Bone is a complex, hierarchically organized tissue1,2. Understanding its structural organization is essential for addressing fundamental questions related to how cells contribute to tissue architecture and how structural changes impact mechanical function3. In this context, bone remodeling after injury remains a central topic of investigation. Fracture healing is a complex multistage process, beginning with an initial inflammatory response, followed by angiogenesis and the differentiation of progenitor cells into chondrocytes and osteoblasts4,5. Much has been debated regarding the role of biomaterials to stimulate osteointegration, particularly about biocompatibility and their ability to prevent fibrous encapsulation6.
A commonly adopted method to assess biomaterial-stimulated osteointegration is optical microscopy, which enables both quantitative and qualitative histological analyses. This approach has been shown to be a reliable and effective way to evaluate the success of bone implants7,8. Nevertheless, several issues related to bone hierarchical organization cannot be explored by using a single imaging approach3, particularly the histological one, which is limited at least in part by the magnification restriction of the method.
Advances in three-dimensional imaging have proven essential for understanding bone hierarchical organization. Techniques such as microcomputed tomography (microCT; either synchrotron radiation or conventional laboratory-based X-ray sources)9,10,11, dual-beam scanning electron microscopy (FIB-SEM) associated with slice-and-view tomography12,13,14 and transmission electron tomography15,16 provided new insights into bone architecture at multiple scales.
Based on different 3D imaging approaches, a model of bone hierarchical organization has been proposed, which initially identified nine distinct levels of bone structural organization1. This scheme has recently been revised and expanded to encompass approximately 12 hierarchical levels, serving as a valuable reference for exploring the relationship between structure and function in bone tissue2,17. Importantly, this proposed hierarchical model provides a foundation for evaluating the success of bone repair following injury. Evaluating whether the regenerated bone tissue reproduces the native hierarchical structure is a key standard in assessing the quality of bone healing, particularly when biomaterials are used within the damaged site.
Although the use of multiple imaging techniques to investigate bone organization has been discussed6,7,18,19,20, little has been explored regarding a workflow that integrates different imaging modalities to assess bone mineral organization across multiple spatial scales. As mentioned, bone is a complex composite tissue, consisting of an organic phase (cells, collagen, and macromolecules) and a mineralized matrix. From a sample preparation perspective, this complexity presents technical challenges regarding solvent permeation through the sample, sectioning of a hard specimen, and the integration of different imaging strategies on the same sample.
The key innovation in this work is the establishment of a reproducible, correlative, and multiscale imaging workflow using three-dimensional imaging and complementary techniques. As illustrated in Figure 1A, the workflow is designed so that the same sample can be sequentially examined using different imaging modalities. Because some of these techniques are destructive or require sectioning, the workflow follows an order that begins with non-destructive methods and progresses to approaches that involve cutting or material removal.
This sample preparation and analysis workflow is broadly applicable to hard or mineralized specimens, including cortical and trabecular bone, pre-implantation scaffolds, and the evaluation of the bone/biomaterials interface after implantation. By combining imaging modalities that span different fields of view and resolution ranges (Figure 1B), the workflow enables the analysis of samples from the centimeter scale down to the micrometer and nanometer scales. While the sequence is optimized to support correlative analysis of a single specimen, each technique can also be applied independently according to the experimental goal. This approach enables a detailed assessment of biomaterials' osteointegration and the evaluation of the regenerated bone growth in the healing area.