## 1 Introduction  
Significant advancements in bone tissue engineering over the past decades have addressed many challenges associated with hard tissue augmentation procedures, yet notable gaps persist [1]. Bone grafts remain a cornerstone of clinical practice and are classified into biological types (autografts, allografts, xenografts) or synthetic alternatives [2]. Among these, autografts are considered the \"gold standard\" due to their superior osteogenic properties. Nonetheless, the limited availability of these materials and the associated donor site morbidity underscore the need to explore alternative options that offer comparable efficacy while minimizing risks [3]. Allografts and xenografts have emerged as viable alternatives, offering improved osteeconductive properties and widespread availability.  
However, challenges such as immune response and high costs persist [4; 5]. Advancements in tissue engineering and material science have also facilitated the development of synthetic materials, particularly hydroxyapatite (HA) which mimics the composition and function of natural bone tissue [6].  
Bone tissue regeneration depends on the interplay of osteogenic, osteoinductive, and osteoconductive properties, along with sufficient vascularization--all key elements that are encapsulated in the \"diamond concept\" of bone healing [7]. Biomaterials for bone repair must meet specific criteria: biocompatibility; adequate mechanical stability; macroporosity to support cell infiltration and vascularization; and controlled degradation rates to integrate seamlessly with native bone [4; 8; 9; 10]. Hydroxyapatite, a calcium phosphate-based biocearamic, has become a focal point in bone tissue engineering due to its excellent bioactivity and chemical similarity to bone mineral. Its widespread application, however, is limited by inherent brittleness, porosity, and poor mechanical properties [11].  
To address these limitations, ionic doping has emerged as a transformative approach to enhancing the structural and functional properties of hydroxyapatite. Doping introduces specific ions into the HA lattice, modifying its crystal structure and influencing key properties such as solubility, mechanical strength, bioactivity, and antimicrobial efficacy [12]. For instance, strontium (Sr2+) enhances bioactivity and promotes bone cell proliferation, while magnesium (Mg2+) improves bone density and mechanical strength. Zinc (Zn2+) and cerium (Ce3+) contribute to osteogenesis and antibacterial properties, while silver (Ag+) provides antimicrobial protection, reducing the risk of peri-implant infections. These modifications bring hydroxyapatite closer to mimicking the hierarchical structure and functionality of native bone, providing customized solutions for clinical applications.  
This review focuses on the role of ionic doping in hydroxyapatite, emphasizing its impact on structural, mechanical, and biological properties. By examining the influence of specific ions on hydroxyapatite characteristics, we aim to highlight advancements in the material's potential for orthopedic and regenerative applications. The integration of ionic doping into HA represents a significant step forward in tailoring biomaterials to meet diverse clinical needs, bridging the gap between synthetic substitutes and the complex requirements of natural bone tissue [12].  
The schematic placement of doped hydroxyapatite within the classification of biomaterials is presented in Figure 1.  
Figure 1: Schematic placement of hydroxyapatite within the classification of biomaterials.