Elastic and Viscoelastic Properties of the Human Pubic Symphysis Joint: Effects of Lateral Impact Loading

[+] Author and Article Information
Greg J. Dakin, Raul A. Arbelaez, Fred J. Molz, Kenneth A. Mann, Alan W. Eberhardt

Department of Biomedical Engineering, University of Alabama at Birmingham, Birmingham, AL 35294

Jorge E. Alonso

Division of Orthopaedic Surgery, University of Alabama at Birmingham, Birmingham, AL 35294

J Biomech Eng 123(3), 218-226 (Dec 21, 2000) (9 pages) doi:10.1115/1.1372321 History: Received February 13, 2000; Revised December 21, 2000
Copyright © 2001 by ASME
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Average compressive creep curves comparing (a) gender and (b) trauma. The creep curves were calculated from the mean step loads (P) and mean Bailey–Norton variables (A, B, and C) with the “instantaneous displacement” (di) normalized to 0.6 mm, which was a typical value for the specimens tested. Graph (a) reflects the greater “steady-state creep” rate for females and (b) illustrates the larger “primary creep” for the impacted joints.
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Tension-to-failure tests resulted in three types of fracture: (a) combination bone fracture with joint cracking, (b) bone fracture, and (c) disc failure
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Assembly drawing of the bending fixture. Distance m (19.5±4.5 mm) varied slightly for each joint depending on the joint size. Length n was adjusted to move the fixture’s center of gravity directly over the load cell. Angle θ was the degree of joint bending recorded by the goniometer.
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Analysis of “primary creep”: For all three curves, the Bailey–Norton variable C=0.25 and the “instantaneous displacement” di=0.6
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Analysis of “steady-state creep”: For all three curves, APB=0.04 and the “instantaneous displacement” di=0.6
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Anterior view of the human pelvis. In the present study, the six loading directions were defined as: tension (perpendicular to the joint), compression, posterior bending (the rami move away from the reader), anterior bending (the rami move toward the reader), superior bending, and inferior bending.
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Anteroposterior x-ray view of a pubic symphysis specimen. The joint is oriented with the superior end of the specimen at the top of the x-ray. The arrow points to the gap between the bones where the fibrocartilage disc is located.
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Schematic of the two modes of the testing fixture. During tension/compression testing (a), the base plates attached directly to the MTS actuator and load cell. For the bending tests (b), pin and yolk attachments connected the lever arms to the MTS actuator and load cell. In both cases, the pubic symphysis specimen was oriented on its side, with the interpubic disc lying horizontally between the two pots.
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Slopes of the load-displacement curve of the tenth loading cycle were obtained as a measure of toe region and linear region axial stiffness. For consistency, linear regression was performed at the same displacement levels (0.0 mm to 1.0 mm and 0.65 mm to 0.75 mm for the axial toe and linear regions, respectively) for each specimen. The same method was used on the bending moment–bending angle curves to determine bending stiffness.
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Peak load (fp), displacement at peak load (dp), and stiffness were obtained for the tension-to-failure tests. The same method from determining linear region tensile stiffness was used by performing a regression analysis at the 0.65 mm to 0.75 mm level of displacement.
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Total displacement function d(t)=di+dc(t) fitted to the experimental creep data




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