There is growing evidence that RANKL (also known as osteoclast differentiation factor), its receptor RANK and its natural inhibitor osteoprotegerin (OPG) are involved in bone loss in a number of pathologies. The aim of this study was to determine if these factors are expressed in a number of bone loss pathologies and what cell types were producing these factors in the tissues using reverse transcription polymerase chain reaction (RT-PCR), in situ hybridisation and immunostaining techniques. Periarticular tissue was obtained from 15 patients undergoing revision of aseptic loose implants. Rheumatoid joint tissue was obtained from the pannus region of 12 patients diagnosed with rheumatoid arthritis undergoing joint replacement or joint fusion. Inflamed gingival tissue from sites near bone erosion were obtained from 11 patients with periodontal disease. 6 normal periodontal and periarticular tissue from 6 osteoarthritic patients was used as controls.

RANK, RANKL, OPG and M-CSF mRNA were expressed in tissues obtained from all the pathologies. Higher ratio’s of RANKL to OPG were observed in the pathological tissues compared to their respective controls. In revision tissues many multinucleated giant cell containing particles expressed RANK mRNA. The pattern of staining of RANK mRNA was markedly different in the rheumatoid and periodontal tissues. Differences were also seen in the pattern of expression for RANKL using both in situ and immunostaining. Overall our results indicate that although similar osteoclastogenic factors are fundamentally involved in these bone loss pathologies, different cell types may be producing and/or responding to these factors. Identifying fundamental mechanisms such as these may indicate that similar treatments, such as using OPG or related compounds, may be used for a diverse range of bone loss diseases.

Wear particles are thought to be a major factor causing osteolysis that leads to aseptic loosening. The aim of this study was to investigate the role of primary regulators of osteoclast development, RANKL (also known as osteoclast differentiation factor), its receptor RANK and natural inhibitor osteoprotegerin (OPG) in aseptic loosening. Cells were isolated from periprosthetic tissues taken at revision from more than 30 patients and the expression of these mediators in vivo was assessed using semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). These cells were also cultured on dentine to determine their ability to become mature osteoclasts. In situ hybridisation using DIG labelled riboprobes specific for RANK mRNA was used to identify cells likely to become osteoclasts. We were able to compare revison tissues containing several different types of prosthetic wear particles.

RANKL, RANK and OPG mRNA were found in samples of periprosthetic revision tissues. Cells derived from this tissue developed into mature osteoclasts capable of resorbing dentine. Cells that rapidly formed osteoblasts expressed a fifteen fold higher ratio of RANKL:OPG mRNA. In situ hybridisation showed RANK expression by macrophages and giant cells, many of which contained wear particles. Significantly, cells from tissues containing silastic wear particles expressed higher levels of RANKL relative to OPG and more produced large numbers of osteoclasts in vitro. This study shows that different bio materials in a particulate form may differ in their ability to form osteoclasts and that the relative levels of RANKL and OPG are likely to be important in determining if osteolysis will occur. In the future molecules that inhibit RANKL binding, such as OPG, may be considered for therapy of periprosthetic osteolysis.