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Occipital Morphology: An Anatomic Guide to Internal Fixation

Zipnick, Richard I., MD; Merola, Andrew A., MD; Gorup, John, MD; Kunkle, Kevin, MD; Shin, Tae, MD; Caruso, Steven A., ME; Haher, Thomas R., MD

Anatomy
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Study Design The authors present the results of an anatomic study of the human occiput to delineate appropriate screw placement sites.

Objectives Occipital bone morphologic characteristics were evaluated to determine whether significant variability exists and to determine the position of greatest bone thickness for safe and effective internal fixation.

Summary of Background Data New instrumentation and techniques for occipital fixation are being developed in response to concerns about occipital bone variability. Thirty cadaveric occiputs were evaluated to determine if such variability exists and the location of greatest bone thickness. Radial thickness, occipital locations, and gender differences, were determined.

Methods Twenty-six skulls were sectioned sagittally to determine the contributions of the inner, middle, and outer tables to overall occipital thickness. The angle required to gain maximal cortical purchase was determined. Mean values and variance were analyzed statistically to determine variability and thickness. Data was plotted in three dimensions. Variability in morphologic features was minimal.

Results The internal occipital protuberance-external occipital protuberance was thickest at 17.55 mm (SD = 3.18 mm) and was consistently located on the superior nuchal line 43° from the horizontal skull base line. Bone thickness decreased radially from the central internal occipital protuberance position. Bone thickness above the superior nuchal line exceeded that below by 2.74 mm (P < 0.05) vertically and at the oblique positions (P < 0.05). Bone to the right of the midline was only 1 mm thicker than that to the left. Gender differences were minimal. The inner table contributed only 10% to overall occipital thickness. As occipital thickness decreased, the optimal purchase angle increased.

Conclusions Unicortical purchase at and above the superior nuchal line is warranted with a low risk of intracranial venous penetration. Internal fixation devices developed in response to occipital bone variability should be considered with respect to occipital bone thickness distributions. Attention to cervical morphologic characteristics should result in higher success rates in occipitocervical arthrodesis.

From the *Department of Orthopaedic Surgery, State University of New York, Health Science Center at Brooklyn, Brooklyn, New York, Department of Biomechanical Engineering, St. Vincent's Hospital and Medical Center of New York, and New York Medical College, Valhalla, New York, and Department of Orthopaedic Surgery, St. Vincent's Hospital and Medical Center of New York, New York, New York.

Acknowledgment date: May 26, 1995.

First revision date: October 18, 1995.

Acceptance date: November 15, 1995.

Device status category: 1.

Address reprint requests to: Thomas R. Haher, MD; 153 W. 11th St.; O'Toole, Room 2056; New York, NY 10011

Copyright © 1996 Wolters Kluwer Health, Inc. All rights reserved.