Subgingival caries margin on a root-filled first molar: crown lengthening, forced eruption or extraction
A 55-year-old woman is referred for a crown on tooth 16. The tooth had root canal treatment 8 years ago and was never restored with a post; it now has a mesial caries lesion whose deepest point lies 2 mm apical to the bone crest. The mesiopalatal probing depth is 6 mm with bleeding on probing, while the other sulci are 2–3 mm. On the radiograph the mesial bone crest lies 3 mm apical to the cementoenamel junction, the root filling is 1 mm short of the apex, and there is no periapical radiolucency. Teeth 15 and 17 are present and sound; 17 carries a large MOD amalgam. She grinds severely and has worn through two acrylic splints in three years. The tooth is asymptomatic, percussion is normal, mobility is grade I, and she says she wants 'a crown like the one that lasted twelve years on the other side'.
1. Quantify the biologic-width requirement, and evaluate crown lengthening, orthodontic forced eruption and extraction. Give a recommendation.
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The restoration margin must sit about 3 mm coronal to bone: roughly 1 mm of sulcular depth plus a 2 mm biologic width (1 mm junctional epithelium, 1 mm connective tissue attachment). If the caries is 2 mm apical to the crest, the final margin sits about 2.5 mm apical to the crest after removing all caries, which means the bone would have to be resected about 5–5.5 mm apically to recreate that 3 mm distance.
That is a very large resection on a mesial surface. It would produce an unfavourable crown-to-root ratio on an already root-filled tooth, would very likely expose the buccal furcation of 16, would lower the bone between 15 and 16 (creating a defect on a sound tooth), and would leave an unesthetic long crown. Crown lengthening alone is therefore a poor choice here.
Orthodontic forced eruption with fibreotomy is more attractive: 2.5–3 mm of eruption moves the caries margin coronally while the bone largely stays, giving sound tooth structure for the margin without sacrificing bone or creating a furcation defect. It does, however, require healthy periodontal support, several months of treatment, and endodontic monitoring. Extraction followed by an implant or a 15–17 fixed partial denture is the honest alternative, and in a severe bruxer with grade I mobility I would put both options to her with the trade-offs stated rather than commit her silently to the most heroic one.
2. If she chooses to retain the tooth, describe the restorative sequence and design, including your decision about a post and the ferrule.
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First, remove all caries and the old restorative material and re-evaluate under rubber dam, because the final margin position and the amount of remaining axial wall determine everything else. Caries excavation must reach sound dentine at the margin; if it does not, the tooth is not restorable and she should be redirected.
No post is indicated. There is a 1 mm short but otherwise acceptable root filling with no periapical pathosis, the coronal walls can carry retention, and a post does not strengthen a root - it only adds perforation and strip-perforation risk. If the tooth is retained, an immediate dentine seal followed by a composite core build-up is appropriate. What I do insist on is 1.5–2 mm of circumferential ferrule; if that is not achievable after the eruption or resection, the tooth should not be crowned and extraction is the correct decision.
The provisional restoration should be made before the definitive crown so the tissues can be shaped and the occlusion tested. The definitive restoration is full coverage (zirconia or metal-ceramic) with supragingival margins where possible, a flat occlusal scheme and even contacts, plus a new hard splint - in a patient who has worn through two splints in three years, the occlusion must be treated as part of the crown, not an afterthought.
3. She asks about replacing the tooth with an implant instead of going through months of treatment. Compare the two paths and outline the sequence if she chooses extraction.
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Both are defensible; the comparison should be explicit. Retaining the tooth via forced eruption preserves bone and proprioception and avoids surgery, but it takes several months, leaves a root-filled tooth under a heavy bruxer, and depends on the tooth remaining periodontally stable. An implant avoids the biologic-width problem entirely and gives a restoration not dependent on a compromised tooth, but it introduces surgery, is itself vulnerable to peri-implantitis, and in the 16 region the maxillary sinus floor and the residual ridge width must be assessed before promising anything.
If she chooses extraction: I would take a cone-beam or at least a careful radiographic assessment of the sinus and socket, perform a minimally traumatic extraction, and decide on socket preservation based on the buccal wall. In a bruxer, immediate placement is tempting but risky if there is any buccal wall loss or infection. I would wait for soft-tissue healing (about 8–12 weeks), place the implant with adequate primary stability, and load it only after osseointegration with a protected occlusion and a new splint. She should also be warned that bruxism increases mechanical complications on both paths - fracture of the crown on the tooth, and ceramic or screw loosening on the implant.