Design And Feasibility Study For Posterior Atlantoaxial Restricted Non-fusion Fixation System

BackgroundAtlantoaxial dislocation (AAD) is a common disease in the cervical spine, and mostly needs a surgical intervention in form of reduction and fusion by anterior and/or posterior approach. However, such fusion results in considerable loss of head and neck rotation and might adversely impact the quality of life of the patient. To address this issue, a new method is explored to not only stabilize the atlantoaxial complex but also restore the C1–C2 axial rotation and lateral bending. The successful usage of total disc arthroplasty (TDA) in subaxial cervical spine and lumbar spine and other dynamic fixations in lumbar spine could be a good reference. Some designed an artificial atlanto-odontoid joint that can be implanted by transoral approach, which was a good exploration. Indeed, head and neck flexion, extension, and lateral bending occur primarily as a result of the combined action. This artificial joint replacement has some limitation, such as small performance field, technical demanding and high potential complications. The posterior approach is a most common way for spinal surgeon. Especially for those who need a C1 posterior arch removal. Also, pedicle screws for both C1 and C2 make its possibility for posterior atlantoaxial dynamic fixation because of their excellent biomechanics.ObjectivesThe purpose of this research was to design a novel posterior atlantoaxial restricted non-fusion fixation (PAARNF), with advantages of stabilize the atlantoaxial flexion-extension while preserve the other motion of atlantoaxial joint. The aims of five experiments including design and related study were:1. The aim of imaging measurements of human CT scans was to obtain the parameters related to new PAARNF design.2. To design new type PAARNF that can stabilize the atlantoaxial flexion-extension and preserve the axial rotation and lateral bending motion of atlantoaxial joint.3. To construct three-dimensional (3D) finite element model (FEM) of PAARNF and compare with the biomechanical stability and rang of motion (ROM) of normal upper cervical spine model, AAD model and C1-C2 pedicle screw-rod fixation. The stress distribution of PAARNF was also investigated.4. To give data support for further revision of PAARNF. Materials and Methods1. Fifty CT scans and 3D reconstruction of the cervical spine of patients from shanghai changzheng hospital were used for morphometric study of the atlantoaxial complex. The following parameters were evaluated, including:The height of C1 posterior tubercle, distance between superior margins of C1 posterior tubercle and C2 lamina, distance from superior margin of C1 posterior tubercle to inferior margin of C2 lamina, distance from the entry point of C2 pedicle screw to superior margin of C2 lamina, sagittal height difference between C1 posterior tubercle and C2 spinous process and angle of each side of C2 pedicle screw entry point and C2 lamina superior margin.2. To design and produce a new posterior atlantoaxial restricted non-fusion fixation (PAARNF) according to the parameters of imaging measure.3. To construction the FEM of normal upper cervical spine, CT scans of cervical spine from C0 to C5 of a healthy man of 31 years old were obtained. The Geomagic8.0 software system and Abaqus6.9 software were applied to model the spinal segment and the material properties were deferent according to literatures. At the basis of the normal upper cervical spine FEM, AAD model was constructed by removal of transverse ligament of C1. Then, C1-C2 pedicle screw-rod fixation model and PAARNF model were built base on the AAD model.4. The ROM of C1-2 and the stress distribution of PAARNF were recorded and compare the biomechanical stability and the ROM with normal upper cervical spine model, AAD model and C1-C2 pedicle screw-rod fixation.Results1. The height of C1 posterior tubercle was (10.99±1.56) mm. Distance between superior margins of C1 posterior tubercle and C2 lamina was(18.42±3.18)mm,distance from superior margin of C1 posterior tubercle to inferior margin of C2 lamina was(31.33±3.62)mm, distance from the entry point of C2 pedicle screw to superior margin of C2 lamina was(25.38±2.49)mm. Sagittal height difference between C1 posterior tubercle and C2 spinous process was(13.1±2.45)mm. Angle of each side of C2 pedicle screw entry point and C2 lamina superior margin was 90.4±4.8 degree.2. A new posterior atlantoaxial restricted non-fusion fixation (PAARNF) was designed and was produced according to the parameters of imaging measure. The fixation is compose of an universal titanium alloy connector,four polyaxial screws,an curved rod, and two rods with thread at one side.3. The normal upper cervical spine FEM was built and was validated according to the biomechanics in vitro. The other FEMs were also successful constructed. The PAARNF has little dislodgement in either flexion or extension, while it could move synchronously in lateral bending motion and has a good axial rotation with 33 degrees. Stress concentration mostly at the end of screws of C1 and C2 that were conducted with cortical bone, which could be seen in most screw-rod fixation of spinal surgery. Only a significant concentration at the ventral part of universal titanium alloy connector in flexion.ConclusionA novel posterior atlantoaxial restricted non-fusion fixation, which might fit Chinese adult patients, was designed. This non-fusion fixation could restrict the atlantoaxial joint motion both in flexion and in extension, and could preserve the C1–C2 axial rotation and lateral bending. No significant concentration was found in mostly motions. This PAARNF could be place through a normal posterior approach with a relatively simple performance. It is a feasible and performable design. However, a further revision is needed to perfect the product and more relative research should be carried on the agenda.