objective of this study was to establish the clinical classification of cervical hyperextension injury based on the magnetic resonance imaging characteristics.The data of the clinical efficacy of patients with cervical hyperextension injury treated according to the image classification were analyzed retrospectively and a cervical model was established by using the three-dimensional finite element method to simulate the stress response mechanism of DLC in hyperextension injury under different cervical degenerative states to verify the effectiveness of the established clinical classification of cervical hyperextension injury.Methods:Part I:imaging and clinical study of cervical hyperextension injuryThe data of patients hospitalized for cervical hyperextension injury in Shanghai Tongji Hospital Hospital from April 2009 to December 2016 were retrospectively analyzed,including their gender,age,injury factors and neurological function.The relationship among degenerative spinal stenosis,spinal cord injury segments and DLC injury segments in cervical hyperextension injury was summarized.A classification of cervical hyperextension injury was established based on imaging.The differences in age,injury factors and neurological function and life function among patients with different types were analyzed and compared,as well as the relationship with the degree of degenerative spinal stenosis.Secondly,the data of patients treated according to the established imaging classification and surgical indications of cervical hyperextension injury were analyzed retrospectively.Before treatment and at the last follow-up,the neurological function of patients between without degeneration group and degeneration group,between conservative treatment group and surgical treatment group was evaluated by comparing the American Spinal Cord Injury Association Injury Scale（ASIA score）and ASIA Impairment Scale Grade（AIS Grade）,separately.The Japanese Orthopaedic Association score（JOA score）before treatment and at the last follow-up were recorded and compared.The JOA improvement rate was calculated to evaluate the patients’life function and verify the effectiveness of the classification.（the score of last follow-up-the score before treatment）/（17-the score before treatment）×100%,when the improvement rate is 100%,it was cured,>60%was markedly effective,25%～60%was effective,and<25%was ineffective.Part II:study on the effect of cervical degenerative states on the injury mechanism of DLC in cervical hyperextension injury by finite element analysisThe CT scanning image data of the cervical vertebra of a volunteer without cervical degenerative changes was obtained to reconstruct the three-dimensional model of the cervical vertebra through the finite element software.According to the literature,the material properties,constraints and contact effects were set and then the torque was applied to the model.The range of motion（ROM）of the model is compared with the literature data to verify the effectiveness of the model.Then,the stress distribution and ROM of the model were tested under different degenerative states.（1）The material properties of nucleus pulposus and annulus fibrosus of intervertebral disc were modified according to the literature,and the height of intervertebral disc was reduced.The stress distribution and ROM of the model under the condition of intervertebral disc degeneration in C_5/6 segment,C_6/7 segment and C_5/6 with C_6/7 segments were tested respectively;（2）The morphological parameters of osteophyte were obtained by retrospective measurement of clinical data,and then the model with osteophyte was constructed to test the stress distribution and ROM of the model under the formation of osteophyte in C_5/6 segment,C_6/7 segment and C_5/6 with C_6/7 segments respectively.（3）The morphological parameters of ossified posterior longitudinal ligament were obtained by retrospective measurement of clinical data,and then the model with ossified posterior longitudinal ligament was constructed to test the stress distribution and ROM of the model under the formation of ossified posterior longitudinal ligament in C_4-C₅ segment,C_5-C₆ segment and C_4-C₆segment respectively.Results:Part I:imaging and clinical study of cervical hyperextension injuryA total of 88 patients with cervical hyperextension injury were included in this study,with an average age of 52.9±14.8 years and an average follow-up of 1-9.1years[4（1,6）years],mainly in the age group of 40-69 years.The main injury factors were traffic accidents,accounting for 47.7%,followed by flat falls,accounting for 22.7%.Among them,25 patients（29 segments）were accompanied by DLC injury.The coincidence rate between the central area of abnormal spinal cord signal and the injured segment of DLC was only 40%（10/25 cases）.According to the relationship among degenerative state,spinal cord signal changes and DLC injured segments,a clinical classification of cervical hyperextension injury based on magnetic resonance imaging was established:type A without degenerative spinal canal stenosis.Type B,C and D are accompanied by degenerative spinal stenosis.Type B is simple disc herniation,type C is disc herniation and osteophyte,and type D is ossification of posterior longitudinal ligament.Each type was subdivided into three subtypes.The average age,the incidence of abnormal spinal cord signal and the incidence of DLC injury in degeneration group（type B,C and D）were significant higher than those in without degeneration group（type A）（P<0.05）.The without degeneration group was mainly high-energy injury,and the proportion of low-energy injury increased in degeneration group.The ASIA score,AIS grade and JOA score in the without degeneration group were better than those in the degeneration group（P<0.05）before treatment.The ASIA score and JOA score were negatively correlated with the degree of spinal canal stenosis and the number of stenosis segments.Of the 88 patients,66（type A3,B,C and D）were treated with surgery and 22（A1 and A2）were treated conservatively.At the last follow-up,ASIA score,AIS score and JOA score were significantly improved in conservative treatment group and surgical treatment group,without degeneration group and degeneration group.Part II:study on the effect of cervical degenerative states on the injury mechanism of DLC in cervical hyperextension injury by finite element analysis（1）The established three-dimensional finite element model of normal cervical spine contained 160849 nodes and 497859 elements.After the torque was applied,the ROM between each vertebral body was measured and compared with the data in the literature.The data were in good consistency,which verifies that the model was accurate and reliable.（2）Three finite element conditions of simultaneous disc degeneration in C_5/6 segment,C_6/7 segment and C_5/6 with C_6/7 segments were established respectively.The ROM of degenerative segments were lower than that of the normal model.In the simultaneous disc degeneration models of C_5/6 segment and C_5/6 with C_6/7 segments,the maximum von Mises stresses in front of the vertebral body were distributed in the adjacent intervertebral discs.In the C_6/7 segment degeneration model,the maximum von Mises stress in front of the vertebral body was distributed in the C_4/5 intervertebral disc.（3）Three finite element conditions of simultaneous osteophyte in C_5/6 segment,C_6/7 segment and C_5/6 with C_6/7 segments were established respectively.The ROM of segments with osteophytes were lower than that of the normal model.In the simultaneous osteophyte models of C_5/6 segment and C_5/6 with C_6/7 segments,the maximum von Mises stresses in front of the vertebral body were distributed in the adjacent intervertebral discs.In the C_6/7 segment osteophyte model,the maximum von Mises stress in front of the vertebral body was distributed in the C_4/5 intervertebral disc.（4）Three finite element conditions of simultaneous ossified posterior longitudinal ligament in C_4-C₅ segment,C_5-C₆segment and C_4-C₆segment were established respectively.The ROM of segments with ossified posterior longitudinal ligament were lower than that of the normal model.The maximum von Mises stresses in front of the vertebral body were distributed in the adjacent intervertebral discs.Conclusions:（1）The established clinical classification of cervical hyperextension injury based on magnetic resonance imaging had certain clinical guiding significance for the diagnosis and treatment of cervical hyperextension injury.Patients with type A1 and A2 could be treated conservatively,while patients with type A3,B,C and D could be operated early.Satisfactory clinical effects were obtained.Satisfactory clinical effects could be obtained.（2）In the degenerative state,the spinal cord injury segments were not completely consistent with the DLC injury segments.It was necessary to comprehensively consider the classification and explore the suspicious segments during operation to avoid imaging negative DLC injury.（3）The three-dimensional finite element model of cervical spine was reverse reconstructed by using CT scanning data in this study.It was accurate and detailed and the material properties were true and reliable.It is verified that it can be applied to the biomechanical study of cervical spine.（4）After intervertebral disc degeneration,the nucleus pulposus of intervertebral disc dehydrated,the height of intervertebral space decreased,and the biomechanical stability of the degenerative segment was changed.This segment or its adjacent segments became the weak point of DLC and were easy to be damaged in cervical hyperextension injury.（5）When the osteophyte and the ossification of the posterior longitudinal ligament form a bone bridge,the unstable segments of the cervical spine could obtain stability again.During the extension process,the adjacent segments were easy to become the concentration point of stress,resulting in injury.At the same time,it was necessary to pay attention to the possibility of DLC injury at the vertex position(C_4/5)during cervical vertebra extension.（6）The next work plan is to accumulate more cases of cervical hyperextension injury to increase the sample size and further verify the accuracy and practicability of clinical classification of cervical hyperextension injury based on magnetic resonance imaging.At the same time,we will optimize the finite element model of cervical spine,establish the spinal cord model,and integrate the two models to provide a detailed and reliable cervical spine and spinal cord model for the study of cervical biomechanics.