Retrieval analysis of orthopaedic implants. Tissue reactions around a hydroxyapatite coated hip prosthesis

For the evaluation of newly developed orthopaedic implantations, it is of utmost importance to collect all available information with respect to the behaviour of the implant. Retrieval analysis of well-functioning implants (post-mortem) and of failures (during revision) may give very valuable information with respect to short term reactions of the organism to the implant.In this case, bone reactions were studied around a titanium, hydroxyapatite (HA) coated Osteonics bipolar hip prosthesis, which was revised for severe mid thigh pain four years after implantation. Scarce remnants of a coating-like material on the surface of the prosthesis were found after inspection of the retrieved prosthesis using a dissecting microscope. Histology of these remnants and of the bony side of the bone-HA interface, failed to reveal any remnants of a HA coating. Trabecular bone which closely followed the contour of the prosthesis was found at the bone-prosthesis interface. Facing the prosthesis, this trabecular bone was mainly immature callous bone. At other locations, where mineralized bone faced the prosthesis, many small dark titanium wear particles were found. Similar particles were found in macrophages in the intertrabecular medullary space. Polyethylene wear particles were specifically located in macrophages in a soft tissue interface at more distal levels along the stem of the prosthesis.Although the observations presented in this case cannot be generalized, it clearly shows that the HA coating layer had completely disappeared after four years. More detailed retrieval studies and longer clinical follow-up studies are needed before a final evaluation of the behaviour of HA coatings and long term fixation of HA-coated prostheses can be made.