A biomechanical study of top screw pullout in anterior scoliosis correction constructs.
STUDY DESIGN.: Biomechanical testing of vertebral body screw pullout resistance with relevance to top screw pullout in thoracoscopic anterior scoliosis constructs.


OBJECTIVE
To analyze the effect of screw positioning and angulation on pullout resistance of vertebral body screws, where the pullout takes place along a curved path as occurs in anterior scoliosis constructs.


BACKGROUND
Top screw pullout is a significant clinical problem in thoracoscopic anterior scoliosis surgery, with rates of up to18% reported in the literature.


METHODS
A custom-designed biomechanical test rig was used to perform pullout tests of Medtronic anterior vertebral screws where the pullout occurred along an arc of known radius. Using synthetic bone blocks, a range of pullout radiuses and screw angulations were tested, in order to determine an "optimal" configuration. The optimal configuration was then compared with standard screw positioning using a series of tests on ovine vertebrae (n=29).


RESULTS
Screw angulation has a small but significant effect on pullout resistance, with maximum strength being achieved at 10-degree cephalad angulation. Combining 10-degree cephalad angulation with maximal spacing between the top 2 screws (maximum pullout radius) increased the pullout resistance by 88% compared with "standard" screw positioning (screws inserted perpendicular to rod at midbody height).


CONCLUSIONS
The positioning of the top screw in anterior scoliosis constructs can significantly alter its pullout resistance.
Mayo
Andrew E
AE
Paediatric Spine Research Group, Queensland University of Technology and Mater Health Services Brisbane Ltd, Queensland, Australia.
Labrom
Robert D
RD
Askin
Geoffrey N
GN
Adam
Clayton J
CJ
D000818:Animals; D001696:Biomechanical Phenomena; D001863:Bone Screws; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery; D012600:Scoliosis / Q000503:physiopathology / Q000601:surgery*; D012756:Sheep; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D013904:Thoracic Vertebrae / Q000601:surgery