Final answer:
The preferred action when an endodontic instrument breaks in the root canal is to complete the root canal filling and monitor the patient, with extraction being the last resort.
Step-by-step explanation:
When an endodontic instrument is broken in the apical third of a root canal and lodged tightly without radiographic changes at the apex, the procedure of choice is to perform a routine root canal filling in the remaining root canal space and place the patient on recall. This approach minimizes intervention while attempting a less invasive resolution to the issue. It is important to monitor the patient closely to ensure that no complications arise from the broken instrument fragment.
If complications develop or if symptoms arise, further actions such as an apicoectomy with a reverse filling or the resection of the apical section of the root may be considered. Extraction is generally seen as a last resort, when the structural integrity of the tooth cannot be salvaged or if pathology develops. However, in the absence of apical pathology and with a fragment well lodged, careful observation after completing the root canal is advised.