Micro-CT evaluation of bone defects: Applications to osteolytic bone metastases, bone cysts, and fracture
Introduction
Defects or flaws in a material reduce its strength by magnifying local stresses [1]. In the case of bone, defects may include bone metastases, cysts, and drill holes for stabilizing or studying fracture repair. These defects can occur in the cortex, the dense material that surrounds bone, or in the interior trabecular bone network. Cellular solids, such as trabecular bone, wood, and engineered foams, are particularly sensitive to structural defects [2]. The loss of structural members has been shown to be more detrimental to strength than an equivalent reduction in apparent density through uniform thinning of its members [2], [3], and failure of trabecular bone samples has been found to localize in regions of low bone volume fraction and connectivity density [4]. The evaluation of defects is, therefore, an important aspect of evaluating a material's strength, in particular in the case of bone where these defects are associated with pain and fragility [5].
Micro-computed tomography (micro-CT) is a non-destructive three-dimensional (3D) imaging modality and the gold standard for evaluating bone microarchitecture; however, its application to bone metastasis research, including evaluation of bone destruction and treatment, has been challenging. Common human cancers, including lung, breast, and prostate, have the potential to spread to bone, and often the skeleton becomes the most heavily burdened site of tumors in the advanced stages of these diseases [5]. Bone metastases can be induced in immune-compromised mice by intra-cardiac injection with tumor cells [6], the more common method, or by direct injection into the bone [7], [8], [9]. Regardless of the method used, when the tumor burden is severe, and in particular when the tumor has interrupted the cortex, it becomes difficult to define the region of interest required to perform a standard micro-CT bone morphological analysis and to evaluate bone quality [8]. Due to the high inter-individual variability in bone properties [10], the intra-cardiac method, which does not allow for control for the site or tumor burden, poses the additional challenge of how one identifies a potential metastasis and quantifies changes that occur with time or treatment. Scoring of X-rays is currently the most common procedure for evaluating osteolytic bone metastases in both clinical practice and animal research, but it is only useful when osteolytic damage is severe. Furthermore, because radiological images are two-dimensional, measurements will be sensitive to the image's orientation, and this imaging modality is limited in its ability to provide a detailed evaluation of bone quality.
Evaluation of bone defects, including osteolytic bone metastases, cysts, and burr-hole fractures, presents challenges when evaluating bone quality by micro-CT. Defects can be highly variable across individuals and it has been unclear which measures of bone quality provide the best indication of the presence of these types of defects. Variability in defect location and severe deterioration of the bone make it difficult to determine an appropriate region of interest for analysis, in particular for cross-sectional studies where there is no internal control available, nor repeat measurements over time, to mitigate some of the variability. Bone mineral density (BMD), as measured by micro-CT, has been shown to have a stronger correlation with bioluminescence imaging, indicative of tumor growth, over radiological scoring [11]; however, direct quantitative metrics are needed. Therefore, the goal of this research is to establish a new methodology based on a micro-CT measure of microarchitectural bone spacing as a sensitive, quantitative tool to quantify and monitor bone defects. These quantitative data will be important for the development of treatments related to burr-hole fracture healing and metastatic tumor burden.
Section snippets
Materials and methods
Micro-CT datasets for illustration of the methodology were selected from two independent mouse studies, both resulting in focal defects, performed in our laboratory between 2005 and 2009. The first study examined fracture healing in mice subjected to a burr-hole defect in the proximal tibia [12]. The second study examined bone metastatic growth in mice subjected to intra-cardiac injection with human breast cancer cells [13]. These studies were carried out in compliance with the Canadian Council
Burr-hole defect
The absolute difference in bone quality parameters between limbs containing the burr-hole defect and the intact limb are presented as box plots (Fig. 3). The presence of the defect increased Sp by 27.6% (p < 0.0001), SpSD by 113% (p < 0.0001), and Spmax by 72.8% (p < 0.0001). It significantly increased BV/TV (p = 0.013), but only by 3%, indicating that is a much less sensitive parameter than Sp, SpSD, and Sp_max. The other morphological parameters were not significantly influenced by the defect,
Discussion
This study demonstrates how standard micro-CT morphological analysis can be adapted to overcome the common problem of evaluating bone defects, which is complicated by the high variability in defect size and location; traditional morphological analysis variables typically mask these important changes. Our results show that both types of defects (burr-hole and osteolytic metastases) greatly influence the distribution of bone spacing (Sp) whereas other parameters showed limited sensitivity to
Conclusions
The technique we propose here is a fast and quantitative approach to measuring bone defects. These can occur in various forms and present challenges to adequately quantify when using a standard micro-CT evaluation. The method using spacing as a quantitative basis for assessing burr-hole defects and bone metastases in a mouse model is an efficient approach with potential for other applications using 3D imaging modalities.
Acknowledgements
The authors would like to thank R.J. Klinck and J. Taiani, for their assistant with micro-CT data collection and for performing the burr-hole surgical procedure.
Conflict of interest
None declared.
Funding
This study was supported by a Translational Group Grant from the Alberta Cancer Foundation. FRJ was the recipient of a Canada Research Chairs award; SKB holds the Rintoul Chair in Bone and Joint Research.
Ethical approval
These studies were carried out in compliance with the Canadian Council of
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