The purposes of this in vitro study are that to investigate if the cord pretension and spacer in posterior dynamic stabilization affects the flexibility in sheep lumbar spine. Methods Eight sheep lumbar spine in the intact state, Dynesys with neutral spacer and 300N cord pretension, Dynesys with neutral spacer and no cord pretension, Dynesys with 300N without spacer, and rigid rod fixation were evaluated. The range of motion (ROM) and neutral zone (NZ) of flexion-extension, lateral bending and axial rotation were measured calculated to evaluate the flexibility of sheep lumbar spine. Results There were significant decrease the ROM and NZ in flexion -extension, in lateral bending (ROM, p<0.001; NZ, p<0.001) after implantation of Dynesys or rigid fixator. On comparing the effect of the cord pretension, the ROM and NZ in flexion-extension was significant difference between the instrumentation of Dynesys with 300N pretension and Dynesys without pretension was found (ROM, p=0.013; NZ, p=0.037). On comparing the effect of the spacer, there was no significant difference in ROM and NZ between the instrumentation of Dynesys with spacer and Dynesys without spacer in all principal motion direction. Dynesys assembly without spacer showed more stiffness than rigid fixator. Conclusions On the basis of this in vitro study, the kinematic behavior of Dynesys is similar to the rigid rod fixation. The flexibility of Dynesys was reduced when assembly with 300 N cord pretension. The cord pretension and spacer plays important role in flexibility of Dynesy

Keywords : Dynamic Neutralization Stabilization System, In Vitro, Sheep, Lumbar Vertebrae