On periodontal probing, a tooth has an isolated, narrow, 8 mm deep periodontal pocket with no adjacent teeth showing similar depth. The tooth has been endodontically treated and has no occlusal fremitus. What does this isolated deep pocket most likely indicate?

An isolated, narrow, deep periodontal pocket in an endodontically treated tooth — with no similar involvement of adjacent teeth — is pathognomonic for a vertical root fracture (VRF). VRF causes a narrow 'sinus tract' pocket that extends apically along the fracture line, rather than the broad, generalized bone loss seen in periodontitis. Additional signs of VRF include: localized sinus tract, J-shaped radiolucency on periapical radiograph, and bone loss on two opposite surfaces of the root. The tooth is often restored with a post. CBCT is the most sensitive imaging tool for detecting VRF. Once confirmed, prognosis is poor and extraction is typically indicated.