C1 pedicle screws versus C1 lateral mass screws: comparisons of pullout strengths and biomechanical stabilities.
METHODS
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.


BACKGROUND
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.


CONCLUSIONS
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.
Ma
Xiang-Yang
XY
Department of Orthopedics, Guangzhou Liu Hua Qiao Hospital, Guangzhou, People's Republic of China. maxy1001@126.com
Yin
Qing-Shui
QS
Wu
Zeng-Hui
ZH
Xia
Hong
H
Liu
Jing-Fa
JF
Xiang
Ming
M
Zhao
Wei-Dong
WD
Zhong
Shi-Zhen
SZ
D000328:Adult; D001268:Atlanto-Axial Joint / Q000601:surgery; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D001270:Cervical Atlas / Q000000981:diagnostic imaging / Q000601:surgery*; D005593:Fracture Fixation, Internal / Q000295:instrumentation*; D006801:Humans; D066298:In Vitro Techniques; D007593:Joint Instability / Q000601:surgery; D008297:Male; D008422:Materials Testing; D011474:Prosthesis Design; D011859:Radiography; D016059:Range of Motion, Articular