Safety entry point, size and direction for placement of thoracic pedicle screw--a cadaveric study.

BACKGROUND Understanding of the pedicle anatomy is necessary for pedicle screw placement; however, there have been few Thai studies reporting on thoracic pedicle morphometry. OBJECTIVE To determine important anatomical measurements for thoracic pedicle screw fixation. MATERIAL AND METHOD T1-T12 vertebral pedicles were evaluated in 27 cadavers. The diameter angle, length and entry point of the pedicle were measured. RESULTS The transverse diameter was smaller than the sagittal diameter at all levels. The narrowest transverse diameter was at the T5 level in males and T4 level in females. The pedicle diameter in males was statistically significantly greater than in females at all levels except at the T12 level. Most pedicles with a transverse diameter of 5.0 mm or less were found at the T4, T5, T6 and T7 levels. The transverse angle was widest at the T1 level; it faced medially at all levels. The sagittal angle was widest at the T12 level andfaced cephaladly at all levels. The axis length was shortest at the T1 level. Most entry points were at the inferolateral zone. CONCLUSION A 4-mm diameter screw should be used carefully at the mid-thoracic levels for Thai people, especially females. Screw lengths of less than 30 mm are safe for placement at all levels. The safe zone for the pedicle entry point is in the inferolateral zone.