Aseptic loosening is generally associated with the presence of wear particle-associated macrophages in the pseudomembrane commonly formed around failed prosthetic implants. The extent of the macrophage response evoked by the wear particles has been shown to correlate with the amount of periprosthetic osteolysis. Numerous studies have shown that wear particle-associated macrophages contribute to osteolysis by (i) releasing inflammatory cytokines and/or (ii) differentiating into bone resorbing osteoclasts. Although macrophages and macrophage polykaryons are the main inflammatory cells found in periprosthetic tissues, numerous fibroblasts are also present in the connective tissue pseudomembrane. The recently identified molecule, RANKL has been shown to play a central role in the osteoclast formation and bone resorption observed in aseptic loosening. We have shown that arthroplasty macrophages, which express RANK, the receptor for RANKL, are capable of osteoclast differentiation; this process is inhibited by osteoprotegerin (OPG), the soluble decoy receptor for RANKL. As fibroblasts are known to express RANKL, the aim of the present study was to determine whether fibroblasts isolated from periprosthetic tissues could induce the generation of bone resorbing osteoclasts that would contribute to the osteolysis commonly seen in the periprosthetic loosening.

Fibroblast-like cells were isolated from pseudomembrane from patients (n=5) undergoing hip revision due to aseptic loosening, by routine collagenase enzyme digestion. The isolated cells were seeded in flasks for 2–4 weeks before being passaged for a further 3–4 times. Generated fibroblast-like cells (10⁴) were then co-cultured with 5x10⁵ normal human peripheral blood monocytes (n=5) on glass coverslips and dentine slices in the presence of (i) no added factors, (ii) macrophage colony stimulating factor (M-CSF) and (iii) M-CSF plus OPG. All cultures were maintained for 1,17 and 21 days. The extent of osteoclast differentiation was then determined by the expression of specific osteoclast markers including tartrate-resistant acid phosphatase (TRAP) and vitronectin receptor (VNR) and evidence of lacunar resorption.

In the absence M-CSF, no osteoclast formation was noted in 24 hours, 17 or 21 days in fibroblast/monocyte cultures. However, in the presence of M-CSF alone, large numbers of TRAP⁺ and VNR⁺ multinucleated cells capable of lacunar resorption were noted in these co-cultures. The addition of OPG, which is known to inhibit RANKL-mediated osteoclast formation, significantly reduced the extent of osteoclast formation and lacunar resorption in these co-cultures.

These results indicate that one means whereby peri-prosthetic osteolysis may occur is by fibroblasts in the arthroplasty pseudomembrane inducing macrophage-osteoclast differentiation. Fibroblasts express RANKL and interact with arthroplasty macrophages, which express RANK and function as osteoclast precursors. These findings indicate that suppression of osteoclast formation by OPG may be a possible form of therapy for reducing prosthetic loosening.