X-ray diffraction of bone at the interface with hydroxyapatite-coated versus uncoated metal implants

The microstructural characteristics of the newly formed bone tissue at the interface with hydroxyapatite-coated and uncoated stainless steel pins used in an external fracture fixation system have been evaluated. The bone far from the interface was used as a control. Pins were transversally inserted into the diaphyses of sheep tibiae and were loaded in for six weeks. Three sheep received coated pins and two received uncoated pins. Crystallographic habit and mineralization of the implant-facing bone were evaluated. Moreover, lattice parameters of bone apatite were measured and hydroxyapatite (HA) coating degradation was investigated, by means of conventional and microbeam X-ray diffraction (XRD). In coated pins, six weeks after the implantation the newly formed bone tissue at the interface did not reach complete maturation, but the presence of the implant did not alter the apatite lattice structure; the lattice parameters did not show statistically significant variations with respect to those observed in the control bone. In uncoated pins, bone tissue rarely appeared totally mineralized and lattice parameters were significantly different with respect to those observed in the bone far from the implant. HA particles were observed spreading in the bone-facing coated pins; the XRD pattern of bone apatite surrounding HA particles was unmodified. It was concluded that HA coatings improved the bone remodelling process during pin fixation in comparison to uncoated pins and did not alter the crystallographic habit of apatite.