Although infection rates following bone-implant surgery remain fairly low, the total number of OM cases is high3
, and the recent surge in MRSA further underscores the significance of this clinical problem4
. At best, this catastrophic outcome requires a challenging revision surgery that is further complicated by deteriorated bone stock due to osteolysis. Thus, an important unanswered question regarding the treatment of patients with implant-associated OM is whether or not they would benefit from anti-resorptive therapy. Despite the successful case reports in the literature5,6
, our study indicates that this practice may be contraindicated since Aln and OPG: i) increase the amount of necrotic cortical bone around the implant that serves as a nidus for infection (), ii) significantly increases the incidence of high-grade infections in mice during the establishment of OM (), and iii) reduce the cortical hole () through which the opsonized bacteria and lymph must drain out of the wound ().
Equally important to our positive findings are results that refute claims regarding the anti-angiogenic and immunosuppressive effects of anti-resorptive therapies. In search of the etiology of ONJ, others have reported that bisphosphonates inhibit capillary tube formation, vessel sprouting and circulating endothelial cells 14–16
, and cited this as a mechanistic factor in the condition. In contrast, our vascular micro-CT analyses failed to demonstrate any drug effects on perfusable vessels >10μm (data not shown). Thus, our findings are more consistent with reports from others who have argued against this theory based on the fact that drugs such as thalidomide, with far greater anti-angiogenic capacity, do not result in ONJ 14
. Similarly, the findings that RANK signaling is required for lymph node development 32,33
and immunoregulatory co-stimulation 34,35
have led to a suspicion that RANK-ligand inhibition is immunosuppressive. However, our studies corroborate the prior findings of Stolina et al
, who demonstrated that OPG treatment does not affect cell-mediated immune responses and granuloma formation 36
, and the recent finding that humans genetically deficient in RANKL have severe osteopetrosis without immunodeficiency 37
. Therefore, we conclude that RANKL inhibition has insignificant immunoregulatory effects in the setting of in vivo
pathogenic challenge, and that its ability to increase the incidence of high-grade OM can be entirely attributed to its effects on osteoclastic bone resorption, as these effects could be replicated with Aln (–). To further abate concerns regarding the clinical use of a biologic RANKL inhibitor (i.e. denosumab 38
), we demonstrated that OPG and TNFR:Fc mediate similar effects on OM via different mechanisms, as evidenced by their differential effects on osteolysis, granuloma formation, and lymphatics. Given the remarkable clinical success of anti-TNF therapy over the last decade, and its much greater effects on the immune system compared to OPG, it is becoming more apparent that immunosuppression is unlikely to be a major limitation of biologic anti-resorptive therapy.
The most surprising results of our study was the observation that the massive bone void around the transtibial implant was largely due to inhibition of reactive bone formation rather than osteoclastic bone resorption, as evidenced by the micro-CT images of the Aln and OPG treated samples (). Most interestingly is that this inhibition of bone formation appears as a perfectly symmetrical ring that extend >1mm from the biofilm adjacent to the pin. Thus, it is clear that this is mediated by an indirect mechanism that likely involves a secreted factor (i.e. noggin, DKK1, SOST) that has a gradient expression pattern originating from the infected bone. Elucidation of this factor(s) is now the subject of ongoing investigation, as it could also prove to be important for preventing inflammation mediated bone loss in aseptic conditions such as erosive arthritis and periprosthetic osteolysis.
While we do not want to overstate the implications of our findings as they relate to the emerging ONJ epidemic, we feel that there are several parallels that warrant discussion. The first and most frustrating is the absence of etiologic factors that are necessary and sufficient to faithfully reproduce OM and ONJ in similar individuals. Even though a correlation between ONJ and bisphosphonate use in cancer patients following oral surgery has been established, its overall incidence in this population is >10% 9,10
. Similarly, despite our best efforts in a controlled experiment model, we have been unable to establish conditions that lead to equivalent OM in genetically identical littermates, in which many of the mice that receive a Xen29 coated pin remain uninfected (). This indicates that stochastic epigenetic factors, such as the amount of necrotic bone available for bacterial adhesion, and the temporally restricted period for this event to occur, may be critical to the establishment of both OM and ONJ.
The other parallel that deserves mention is the potential association between inhibited lymphatics during the establishment of bone infection and bone pain. One potential mechanism by which anti-resorptive agents could inhibit lymphangiogenesis is by reducing the amount of VEGF-C produced during the infection through its effects on osteoclast precursors and osteoclasts, which produce high levels in response to RANKL39
. Clinically, OM and ONJ are primarily discovered as a result of symptomatic bone pain, whose nature is largely unknown. Our novel finding that OPG and Aln drug inhibition of resorption is associated with significant decreased lymphatics during the establishment of bone infection implies that there must be a marked increase in intraosseous pressure at this time, which would be expected to induce bone pain characteristic of that seen in OM and ONJ. Interestingly, recent clinical studies have demonstrated that surgery-induced lymphedema is a major cause of local pain, and improvement of lymphatic drainage is an effective therapy in these patients 40
. Thus, the association of decreased lymph node size and drainage in our model of OM warrants further investigation.