Biomechanic analysis of a double hybrid fixation in the lumbar spine

Objetive: to evaluate the biomechanics effects of the double hybrid fixation, rigid and dynamic, over the adjacent non-fixated level in the lumbar spine. Methods: using six specimens, L3-S1, from human lumbar spine, the measurement of the intradiscal pressure (IDP) in the disc L3-L4, L4-L5 and L5-S1, were performed in the following conditions: non-fixated specimen, with rigid fixation in L5-S1 and double hybrid fixation in L4-S1. The six specimens were subjected to progressive load until 750 N in axial compression, flexion and extension. Results: in the upper adjacent disc to the dynamic stabilization, L3-L4 disc, the IDP increased about 10%. In disc L4-L5 the posterior dynamic fixation the IDP decreased 50% and the IDP of disc with rigid fixation L5-S1 was reduced over 65%. Conclusion: the IDP of non-fixated segment adjacent to a dynamic implant is much lesser that in rigid fixations and the system provided protection to the disc over the rigid fixation.