The Effect of Pedicle Expansion on Pedicle Morphology and Biomechanical Stability in the Immature Porcine Spine

Abstract

Biomechanical study in an animal model. To evaluate the feasibility of sequential dilation of the immature pedicles by dilators and to determine the biomechanical stability of screws placed in these expanded pedicles. Pedicle screws have become the implant of choice in spinal fixation. Secondary to the small vertebra sizes of pediatric patients and difficulty in finding appropriate screw sizes, they have found limited use in pediatric spine. Dilation of the pediatric pedicles may overcome the limitation secondary to discrepancy between screw sizes. However, there are no data in the literature regarding dilation capacity of pediatric pedicles to enable larger pedicle screw fixation. Two-month-old domestic pig vertebrae were used. The right pedicles were dilated with stainless steel dilators just before there is visual evidence of pedicle failure. The left pedicles served as a control group. The inner and outer diameters of the pedicles were measured on the CT scans before and after dilation. The pedicles were instrumented with 3.5-mm pedicle screws at the thoracic level and 4.0-mm pedicle screws at the lumbar level. The pullout strength of each pedicle was measured. The dilation procedure resulted in an increase in both inner (2.59 +/- 0.75 to 3.32 +/- 0.58 mm) and outer diameters (5.43 +/- 0.95 to 6.21 +/- 0.96 mm) (P < 0.05). The inner diameters dilated more than the outer diameters (34.3% vs. 15.0%). The pullout strength of the expanded pedicles (320.1 +/- 83.9 N) was significantly lower than the nonexpanded ones (408.1 +/- 102.0 N) (P < 0.01). This study demonstrated that immature pedicles can be expanded by application of serial dilators. However, dilation significantly decreases the pullout strength of the pedicle.