**1. Introduction**

Osteonecrosis of the jaw is a common side effect of antiresorptive drugs that are administered to cancer patients for bone metastasis, multiple myeloma, and osteoporosis. Antiresorptive

© 2017 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

medications contain bisphosphonates and the receptor activator nuclear factor kB ligand (RANKL) inhibitor [1].

On the other hand, denosumab, a human IGG2 monoclonal antibody against RANKL, is a therapeutic agent for bone metastasis and osteoporosis, with a half-life of approximately 1 month. Unlike bisphosphonates (BPs), which promotes apoptosis in osteoclasts, denosumab inhibits osteoclastic bone resorption without causing apoptosis. Furthermore, denosumab is not deposited in the bone and thus does not persist for long periods of time, as is the case with BPs, and so the effects of denosumab are reversible [2]. However, patients treated with denosumab also developed ONJ (denosumab-related ONJ [DRONJ]), which was clinically indistinguishable from BRONJ and occurred at almost the same rates [3].

Since both BP and denosumab which show anti-bone resorption effects through different molecular mechanisms of action are associated with ONJ, antiresorptive agent-related ONJ (ARONJ) has been suggested as a comprehensive term encompassing both BRONJ and DRONJ [4]. The American Association of Oral and Maxillofacial Surgeons (AAOMS) has proposed the term "medication-related osteonecrosis of the jaw" (MRONJ), based on observations that antiangiogenic inhibitors and molecularly targeted drugs such as tyrosine kinase inhibitors are also infrequently associated with ONJ or increase the incidence of BRONJ/ DRONJ in cancer patients receiving BPs or denosumab, although global consensus has not yet been established [5].

The number of patients with MRONJ has grown recently. Medication-related osteonecrosis of the jaw (MRONJ) is defined as exposed bone in the oral cavity or extra-oral fistula in the maxillofacial region for more than 8 weekswith the treatment of antiresorptive or antiangiogenic agents for more than 8 weeks. These patients have no history of radiotherapy or metastatic disease in the jaw. We present about the details, pathophysiology, diagnosis, staging and treatment, risk factor, and prevention of MRONJ in below paragraphs in this chapter.
