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Related Experiment Videos

Multiaxial pedicle screw designs: static and dynamic mechanical testing.

Ralph Edward Stanford1, Andreas Herman Loefler, Philip Mark Stanford

  • 1Orthopaedic Research Laboratory, University of New South Wales, Sydney, Australia.

Spine
|April 20, 2004
PubMed
Summary

Multiaxial pedicle screws offer surgical versatility but may compromise strength. This study tested six designs, finding that while some met in vivo load expectations, specific locking mechanisms were prone to fatigue failure.

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Area of Science:

  • Spinal surgery
  • Biomechanical engineering
  • Orthopedic implants

Background:

  • Multiaxial pedicle screws offer enhanced surgical versatility.
  • The complex design of multiaxial screws may compromise mechanical strength and fatigue resistance.
  • Limited published data exists on the mechanical properties of these implants.

Purpose of the Study:

  • To measure the static yield and ultimate strengths, yield stiffness, and fatigue resistance of multiaxial pedicle screws.
  • To compare the mechanical properties of these screws with expected in vivo loads.

Main Methods:

  • A randomized investigation was conducted using a vertebrectomy model for mechanical testing.
  • Six different designs of multiaxial pedicle screws were tested under static tension, static compression, and cyclical loading.

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  • Failure modes, strength, stiffness, and cycles to failure were determined for each design.

    • Main Results:

    • Static compression yield loads ranged from 217.1 to 388.0 N, with yield stiffness from 23.7 to 38.0 N/mm.
    • Fatigue testing showed cycles to failure ranging from 42,000 to over 4.7 million at 75% of static ultimate load.
    • Significant differences in mechanical properties were observed between the tested screw designs, with failure often occurring at the multiaxial link.

    Conclusions:

    • The static bending yield strengths of the tested screws generally exceeded expected in vivo loads.
    • Multiaxial screw designs exhibited lower static bending yield strength compared to fixed screw designs.
    • "Ball-in-cup" locking mechanisms demonstrated vulnerability to fatigue failure, while smoother surfaces and thicker materials appeared to improve fatigue resistance.