Expandable pedicle screw trajectory in cadaveric lumbar vertebra: an evaluation using microcomputed tomography.

STUDY DESIGN: An in-vitro cadaver-based measurement using microcomputed tomography. OBJECTIVES: To evaluate the safety of removal of expandable pedicle screws (EPSs) in human vertebral bodies after a long (6 mo) insertion time. SUMMARY OF BACKGROUND DATA: Transpedicular fixation is commonly used for posterior fixation of the spine. Osteoporotic patients are in high risk of internal fixation failure. EPS is an available device for increasing transpedical fixation in the osteoporotic spine. However, it is unclear that how much the diameter of the expanding screw tip increases in the human vertebral body, and no reports have assessed the potential for fracture of the pedicle when the EPS is removed. Therefore, the safety of removal of EPS needs to be evaluated. METHODS: In this study, 6.50 mm EPSs were inserted into cadaveric lumbar vertebrae. The maximum diameters of expansion, the maximum diameters of the EPS trajectories, and the diameters of the EPS trajectories in the pedicles were measured using microcomputed tomography imaging and 3-dimensional reconstruction. Regions of interest of the same size adjacent to the EPS in the pedicle and in the vertebral body were reconstructed and analyzed using the same thresholds (1000). Bone volume divided by total volume (BV/TV) in these regions of interest was determined. RESULTS: The maximum diameter of expansion of the EPSs in the vertebral body was 7.63 mm, a 17.38% increase from the original diameter. The maximum diameter of the EPS trajectory and the diameter of the EPS trajectory in the pedicle were 7.56 mm and 7.09 mm, respectively, increases of 16.31% and 9.08%. The differences were statistically significant (P < 0.05). In the pedicle, there was a negative correlation between the diameter of the EPS trajectory and the BV/TV along the EPS trajectory (r = -0.88; P = 0.002). In the vertebral body, a negative correlation occurred between the maximum expansion diameter of the EPS and vertebral BV/TV (r = -0.85; P = 0.004). CONCLUSIONS: The EPS could be reliably removed after expansion without fracturing the pedicle. Furthermore, the BV/TV had an important influence on the degree of expansion of EPS and whether or not pedicle fracture occurs.