Can modal analysis replace the destructive insertion torque and pull-out tests in primary fixation stability assessment of spinal pedicle screw?

Background context: Pedicle screw fixation devices are the predominant stabilization systems 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. Objective: A non-destructive method was proposed to evaluate the mechanical stability of screw. 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 were inserted into two different polyurethane (PU) foams with densities of 0.16 and 0.32 g/cm3. The PIT and PPT were measured. 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 obtained 2186 Hz, 123.75 N.cm and 981.50 N, respectively when the screw was inserted into the high-density foam at depth of 30 mm. The minimum values were measured 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 (ωn) was linearly dependent on the peak insertion torque (PIT) (ωn=14PIT) and also on the peak pull-out force (PPF) (ωn=1.7PPF). Conclusions: The modal analysis was found to be a reliable, repeatable and non-destructive method, which could be considered as a prospective alternative to destructive pull-out test, which is limited to in-vitro application only. The modal analysis could be applied to assess the stability of implantable screws like orthopedic and spinal screws.