Analysis of pullout strength and pedicle screw loosening from loading rigid rod construct

Abstract

Introduction: Pedicle screws have been used for many different purposes in order to provide effective fixation between a metal interface like a rod or plate to a vertebral body. These screws generally fixate in a proper and secure manner in patients with normal bone density but in patients with osteoporosis, these screws can easily come out of the bone and lead to complications. Methods: Two pedicle screws each were applied in osteoporotic thoracic vertebral bodies from human cadavers. Three cc of Kyphplasty cement was injected through one screw and not through the other screw. Half were loaded in a cantilever setup to simulate the load as well as a 5 Nm moment to measure screw loosening that occurs. The other half had the screw heads were gripped in a self centering vice, secured in an aligning fixture, and subjected to pullout tests at a rate of 1 mm/sec on MTS 858 Minibionics until failure occurred and peak load is recorded. Results: For non-cemented screws, the angles increased for 5 of the 6 specimens and the translations increased for all 6 specimens tested. The average peak force recorded for the five vertebras tested is 320.24±40.38N. When compared to the pullout forces of cemented pedicle screws inserted into the opposite pedicle of the same vertebral bodies, the average increases dramatically to 659.84±102.6N. The average percent increase in pullout strength is 109.75% (P < 0.003). Discussion: A t-test showed significantly more loosening (p