Kinetics of crystal growth and dissolution of natural and synthetic apatites

Using the constant composition method, growth and dissolution kinetics of synthetic and naturally occurring calcium phosphate minerals were studied in the presence and absence of additives. Since citric acid (CA) is a major cause of tooth erosion its influence on stoichiometric solutions of fluorapatite, FAP and tricalcium phosphate, TCP as well as non-stoichiometric FAP was studied. Results showed that CA inhibited FAP but accelerated TCP dissolution, most likely due different dissolution mechanisms. Bisphosphonates inhibit bone resorption and have become the treatment of choice for osteoporosis. Therefore, hydroxyapatite, HAP dissolution in the presence of six bisphosphonates was studied under osteoclastic resorption conditions and were compared to growth experiments. Bisphosphonates markedly inhibited HAP dissolution and growth, with dissolution being ten times slower than growth. Variations in inhibition effectiveness may be due to the extent of protonation and size of the bisphosphonate compound. The influence of bone osteopontin and other urinary macromolecules [uropontin and Tamm-Horsfall protein (THP)] on the growth of hydroxyapatite was also studied. Rat bone osteopontin was more effective at inhibiting HAP growth than either human uropontin or THP. Dissolution differences may be related to acidic amino acid content, protein size and/or extent of post-translational modifications. Different purification methods were also found to effect dissolution.; Solubility sensitivities of powdered human dentin and enamel were investigated as a function of solution composition (i.e. ionic strength, IS and pH). Experiments were also performed using heated powdered enamel surfaces, to mimic those following dental laser irradiation.{09}These tissues dissolved faster in a solution of high IS or low pH and dentin was more soluble than enamel. Heating enamel powder (above 800°C) markedly decreased its dissolution abilities. Dissolution of powdered canine and primate vertebrae and humerus bone in the presence and absence of three bisphosphonates was examined. The overall order of dissolution inhibition effectiveness was similar to the synthetic HAP results. Canine vertebrae dissolved faster than the primate vertebrae, while the latter dissolved faster than primate humerus bone. Also, dissolution of surgically induced postmenopausal osteoporosis (ovariectomy) primates was found to be slower than nonovariectomized samples.