An experimental investigation on the biomechanics of bone fragility in type 2diabetes

Type 2 diabetic patients experience up to a 3-fold increase in bone fracture risk. Paradoxically, type 2 diabetes is associated with a normal or increased bone mineral density (BMD) when compared to non-diabetic patients. The current leading hypothesis is that the hyperglycaemic state leads to non-enzymatic glycation in collagen causing the formation of crosslinks, known as Advanced Glycation End Products (AGEs), stiffening the overall collagen network leading to more brittle behaviour. While the relationship between AGE accumulation and bone biomechanics has been widely suggested, a causal relationship has not yet been established, suggesting that other tissue-level mechanisms may be responsible for fragility. The objective of this thesis is to investigate the biomechanics of type 2 diabetic bone fragility through a multiscale experimental strategy that considers structural, mechanical and compositional features of in vitro and ex vivo human tissue samples. Initially, an in vitro glycation model was used to simulate diabetic conditions in twenty anatomically adjacent pairs of cortical bone from a single bovine femur. Mechanical characterisation was carried out using 3-point bend, fracture toughness, and nanoindentation testing, while bone composition was analysed by quantifying the accumulation of fluorescent AGEs. A study was also carried out on human trabecular and cortical bone tissue, obtained from femoral heads of patients undergoing total hip replacement, to evaluate the effect of type 2 diabetes on bone biomechanics. Mechanical testing was carried out on isolated trabecular cores using monotonic and cyclic compression loading and nanoindentation experiments, with bone microdamage analysed using micro-computed tomography (micro-CT) imaging. Bone composition was evaluated using Raman spectroscopy, high-performance liquid chromatography, fluorometric spectroscopy. Finally, the effect of type 2 diabetes on the trabecular microarchitecture in the femoral head was also evaluated through a macro- and micro-regional analysis of micro-CT based images. It was found that AGEs were not detrimental to the mechanical properties of bone tissue, with AGE accumulation actually found to enhance several pre- and post-yield properties of the in vitro glycated bovine tissue. It was also found that human type 2 diabetic bone had altered mechanical, compositional, and morphological properties compared to non-type 2 diabetic bone. High-resolution (10μm) micro-CT imaging showed that cores taken from the central trabecular region of femoral head had higher bone mineral density, bone volume, trabecular thickness and reduced trabecular separation. These samples of human type 2 diabetic bone also had enhanced macro-mechanical compressive and fatigue properties, with many significant differences remaining even when normalised against the bone volume. Using nanoindentation, tissue-level mechanical properties of cortical and trabecular bone was unchanged in type 2 diabetic samples compared to controls. Through compositional analysis, higher levels of furosine were found in type 2 diabetic trabecular bone and an increase in both furosine and carboxymethyl-lysine (an AGE) was found in cortical bone. Raman spectroscopy showed that type 2 bone had a higher mineral-to-matrix ratio, carbonate substitution and reduced crystallinity compared to the controls. Finally, regional differences within micro regions of the femoral head of type 2 diabetic samples compared to non-type 2 diabetic samples were found, along with regional differences within each macro region within each group were found. In conclusion, this thesis shows that type 2 diabetes leads to distinct changes in both organic and mineral phases of the bone tissue matrix, but these changes did not coincide with any reduction in the mechanical properties of the tissue under either monotonic or cyclic loading. While this enhances the current understanding of type 2 diabetic bone, this thesis provides no evidence that AGE accumulation is responsible for diabetic bone fragility and further investigations are required to elucidate the mechanisms responsible for bone fragility in type 2 diabetes.

Publisher

NUI Galway

Rights

Attribution-NonCommercial-NoDerivs 3.0 Ireland
CC BY-NC-ND 3.0 IE

cbn

Collections

University of Galway Theses (PhD Theses)