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Research Papers

In Vitro Fatigue Failure of Cemented Acetabular Replacements: A Hip Simulator Study

[+] Author and Article Information
N. P. Zant

Department of Mechanical and Design Engineering, University of Portsmouth, Anglesea Road, Anglesea Building, Portsmouth PO1 3DJ, United Kingdom

P. Heaton-Adegbile

Department of Mechanical and Design Engineering, University of Portsmouth, Anglesea Road, Anglesea Building, Portsmouth PO1 3DJ, United Kingdom; The North Hampshire Hospital, Basingstoke RG24 9NA, United Kingdom

J. G. Hussell

 Queen Alexandra Hospital, Portsmouth NHS Trust PO6 3LY, United Kingdom

J. Tong

Department of Mechanical and Design Engineering, University of Portsmouth, Anglesea Road, Anglesea Building, Portsmouth PO1 3DJ, United Kingdomjie.tong@port.ac.uk

J Biomech Eng 130(2), 021019 (Apr 07, 2008) (9 pages) doi:10.1115/1.2904466 History: Received January 15, 2007; Revised October 03, 2007; Published April 07, 2008

Although hip simulators for in vitro wear testing of prosthetic materials used in total hip arthroplasty (THA) have been available for a number of years, similar equipment has yet to appear for endurance testing of fixation in cemented THA, despite considerable evidence of late aseptic loosening as one of the most significant failure mechanisms in this type of replacements. An in vitro study of fatigue behavior in cemented acetabular replacements has been carried out, utilizing a newly developed hip simulator. The machine was designed to simulate the direction and the magnitude of the hip contact force under typical physiological loading conditions, including normal walking and stair climbing, as reported by Bergmann (2001, Hip 98, Freie Universitaet, Berlin). A 3D finite element analysis has been carried out to validate the function of the hip simulator and to evaluate the effects of boundary conditions and geometry of the specimen on the stress distribution in the cement mantle. Bovine pelvic bones were implanted with a Charnley cup, using standard manual cementing techniques. Experiments were carried out under normal walking and descending stairs loading conditions with selected load levels from a body weight of 75125kg. Periodically, the samples were removed from the test rigs to allow CT scanning for the purpose of monitoring damage development in the cement fixation. The hip simulator was found to be satisfactory in reproducing the hip contact force during normal walking and stair climbing, as reported by Bergmann Finite element analysis shows that the stress distributions in the cement mantle and at the bone-cement interface are largely unaffected by the geometry and the boundary conditions of the model. Three samples were tested up to 17×106cycles and sectioned post-testing for microscopic studies. Debonding at the bone-cement interface of various degrees in the posterior-superior quadrant was revealed in these samples, and the location of the failures corresponds to the highest stressed region from the finite-element analysis. Preliminary experimental results from a newly developed hip simulator seem to suggest that debonding at the bone-cement interface is the main failure mechanism in cemented acetabular replacements, and descending stairs seem to be more detrimental than normal walking or ascending stairs with regard to fatigue integrity of cement fixation.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

The resultant hip contact force relative to the cup coordinate system. Note that the magnitude and the orientation of the hip contact force may be achieved by two rotationary actuators about y and z′ axes, respectively. z′ is perpendicular to the obd plane. The rotations are represented by angle φ and θ.

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Figure 2

Schematic diagram of the motions of the Portsmouth hip simulator with the coordinate systems shown in the machine frame (X,Y,Z) and cup frame (x,y,z), as used by Bergmann (12)

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Figure 3

The Portsmouth four station hip simulator with a specimen loaded in one of the test cells

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Figure 4

Comparison of the hip contact forces as demand signal (line) and response of the machine (symbol) for normal walking (a), ascending stairs (b), and descending stairs (c)

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Figure 5

Comparison of the motions as demand signal (line) and response of the machine (symbol) for normal walking (a), ascending stairs (b), and descending stairs (c)

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Figure 6

Constraints of the Models A and B, simulation of the constraint conditions of the hip simulator (a) and the constraint conditions of the bovine bone Model D (b)

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Figure 7

A summary of von Mises stresses in the cement mantle near the bone-cement interface from selected FE models

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Figure 8

Comparison of von Mises stresses in the cement mantle near the bone-cement interface under normal walking (E), ascending stairs (F), and descending stairs (G) loading conditions

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Figure 9

Results from the preliminary hip simulator tests. (a) Partial failure in the posterior-superior quadrant at the bone-cement interface (normal walking, 15×106cycles, BW 100kg); (b) multiple discrete failures at the bone-cement interface (normal walking, 17×106cycles, BW 100kg); (c) near-complete failure at the bone-cement interface (descending stairs, 2.2×106cycles, BW 125kg); (d) a CT scan image showing extensive radiolucent line at the bone-cement interface of sample shown in (c).

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