Abstract
Introduction
Fixation failure and pseudoarthrosis at the lumbosacral junction continue to be a big problem for spine surgeons. The poor bone quality of the sacrum, relative displacement between a screw and a screw head of a polyaxial screw and between a screw head and a rod and the substantial biomechanical forces at the lumbosacral junction are major risk factors for the high rates of instrumentation failure. It has already been believed that a transverse connector is necessary to achieve lumbosacral fusion. The purpose of our study is to investigate the dynamic stability of lumbosacral fixation using L5/S1 pedicle screws and a transverse connector with CT/finite element analysis quantitatively.
Material and Methods
CT Dicom data including L4, L5, and pelvis was obtained from the patient who had kyphoscoliotic spine and prepared for the correction surgery. The data were changed to the biomechanical model for analysis. On the computer, four pedicle screws (6 × 40 mm) were applied to L5 and S1 (PS) of the model. In addition to PS, a transverse connector was applied to bilateral rods (PS–TR). The flexion, extension, bending, and rotation force was loaded on L4 and the displacement of L5 was calculated with finite element analysis. The larger displacement was defined as the lower stability. The software of finite element analysis was MechincalFinder7.0EE.
Results
As to flexion, the displacement of L5 was reduced to 82% (PS–TR) compared with PS. As to extension, the displacement of L5 was reduced to 85% (PS–TR) compared with PS. As to right bending, the displacement of L5 was reduced to 77% (PS–TR) compared with PS. As to right rotation, the displacement of L5 was reduced to 13% (PS–TR) compared with PS.
Conclusion
As to flexion, extension, bending, and rotation, PS–TR tended to have higher stability than PS. As to rotation, the displacement of L5 (PS) is remarkably larger than that of PS–TR and it indicates that as to L5/S1 pedicle screw fixation, a transverse connector plays a significant role for lumbosacral stability and L5/S1 pedicle screw fixation without a transverse connector may be one reason for the higher rate of pseudoarthrosis.