**2.1 Calcium phosphates family**

Calcium phosphate-based bioceramics have been used in medicine and dentistry for decades. Applications include dental implants, percutaneous devices, periodontal treatment, alveolar ridge augmentation, orthopedics, maxillofacial surgery, otolaryngology and spinal surgery (Hench, 1991).

Bone is a natural nano-composite composed of organic (40%) and inorganic (60%) components. The inorganic constituent of bone is made up of biological apatites, which provide strength to the skeleton and act as a storehouse for calcium, phosphorus, sodium, and magnesium. These biological apatites are structurally similar, though not identical, to the mineral apatite hydroxyapatite (HAp, Ca10(PO4)6(OH)2). Hydroxyapatite is the most ubiquitous and well-known phase of calcium phosphate. It has the Ca/P ratio of 1.67 (Narayan et al., 2004). Different phases of calcium phosphate ceramics are used depending upon whether a resorbable or bioactive material is desired. The stable phases of calcium phosphate ceramics depend considerably upon temperature and the presence of water, either during processing or use in the environment (See Fig. 1) (Hench, 1991).

Going through aforementioned properties, it can be realized that the calcium phosphates family includes several members with different characteristics. Calcium phosphate ratio, Ca/P, has been found as the best way to distinguish among these members. In table 2 these members are shown based on their Ca/P ratio.
