Curved mesiobuccal canal and a misleading apex locator in the practical test
In the practical test you are given an extracted maxillary first molar mounted in a typodont and 90 minutes to complete root canal treatment. The preoperative radiograph shows three root canals, with the mesiobuccal canal curving sharply, at about 40 degrees, in the apical third. Your estimated working length is 19 mm. You make the access cavity and locate the MB, DB and palatal orifices. When you place a size 10 K-file into the MB canal it binds at about 15 mm, and the electronic apex locator then reads 'apex' although the file is visibly short. The canal is not bleeding. You have 60 minutes remaining, and you must also produce a radiograph for the examiner and obturate the tooth.
1. Why might the apex locator read 'apex' when the file is only 15 mm in, and how would you establish a reliable working length?
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Why it happens - a reading of 'apex' with a short file usually means the tip is not following the canal. The common causes are:
- the file is against a wall or has created a ledge at the curve;
- the file is in a lateral canal or a perforation;
- the canal contains blood, pus or conductive irrigant, which shortens the electrical path;
- a metallic restoration or a file touching metal contacts the file and gives a false reading;
- a wide open apex, less likely here.
What I would do - remove the file, clean the flutes, irrigate, and precurve a size 08 or 10 file to match the curvature before re-entering. Use a crown-down approach to remove coronal interference first. Take a radiograph with the file held at the binding point using a paralleling technique. Repeat the locator reading only in a reasonably dry canal, after suction and with the canal free of excess sodium hypochlorite, and compare it with the radiograph and with tactile feedback.
Principle - a working length is never a single instrument reading. It is the agreement of a good radiograph, a reproducible locator reading and tactile sense, documented on the chart. If the true length is 19 mm, I instrument to 19 mm, not to 15 mm.
2. How would you negotiate and shape the curved MB canal without transporting it or separating a file?
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Glide path first - establish a smooth reproducible path to length with precurved size 08 and 10 hand files before any rotary instrument. If a 10 will not reach length passively, I do not proceed to rotary.
Technique
- Precurve every file to slightly more than the canal curvature.
- Use light apical pressure with a watch-winding or balanced-force motion; never force a file.
- Recapitulate and irrigate frequently; clean the flutes every time the file comes out.
- Use a crown-down sequence so the coronal third is opened before the apex is approached.
- Watch for signs of stress: the file unwinding, a squeaking sensation, or the file not returning to its original shape after removal means it must be discarded immediately.
Irrigation - sodium hypochlorite 1 to 3%, delivered with a side-vented 27-gauge needle kept 1 to 1.5 mm short of working length to avoid extrusion, with gentle agitation. Use EDTA 17% to remove the smear layer and aid negotiation. Never wedge a needle.
Shape - apical enlargement appropriate to the canal, typically 25/04 to 30/04 in a curved canal, with a patency file used passively and only if it passes without force. Document the final apical size and length.
3. If you cannot reach the full working length within the time available, what would you do, and how would you close the tooth?
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Do not force to length. Pushing a file past a blockage transports the canal, strips the outer wall of the curve, or separates the instrument. A short, clean, fully irrigated canal is better than a perforated one.
Options
- Negotiate further with a precurved 08 file and EDTA; if length is gained, complete in this visit.
- If length is not gained, accept the achieved length, instrument and irrigate thoroughly, dry, and obturate to that length with a matched cone, documenting the short obturation and the reason.
- If the canal is contaminated or time runs out, place calcium hydroxide, seal the access with a well-adapted temporary restoration, and complete at a second visit.
Obturation - match the master cone to the achieved length and verify with a radiograph. Warm vertical compaction or a single-cone technique with a suitable sealer is acceptable; the priority in a curved canal is not to overfill or to extrude sealer.
Coronal seal - a temporary or core build-up that seals the access is essential; leakage between visits is a common cause of failure.
Reflection - if the curvature is beyond what I can manage safely within the session, the correct professional judgement is to refer rather than persist. I would record the difficulty and inform the patient.