Stage	Exposed or necrotic bone	History and clinical findings
At risk	No clinical evidence
Stage 0	No clinical evidence	Non-specific clinical and radiographic findings
Stage 1	Exposed and necrotic bone, or fistulae that probes to bone	Asymptomatic with no evidence of infection
Stage 2	Exposed and necrotic bone, or fistulae that probes to bone	Associated with infection, Pain and erythema in the region of the exposed bone with or without purulent drainage
Stage 3	Exposed and necrotic bone, or fistulae that probes to bone	Pain, infection, and one or more of the following: Exposed, necrotic bone extending beyond the region of alveolar bone resulting in pathologic fracture Extra-oral fistula, oro-antral or oro-nasal communication Lytic changes that extend to the inferior border of the mandible or sinus floor

^*Adapted from Ruggiero et al.¹⁶