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TECHNICAL PAPERS: Bone/Orthopedic

The Effect of Cross-Sectional Stem Shape on the Torsional Stability of Cemented Implant Components

[+] Author and Article Information
Angela E. Kedgley

Biomechanical Testing Laboratory, Department of Mechanical and Materials Engineering,  University of Western Ontario, London, Ontario N6A 5B9, Canadaakedgley@uwo.ca

Sarah E. Takaki

Biomechanical Testing Laboratory, Biomedical Engineering Graduate Program,  University of Western Ontario, London, Ontario N6A 5B9, Canadastakaki@uwo.ca

Pencilla Lang

Biomechanical Testing Laboratory, Department of Mechanical and Materials Engineering,  University of Western Ontario, London, Ontario N6A 5B9, Canadaplang3@uwo.ca

Cynthia E. Dunning

Department of Mechanical and Materials Engineering,  University of Western Ontario, London, Ontario N6A 5B9, Canadacdunning@eng.uwo.ca

J Biomech Eng 129(3), 310-314 (Nov 14, 2006) (5 pages) doi:10.1115/1.2720907 History: Received March 29, 2006; Revised November 14, 2006

Stability of a cemented implant, once the stem-cement interface has debonded, is reliant upon stem geometry and surface finish. There are relatively few studies addressing the effect of cross-sectional stem shape on cemented implant fixation. The purpose of this investigation was to compare the torsional stability of five different stem cross-sectional shapes—circular, oval, triangular, rectangular with rounded edges, and rectangular with sharp edges—under monotonically increasing and cyclic loading conditions. Seven samples of each stem geometry were tested. Stems were potted in bone cement and loaded to 5 deg of rotation. For monotonic loading, torque was applied at a constant rate of 2.5 deg/min. For cyclic loading, a sine wave torque pattern was applied, with a maximum magnitude that began at 4.5 Nm for 1500 cycles and then increased by 2.25 Nm every 1500 cycles until 5 deg of rotation. The rectangular stem with the sharp edges always provided the greatest resistance to torque, followed by the rectangular with rounded edges, triangular, oval, and circular. These results, including the effects of sharp corners, may differ for modes of loading other than torsion. These experimental results support the findings of earlier finite element models, indicating stem shape has a significant effect on resistance to torsional loading.

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Copyright © 2007 by American Society of Mechanical Engineers
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Figures

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Figure 1

The stem shapes. Five stem shapes were tested: (a) circular, (b) oval, (c) triangular, (d) round rectangular, and (e) sharp rectangular. The cross-sectional dimensions were as indicated, with all dimensions in millimeters (8.0mm≈0.315in.; 6.0mm≈0.236in.).

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Figure 2

The test jig. The round rectangular stem is shown cemented in the aluminum tube and held in the loading fixture. The Instron applied a compressive load to the torque arm, which was attached to the end of the stem and supported by a Delrin® block. This resulted in pure torsion being applied to the stem.

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Figure 3

Representative torque-displacement plots for monotonic loading. Torque-displacement curves shown for one representative trial of each stem shape. Failure occurred at the first sharp decrease in torque. Loading was stopped once the stems reached 5deg of stem rotation.

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Figure 4

Cycles to 5deg of stem rotation for cyclic loading. Mean (±1 SD) cycles to 5deg of stem rotation for seven trials of each stem shape. Significant differences (p<0.05) are indicated by a star (☆).

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Figure 5

Torque at 5deg of stem rotation for cyclic loading. Mean (±1 SD) torque at 5deg of stem rotation for seven trials of each stem shape. Significant differences (p<0.05) are indicated by a star (☆).

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