Mechanisms and structure of the bond between bone and hydroxyapatite ceramics.

Two assays were carried out to investigate the postulates that dissolution/reprecipitation phenomena and epistaxis are involved in the formation of the bond between sintered hydroxyapatite (HA) and bone. HA was exposed to a physiologic solution and the ions going into solution quantified using inductively coupled plasma (ICP) emission spectroscopy. The HA was retrieved and examined using scanning electron microscopy (SEM). In the second assay, HA was implanted into Alsatian femura. Following retrieval, specimens were prepared and studied with SEM, at magnifications usually reserved for transmission electron microscopy (TEM). In vitro the HA surface composition appeared to shift to the acidic (calcium-deficient) direction. The HA surfaces in both assays underwent a dissolution/reprecipitation degradation of varying severity, forming a recrystallization layer of spherocrystallites. Two modes of bone bonding to implanted HA were identified: (1) bone tissue components bonded to HA via a recrystallization zone similar in structure to the reprecipitation layer in the corrosion assay, and (2) bone tissue components bonded directly to HA crystals with no morphologically discernible signs of dissolution embarrassment. The formation of the bone/HA bond seems to involve dissolution/reprecipitation phenomena. What is believed to be the first morphological evidence of epitaxial growth involvement in the formation of this bond is presented.