Accuracy of compression test on excised specimens

Accuracy of compression test on excised specimens

Citation for published version (APA):

Terlouw, M. A., Rietbergen, van, B., & Huiskes, H. W. J. (2002). Accuracy of compression test on excised specimens. Poster session presented at Mate Poster Award 2002 : 7th Annual Poster Contest.

Document status and date: Published: 01/01/2002 Document Version:

Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers) Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.

• The final author version and the galley proof are versions of the publication after peer review.

• The final published version features the final layout of the paper including the volume, issue and page numbers.

Link to publication

General rights

Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain

• You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement:

www.tue.nl/taverne Take down policy

If you believe that this document breaches copyright please contact us at: openaccess@tue.nl

providing details and we will investigate your claim.

12

/

department of biomedical engineering

PO Box 513, 5600 MB Eindhoven, the Netherlands

Accuracy of compression test on excised specimens.

Numerical evaluation of cancellous bone elastic behaviour.

M.A.Terlouw, B. van Rietbergen, R.Huiskes

Eindhoven University of Technology, Department of Biomedical Engineering

Problem

• Strain analysis in bone, is required for evaluation of bone strength and analysis of implants.

• Until recently, a porous composite material like bone could only be analysed using apparent properties.

• These are obtained through experimental testing of excised specimen. This method contains (inherent) errors [1].

unsupported side trabeculae
experimental artefacts

• Recent development of µCT and µFE methods (Fig 1.) enables numerical analysis of real-life bone architecture [2]. • Numerical analysis of excised specimen solves some

experimental problems, but creates some others [2].

assumption of Hookean material behaviour
numerical errors

• When whole bone model is subjected to physiological loading, the in-situ strains and stresses can be obtained.

Figure 1 Creation of theµFE model

Aims

• Numerical quantification of the inherent error in the computed strain, for

a numerical representation of the experimental method.
a numerical uni-axial strain test, in six directions

Method

• Femur is scanned using µCt, and the image of the trabecular architecture is converted to aµFE model.

• One healthy and one osteoporotic femur were scanned and

digitized (Fig 2).

• In each femurhead, seven VOIS were defined.

• Each VOI consisted of 1003_voxels.

• Three analyses were performed:

Figure 2 Position of the VOIs

In-situ analysis

• Femur was subjected to physiological forces; local stresses and strains were computed.

• The VOIS were extracted from the deformed geometry. • For every VOI, computed strains and stresses were averaged. • The averaged values acted as the ’Gold standard’ reference.

Standard compression test (Uniaxial stress)

• The VOIs were extracted from the unloaded geometry. • Every extracted VOI was subjected to free compressions

in three directions.

• Side surfaces of the specimen were not prescribed. • Stiffness matrix was computed from stresses and strains.

Uniaxial strain

• The VOIs were extracted from the unloaded geometry. • Every extracted VOI was subjected to confined

compression in six directions.

• Side surfaces of the specimen were prescribed.

• Stiffness matrix was computed from stresses and strains.

Quantification of the error

Cuni−strain∗ σappin−situ

µFE model of whole femur Ccomp−test∗ σappin−situ

σapp in−situ app uni−ax app in−situ appcomp−test

Figure 5 Computational scheme

Results

−1 −0.5 0 0.5 1 1.5 2 2.5 3 x 10−3 −1 −0.5 0 0.5 1 1.5 2 2.5 3x 10 −3 εapp in−situ ε app uni − strain and ε app comp − test

gold standard reference normal, comp−test osteo, comp−test normal, uni−strain osteo, uni−strain least−square fit, compression least−square fit, uni−axial

Figure 4 Correlation plot

Correlation of in-situ strain versus comp-test strain (blue) and cor-relation of in-situ strain versus uni-axial (red)

Discussion

• Both methods give a good result

• Compressive forces have absolute error of 10% • Six compressions gives relative error of 1%

References

[1] KEAVENY, T.M.VAN ET.AL. (1997), J Orth Res, 15, 101 [2] RIETBERGEN, B.VAN ET.AL. (1996), J Biomech, 29, 1653