Anatomic Relationship of Bony Structures in Pedicle-Rib Unit and its Significance

OBJECTIVE: The pedicle-rib unit is regarded as an expanded pedicle and is a new approach to thoracic pedicle fixation. Previous studies were mostly focused on anatomic, radiographic, and biomechanical assessment. However, there is no study on anatomic relationship of bony structures in the pedicle-rib unit. This article investigates the anatomic relationships between transverse process, pedicle, rib, and corresponding vertebrae body in the pedicle-rib unit, so as to improve the clinical safety of pedicle-rib unit screw placement. METHODS: Forty-five normal dry adult cadaver specimens were included. Anatomic parameters were measured. Anatomic parameters were 1) the anatomic relationship between the transverse process and the pedicle (the distance from the horizontal center line of the transverse process to the center and the superior and inferior border of the pedicle); 2) the anatomic relationship between the transverse process and the rib (the overlap portion between the costal neck and the transverse process); 3) the anatomic relationship between the transverse process and the vertebral body (the relative position of the horizontal midline of the transverse process and the vertebral body); and 4) the anatomic relationship between the pedicle and the rib (the overlap portion between the costal neck and the pedicle). RESULTS: The midline of the transverse process was located within the range of the superior and inferior borders of the pedicle in the thoracic spine. The distance between the midline of the transverse process and the center of the pedicle was the closest at T6 and T7, but the farthest at T11 and T12. The anterolateral part of the transverse process was mostly covered by rib from T1 to T8, but it was less covered from T9 to T12. The midline of the transverse process corresponded to the upper third or middle third of the vertebral body. The pedicle is completely or mostly overlapped by the rib from T1 to T9 on the anterolateral side, but from T10 to T12, the rib overlapped the pedicle partially. CONCLUSIONS: The pedicle-rib unit is a three-dimensional anatomic structure. The pedicle, transverse process, and rib are not completely in the same plane, and their positions vary in different segments. Pedicle-rib unit screw fixation is anatomically feasible. Setting the screw in the upper-middle thoracic spine is safer than setting it in the lower thoracic spine.