Comparative Study on Measured Variables and Sensitivity to Bone Microstructural Changes Induced by Weightlessness Between In Vivo and Ex Vivo Micro-CT Scans

Depending on the experimental design, micro-CT can be used to examine bones either in vivo or ex vivo (excised fresh or formalin-fixed). In this study we investigated if differences exist in the variables measured by micro-CT between in vivo and ex vivo scans and which kind of scan is more sensitive to the changes of bone microstructure induced by simulated weightlessness. Rat tail suspension was used to simulate the weightless condition. The same bone from either normal or tail-suspended rats was scanned by micro-CT both in vivo and ex vivo (fresh and fixed by formalin). Then, bone mineral density (BMD) and microstructural characteristics were analyzed. The results showed that no significant differences existed in the microstructural parameters of trabecular bone among in vivo, fresh, and formalin-fixed bone scans from both femurs and tibias, although BMD exhibited differences. On the other hand, most parameters of the tail-suspended rats measured by micro-CT deteriorated compared with controls. Ex vivo scanning appeared to be more sensitive to bone microstructural changes induced by tail suspension than in vivo scanning. In general, the results indicate that values obtained in vivo and ex vivo (fresh and fixed) are comparable, thus allowing for meaningful comparison of experimental results from different studies irrespective of the type of scans. In addition, this study suggests that it is better to use ex vivo scanning when evaluating bone microstructure under weightlessness. However, researchers can select any type of scan depending upon the objective and the demands of the experiment.