The effect of surgical revascularization on the mechanical properties of cryopreserved bone allograft in a porcine tibia model

Cryopreserved bone allografts(CBA) are susceptible to infection, nonunion, and late stress fracture. Although surgical revascularization by intramedullary implantation of an arteriovenous bundle (AV bundle) generates a neoangiogenic blood supply, there is potential for vascular ingrowth-mediated bone resorption to weaken the graft. For this reason, we have evaluated changes in CBA mechanical properties of structural tibial allografts with and without surgically induced angiogenesis. Cryopreserved tibia bone allografts were transplanted to reconstruct a 3.5 cm segmental tibial defect in 16 Yucatan mini pigs. Surgical revascularization was performed in half by implantation of a cranial tibial AV bundle, (revascularization group). A control group of identical size had a ligated AV bundle implanted, (ligated group). At 20 weeks micro-computed tomography (CT) measured bone mineral density (BMD) as well as bone union. Reference point indentation (RPI) compared cortex material properties, and axial compression determined the allotransplant compressive modulus. Seven of eight tibiae in the angiogenesis group were healed at both junction points at 20 weeks. Only four of eight tibiae healed in the ligated control group. There was no significant difference between the revascularization and ligated control groups in BMD and axial compression test. Similarly, RPI parameters were statistically equal. In paired comparisons with contralateral tibias, however, some RPI values were significantly worse in the ligated control group tibiae. This study demonstrates no adverse effect of surgical angiogenesis on cryopreserved structural bone allograft biomechanical properties in a large animal orthotopic segmental tibial defect model. These data suggest the potential value of surgical angiogenesis in clinical limb-sparing reconstructive surgery.

Determination of the Depth- and Time- Dependent Mechanical Behavior of Mouse Articular Cartilage Using Cyclic Reference Point Indentation

Mouse models of osteoarthritis and cartilage degeneration are important and powerful tools for investigating the molecular mechanisms of the disease pathology. Because of the vast number of genetically modified mouse models that are available for research, the ability to use these models is particularly attractive for the mechanobiologic interactions in the pathogenesis of osteoarthritis. However, the very small scale of mouse articular cartilage, where the healthy tissue is only 80 µm in thickness, poses challenges in quantifying mechanical characteristics of the tissue. We introduce here a novel approach that combines experimental and analytical methods to quantify the nuanced mechanical changes during cartilage degeneration at this scale. Cyclic reference point indentation is used to directly test the murine articular cartilage to obtain the force-deformation and the phase-shift characteristics of the tissue. The cartilage zonal thicknesses are confirmed from histology. These data are then fitted to a parallel spring model to determine the depth-dependent tissue stiffness and modulus. Using this approach, we investigated the effects of trypsin degradation on the zonal mechanical behavior of mouse articular cartilage. We observe a decline of the superficial zone stiffness coupled with the loss of the superficial layer. Subsequent degradation by trypsin allowed the identification of middle- and deep- zone properties. Taken together, this approach can be a useful tool for understanding the disease mechanisms of cartilage homeostasis and degeneration, and for monitoring of therapies for osteoarthritis.

Microhardness distribution of the tibial diaphysis and test site selection for reference point indentation technique

Indentation hardness test is a good in vitro method of bone quality assessment. The purpose of this study is to explore the distribution characteristics of bone tissue microhardness in tibial diaphysis and provide theoretical support for the test site selection of the reference point indentation technique.Three fresh right tibias were obtained from 3 cadaver donors. The tibial diaphysis was evenly divided into 6 sections. Bone specimens with a thickness of 3 mm were cut from each part. After appropriate management, micro-indentation tests were performed in various regions of the specimens to acquire the microhardness values of the tibial diaphysis. Statistical analysis was performed by randomized block design variance analysis to study the distribution characteristics of bone microhardness.72 regions were selected for 360 effective indentations. We found that the bone microhardness is inhomogeneous in tibia diaphysis. Mean hardness value of the anterior, medial, posterior, lateral region of tibia diaphysis was 45.58 ± 4.39 Vickers hardness (HV), 52.33 ± 3.93 HV, 54.00 ± 4.21 HV, 52.89 ± 4.44 HV, respectively. The anterior cortex exhibits lower microhardness value than the other regions (P < .001). Within the same region, microhardness varies significantly with positions in the tibial diaphysis. The variations in indentation hardness are bound to have a significant impact on the comparability of different reference point indentation (RPI) studies.The results of this study indicated the regional microhardness difference in the human tibia diaphysis. The microhardness of different planes in the same region is also inconsistent. Inhomogeneous distribution of indentation microhardness would have considerable influence in the test site selection of RPI technique. The data collected in our study would contribute to the design of highly precise 3D printing implants and bionic bones with gradient elastic modulus.

Clinical measurements of bone tissue mechanical behavior using reference point indentation

Over the last thirty years, it has become increasingly clear the amount of bone (e.g. 'bone quantity') and the quality of the bone matrix (e.g. 'bone quality') both critically contribute to bone's tissue-level mechanical behavior and the subsequent ability of bone to resist fracture. Although determining the tissue-level mechanical behavior of bone through mechanical testing is relatively straightforward in the laboratory, the destructive nature of such testing is unfeasible in humans and in animal models requiring longitudinal observation. Therefore, surrogate measurements are necessary for quantifying tissue-level mechanical behavior for the pre-clinical and clinical evaluation of bone strength and fracture risk in vivo. A specific implementation of indentation known as reference point indentation (RPI) enables the mechanical testing of bone tissue without the need to excise and prepare the bone surface. However, this compromises the ability to carefully control the specimen geometry that is required to define the bone tissue material properties. Yet the versatility of such measurements in clinical populations is provocative, and to date there are a number of promising studies that have utilized this tool to discern bone pathologies and to monitor the effects of therapeutics on bone quality. Concurrently, on-going efforts continue to investigate the aspects of bone material behavior measured by RPI, and the compositional factors that contribute to these measurements. There are currently two variants, cyclic- and impact- RPI, that have been utilized in pre-clinical and clinical studies. This review surveys clinical studies that utilize RPI, with particular emphasis on the clinical instrument, as well as the endeavors to understand the fundamental mechanisms of such measurements. Ultimately, an improved awareness in the tradeoffs and limitations of in vivo RPI is critical towards the effective and successful utilization of this tool for the overall improvement of fragility determination in the clinic.

Differences in sensitivity to microstructure between cyclic- and impact-based microindentation of human cortical bone

Unlike the known relationships between traditional mechanical properties and microstructural features of bone, the factors that influence the mechanical resistance of bone to cyclic reference point microindention (cRPI) and impact microindention (IMI) have yet to be identified. To determine whether cRPI and IMI properties depend on microstructure, we indented the tibia mid-shaft, the distal radius, and the proximal humerus from 10 elderly donors using the BioDent and OsteoProbe (neighboring sites). As the only output measure of IMI, bone material strength index (BMSi) was significantly different across all three anatomical sites being highest for the tibia mid-shaft and lowest for the proximal humerus. Total indentation distance (inverse of BMSi) was higher for the proximal humerus than for the tibia mid-shaft but was not different between other anatomical comparisons. As a possible explanation for the differences in BMSi, pore water, as determined by ¹ H nuclear magnetic resonance, was lowest for the tibia and highest for the humerus. Moreover, the local intra-cortical porosity, as determined by micro-computed tomography, was negatively correlated with BMSi for both arm bones. BMSi was also positively correlated with peak bending stress of cortical bone extracted from the tibia mid-shaft. Microstructural correlations with cRPI properties were not significant for any of the bones. The one exception was that average energy dissipated during cRPI was negatively correlated with local tissue mineral density in the tibia mid-shaft. With higher indentation force and larger tip diameter than cRPI, only IMI appears to be sensitive to the underlying porosity of cortical bone. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1442-1452, 2017.

Bone quality assessment techniques: geometric, compositional, and mechanical characterization from macroscale to nanoscale

This review presents an overview of the characterization techniques available to experimentally evaluate bone quality, defined as the geometric and material factors that contribute to fracture resistance independently of areal bone mineral density (aBMD) assessed by dual energy x-ray absorptiometry. The methods available for characterization of the geometric, compositional, and mechanical properties of bone across multiple length scales are summarized, along with their outcomes and their advantages and disadvantages. Examples of how each technique is used are discussed, as well as practical concerns such as sample preparation and whether or not each testing method is destructive. Techniques that can be used in vivo and those that have been recently improved or developed are emphasized, including high resolution peripheral quantitative computed tomography to evaluate geometric properties and reference point indentation to evaluate material properties. Because no single method can completely characterize bone quality, we provide a framework for how multiple characterization methods can be used together to generate a more comprehensive analysis of bone quality to complement aBMD in fracture risk assessment.

Assessing the inter- and intra-animal variability of in vivo OsteoProbe skeletal measures in untreated dogs

The OsteoProbe is a second-generation reference point indentation (RPI) device without a reference probe that is designed to simplify RPI testing for clinical use. Successful clinical implementation of the OsteoProbe would benefit from a better understanding of how its output, bone material strength index (BMSi), relates to the material properties of bone and under what conditions it reliably correlates with fracture risk. Large animal models have the potential to help fill this knowledge gap, as cadaveric studies are retrospective and limited by incomplete patient histories (including the potential use of bone matrix altering drugs such as bisphosphonates). The goal of this study was to assess the intra and inter-animal variability of OsteoProbe measures in untreated beagle dogs (n = 12), and to evaluate this variability in comparison to traditional mechanical testing. OsteoProbe measurements were performed in vivo on the left tibia of each dog and repeated 6 months later on the day of sacrifice. Within-animal variation of BMSi (CV of 5–10 indents) averaged 8.9 and 9.0% at the first and second timepoints, respectively. In contrast, inter-animal variation of BMSi increased from 5.3% to 9.1%. The group variation of BMSi was on par with that of traditional 3-point mechanical testing; inter-animal variation was 10% for ultimate force, 13% for stiffness, and 12% for total work as measured on the femur. There was no significant change in mean BMSi after 6 months, but the individual change with time across the 12 dogs was highly variable, ranging from − 12.4% to + 21.7% (mean 1.6%, SD 10.6%). No significant correlations were found between in vivo tibia BMSi and femur mechanical properties measured by ex vivo 3-pt bending, but this may be a limitation of sample size or the tests being performed on different bones. No relationship was found between BMSi and tissue mineral density, but a strong positive correlation was found between BMSi and tibia cortical thickness (ρ = 0.706, p = 0.010). This report shows that while the OsteoProbe device has inter-individual variability quite similar to that of traditional mechanical testing, the longitudinal changes show high levels of heterogeneity across subjects. We further highlight the need for standardization in post-testing data processing and further study of the relationships between OsteoProbe and traditional mechanical testing.

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OsteoProbe bone material strength index (BMSi) was assessed in female beagle dogs.

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The inter-subject variance of BMSi was comparable to traditional mechanical tests.

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Longitudinal changes in BMSi over 6 months were heterogeneous across subjects.

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Femur mechanical properties measured by bending did not correlate with tibia BMSi.

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The need for standardization of OsteoProbe post-test data processing is discussed.

The use of traditional and novel techniques to determine the hardness and indentation properties of immature radicular dentin treated with antibiotic medicaments followed by ethylenediaminetetraacetic acid

Objective:

The objective was to investigate the effect of intracanal antibiotic medicaments followed by ethylenediaminetetraacetic acid (EDTA) on the indentation properties and hardness of radicular dentin using a BioDent reference point indenter and a traditional microhardness technique, respectively.

Materials and Methods:

Specimens with intact root canal dentin surfaces and polished radicular dentin specimens were obtained from immature human premolars. Each type of specimen was randomly assigned (n = 10 per group) and treated with either double antibiotic paste (DAP) for 4-week followed by EDTA for 5 min, triple antibiotic paste (TAP) for 4-week followed by EDTA for 5 min, EDTA for 5 min or Hank's balanced salt solution (control). The BioDent reference point indentor and Vickers microhardness tester were used to measure the indentation properties of root canal surfaces and the hardness of polished dentin specimens, respectively. One-way ANOVA followed by Fisher's protected least significant differences were used for statistical analyses.

Results:

Both types of radicular dentin treated with antibiotic pastes and/or EDTA had a significant increase in the majority of indentation properties and a significant reduction in hardness compared to the untreated dentin. Furthermore, treatment of dentin with antibiotic pastes and EDTA caused significant increases in indentation properties and a significant reduction in hardness compared to EDTA-treated dentin. However, the RPI technique was not able to significantly differentiate between DAP + EDTA and TAP + EDTA-treated dentin.

Conclusion:

Dentin treated with antibiotic medicaments followed by EDTA had a significant increase the indentation properties and significantly reduction in hardness of radicular dentin.

Novel assessment tools for osteoporosis diagnosis and treatment

This review describes new technologies for the diagnosis and treatment, including fracture risk prediction, of postmenopausal osteoporosis. Four promising technologies and their potential for clinical translation and basic science studies are discussed. These include reference point indentation (RPI), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and magnetic resonance imaging (MRI). While each modality exploits different physical principles, the commonality is that none of them require use of ionizing radiation. To provide context for the new developments, brief summaries are provided for the current state of biomarker assays, fracture risk assessment (FRAX), and other fracture risk prediction algorithms and quantitative ultrasound (QUS) measurements.

Multi-scale analysis of bone chemistry, morphology and mechanics in the oim model of osteogenesis imperfecta

Osteogenesis imperfecta is a congenital disease commonly characterized by brittle bones and caused by mutations in the genes encoding Type I collagen, the single most abundant protein produced by the body. The oim model has a natural collagen mutation, converting its heterotrimeric structure (two α1 and one α2 chains) into α1 homotrimers. This mutation in collagen may impact formation of the mineral, creating a brittle bone phenotype in animals. Femurs from male wild type (WT) and homozygous (oim/oim) mice, all at 12 weeks of age, were assessed using assays at multiple length scales with minimal sample processing to ensure a near-physiological state. Atomic force microscopy (AFM) demonstrated detectable differences in the organization of collagen at the nanoscale that may partially contribute to alterations in material and structural behavior obtained through mechanical testing and reference point indentation (RPI). Changes in geometric and chemical structure obtained from µ-Computed Tomography and Raman spectroscopy indicate a smaller bone with reduced trabecular architecture and altered chemical composition. Decreased tissue material properties in oim/oim mice are likely driven by changes in collagen fibril structure, decreasing space available for mineral nucleation and growth, as supported by a reduction in mineral crystallinity. Multi-scale analyses of this nature offer much in assessing how molecular changes compound to create a degraded, brittle bone phenotype.