Three-unit bridge or implant for a missing mandibular molar
A 62-year-old man requests replacement of a missing mandibular right first molar. The adjacent teeth are the second premolar, which has a 4 mm clinical crown and a small occlusal composite, and the second molar, which is root-filled with a large mesio-occlusodistal amalgam, 1 mm of ferrule, and a hairline crack in the distal cusp. Both are asymptomatic and have probing depths ≤3 mm. The ridge is 8 mm wide with 6 mm of bone height above the inferior alveolar canal. He grinds at night and has worn occlusal surfaces. He prefers a fixed solution and asks for a three-unit bridge. He has no medical contraindications and takes no medications. His maximum opening is 45 mm and he has a low smile line. He has good oral hygiene.
1. Evaluate whether a three-unit fixed partial denture is appropriate, focusing on the suitability of each abutment.
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A three-unit fixed partial denture is possible but not the first choice in this mouth. The second premolar has a short 4 mm clinical crown but is otherwise sound; a short crown can still be prepared with a supragingival finish line and adequate retention, but retention and resistance form are less favorable. The second molar is the weak link: it is root-filled, has a large MOD amalgam, only 1 mm of ferrule, and a cracked distal cusp. Using it as an abutment for a fixed partial denture loads a compromised tooth and risks catastrophic fracture. A crown may protect it, but 1 mm ferrule and a crack mean the prognosis is guarded even before it is used as an abutment. Night grinding adds occlusal risk.
The bone height of 6 mm above the canal makes a single implant challenging but not impossible; a short implant may be feasible, or site development may be needed. The key judgment is that the bridge is not automatically wrong, but it is abutment-driven and risk-heavy, whereas a single implant replaces the missing tooth without loading the cracked molar.
2. If the patient still chooses a bridge, describe the design, material, and occlusal considerations.
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If a bridge is chosen, I would use full-coverage retainers on both abutments, not a partial-coverage premolar retainer, because retention and resistance form are limited. The second molar needs cuspal coverage and a design that protects the cracked cusp; if the crack extends subgingivally or the tooth cannot be adequately ferrule-restored, the bridge plan should be abandoned.
For a bruxer, monolithic zirconia is a reasonable material because of its strength and wear resistance, provided the connector dimensions are adequate; a molar connector should be generous, and the occlusion should be checked in excursive movements. Margins should be supragingival where possible, and the occlusion should be adjusted to avoid heavy working-side contacts on the pontic and retainers. A night guard should be part of the plan.
The patient must understand that the bridge does not treat the cracked molar; it converts a questionable tooth into a load-bearing abutment. The consent discussion should include fracture, debonding, pulp complications, and the possibility that the second molar is lost later.
3. What would you tell the patient about alternatives and contingency planning if the second molar fails?
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I would present three realistic options: single implant crown, three-unit fixed partial denture, and removable partial denture, with the latter being the least preferred for a molar space but still an option. The implant option avoids preparing the premolar and avoids loading the cracked molar, but it requires adequate bone and a cooperative patient; 6 mm of bone above the canal means a short implant or staged bone augmentation may be needed, with a small risk of nerve injury.
Contingency planning matters. If the second molar fails after being used as an abutment, the bridge fails too, and the patient may then need an implant and a new prosthesis. That is a larger burden than treating the missing tooth with an implant now. I would explain that the cracked, root-filled molar may need a crown regardless, and that its long-term survival is uncertain.
I would document the discussion, including the patient's preference for a fixed solution and his bruxism, and recommend a night guard and regular review. A decision is not informed unless he understands that the bridge's weakest point is not the missing tooth but the abutment.