Alloplastic bone substitutes may be ceramic, hydroxyapatite (naturally occurring mineral form of calcium apatite), tricalcium phosphate (calcium salt of phosphoric acid), or calcium sulfate. Polymer-based bone grafts, for example, Healos, use degradable and nondegradable polymers. Alloplastic substitutes come into the scene when patients refuse to accept tissues from another human being for ethical or religious reasons or an animal, fearing transmission of diseases.