A biomechanical approach to improved fracture risk assessment with a focus on reference point microindentation

Abstract

Osteoporosis is a bone disease with two primary definitions: 1) the World Health Organisation definition of low Bone Mineral Density (BMD) and 2) the National Institutes of Health definition of increased bone fragility and fracture risk. Though BMD, alongside clinical factors, is the current gold standard for diagnosing osteoporosis, these measures are risk factors that only define a small proportion of individuals who fracture. Changes in the biomechanical properties of the bone may relate to fracture risk and bone disease and, if they can be clinically assessed, could be useful in supplementing existing techniques to improve future diagnosis and treatment.

This thesis addresses whether there are biomechanical changes in osteoporotic as well as osteoarthritic bone and whether these can be measured through Reference Point Microindentation (RPI). RPI has previously been applied in vivo with no reported complications and the hypothesis that a higher indentation depth of a needle-like test probe implies increased fracture risk. Despite emerging implementation of RPI, there is limited research to advise on optimal testing with this technique. Therefore, recommendations are provided to minimise testing variation by studying the variability associated with RPI testing parameters in vitro. Primarily, a best practice would be to: keep maximum load, sample preparation and mode-of-use consistent, fix the device in its stand, remove soft tissue and machine the bone, ensure sufficient cortical thickness and make multiple repeat measurements. These recommendations then guided the main, clinically focussed, study of this thesis. RPI was applied to femoral neck samples extracted from fractured and osteoarthritic patients, finding the surface measured indentation depth to be increased relative to cadaveric control samples. Furthermore, the measured indentation property had minimal correlation with current existing techniques, supplementing BMD and clinical factors to improve discrimination of fractured from control tissue.

Complementary fracture toughness testing allowed for improved understanding of bone disease and interpretation of the RPI results. This study demonstrated that fracture toughness properties of the inferomedial femoral neck seem largely unaffected by osteoporosis or osteoarthritis. Furthermore, correlation between RPI and fracture properties was minimal, implying that the technique was assessing other properties to discriminate the osteoporotic and osteoarthritic from control bone. Indent imaging, through micro-computed tomography and serial sectioning techniques, confirmed this to be the case with RPI being a multifactorial measure comprised of structural as well as fracture mechanics and elastoplastic properties. Altogether, this thesis provides insight into the effects of both osteoporosis and osteoarthritis on the biomechanical properties of the femoral neck and presents how these could potential be clinically assessed through RPI, supplementing existing techniques to improve fracture risk assessment.

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Identifiers

ORCID for Philipp J. Thurner: orcid.org/0000-0001-7588-9041

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