Full-field strain measurement during mechanical testing of the human femur at physiologically relevant strain rates

Grassi, Lorenzo; Väänänen, Sami P.; Yavari, Saber Amin; Jurvelin, Jukka S.; Weinans, Harrie; Ristinmaa, Matti; Zadpoor, Amir A.; Isaksson, Hanna

Affiliations

¹Division of Solid Mechanics, Lund University, Lund 22363, Sweden

²Department of Biomedical Engineering, Lund University, BMC D13, Sölvegatan 19, Lund 22184, Sweden

³Department of Applied Physics, University of Eastern Finland, Kuopio 70211, Finland

⁴Faculty of Mechanical, Maritime, and Materials Engineering, Delft University of Technology, Delft 2628 CD, The Netherlands

⁵Department of Orthopaedics, UMC Utrecht 3508 GA, The Netherlands

⁶Department of Orthopaedics, Lund University, Lund 22184, Sweden

Abstract

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.

Links

DOI: 10.1115/1.4028415

PubMed: 25162941

WoS: 000342750600011

History

Received: 2014-05-08

Revised: 2014-08-13

Accepted: 2014-08-27

Online: 2014-09-17

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2018	Zhao S, Arnold M, Ma S, Abel RL, Cobb JP, Hansen U, Boughton O. Standardizing compression testing for measuring the stiffness of human bone. Bone Joint Res. August 2018;7(8):524-538.
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2016	Grassi L, Väänänen SP, Ristinmaa M, Jurvelin JS, Isaksson H. How accurately can subject-specific finite element models predict strains and strength of human femora? investigation using full-field measurements. J Biomech. March 21, 2016;49(5):802-806.
2020	Katz Y, Yosibash Z. New insights on the proximal femur biomechanics using digital image correlation. J Biomech. March 5, 2020;101:109599.
2020	Grassi L, Kok J, Gustafsson A, Zheng Y, Väänänen SP, Jurvelin JS, Isaksson H. Elucidating failure mechanisms in human femurs during a fall to the side using bilateral digital image correlation. J Biomech. June 9, 2020;106:109826.
2015	Grassi L, Isaksson H. Extracting accurate strain measurements in bone mechanics: a critical review of current methods. J Mech Behav Biomed Mater. October 2015;50:43-54.
2019	Gustafsson A, Khayyeri H, Wallin M, Isaksson H. An interface damage model that captures crack propagation at the microscale in cortical bone using XFEM. J Mech Behav Biomed Mater. February 2019;90:556-565.
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2019	Väänänen SP, Grassi L, Venäläinen MS, Matikka H, Zheng Y, Jurvelin JS, Isaksson H. Automated segmentation of cortical and trabecular bone to generate finite element models for femoral bone mechanics. Med Eng Phys. September 2019;70:19-28.
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