Biomechanical Evaluation of the Pedicle Screw Insertion Depth Effect on Screw Stability Under Cyclic Loading and Subsequent Pullout

STUDY DESIGN A biomechanical ex vivo study of the human lumbar spine. OBJECTIVE To evaluate the effects of transpedicular screw insertion depth on overall screw stability and pullout strength following cyclic loading in the osteoporotic lumbar spine. SUMMARY OF BACKGROUND DATA Although much is known about the clinical outcomes of spinal fusion, questions remain in our understanding of the biomechanical strength of lumbar pedicle screw fixation as it relates to screw sizing and placement. Biomechanical analyses examining ideal pedicle screw depth with current pedicle screw technology are limited. In the osteoporotic spine, optimized pedicle screw insertion depth may improve construct strength, decreasing the risk of loosening or pullout. METHODS A total of 100 pedicles from 10 osteoporotic lumbar spines were randomly instrumented with pedicle screws in mid-body, pericortical, and bicortical depths. Instrumented specimens underwent cyclic loading (5000 cycles of ±2 N m pure flexion moment) and subsequent pullout. Screw loosening, failure loads, and energy absorption were calculated. RESULTS Cyclic loading significantly (P