Posterior Single-Unit Implant in a Bruxer with Limited Interocclusal Space
A 58-year-old man requests replacement of the mandibular right first molar (tooth 46) lost 8 months ago. He has a documented history of nocturnal bruxism managed with a hard occlusal splint, and wear facets are visible on the opposing maxillary teeth. Cone-beam CT shows 11 mm of available bone height above the inferior alveolar canal and 6.5 mm buccolingual width. The edentulous span measures 9 mm. Interocclusal space at the proposed site is 6.0 mm from the crest of the ridge to the opposing occlusal plane. The patient is a heavy coffee drinker, takes no medications, and has a HbA1c of 6.1%. He asks whether a single implant crown is 'as strong as a bridge.'
1. Evaluate whether a single implant-supported crown, a conventional three-unit fixed partial denture, or a cantilever resin-bonded option is the most defensible plan, and justify your choice against the specific numbers given.
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A single implant crown is the defensible plan, but the numbers matter:
- Bone: 11 mm height above the canal and 6.5 mm width comfortably accommodate a standard-diameter implant (4.1–4.8 mm) with ≥1.5 mm buccal and lingual bone, so no grafting is required.
- Span: A 9 mm single-tooth gap is ideal for one implant; a three-unit FPD would require preparing two sound adjacent teeth, which is destructive and creates a hygienic burden in a bruxer.
- Interocclusal space: 6.0 mm is the tight constraint. A screw-retained monolithic zirconia crown needs roughly 1.5–2 mm of restorative space above the implant platform plus abutment height; 6.0 mm is workable but leaves little margin for error, so plan a screw-retained design and confirm the platform depth at surgery. If space falls below ~5 mm, consider a hybrid or a shorter abutment.
- Bruxism: Not a contraindication, but it changes the occlusal scheme - flat, shallow anatomy, light centric contact, no working/non-working interferences, and continued splint use. A cantilever resin-bonded option is rejected because a 9 mm cantilever in a bruxer invites debonding and overload.
Conclusion: single implant, screw-retained monolithic zirconia, protective occlusal scheme, splint continued.
2. The patient asks what happens if he does not wear his splint. Explain the biomechanical risk specific to a single implant crown in this patient.
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A single implant lacks a periodontal ligament, so it has no proprioceptive feedback and no viscoelastic cushioning. In a bruxer, parafunctional loads can be several times higher than functional loads and are delivered axially and laterally with no protective reflex.
Specific risks:
- Cement/microgap and screw loosening - cyclic off-axial loading concentrates stress at the abutment screw and the implant–abutment interface.
- Peri-implant bone loss - occlusal overload is a co-factor (with plaque) for marginal bone loss and, in extreme cases, implant fracture.
- Restorative failure - chipping of the crown, especially if the occlusal scheme has steep cusps or excursive contacts.
This is why the plan includes a flat occlusal scheme and a hard splint. The splint distributes load and protects the restoration; without it, the failure rate rises and the patient should be counseled that the restoration is being asked to do more than it was designed for.