A probabilistic damage model based on direct 3-D correlation of strain to damage formation following fatigue loading of rat femora

Differences in metatarsal structure and mechanical behavior are small in runners with and without acute metatarsal bone stress injury

Purpose

To investigate differences in metatarsal bone structure and training habits in runners with and without a recent metatarsal bone stress injury (BSI).

Methods

Fifty-four runners (14 male/40 female, age 25.8±7.3 yrs) who ran 47±32 kilometers weekly participated in this study. Training and injury history data were collected, along with CT images from metatarsals 2-4 of the non-injured foot of recently injured runners (n=11, 5 male), and the left foot from the healthy runners (n=43, 9 male). Quantitative CT analysis was performed and subject-specific finite element (FE) models simulated a "virtual mechanical test" on each bone at a range of biomechanically relevant angles. Key FE outcomes included principal strains and a measure of total damaged volume, which is related to fatigue life.

Results

Injured runners reported significantly higher training volume (78.9±33.9 km/week) than healthy runners (39.2±20.2 km/week) and had lower BMI (21.3±1.7 vs. 22.7±2.6 kg/m2) but the groups were otherwise similar. In the female group, injured runners had significantly larger bone volume and BMC, similar bone strains, and significantly higher damaged volume metrics than healthy females. The FE simulations showed that decreasing the loading angle of the metatarsals by 10 degrees was associated with a 22% decrease in strain and damaged volume.

Conclusion

The metatarsals of injured and healthy runners are only slightly different from each other, and there are no obvious structural deficits in the injured runners. Other factors including training volume, footstrike biomechanics, and sex differences may explain BSI in this cohort. Interventions that decrease metatarsal loading angle or magnitude may reduce BSI risk by reducing bone microdamage.

Generating Virtual Bone Scans for the Purpose of Investigating the Effects of Cortical Microstructure

Evaluating the contribution of microstructure to overall bone strength is tricky since it is difficult to control changes to pore structure in human or animal samples. We developed an open-source program that can generate three-dimensional (3D) models of micron-scale cortical bone. These models can be highly customized with a wide array of variable input parameters to allow for generation of samples similar to micro-computed topography scans of cortical bone or with specific geometric features. The program can generate samples with specific desired porosities and minor deviations in pore diameter from human samples: 1.67% (±4.90) using literature values, and 1.36% (±2.39) with optimized values. When coupled with finite element analysis, this open-source program could be a useful tool for evaluating stress distributions caused by microstructural changes.

Zoledronate reduces loading-induced microdamage in cortical ulna of ovariectomized rats

As a daily physiological mechanism in bone, microdamage accumulation dissipates energy and helps to prevent fractures. However, excessive damage accumulation might bring adverse effects to bone mechanical properties, which is especially problematic among the osteoporotic and osteopenic patients treated by bisphosphonates. Some pre-clinical studies in the literature applied forelimb loading models to produce well-controlled microdamage in cortical bone. Ovariectomized animals were also extensively studied to assimilate human conditions of estrogen-related bone loss. In the present study, we combined both experimental models to investigate microdamage accumulation in the context of osteopenia and zoledronate treatment. Three-month-old normal and ovariectomized rats treated by saline or zoledronate underwent controlled compressive loading on their right forelimb to create in vivo microdamage, which was then quantified by barium sulfate contrast-enhanced micro-CT imaging. Weekly in vivo micro-CT scans were taken to evaluate bone (re)modeling and to capture microstructural changes over time. After sacrifice, three-point-bending tests were performed to assess bone mechanical properties. Results show that the zoledronate treatment can reduce cortical microdamage accumulation in ovariectomized rats, which might be explained by the enhancement of several bone structural properties such as ultimate force, yield force, cortical bone area and volume. The rats showed increased bone formation volume and surface after the generation of microdamage, especially for the normal and the ovariectomized groups. Woven bone formation was also observed in loaded ulnae, which was most significant in ovariectomized rats. Although all the rats showed strong correlations between periosteal bone formation and microdamage accumulation, the correlation levels were lower for the zoledronate-treated groups, potentially because of their lower levels of microdamage. The present study provides insights to further investigations of pharmaceutical treatments for osteoporosis and osteopenia. The same experimental concept can be applied in future studies on microdamage and drug testing.

Mechanical fatigue of whole rabbit-tibiae under combined compression-torsional loading is better explained by strained volume than peak strain magnitude

The mechanical fatigue behavior of whole bone is poorly defined, particularly for the combined loading modes that occur in vivo. The purpose of this study was to quantify the fatigue life of whole rabbit-tibiae under cyclic uniaxial compression and biaxial (compression and torsion) loading, and to explore the relationship between fatigue life and specimen-specific finite element (FE) predictions of stress/strain. Twelve tibiae were tested cyclically until failure across a range of uniaxial-compressive loads. Another twenty-two tibiae were separated into three groups and loaded biaxially; peak compressive load was constant in all three groups (50% ultimate force) but torsion was varied (0%, 25%, or 50% of ultimate torque). FE models with heterogeneous linear-elastic material properties were developed from computed tomography. We assessed peak stress/strain and stressed/strained volume based on principal stress/strain, as well as von Mises and pressure modified von Mises criteria. A logarithmic (r² = 0.68; p < 0.001) relationship was observed between uniaxial-compressive load and fatigue life. Biaxial tests demonstrated that fatigue life decreased with superposed torsion (p = 0.034). Strained volume, based on a maximum principal strain or pressure modified von Mises strain criteria, were strong predictors of fatigue life under both uniaxial (r² = 0.73-0.82) and biaxial (r² = 0.59-0.60) loads, and these outperformed equivalent peak stress- and strain-based measures. Our findings highlight the importance of evaluating strain distributions, rather than peak stress or strain, to predict the fatigue behavior or whole bone, which has important implications for the study of stress fracture.

Crack propagation in cortical bone is affected by the characteristics of the cement line: a parameter study using an XFEM interface damage model

Bulk properties of cortical bone have been well characterized experimentally, and potent toughening mechanisms, e.g., crack deflections, have been identified at the microscale. However, it is currently difficult to experimentally measure local damage properties and isolate their effect on the tissue fracture resistance. Instead, computer models can be used to analyze the impact of local characteristics and structures, but material parameters required in computer models are not well established. The aim of this study was therefore to identify the material parameters that are important for crack propagation in cortical bone and to elucidate what parameters need to be better defined experimentally. A comprehensive material parameter study was performed using an XFEM interface damage model in 2D to simulate crack propagation around an osteon at the microscale. The importance of 14 factors (material parameters) on four different outcome criteria (maximum force, fracture energy, crack length and crack trajectory) was evaluated using ANOVA for three different osteon orientations. The results identified factors related to the cement line to influence the crack propagation, where the interface strength was important for the ability to deflect cracks. Crack deflection was also favored by low interface stiffness. However, the cement line properties are not well determined experimentally and need to be better characterized. The matrix and osteon stiffness had no or low impact on the crack pattern. Furthermore, the results illustrated how reduced matrix toughness promoted crack penetration of the cement line. This effect is highly relevant for the understanding of the influence of aging on crack propagation and fracture resistance in cortical bone.

The fracture toughness of small animal cortical bone measured using arc-shaped tension specimens: Effects of bisphosphonate and deproteinization treatments

Small animal models, and especially transgenic models, have become widespread in the study of bone mechanobiology and metabolic bone disease, but test methods for measuring fracture toughness on multiple replicates or at multiple locations within a single small animal bone are lacking. Therefore, the objective of this study was to develop a method to measure cortical bone fracture toughness in multiple specimens and locations along the diaphysis of small animal bones. Arc-shaped tension specimens were prepared from the mid-diaphysis of rabbit ulnae and loaded to failure to measure the radial fracture toughness in multiple replicates per bone. The test specimen dimensions, crack length, and maximum load met requirements for measuring the plane strain fracture toughness. Experimental groups included a control group, bisphosphonate treatment group, and an ex vivo deproteinization treatment following bisphosphonate treatment (5 rabbits/group and 15 specimens/group). The fracture toughness of ulnar cortical bone from rabbits treated with zoledronic acid for six months exhibited no difference compared with the control group. Partially deproteinized specimens exhibited significantly lower fracture toughness compared with both the control and bisphosphonate treatment groups. The deproteinization treatment increased tissue mineral density (TMD) and resulted in a negative linear correlation between the measured fracture toughness and TMD. Fracture toughness measurements were repeatable with a coefficient of variation of 12-16% within experimental groups. Retrospective power analysis of the control and deproteinization treatment groups indicated a minimum detectable difference of 0.1MPa·m^1/2. Therefore, the overall results of this study suggest that arc-shaped tension specimens offer an advantageous new method for measuring the fracture toughness in small animal bones.

Microcomputed tomography: approaches and applications in bioengineering

Microcomputed tomography (microCT) has become a standard and essential tool for quantifying structure-function relationships, disease progression, and regeneration in preclinical models and has facilitated numerous scientific and bioengineering advancements over the past 30 years. In this article, we recount the early events that led to the initial development of microCT and review microCT approaches for quantitative evaluation of bone, cartilage, and cardiovascular structures, with applications in fundamental structure-function analysis, disease, tissue engineering, and numerical modeling. Finally, we address several next-generation approaches under active investigation to improve spatial resolution, acquisition time, tissue contrast, radiation dose, and functional and molecular information.