Histological and radiological analyses of hydroxyapatite orbital implants in rabbits

To date, only anectodal clinical data exist pertaining to the histological changes of hydroxyapatite within an enucleated socket. This study was conducted to determine the histological and radiological changes in a coralline hydroxyapatite sphere placed into the central socket, in a controlled fashion. Rabbits underwent simple enucleation with implantation of an autologous sclera-wrapped hydroxyapatite spheres with extraocular muscle reattachment. Preoperatively, the mineral density of each sphere was determined using quantitative computed tomography (CT) that was repeated 2- and 6 weeks postoperatively. The implants were harvested at 2- and 6 weeks and submitted for light and electron microscopic analysis. The results demonstrated a uniform influx of fibrovascular tissue that did not reach the center of the implant, even at 6 weeks. A marked mixed-cell inflammatory response was noted at the interface between the fibrovascular tissue and the hydroxyapatite. Giant cells were noted only at the scleral windows. This study demonstrated that the early response to hydroxyapatite implants was fibrovascular ingrowth with mixed-cell inflammation. These histological observations correlated with findings observed with quantitative CT. Quantitative CT appears to be an ideal modality for observing the early temporal tissue density changes in hydroxyapatite implants.