Pedicle instrumentation in the thoracic spine - A morphometric and cadaveric study for placement of screws

Study Design. In part 1 of the study, the morphometry of thoracic pedicles and bony landmarks for pedicle screw placement were evaluated. In part 2, p edicle screws were inserted in fresh cadavers, using a different entry poin t in the left and right pedicles. Objectives. To identify the safest entry point and screw orientation for pe dicle screws in the thoracic spine. Summary of Background Data. A few morphometric investigations have been per formed on thoracic vertebrae, but the safest technique for screw insertion in thoracic pedicles has not been analyzed. Methods. Mean, range, and standard deviations of pedicle transverse diamete r and pedicle orientation were measured in 99 dried thoracic vertebrae. We evaluated the position of the bottom of the superior facet and that of the superior border of the transverse process in relation to the center of the pedicle. The relation between the pedicle axis and the superior facet in th e frontal plane was also assessed. In part 2 of the study, pedicle screws w ere inserted in fresh cadavers at the intersection between the superior bor der of the transverse process and the middle of the superior facet (entry p oint A) and between the former and the lateral two thirds of the facet (ent ry point B). Results. The smallest transverse diameter was found at T6 (mean 4.3 mm) whe re pedicles measured less than 5 mm in 68% of the specimens. In the frontal plane, the pedicle axis intersected the middle of the superior facet inn 1 5% of specimens, the lateral two-thirds in 62%, and the lateral border of t he facet in 23%. Of the 126 screws inserted in fresh human cadavers, 15 (24 %) of the screws inserted using entry point A and 10 (16%) of those inserte d using entry point B violated the pedicle cortex (P < 0.05). Six (10%) of the screws inserted using entry point A compared with no screw inserted usi ng entry point B penetrated the anterior vertebral cortex (P = 0.03). Conclusions. Pedicles between T4 and T8 may not be wide enough for screw fi xation. An entry point for pedicle screws located at the intersection betwe en the superior border of the transverse process and the lateral two thirds of the superior facet seems more likely to be line with the pedicle axis t han do other entry points. In the lower thoracic vertebrae this entry point , in combination with insertion of the screws more medially oriented than t he pedicle axis, significantly reduces the risk of violating the anterior v ertebral cortex.