The Case at a Glance
Field | Value |
|---|---|
Site | #31 — lower left second molar (FDI 37) |
Patient | Male, late-40s, non-smoker, no systemic contraindications noted on referral |
Implant (already in place) | 5.0 × 10 mm, bone-level, platform-switched. Osseointegrated and stable per the referring dentist's assessment |
Original restoration | Cement-retained monolithic zirconia crown on a stock titanium abutment. Delivered elsewhere. In service for about 3 years |
Reason for return | Repeated screw loosening (3 episodes in 14 months), proximal contact opening with food impaction, visible occlusal wear facets |
What we were asked to make | A new crown, this time screw-retained, with a plan that addresses why the first one failed |
Case type | Remake with redesign — not a replacement of the same part |
Patient Situation, Only the Parts That Changed the Design
A few things in the chart changed the way we drew this restoration. The rest sat in the file for context.
- Site is a lower second molar. Distal access is tight for the patient's hygiene and for us during try-in. That alone pushes the design toward screw retention — if we ever need to retrieve the crown, we do not want to be sectioning it from behind the first molar.
- Opposing dentition is a natural upper second molar with heavy wear facets. The patient grinds. The dentist's referral did not include a formal bruxism diagnosis, but the wear on the opposing enamel and on the previous crown was obvious on the models.
- Mesio-distal space between #30 and the implant platform is 11.2 mm. Not generous, not tight. Enough to build a proper emergence profile if we keep the contour disciplined.
- Soft-tissue height around the implant platform is 3 mm buccal / 2 mm lingual. The dentist flagged that food had been catching on the lingual side, which lines up with how the old crown was contoured.
Why the First Crown Failed — What We Could Reconstruct
The old restoration arrived with the dentist's cover note and a set of intra-oral photos. We disassembled it on our bench and logged what we saw. We are not diagnosing anyone's work — we are writing down what the piece told us.
Observation | What it suggests about the original design |
|---|---|
Stock abutment, 2 mm cuff height, straight profile | Emergence came out of the gingiva too wide, with a sharp shoulder transition. That shelf is where food was catching |
Cement line visible at the sulcus floor | Residual cement sites are a known plaque trap on implant restorations (EFP S3 clinical practice guideline) |
Wear facets on buccal cusps, mesio-lingual cusp flattened | Occlusal scheme concentrated load on the mesial half of the table — not a balanced wide contact |
Interproximal to #30 open at 0.4 mm with floss passing without resistance | Contact had drifted. Repeated remove/retighten cycles can relax a cement-retained setup faster than a screw-retained one |
Screw thread shows plastic deformation at mid-shaft | Consistent with repeated retorquing. The screw itself is a wear item, not a structural failure |
Screw and abutment loosening is reported in roughly 12.7% of implant single crowns at 5 years in a systematic review of 26 cohorts (PubMed 18067597). So the mode is common. What is not common is doing something about it before the third tightening appointment.
What the Dentist Asked For
The referral came in with five requirements. Four of them were firm; one was negotiable. We read that split as the start of the brief, not the end.
# | Request | Firm or negotiable | What it drives in the lab |
|---|---|---|---|
1 | Screw-retained, retrievable | Firm | Access hole position has to clear the central fossa and the contact zones |
2 | No residual cement anywhere subgingival | Firm | Rules out cementation; also informs how we finish the margin on the Ti-base |
3 | Material that will tolerate heavy occlusal load on the opposing natural dentition | Firm | Monolithic high-strength zirconia; no veneering porcelain on functional cusps |
4 | Emergence profile designed for hygiene access — the patient uses floss and a tufted brush | Firm | S-shape transition, no shoulders, lingual contour matched to the 2 mm soft-tissue height |
5 | Shade A3, with mild characterization on the buccal | Negotiable | Low priority on a second molar; we delivered it, but we did not spend design time on it |
We also sent one proposal back before starting. The dentist had asked for the access hole to come out in the central fossa, which is textbook. In this specific patient the planned implant angulation put that hole 0.9 mm inside the mesio-palatal functional cusp — close enough that the composite seal would be under load every bite. We proposed an angled screw channel abutment that moved the exit 15° to the distal, keeping the hole in the central fossa but out of the occlusal contact zone. The dentist confirmed by email the next day. Both the first design and the revised design are in the case folder.
How We Satisfied Those Requirements
This is the part that is only ours. Everything below is from the work order and from measurements on the finished piece.
1. Retention: screw, by way of an angled channel
- Platform: implant-level, bonded to a titanium base (Ti-base) with GC Multi-Link Hybrid Abutment adhesive, screw channel at 15° distal to the long axis
- Access hole final diameter: 2.8 mm, closed with PTFE tape + Fuji II LC + light-cured composite in three increments
- Torque spec on delivery: per the implant system's prosthetic manual, confirmed by the dentist chairside
Screw retention gave us something more useful than a slogan. It gave us retrievability — if anything in the next five years needs attention (contact point, soft-tissue change, hygiene re-evaluation), the dentist can unscrew the crown in-chair rather than coring and remaking.
2. Material: monolithic multilayer zirconia
Spec | Value |
|---|---|
Material class | High-strength multilayer zirconia, 3Y–TZP at cervical, graduated transparency to incisal |
Minimum occlusal thickness | 1.5 mm at centric contacts |
Minimum axial thickness | 1.0 mm |
Finishing | Glazed and polished on functional surfaces; no feldspathic layering on cusps or marginal ridges |
High-strength zirconia is what our catalogue uses for posterior crowns on patients with heavy occlusion and parafunction (see our internal material guide on monolithic vs layered zirconia and PFM). Keeping the functional cusps monolithic removes the main failure mode of layered posterior work — veneer chipping — and keeps the design headroom you have inside normal clinical thickness.
3. Occlusion: redesigned, not re-copied
The old crown had its heaviest contacts on the mesial-buccal cusp tip. We rebuilt the scheme from scratch:
- Centric contacts on inner incline surfaces, three contacts, widely distributed, each 0.5–1.0 mm wide — not pinpoint
- Mild infraocclusion of 20–30 µm at the implant crown compared with the adjacent natural first molar (teeth under PDL settle under load; the implant does not)
- No working or balancing contacts in protrusive or lateral excursions. In this patient's envelope of function the implant crown is passive during lateral movement; load gets transferred to canine guidance
- Cusp inclines kept shallower than the original by about 10°, to reduce the horizontal load vector that was likely contributing to the screw unwinding
The infraocclusion target comes from mechanical reasoning about implant restorations and from the clinician's chairside call, not from a specific guideline citation. We flagged it in the delivery sheet so it would be re-evaluated at the first post-insertion check.
4. Emergence profile and interproximal contact
- Emergence rebuilt as an S-curve, not a straight ramp. From the implant platform (5.0 mm diameter) widening through a concave zone at the mid-sulcus, then flaring to full crown contour above the gingival margin
- Interproximal contact point reconstructed at 2 mm below the marginal ridge, 0.8 mm wide bucco-lingually, firm floss resistance verified on the stone model
- Lingual contour pulled in by about 0.3 mm compared with the previous crown, specifically so the patient's tongue + floss combo can reach the lingual sulcus. This was the detail the referral note had asked about
The point of this is not aesthetics. It is cleanability. The EFP/AAP consensus on peri-implant mucositis states it outright: implant placement and prosthetic reconstructions need to allow proper personal cleaning, diagnosis by probing and professional plaque removal (Jepsen et al. 2015, PubMed 25626479). Peri-implant mucositis is the common early finding. A 2025 AO/AAP meta-analysis of 102 studies covering 13,030 patients puts patient-level prevalence at 46% for mucositis and 21% for peri-implantitis (AO/AAP systematic review, 2025). Those are population numbers, not a prediction for this patient. We read them as the reason emergence design is a design question, not a cosmetic one.
5. Workflow at the lab
Every step on this case was done inside our own facility. No case data was outsourced.
Step | Where | Working days |
|---|---|---|
Digital case intake, scan + chart review, remake notes | Case Coordination | 1 |
CAD design, including angled channel abutment + crown as one project file | CAD Department | 1 |
Design review with the dentist, revision #1 (contact point width) | Case Coordination ↔ Dentist | 1 |
CAM milling (Ti-base + zirconia blank) | Milling | 1 |
Zirconia sintering, staining, glaze | Zirconia processing | 2 |
Polish on functional surfaces, Ti-base bonding | Finishing | 1 |
QC inspection — fit on model, screw seat, contact, occlusal map, surface | QC | 1 |
Pack and ship | Logistics | 1 |
Total working days in the lab | 9 |
The intake-to-shipment window on this case was 9 working days. On remakes we flag the file to Case Coordination at receipt so revisions do not stack up at the end.
Result
The crown was delivered to the dentist, torqued per the implant system's prosthetic manual, and the access hole was sealed in the chair. The dentist sent us the post-delivery check at 48 hours, at 1 week, and at 3 months.
Check-in | What came back |
|---|---|
48 hours | Patient comfortable; occlusion stable on articulating paper; no sensitivity to pressure |
1 week | Soft tissue adapted around the new emergence; no food impaction reported by the patient |
3 months | Baseline peri-implant probing by the dentist (four sites). Values stable, no bleeding on probing. No retorque needed. Three months is where we set the first baseline in our own follow-up sheet. What the current guideline does set out is a structured supportive programme with periodic assessment of peri-implant tissue health (EFP S3, 2023), and the 2025 AO/AAP consensus describes maintenance as essential to keeping the tissue stable long term (AO/AAP consensus, 2025) |
We have the three-month follow-up in writing. We do not have five-year data on this specific crown, and we are not going to claim we do. What we can say is that the mechanical reason the first crown kept failing — load concentrated on a steep mesial cusp, access hole positioned where occlusal force would hit it, cement-retained setup that could not be retrieved and inspected cleanly — has been changed in the new one.
If the next three or five years show that any of these design choices needs to be revisited, we will document it on this page.
A Short Note on "Molar Implant" Restorations in General
A dental implant molar restoration lives in the hardest-working part of the mouth. Posterior bite forces are higher than anterior forces, the bone quality in the posterior maxilla is the lowest of the four quadrants of the jaw (CBCT bone-density study, PubMed 24867520), and the crown itself is the component that most often needs attention first. A systematic review of implant single crowns gives the crown a 5-year survival of 94.5%, compared with the implant body at 96.8% (PubMed 18067597) — in other words, the restoration is more likely to need a revisit than the implant underneath it. At ten years the implant body is still around 96.4% across 18 prospective cohorts (10-year survival review, PubMed 30904559). The part that ages is the part we make.
That gap is where a lab earns its place in the workflow. The surgeon decides whether a tooth can be replaced. The clinician decides when to load it — placement timing and loading timing are two separate protocol decisions, and the ITI consensus sets them out that way (ITI Consensus — Implant Placement and Loading Protocols). The lab decides, within those constraints, what the thing on top is going to look like, bite like, and clean like for the years it is in service. Materials, retention method, emergence shape, occlusal scheme — those are our side of the table.
For our part, we build molar implant crowns and dental implant molar restorations as a full in-house workflow at Istar: digital case intake, CAD design, in-house milling and zirconia processing, ceramic work, polish, QC, pack, and ship. Approximately 460 team members, roughly 80 of them in the implant restoration department. No outsourcing of case files to third-party labs.
Working With Us on a Similar Case
If you have a case that looks like this one — an implant crown coming back with screw loosening, contact drift, or an emergence profile that is not letting the patient keep the site clean — the useful first exchange is not a quote. It is the piece itself.
Send us: the chairside photos, the pre- and post-insertion radiographs, the implant system and fixture reference, the opposing dentition, and anything the patient has reported since delivery. We will come back with a redesign proposal before we quote. Pricing is based on the final design plan, not on the inquiry.
- Submit a case file: [email protected] (replace with your real contact or form)
- Factory visit (Shenzhen or Huizhou): by appointment through your case coordinator
References Used on This Page
- Herrera D, Berglundh T, Schwarz F, et al. Prevention and treatment of peri-implant diseases — The EFP S3 level clinical practice guideline. J Clin Periodontol 2023;50(Suppl 26):4–76. https://onlinelibrary.wiley.com/doi/full/10.1111/jcpe.13823
- Jung RE, Pjetursson BE, Glauser R, et al. A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Implants Res 2008;19(2):119–130. PubMed 18067597
- Jepsen S, Berglundh T, Genco R, et al. Primary prevention of peri-implantitis: managing peri-implant mucositis. J Clin Periodontol 2015;42(Suppl 16):S152–S157. PubMed 25626479
- Galarraga-Vinueza M, Pagni S, Finkelman M, et al. Prevalence, incidence, systemic, behavioral, and patient-related risk factors and indicators for peri-implant diseases: an AO/AAP systematic review and meta-analysis. J Periodontol 2025;96:587–633. PMC12273760
- Wang HL, Avila-Ortiz G, Monje A, et al. AO/AAP consensus on prevention and management of peri-implant diseases and conditions: Summary report. J Periodontol 2025;96(6):519–541. PMC12273748
- Hao Y, Zhao W, Wang Y, et al. Assessments of jaw bone density at implant sites using 3D cone-beam computed tomography. Eur Rev Med Pharmacol Sci 2014;18(9):1398–1403. PubMed 24867520
- Howe MS, Keys W, Richards D. Long-term (10-year) dental implant survival: a systematic review and sensitivity meta-analysis. J Dent 2019;84:9–21. PubMed 30904559
- Gallucci G, Hamilton A, Zhou W, Buser D, Chen ST. Implant Placement and Loading Protocols. ITI Consensus Conference 2018. ITI Consensus Database