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Research Papers

Full-Field Strain Measurement During Mechanical Testing of the Human Femur at Physiologically Relevant Strain Rates

[+] Author and Article Information
Lorenzo Grassi

Division of Solid Mechanics,
Lund University,
Lund 22363, Sweden
Department of Biomedical Engineering,
Lund University,
BMC D13, Sölvegatan 19,
Lund 22184, Sweden
e-mail: lorenzo.grassi@bme.lth.se

Sami P. Väänänen, Jukka S. Jurvelin

Department of Applied Physics,
University of Eastern Finland,
Kuopio 70211, Finland

Saber Amin Yavari, Amir A. Zadpoor

Faculty of Mechanical, Maritime, and
Materials Engineering,
Delft University of Technology,
Delft 2628 CD, The Netherlands

Harrie Weinans

Faculty of Mechanical, Maritime, and
Materials Engineering,
Delft University of Technology,
Delft 2628 CD, The Netherlands
Department of Orthopaedics,
UMC Utrecht 3508 GA, The Netherlands

Matti Ristinmaa

Division of Solid Mechanics,
Lund University,
Lund 22363, Sweden

Hanna Isaksson

Division of Solid Mechanics,
Lund University,
Lund 22363, Sweden
Department of Biomedical Engineering,
Lund University,
Lund 22184, Sweden
Department of Orthopaedics,
Lund University,
Lund 22184, Sweden

Manuscript received May 8, 2014; final manuscript received August 13, 2014; accepted manuscript posted August 27, 2014; published online September 17, 2014. Assoc. Editor: David Corr.

J Biomech Eng 136(11), 111010 (Sep 17, 2014) (8 pages) Paper No: BIO-14-1199; doi: 10.1115/1.4028415 History: Received May 08, 2014; Revised August 13, 2014; Accepted August 27, 2014

Understanding the mechanical properties of human femora is of great importance for the development of a reliable fracture criterion aimed at assessing fracture risk. Earlier ex vivo studies have been conducted by measuring strains on a limited set of locations using strain gauges (SGs). Digital image correlation (DIC) could instead be used to reconstruct the full-field strain pattern over the surface of the femur. The objective of this study was to measure the full-field strain response of cadaver femora tested at a physiological strain rate up to fracture in a configuration resembling single stance. The three cadaver femora were cleaned from soft tissues, and a white background paint was applied with a random black speckle pattern over the anterior surface. The mechanical tests were conducted up to fracture at a constant displacement rate of 15 mm/s, and two cameras recorded the event at 3000 frames per second. DIC was performed to retrieve the full-field displacement map, from which strains were derived. A low-pass filter was applied over the measured displacements before the crack opened in order to reduce the noise level. The noise levels were assessed using a dedicated control plate. Conversely, no filtering was applied at the frames close to fracture to get the maximum resolution. The specimens showed a linear behavior of the principal strains with respect to the applied force up to fracture. The strain rate was comparable to the values available in literature from in vivo measurements during daily activities. The cracks opened and fully propagated in less than 1 ms, and small regions with high values of the major principal strains could be spotted just a few frames before the crack opened. This corroborates the hypothesis of a strain-driven fracture mechanism in human bone. The data represent a comprehensive collection of full-field strains, both at physiological load levels and up to fracture. About 10,000 points were tracked on each bone, providing superior spatial resolution compared to ∼15 measurements typically collected using SGs. These experimental data collection can be further used for validation of numerical models, and for experimental verification of bone constitutive laws and fracture criteria.

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Figures

Grahic Jump Location
Fig. 2

Force–displacement curves for the three specimens tested. Specimen #1 is shown in red (dotted line), #2 in green (dashed line), and #3 in blue (continuous line).

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Fig. 1

(a) Experimental setup. (b) A raw image from the master DIC camera, showing one sample with its control plate. (c) Sketch of the specimen preparation. Frontal plane was defined by the three contact points on the frontal side of the femur, depicted with red circles. The shaft axis was defined by the two most lateral points at the smaller trochanter level and at the cutting plane level, respectively (blue triangles). This reference system was used to guide the insertion of the distal pot, and the application of the protective cap on the femoral head. The position of the three virtual SGs is shown with green squares.

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Fig. 3

Evolution of the major (positive sector) and minor (negative sector) principal strains, as a function of the applied force, in the simulated SGs for the three femora. The strain gage on the head is depicted in blue (continuous line), the one on the neck in green (dashed line), and the one at the diaphysis level in red (dotted line).

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Fig. 4

The orientation of the major principal strain direction for the three femora at four different load levels, defined by 75%, 100%, and 150% of the BW, and at the frame immediately before the crack formation. Only a subsampling of the points was depicted with arrows, and the direction of the other points is indicated by the background color of the femur, according to the legend on the right.

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Fig. 5

The orientation of the minor principal strain direction for the three femora at four different load levels, defined by 75%, 100%, and 150% of the BW, and at the frame immediately before the crack formation. Only a subsampling of the points was depicted with arrows, and the direction of the other points is indicated by the background color of the femur, according to the legend on the right.

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Fig. 6

Major principal strain distribution for the three femora at four different load levels, defined by 75%, 100%, and 150% of the BW, and at the frame immediately before the crack formation

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Fig. 7

Minor principal strain distribution for the three femora at four different load levels, defined by 75%, 100%, and 150% of the BW, and at the frame immediately before the crack formation

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Fig. 8

Crack formation and propagation for specimens #2 and #3: the major principal strain distribution is superimposed on the raw pictures recorded. The major principal strains after the crack opened are not shown, as the fast surface motion resulted in a slightly out-of-focus picture. This impaired the correct image correlation.

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Fig. 9

Fracture limit plot for specimens #2 and #3 at the last frame before the crack is detected. The 5% of the points with a higher sum of the principal strains is evidenced in black in the plots (left), and their anatomical location is shown with the same color (right).

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Fig. 10

The orientation of the major and minor principal strain directions for specimen #2 at the frame immediately before the crack formation. The orientations are reported over a sagittal section of the femur obtained with a high-resolution CT scanner (Verity CT scanner, Planmend, Finland). The CT images and the full-field orientation data were coregistered manually, with aim of qualitatively showing the correspondence between the principal strain directions on the bone surface, and the internal trabecular orientation.

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