Pedicle screw diameter selection for safe insertion in the thoracic spine

Objective Many thoracic pedicles are too small for the safe acceptance of a transpedicular screw. However, few studies have so far reported on the methods to select a proper pedicle screw size and to conWrm the morphologic changes for such a small thoracic spine pedicle. The objective of this work was to determine the potential limits of a pedicle screw diameter for transpedicular screw placement in the thoracic spine. Methods T2–T9 vertebrae from eleven patients that underwent posterior thoracic instrumentation with the use of Xuoroscopically assisted insertion method were analyzed. The outcome measures were the pedicle widths, the gap between the outer pedicle width and the selected pedicle screw diameter, and the penetration length of the pedicle screws using computed tomography. The screws were distributed into two groups according to the pedicle width and screw diameter, and the screw perforation rate of the two groups was compared. The relationships of the gap and the distance of the screw penetration were compared and investigated in regard to the pedicle screw diameter selection. Results A total of 16 screws demonstrated a smaller diameter than the inner pedicle widths, while 22 screws had a larger diameter than the inner pedicle widths. One screw (6.3%) perforated the pedicle cortex in the smaller screw group, and twelve screws (54.5%) perforated the pedicle cortex in the larger screw group (P = 0.006). A linear regression analysis in the larger screw group revealed that when the gap was less than 0.5 mm, a risk of a pedicle wall violation was observed. Conclusions When the screws with a larger diameter than the inner pedicle width are selected, the screw perforation rate increases. Therefore, the size of the screw diameter must be at least 0.5 mm less than the outer pedicle width to ensure safe transpedicular screw placement.