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Research Papers

The Effect of Under-Reaming on the Cup/Bone Interface of a Press Fit Hip Replacement

[+] Author and Article Information
I. Zivkovic

Biomechanics Research Laboratory, University of Illinois at Chicago, 842 West Taylor Street, Room 1039, Chicago, IL 60607-7039

M. Gonzalez1

Biomechanics Research Laboratory, University of Illinois at Chicago, 842 West Taylor Street, Room 1039, Chicago, IL 60607-7039; Department of Orthopedic Surgery, University of Illinois at Chicago, 1801 West Taylor Street, Room 2-A, Chicago, IL 60612-4319

F. Amirouche

Biomechanics Research Laboratory, University of Illinois at Chicago, 842 West Taylor Street, Room 1039, Chicago, IL 60607-7039; Department of Orthopedic Surgery, University of Illinois at Chicago, 1801 West Taylor Street, Room 2-A, Chicago, IL 60612-4319

1

Corresponding author.

J Biomech Eng 132(4), 041008 (Mar 19, 2010) (8 pages) doi:10.1115/1.2913228 History: Received December 22, 2006; Revised December 27, 2007; Published March 19, 2010; Online March 19, 2010

This paper explores the effect of under-reaming on micromotion at the cup/bone interface of a press-fit acetabular cup. A cadaver experiment was performed on 11 acetabuli implanted with a cementless acetabular cup. The loading profile simulated hip impingement at the extremes of motion and subluxation relocation of the hip joint. Micromotion of each cup was measured in a custom made jig with linear variable differential transducers. A CAT scan and DEXA scan of the acetabulum and femoral head respectively were used to construct a three-dimensional patient specific finite element model of the hemi-pelvis. The model predicted cup micromotion under loading conditions and stresses in the acetabulum as a result of cup insertion. Micromotion was then calculated as a function of variable bone density and variable degree of underreaming. Simulated cup insertion with under-reaming of 2 mm or more approached or exceeded the yield strength of bone in acetabula with reduced bone mass density.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Experimental setup showing hemipelvis mounted in custom made jig with sensors in place

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Figure 2

Schematic of the experimental setup with inset showing the position of LVDT sensors

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Figure 3

Finite element model of the hemipelvis implanted with the acetabular cup

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Figure 4

Illustration of bone cross section of the finite element model of hemipelvis showing how different layers of bones were constructed. The subchondral and cortical bones were modeled as shell overlaying trabecular bone.

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Figure 5

Displacement constraints applied to the model. The lower part of the ilium is fully constrained (DX=DY=DZ=0), and the cup is unconstrained.

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Figure 6

Simulation of hammering of the cup in insertion. The acetabular cup was initially displaced penetrating the acetabular dome by 0.75mm.

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Figure 7

The graph shows relative micromotion between the acetabular cup and the subchondral bone measured with six placed LVDT sensors

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Figure 8

Contact status for 2mm and 1.75mm

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Figure 9

Finite element prediction of maximum stress in insertion (hoop stress)...for different bone densities. Transverse lines represent range of yield strength for human bone (19)

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Figure 10

Variation in cup rotation as a function of change in elastic modulus of a pelvis bone. Variations in cup’s rotation and BMD are expressed as a percent change.

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Figure 11

Three dimensional plot showing peak stresses as a function of bone mineral density and degree of under-reaming

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