Cast Post, Separated File, and an Untreated Middle Mesial

Option 1: nonsurgical retreatment

• The lesion is centered on the mesial root and there is an untreated middle mesial canal, a classic cause of failure. The crown margins are closed, probing is normal, and the buccal plate is intact, so the tooth is restorable and periodontally sound.
• The barrier is the 10 mm cast post. It can usually be removed atraumatically with ultrasonics and a post puller under magnification, but there is a real risk of root fracture.

Option 2: apical surgery with root-end resection

• The file sits about 3 mm from the apex, exactly where a 3 mm beveled resection would remove it. That is a decisive advantage.
• The untreated middle mesial canal is addressed only if it exits within the resected segment or is incorporated in the retropreparation.
• Surgery does not remove coronal biofilm in a canal that exits above the resection.

Option 3: extraction and implant

• Defensible if the post cannot be removed and the tooth is unrestorable, but premature here: good crown, healthy periodontium, localized treatable lesion.

Option 4: intentional replantation

• A last resort; the post and fragment add complexity and extraoral time adds resorption risk.

Ranking and drivers

• Retreatment first if the post can be removed; surgery if it cannot, or if retreatment fails. The decisive findings are post removability, the fragment's level relative to the resection, and whether the middle mesial exits within the resected segment.

Sequence

1. Review the CBCT to map post length, curvature, fragment position, and middle mesial angulation.
2. Access through the crown if margins allow; remove the crown only if access demands it.
3. Remove the post: trough around it with an ultrasonic tip under the microscope, then apply a post puller with pure axial traction. Avoid levering, which fractures roots.
4. Remove gutta-percha with rotary files, heat, and solvent.
5. Negotiate the middle mesial with precurved 8-10 files, using the CBCT to anticipate the angle.
6. Bypass the fragment with a 10/.02 hand file and loosen it with ultrasonics. If it can be bypassed and the canal cleaned, it may be left; a retained fragment that does not block cleaning is not itself a cause of failure.

Stop and convert if

• The root fractures.
• A perforation is created that cannot be sealed.
• The post will not move after 20-30 minutes of careful effort.
• The middle mesial cannot be negotiated and the lesion is clearly driven by it.

In those situations, proceed to surgery or extraction rather than persist and destroy the tooth.

Sequence

• Anesthesia: buccal and lingual infiltration plus an inferior alveolar block; check the CBCT distance from the apices to the mandibular canal first.
• Flap: sulcular full-thickness flap with a vertical release over sound bone; retract carefully to protect the mental nerve.
• Osteotomy with a round bur under copious saline and the microscope, wide enough to see the root end; curette the lesion and send it for histopathology.
• Resect 3 mm of the mesial root with a 0-degree bevel. The fragment at 3 mm should be exposed and removed with an ultrasonic tip or microexplorer.
• Identify the middle mesial canal and the isthmus with methylene blue, prepare 3 mm deep with ultrasound following the canal, and fill with a bioceramic.
• Reposition and suture; prescribe analgesia and give wound-care instructions.

Prognosis depends on

• A durable coronal seal and a ferrule.
• Remaining root length and crown-root ratio after resection: at least 4-5 mm of root and a favorable ratio.
• Complete cleaning of the isthmus, since this is where the original failure originated.
• Periodontal status and smoking.

Also

• Document nerve proximity and consider a CBCT-guided depth measurement; a lesion overlying the mandibular canal changes the risk calculus for both surgery and retreatment.