Cervical pedicle screws vs. lateral mass screws: uniplanar fatigue analysis and residual pullout strengths.
BACKGROUND
Although successful clinical use of cervical pedicle screws has been reported, anatomical studies have shown the possibility for serious iatrogenic injury. However, there are only a limited number of reports on the biomechanical properties of these screws which evaluate the potential benefits of their application.


OBJECTIVE
To investigate if the pull-out strengths after cyclic uniplanar loading of cervical pedicle screws are superior to lateral mass screws.


METHODS
An in vitro biomechanical study.


METHODS
Twenty fresh-frozen disarticulated human vertebrae (C3-C7) were randomized to receive both a 3.5 mm cervical pedicle screw and lateral mass screw. The screws were cyclically loaded 200 times in the sagittal plane. The amount of displacement was recorded every 50 cycles. After cyclical loading, the screws were pulled and tensile load to failure was recorded. Bone density was measured in each specimen and maximum screw insertion torque was recorded for each screw.


RESULTS
During loading the two screw types showed similar stability initially, however the lateral mass screws rapidly loosened compared to the pedicle screws. The rate of loosening in the lateral mass screws was widely variable, while the performance of the pedicle screws was very consistent. The pullout strengths were significantly higher for the cervical pedicle screws (1214 N vs. 332 N) and 40% failed by fracture of the pedicle rather than screw pullout. Pedicle screw pullout strengths correlated with both screw insertion torque and specimen bone density.


CONCLUSIONS
Cervical pedicle screws demonstrated a significantly lower rate of loosening at the bone-screw interface, as well as higher strength after fatigue testing. These biomechanical strengths may justify their use in certain limited clinical applications.
Johnston
Todd L
TL
Cedar Valley Medical Specialists, 1753 West Ridgeway Avenue, Suite 103B, Waterloo, IA 50701, USA.
Karaikovic
Eldin E
EE
Lautenschlager
Eugene P
EP
Marcu
David
D
D000368:Aged; D000369:Aged, 80 and over; D001863:Bone Screws; D002102:Cadaver; D002574:Cervical Vertebrae / Q000601:surgery*; D005260:Female; D006801:Humans; D008297:Male; D008422:Materials Testing; D008875:Middle Aged; D013123:Spinal Fusion / Q000295:instrumentation; D013314:Stress, Mechanical