Replacement of Destructive Pull-out Test with Modal Analysis in Primary Fixation Stability Assessment of Spinal Pedicle Screw

Document Type : RESEARCH PAPER


1 1 Biomechanical Engineering Group, Department of Biomedical Engineering, Amirkabir University of Technology, Hafez Avenue, Tehran, Iran 2 Biomechanics Section, KU Leuven- University of Leuven, Celestijnenlaan, Leuven, Belgium

2 Biomechanical Engineering Group, Department of Biomedical Engineering, Amirkabir University of Technology, Hafez Avenue, Tehran, Iran

3 Biomechanics Section, KU Leuven- University of Leuven, Celestijnenlaan, Leuven, Belgium


Background: Pedicle screw fixation devices are the predominant stabilization systems adopted for a wide variety of
spinal defects. Accordingly, both pedicle screw design and bone quality are known as the main parameters affecting the
fixation strength as measured by the pull-out force and insertion torque. The pull-out test method, which is recommended
by the standards of the American Society for Testing and Materials (ASTM), is destructive. A non-destructive test method
was proposed to evaluate the mechanical stability of the pedicle screw using modal analysis. Natural frequency (ωn)
extracted from the modal analysis was then correlated with peak pull-out force (PPF) and peak insertion torque (PIT).
Methods: Cylindrical pedicle screws with a conical core were inserted into two different polyurethane (PU) foams with
densities of 0.16 and 0.32 g/cm3. The PIT and PPF were measured according to the well-established ASTM-F543
standard at three different insertion depths of 10, 20, and 30 mm. Modal analysis was carried out through recording
time response of an accelerometer attached to the head of the screw impacted by a shock hammer. The effect of the
insertion depth and foam density on the insertion torque, natural frequency, and pull-out force were quantified.
Results: The maximum values of ωn, PIT, and PPT were obtained at 2,186 Hz, 123.75, and 981.50 N, respectively,
when the screw was inserted into the high-density foam at the depth of 30 mm. The minimum values were estimated
at 332 Hz, 16, and 127 N, respectively, within the low-density PU at the depth of 10 mm. The higher value of ωn
was originated from higher bone screw stability and thus more fixation strength. According to the regression analysis
outcomes, the natural frequency (ωn) was linearly dependent on the PIT (ωn=14 PIT) and also on the PPF (ωn=1.7
PPF). Coefficients of variation as the results of the modal analysis were significantly less than those in conventional
methods (i.e. pull-out and insertion torque).
Conclusion: The modal analysis was found to be a reliable, repeatable, and non-destructive method, which could be
considered a prospective alternative to the destructive pull-out test that is limited to the in-vitro application only. The
modal analysis could be applied to assess the stability of implantable screws, such as orthopedic and spinal screws.
Level of evidence: V


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