A patient complains of carious cavity in tooth 11. The filling was lost one week ago. The tooth crown is dark, there is residual filling material at the bottom of the carious cavity. Vertical percussion is painless. X-ray shows an oval area of bone tissue resorption with clear margins, 0.4x0.3 cm in size. The root canal is filled by 2/3 of its length. What is the most likely diagnosis?