A Biomechanical Study of Screw Designs of Transpedicular Screw on the Fixation Strength

Kim, Ki Tack; Lee, Sang Un; Kim, Young Woo; Hong, Gyu Pyo; Mun, Mu Sung

Abstract

INTRODUCTION: The fixation strength of transpedicular screw system in the vertebral hody relied on bone quality and anatomical characteristics of vertebral pedicle, designs of screw and types of connection(rod or plate) with screw. The purpose of this study is to verify the biomechanical nature of the transpedicular fixation in spine under various conditions with porcine vertebrae. MATERIAL AND METHOD: Fresh porcine vertebrae and the custom-made screws were used in this experiment. To reduce the errors caused by vertebral bodies of different size and quality, vertebral bodies having regular range of pedicular width(10.0 to 11.5mm) and hone density(more than 1.0 gm/cm2) were used. The pedicle screws were inserted in the same procedure and axial pull out test was performed with using the Material Testing System(lntron8511, Canton, USA). The experiments were performed in four types to assess the difference of strength accroding to designs of the screw hy using two group of screws. The first group of screw was designed according to the outer and inner diameter and the second group was designed according to the shape, pitch, and thread profile of screw. Experiment I was perfomed to evaluate the effect of screw diameters on the biomechanical pull-out strength hy using the first group of custom-made pedicle screw which fixed all other factors except the diameter of screw. Experiment I was to verify the effect of screw shape, experiment III to verify the effect of pitch and experiment IV to verify the effect of thread profile.
RESULTS
The results of experiments were summarized as follows: Experiment I showed that the screw of larger outer diameter had greater holding strength. Experiment II showed that the holding strength of cylindrical shaped screw is superior to that of conical shaped screw. Experiment III showed that there is no statistical significance between different modes of pitch. Experiment IV showed that the holding strength of buttress shape of thread profile is superior to that of V-shape.
CONCLUSION
It seemed that the fixation strength of the screw was more powerful with 1 mm increment of outer diameter in 4-7mm of outer diameter, 3mm of pitch and buttress shape of thread of the screw with the same operation technique.