MECHANICAL STABILITY OF THORACOLUMBAR PEDICLE SCREW FIXATION - THE EFFECT OF CROSS-LINKS

Study Design. Pedicle screw fixation for unstable thoracolumbar spine injuries is relatively new. The effect of one or two crosslinks on rot ational and lateral bending stiffness was studied. Objective. To deter mine the rotational and bending stiffness values of thoracolumbar frac tures fixed by the AO's internal fixation system with zero, one, or tw o crosslinks. Methods. Eight embalmed thoracolumbar spine segments (T1 2-L2) were instrumented at T12 and L2 with a pedicle screw-rod system. Rotational stiffness was determined for 10 cycles to 2.5 degrees, 3.5 degrees, and 5 degrees of rotation, with and without one or two cross links, and lateral bending stiffness for 10 cycles to 0.25, 0.40, and 0.50 inch. The results showed a clear trend toward increased stiffness with crosslinks. Results. The stiffness values of the two-crosslink c onstruct at 2.5 degrees and 3.5 degrees of rotation were significantly higher than those of the zero-crosslink system. Also, the bending sti ffness of the two-crosslink construct was significantly higher than th at of no-crosslink system at all of the displacements. Conclusions. Ro tational stiffness values of the two-crosslink construct were signific antly higher than those of the zero-crosslink system, at 2.5 degrees a nd 3.5 degrees of rotation. Lateral bending stiffness of the two-cross link system was higher than that of the zero-crosslink system at all l evels of displacement.