Feasibility And Efficacy Of C2 Trans-laminar Screws Technique

Objectives. Different atlantoaxial fusion techniques are used for instability of the upper cervical spine. C2 laminar screws are becoming an increasingly used method of fixation, avoiding the fixation through the C2 pedicle and eliminating the risk of vertebral artery injury. In this research, we performed the anatomical and CT measurement of C2 lamina in North Chinese people and established an anatomic detailed finite element model of C1-C2 complex in order to provide the anatomical and radiographic data and to verify the clinical applicability of trans-lamina screws in North Chinese people.Methods. (1) 96 dry samples of cadaveric atlas were measured with vernier calipers and protractors (group A). The parameters included the length of C2 laminar, the height of C2 laminar, the thickness of C2 lamina, the height of spinous process root the length from entry point to lateral border of lateral mass, the length from entry point to lateral border of lamina and the angle between lamina and spinous process(°). Radiographic measurements were then performed on 112 volunteers and all measurements were repeated from these images (group B). Correlation coefficients were calculated to determine the ability of radiographic measurement to accurately determine these measurements. (2) The coordinate data of the vertebras were obtained from the CT scan images of Chinese healthy male adult volunteers' cervical spine, VTK(Visualization Toolkit) software was used to preprocess and establish the geometry model of the C1-C2 cervical spine. The geometry model was meshed by ABAQUS software. Some material parameters were defined from other available material parameters using proportion and function scale method. The changes of theoretical stress in different position of atlas were analyzed when the skull was in neutral position, flexion/extension, right and left lateral bending, axial rotation and anterior-posterior (AP) translation.Results. (1) The data of males were significant higher than those of female (P<0.05) except angle A (P>0.05) according to our anatomic and radiographic measurement. The thickness in 45 percent specimens is less than 6mm. The length of lamina in all specimens is less than 2.5cm, while length from entry point to rim of lamina is more than 3cm only in 5 percent specimens. The length from entry point to the lateral rim of lateral mass range from 2.5-4cm, however only 5% percent is longer than 4cm. There was no significant difference in the data of bilateral lamina (P>0.05).There was no significant difference of age, height in group A and B. The data of group B have no significant statistic difference with those of group A. (2) The model of C1-C2 complex consists of 130982 nodes and 83776 elements, that of the rod-cantilever construction is composed of 52381 nodes and 32306 elements. The data indicates that C1 lateral mass-C2 intralaminar screw-rod fixation significantly reduced the C1-C2 motion in flexion/extension, lateral bending and axial rotation compared to the intact cases. The results showed that there was concentration of stress at the upper bone-screw interface and the inferior transition area between rod and screw in extension. Stress concentration in flexion positions occurred in both the upper rod-screw transition area and bone-screw interface. In rotation positions the inferior conjunction of screw-rod has stress convergence, the stress of the screw system is well-distributed.Conclusion. (1) This result demonstrates that the radiographic measurement method we used is simple, accurate and reliable. The preoperative radiographic evaluation is very important to determine the suitable size of screws. The appropriate diameter of screws is 3mm to 3.5mm in Chinese people. It is safe to use less than 30mm longer screws in Chinese people. (2) The finite element model of the atlantoaxial spine realistically simulates the atlantoaxial motion, which can simulate the natural condition and facilitate the further biomechanical research. (3) This novel fixation method could be used as an option for atlantoaxial instability of posterior approach techniques.