Objective: The pathogenesis of periprosthetic bone loss in aseptic and septic prosthesis loosening is unclear. There is considerable evidence that macrophages and osteoclasts play a key role in focal bone erosion and osteolysis around the prosthesis. RANKL (receptor activator of nuclear factor kappaB ligand) was shown to be a potent osteoclastogenic factor, and to be involved in bone destruction of myeloma and rheumatoid arthritis patients. Osteoprotegerin (OPG) is the natural RANKL inhibitor and may prevent periprosthetic bone loss.
Methods: The presence and distribution of RANKL, its receptor RANK and OPG in the periprosthetic interface of septically (n = 5) and aseptically (n = 6) loosened prostheses was examined by immunohistochemistry and immunoblotting. Additionally, the immunophenotype of the inflammatory infiltrate was determined [CD3, CD68, Ki-67, tartrate-resistant acid posphatase (TRAP)].
Results: Aseptic and septic cases revealed a different histopathologic pattern. However, in all cases RANKL and RANK could be demonstrated in macrophages and giant cells. In addition, RANKL detected by immunoblot analysis proved to have the same molecular weight as a recombinant RANKL used as a control (31 kD and approximately 48 kD). OPG was detected in aseptic loosening, where macrophages showed a strong staining, but multinucleated giant cells were only weakly stained. A weak OPG staining was also observed in septic loosening.
Conclusion: The pathogenesis of bone loss in septic loosening remains unclear, because the septic membrane bears few macrophages and giant cells, and half of them express OPG. In aseptic loosening, macrophages might not be stimulated by RANKL as a result of OPG expression. But multinucleated giant cells may be activated, as they hardly express OPG. They might be responsible for periprosthetic bone loss in aseptic loosening as a result of their RANKL and RANK expression.