SPINE Volume 25, Number 10, pp 1197–1205 ©2000, Lippincott Williams & Wilkins, Inc. Internal Morphology of Human Cervical Pedicles Manohar M. Panjabi, PhD, Eon K. Shin, BA, Neal C. Chen, BS, and Jaw-Lin Wang, PhD Study Design. The internal architecture of cervical spine pedicles was investigated by thin sectioning and digitization of radiographic images. Objectives. To provide quantitative information on the internal dimensions and cortical shell thicknesses of the middle and lower cervical pedicles. Summary of Background Data. Although there have been a number of studies presenting data on the external dimensions of the cervical pedicle, little is known regard- ing its internal architecture and cortical shell thickness along the pedicle axis. Methods. Twenty-five human cervical vertebrae (C3– C7) were secured to a thin-sectioning machine to produce three 0.7-mm-thick pedicle slices along its axis. Plain ra- diographs of the pedicle slices were scanned and digi- tized to facilitate measurement of the internal dimen- sions. Computer software was specifically developed to determine the external dimensions (i.e., pedicle height and width) and the internal dimensions (i.e., cortical shell thicknesses of the superior, inferior, lateral, and medial walls and the cancellous core height and width) of cervi- cal pedicles. Results. Superior and inferior wall cortical thicknesses of pedicle thin slices were similar, whereas the lateral wall cortical thickness was significantly smaller than the me- dial wall thickness. The medial cortical shell (average value range: 1.2–2.0 mm) was measured to be 1.4 to 3.6 times as thick as the lateral cortical shell (average value range: 0.4 –1.1 mm). When medial and lateral cortical thicknesses were normalized for external dimensions, the combined cortical shell thickness was thinnest at C7 (av- erage value range: 18.6 –25.6% of the external width), and this result was statistically significant when compared with other vertebral levels. Conclusions. The cervical pedicle is a complex, three- dimensional structure exhibiting extensive variability in internal morphology. Characteristics of the cervical pedi- cle at different spinal levels must be noted before trans- pedicular screw fixation. [Key words: anatomy, cancel- lous bone, cervical pedicle, cortical bone, transpedicular fixation] Spine 2000;25:1197–1205 Transpedicular screw fixation is one of the most sophis- ticated procedures currently in use to treat spinal insta- bilities. 24 Common causes of instability include spinal trauma, extensive laminectomies, and destruction of bony elements by neoplasm. 7 The benefits of transpe- dicular screws have been recognized for treating these conditions in the thoracolumbar vertebrae. Recently, clinical studies have been published that record its use for the cervical spine. 1–6 Although other surgical techniques such as clamp and hook plating, lateral screw and plate fixation, and interspinous wiring have been effective in stabilizing the cervical column, pedicle screws have been found to provide superior fixation with the least likeli- hood of hardware loosening. 5,12,14 In spite of the benefits conferred by transpedicular screw fixation in the cervical spine, controversy exists regarding its potential risks. The smaller size of cervical pedicles and variability in pedicle morphometry demand careful assessment of the angle and placement of trans- pedicular screws. Incorrect insertion of pedicle screws can cause damage to adjacent vital structures such as the spinal cord, nerve roots, and vertebral arteries. Miller et al 17 contend that the procedure poses a serious risk to patients because of the high percentage of pedicle wall violations observed experimentally. It is their recommen- dation that the technique not be used routinely in the cervical spine and be limited to use in patients with severe spinal trauma. Other investigators, however, have re- corded numerous instances in which no significant spinal cord, nerve root, or vertebral artery complications have arisen as a result of cervical screw fixation. 2,11 With a quantitative understanding of cervical pedicle morphol- ogy along its axis and at different spinal levels, it should be possible to improve the percentage of successful sur- gical outcomes while minimizing risk to the patient. Several anatomic studies have already documented the external dimensions and angular parameters of the cervical pedicle. 9,12,13,19,22 Although such investigations provide the spinal surgeon with useful information on the external morphometry of the cervical pedicle, other investigators have suggested that it is the internal archi- tecture of the pedicle that is complex and requires further elucidation. 8,10,15,16,18,25 Investigations of the thoracic pedicle have shown that its shape is not simply “a cylin- der of cortical bone with some cancellous bone in its center,” 21 but rather a complex three-dimensional struc- ture most often shaped like a teardrop when viewed in cross-section. 15,20 In addition, it has been found that screw stability and screw pull-out strength depend largely on the structural and internal characteristics of the pedicle, 10,23 not on its external dimensions. 12 These findings, combined with the spinal surgeon’s need to have more detailed information, indicate that there is a necessity for the internal architecture of the cervical pedicle to be clearly elucidated. Karaikovic et al 13 examined the cross-sectional anatomy of the cervical pedicle with digital caliper and computed tomography to obtain readings of gross external and internal pedicle From the Biomechanics Research Laboratory, Department of Ortho- paedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut. Supported in part by the Yale University School of Medicine Fellow- ship Program. Acknowledgment date: March 9, 1999. Acceptance date: August 19, 1999. Device status category: 1. Conflict of interest category: 12. 1197