Comparison of biomechanical properties of single- and two-segment fusion for Denis type B spinal fractures.

OBJECTIVE: To compare the biomechanical properties of single- and two-segment fusion for Denis type B spinal fracture.
METHODS: Two female patients with Denis type B L1 vertebral fractures were studied. Both patients had achieved intervertebral fusion by 1 year postoperatively, at which time CT data were collected, including data of one patient before and after removal of pedicle screws and of another whose pedicle screws were not removed. The data were imported into Mimics software and T11 -L2 three dimensional models established. After construction of the models, they were imported into ANSYS software. An axial load (260 N) and 10 Nm torque were loaded to simulate the flexion, extension, lateral bending and rotation of the spine, respectively.
RESULTS: There was no significant difference in the average displacement of the spine motion between the two-segment and single-segment fusion patients without removal of pedicle screws. However, for all motion forms, the average displacement of the single-segment fusion patient' spine after removal of pedicle screws was significantly greater than that before removal of pedicle screws and that of the two-segment fusion patient. The average Von Mises stress of T11-12 intervertebral disc of two-segment fusion patient was significantly greater than that of the one-segment fusion patient. Moreover, the average Von Mises stress of T11-12 intervertebral disc of single-segment fusion patient was greater before than after removal of pedicle screws.
CONCLUSION: Provided there is satisfactory interbody fusion, removal of pedicle screws after one-segment fusion can increase spinal motion, reduce the stress on adjacent intervertebral discs and delay disc degeneration.