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

Background: Pedicle screw fixation devices are the predominant stabilization systems adopted for a wide variety ofspinal defects. Accordingly, both pedicle screw design and bone quality are known as the main parameters affecting thefixation strength as measured by the pull-out force and insertion torque. The pull-out test method, which is recommendedby the standards of the American Society for Testing and Materials (ASTM), is destructive. A non-destructive test methodwas 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 withdensities of ۰.۱۶ and ۰.۳۲ g/cm۳. The PIT and PPF were measured according to the well-established ASTM-F۵۴۳standard at three different insertion depths of ۱۰, ۲۰, and ۳۰ mm. Modal analysis was carried out through recordingtime response of an accelerometer attached to the head of the screw impacted by a shock hammer. The effect of theinsertion 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 ۲,۱۸۶ Hz, ۱۲۳.۷۵ N.cm, and ۹۸۱.۵۰ N, respectively,when the screw was inserted into the high-density foam at the depth of ۳۰ mm. The minimum values were estimatedat ۳۳۲ Hz, ۱۶ N.cm, and ۱۲۷ N, respectively, within the low-density PU at the depth of ۱۰ mm. The higher value of ωnwas originated from higher bone screw stability and thus more fixation strength. According to the regression analysisoutcomes, the natural frequency (ωn) was linearly dependent on the PIT (ωn=۱۴ PIT) and also on the PPF (ωn=۱.۷PPF). Coefficients of variation as the results of the modal analysis were significantly less than those in conventionalmethods (i.e. pull-out and insertion torque).Conclusion: The modal analysis was found to be a reliable, repeatable, and non-destructive method, which could beconsidered a prospective alternative to the destructive pull-out test that is limited to the in-vitro application only. Themodal analysis could be applied to assess the stability of implantable screws, such as orthopedic and spinal screws.Level of evidence: V

Keywords:

Bone screw fixation , Insertion torque , Non-destructive modal analysis , Polyurethane foam , Primary stability , Pull-out strength