Effect of Insertion Status of a Pedicular Screw on its Mechanical Properties using the Finite Element Method

Ahn, Myun Whan; Ahn, Chang Soo; Choi, Yong Suk; Seo, Jae Sung; Lee, Choon Yeol; Chung, Il Sub

Abstract

PURPOSE: The structural performance of a fixation system was investigated in the case of screw insertion into the pedicular canal with or without its threads fastened in the cortex and in the broken pedicle.
MATERIALS AND METHODS
The symmetric geometric model was used for the pull-out test, and the simplified 3-dimensional model at 3 level, namely, the 3rd, 4th and 5th lumbar vertebrae, was used for the axial compressive loading test.
RESULTS
According to the result of the axial-loading test, bending moments and Von-Mises stresses were increased at the junction between the head and the 2nd and 3rd pitches of the lumbar pedicle screw and at the center of 4th lumbar vertebral body, in the models of intact, posterior elements removed and unstable fractured spines. As the results of flexibility, there is no difference in intra-cortical insertion compared to intra-medullary pedicle insertion in the intact spine model. However, flexibility of the laminectomy model was 8% better that of the intra-cortical insertion, and the unstable fracture model 2% better.
CONCLUSION
There is no difference in the flexibility of intra-cortical and intra-medullary pedicle screw insertion. The stability of the posterior structure is more important than anterior pedicle screw fixation.