An experimental investigation on the biomechanics of bone fragility in type 2diabetes
Date
2023-10-11Author
Britton, Marissa
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Abstract
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.