OBJECTIVETo study the biomechanical change of the craniovertebral junction in conditions of atlas assimilation.

METHODSMimics software was used to process CT data of the craniovertebral junction in a health adult to obtain the three-dimensional reconstruction and the cloudy points of C1, C2 and part of the occipital bone. Then the cloudy points were imported into the Abaqus 6. 8 software to establish the occipito-atlantoaxial finite element model in normal structure. According to the established model in normal structure, the model in conditions of atlas assimilation was set by changing the model parameters. Both models of normal structure and atlas assimilation were loaded with 1. 5 N . m static moment to simulate four motions of flexion, extension, lateral bending and axial rotation respectively. The movement characteristics,joint stress force and ligament deformation was analyzed.

RESULTSUnder 1. 5 N . m moment, in model of atlas assimilation the C1-C2 range of movement decreased from 13. 55° to 11.88° in flexion,increased from 13. 22° to 15. 24° in extension and from 4. 05° to 4. 23° in lateral bending and remained unchanged in axial rotation when compared with the normal model. In flexion movement, the contact force of the atlanto-dental joint increased from 1. 59 MPa to 3. 28 MPa and the deflection of apical ligament, tectorial membrane and alar ligament increased 129. 1%, 157. 6% and 75. 1% respectively when compared with the normal model.

CONCLUSIONSThe normal C1-C2 motion mode is destructed in conditions of atlas assimilation, leading to the changes of the range of movement,joint stress force and the ligament deformation at C1 C2 junction. The atlantoaxial instability will likely occur in flexion motion.