Osseous metaplasia with functioning bone marrow in hydroxyapatite orbital implants

Background: Bone formation within the hydroxyapatite implant has been repor ted in explanted spheres in humans. Bone-specific differentiation was obser ved to occur earlier in the pores of spherical hydroxyapatite implants enha nced with osteogenin within the rabbit socket. We observed previously unrep orted bone marrow formation in a coralline hydroxyapatite implant placed in to the rabbit orbit after evisceration. Methods: One eye of each of 10 New Zealand white rabbits weighing between 2 and 3 kg was eviscerated and impla nted with hydroxyapatite spheres. The explanted hydroxyapatite spheres 20 w eeks after surgery were examined under the microscope. Results: Histopathol ogic examination of the excised implants showed the presence of trabeculae of mature bone with fatty marrow and hematopoietic elements. Scattered thro ughout the fatty tissue were bone marrow elements consisting of precursors of the granulocytic and erythroid series and also megakaryocytes. Conclusio n: The osseous metaplasia with functioning bone marrow was incidentally obs erved in the coralline hydroxyapatite implant without the addition of any o steogenesis-inducing agents.