Conservative Management of External Replacement Resorption in a Posterior Tooth: A Case Report

External replacement resorption (ERR) is an uncommon yet aggressive form of root resorption in which bone replaces the root following damage to the periodontal ligament. Early recognition is critical, as progression is typically silent and may ultimately result in tooth loss. This report describes a 25-year-old male patient who presented with a "sunken" lower right first molar teeth three years after biting a hard object. Clinical examination revealed infra-occlusion, a high-pitched metallic percussion tone, and delayed pulp responses. Periapical radiography and cone beam computed tomography demonstrated loss of periodontal ligament space and trabecular bone within the distal root, confirming progressive ERR with pulpal necrosis and asymptomatic apical periodontitis. Conservative non-surgical root canal therapy was undertaken, during which calcium hydroxide was placed as an intracanal medicament for four weeks to suppress clastic activity. Subsequently, the distal canal was obturated incrementally with mineral trioxide aggregate, while the mesial canals were filled with gutta-percha and a calcium-silicate bioceramic sealer. A composite core was built, and a computer-aided design and computer-aided manufacturing (CAD/CAM)-milled zirconia crown was cemented to restore occlusion. Follow-up at six and twelve months demonstrated resolution of symptoms, stable occlusion, and radiographic evidence of periapical healing without further resorptive progression. Early diagnosis aided by cone beam computed tomography (CBCT) and a biologically based, calcium-silicate obturation approach allowed successful preservation of a mandibular molar affected by progressive ERR, highlighting that conservative endodontic management with bioactive materials can be a predictable option even in advanced cases.