Mechanical Analysis of Insertion Depth and Insertion Angle on Pedicle Screw Strain

The purpose of this study was to determine the effects of incomplete insertion and insertion angle on pedicle screws strain. Each of four stainless steel pedicle screws (7mm x 40mm) were equipped with two strain gauges on the superior and inferior surfaces of the screw head. The incomplete screw insertion test protocol was proceeded by screws inserted at sequential halfturn intervals until full insertion. The variable insertion angle protocol was followed with screw insertion angles of 0°, 10°, and 20° posterior-inferiorly to the transverse plane. Strain was computed for each gage at each depth and/or insertion angle as the construct was compresed, and average values for the strain-to-force ratio were determined. As the pedicle screws were inserted at progressively larger posterior-inferior angles to the transverse plane, the strain-to-force ratio decreased. Pedicle screw strain was significantly decreased when screws were inserted 20 degrees cephalad-caudal plane (anatomical y-axis). Screw prominence, not screw angulation, has a significant detrimental effect on screw strain.