Subaxial Cervical Intradiscal Pressure and Segmental Kinematics Following Atlantoaxial Fixation in Different Angles.

OBJECTIVE

To evaluate in a comprehensive biomechanical study the influences of fixed C1-C2 and different C1-C2 angles on the range of motion (ROM) and the intradiscal pressure (IDP) of subaxial cervical spine.

METHODS

We simulated three-dimensional cervical motions on 8 human specimens with C1-C2 fixed in 3 different angles (neutral position, neutral position -10°, neutral position +10°) following intact analysis in the material test system. The ROM changes of each motion segment and the IDP changes of 4 subaxial motion segments (C2-C3, C3-C4, C4-C5, and C5-C6) were monitored.

RESULTS

ROM change patterns at all subaxial segments were similar. Fixed C1-C2 led to a significant ROM increase relative to the intact condition during flexion/extension testing. A larger C1-C2 angle (neutral position +10°) caused an additional ROM increase during flexion, whereas a smaller C1-C2 angle (neutral position -10°) induced a further ROM increase during extension. Axial rotation testing revealed the most striking and similar ROM increases in the instrumented groups relative to the intact group. Lateral bending testing did not reveal significant ROM change between the instrumented groups and the intact group. For IDP analysis, C1-C2 fixed in a larger angle (neutral position +10°) caused significant IDP increases at the C2-C3, C3-C4, and C4-C5 levels during flexion.

CONCLUSIONS

To maintain a physiologic sagittal alignment of subaxial cervical spine, C1-C2 should be fixed in the neutral position or a relatively smaller angle instead of a more lordotic position.