The micro-structure of bone trabecular fracture: an inter-site study

High-resolution local trabecular strain within trabecular structure under cyclic loading

Trabecular bone structure is a complex microstructure consisting of rods and plates, which poses challenges for its mechanical characterization. Digital image correlation (DIC) offers the possibility to characterize the strain response on the surface of trabecular bone. This study employed DIC equipped with a telecentric lens to investigate the strain state of individual trabeculae within their trabecular structure by assessing the longitudinal strain of the trabeculae at both the middle and near the edges of the trabeculae. Due to the high-resolution of the used DIC system, local surface strain of trabeculae was analyzed too. Lastly, the correlation between longitudinal trabecular strain and the orientation and slenderness of the trabeculae was investigated. The results showed that the strain magnification close to the edge of the trabeculae was higher and reached up to 8-folds the strain along the middle of the trabeculae. On the contrary, no strain magnification was found for most of the trabeculae between the longitudinal trabecular strain along the middle of the trabeculae and the globally applied strain. High-resolution full-field strain maps were obtained on the surface of trabeculae showing heterogeneous strain distribution with increasing load. No significant correlation was found between longitudinal trabecular strain and its orientation or slenderness. These findings and the applied methodology can be used to broaden our understanding of the deformation mechanisms of trabeculae within the trabecular network.

Full-field experimental analysis of the influence of microstructural parameters on the mechanical properties of humeral head trabecular bone

Understanding the mechanical properties of trabecular bone within the metaphysis of the proximal humerus is becoming increasingly important for the design of humeral head joint replacement components that prioritize bone preservation. The aim of this study was to perform full-field mechanical testing methods on isolated trabecular bone cores from the humeral head to experimentally measure the local magnitude of strain before macroscopic failure and to characterize the ultimate strength of each core. Isolated cubic trabecular bone cores were extracted from the center of humeral head osteotomies retrieved from (1) patients with end-stage osteoarthritis (OA) undergoing total shoulder arthroplasty (TSA) and (2) normal nonpathologic cadaveric humeral heads. A custom computed tomography (CT)-compatible loading device was used to perform compressive mechanical testing. For 10 of the OA specimens, stepwise loading was performed directly within a microCT scanner and digital volume correlation (DVC) was used to measure full-field strains throughout the trabecular structure. A higher variability in ultimate strength was measured for the trabecular cores retrieved from OA humeral heads (range: 2.8-7.6 MPa) compared to the normal cadaveric humeral heads (range: 2.2-5.4 MPa), but no statistically significant difference between the groups was found (p = 0.06). Ultimate strength was strongly correlated with bone volume fraction (OA r²  = 0.72; normal r²  = 0.76) and bone mineral content (OA r²  = 0.79; normal r²  = 0.77). At the trabecular level, 95th percentile of third principal strains, measured at a subvolume size of 152 µm, exceeded 19,000 µε for each of the 10 specimens (range: -19,551 to -36,535 µε) before macroscopic failure of the cores occured. No strong linear correlations (r²  ≥ 0.50) were found between the median or 95th percentile of DVC third principal strain and the corresponding morphometric parameters of each individual bone core. The results of this study indicate that bone volume fraction and bone mineral content heavily influence the apparent ultimate strength of trabecular bone cores collected from OA patients undergoing TSA. Clinical significance: The strong correlations observed within this study further emphasize the importance of considering bone mineral content or bone volume fraction measurements in assessing the localized risk of trabecular bone fracture for orthopedic applications.

Morphobiochemical diagnosis of acute trabecular microfractures using gamma correction Tc-99m HDP pinhole bone scan with histopathological verification

We prospectively performed gamma correction pinhole bone scan (GCPBS) and histopathologic verification study to make simultaneous morphobiochemical diagnosis of trabecular microfractures (TMF) occurred in the femoral head as a part of femoral neck fracture.Materials consisted of surgical specimens of the femoral head in 6 consecutive patients. The specimens were imaged using Tc-99m hydroxymethylene diphosphonate (HDP) pinhole scan and processed by the gamma correction. After cleansing with 10% formalin solution, injured specimen surface was observed using a surgical microscope to record TMF. Morphological findings shown in the photograph, naive pinhole bone scan, GCPBS, and hematoxylin-eosin (H&E) stain of the specimen were reciprocally correlated for histological verification and the usefulness of suppression and enhancement of Tc-99m HDP uptake was biochemically investigated in TMF and edema and hemorrhage using gamma correction.On the one hand, GCPBS was able to depict the calcifying calluses in TMF with enhanced Tc-99m HDP uptake. They were pinpointed, speckled, round, ovoid, rod-like, geographic, and crushed in shape. The smallest callus measured was 0.23 mm in this series. On the other hand, GCPBS biochemically was able to discern the calluses with enhanced high Tc-99m HDP uptake from the normal and edema dipped and hemorrhage irritated trabeculae with washed out uptake.Morphobiochemically, GCPBS can clearly depict microfractures in the femoral head produced by femoral neck fracture. It discerns the microcalluses with enhanced Tc-99m HDP uptake from the intact and edema dipped and hemorrhage irritated trabeculae with suppressed washed out Tc-99m HDP uptake. Both conventional pinhole bone scan and gamma correction are useful imaging means to specifically diagnose the microcalluses naturally formed in TMF.

An Independent Active Contours Segmentation framework for bone micro-CT images

Micro-CT is an imaging technique for small tissues and objects that is gaining increased popularity especially as a pre-clinical application. Nevertheless, there is no well-established micro-CT segmentation method, while typical procedures lack sophistication and frequently require a degree of manual intervention, leading to errors and subjective results. To address these issues, a novel segmentation framework, called Independent Active Contours Segmentation (IACS), is proposed in this paper. The proposed IACS is based on two autonomous modules, namely automatic ROI extraction and IAC Evolution, which segments the ROI image using multiple Active Contours that evolve simultaneously and independently of one another. The proposed method is applied on a Phantom dataset and on real datasets. It is tested against several established segmentation methods that include Adaptive Thresholding, Otsu Thresholding, Region Growing, Chan-Vese (CV) AC, Geodesic AC and Automatic Local Ratio-CV AC, both qualitatively and quantitatively. The results prove its superior performance in terms of object identification capability, accuracy and robustness, under normal circumstances and under four types of artificially introduced noise. These enhancements can lead to more reliable analysis, better diagnostic procedures and treatment evaluation of several bone-related pathologies, and to the facilitation and further advancement of bone research.

A New Ensemble Classification System For Fracture Zone Prediction Using Imbalanced Micro-CT Bone Morphometrical Data

Trabecular bone fractures constitute a major health issue for the modern societies, with the currently established prediction methods of fracture risk, such as bone mineral density (BMD), resulting in errors up to 40%. Fracture-zone prediction based on bone's microstructure has been recently proposed as an alternative prediction method of fracture risk. In this paper, a classification system (CS) for the automatic fracture-zone prediction based on an Ensemble of Imbalanced Learning methods is proposed, following the observation that the percentage of the actual fractured bone area is significantly smaller than the intact bone in the case of a fracture event. The sample is divided into Volumes of Interest (VOIs) of specific size and 29 morphometrical parameters are calculated from each VOI, which serve as input features for the CS in order for it to separate the input patterns in to two classes: fractured and nonfractured. To this end, two well-established Imbalanced Learning methods, namely Random Undersampling and Synthetic Minority Oversampling, and two popular classification algorithms, namely Multilayer Perceptrons and Support Vector Machines, are tested and combined accordingly, to provide the best possible performance on a dataset that contains 45 specimens' pre- and postfailure scans. The best combination is then compared with three well-established Ensembles of Imbalanced Learning methods, namely RUSBoost, UnderBagging and SMOTEBagging. The experimental results clearly show that the proposed CS outperforms the competition, scoring in some occasions more than 90% in G-Mean and Area under Curve metrics. Finally, an investigation on the significance of the various trabecular bone's biomechanical parameters is made using the sequential forward floating selection technique, in order to identify possible biomarkers for fracture-zone prediction.

Micro-Finite Element analysis will overestimate the compressive stiffness of fractured cancellous bone

Recently, micro-Finite Element (micro-FE) analysis based on High Resolution peripheral Quantitative CT (HRpQCT) images was introduced to quantify the state of fracture healing (de Jong et al., 2014). That study suggested that the direct post-fracture stiffness may be overestimated by micro-FE. The aim of this study was to investigate this further by measuring the loss in stiffness of cancellous bone samples under compressive loading and to compare this with predictions based on micro-FE analyses and bone microstructural and fracture morphology. Sixty porcine trabecular cores were micro-CT scanned and tested in compression before and after inducing a fracture in 4 different manners. The loss in stiffness as measured in the experiment was compared to that calculated from micro-FE analysis. Additionally, bone morphology parameters and fracture thickness were calculated. The experimentally measured loss in stiffness ranged from 37% to 80%. The losses calculated from the micro-FE analyses were lower and ranged from 36% to 61%, while in one case an increase in stiffness was calculated. For 2 of the 4 experiments, the results of the experiment and micro-FE analyses were significantly different. Only for very smooth fractures good agreement was obtained between FE and experimental results. The loss in stiffness did not correlate with any investigated bone morphology parameter or the thickness of the fracture region. It was concluded that micro-FE analysis can severely overestimate the stiffness of fractured bone depending on the type of fracture, but in the case of smooth fractures good estimates are possible.

Inhibition of CaMKK2 reverses age-associated decline in bone mass

Decline in bone formation is a major contributing factor to the loss of bone mass associated with aging. We previously showed that the genetic ablation of the tissue-restricted and multifunctional Ca(2+)/calmodulin (CaM)-dependent protein kinase kinase 2 (CaMKK2) stimulates trabecular bone mass accrual, mainly by promoting anabolic pathways and inhibiting catabolic pathways of bone remodeling. In this study, we investigated whether inhibition of this kinase using its selective cell-permeable inhibitor STO-609 will stimulate bone formation in 32 week old male WT mice and reverse age-associated of decline in bone volume and strength. Tri-weekly intraperitoneal injections of saline or STO-609 (10 μM) were performed for six weeks followed by metabolic labeling with calcein and alizarin red. New bone formation was assessed by dynamic histomorphometry whereas micro-computed tomography was employed to measure trabecular bone volume, microarchitecture and femoral mid-shaft geometry. Cortical and trabecular bone biomechanical properties were assessed using three-point bending and punch compression methods respectively. Our results reveal that as they progress from 12 to 32 weeks of age, WT mice sustain a significant decline in trabecular bone volume, microarchitecture and strength as well as cortical bone strength. However, treatment of the 32 week old WT mice with STO-609 stimulated apposition of new bone and completely reversed the age-associated decrease in bone volume, quality, as well as trabecular and cortical bone strength. We also observed that regardless of age, male Camkk2(-/-) mice possessed significantly elevated trabecular bone volume, microarchitecture and compressive strength as well as cortical bone strength compared to age-matched WT mice, implying that the chronic loss of this kinase attenuates age-associated decline in bone mass. Further, whereas STO-609 treatment and/or the absence of CaMKK2 significantly enhanced the femoral mid-shaft geometry, the mid-shaft cortical wall thickness and material bending stress remained similar among the cohorts, implying that regardless of treatment, the material properties of the bone remain similar. Thus, our cumulative results provide evidence for the pharmacological inhibition of CaMKK2 as a bone anabolic strategy in combating age-associated osteoporosis.

Using Micro-CT Derived Bone Microarchitecture to Analyze Bone Stiffness - A Case Study on Osteoporosis Rat Bone

Micro-computed tomography (Micro-CT) images can be used to quantitatively represent bone geometry through a range of computed attenuation-based parameters. Nonetheless, those parameters remain indirect indices of bone microarchitectural strength and require further computational tools to interpret bone structural stiffness and potential for mechanical failure. Finite element analysis (FEA) can be applied to measure trabecular bone stiffness and potentially predict the location of structural failure in preclinical animal models of osteoporosis, although that procedure from image segmentation of Micro-CT derived bone geometry to FEA is often challenging and computationally expensive, resulting in failure of the model to build. Notably, the selection of resolution and threshold for bone segmentation are key steps that greatly affect computational complexity and validity. In the following study, we evaluated an approach whereby Micro-CT derived grayscale attenuation and segmentation data guided the selection of trabecular bone for analysis by FEA. We further correlated those FEA results to both two- and three-dimensional bone microarchitecture from sham and ovariectomized (OVX) rats (n = 10/group). A virtual cylinder of vertebral trabecular bone 40% in length from the caudal side was selected for FEA, because Micro-CT based image analysis indicated the largest differences in microarchitecture between the two groups resided there. Bone stiffness was calculated using FEA and statistically correlated with the three-dimensional values of bone volume/tissue volume, bone mineral density, fractal dimension, trabecular separation, and trabecular bone pattern factor. Our method simplified the process for the assessment of trabecular bone stiffness by FEA from Micro-CT images and highlighted the importance of bone microarchitecture in conferring significantly increased bone quality capable of resisting failure due to increased mechanical loading.