Contributions of Material Properties and Structure to Increased Bone Fragility for a Given Bone Mass in the UCD-T2DM Rat Model of Type 2 Diabetes

Abstract

Adults with type 2 diabetes (T2D) have a higher fracture risk for a given bone quantity, but the mechanisms remain unclear. Using a rat model of polygenic obese T2D, we demonstrate that diabetes significantly reduces whole-bone strength for a given bone mass (μCT-derived BMC), and we quantify the roles of T2D-induced deficits in material properties versus bone structure; ie, geometry and microarchitecture. Lumbar vertebrae and ulnae were harvested from 6-month-old lean Sprague-Dawley rats, obese Sprague-Dawley rats, and diabetic obese UCD-T2DM rats (diabetic for 69 ± 7 days; blood glucose >200 mg/dL). Both obese rats and those with diabetes had reduced whole-bone strength for a given BMC. In obese rats, this was attributable to structural deficits, whereas in UCD-T2DM rats, this was attributable to structural deficits and to deficits in tissue material properties. For the vertebra, deficits in bone structure included thinner and more rod-like trabeculae; for the ulnae, these deficits included inefficient distribution of bone mass to resist bending. Deficits in ulnar material properties in UCD-T2DM rats were associated with increased non-enzymatic crosslinking and impaired collagen fibril deformation. Specifically, small-angle X-ray scattering revealed that diabetes reduced collagen fibril ultimate strain by 40%, and those changes coincided with significant reductions in the elastic, yield, and ultimate tensile properties of the bone tissue. Importantly, the biomechanical effects of these material property deficits were substantial. Prescribing diabetes-specific tissue yield strains in high-resolution finite element models reduced whole-bone strength by a similar amount (and in some cases a 3.4-fold greater amount) as the structural deficits. These findings provide insight into factors that increase bone fragility for a given bone mass in T2D; not only does diabetes associate with less biomechanically efficient bone structure, but diabetes also reduces tissue ductility by limiting collagen fibril deformation, and in doing so, reduces the maximum load capacity of the bone. © 2018 American Society for Bone and Mineral Research.

Keywords: BIOMECHANICS; BONE μCT; COLLAGEN; METABOLISM; PRECLINICAL STUDIES.

Figure 1

L4 vertebrae (without endplates) and right ulnae (15-mm long region from the mid-shaft) from lean control rats, obese non-diabetic rats, and diabetic obese rats. Bones were scanned with micro-CT prior to mechanical testing, and the scans were then used to create high-resolution finite element models. Mineral density-shaded cross-sections from obese rats and those with diabetes illustrate differences in bone geometry and trabecular microarchitecture compared to those from lean controls.

Figure 2

Obese rats and those with diabetes showed significant reductions in whole-bone biomechanical properties per unit bone mass, BMC. Vertebral properties (A-C) were measured in compression; unlar mid-shaft properties (D-F) were measured in 3-point bending. Data are mean ± SEM for n = 5–6 rats/group. ^a p < 0.01 vs. control; ^b p < 0.01 vs. obese; ^c p < 0.05 vs. control; ^d p < 0.05 vs. obese

Figure 3

Collagen fibril deformation during ulna tensile testing was measured by SAXS. Diabetes reduced fibril strain, particularly at tissue strains above 0.5% (A) and culminating with a 40% reduction in fibril ultimate strain (B). Reductions in fibril strain coincided with a 27% increase in AGEs (C). Data are mean ± SEM for n = 4–5 rats/group. ^a p < 0.05 vs. control

Figure 4

High-resolution finite element analysis of the vertebrae (A & B) and ulnae (C & D) was used to estimate the relative roles of diabetes-induced deficits in bone geometry and architecture vs. material properties. Vertebral behavior was evaluated in compression; unlar mid-shaft behavior was evaluated in 3-point bending. For the elastic biomechanical properties (A & C), models of the bones from lean control rats and those with diabetes were assigned either the average tissue modulus of the bones in the control group or the specimen-specific tissue modulus derived from the micro-CT-based measurements of tissue mineral density. For the biomechanical properties at yield (B & D), models of the bones were assigned either the average tissue modulus and tissue yield strain of the bones in the control group or the specimen-specific tissue modulus and yield strain measured from the SAXS experiments.