Biomechanical Study of Pedicle Screw Fixation Comparing Intracortical Pedicle Screw and Oversized Pedicle Screw.

Introduction: Pedicle screw placement can be technically challenging in anatomically narrow pedicles, such as those in the thoracic spine or in older patients. Although smaller screws preserve cortical integrity, they may compromise fixation strength. Conversely, oversized pedicle screws that breach the pedicle cortex may enhance anchorage but raise concerns about potential neurovascular injury. Methods: We performed a cadaveric biomechanical study using 36 thoracic and lumbar vertebrae harvested from five older donors. Each vertebra received an oversized screw that intentionally breached the pedicle cortex on one side and a smaller intracortical screw on the contralateral side. Four vertebrae were excluded owing to fracture during testing, leaving 32 vertebrae for analysis. After cyclic loading, pullout strength was measured. Subgroup and multivariate analyses were conducted based on pedicle diameter, vertebral level, and insertion side. Results: Oversized screws indicated superior pullout strength in 20 of the 32 analyzed vertebrae. When the breach percentage exceeded 40%, oversized screws generally outperformed intracortical screws. Subgroup analysis revealed a significant advantage of oversized screws in narrow pedicles (<5 mm), where they increased pullout strength. In contrast, a decrease in strength was observed in wide pedicles (≥5 mm). Multivariate analysis identified pedicle diameter as the only independent predictor of strength improvement. Conclusions: Oversized screws provide stronger fixation than do intracortical screws, particularly in anatomically narrow pedicles where cortical contact is limited. These findings suggest a potential biomechanical advantage of oversized screws in selected patients. However, careful consideration of anatomical risk and patient-specific factors is essential to minimize neurological complications.