Predicting Bone Adaptation in Astronauts during and after Spaceflight.

A method was previously developed to identify participant-specific parameters in a model of trabecular bone adaptation from longitudinal computed tomography (CT) imaging. In this study, we use these numerical methods to estimate changes in astronaut bone health during the distinct phases of spaceflight and recovery on Earth. Astronauts (N = 16) received high-resolution peripheral quantitative CT (HR-pQCT) scans of their distal tibia prior to launch (L), upon their return from an approximately six-month stay on the international space station (R+0), and after six (R+6) and 12 (R+12) months of recovery. To model trabecular bone adaptation, we determined participant-specific parameters at each time interval and estimated their bone structure at R+0, R+6, and R+12. To assess the fit of our model to this population, we compared static and dynamic bone morphometry as well as the Dice coefficient and symmetric distance at each measurement. In general, modeled and observed static morphometry were highly correlated (R2> 0.94) and statistically different (p < 0.0001) but with errors close to HR-pQCT precision limits. Dynamic morphometry, which captures rates of bone adaptation, was poorly estimated by our model (p < 0.0001). The Dice coefficient and symmetric distance indicated a reasonable local fit between observed and predicted bone volumes. This work applies a general and versatile computational framework to test bone adaptation models. Future work can explore and test increasingly sophisticated models (e.g., those including load or physiological factors) on a participant-specific basis.

Exogenously delivered iPSCs disrupt the natural repair response of endogenous MPCs after bone injury.

Promoting bone healing including fracture non-unions are promising targets for bone tissue engineering due to the limited success of current clinical treatment methods. There has been significant research on the use of stem cells with and without biomaterial scaffolds to treat bone fractures due to their promising regenerative capabilities. However, the relative roles of exogenous vs. endogenous stem cells and their overall contribution to in vivo fracture repair is not well understood. The purpose of this study was to determine the interaction between exogenous and endogenous stem cells during bone healing. This study was conducted using a standardized burr-hole bone injury model in a mesenchymal progenitor cell (MPC) lineage-tracing mouse under normal homeostatic and osteoporotic conditions. Burr-hole injuries were treated with a collagen-I biomaterial loaded with and without labelled induced pluripotent stem cells (iPSCs). Using lineage-tracing, the roles of exogenous and endogenous stem cells during bone healing were examined. It was observed that treatment with iPSCs resulted in muted healing compared to untreated controls in intact mice post-injury. When the cell populations were examined histologically, iPSC-treated burr-hole defects presented with a dramatic reduction in endogenous MPCs and cell proliferation throughout the injury site. However, when the ovaries were removed and an osteoporotic-like phenotype induced in the mice, iPSCs treatment resulted in increased bone formation relative to untreated controls. In the absence of iPSCs, endogenous MPCs demonstrated robust proliferative and osteogenic capacity to undertake repair and this behaviour was disrupted in the presence of iPSCs which instead took on an osteoblast fate but with little proliferation. This study clearly demonstrates that exogenously delivered cell populations can impact the normal function of endogenous stem/progenitor populations during the normal healing cascade. These interactions need to be better understood to inform cell and biomaterial therapies to treat fractures.

Automatic segmentation of trabecular and cortical compartments in HR-pQCT images using an embedding-predicting U-Net and morphological post-processing.

High-resolution peripheral quantitative computed tomography (HR-pQCT) is an emerging in vivo imaging modality for quantification of bone microarchitecture. However, extraction of quantitative microarchitectural parameters from HR-pQCT images requires an accurate segmentation of the image. The current standard protocol using semi-automated contouring for HR-pQCT image segmentation is laborious, introduces inter-operator biases into research data, and poses a barrier to streamlined clinical implementation. In this work, we propose and validate a fully automated algorithm for segmentation of HR-pQCT radius and tibia images. A multi-slice 2D U-Net produces initial segmentation predictions, which are post-processed via a sequence of traditional morphological image filters. The U-Net was trained on a large dataset containing 1822 images from 896 unique participants. Predicted segmentations were compared to reference segmentations on a disjoint dataset containing 386 images from 190 unique participants, and 156 pairs of repeated images were used to compare the precision of the novel and current protocols. The agreement of morphological parameters obtained using the predicted segmentation relative to the reference standard was excellent (R2 between 0.938 and > 0.999). Precision was significantly improved for several outputs, most notably cortical porosity. This novel and robust algorithm for automated segmentation will increase the feasibility of using HR-pQCT in research and clinical settings.

High spatial resolution analysis using automated indentation mapping differentiates biomechanical properties of normal vs. degenerated articular cartilage in mice.

Characterizing the biomechanical properties of articular cartilage is crucial to understanding processes of tissue homeostasis vs. degeneration. In mouse models, however, limitations are imposed by their small joint size and thin cartilage surfaces. Here we present a three-dimensional (3D) automated surface mapping system and methodology that allows for mechanical characterization of mouse cartilage with high spatial resolution. We performed repeated indentation mappings, followed by cartilage thickness measurement via needle probing, at 31 predefined positions distributed over the medial and lateral femoral condyles of healthy mice. High-resolution 3D x-ray microscopy (XRM) imaging was used to validate tissue thickness measurements. The automated indentation mapping was reproducible, and needle probing yielded cartilage thicknesses comparable to XRM imaging. When comparing healthy vs. degenerated cartilage, topographical variations in biomechanics were identified, with altered thickness and stiffness (instantaneous modulus) across condyles and within anteroposterior sub-regions. This quantitative technique comprehensively characterized cartilage function in mice femoral condyle cartilage. Hence, it has the potential to improve our understanding of tissue structure-function interplay in mouse models of repair and disease.

An inverse technique to identify participant-specific bone adaptation from serial CT measurements.

Simulated bone adaptation is framed as an interface evolution problem. The interface is extracted from a high-resolution computed tomography (CT) image of trabecular bone microarchitecture and modified by the level set equation. A model and its parameters determine the bone adaptation rate and thus the bone structure at any future time. This study develops an inverse problem and solver to identify model parameters from multiple high-resolution CT images of bone within the level set framework. We demonstrate the technique on a model of advection and mean curvature flow, termed curvature-driven adaptation. The inverse solver uses two CT scans to estimate model parameters, which map the bone surface from one image to the next. The solver was tested with synthetic images of bone changing according to the curvature-driven model with known model parameters. The algorithm recovered known model parameters excellently (R2 > .99, p < .001). A grid search indicated that the estimated model parameters were insensitive to hyper-parameter selection for learning rate 1e-5≤η≤ 5e-5 and gradient scaling factor 5e-5≤γ≤ 5e-4 . Finally, we tested the algorithm's sensitivity to salt-and-pepper noise of probability P , where .0 ≤P≤ .9. Model parameter accuracy did not change for P < .7, corresponding to Dice coefficients greater than .7. The inverse problem estimates bone adaptation parameters from multiple CT images of changing bone microarchitecture. In the future, this technique could be used to determine participant-specific bone adaptation parameters in vivo, validate bone adaptation models, and predict bone health.

Proximal Tibia Bone Stiffness and Strength in HR-pQCT- and QCT-Based Finite Element Models.

Injury to the ACL significantly increases the risk of developing post-traumatic osteoarthritis. Following injury, considerable focus is placed on visualizing soft tissue changes using MRI, but there is less emphasis on the alterations to the underlying bone. It has recently been shown using high-resolution peripheral quantitative computed tomography (HR-pQCT) that significant reductions in bone quality occur in the knee post ACL-injury. Despite the ability of HR-pQCT to show these changes, the availability of scanners and computational time requirements required to assess bone stiffness and strength with HR-pQCT limit its widespread clinical use. As such, the objective of this study was to determine if clinical quantitative CT (QCT) finite element models (QCT-FEMs) can accurately replicate HR-pQCT FEM proximal tibial stiffness and strength. From FEMs of 30 participants who underwent both QCT and HR-pQCT bilateral imaging, QCT-FEMs were strongly correlated with HR-pQCT FEM stiffness (R2 = 0.79). When QCT-FEM bone strength was estimated using the reaction force at 1% apparent strain, strong correlations were observed (R2 = 0.81), with no bias between HR-pQCT FEMs and non-linear QCT-FEMs. These results indicate that QCT-FEMs can accurately replicate HR-pQCT FEM stiffness and strength in the proximal tibia. Although these models are not able to replicate the trabecular structure or tissue-level strains, they require significantly reduced computational time and use widely available clinical-CT images as input, which make them an attractive choice to monitor bone density, stiffness and strength alterations, such as those that occur post ACL-injury.

Quantitative measures of bone shape, cartilage morphometry and joint alignment are associated with disease in an ACLT and MMx rat model of osteoarthritis.

Multi-scale, subject-specific quantitative methods to characterize and monitor osteoarthritis in animal models and therapeutic treatments could help reveal causal relationships in disease development and distinguish treatment strategies. In this work, we demonstrate a reproducible and sensitive quantitative image analysis to characterize bone, cartilage and joint measures describing a rat model of post-traumatic osteoarthritis. Eleven 3-month-old male Wistar rats underwent medial anterior cruciate ligament (ACL) transection and medial meniscectomy on the right knee to destabilise the right tibiofemoral joint. They were sacrificed 6 weeks post-surgery and a silicon-based micro-bead contrast agent was injected in the joint space, before scanning with micro-computed tomography (microCT). Subsequently, 3D quantitative morphometric analysis (QMA), previously developed for rabbit joints, was performed. This included cartilage, subchondral cortical and epiphyseal bone measures, as well as novel tibiofemoral joint metrics. Semi-quantitative evaluation was performed on matching two-dimensional (2D) histology and microCT images. Reproducibility of the QMA was tested on eleven age-matched additional joints. The results indicate the QMA method is accurate and reproducible and that microCT-derived cartilage measurements are valid for the analysis of rat joints. The pathologic changes caused by transection of the ACL and medial meniscectomy were reflected in measurements of bone shape, cartilage morphology, and joint alignment. Furthermore, we were able to identify model-specific predictive parameters based on morphometric parameters measured with the QMA.

A quantitative assessment of dual energy computed tomography-based material decomposition for imaging bone marrow edema associated with acute knee injury.

PURPOSE: This study developed methods to quantify and improve the accuracy of dual-energy CT (DECT)-based bone marrow edema imaging using a clinical CT system. Objectives were: (a) to quantitatively compare DECT with gold-standard, fluid-sensitive MRI for imaging of edema-like marrow signal intensity (EMSI) and (b) to identify image analysis parameters that improve delineation of EMSI associated with acute knee injury on DECT images. METHODS: DECT images from ten participants with acute knee injury were decomposed into estimated fractions of bone, healthy marrow, and edema based on energy-dependent differences in tissue attenuation. Fluid-sensitive MR images were registered to DECT for quantitative, voxel-by-voxel comparison between the two modalities. An optimization scheme was developed to find attenuation coefficients for healthy marrow and edema that improved EMSI delineation, compared to MRI. DECT method accuracy was evaluated by measuring dice coefficients, mutual information, and normalized cross correlation between the DECT result and registered MRI. RESULTS: When applying the optimized three-material decomposition method, dice coefficients for EMSI identified through DECT vs MRI were 0.32 at the tibia and 0.13 at the femur. Optimization of attenuation coefficients improved dice coefficient, mutual information, and cross-correlation between DECT and gold-standard MRI by 48%-107% compared to three-material decomposition using non-optimized parameters, and improved mutual information and cross-correlation by 39%-58% compared to the manufacturer-provided two-material decomposition. CONCLUSIONS: This study quantitatively evaluated the performance of DECT in imaging knee injury-associated EMSI and identified a method to optimize DECT-based visualization of complex tissues (marrow and edema) whose attenuation parameters cannot be easily characterized. Further studies are needed to improve DECT-based EMSI imaging at the femur.

The Assessment of Skeletal Muscle and Cortical Bone by Second-generation HR-pQCT at the Tibial Midshaft.

BACKGROUND: Peripheral quantitative computed tomography (pQCT) is the current densitometric gold-standard for assessing skeletal muscle at the 66% proximal tibia site. High resolution peripheral quantitative computed tomography (HR-pQCT) is a leading technology for quantifying bone microarchitecture at the distal extremities, and with the second-generation HR-pQCT it is possible to measure proximal limb sites. Therefore, the objectives of this study were to: (1) assess the feasibility of using HR-pQCT to assess skeletal muscle parameters at the 66% proximal tibia site, and (2) test HR-pQCT skeletal muscle measurement reproducibility at this site. METHODS: Adult participants (9 males; 7 females; ages 31-75) received 1 pQCT scan and 2 HR-pQCT scans at the 66% proximal site of the nondominant tibia. Participants were repositioned between HR-pQCT scans to test reproducibility. HR-pQCT and pQCT scans were analyzed to quantify muscle cross-sectional area (CSA) and muscle density. Coefficients of determination and Bland-Altman plots compared muscle parameters between pQCT and HR-pQCT. For short-term reproducibility, root-mean-square of coefficient of variance and least significant change were calculated. RESULTS: HR-pQCT and pQCT measured muscle density and muscle CSA were positively correlated (R2 = 0.66, R2 = 0.95, p < 0.001, respectively). Muscle density was equivalent between HR-pQCT and pQCT; however, there was systematic and directional bias for muscle CSA, such that muscle CSA was 11% lower with HR-pQCT and bias increased with larger muscle CSA. Root-mean-square of coefficient of variance was 0.67% and 0.92% for HR-pQCT measured muscle density and muscle CSA, respectively, while least significant change was 1.4 mg/cm3 and 174.0 mm2 for muscle density and muscle CSA, respectively. CONCLUSION: HR-pQCT is capable of assessing skeletal muscle at the 66% site of the tibia with good precision. Measures of muscle density are comparable between HR-pQCT and pQCT.

Bone and joint enhancement filtering: Application to proximal femur segmentation from uncalibrated computed tomography datasets.

Methods for reliable femur segmentation enable the execution of quality retrospective studies and building of robust screening tools for bone and joint disease. An enhance-and-segment pipeline is proposed for proximal femur segmentation from computed tomography datasets. The filter is based on a scale-space model of cortical bone with properties including edge localization, invariance to density calibration, rotation invariance, and stability to noise. The filter is integrated with a graph cut segmentation technique guided through user provided sparse labels for rapid segmentation. Analysis is performed on 20 independent femurs. Rater proximal femur segmentation agreement was 0.21 mm (average surface distance), 0.98 (Dice similarity coefficient), and 2.34 mm (Hausdorff distance). Manual segmentation added considerable variability to measured failure load and volume (CVRMS > 5%) but not density. The proposed algorithm considerably improved inter-rater reproducibility for all three outcomes (CVRMS < 0.5%). The algorithm localized the periosteal surface accurately compared to manual segmentation but with a slight bias towards a smaller volume. Hessian-based filtering and graph cut segmentation localizes the periosteal surface of the proximal femur with comparable accuracy and improved precision compared to manual segmentation.

p21-/- Mice Exhibit Spontaneous Articular Cartilage Regeneration Post-Injury.

OBJECTIVE: Recent studies have implicated the cyclin dependent kinase inhibitor, p21, in enhanced tissue regeneration observed in MRL/MpJ "super-healer" mice. Specifically, p21 is downregulated in MRL cells and similar ear hole closure to MRL mice has been observed in p21-/- mice. However, the direct implications of p21 deletion in endogenous articular cartilage regeneration remain unknown. In this study, we investigated the role of p21 deletion in the ability of mice to heal full-thickness cartilage defects (FTCDs). DESIGN: C57BL/6 and p21-/- (Cdkn1atm1Tyj) mice were subjected to FTCD and assessment of cartilage healing was performed at 1 hour, 3 days, 1 week, 2 weeks, and 4 weeks post-FTCD using a 14-point histological scoring system. X-ray microscopy was used to quantify cartilage healing parameters (e.g., cartilage thickness, surface area/volume) between C57BL/6 and p21-/- mice. RESULTS: Absence of p21 resulted in increased spontaneous articular cartilage regeneration by 3 days post-FTCD. Furthermore, p21-/- mice presented with increased cartilage thickness at 1 and 2 weeks post-FTCD compared with uninjured controls, returning to baseline by 4 weeks post-FTCD. CONCLUSIONS: We report that p21-/- mice display enhanced articular cartilage regeneration post-FTCD compared with C57BL/6 mice. Furthermore, cartilage thickness was increased in p21-/- mice at 1 week post-FTCD compared with uninjured p21-/- mice and C57BL/6 mice.

CT-based internal density calibration for opportunistic skeletal assessment using abdominal CT scans.

CT-based opportunistic skeletal assessment complements current osteoporosis diagnosis. Quantitative assessment by internal density calibration overcomes the limitations of phantom-based calibration. We sought to establish and validate an internal calibration technique using abdominal CT scans and establish reproducibility precision for three density calibration techniques. Ten full-body cadavers were CT scanned at the spine and pelvis with a calibration phantom. Internal calibration was performed using in-scan tissue references and deriving a voxel-specific calibration. Bone mineral density (BMD) and finite element (FE) failure load assessed skeletal health. Three independent users measured intra-exam precision by manual tissue selection. To verify results, ten subjects were imaged using an abdominal imaging protocol. Internal calibration performed equivalently to gold-standard phantom-based calibration in the cadaver spine and hip. Internal calibration BMD precision in the spine was 7 mg/cc (4.9%) and FE precision was 163 N (7.2%), whereas phantom-based precision was 3 mg/cc (1.8%) and 77 N (3.8%). Internal calibration hip BMD and FE precision was 11 mg/cc (5.3%) and 84 N (6.0%), whereas phantom-based precision was 2 mg/cc (1.3%) and 30 N (3.4%). Using the abdominal imaging protocol, internal calibration performed comparably to phantom-based calibration. Internal calibration provides BMD and FE outcome precision within 7.2% for opportunistic skeletal health assessment.

Longitudinal Effects of Acute Anterior Cruciate Ligament Tears on Peri-Articular Bone in Human Knees Within the First Year of Injury.

Anterior cruciate ligament (ACL) tears are common sports-related knee injuries that increase the risk of developing post-traumatic osteoarthritis. ACL tears are rarely an isolated injury but are often associated with traumatic bone marrow lesions (BMLs). While early loss of bone mass following the ACL injury has been previously described, to date, microarchitectural information has not been reported due to the limited resolution of clinical imaging systems. In this study, we provide the first evidence of detailed bone mass and microarchitectural changes in the first 10 months following an acute ACL tear, and localized to traumatic BMLs. Fifteen participants with an acute unilateral ACL tear were assessed at four-time points using dual-energy X-ray absorptiometry and high-resolution peripheral quantitative computed tomography, and traumatic BMLs were identified with magnetic resonance imaging. Loss of bone mass was localized to the injured knee (-4.6% to -15.8%, depending on bone and depth) and was accelerated immediately following the injury before suggesting a recovery phase. This loss of bone was accelerated even greater in traumatic BMLs (-18.2% to -20.6%, depending on bone). Bone loss was accompanied by microstructural degeneration of trabecular bone. For example, in the lateral femur of the injured knee, the subchondral bone plate decreased in thickness (-9.0%). This study confirmed loss of bone mass in the months following ACL tears and described the underlying bone microstructural changes. The presented bone changes were accelerated in regions of traumatic BMLs. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:2325-2336, 2019.

Three-Dimensional Quantitative Morphometric Analysis (QMA) for In Situ Joint and Tissue Assessment of Osteoarthritis in a Preclinical Rabbit Disease Model.

This work utilises advances in multi-tissue imaging, and incorporates new metrics which define in situ joint changes and individual tissue changes in osteoarthritis (OA). The aims are to (1) demonstrate a protocol for processing intact animal joints for microCT to visualise relevant joint, bone and cartilage structures for understanding OA in a preclinical rabbit model, and (2) introduce a comprehensive three-dimensional (3D) quantitative morphometric analysis (QMA), including an assessment of reproducibility. Sixteen rabbit joints with and without transection of the anterior cruciate ligament were scanned with microCT and contrast agents, and processed for histology. Semi-quantitative evaluation was performed on matching two-dimensional (2D) histology and microCT images. Subsequently, 3D QMA was performed; including measures of cartilage, subchondral cortical and epiphyseal bone, and novel tibio-femoral joint metrics. Reproducibility of the QMA was tested on seven additional joints. A significant correlation was observed in cartilage thickness from matching histology-microCT pairs. The lateral compartment of operated joints had larger joint space width, thicker femoral cartilage and reduced bone volume, while osteophytes could be detected quantitatively. Measures between the in situ tibia and femur indicated an altered loading scenario. High measurement reproducibility was observed for all new parameters; with ICC ranging from 0.754 to 0.998. In conclusion, this study provides a novel 3D QMA to quantify macro and micro tissue measures in the joint of a rabbit OA model. New metrics were established consisting of: an angle to quantitatively measure osteophytes (σ), an angle to indicate erosion between the lateral and medial femoral condyles (ρ), a vector defining altered angulation (λ, α, ß, γ) and a twist angle (τ) measuring instability and tissue degeneration between the femur and tibia, a length measure of joint space width (JSW), and a slope and intercept (m, Χ) of joint contact to demonstrate altered loading with disease progression, as well as traditional bone and cartilage and histo-morphometry measures. We demonstrate correlation of microCT and histology, sensitive discrimination of OA change and robust reproducibility.

Reproducibility of compartmental subchondral bone morphometry in the mouse tibiofemoral joint.

AIM: Evidence suggests that subchondral bone can be used as a predictor for the onset of osteoarthritis. As such, there is a need to accurately and reproducibly quantify subchondral bone in areas where osteoarthritis develops. In this paper, we present a novel technique for the segmentation of subchondral bone in the tibiofemoral joint and assess the reproducibility of this method with multiple measures and users. METHODS: The right hind leg of seven C57BL/6 mice were excised and imaged in µCT. The menisci and patella were manually segmented and the image data was Gaussian filtered and binarized. An in-house algorithm was used to generate cortical and epiphyseal volumes of interest and standard morphometric indices for bone were computed. The intraclass correlation coefficient (ICC), absolute precision error (PE(SD)), and precision error as a percentage of the coefficient of variation of the repeated measurements (PE(%CV)) were calculated for each index. Additionally, an inter-user study was performed using the same indices and statistics. RESULTS: For repeated measures, ICC ranged from 0.869 (cortical bone volume fraction, femur) to 0.994 (degree of anisotropy, femur). Similarly, PE(%CV) ranged from 0.84% (cortical bone volume fraction, femur) to 5.11% (connectivity density, tibia). For repeated users, no effect was seen in the femur with a slight effect in the tibia. CONCLUSIONS: A novel method for the automatic segmentation of cortical and epiphyseal bone is presented and is shown to be reproducible in C57BL/6 mice. This tool will allow for high-throughput studies of osteoarthritis in animal models.