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

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 (ω_n) extracted from the modal analysis was then correlated with peak pull-out force (PPF) and peak insertion torque (PIT).

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.

The maximum values of ω_n, PIT, and PPT were obtained at 2,186 Hz, 123.75 N.cm, 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 N.cm, 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 (ω_n) was linearly dependent on the PIT (ω_n=14 PIT) and also on the PPF (ω_n=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).

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.