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    Effects of angle and laminectomy on triangulated pedicle screws
    (Churchill Livingstone, 2007) Kilincer, Curnhur; Inceoglu, Serkan; Sohn, Moon Jun; Ferrara, Lisa A.; Benzel, Edward C.
    We aimed to demonstrate the effect of angle and laminectomy on paired pedicle screws to determine whether a 90 degrees screw angle is optimal as has been previously suggested. According to the angle between right and left screws, 28 calf vertebrae were divided into three groups and instrumented as follows: Group I: 60 degrees screw angle; Group II: 90 degrees angle; Group III: 60 degrees angle with laminectomy. The screws were connected using rods and cross-fixators and tested to peak pullout force. Triangulated pedicle screws provided 76.5% more pullout strength than single screws. Most of the specimens failed through loss of convergence angle (toggling of screws on the rods) and subsequent uni- or bilateral screw pullout. Mean +/- SD peak loads were: Group I: 2071 +/- 622 N; Group II: 1753 +/- 497 N; Group III: 2186 +/- 587 N. The differences were not significant (p > 0.05). 90 degrees triangulation was not associated with a superior pullout performance versus conventional 60 degrees triangulation, suggesting that achieving additional triangulation angle is not necessary to obtain increased pullout strength. Laminectomy did not alter the effect of triangulation on fixation strength. (c) 2006 Elsevier Ltd. All rights reserved.
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    Load sharing within a human thoracic vertebral body: An in vitro biomechanical study
    (Turkish Neurosurgical Soc, 2007) Kilincer, Cumhur; Inceglu, Serkan; Sohn, Moon Jun; Ferrara, Lisa A.; Bakirci, Nadi; Benzel, Edward C.
    OBJECTIVE: The vertebral body is the major load bearing part of the vertebra and consists of a central trabecular core surrounded by a thin cortical shell. The aim of this in vitro biomechanical study is to determine the debated issue of load sharing in a vertebral body. METHODS: A series of non-destructive compressive testing on excised human thoracic vertebral bodies were performed. The testing process consisted of a stepwise removal of the vertebrae's trabecular centrum and measurement of surface strains. RESULTS: Load sharing of cortical shell of osteopenic vertebrae (48.1+/-7.6) was significantly higher than that of normal vertebrae (44.3+/-10.6). Load sharing of middle thoracic vertebrae (49.4+/-10.0) was significantly higher than that of lower thoracic vertebrae (42.4+/-8.5). According to general linear model analysis, test speed and load were not found to be effectual on load sharing with the exception that osteopenic vertebrae showed lower cortical load sharing under higher loads. CONCLUSIONS: The cortical shell takes nearly 45% of physiological loads acting upon an isolated thoracic vertebra. Load sharing between cortical shell and trabecular centrum is significantly affected by spinal level and bone mineral density. The load borne by trabecular bone increases towards the lower spinal levels, and decreases by osteoporosis.