Reevaluating the Fennell Technique: The Necessity of a Unified Cortical Entry Depth and Medial Adjustment for Thoracic Pedicle Screw Placement

Objective To quantitatively evaluate the accuracy of the unified anatomical landmark (Fennell) method for pedicle screw placement and propose supplemental and modified techniques. Methods CT scans of 200 thoracic vertebral segments were collected using a random number table method (remainder method). The pedicle transverse diameter was measured, and two entry points were identified: the Fennell point (F-point, 3 mm below the junction of the lateral edge of the superior articular process and the transverse process) and the optimized point (O-point, within the transverse process-lamina groove). A rectangular frame—representing screw diameter and length—was inserted along the pedicle axis with 30° lateral angulation for T1–2 and 20° for T3–12. The internal migration distance (IMD) was defined as the distance between the midpoint of the caudal edge of the frame and the O-point. The optimal lateral angulation was measured from both the F- and O-points toward the pedicle center. Statistical analyses were performed for pedicle diameter, ideal screw dimensions, optimal angulation, and IMD values, with p < 0.05 considered statistically significant. Results Significant left-right and gender-related differences were observed across various parameters and thoracic levels. The pedicle transverse diameter, optimal lateral angulation from both the F-point and O-point, and internal migration distance (IMD) exhibited significant side-to-side differences at multiple levels (T1-T11). Gender-based comparisons further revealed significant differences in pedicle morphology (transverse diameter), implant dimensions (screw diameter and length), optimal trajectory angulation, and IMD values, particularly at T1, T4, T5, T7, T8, T9, T10, and T12 (p < 0.05 for all). Conclusions When placing thoracic pedicle screws, using the transverse process-lamina groove as a consistent depth reference for cortical bone preparation can serve as a valuable supplement to the uniform anatomical landmark (Fennell) technique. Additionally, performing a appropriate medial adjustment from this unified depth during needle insertion represents a practical modification to enhance the accuracy and safety of the technique.