A Biomechanical and SEM Analysis on Repetitive Use of Spinal Screws

The goal of this study was to quantify the effect of repetitive use of spinal screws on screw pullout resistance. Synthetic cancellous material with a medium porosity and 20 inch–pound (in.–lb.) of compressive strength (25 x 25 x 50 mm) was used to assess repetitive pullout resistances of the screws. Three different screws were tested. Cancellous lateral mass screws, cortical lateral mass screws, and pedicle screws. The insertional torques of the screws were measured with a torque wrench.

The screws were axially extracted from the host material, at a constant speed of 2.5 mm/min. From this data, the pullout resistance was quantified. Screw extraction was repeated for 3 trials in the synthetic cancellous blocks. The control (non–tested) and tested cortical screws were examined microscopically with a scanning electron microscope (SEM).

A statistically significant decrease in pullout resistance was observed for the cortical lateral mass screws, p<0.05. No statistical differences were noted in pullout resistances for the remaining screw types and insertional torques for all screws. SEM analysis of the cortical screws showed a smoothing of the thread tip and entire screw surface, as well as, small–pitted deformations within the individual threads. It is concluded that screw surface characteristics and performance may be effected by repeated screw use. This phenomenon appears to be particularly apparent with cortical screws.