Identification of the mechanical behaviour of bone using digital image correlation and the virtual fields method.

Abstract

Bone fracture, and especially hip fracture, is very common in the elderly. As the UK's ageing population is increasing, hip fracture is becoming a serious and costly public health problem. Accurate models are needed to identify individuals at risk and reduce fracture risk. The quality of these models relies on the quality of the bone material properties. Bone is known as a heterogeneous and anisotropic material. However, its material parameters are typically obtained from simple uniaxial tests, measured with extensometers or the crosshead displacement of a test machine, methods which cannot capture bone's complex behaviour.

This project used full-field measurements and inverse identification to design and validate a methodology for the identification of the four orthotropic stiffness parameters of bone and its orthotropy angle in a single test. A test capable of activating all stiffness components was designed. The test was verified with simulations on synthetic data, and validated with experiments on glass fibre specimens. By testing human cortical bone samples, the four stiffness parameters of bone were identified, along with a first estimation of its orthotropy angle.

A significant part of this project relied on uncertainty quantification. A synthetic model was generated for each test, accounting for differences in the experimental conditions, and errors associated with each individual test were predicted. These predicted errors were used to eliminate imaging and experimental procedure errors. The identified stiffness properties were the following: 16.24±1.33 GPa for E11, 7.73±0.73 GPa for E22, 0.33 ± 0.04 for ν12 and 4.46±0.32 GPa for G12.

The next step would be to repeat this test on a larger population and obtain representative averages. The identified stiffness properties can then be applied to finite element models for assessing surgical treatment outcomes and fracture risk in individuals.

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Identifiers

ORCID for Lloyd Fletcher: orcid.org/0000-0003-2841-8030

ORCID for Fabrice Pierron: orcid.org/0000-0003-2813-4994

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