Effects of hydroxylapatite coating crystallinity on biosolubility, cell attachment efficiency and proliferation in vitro.

A hydroxylapatite (HA) coating with approximately 97% crystalline HA content (MP-1 treated HA coating, MP-HA) was tested in vitro for its biosolubility and cellular biocompatibility. The MP-HA coating was compared with a standard HA coating with approximately 63% crystallinity (SHA) and an amorphous HA coating with approximately 25% crystallinity (AHA), as well as a titanium (Ti) surface without HA coating as a control. The topographic study with scanning electron microscopy indicated that MP-HA appeared more coarse, with projected nodules which altered the shape of cells attached to the substrate. Biosolubility study indicated that MP-HA had the least effect on the culture medium pH, while AHA (P < 0.01) and SHA (P < 0.05) significantly raised the medium pH up to 8.2 and 7.75, respectively. X-ray diffraction (XRD) analysis showed essentially unchanged levels of the total soluble phases of all coatings after incubation with culture medium, except that the CaO phase was rapidly dissolved from AHA coatings and completely eliminated from SHA coatings. Cultures of human gingival fibroblasts on these HA coatings showed that MP-HA and SHA had about the same cell attachment efficiency which was relatively lower than that of AHA coatings. MP-HA generated significant higher cell proliferation rate relative to AHA (P < 0.01) and SHA (P < 0.05). This study indicated that surface chemistry and topography of lower crystallinity might be favorable to cell attachment, but that elevated medium pH might result in a cytotoxic effect that inhibits the proliferation of attached cells on coating surfaces.