Anatomic considerations of costotransverse screw placement in the thoracic spine

BACKGROUND Numerous techniques have been reported to restore spinal stability and to correct spinal deformities, including rods with wires/hooks, and rods or plates with pedicular screws. It was thought that posterior fixation of the thoracic spine through the costotransverse joint may be another alternative. METHODS Nine cadavers were obtained for study of screw fixation of the costotransverse joint for posterior thoracic instrumentation. The entrance point for screw insertion was designed to be at the posterior center of the clubbed extremity of the transverse process. From this point, a 3-mm drill bit was used to create the screw path penetrating the costotransverse joint and the ventral cortex of the rib. Under direct visualization of the costotransverse joint, the drill bit was directed parallel to the sagittal plane and toward the upper portion of the rib. Measurements included the screw path length and sagittal angulation. Also, the distance between the superior borders of the transverse process and the tubercle of the rib and the anatomic relationship of the drill bit exit to the intercostal vessels and nerves were evaluated. RESULTS The maximum length of the screw path was found at T1 (19.7 mm), whereas the minimum length was noted at T4-T5 (13.9 mm), This value decreased gradually from T1 to T4-T5, and slightly increased to T10. The larger sagittal angles of the screw path were found at the levels of T1-T4 (78-86 degrees), whereas the smaller were noted at the levels below T5 (53-61 degrees). The mean distance between the superior borders of the transverse process and the tubercle of the rib was smaller at T1-T5 (0.2-0.4 mm), and significantly increased to T8 (5.1 mm), and then slightly decreased to T10, The variation of this parameter was remarkable. All of the exit points for the drill bit were located in the upper half of the rib, and away from the intercostal vessels. CONCLUSIONS The ideal screw orientation is parallel to the sagittal plane, and angled 80-90 degrees relative to the frontal plane for T1-T4 and 50-70 degrees for T5-T10 superiorly starting at the posterior center of the transverse process. Costotransverse screw fixation in the thoracic spine may be an alternative to pre-existing methods. (C) 2000 by Elsevier Science Inc.