Multiaxial pedicle screw designs: static and dynamic mechanical testing.
METHODS
Randomized investigation of multiaxial pedicle screw mechanical properties.


OBJECTIVE
Measure static yield and ultimate strengths, yield stiffness, and fatigue resistance according to an established model. Compare these measured properties with expected loads in vivo.


BACKGROUND
Multiaxial pedicle screws provide surgical versatility, but the complexity of their design may reduce their strength and fatigue resistance. There is no published data on the mechanical properties of such screws.


METHODS
Screws were assembled according to a vertebrectomy model for destructive mechanical testing. Groups of five assemblies were tested in static tension and compression and subject to three cyclical loads. Modes of failure, yield, and ultimate strength, yield stiffness, and cycles to failure were determined for six designs of screw.


RESULTS
Static compression yield loads ranged from 217.1 to 388.0 N and yield stiffness from 23.7 to 38.0 N/mm. Cycles to failure ranged from 42 x 10(3) to 4,719 x 10(3) at 75% of static ultimate load. There were significant differences between designs in all modes of testing. Failure occurred at the multiaxial link in static and cyclical compression.


CONCLUSIONS
Bending yield strengths just exceeded loads expected in vivo. Multiaxial designs had lower static bending yield strength than fixed screw designs. Five out of six multiaxial screw designs achieved one million cycles at 200 N in compression bending. "Ball-in-cup" multiaxial locking mechanisms were vulnerable to fatigue failure. Smooth surfaces and thicker material appeared to be protective against fatigue failure.
Stanford
Ralph Edward
RE
Orthopaedic Research Laboratory, University of New South Wales, Sydney, Australia.
Loefler
Andreas Herman
AH
Stanford
Philip Mark
PM
Walsh
William R
WR
D001863:Bone Screws / Q000592:standards* / Q000706:statistics & numerical data*; D019544:Equipment Failure Analysis / Q000379:methods / Q000706:statistics & numerical data; D008422:Materials Testing / Q000379:methods* / Q000706:statistics & numerical data*; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D019415:Torque