STUDY DESIGN: In vitro biomechanical testing of transvertebral body screws in different positions in both axial pull-out and toggle. OBJECTIVES: 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. SUMMARY OF BACKGROUND DATA: 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.
STUDY DESIGN: In vitro biomechanical testing of transvertebral body screws in different positions in both axial pull-out and toggle. OBJECTIVES: 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. SUMMARY OF BACKGROUND DATA: 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.
Authors: Weijun Wang; Zezhang Zhu; Feng Zhu; Bin Wang; Winnie C W Chu; Jack C Y Cheng; Yong Qiu Journal: Eur Spine J Date: 2008-05-31 Impact factor: 3.134
Authors: Daniel Kyle Palmer; David Rios; Wyzscx Merfil Patacxil; Paul A Williams; Wayne K Cheng; Serkan İnceoğlu Journal: Int J Spine Surg Date: 2012-12-01