Ira C. Ison

Histological, chemical, and crystallographic analysis of four calcium phosphate cements in different rabbit osseous sites

Four calcium phosphate cement formulations were implanted in the rabbit distal femoral metaphysis and middiaphysis. Chemical, crystallographic, and histological analyses were made at 2, 4, and 8 weeks after implantation. When implanted into the metaphysis, part of the brushite cement was converted into carbonated apatite by 2 weeks. Some of the brushite cement was removed by mononuclear macrophages prior to its conversion into apatite. Osteoclastlike cell mediated remodeling was predominant at 8 weeks after brushite had converted to apatite. The same histological results were seen for brushite plus calcite aggregate cement, except with calcite aggregates still present at 8 weeks. However, when implanted in the diaphysis, brushite and brushite plus calcite aggregate did not convert to another calcium phosphate phase by 4 weeks. Carbonated apatite cement implanted in the metaphysis did not transform to another calcium phosphate phase. There was no evidence of adverse foreign body reaction. Osteoclastlike cell mediated remodeling was predominant at 8 weeks. The apatite plus calcite aggregate cement implanted in the metaphysis that was not remodeled remained as poorly crystalline apatite. Calcite aggregates were still present at 8 weeks. There was no evidence of foreign body reaction. Osteoclastlike cell remodeling was predominant at 8 weeks. Response to brushite cements prior to conversion to apatite was macrophage dominated, and response to apatite cements was osteoclast dominated. Mineralogy, chemical composition, and osseous implantation site of these calcium phosphates significantly affected their in vivo host response.

Measurements of the solubilities and dissolution rates of several hydroxyapatites.

Calcium phosphate based materials, such as apatites, are increasingly being developed and used in implants for orthopedic and dental applications. Previous investigation of various calcium phosphate ceramics has demonstrated great variability in the solubility characteristics in solution between materials with similar stoichiometric composition. Therefore, in this study, the solubility and rate of dissolution of three apatite sources, BoneSource, Norian cranial repair system (CRS), and a sintered hydroxyapatite (Calcitite) are evaluated in a thermodynamically closed system. The measured solubility under physiological conditions (tris buffer solution, pH 7.4, 37 degrees C) of BoneSource, Norian CRS and Calcitite is 7.5, 7.4 and 1.4 ppm, respectively. Initial dissolution rates at 10 min of BoneSource, CRS, and Calcitite were 0.0465, 0.1589, and essentially 0 mg/min respectively. Solubility product constants at 37 degrees C were calculated to be 1.49 x 10(-35) for CRS, 1.19 x 10(-35) for BoneSource, and 2.92 x 10(-42) for Calcitite. The increased solubility of the BoneSource and Norian CRS materials over that of Calcitite is related to their poor crystallinity compared to sintered hydroxyapatite.