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Study Design: Morphometric, radiographic, and computed tomographic evaluation of the pedicle of the first sacral vertebra was performed, and the pedicle's spatial relation with the posterior surface of the ilium was defined., Objectives: To facilitate accurate localization of the entry site of the iliosacral pedicular screw on the posterior surface of the ilium, to provide optimal length and direction of iliosacral screw placement, and to investigate the feasibility of inserting two screws through the first sacral vertebral pedicle for unstable posterior pelvic fixation., Methods: Anterior and posterior pedicular height, pedicular depth, alar depth, and posterior alar height of S1 vertebrae were measured in 11 body pelves bilaterally. Sacral pedicular height was also measured on the outlet view radiograph as visualized during intraoperative fluoroscopy, and compared with actual anatomic pedicular height. The distance from the posterior limit of the ilium to the S1 ala, pedicle, and pedicle axis, and the distance between the outer table of the ilium and anterior cortex of the sacrum were measured on axial computed tomography scans. Finally, parasagittal sections of the sacral were made to assess the safety zone for placement of two pedicular screws into the vertebral body., Results: The mean anterior and posterior pedicular heights were 30.2 and 26.1 mm, respectively. The depths of the pedicle and ala were 27.8 and 45.8 mm, respectively. The mean posterior alar height was 28.7 mm. The mean first sacral pedicular height measured on the outlet-view radiographs was 20 mm, which was significantly less (P < 0.0001) than the actual gross anatomic pedicular height. The mean distance from the posterior limit of the ilium to the pedicle axis projection point on axial computed tomography scans was 32.5 mm, and the mean distance from this point to the greater sciatic notch was 38.6 mm. The mean distance between the outer table of the ilium and the anterior cortex of the sacrum was 105.2 mm. The safety margin for two closely inserted pedicular screws was only 4 to 6 mm., Conclusions: This study suggests that placement of one screw through the S1 pedicle into the vertebral body is safer, and routine placement of two sacral pedicular screws may be difficult. The optimal starting point for placement of single iliosacral screw is 3 to 3.5 cm anterior to the posterior border of the iliac bone in the sagittal plans, and 3.5 to 4 cm cephalad to the greater sciatic notch. The screw should be directed perpendicular to the outer surface of the table from this entry point. The safe length of the iliosacral pedicular screw is up to 80 mm.

Study Design: This study analyzed anatomic parameters between the midpoint of cervical vertebral lateral masses as seen on the superficial, posterior aspect of the mass and cervical nerve roots. Posterior cervical dissection was performed, with the midpoint of the lateral masses kept intact and the nerve roots exposed., Objective: To quantitatively determine the location of the cervical nerve roots and the transverse foramina, indicating vertebral artery placement relative to the posterior aspect of the cervical spine., Summary of Background Data: Posterior plate-screw fixation of the cervical spine has been widely used to treat unstable fractures of the cervical spine. However, injury to the spinal nerve roots during the procedure remains an important concern. No previous anatomic study regarding the location of the cervical nerve roots relative to the posterior aspect of the cervical spine has been reported., Methods: Fifteen specimens were obtained for study of the cervical spine. Laminectomy and partial removal of the superior and inferior articular facets then were performed on C2-C3 through C7-T1 to expose the nerve roots and dura. Photographs, containing a reference scale, were taken simultaneously perpendicular to the sagittal and transverse planes of the specimen. Using enlarged versions of the photographs, independent measurements by several observers were taken from the superficial, posterior center of each lateral mass to the nerve root superiorly and inferiorly, and to the lateral limits of the dura. Vertebrae from an additional 20 spines were examined to determine the position of the transverse foramina relative to the lateral mass of the vertebrae., Results: The results showed that for C3-C7, the average distance from the superficial, posterior center of the lateral mass to the nerve root superiorly was 5.7 +/- 1.5 mm. Inferiorly, the average distance was 5.5 +/- 0.8 mm. The average distance from the lateral mass to the spinal cord dura was 9.2 +/- 1.4 mm, and the average medial angle of the nerve root was 76.3 degrees +/- 4.4 degrees. For cervical vertebrae C3-C5, the transverse foramina were situated medial to the posterior center of the lateral mass. At the C6 level, the transverse foramina were situated anterior to the posterior midpoint of the lateral mass., Conclusion: This study demonstrates that the posterior midpoint of the lateral mass is a safe point for initiating screw insertion.

Study Design: This study assessed numerous structural features of the second cervical vertebra (C2), describing the projection point of the pedicle on its posterior aspect., Objectives: Evaluation of the specimens included quantitative description of 18 linear and four angular parameters, which then were correlated between male and female specimens. The point of projection of the C2 pedicle axis was described with an emphasis on a perspective relevant to a posterior approach., Summary of Background Data: The literature regarding the anatomy of the axis focuses mainly on the dens. Very little research regarding the quantitative study of the C2 pedicle has been reported., Methods: Fifty dry C2 cervical vertebrae (30 male, 20 female) were obtained for anatomic measurements. Anatomic evaluation focused on the pedicle, vertebral body, dens, superior facet, and vertebral canal. All measurements were made using calipers and a standard rule linear measurements and a goniometer for angular measures. Based on the measurement of 50 specimens, including 18 linear and four angular parameters, the mean, range, and standard deviation were calculated for all of the specimens and for male and female separately., Results: A significant difference was found to exist for 11 of 18 linear measurements and one of four angular parameters. The projection point of the pedicle axis on the posterior aspect of the lateral mass was described with an emphasis on a perspective relevant to a posterior surgical approach. The location of the projection point of the pedicle axis was found to be 5.4 +/- 1.2 mm inferior to the horizontal line, and 7.2 +/- 1.3 mm lateral to the vertical line. The pedicle axis was found to lie at 33 degrees in the medial direction and 20 degrees in the superior direction form the point of pedicle axis projection., Conclusions: When the techniques described here are used, the findings may be helpful in cases involving C2 when surgical intervention and instrumentation are desired.