Hydroxyapatite fully coated conic hip prosthetic stem: a long term animal study

The purpose of this paper is to evaluate in an animal model the long term results obtained with a prosthetic stem fully coated with hydroxyapatite. The cup was manufactured in polyethylene and was cemented. Six arthroplasties were performed in six sheep. After twelve months, the animals were euthanized and the femurs were harvested and processed for undecalcified sectioning. Twelve cross sections were cut perpendicularly to the longitudinal stem axis. Sections one to five corresponded to the area of the stem which, at the time of surgery, had a full initial contact between the bone and the prosthesis; sections six to ten corresponded to the area of the stem which, at the time of surgery, had a gap from 0 to 2 mm between the bone and the prosthesis; sections eleven and twelve had an initial gap larger than 2 mm. At one year after implantation, in the sections one to five, morphological analyses showed extensive direct contact between the bone and the hydroxyapatite coating. Bone prosthesis contact was lower in the sections six to ten. No contact was seen in sections eleven and twelve. Comparing bone to prosthesis contact of each subsequent section, from proximal to distal, the difference becomes significant with section five compared to section six (p < 0.00005). No detachment of the hydroxyapatite coating from the metallic substrate was observed in any section. In conclusion, this study shows that a conic shaped femoral stem, fully coated with hydroxyapatite gives very good histological and histomorphometric results at one year. Prosthesis osteointegration showed to be influenced by the initial bone to prosthesis contact. No direct bone to prosthesis contact was achieved if the initial bone to prosthesis gap was larger than 2 mm.