Cobalt Chrome Lower Denture Case: A Mandibular Kennedy Class I, From Rx to Fit

The referring dentist had already tried to address this with a flexible partial. That approach had not held up: the appliance could not be adjusted without remaking it, and it did not give the rigidity needed across an arch with bilateral free ends. The reason we are showing this case is not that the flexible option was wrong — it is that it ran into exactly the limit that a cast Co-Cr framework is built to solve, and the dentist knew it before she walked in.

1.2 What was clinically documented at impression

The information below is what reached us on the work order and in the accompanying records. We restate it because it is the input that drove every design decision in this case.

Remaining dentition. The lower arch retained the six anterior teeth — both central incisors, both lateral incisors, and both canines. The canines were the terminal abutments on each side. The remaining teeth were recorded as unrestored, with no active caries and no mobility noted at the time of impression.

Edentulous areas. Both posterior segments were edentulous, extending distally from the canines. Ridge resorption in those areas was moderate. The clinical significance for us was that these are distal-extension saddles, which means the base will sit on tissue that compresses under load, and the framework has to be designed around that rather than built as if the seat were static.

Mandibular tori. Both lingual tori were prominent enough to be a real design constraint, not a minor landmark. A mandibular major connector cannot simply run across them unrelieved. This single feature is what ultimately ruled out the lingual bar in this case, and it drove the blockout work described in Step 4 below.

Floor of mouth. The floor of the mouth was high, with a shallow lingual sulcus. This matters for the same reason the tori do: it reduces the vertical envelope available to a lingual bar, because the bar has to clear the functional depth of the tongue.

Interarch space. Space between the ridge and the opposing occlusal plane was limited in the posterior segments. This constrained saddle design and tooth selection rather than the framework itself, but it was noted on the Rx.

Implants. The dentist recorded that implant placement was not an option for this patient at this time. The Rx asked for a tooth-supported and tissue-supported partial instead, with a framework rigid enough to distribute load across the arch. We did not participate in that assessment and we are not commenting on it — we simply built the framework the dentist specified.

1.3 What the patient wanted out of the finished piece

Worth stating plainly, because it shaped the setup: she wanted to eat normal food without the appliance shifting, and she wanted the metal to stay out of sight in the lower front. She did not want to talk about a second removable in five years. That last point is the reason the dentist asked for the mesh areas described in the Rx — it is the one thing that can be done at the design stage to make a future addition less disruptive.

2. The prescription

This is the work order as we received it. We have reproduced it in full because every line of it shows up later in our process, and because the level of detail in an Rx is the single biggest driver of how smoothly a case runs.

2.1 The Rx as written

2.2 Reading the Rx

Three lines in that order deserve a note, because they are where the fabrication decisions actually live.

The connector choice is a decision, not a preference. A lingual bar is the textbook default for a mandibular partial when there is room for it. It was ruled out here on two independent grounds — the tori it would have to cross, and the high floor of mouth that leaves it nowhere to sit. The lingual plate solves both, and it also supplies the indirect retention the case needs without a separate rest. The design guidance we worked from frames a lingual plate as a legitimate choice with its own requirements rather than a fallback, and we followed the dimensional requirements in that guidance rather than eyeballing the borders. As a reference point, the plate's upper border typically sits about 1.5 to 2 mm below the free gingival margin, with 1 mm as the absolute minimum, and its lower border is set by the functional depth of the floor of the mouth — the Dalhousie Removable Partial Denture Manual is the reference we work from on these numbers.

The clasp specification is the part of the Rx that most affects fit. The RPA assembly on each canine is not an interchangeable choice. A mesial rest on a distal-extension abutment is deliberate: it moves the fulcrum axis in a direction that reduces the torque the base takes when the saddle sinks into the mucosa. The proximal plate and clasp arm have to match the undercut we survey, and the cast arm has to have the flex the undercut depth allows. Get the undercut relationship wrong and the clasp either does not engage or binds on the way in.

The mesh areas are a small line with a large consequence. Two lattice areas, one per saddle, positioned for a molar. A framework with a lattice section can accept a tooth added later without fabricating a new framework — the survival data on clasp-retained Co-Cr partials notes that mesh or lattice areas can accommodate later tooth addition. Worth being accurate about what that does and does not do: it defers a remake, it does not remove one.

3. How we built it

Our process for a cast framework runs in ten steps. Here is this case, step by step, with the Rx line each step answers to.

Step 1 — Intake and data verification

The case arrived as a digital file. We run a completeness check before anything else: does the data match the Rx, and is everything the Rx asks for actually present. For this case the file, the shade, and the arrangement guidance were all present and consistent, and the case entered the design queue the same day.

Step 2 — Working cast and survey

The scan data was used to produce a working cast, which was then surveyed. The survey is the step that fixes the path of insertion and identifies the undercuts that the clasp arms will engage, and it is the point where a design either holds together or falls apart. We marked the undercut on both canines and confirmed the approach to the interdental areas was clear for the proximal plates.

Step 3 — Design drawing, sent for approval

For a framework of this complexity we send a labelled design drawing to the dentist for written approval before any material is committed. The drawing carried the clasp type on each canine with the undercut side marked, the mesial rest positions, the lingual plate's upper and lower borders dimensioned against the free gingival margin, the relief ranges across the tori and ridge crests, and the two lattice areas with their coverage.

This is standard practice on our side for cases of this type, and it is also the step that catches the most. The dentist came back with one change: they wanted the plate's upper border sitting slightly lower across the central incisors, for esthetics. We revised the drawing, it came back approved, and only then did the file move to production.

Step 4 — Blockout and relief

This is where the tori get dealt with, and it is the step that took the most care on this case.

Blockout is the process of filling in the undercuts on the master cast so the wax pattern can be withdrawn without pulling the investment with it, and it is where relief is added where the framework is not supposed to touch tissue. On this case the two lingual tori required a relief zone on the master cast before anything else could proceed, so that the plate could pass over them without pressing on them. The ridge crests under the distal extensions were relieved for the same reason.

The dimensional requirements the plate has to meet — upper border clearance from the free gingival margin, lower border set by the floor of the mouth — were applied at this stage, and the borders were drawn to match the approved drawing rather than approximated.

Step 5 — Pattern, refractory model, and casting

The framework was waxed up to the approved drawing, the pattern was invested in a refractory mold, and the framework was cast in a dental cobalt-chromium alloy.

The alloy meets the Type 5 classification under ISO 22674, the dentistry metallic materials standard, which is the right reference for a removable framework in this class of alloy. ASTM F75 also comes up in Co-Cr conversations, and we mention it because customers ask: it covers cobalt-chromium-molybdenum castings for surgical implants, which is a different application from removable prosthodontic work.

Step 6 — Framework try-in, before the acrylic

The cast framework was tried in on the master cast and checked for passive seating — no rocking, no need for finger pressure, and the lingual plate sitting clear of the tori. Rest seats were confirmed fully home one abutment at a time rather than by overall impression.

This step is deliberately placed before the saddles are processed. Once acrylic is on the framework, a fit problem stops being a simple correction and starts being a rebuild, and we have found it far better to spend the time here.

Step 7 — Saddle processing

The two lattice areas were prepared for mechanical retention of the acrylic, and the saddle bases were processed onto the framework. The teeth were set to the VITA shade and mould on the Rx, with the anterior positions following the dentist's arrangement guidance and the posterior teeth carrying the occlusal contact as specified — the framework itself was kept clear of the opposing contacts.

Step 8 — Finishing

The metal-to-acrylic finish line was finished to a clean junction, and the whole assembly was polished in stages to a smooth surface with no scratches or burrs. Surface finish matters here more than it does on a fixed restoration, because the patient is handling this piece against the gingiva and the tongue every day.

Step 9 — Independent final inspection

Final release is done by a quality inspector who did not work on the case. That separation is a fixed rule in our process, not a formality. For this case the inspection covered:

• Framework seated passively on the master cast, no rocking
• Both mesial rests fully home, checked separately
• Lingual plate borders matching the approved drawing
• Relief maintained over both tori and the ridge crests
• Clasp arms engaging the surveyed undercut with appropriate flex, no occlusal interference
• Occlusion verified in centric, protrusive, and lateral excursions, with no early contacts
• Tooth position and shade matching the Rx
• Metal-to-acrylic junction clean, with the retention features intact
• Surface finish free of scratches and burrs

Step 10 — Delivery

The finished partial was shipped with the case documentation, along with the alloy batch and composition certificate for the metal it was cast from.

4. The fitting appointment

The dentist scheduled the patient for a fit and adjustment appointment when the partial arrived. This is where the work gets tested against a real mouth, and where a few things showed up that no amount of pre-insertion checking would have caught.

4.1 What the dentist checked

• Framework seating and border extension, checked against the approved drawing
• Occlusal contacts in all three movements, with early contacts removed
• Clearance between the lingual plate and the gingival margins of the anterior teeth
• Pressure from the plate over the tori — none present
• Tongue space and speech
• The two canine abutments under load, checked for any reaction in the supporting tissues

4.2 The dentist's feedback

The dentist's assessment at the fit appointment was that the framework seated fully and passively on the first appointment, with no adjustment required for retention. The border positions and the relief over the tori matched the approved drawing, which she noted was the single thing she had been most concerned about and the reason she had asked to see the drawing before the framework was cast.

Her one comment on the finished piece was a request, which we logged as a design note for the patient's next appointment rather than a fault: the acrylic over the anterior lattice was running slightly close to the gingival margin, and she wanted the patient to be shown how to clean the transition. She also asked us to record in the case file that the metal areas are exposed by design, so that if the patient reports tenderness in a specific spot the dentist knows to look there first rather than assume a misfit.

4.3 The patient's feedback

At the fitting appointment she said the partial felt "solid" — the first thing she mentioned, unprompted, was that it did not move when she moved her jaw around, which was the thing she had been chasing for two years. She reported that the lower front felt noticeably less bulky than the flexible appliance she had been wearing, and that she had not noticed the metal at all until she was told to look for it.

The things she flagged were practical rather than structural: that the metal felt a little rough at one spot against her tongue and she kept rubbing it with her tongue, and that she was aware of the appliance sitting lower than she expected. The first was a finishing matter and was smoothed at the appointment. The second is the honest trade-off of a lingual plate, and it is a point we raise at delivery with every case of this type.

4.4 Two weeks later

The dentist reviewed the patient at two weeks. The partial was still seating fully with no adjustment. The sore spot on the tongue had not returned after the polish, which confirmed it was a surface finish issue rather than a fit issue. There was no tissue reaction under the abutments and no pressure mark at the tori. The patient reported that she had gone back to foods she had been avoiding — things that needed to be cut and chewed — and that she was wearing the partial through meals rather than taking it out.

She did report one thing that was genuinely useful to us, and it is the kind of detail only a patient surfaces. The plate is comfortable, but she had noticed that food collects along the border where it meets the teeth, and she had started to think about it. The dentist reinforced cleaning at the two-week visit, and we have since started flagging the border-cleaning point more explicitly at delivery for lingual plate cases, since it is a predictable consequence of the design rather than a fault in it.

5. About the lab behind this case

Plain facts, because they affect what we can do for a case like this one:

• We have been running since 2012, with manufacturing in Shenzhen and Huizhou, and we are a full-service lab — cases are not passed to another lab.
• Our removable department is around 55 technicians, inside a team of roughly 460 people, with a quality group kept separate from production.
• Removable case volume runs around 4,000 a month in normal season and we have scaled to about 6,500 in peak season, so turnaround does not quietly slip when orders stack up.
• We cast and finish Co-Cr frameworks in-house, and we can also work in Vitallium, titanium, and PEEK when the dentist specifies it. Alloy batch and composition certificates are available on request.
• We hold case records — prescriptions, approved designs, production notes — for the standard 3 months by default. If a dentist wants a case preserved longer, for a long-term follow-up or a write-up, tell us at the time of the order and we will flag it for extended retention.
• Communication runs in English: email, WhatsApp, and video case reviews, with English material certificates and labels available.
• On lab-side remakes, the position is simple. If the fault is ours — a framework that will not seat, a clasp set wrong, poor finish, a shade that does not match the Rx — we redo it, and remake cases are prioritised. Shipping and third-party implant parts are not covered under a lab-side remake, and changes requested after the fact (a new shade, a different material, proceeding on an impression we have flagged) are quoted separately.

On alloy content, since it comes up on nearly every case: Co-Cr dental alloys contain cobalt and chromium with small amounts of other elements, and there are documented reports of type IV delayed hypersensitivity to cobalt, chromium and nickel. We do not make allergy-free claims about our alloys. If a patient has a known metals history, the dentist can request a specific alloy up front, and under ISO 22674 an alloy with nickel content above 0.1% by mass has to be declared on the label.

6. If you have a case that looks like this one

A few things that help us move fastest:

• Send the Rx and the scan or model, and we will come back with a framework proposal and the questions we would normally ask on a lower-arch case before anything is cast.
• Tell us if the impression or the scan was taken under compromised conditions. We would rather ask for a rescan than build on data we do not trust — particularly in the distal extension areas, where direct intraoral scanning of free-end saddles is still considered a more cautious application.
• If a flexible or acrylic partial has already been tried and the case is moving to a cast framework, say so in the notes. It changes how we approach the connector and the saddles.
• If the case needs lattice areas for future tooth addition, flag it at the order. It has to be in the original design; it cannot be added to a finished framework.

What we will not do: send a framework to be cast before the design drawing is approved, make allergy-free claims, promise an outcome, or present a lab-side mistake as a clinical variable.

And a line worth passing on to the patient, in her own words: the metal is the part of this partial that holds everything steady — the plastic and the teeth are built onto it, not instead of it.

References

• Loney RW. Removable Partial Denture Manual. Dalhousie University, 2018.
• ISO 22674:2022 — Dentistry — Metallic materials for fixed and removable restorations.
• ASTM F75-23 — Standard specification for cobalt-28 chromium-6 molybdenum alloy castings for surgical implants.
• Co-Cr dental alloys: toxicological risks, systematic review.
• Intraoral scanning across Kennedy classes of RPDs — feasibility review.
• 10-year retrospective of 1,893 RPDs in 1,246 patients.
• 842-patient retrospective of clasp-retained Co-Cr RPDs, mandibular survival signal.