A Finite Element Investigation And A Clinical Analysis Of Four Posterior C1-2 Fixation Techniques

Objective: To establish and validate a three-dimensional finite element model of cervical vertebra.Base on this model,establish a three-dimensional finite element model of four types of atlantoaxial complex fracture.Stimulate FEM of Jefferson type ? odontoid fracture.A compueter-aided software according to a fixed system specification was used to build the FEM of different fixation techniques.Further,by evaluating the clinical outcome of 17 cases adopting the C1-2fixation techniques in long term follow-up,we aim at verifying the safety and efficacy in practical work.Materials and Methods: Three-dimension finite element analysis,the cervical spine were determined from CT images of a healthy volunteer.Finite element modeling softwares were used to develop a geometric model of cervical spine.Based on the finite element model of a normal cervical spine,establish a three-dimensional finite element model of four types of atlantoaxial complex fracture.Further,four different posterior fixation techniques including occipital-cervical fusion technique,C1-2 transarticular screw technique,C1-2 pedicle screw technique and C1-2 pedicle screw technique+ odontoid screw technique.Flexion,extension,lateral bending and rotation were imposed on the intact,fracture and four fixation models respectively.The Von Mises stress distribution and range of motion(ROM)of each segment in different models was compared.17 patients with efferson type ? odontoid fracture who adopted variable posterior fixation techniques were followed up for mean 13 months.We performed X-rays and 3-D CT to evaluate the postoperative fusion and JOA score system is used to evaluate the nerve improvement.Results:The model of cervical spine was built clearly and distinctly.,with good geometric similarity.The results of the normal model were matched to the results of the vitro experiment of biomechanics by Panjabi et al.The finite element model of four types of atlantoaxial complex fractures was similar with clinical case.The range of motion was larger in abnormal model than in the normal model.ROM of C0-C1 and C1-C2 in C1 fracture combined ? odontoid fracture model and C1 fracture combined ? odontoid fracture model increase significantly in anterior flexion,posterior extension,lateral flexion and rotation.ROM of C0-C1 in C1 fracture combined hangman fracture model reduce in anterior flexion,posterior extension and increase in lateral flexion,rotation.ROM of C1-C2 in C1 fracture combined hangman fracture model increase significantly in anterior flexion,posterior extension,lateral flexion and rotation.ROM of C0-C1 in C1 fracture combined stability C2 fracture mode increase in anterior flexion,posterior extension and rotation,reduce in lateral flexion.When fall with neutral position,it cause Jefferson fracture and hangman fracture or ? odontoid fracture.When fall with posterior extension position,it cause posterior arch of C1 fracture and hangman fracture.When fall with anterior flexion position,it cause C1 fracture and ? odontoid fracture or stable C2 fracture.When fall with rotation position,it cause lateral mass,anterior arch and posterior arch of C1 fracture and hangman fracture or ? odontoid fracture.ROM of C0-3 was lower in occipitocervical fusion model than the intact model.ROM of C0-1 was largerl and ROM of C1-2 was lower in Margel fixation,pedicle screw fixation and odontoid screw add pedicle screw fixation model than the intact model.In the mean 13 month's follow-up,all the patients get neurological improvement to some extent.JOA score increased from preoperative mean 6.5 to 14.9 at postoperative 3 months.The improve rate was 81.7%.No interal fixation realted complications such as screw broken,second dislocation was detected.The imaging examination showed no C1-C2 instability.All the patients achieved bony fusion in 3 months.Conclusions:Finite element modeling are feasible,which is helpful to determine biomechanical research and biomechanical analysis of internal fixation.The four fixation techniques are safe,effective and can be applied to selected cases according individual conditions.The four different fixation techniques can limit ROM of C1-2 in all the directions.The other three fixation techniques can enlarge the ROM of C0-C1 except occipitocervical fusion model and may be cause cervical degeneration.Different fixation techniques have different features.We should select rational method according individual conditions.