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A BIO MECHANICAL STUDY OF TWO DIFFERENT PEDICLE SCREW METHODS FOR FIXATION IN OSTEOPOROTIC AND NON‐OSTEOPOROTIC VERTEBRAE: GP42.

Higashino, Kosaku1; Kim, Jin Hwan2; Horton, William2; Hutton, William2

Spine Journal Meeting Abstracts: October 2011 - Volume - Issue - [no page #]
GENERAL POSTERS
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1Tokushima University, Orthopedics, Tokushima, Japan; 2Emory University School of Medicine, Orthopedics, Atlanta, US

INTRODUCTION: In reconstruction of the osteoporotic spine patients often show poor outcome due to pedicle screw failure. We carried out a study, using osteoporotic and non‐osteoporotic vertebrae, to determine the difference in fixation strength between pedicle screws inserted straight forward and pedicle screws inserted in an upwards trajectory so that the tip lay below the superior endplate (i.e. endplate screw). There is some evidence to suggest that endplate screws have a strength advantage. Our particular focus was on osteoporotic vertebrae.

METHODS: Thirty three vertebrae (T10‐ L2) were harvested. The BMD was measured: 15 vertebrae were greater than 0.8 g/cm2 and designated as non‐osteoporotic (average BMD 1.146 ± 0.186 g/cm2) and 18 vertebrae were designated as osteoporotic (average BMD 0.643 ±0.088 g/cm2). On one pedicle the screw was inserted straight forward and on the other pedicle the screw was inserted as an endplate screw. Then, using a an MTS Mini Bionix two types of mechanical testing were carried out on each pedicle screw: 1) Cephalocaudad toggling was first carried out to simulate some physiological‐type loading: 500 cycles at 0.3 Hz, at±50 N; 2) The screw was then pulled out (along its long axis) at a displacement rate of 12.5 cm/minute.

RESULTS: 1) There was no difference in pullout force between the pedicle screws inserted straight forward and the pedicle screws inserted as endplate screws. This result applies whether the vertebrae were osteoporotic or non‐osteoporotic (p = 0.7969); 2) There was a big difference in pullout strength between the osteoporotic and the non‐osteoporotic vertebrae (c.f. 620 N with 435 N; p = 0.0013).

DISCUSSION: Pedicle screws inserted as endplate screws do not provide a strength advantage over pedicle screws inserted straight forward, whether the vertebrae are osteoporotic or not.

© 2011 Lippincott Williams & Wilkins, Inc.