Internal architecture of the thoracic pedicle - An anatomic study

Study Design. In this study, data are presented that provide the surgeon with additional information about the internal structure of the thoracic pedicle, which is especially useful for pedicle screw fixation in the thoracic spine. Objectives. To quantify the internal structure of the pedicle in the thoracic spine. Summary of Background Data. There are many studies describing the external dimensions of the thoracic pedicle (i.e., pedicle height, pedicle width, and pedicle axis in the transverse and sagittal planes). However, there is little reliable information concerning the internal structure of the pedicle. Methods. Eighteen thoracic vertebrae were attached to a thin-sectioning machine and both pedicles were cut in six 1.0-mm thin slices. Slides of contact radiographs were rear-projected to a digitizer and the internal and external borders of the pedicle were digitized. Using special computer software, two external dimensions (i.e., pedicle height and pedicle width) and four internal dimensions (i.e., cortical thicknesses of the superior, inferior, medial, and lateral walls) were calculated. Results. The cancellous core was more than twice as large as the cortical shell, with a range from 65.6% to 78.6% with respect to the pedicle height, and 61.3% to 71.6% with respect to the pedicle width. The medial wall was between two and three times thicker than the lateral wall throughout all the pedicle slices and thoracic levels. These differences were highly significant (P<0.001). Conclusions. The thoracic pedicle is a complex three-dimensional structure that is mostly filled with cancellous bone. The medial wall is significantly thicker than the lateral wall, which could explain the fact that most of the pedicle fractures related to pedicle screw insertion occur laterally.