Locating MB2 Through a Zirconia Crown With Limited Opening
Fatima, 38, is referred to a specialist endodontic clinic in Salmiya for retreatment of 26. The tooth has a 6-year-old zirconia crown and a history of two previous access attempts through the crown; the palatal access cavity is filled with composite. She reports no pain, but the referring dentist cannot locate the second mesiobuccal canal and has seen a 'hollow' on the floor. Mouth opening is 31 mm; the tooth is buccally displaced and rotated 15 degrees. Preoperative radiograph shows three canals treated to length, a 30-degree mesiobuccal curvature, and a 5 mm periapical radiolucency on the mesiobuccal root. A small-field CBCT shows two mesiobuccal canals: MB1 treated, MB2 calcified and joining MB1 3 mm short of the apex; the distobuccal canal is separate. The crown margin is intact, and the patient wants to keep the tooth. She has no medical conditions and takes no medications.
1. How would you modify the access to locate MB2 through the existing zirconia crown, given 31 mm opening and 15 degrees of rotation?
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The first decision is whether the existing access is usable. I would remove the composite from the palatal cavity, inspect the floor with magnification and a dental operating microscope, and check that the access outline reaches the mesial marginal ridge. A small access that does not include the mesial slope of the occlusal surface will hide MB2 regardless of effort.
I would modify through the zirconia with a coarse diamond bur under water, removing restorative material and extending the outline mesially and buccally. The goal is a rhomboid or trapezoidal outline that gives a straight line to the mesiobuccal floor, not a larger hole for its own sake. I would use an ultrasonic tip to trough gently from MB1 toward the mesial floor, following the CBCT depth and direction, and I would rotate the patient's head and reposition myself to compensate for the 15-degree rotation and 31 mm opening.
If the crown margin is intact and the access can be sealed predictably, treatment through the crown is reasonable. If the angle prevents a safe path, I would remove the crown, place a provisional, and re-access the natural tooth. I would explain that a new crown may be needed and record that in the consent. Forcing access through a zirconia crown without vision risks perforation and an irreparable access.
2. The CBCT shows MB2 calcified and joining MB1 3 mm short of the apex. How does that alter your preparation, and what risk do you specifically watch?
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A joined canal changes the objective. I would not try to create a separate apical path for MB2. I would treat the system as a single apical canal with two coronal orifices: negotiate MB2 to the confluence, clean and shape MB1 to the apex, and ensure both orifices are irrigated and activated. The apical preparation should be kept conservative because the mesiobuccal root is thin and curved; over-enlargement near the join is how strip perforations occur.
The specific risk is strip perforation of the danger zone on the distal wall of the mesiobuccal root, especially if I try to force a file along a calcified MB2 that curves into MB1. I would use small hand files with chelating agent, verify patency only where it is safe, and use ultrasonic activation of irrigant in the coronal half of MB2 rather than aggressive filing. If MB2 cannot be negotiated beyond the calcification, I would document that, disinfect the accessible portion, and rely on the MB1 preparation and irrigation; the confluence means a separate apical preparation is not required. Obturation should fill both orifices without extruding material into the isthmus or perforation.
3. If straight-line access cannot be achieved, what alternatives would you present, and how would you decide?
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The alternatives are: continue through the crown with a modified access and accept a compromised angle; remove the crown and re-access the natural tooth; refer to an endodontist with better microscope access and experience with calcified MB2; or extract the tooth if restorability or access is genuinely impossible. Which is best depends on how much tooth structure remains under the crown, the position of the margin, the periodontal support and the patient's willingness to pay for a new crown.
I would not present extraction as the first option just because access is difficult. A 5 mm lesion and a missed MB2 are treatable if the canal can be disinfected; the question is whether I can do it safely. I would explain the trade-offs in terms of risk of perforation, risk of leaving untreated anatomy, cost and time.
My decision rule: if I cannot see the floor or trough toward MB2 without cutting into the root wall, I stop and remove the crown or refer. If the access is adequate after crown removal, I proceed. The patient should know before treatment starts that a new crown may be required, because that changes the cost and the consent.