## 1 Introduction  
With the advancement of medical technology, fractures have emerged as some of the most prevalent injuries in the human body. While external fixation techniques are suitable for stable fractures, patients with unstable fractures (such as comminuted fractures), advanced ages, abnormal bone metabolism, or osteoporosis exhibit a relatively increased risk of non-union or delayed union due to their limited fracture healing ability or compromised osteogenic capacity [1]. Consequently, bone reconstruction surgery utilizing artificial implant skeletons remains the primary treatment approach [2]. Currently, hydroxyapatite (HA) stands out as the most commonly employed biomaterial for fabricating these artificial implant skeletons. Following bone reconstruction surgery involving porous HA skeletons,bodily fluids come into contact with and gradually infiltrate these structures, initiating biomineralization [3; 4; 5; 6]. apatite nanocrystals are slowly formed after the beginning of biomineralization. Due to the gradual infiltration of body fluids onto the porous HA skeleton, biomineralization occurs not only on the surface of the artificial porous HA skeleton but also, more importantly, inside the artificial porous HA skeleton [7; 8; 9; 10]. In the later stage of the implantation of the artificial porous HA skeleton, bone cells grow onto the skeleton, and the microstructure of the skeleton plays an important role in the success of implantation. Therefore, this process will inevitably depend on the internal porous structure modification of the HA skeleton [11]. However, the apatite nanocrystals on the inner surface of the artificial porous HA skeleton are rarely studied, and their key role in bone growth and regeneration is unclear. Therefore, the study of the microstructure diversity of apatite nanocrystals formed by the biomineralization between the inner surface of the skeleton and body fluids and the influence of microstructure diversity on osteocyte adhesion play important roles in exploring the mechanism of bone growth and cell regeneration in an artificial porous skeleton [12; 13; 14]. At present, many studies [15; 16; 17; 18; 19] have investigated the biomineralization process of apatite nanocrystal formation in simulated body fluids (SBFs) and the related benefits of apatite nanocrystals in bone integration and regeneration. But, typically, such studies select fully exposed HA surfaces with non-porous characteristics, both experimentally and in modeling, to create controlled SBF contact environments. This is the most common way to study the basic principles of apatite crystal formation. After bone reconstruction surgery, it is certain that the infiltration of the three-dimensional porous skeleton in body fluids is not synchronous [20]. The main reason is that the sizes, shapes, and surface texture states of the micro-channels, micro-pores, and micro-holes of the three-dimensional porous skeleton are different, so the microstructures of apatite nanocrystals formed by mineralization are diverse. Theoretically, different SBF contact environments lead to different types of nanocrystal structures that cannot be observed on HA surfaces soaked in a smooth, well-controlled SBF contact environment. Since the internal environment of the three-dimensional porous skeleton after undergoing biomineralization is more relevant to the growth of bone cells, the in-depth and detailed study of the unique nanocrystal structure \"hidden\" inside the three-dimensional porous skeleton will provide valuable insights into the nature of bone cell growth and bone tissue regeneration [20]. Although the study of micropores with different surface sizes (from a few \(\upmu\)m to a few hundred \(\upmu\)m) on the three-dimensional porous skeleton is important for the function of bone growth [21; 22; 23; 24], it is equally important, or even more important, to study the biomineralization process and its microscopic diversity on the internal surface of the three-dimensional porous skeleton.