Full-Arch Mandibular Rehabilitation: Which Prosthesis and Which Loading Protocol?
A 67-year-old woman with an edentulous mandible and a partially edentulous maxilla (teeth 13–23 remaining, all with guarded prognosis) requests a 'permanent' lower solution. CBCT shows 6–8 mm of bone above the inferior alveolar nerve in the posterior mandible, 14 mm in the anterior mandible, and a 3 mm buccal concavity in the posterior. She has a history of well-controlled hypertension, takes a daily aspirin, and has a maximum mouth opening of 38 mm. She is edentulous in the lower arch and currently wears a complete denture that is 12 years old. She is a bruxer with visible wear facets on the maxillary teeth.
1. Compare a two-implant overdenture, a four-implant fixed complete denture, and an all-on-four-type design for this patient. Which do you recommend and why?
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Option comparison against her specific constraints:
| Option | Fit to her anatomy | Pros | Cons |
|---|---|---|---|
| 2-implant overdenture | Anterior mandible has 14 mm - easy | Lowest cost, least surgery, retrievable, easiest hygiene | Removable (she asked for 'permanent'), needs denture space, wear of attachments, not fixed |
| 4-implant fixed complete denture (straight) | Posterior 6–8 mm limits implant length; 3 mm buccal concavity risks dehiscence | Fixed, well-documented, cleansable | Requires graft or short implants posteriorly; high load in a bruxer |
| All-on-4 (tilted posterior implants) | Tilting avoids the 3 mm concavity and the nerve; 6–8 mm posterior bone is workable with tilted implants | Avoids grafting, fixed, fewer implants | Technique-sensitive; cantilever load; bruxism is a relative contraindication to cantilevers |
Recommendation: A four-implant fixed complete denture with distally tilted posterior implants (or short posterior implants if tilting is not feasible) is the best fit if the bruxism is managed. The 3 mm buccal concavity and 6–8 mm posterior height make a straight posterior implant risky; tilting follows the anterior wall of the sinus/nerve and gains anterior-posterior spread. The 2-implant overdenture is a legitimate fallback if she cannot accept the cost or the surgical extent.
Critical caveat: Bruxism is a relative contraindication to a cantilevered fixed prosthesis. Either eliminate the cantilever, use a night guard, or steer her toward the overdenture. Document this trade-off explicitly.
2. What loading protocol would you choose, and what determines whether immediate loading is safe here?
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Immediate loading criteria (all must be met):
- Insertion torque ≥35 Ncm (or ISQ ≥65–70) for all implants, especially the posterior tilted ones.
- No parafunction - she is a bruxer, so this criterion fails.
- Adequate bone quality and quantity; anterior mandible is type II–III, generally favorable, but posterior is type III–IV.
- Ability to fabricate a rigid, passively fitting provisional with even occlusal contacts and no cantilever.
Judgment: Given her bruxism, immediate loading of a full-arch fixed prosthesis is not the default. The defensible plan is:
- Conventional loading (3–4 months) with a well-fitting, relieved interim denture, or
- Immediate loading only if all implants achieve ≥35 Ncm and the provisional is designed with no cantilever, bilateral balanced occlusion, and a night guard - and even then, the bruxism makes this a calculated risk.
What the exam is testing: You cannot separate the loading decision from the parafunction history and the prosthesis design. The correct answer is not 'immediate is better' or 'delayed is safer' in the abstract - it is that the bruxism converts this from a routine immediate-load case to a case where delayed loading (or a removable prosthesis) is the lower-risk choice, and the patient must be told why.
3. She asks whether the maxillary teeth should be extracted and a maxillary prosthesis placed at the same time. How do you sequence this?
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Sequencing principles:
- Do not treat both arches simultaneously unless the patient can tolerate it and the prosthodontic plan for both is finalized. Simultaneous full-arch treatment in a bruxer with guarded maxillary teeth is high-risk and hard to stage.
Recommended sequence:
- Stabilize the maxilla first - the remaining maxillary teeth (13–23) have guarded prognosis and will be the opposing arch. Decide: retain with splinting, or extract and convert to a maxillary prosthesis. If they are lost later, the mandibular prosthesis's occlusal scheme changes.
- Mandibular surgery with a conventional-loading protocol and an interim denture.
- Maxillary rehabilitation after the mandible is stable and the patient has demonstrated hygiene and compliance with a night guard.
If she insists on simultaneous treatment:
- Plan the maxillary prosthesis before mandibular surgery so the final occlusal scheme is designed once, not twice.
- Use a provisional maxillary prosthesis (implant-supported or tissue-supported) that can be relined as the mandible heals.
- Confirm the patient's commitment to a night guard and to maintenance before starting.
Documentation: Record the guarded prognosis of the maxillary teeth, the rationale for staging, and the patient's understanding that the maxillary plan may change if those teeth fail.