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C1 Pedicle Screws Versus C1 Lateral Mass Screws: Comparisons of Pullout Strengths and Biomechanical Stabilities

Ma, Xiang-Yang MD, PhD*; Yin, Qing-Shui MD*; Wu, Zeng-Hui MD*; Xia, Hong MD*; Liu, Jing-Fa MD*; Xiang, Ming MD; Zhao, Wei-Dong MD; Zhong, Shi-Zhen PhD

doi: 10.1097/BRS.0b013e318193a21b

Study Design. In vitro biomechanical study.

Objective. To compare the pullout strengths and the biomechanical stabilities afforded by C1 lateral mass screws and C1 pedicle screws using bicortical and unicortical fixation techniques.

Summary of Background Data. Posterior screw fixation techniques in the atlas including C1 lateral mass screw and C1 pedicle screw. The shortcomings of C1 lateral mass screw technique and potential risks of bicortical fixation method were recently described; C1 pedicle screw technique with unicortical fixation might overcome these anatomic and clinical drawbacks. However, it is unknown whether the biomechanical characteristics of unicortical C1 pedicle screw are comparable with that of bicortical C1 lateral mass screw.

Methods. Bicortical or unicortical C1 pedicle screws and C1 lateral mass screws were inserted into 12 adult fresh human C1 specimens. Pullout strength was evaluated using a material testing machine. The construct’s stability of bicortical C1 lateral mass screws or unicortical C1 pedicle screws incorporating unicortical C2 pedicle screws was compared with bilateral transarticular screws using another 6 fresh cervical cadaver spines. Pullout strength and biomechanical stability differences were compared statistically.

Results. Bicortical C1 pedicle screws provided the biggest pullout strength (1757.0 ± 318.7 N) of all 4 methods, whereas unicortical C1 lateral mass screws provided the weakest(794.5 ± 314.8 N). However, there were no statistically significant differences between bicortical C1 lateral mass screws (1243.8 ± 350.0 N) and unicortical C1 pedicle screws (1192.5 ± 172.6 N). Furthermore, there was no statistically significant difference of biomechanical construct stability between unicortical C1 pedicle screw-rod constructs and bicortical C1 lateral mass screw-rod constructs.

Conclusion. C1 pedicle screws are stiffer than C1 lateral mass screws. Unicortical C1 pedicle screw provided the same pullout resistance and three-dimensional stability as bicortical C1 lateral mass fixation. Although lateral mass screw placement into C1 requires bicortical purchase, pedicle screw insertion into the atlas only requires unicortical fixation.

The pullout strength and biomechanical construct stability of C1 lateral mass screws and C1 pedicle screws was evaluated with fresh adult human cervical cadaver. Our results indicated that C1 pedicle screws were stiffer than C1 lateral mass screws, the transpedicular procedure with unicortical fixation provides a similar pullout resistance and biomechanical construct stability as C1 lateral mass screws with bicortical fixation.

From the *Department of Orthopedics, Guangzhou Liu Hua Qiao Hospital, Guangzhou, People’s Republic of China; †Department of Orthopedics, Shenzhen Shekou People’s Hospital, Guangdong, Peoples Republic of China; and ‡Department of Anatomy, Southern Medical University, Guangzhou, People’s Republic of China.

Acknowledgment date: February 25, 2008. Revision date: September 17, 2008. Acceptance date: September 17, 2008.

The medical device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Federal funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Supported by Natural Sciences of Guangdong Province of China (No: 20023001).

Address correspondence and reprint requests to Xiangyang Ma, MD, PhD, Department of Orthopedics, Guangzhou Liu Hua Qiao Hospital, 111 Liu Hua Road, Guangzhou 510010, P. R. China; E-mail:

© 2009 Lippincott Williams & Wilkins, Inc.