Biomechanical study of pedicle screw fixation in severely osteoporotic bone.
BACKGROUND
Obtaining adequate purchase with standard pedicle screw techniques remains a challenge in poor quality bone. The development of alternate insertion techniques and screw designs was prompted by recognition of potential fixation complications. An expandable pedicle screw design has been shown to significantly improve fixation compared to a conventional screw in poor quality bone.


OBJECTIVE
The purpose of this study was to determine if polymethylmethacrylate (PMMA) bone cement augmentation of an expandable pedicle screw can further improve fixation strength compared to the expandable screw alone in severely osteoporotic bone. A technique for cement insertion into the pedicle by means of the cannulated central portion of the expandable screw is also described.


METHODS
The axial pullout strength, stiffness and energy absorbed of cemented and noncemented expandable pedicle screws was determined in cadaveric vertebrae.


METHODS
Twenty-one fresh unembalmed vertebrae from the thoracolumbar spine were used. Radiographs and bone mineral density measurements (BMD) were used to characterize bone quality. Paired cemented and noncemented pedicle screw axial pullout strength was determined through mechanical testing. Mechanical pullout strength, stiffness and energy to failure was correlated with BMD.


RESULTS
Overall, there was a 250% increase in mean pullout strength with the cemented expandable screw compared with a noncemented expandable screw including a greater than twofold increase in pullout strength in the most severely osteoporotic bone. The mean stiffness and energy absorbed to failure was also significantly increased. A cemented conventional screw achieved a pullout strength similar to the noncemented expandable screw.


CONCLUSIONS
PMMA cement augmentation of the expandable pedicle screw may be a viable clinical option for achieving fixation in severely osteoporotic bone.
Cook
Stephen D
SD
Tulane University Health Sciences Center, Department of Orthopaedic Surgery, 1430 Tulane Avenue, SL-32, New Orleans, LA 70112, USA. scook2@tulane.edu
Salkeld
Samantha L
SL
Stanley
Tom
T
Faciane
Albert
A
Miller
Scot D
SD
D000368:Aged; D001696:Biomechanical Phenomena; D001843:Bone Cements; D001863:Bone Screws; D002102:Cadaver; D019245:Compressive Strength; D004867:Equipment Design; D004869:Equipment Safety; D005260:Female; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000503:physiopathology / Q000601:surgery; D008297:Male; D008875:Middle Aged; D010024:Osteoporosis / Q000601:surgery*; D019904:Polymethyl Methacrylate / Q000627:therapeutic use*; D012680:Sensitivity and Specificity; D013904:Thoracic Vertebrae / Q000503:physiopathology / Q000601:surgery
D001843:Bone Cements; D019904:Polymethyl Methacrylate