Osteonecrosis of the jaw (ONJ) is a well-recognized complication of antiresorptive medications, such as bisphosphonates (BPs). Although ONJ is most common after tooth extractions in patients receiving high dose BPs, many patients do not experience oral trauma. Animal models utilizing tooth extractions and high BP doses recapitulate several clinical, radiographic and histologic findings of ONJ. We and others have reported on rat models of ONJ utilizing experimental dental disease in the absence of tooth extraction. These models emphasize the importance of dental infection/inflammation for ONJ development. Here, we extend our original report in the rat, and present a mouse model of ONJ in the presence of dental disease. Mice were injected with high dose zoledronic acid and pulpal exposure of mandibular molars was performed to induce periapical disease. After 8 weeks, quantitative and qualitative radiographic and histologic analyses of mouse mandibles were executed. Periapical lesions were larger in vehicle- vs. BP treated mice. Importantly, radiographic features resembling clinical ONJ, including thickening of the lamina dura, periosteal bone deposition and increased trabecular density, were seen in the drilled site of BP treated animals. Histologically, osteonecrosis, periosteal thickening, periosteal bone apposition, epithelial migration and bone exposure were present in the BP treated animals in the presence of periapical disease. No difference in TRAP+ cell numbers was observed, but round, detached, and removed from the bone surface cells were present in BP animals. Although 88% of the BP animals showed areas of osteonecrosis in the dental disease site, only 33% developed bone exposure, suggesting that osteonecrosis precedes bone exposure. Our data further emphasize the importance of dental disease in ONJ development, provide qualitative and quantitative measures of ONJ, and present a novel mouse ONJ model in the absence of tooth extraction that should be useful in further exploring ONJ pathophysiological mechanisms.

Antiresorptive medications are essential in treating diseases of pathologic osteoclastic bone resorption, including bone cancer and osteoporosis. Bisphosphonates (BPs) are the most commonly used antiresorptives in clinical practice. Although inhibition of bone resorption is important in regulating unwanted malignant and metabolic osteolysis, BP treatment is associated with potential side effects, including osteonecrosis of the jaws (ONJ). Recently, non-BP antiresorptive medications targeting osteoclastic function and differentiation, such as denosumab, have entered the clinical arena. Denosumab treatment results in a similar rate of ONJ as BPs. Animal models of ONJ, using high-dose BP treatment in combination with tooth extraction or dental disease, provide valuable tools and insight in exploring ONJ pathophysiology. However, the ability of other antiresorptives to induce ONJ-like lesions in animal models has not been explored. Such studies would be beneficial in providing support for the role of osteoclast inhibition in ONJ pathogenesis versus a direct BP effect on oral tissues. Here, we tested the ability of the receptor activator of NF-kB ligand (RANKL) inhibitors RANK-Fc (composed of the extracellular domain of RANK fused to the fragment crystallizable [Fc] portion of immunoglobulin G [IgG]) and OPG-Fc (composed of the RANKL-binding domains of osteoprotegerin [OPG] linked to the Fc portion of IgG) to induce ONJ in mice in the presence of periapical disease, but in the absence of dental extractions. We demonstrate radiographic evidence of ONJ in RANK-Fc–treated and OPG-Fc–treated mice, including inhibition of bone loss, increased bone density, lamina dura thickening, and periosteal bone deposition. These findings closely resembled the radiographic appearance of an ONJ patient on denosumab treatment. Histologic examination revealed that RANK-Fc treatment and OPG-Fc treatment resulted in absence of osteoclasts, periosteal bone formation, empty osteocytic lacunae, osteonecrosis, and bone exposure. In conclusion, we have successfully induced ONJ in mice with periapical disease, using potent osteoclast inhibitors other than BPs. Our findings, coupled with ONJ animal models using high-dose BPs, suggest that osteoclast inhibition is pivotal to the pathogenesis of ONJ.

Although osteonecrosis of the jaws (ONJ), a serious complication of antiresorptive medications, was reported a decade ago, the exact mechanisms of disease pathophysiology remain elusive. ONJ-like lesions can be induced in animals after antiresorptive treatment and experimental interventions such as tooth extraction or periapical or periodontal disease. However, experimental induction and manipulation of disease progression does not always reflect clinical reality. Interestingly, naturally occurring maxillofacial abscesses, inducing aggressive inflammation of the peri-radicular mucosa with significant osteolysis and alveolar bone expansion, have been reported in mice. Here, we aimed to explore whether osteonecrotic lesions would develop in areas of maxillary peri-radicular infections, in mice on antiresorptive medications with distinct pharmacologic action, thus establishing a novel ONJ animal model. Mice were treated with RANK-Fc or OPG-Fc that bind to RANKL or with the potent bisphosphonate zoledronic acid (ZA). Maxillae were assessed radiographically and histologically. μCT imaging of vehicle mice revealed several maxillae with altered alveolar bone morphology, significant ridge expansion and large lytic areas. However, in RANK-Fc, OPG-Fc and ZA treated animals the extent of bone loss was significantly less, but exuberant bone deposition was noted at the ridge periphery. BV and BV/TV were increased in the diseased site of antiresorptive vs. veh animals. Histologically, extensive inflammation, bone resorption and marginal gingival epithelium migration were seen in the diseased site of vehicle animals. Rank-Fc, OPG-Fc and ZA reduced alveolar bone loss, increased periosteal bone formation, and induced areas of osteonecrosis, and bone exposure that in many animals covered significant part of the alveolar bone. Collectively, our data demonstrate ONJ-like lesions at sites of maxillary peri-radicular infection, indistinguishable in mice treated with RAKL inhibitors vs. zoledronate. This novel mouse model of spontaneous ONJ supports a central role of osteoclast inhibition and infection/inflammation in ONJ pathogenesis and validates and complements existing animal models employing experimental interventions.

Purpose Bisphosphonates (BPs) are widely used for the management of bone diseases such as osteoporosis and bone malignancy. However, osteonecrosis of the jaws (ONJ) is a serious complication of BP treatment. ONJ lesions mainly occur after extraction of teeth deemed unrestorable or around teeth with active periodontal or periapical disease. Because socket healing or dental disease shows higher bone turnover, the authors hypothesized that preferentially high BP accumulation would be observed in these areas. Materials and Methods The authors tested the uptake of fluorescein-labeled zoledronic acid (5-FAM-ZOL) in sites of tooth extraction or experimental periapical disease in mice. Maxillary molars were extracted or the crowns of mandibular molars were drilled to induce pulp exposure. Animals were injected with 5-FAM-ZOL 200 μg/kg at various times after intervention and fluorescence was measured at healthy versus intervention sites. Fluorescein injections were used as controls. Data were analyzed by t test and mixed effects linear models were constructed because the animals had repeated measurements over time and at the 2 sites. Results A statistically significant (P ≤ .001 to .002) time-dependent uptake of 5-FAM-ZOL was detected in the areas of extraction socket and in the alveolar ridge around teeth with periapical disease compared with the healthy contralateral sites of the same animals. For the 2 conditions, the uptake reached a maximum 3 days after experimental intervention and decreased thereafter. Conclusions These data suggest that sites with increased bone turnover, such as extraction sites or areas of periapical inflammation, are exposed to higher BP doses than the remaining alveolar ridge and could explain, at least in part, the susceptibility of such areas to ONJ.