Biomechanical analysis of 4 types of pedicle screws for scoliotic spine instrumentation.
METHODS
Numerical modeling and simulations of scoliotic spine instrumentation using monoaxial, uniaxial, polyaxial, and multiple-degrees-of-freedom (6DOF) postloading pedicle screws.


OBJECTIVE
To biomechanically analyze the general curve reduction effects and bone-screw force levels of monoaxial, uniaxial, polyaxial, and 6DOF pedicle screws for scoliotic spine instrumentation.


BACKGROUND
The ideal spinal fusion construct for treating scoliosis is still debatable. Studies on the effects of different types of implants were mainly based on postoperative radiograph measurements. Systematic studies are yet to be done on how bone-screw forces are correlated with screw types.


METHODS
Computer biomechanical models were built using 3-dimensional geometry and spine stiffness of 10 patients with adolescent idiopathic scoliosis having undergone spinal instrumentation. The surgical instrumentations were simulated each time, using a different type of screw. For each case and screw type, 15 screw placement variations were simulated to investigate their effects on bone-screw forces.


RESULTS
The maximum differences between different screw types were 6.4°, 1.1°, and 4.7°, respectively, for main thoracic Cobb angles, main thoracic apical vertebral rotation, and thoracic kyphosis (1.2°, 0.3°, and 0.3° on average). The average bone-screw forces were higher for monoaxial (229 N ± 140 N) than uniaxial (206 N ± 122 N), polyaxial (141 N ± 99 N), and 6DOF screws (103 N ± 42 N). Bone-screw forces with monoaxial screws were, respectively, 1.1, 2.5, and 25 times more sensitive to screw placement variation than uniaxial, polyaxial, and 6DOF screws.


CONCLUSIONS
The bone-screw loads of different screws were significantly different. The descending order of bone-screw loads was monoaxial, uniaxial, polyaxial, and 6DOF screws. For patients with large and stiff spinal deformities or for patients with compromised bone quality, screws with more degrees of freedom offer better perspective to reduce bone-screw connection failure.
Department of Mechanical Engineering, Ecole Polytechnique de Montréal, Montreal, Quebec, Canada.
Xiaoyu
X
Wang
Carl-Eric
CE
Aubin
Dennis
D
Crandall
Stefan
S
Parent
Hubert
H
Labelle
D000293:Adolescent; D001696:Biomechanical Phenomena; D001863:Bone Screws / Q000145:classification / Q000592:standards*; D003198:Computer Simulation; D006801:Humans; D007738:Kyphosis / Q000503:physiopathology / Q000601:surgery; D008954:Models, Biological; D016059:Range of Motion, Articular; D015203:Reproducibility of Results; D012399:Rotation; D012600:Scoliosis / Q000503:physiopathology / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013904:Thoracic Vertebrae / Q000503:physiopathology / Q000601:surgery*