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TECHNICAL BRIEFS

Cortical Screw Pullout Strength and Effective Shear Stress in Synthetic Third Generation Composite Femurs

[+] Author and Article Information
Radovan Zdero

 Martin Orthopaedic Biomechanics Lab, St. Michael’s Hospital, Toronto, ON, Canada

Shaun Rose

Department of Mechanical and Industrial Engineering,  Ryerson University, 350 Victoria St., Toronto, ON, M5B 2K3, Canada

Emil H. Schemitsch

 Martin Orthopaedic Biomechanics Lab, St. Michael’s Hospital, Toronto, ON, Canada and Department of Surgery, Faculty of Medicine,  University of Toronto, Toronto, ON, Canada

Marcello Papini

Department of Mechanical and Industrial Engineering,  Ryerson University, 350 Victoria St., Toronto, ON, M5B 2K3, Canadampapini@ryerson.ca

J Biomech Eng 129(2), 289-293 (Aug 28, 2006) (5 pages) doi:10.1115/1.2540926 History: Received June 06, 2005; Revised August 28, 2006

Background: The use of artificial bone analogs in biomechanical testing of orthopaedic fracture fixation devices has increased, particularly due to the recent development of commercially available femurs such as the third generation composite femur that closely reproduce the bulk mechanical behavior of human cadaveric and∕or fresh whole bone. The purpose of this investigation was to measure bone screw pullout forces in composite femurs and determine whether results are comparable to cadaver data from previous literature. Method of Approach: The pullout strengths of 3.5 and 4.5mm standard bicortical screws inserted into synthetic third generation composite femurs were measured and compared to existing adult human cadaveric and animal data from the literature. Results: For 3.5mm screws, the measured extraction shear stress in synthetic femurs (23.7033.99MPa) was in the range of adult human femurs and tibias (24.438.8MPa). For 4.5mm screws, the measured values in synthetic femurs (26.0434.76MPa) were also similar to adult human specimens (15.938.9MPa). Synthetic femur results for extraction stress showed no statistically significant site-to-site effect for 3.5 and 4.5mm screws, with one exception. Overall, the 4.5mm screws showed statistically higher stress required for extraction than 3.5mm screws. Conclusions: The third generation composite femurs provide a satisfactory biomechanical analog to human long-bones at the screw-bone interface. However, it is not known whether these femurs perform similarly to human bone during physiological screw “toggling.”

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

Grahic Jump Location
Figure 1

Screw insertion points on third generation composite femurs

Grahic Jump Location
Figure 2

Screw pullout test setup. (1) base plate, (2) side mounting plates equipped with horizontal support plate atop the femur to prevent it from flexing at the screw site, (3) thumb screw position for fixing femurs, (4) U-channel screw extraction grip with tapered groove for multiple screw head sizes, and (5) universal joint swivel pin to minimize eccentric loads on load cell.

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