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Background
Mandibular osteonecrosis, also known as osteonecrosis of the jaw, is an iatrogenic disease of devitalized mandible secondary to radiotherapy and medications in most cases. It encompasses two distinct entities, i.e., osteoradionecrosis (ORN) and medication related osteonecrosis of jaw (MRONJ). ORN of the jaws is defined as exposed irradiated bone that fails to heal over a period of 3 months without any evidence of persisting or recurrent tumor. MRONJ is defined as an area of exposed bone in the maxillofacial region that has not healed within 8 weeks after identification by a healthcare professional, in a patient who was receiving or had been exposed to a bisphosphonate and had not had radiation therapy to the craniofacial region. MRONJ affects up to 26 % of patients on antiresorptive drugs and up to 10 % of patients receiving IV bisphosphonates. This exhibit aims to review the pathogenesis and clinical presentation of this entity and demonstrate the spectrum of imaging findings across modalities. This would enable radiologists to glean a set of take-home messages that would serve handy in diagnosing and managing mandibular osteonecrosis. Salient teaching points with regards to the imaging presentation.

Educational Goals / Teaching Points
Review the practice of imaging in cases of ORN and MRONJ. Review the current understanding of the pathogenesis of mandibular osteonecrosis.

Key Anatomic/Physiologic Issues and Imaging Findings/Techniques
The pathogenesis of mandibular osteonecrosis is not completely understood. It is believed that a combination of hypoxia and hypovascularity leads to a disturbance in the bone homeostasis leading to a chronic nonhealing wound. Clinically, mandibular osteonecrosis manifests as an area of exposed bone that has failed to heal for a period of 3 months. The patient will typically have a history of recurrent or residual tumour in the same area in case of ORN, or history of long-term medication with bisphosphonates or antiresorptive drugs in case of MRONJ. Patients present with ORN of jaw secondary to a late complication of radiotherapy to head and neck cancers, seen in up to 20% of patients. On radiographs, pathological areas demonstrate radiolucency and cortical irregularity. They demonstrate lytic areas, cortical breaks, and trabecular loss. MRI demonstrates decreased T1 marrow signal, cortical irregularity, and contrast enhancement. FDG PET may demonstrate increased uptake in the region. Scintigraphy (^99mTc-MDP) demonstrates increased uptake and is used to determine extent and location.

Conclusion
Two distinct aetiologies of mandibular osteonecrosis, both resulting from disrupted bone homeostasis. Good history and communication with clinicians helps radiologists in their diagnosis. Radiologists should be aware of mandibular osteonecrosis and consider it in the differential diagnosis for patients with a history of antiresorptive and antiangiogenesis treatment, or with history of radiotherapy to maxillofacial region, so as to avoid unnecessary biopsies and potential complications.