Central and juxta-endplate vertebral body screw placement: a biomechanical analysis in a human cadaveric model.
METHODS
In vitro biomechanical testing of transvertebral body screws in different positions in both axial pull-out and toggle.


OBJECTIVE
To determine the relative strength of unicortical versus bicortical screw fixation within the vertebral body and to determine comparative strength of juxta-endplate and central screw positions with and without staples in both axial pull-out and toggle modes.


BACKGROUND
Loss of fixation is common in centrally placed screws at the rostral end of a construct. To preserve segmental vessels, juxta-endplate screw positions are often used. The biomechanical strength of such screw placement methods has not been measured.


METHODS
Eighty-three human cadaveric vertebral bodies were tested for axial pull-out and toggle with and without staples. Screw positions included central, juxta superior, and inferior endplate. Juxta-endplate screws were toggled in both the rostral and caudal directions perpendicular to the screw axes.


RESULTS
Unicortical fixation resulted in a 93% decrease in axial pull-out strength compared with bicortical fixation. Centrally placed screws and juxta-endplate screws were equivalent in axial pull-out if no staples were used. The juxta-endplate screw with a staple that was toggled away from the endplate had the highest yield strength, followed by the central screw with a staple, and then the juxta-endplate screw without a staple toggled away from the endplate.


CONCLUSIONS
Bicortical fixation is much stronger than unicortical fixation. Centrally placed screws are significantly stronger when used with a staple. When preservation of segmental vessels is desirable, juxta-endplate screws should be placed in such a manner that compressive forces are directed away from the endplate.
Lowe
Thomas
T
M & M Orthopaedics, Naperville, Illinois, USA. Denverorth@aol.com
O'Brien
Michael
M
Smith
David
D
Fitzgerald
David
D
Vraney
Robert
R
Eule
James
J
Alongi
Paul
P
D000293:Adolescent; D000328:Adult; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000706:statistics & numerical data; D006801:Humans; D008159:Lumbar Vertebrae / Q000033:anatomy & histology / Q000601:surgery*; D008422:Materials Testing / Q000379:methods* / Q000706:statistics & numerical data; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013904:Thoracic Vertebrae / Q000033:anatomy & histology / Q000601:surgery*