Q: Why do most dentists use plastic teeth instead of porcelain teeth for dentures and at the same time prefer porcelain over acrylic for C&B? The only reason I can think of is that they do not know how to take a proper vertical registration. The esthetics of plastic teeth are as good as porcelain teeth, especially over time. Also, the rate of plastic tooth breakage is equal to that of porcelain. What is your opinion?
A: Your perceptions are absolutely correct. A few decades ago, porcelain denture teeth were practically the only prosthetic teeth used. In the early 1960's, great improvements were made to the esthetics of plastic replacement teeth and, as these teeth were more widely used, a reduction in the resorption of residual ridges was observed. It was theorized that porcelain denture teeth were responsible for bone loss in some denture patients and therefore many practitioners shunned the use of porcelain teeth.
It has since been discovered that the most likely culprit for ridge resorption when using porcelain teeth is incorrect centric relation. As we are now aware, muscle will prevail over bone or joint. If centric relation is incorrect when plastic teeth are used, the teeth simply wear to compensate for that. However, porcelain teeth will not wear to compensate for incorrect centric relation and usually cause failure of the alveolar processes.
Another reason many dentists don't favor porcelain teeth is the notorious "clacking" some patients experience. This is caused by a lack of posterior speaking or freeway space, and can easily be remedied by reduction of the vertical dimension of the appliance.
However, the good news is that the pendulum may be beginning to swing back toward the use of porcelain teeth thanks to the reappearance of the branching technique. With this method, the denture is delivered as a provisional, and refined by the dentist through a functional impression procedure and a series of occlusal adjustments prior to final processing. Clinicians such as Dr. Peter Dawson and Dr. Walter Turbyfill offer us a golden opportunity to learn more about occlusal analysis and how it applies to denture fabrication. It's our responsibility to seek out these educators and learn how to apply their theories to our everyday work.
Answered by Karen Crace, CDT, January 1998
Q: I have a client whose patient is having trouble with "S" sounds on a new wax try-in. What could be the problem?
A: The "S" position is defined as the closest position of the lower teeth to the upper teeth during normal speech without the teeth contacting each other. The "S" sound is created when air is forced between the hard surfaces of the maxillary and mandibular central incisors. If the distance between the upper and lower incisors is too great, the "S" sound is not crisp. If the space is insufficient, the result is a whistling effect.
Vertical dimension is also a factor. If the freeway space has been encroached upon, the posterior teeth will collide before the optimum "S"position has been achieved.
There are numerous other factors that could affect the "S" sound that only a doctor can diagnose, such as a patient's history of tongue thrusting. In difficult scenarios, it is imperative that the dentist warm a spatula and make necessary changes to the denture in order to give the technician a starting point. Assure your doctor-client that you will correct any resulting esthetic defects, but that you need his help in reestablishing anatomical harmony for the patient.
Answered by Karen Crace, CDT, January 1998
Q: I would appreciate more information on lingualized occlusion for dentures.
A: While the concept and practice of lingualized occlusion has been around for many decades, it has only recently become widely used. In fact, I've heard that it is now even being taught in several leading dental schools. Comparatively speaking, lingualized occlusion is far superior to other occlusal options.
When using other types of occlusal theories and working with edentulous patients, the doctor must attempt to guide the mandible into a repeatable centric relationship in which the condyles are fully seated to an established vertical dimension. To complicate matters, the centric relation may or may not be equal to the patient's centric occlusion. This means that you may experience problems once you transfer this centric relation to an articulator and arrange 33-degree prosthetic teeth into maximum intercuspation.
On the other hand, lingualized occlusion affords a man-made margin in that the denture teeth are arranged and altered to permit some freedom during occlusal function. It does not confine patients to acentric relation which may or may not be the same as their centric occlusion and, unlike monoplane occlusion, it allows them to cut rather than mash their food.
Lingualized occlusion is created by dropping the lingual cusp of the maxillary 33 oposterior tooth into a shallow fossa formed in the mandibular 20 o posterior tooth. Here is a method for achieving lingualized occlusion in edentulous opposing arches:
Arrange the maxillary and mandibular anteriors in phonetic and anatomical harmony.
Establish a lingual control line by drawing an imaginary line from the lingual of the retromolar pad to the mesial of the mandibular canine. Be sure that no part of the lower posterior prosthetic teeth is placed lingual to this line; this would violate the patient's tongue space.
Arrange the maxillary 33 -degree posterior teeth with the assistance of a flat plane template. Begin with the first bicuspid, placing it so that the lingual cusp contacts the flat plane and the buccal cusp is elevated approximately 0.5mm.
Set the second bicuspid in the same way, but elevate the buccal cusp 0.7mm. Continue this sequence until you reach the disto-lingual cusp of the first molar, which should be placed 0.5mm off the plane with a bucco-lingual cusp elevation of approximately 1.5mm. When placing the second molar, the lingual cusp should be 1mm off the plane with its buccal cusp elevated about 2mm. This creates a curve of Spee.
When placing the mandibular posterior teeth, begin with the first bicuspid. Set each tooth in hyperocclusion of about 0.5mm to 1mm, and then mill to the articulator pin contact to achieve a smooth but shallow central fossa. Continue this sequence for each tooth until setup is complete.
Finally, inspect lateral excursions to be certain there are no interferences of the maxillary buccal cusp. This type of occlusion allows the patient to slide around with a certain degree of freedom, yet chew his food on the table created by the surface of the lower denture teeth.
Answered by Karen Crace, CDT, January 1998
Q: What is the best method of processing a cast partial denture reline?
A: Relining an existing appliance seems like an easy proposition, but special care must be taken when working with cast partial acrylic saddle areas. Because new denture base resin is added to the tissue surface of the prosthesis during the reline procedure, changes in the vertical dimension will inevitably occur particularly when traditional compression packing is employed.
In the case of a full denture, the occlusion is simply adjusted clinically at the time of delivery. But a cast partial denture that has been relined by compression packing results in a layer of flash covering the tissue surfaces of the acrylic saddle areas and the major connector and rests. When the appliance is finished back to the original connector/saddle area junctions or finish lines and this flash is removed, the connector and rests most likely won't contact their bearing areas properly when inserted in the mouth.
One simple alternative to compression packing is to use a reline jig. The jig allows the autopolymerizing acrylic to be applied in a more fluid state and the excess acrylic can escape the saddle areas with greater ease. This results in a much thinner flash. The most suitable method of processing a cast partial reline is a closed flask injection technique which creates virtually no additional flash in undesirable areas.
Answered by Karen Crace, CDT, January 1998
Q: What are the various techniques for setting posterior 20° teeth?
A: I assume you're setting 20° teeth opposing each other. When setting any posterior tooth, after the plane of occlusion has been established, I always use a template. In keeping with established principles, I try to set the lingual cusps of the maxillary posteriors over the crest of the lower ridge. This results in all posteriors being set over their respective ridges. This is not always possible, so there are times when you have to modify the alignment somewhat. If I am using a 20° template, I follow the directions on the template.
If I am using a flat template, I set the maxillary teeth in the following manner:
Place the first bicuspid with its long axis at right angles to the occlusal plane. The buccal and lingual cusps are placed on the plane.
Place the second bicuspid in a similar manner.
The mesiobuccal and mesiolingual cusps of the upper first molar touch the occlusal plane. Raise the distobuccal cusp about 1/2 mm; the distolingual cusp will be raised accordingly.
Raise the cusps of the second molar from the occlusal plane following the position of the first molar. The mesiobuccal cusp should be about 1mm from the occlusal plane.
Use a straight edge to align the labial ridge of the canine, the buccal ridges of the first and second bicuspids and the mesiobuccal ridge of the first molar. Align the buccal ridges of the molars in a similar manner, but angle them slightly inward.
Following these guidelines should result in a nice curve of Spee and curve of Wilson. After setting the maxillary posteriors, it is simply a matter of setting the lowers against the uppers in the proper relationship. Correct centric working occlusion and balancing occlusion are critical to the success of the setup. Answered by Bruce Keeling, CDT, September 1998
Q: There are different ways to arrange teeth in fully edentulous cases in a bilateral crossbite. Keeping patient satisfaction (function) as top priority, which method is best?
A: One technique--although not the best one, in my opinion--is to cross the posterior teeth as you would rotate tires on your vehicle. The upper left teeth would be set on the lower right and opposed by the lower left teeth; conversely, the upper right teeth would be set on the lower left and opposed by the lower right teeth. Most of the time this allows you to set the teeth over the ridge.
However, I think this technique severely compromises esthetics. My preference is to use the teeth in their normal position with the following changes:
Keeping the teeth over their respective ridges, use a larger mold on the lower to compensate for the larger arch form.
Spot grind the opposing teeth together to provide good centric, working and balancing occlusion.
According to my clients, this method works very well for their patients.
Answered by Bruce Keeling, CDT, September 1998
Q: What would cause acrylic to change from pink to orange after processing?
A: You don't mention the time frame in which the color change occurs, so I assume it occurs rather quickly. The two most common reasons for it are:
Undercuring, which leaves tinfoil substitute embedded in the denture material.
Cellulose slime in the resin from prolonged soaking of the sheets used to trial pack. If the color change is occurring over a period of time, the patient may be using a strong bleach solution to clean the dentures.
Answered by Bruce Keeling, CDT, September 1998
Q: After boilout, the separator that I apply curdles and leaves craters and run marks in the palate. I have tried several brands and thinned them to several consistencies. What could be the problem?
A: You may not have complete elimination of wax in the mold. Are you using a mild detergent in the boilout and rinsing with clean boiling water? If the wax is not completely eliminated, the separator can't penetrate the stone and it will have a tendency to pool or puddle. As a result, the separator won't dry completely prior to packing. Also, it is very important to apply the separator when the mold is still hot or very warm to allow it to penetrate the open pores in the stone.
Q: What is the simplest and most cost effective way to duplicate a full upper and lower denture?
A: There are at least four different ways with which I'm familiar to duplicate a denture. The first three utilize autopolymerizing acrylic for the teeth and the denture base and also require the use of an autopolymerizing pour resin. They are briefly described below, and can be found in Dental Laboratory Procedures, by Morrow, Rudd and Rhodes.
The first method uses a traditional modified denture flask and alginate to form a mold for the denture:
Paint an adhesive on the inside of the flask to prevent the alginate from separating during the fabrication.
Flask the denture after adding sprues and place it properly in the first pour of the alginate. This captures the tissue side of the denture; do a second pour to capture the cheek side of the denture.
After the alginate has set up, separate the two flask halves, place the tooth-colored acrylic, pour the resin and cure in a warm water bath under pressure.
The second method is very similar except you utilize a special flask designed for reversible hydrocolloid and pour resins. If you already have all of these items in your laboratory, this is probably the most cost-effective way to produce a duplicate denture.
The third method utilizes a coffee cup, wax-treated dental floss, alginate and the appropriate resins:
Add wax sprues to the denture and place the treated dental floss around the periphery of the denture leaving enough to extend over the top of the cup. The dental floss serves to separate the alginate into two halves after it has set up.
The denture is suspended inside the coffee cup and the cup is filled with alginate.
After the alginate sets up, you can remove everything from the cup, pull on the dental floss and separate the two halves.
The fourth method, which is the one I prefer, utilizes laboratory silicone putty as part of the matrix or mold:
Place a mixture of silicone putty in the tissue side of the denture while providing undercuts in the silicone for future retention in plaster or stone.
Place the denture in the bottom half of a conventional denture flask and invest in stone just as you would with a new denture.
When that has set up, place a fairly thin layer of silicone over the outside of the denture, again providing undercuts in the silicone for retention into stone. Then proceed with the flasking in a normal manner.
After the stone has set, open the flask, remove the denture and proceed with the application of the tooth-colored material. Then cure it under pressure in warm water.
Pack the denture with heat-cure acrylic and process according to manufacturer's directions.
I prefer this method because we use everyday materials. Also, we can group these appliances in with other dentures and process them at the same time. While you could argue that our material costs are higher than they would be utilizing one of the other methods, I believe the time we save more than offsets the increased material costs.
Answered by Bruce Keeling, CDT, September 1998
Q: In cases where the vault of the palate is very shallow, what can I do to provide greater suction and prevent the denture from falling out?
A: When dealing with a shallow palatal vault, there are several factors that should receive special attention:
The first begins with the dentist, because the final impression needs to be as accurate as possible. Your dentist-client should definitely use a custom tray in this situation.
The practitioner should carefully check the custom tray for over-extensions and accurately border-mold the tray with compound or a similar material.
In the laboratory, the resulting final impression must be carefully beaded, boxed and poured.
The posterior palatal extension should be determined at the wax try-in visit and transferred to the final cast.
The doctor should also evaluate the amount of soft tissue available, and post-dam the cast accordingly. A more generous post dam is usually desirable when the palatal vault is shallow.
It's imperative that the occlusion be extremely precise to avoid "tripping" the denture during speech or mastication. Obviously, precision processing such as injection molding is the most suitable procedure in this situation because some studies have shown that it helps eliminate the palatal lift that occurs due to shrinkage in conventionally compression-packed denture base resin.
The cast should be remounted after processing and the occlusion milled and balanced prior to separating the denture from the cast.
When finishing and polishing the final prosthesis, be careful not to over finish or over polish the borders.
Following delivery and post-operative adjustments, the dentist may elect to reline the final prosthesis using a functional type impression material. Again, the reline should be processed utilizing a precise method.
Unfortunately, there are a small percentage of patients who--regardless of the care taken by both the doctor and the laboratory--will have no retention in their denture. This is usually due to a combination of shallow palatal vault and severe bone loss, resulting in no labial or buccal undercuts to engage the prosthesis.
Answered by Karen M. Crace, CDT, May 1999
Q: I'm getting a white film on my dentures after processing. It's not visible until I start finishing with a bur. What is causing this and what can I do to prevent it?
A: There are many possible causes for this type of inconsistency. Typically, a back-to-basics approach will help this problem disappear as mysteriously as it developed.
First, assuming you're using a compression packing molding procedure, strictly adhere to the acrylic manufacturer's instructions since this type of processing is less forgiving than newer techniques.
Be certain of the proper liquid/powder ratio and measure the amounts accurately into the mixing cup. As with gypsum products, the liquid should go into the cup before the powder.
Although spatulation times vary, 15 to 30 seconds usually achieves a thorough mix.
The mixing cup should be covered after spatulation with a .150 vacuum form tray sheet or similar covering. Covering the mix during the gelation time prevents the premature evaporation of monomer that results in the formation of a crust of polymer on top of the mix. The cover allows for proper saturation of the tiny particle soft polymer during the gelation time. The actual gelation time can vary depending upon room temperature and humidity, and is generally shorter in the summer than in the winter. Refer to the manufacturer's specifications concerning the exact times.
When the acrylic has reached the proper dough consistency, begin packing immediately. If trial packing is the chosen method, start out by under packing the mold, then gradually adding small amounts of acrylic dough with each packing until uniform flash is achieved. Depending on the size of the denture, this usually takes three to five trial packs. When packing in one single step, the flask must be closed very slowly in the pressing device. If the flask is closed too quickly, the acrylic dough will be displaced resulting in some degree of porosity.
It may be beneficial to consult with a technical representative of the denture base resin manufacturer if the problem continues.
Answered by Karen M. Crace, CDT, May 1999
Q: Three of my new accounts don't know the best method for taking impressions for complete dentures. They insist on taking impressions and pouring models in house. I fabricate to their provided models and fit the model, but not the patient. Help!
A: This is a sticky situation that is going to require some diplomacy on your part, as well as on that of your clients. While it might be difficult to dictate impression-taking techniques to your dentist-clients, you might succeed with some of the following proposals:
First, be certain the problems are not related to any of your fabrication procedures. You should check to be sure the dentures are not being left over-extended when finishing the borders.
If you're reasonably confident that the fit problems are not laboratory related, make arrangements to talk with the individual doctors either by telephone or in person. Diplomatically explain to them that a laboratory can only fabricate and return an appliance that is as accurate as the impression it is made on.
Offer to pour the impressions--at least temporarily--until you determine where the problem lies. Follow up on this by inviting your client's auxiliary staff to visit your laboratory to observe model pouring techniques.
Suggest that your dentist-client contact the manufacturer's representative of his denture impression materials, trays and systems for an in-office demonstration of impression taking and pouring techniques. Since you won't be the one instructing your dentist-client, this may be a more comfortable and diplomatic approach for you both.
If these efforts fail, you should assess whether or not your relationships with these accounts is productive for your business, taking into account the frustrations they likely present for all parties involved. I sincerely hope you're able to resolve this situation in a manner that benefits you, your dentist-clients and, most of all, the patient, who should be our ultimate concern.
Answered by Karen M. Crace, CDT, May 1999
Q: Help! We are putting names in finished dentures and after two or three weeks the name is fogged over and no longer legible. In the course of a month, we process and finish about 400 cases with names. We currently use self-cure monomer with clear acrylic, a PC and a bubble jet printer to print the names on onion skin paper. Can you suggest a better method that's both cost effective and predictable?
A: A fogged over identification may be caused by porosity. However, your method is the standard for putting names on a denture base, so a back-to-basics approach may eliminate such glitches. Make sure you are doing the following:
Cure the autopolymerizing clear acrylic in a pressure pot under 15-20 psi of air pressure. The water should be about 120 degrees F.
Leave the dentures in the pot for the manufacturer's specified amount of time.
If you don't see an improvement, try another resin to overlay the onion skin. Clear orthodontic polymer and monomer are very color stable and the polymer grains are smaller than those in typical base resins, which allows for more density. One final suggestion: treat the name area with a light-cure sealant after pumicing the appliance. This barrier prevents air from reaching the autopolymerized resin.
Answered by Karen M. Crace, CDT, Feburary 2000
Q: What common mistakes or forces outside of the laboratory's control cause a maxillary denture to fracture? Is it possible to prevent this 100% of the time? If so, how?
This is a common problem experienced by most dental technicians fabricating complete dentures. Although not always controllable, here are steps you can take to prevent typical fractures:
Ask your dentist-client to alert you when a patient has a history of fracturing. Then, when fabricating that patient's new maxillary denture, place the posteriors directly over the ridge. If the teeth are placed too far buccally, simple mechanics will allow the patient to "wedge" the appliance laterally, resulting in a midline fracture. A metal palate or internal mesh can also benefit treatment.
Have the dentist research the patient's habits. For example, if the patient must sleep with the denture inserted, consider a clear duplicate; patients with a nocturnal bruxing problem are usually not aware of it.
For existing dentures that fracture repeatedly, these steps will usually eliminate recurrent midline fractures:
First do a simple repair. The dentist should then provide you with a reline impression (functional is best), facebow and a posterior check bite taken in centric relation, along with an opposing model.
Pour and mount the casts, then analyze the occlusion. Check for lateral interferences and solid, even, centric stops. Equilibrate the denture teeth as necessary. Note whether teeth were originally set over the ridge. If not, you may need to remove the posterior teeth in blocks, arranging them correctly, even if this results in posterior cross-bite.
After thoroughly evaluating the occlusion and making corrections, flask the denture and do the reline. When grinding out the reline impression, be sure to open up and remove any old repair material in the fracture area, allowing new denture base material to fill this void. If available, precision processing (not traditional compression packing) is always preferred to ensure the most accurate fit possible. For strength, heat cure the reline rather than using autopolymerizing acrylic.
After processing and deflasking, the denture should be remounted and equilibrated to assure accuracy of the centric stops.
Answered by Karen M. Crace, CDT, Feburary 2000
Q: I'm having problems processing attachments into relines of complete dentures. They are always too low (i.e., they don't "snap" in all the way in the finished product). Do you have any suggestions?
A: This problem probably involves one of two issues: dimensional changes of the newly processed acrylic (more shrinkage than desired) or a bite that is being opened from the tissue side of the denture. Let's examine both scenarios.
If processing is being done specifically according to all manufacturer's instructions, the problem most likely lies in excessive shrinkage of the denture base resin (more than 7-8% typical). When there is excessive shrinkage of the denture base resin, particularly when attachments are involved, dimensional changes occur, resulting in an inaccurate tissue surface of the denture.
When dealing with only a reline of an existing denture, this is unusual because the volume of new resin is quite small and any subsequent shrinkage is relative to that volume. In other words, the larger the volume of acrylic processed, the more shrinkage will occur. This would lead me to believe the problem you are experiencing is more likely attributed to operator error.
If the flask is not securely closed (metal-to-metal contact), this will open the bite of the reline from the tissue side of the appliance, because the tooth side of the denture--including the existing base and attachments--is secured in the top half of the investment/flask. Be certain the flask is completely closed and adequately clamped during polymerization of the acrylic base material. If these measures do not fix your problems, consult your manufacturer's technical representative.
Answered by Karen M. Crace, CDT, Feburary 2000
Q: What is the laboratory technique for the fabrication of a new denture with an existing bar and attachments on an implant superstructure in the patient's mouth?
A: This a great question that has many answers. Here are two possible solutions:
If the existing implant bar is retrievable, the doctor should block out areas of the bar that may cause the impression material to rip or tear when removing the impression from the mouth.
After blocking out any questionable areas, the dentist should syringe a polyvinyl-type impression material around the bar structure, then load the impression tray and take the impression.
After the impression material has set, the doctor should break the suction and pull the impression from the mouth.
He should then retrieve the bar and send both to the laboratory for pouring of the master cast.
Once in the laboratory, examine the impression for imperfections, make the necessary adjustments and fully seat the bar into the impression.
Next, place the appropriate analogs, box and pour the master cast. From this point forward, handle the case as if it were a new implant bar case.
If the bar is not retrievable and must remain in the patient's mouth, follow the same procedures outlined above, except for the pouring technique. Some bar/attachment systems offer laboratory analogs. You should use analogs whenever possible so that the master cast is not so fragile. Consult your system's manufacturer about the availability of analogs.
If analogs are not available, here are two alternatives:
Pour the bar portion of the final impression in an epoxy-type material, then the balance of the impression in die stone.
Use solely vacuum-mixed die stone.
When using either of these methods, be sure to protect the representation of the bar during fabrication procedures. But the cases can essentially be treated as if they are a new bar case (i.e., placing of clips, block-out with plaster).
Answered by Karen M. Crace, CDT, Feburary 2000
Q: How do you retain porcelain denture teeth if the lugs have to be ground off?
A: If teeth need to be butted against the ridge, sometimes it's necessary to grind the metal retaining pins from porcelain anterior denture teeth. Before completely grinding off the pins, first try bending them up toward the incisal edge of the tooth using a tool such as plaster nippers. Many times this provides the small amount of space needed to achieve the desired esthetics and function.
If bending the pins upward is not sufficient, then grind the pins off completely. Use a porcelain etching/bonding product consisting of a hydrofluoric acid solution, followed by silane primer to retain the porcelain teeth in the denture base resin. Treat the teeth after the boilout procedure, according to the bonding kit's instructions. Always use the proper personal protective equipment.
Note: Use caution when grinding posterior porcelain denture teeth. There is a limit to the amount of grinding these teeth can tolerate before becoming structurally weakened. For example, if the posterior teeth are ground completely through the diatorics, they can crack down the central fossae.
Answered by Karen M. Crace, CDT, May 2001
Q: What are the advantages and disadvantages of flexible acrylic partials?
A: Flexible partials can be a great alternative appliance to offer your dentist-clients, an additional profit center for your laboratory and the answer to both esthetically and functionally challenging cases. They don't compromise the patient's appearance with metal clasps, and are considered "temporary" appliances because they don't provide as much support as a cast partial and the material is not as bacteria-resistant.
There are three types of flexible partials: thermoplastic, gasket-retained and elastic/silicone band. Additional benefits of each type include:
Flexible partials fabricated from thermoplastic-type materials are a good alternative when path of insertion problems negate the use of wrought wire clasping. The flexibility of thermoplastic material allows for insertion over extreme opposing undercuts. Since this appliance is completely tissue supported, it is only suitable as an interim partial while a permanent one is being made, but can be worn for a couple of years if the dentist-client feels there is adequate support.
Flexible gasket-retained and elastic/silicone band partials offer the same advantages as the thermoplastic-type partial, but can also be incorporated with a metal substructure that includes tooth rests and plating. By providing both tooth and tissue support, the partial can serve as a more long-term appliance, even up to several years.
Offering flexible partials requires the purchase of special processing equipment and/or undergoing training for the processing technique. If you are interested in fabricating a patented partial, you must also buy a franchise that gives you the right to use that particular technique. Research your options before investing in a particular flexible partial system. Consider which system is financially practical for your laboratory, as well as your ability to market and sell flexible partials.
Answered by Karen M. Crace, CDT, May 2001
Q: How much should a complete lower denture weigh and, if a variance exists, how should it be computed?
There is no formula to compute the weight of the final denture because it depends upon several factors. Two of the most important are the amount of vertical dimension and degree of ridge resorption the patient has experienced. For example, if the patient has had a large amount of ridge resorption along with an excessive amount of vertical space, the denture base must compensate for this, resulting in a heavier appliance. Conversely, if there is little resorption or vertical space, the volume of denture base needed will be smaller and therefore lighter.
Some dentist-clients request certain techniques to make the denture heavier and therefore hold it in place. One, sometimes referred to as the California Technique, calls for extreme sublingual coverage of the tissues while scooping out the lingual surface of the denture base. The tongue rests on top of the resulting concavity and holds the base down. Another more unusual theory calls for the placement of chrome-cobalt weights into the denture base resin so gravity helps hold the base down.
Dr. Tom Shipmon advocates casting a chrome-cobalt base for neutral or negative ridges. In his lectures, he says the base should weigh approximately 15 pennyweights to accommodate for lost hard and soft tissues.
Answered by Karen M. Crace, CDT, May 2001
Q: Why is it important to keep the design of a removable partial denture as simple as possible?
A: There is a tendency for both dentists and technicians to complicate the design of a removable partial denture. However, overly complicated designs create technical difficulties and can make final delivery of the appliance a negative experience for the patient. As long as a removable partial denture includes the following three necessary components of design--positive vertical stops (rests), retention (clasps or attachments), and cross-arch stabilization (major connector and cross-arch retention)--there is no reason to further complicate the framework design.
Whether the removable partial denture is constructed as a wrought wire and acrylic appliance, a gasket type partial, a cast framework with guide planes and I bars, or a fixed/removable combination case including milled ledges and attachments, keep the design as simple as possible.
In addition, when designing a removable partial denture, it's important to consider the patients' expectations and needs. Consider factors such as esthetics, financial means and, in the case of an elderly patient, manual dexterity.
Q: How can I achieve a functional mucus seal in an acrylic partial?
Creating a functional mucus seal is dependant on capturing a functional or physiological impression which is an entirely clinical procedure. It is up to the dentist to evaluate the partial, its borders and occlusion and determine if a functional reline impression would improve the appliance and benefit the patient.
There are many different techniques and materials available to achieve a functional impression. The post palatal seal is very important in retaining a maxillary acrylic appliance and should be evaluated, classified and carved by the dentist. The laboratory is then responsible for reproducing the impression as precisely as possible. Here are some tips:
Use closed flask, injection molded processing--it's the most accurate and consistent procedure currently available.
While finishing and polishing the relined base, use care in finalizing the borders. They should remain completely intact. Use only light pressure with the ragwheel and pumice flour.
Answered by Karen M. Crace, CDT, May 2001
Q: I have two questions. When relining a full upper with a cast palate, how do I gauge the thickness of the steel when pouring? And second, can an F/F reline case be finished with a soft liner?
A: When relining, use calipers to measure the thickness of the denture before and after removing the impression material. This allows you to determine the thickness of the impression and how much material to use for the reline. Since you are relining a cast metal palate, be sure to prep the metal palate and use a metal bonding acrylic.
The answer to your second question about soft liners depends on whether you're working on a mandibular or maxillary denture. Soft liners are preferred for a mandibular denture because they provide better fit and retention. However, maxillary dentures need hard acrylic for proper support and better suction in the posterior palatal seal.
However, there are a few exceptions to this rule.
If the buccal and labial flanges have too much undercut to insert the maxillary denture, you can place soft reline material in the flange area only.
If there's a large torus on the palate, you can cover this area only with soft liner to increase patient comfort.
If there's a large torus on the palate and the patient has a cleft palate, you can use soft reline material in the bulb area of an obturator.
Answered by Doris Anderson, February 2002
Q: How does the number of coats of separator affect the final fit of a denture?
A: The purpose of the separating medium is to create a solid, unbroken calcium and alginate layer on the surface of the model before processing. Most manufacturers recommend applying one or two coats of tin foil substitute on a warm model. One coat may be enough if you are using fresh separator without any contaminants. If there are any areas that do not shine after the first coat dries, apply an additional coat.
If the model was fabricated with an incorrect water-to-powder ratio, its surface will look more porous and you may need up to three coats of separator. Try to avoid puddling of the separating medium because it can cause an ill-fitting denture; use a small paintbrush to remove the excess. On the other hand, undercoated models can cause the stone to adhere to the processed acrylic, making it necessary to grind the tissue surfaces in order to remove the stone. This greatly reduces the quality and fit of the denture.
Answered by Doris Anderson, February 2002
Q: What can I use to opaque metal frameworks on cast partials? Most pink opaques seem too light to mask the framework.
A: Here's my technique for opaquing metal frameworks on cast partials:
Before opaquing, sandblast and silicoat the metal so the opaque will adhere better.
Paint a thin film of a light-cured, tooth-colored opaque (either shade A1 or B1) on the framework and light cure it for 90 seconds. If the framework still shows through, you may need to apply a second coat. However, if the opaque is too thick, the material may not cure. In this case, remove it by sandblasting or shellblasting and start over.
Once the metal is satisfactorily masked, apply a thin coat of pink opaque. The partial is now ready to process in the usual manner.
Answered by Doris Anderson, February 2002
Q: Can you use a microwave to heat hydrocolloid for pour dentures?
A: You should check with the manufacturer of the specific hydrocolloid but, in general, you should not use a microwave to liquify the material. A microwave generates heat too rapidly and there is no way to control it. The gel in the hydrocolloid will break down and, therefore, change its properties.
The best way to heat hydrocolloid is in a double boiler or auto-duplicating machine. Slowly bring the temperature to between 205 degrees F and 210 degrees F and then allow it to cool to a holding temperature of between 125 degrees F and 130 degrees F.
Answered by Doris Anderson, February 2002
Q: A dentist recently requested a name I.D. for a case I just finished. How do I do this?
A: Although compliance has not been universal, many dentist-clients prescribe denture identification for all new prostheses per the ADA's Operation Ident program initiated in 1982. The intent of this initiative includes forensic and humanitarian issues and remains valid and worth promoting to our clients. I always encourage dentists to offer this service to elderly patients who are living in assisted-care facilities because of the horror stories I have heard about "musical" dentures.
There are several different ways to place a name into a denture. Some people insert a handwritten slip of paper into the denture base; however, I think this makes the final prosthetic look cheap. I prefer to add the identification after the appliance is finished but not polished. Following is my technique:
Using a basic word processing program, print the name--I usually use the first initial and last name--in the smallest, legible font possible (the size will vary from font to font and from printer to printer). I recommend using a waterproof ink cartridge in your printer.
Trim the name, leaving a small white border around it to aid in legibility.
Using photo-curing clear varnish, liberally soak or paint the name I.D. It's helpful to handle it with a small hemostat. Run the I.D. through a curing cycle to seal the ink and help prevent fading.
With a thin bur, cut a channel in the denture that's a little wider than the I.D. In the maxillary, I usually position it labially above the last molar; in the mandibular, I place it lingually below the last molar. Some prefer to place the name on the tissue-bearing side of the denture, however, this requires you to remake the I.D. when you reline the case.
Lay a small bead of clear, light-cure gel in the bottom of the channel, place the name on top of the material and submerge it below the level of the finished surface. Smooth it until a slight convex layer covers the I.D. Cure it for 15 seconds, remove, coat with an air barrier and then cure for a final full cycle.
If a light-curing box is not available, clear, autopolymerizing resin is an adequate substitute. Add a layer of resin using the wet-brush technique: wet the brush with monomer and pick up a bead of polymer on the end of the brush. Continue to adapt in this fashion until the bottom of the channel is layered. Place the I.D. on top of the layer and submerge it below the level of the finished surface. Smooth until a slight convex layer covers the I.D. Cure it in a pressure pot.
Finish the area by smoothing it with carbides and pumice. Polish the final denture.
Answered by Tom Zaleske, April 2002
Q: Are there any special considerations when setting up and processing porcelain denture teeth? Since I occasionally crack a tooth, I usually shy away from suggesting them.
A: Unfortunately, porcelain denture teeth are susceptible to breakage. However, by understanding the factors that cause them to break and taking the correct precautions, breakage can become the exception, not the rule.
Following are three types of fractures: impact fractures, thermal fractures and leverage fractures and tips to help you avoid them.
Impact fractures are caused by a tool or instrument used in either the construction or deflasking of the appliance. In the construction phase, impact fractures are most often caused by using mounted stones to modify tooth shapes, incisal characteristics, balance or equilibrium. Once a stone is used on the smooth tooth surface, it creates micro-fractures that--if not smoothed properly--propagate under function until the tooth is destroyed.
Use finely sintered diamond wheels and points. You can also use silicon-bonded rubber wheels and points used for ceramics. They have different levels of abrasiveness--from coarse to fine--to produce a homogeneous surface.
Follow with pumice and a muslin buff. For a final luster, use diamond paste or--for the more economically minded--a liquid metal polisher and a felt wheel. The idea is to restore the surface as closely as possible to its pristine condition so that the tooth possesses as much impact resistance as it did before grinding and modifying.
Impact fractures can also be post-operatively initiated. During function, the ridge undergoes resorption and the denture settles over a period of time, causing a premature contact on a cusp or incisal edge. Incorrect setting propagates the chipping of the posteriors. For example, posteriors that are set too closely edge-to-edge result in a flaking of the buccal surface.
Post-operative impact fractures can be prevented during fabrication. For example, setting the long axes of the uppers and lowers in the same vertical line provides the correct arrangement. However, if the maxillary posteriors are set vertically and the mandibular posteriors are set in at the neck, not vertically, the lingual cusps of the mandibular will be higher than they should be and present a point of impact stress. Conversely, if they are set too far out at the neck, the impact stress will be heavier on the buccal cusps and, again, prone to abnormal impact.
Thermal fractures.Since porcelain teeth are fabricated and designed for use in the oral environment, they can only withstand biologic temperature fluctuations. When they are subjected to an abrupt increase or decrease in temperature of 175 degrees F or more, porcelain teeth can check. Although they may not immediately exhibit damage, the teeth may be weakened by internal strains that won't be seen until later. Larger, bulky teeth have a lower resistance to thermal fracture than smaller ones.
Since thermal fractures commonly occur during final flaming of a setup, always flame away from the teeth--not at them--and never cool your setup in water immediately after flaming.
These fractures can also be caused by using a hard, mostly dry cotton or felt-buff wheel and holding it in one spot too long or running a lathe over 3,000rpm. Always use a wet buff at low speed and lift the wheel on and off the unit to allow the frictional heat to dissipate.
When rushed, some technicians prematurely cool flasks after processing in order to speed the deflasking procedure. Remember: a slow cool down not only prevents thermal shock to porcelain teeth, but is also recommended by acrylic manufacturers to prevent warpage.
Leverage fractures usually occur during the process of deflasking and model removal.
A 50/50 mixture of plaster and lab stone facilitates the process of breaking out the case from the flask. Remember to eject slowly, evenly and smoothly to avoid torque on the investment that, in turn, creates pressure on the labial surfaces of the teeth. Don't assume that a tooth covered in investment is protected from leverage forces.
During this stage, be cognizant of your actions and ask yourself if your next step will produce direct or indirect leverage on the teeth.
Before prying the palatal section of the investment from the posterior to anterior, be sure to remove investment from the linguals of all anterior teeth.
Be careful when prying the cast from the denture in a tuberosity area. The undercut, even a small one, causes the denture base to spring at its thinnest point which is usually in the anterior area where the teeth are forced together laterally, resulting in partially or fully checked teeth.
Don't forget to pack the dentures slowly and fill gradually with resin between trial packs. A mold that's overly full at the initial press, and closed quickly or beyond good packing consistency, can also cause leverage fractures.
Answered by Tom Zaleske, April 2002
Q: My immediate dentures never seem to fit as well or be as esthetic as I would like. Do you have any advice on improving my fabrication technique?
A: Since immediate dentures are a direct replacement for a surgical procedure and the dental team and patient usually nervously anticipate the result, this type of restoration always seems to have an uneasy stigma attached to it. If you think about it, however, this is the only time removable technicians actually work with natural tooth guidelines still in place, making it more likely that replacements will fit exactly as they should. Unless major improvements are needed, you can be confident that the appliance will fit correctly.
Here are some strategies to ensure success:
If there are edentulous areas prior to surgery, request a try-in with whatever teeth you can set on a baseplate. Now is also a good time to evaluate selected tooth shade and shape. Although the dentist-client should select the teeth, this task is often delegated to the technician especially if all or most of the anteriors remain.
The dentition to be replaced is often fixed, multiple-unit restorations that are failing. In this situation, ask the dentist to indicate which units are pontics. This is useful information; since there are no root extractions and, therefore, no resorption or extraction sites in the pontic area, you don't want to remove any die stone under the pontics.
When there is an extreme amount of perio-involvement and gross tissue recession, the large amount of root exposure makes it difficult to determine the tooth shape and size. Try this technique:
Use a pencil to trace the cemento enamel junction (CEJ)
Use baseplate wax to fill in the recessed areas to replace tissue up to the CEJ and then carve the wax back to the outline. This helps reveal the actual shape and size of the clinical crown, which makes it much easier to select a mold of true size and shape.
When shape-characterizing teeth, you can also make a quick alginate duplicate of this modified anterior segment to use as a guideline and make them more closely resemble the original tooth.
When you're ready to remove the teeth from the cast, try this technique:
Use a sharp pencil to mark the transition between tooth and gingiva.
In order to maintain tooth positioning and placement, replace the plaster teeth with denture teeth one tooth at a time; repeat this process until you have removed and replaced every tooth in the anterior arrangement. Be sure to note the angle of the emergence profiles. When removing anteriors, I use the following order: #9, #7, #11, #6, #10 and #8.
After the plaster tooth is removed, use a scalpel to scrape the die stone no more than 2mm deep facially, decreasing gradually to the lingual where you don't remove any die stone. (Since we're trying to position the tooth into the emergence profile, trimming and cupping the die stone allows the denture tooth to look like its emerging from the same tissue spot as the natural tooth.) I like to remove the teeth with a thin-bladed coping saw at the gingival to preserve enough of the tooth to further compare it side by side to the modified denture tooth.
Answered by Tom Zaleske, May 2002
Q: What causes maxillary midline fractures and how can I prevent them?
Maxillary midline fractures seem to be the most prevalent denture repair we encounter--yet the least discernible in cause--and have long been a thorn in the side of technicians and clinicians. While we can repair most midline fractures using a variety of methods, it's also essential to communicate with your dentist-clients to determine and eliminate the factors that contribute to fractures, or they are likely to recur. Let's look at some of those causes:
Anatomic considerations. Some anatomic reasons for fractures include a palatal torus; an over-prominent, mid-palatal suture; deeply vaulted palate; or high or deep frenal attachments. While there are surgical remedies for these conditions, they're costly and not always acceptable to the patient. Therefore, the laboratory needs to strengthen and modify the base to help prevent fractures.
For the palatal torus and prominent midpalatal suture, you should relieve the suture or torus with a spacer during the initial processing or during post-repair relining. If you're relining a fractured denture, piece the denture together using cold-cure resin, then schedule a reline to adapt the spacer. The reline will also strengthen the appliance and increase the longevity of the repair.
For the deeply vaulted palate and high or deep frenum, you can incorporate a cast metal palate prior to initial processing or, after the fracture, embed the denture with either alloy mesh or cast reinforcement. However, these methods add weight to the appliance, are opaque and unesthetic and some technicians find them difficult to reline. Instead, I recommend using the new generation of materials--carbon, glass or aramid fibers--that are mostly translucent, lightweight and infiltrate easily into acrylic during either initial processing or repairs. I find that they reinforce as well as cast alloy; they're also quick and cost effective. To use, place the material in a bi-directional fashion across the potential or existing fracture area.
Again, immediately after any repair, you should reline the denture so it's less likely to refracture.
Occlusal considerations. Occlusal situations that can result in fractures include skeletal classification identity and a denture that opposes any of the following: supra-erupted teeth; uneven, worn or incomplete natural dentition; a worn mandibular denture; or a restoration or appliance made from a different material. Since it's difficult to detect interferences, incorrect contacts or other occlusal anomalies without a mounted analysis, you may need to request a bite registration and opposing model from your client.
For some skeletal Class II patients who have a discrepancy in arch widths, it may be necessary to reset and widen the occlusal table of the maxillary denture to increase stability and eliminate uneven occlusal contacts. In this case, a lingual anterior ramp for protrusive balance may also be indicated if it won't affect phonetics.
Denture teeth that oppose natural dentition or unlike materials--such as crowns or porcelain denture teeth--can cause uneven occlusal contact and may wear away the lingual cusps of the maxillary denture. This leaves long buccal cusps that, when in mastication, act as levers and create an outward wedging effect. This effect produces lines of fatigue that eventually fracture.
In this case, if natural teeth oppose the denture, recommend that your dentist-client correct the natural teeth intraorally. For example, he should recontour natural teeth that exhibit supra-eruptions, unnatural rotations or overly prominent buccal cusps to restore an occlusal plane that is balanced and won't interfere with mastication. Then you should shorten the suspect buccal cusps of the denture; in extreme wear situations, reset new maxillary posteriors and reline to strengthen the correction.
Poor fit or adaptation of the denture base. Contributing factors include: base material shrinkage due to lack of moisture; excessive weight loss by the patient; gross resorption of the residual ridge due to trauma produced by poor occlusion and masticatory function; or non-compliance to recall and reline protocols (such as failing to reline the immediate denture after placement).
To repair midline fractures in these instances, you should piece together the denture using cold-cure resin, then immediately reline to establish proper fit.
Answered by Tom Zaleske, May 2002
Q: What is the fastest way for re-mounting full dentures to refine occlusion?
A: To properly remount full dentures to correct post-processing errors and efficiently refine occlusion, you should specially prepare the master cast prior to the articulation procedure:
Use the appropriate gypsum product to pour up the master cast. For instance, if the denture will incorporate a soft lining, use vacuum-mixed die stone.
The base of the cast should be a minimum of 15mm at its thinnest point so it can withstand extreme pressure during trial packing without fracturing.
A correctly poured master cast that includes proper indexes expedites the remount procedure: invert the impression onto a rubber index pad to create an index in the base of the cast or cut grooves into the bottom of the cast after it has hardened and is trimmed. Use a large cone-shaped bur to cut the grooves, taking care not to cut too deeply.
After the wax try-in is verified and sealed down for processing, soak the mounted cast for at least 20 minutes in tap water. Separate the cast from the mounting plaster at the stone/plaster junction and coat all exposed surfaces of the cast with a gypsum separating medium before investing the waxup. A 1:1 ratio of water and sodium silicate (available at a local pharmacy) provides clean devesting.
Use exact processing methods to minimize post-processing errors. Closed flask, injection-molded processing produces the most accurate remount. The next best choice is compression packing that includes trial packing and tedious trimming of all flash before each closure. In either case, properly clamp the flask during curing to make the remount more efficient. After processing:
Deflask, devest and brush any debris from the bottom of the cast using a stiff denture or toothbrush.
Remove any debris from the index side of the mounting plaster. Also remove any investment that adheres to the occlusal surface of the denture teeth.
Orient the cast to the mounting plaster using the indexes to seat the cast in its correct position. Be sure the master cast is completely seated onto the mounting plaster.
Refine the occlusion with .0025 or thinner carbon paper using a #8 round bur in a handpiece for acrylic teeth and a small stone bur for porcelain teeth. In the case of a full upper over a full lower denture, make the majority of adjustments on the lower denture.
Answered by Karen M. Crace, CDT, March 2003
Q: How do I produce excellent soft liners in dentures? What types of products should I use and what's the process involved?
A: Polyvinyl siloxane (silicone) based soft liner material produces a per manently resilient lining that has excellent properties and produces no known allergic reactions in the patient. There are now two types of silicone-based denture liners. The original heat-cured type contains polyvinyl siloxane, fillers and peroxide, which provide a chemical bond to the peroxide contained in denture base resin. Autopolymerizing silicone-based liners use a primer to help bond to an existing denture base. These liners are quite durable, permanently soft and resistant to debonding and bacterial growth. There are a number of brands at various prices so you should experiment to find the type most suitable for your laboratory.
Because of the durable nature of polyvinyl siloxane-based liners, major adjustments by the dentist are difficult, so it's always best to place a soft liner in an existing denture as a reline procedure. (Note: some manufacturers have special burs for grinding soft liner material chairside; you may want to sell these to your clients along with instructions for use.) You should process the hard denture, then send it to the dentist for adjustment. He should take a reline impression inside the denture and return it to the laboratory for soft liner placement.
While techniques vary slightly from one manufacturer to the next, here's the general procedure for processing heat-cured liners:
Grind out the denture as if performing a reline procedure.
Remove all impression material as well as enough of the existing denture base to accommodate a layer of fresh acrylic and 1-2 mm of soft lining material.
Place a 1-2mm wax shim on all model surfaces to be soft lined.
Trial pack long-working denture base resin against the tissue side of the denture; place a plastic trial-packing sheet between the shim and resin until the optimum density of resin is achieved.
Clamp the flask with the plastic sheet between the wax shim and fresh resin (products contained in the wax will contaminate the fresh resin and disturb the bonding process). Allow it to bench set for 30 to 45 minutes.
Remove the clamp, gently pry open the flask, peel the plastic sheet away and flush off the wax shim with boiling water.
Flatten an amount of soft lining material into the approximate pattern of the tissue surface of the denture to be lined. This pattern should equal the thickness of the wax shim that was previously removed.
Place the pattern onto the fresh layer of resin.
Cover the soft material with a plastic packing sheet and seat the model side of the flask on top.
Slowly close the flask with a pressure device and wait a few seconds for the silicone to properly disperse.
Release the pressure, open the flask and remove the plastic.
Trim all flash.
Inspect the surface of the silicone to locate areas that may need additional silicone.
Add small amounts as necessary and repeat the trial-packing procedure. Continue this sequence until a smooth and uniform layer of soft liner appears.
Paint the model side of the flask with tinfoil substitute, add a small amount of silicone material in the ridge areas of the denture side of the flask and place the two together for final closure (without a plastic sheet). It's acceptable if the tinfoil substitute is still wet.
Apply appropriate maximum pressure to the flask, release, then clamp and cure the flask according to the soft liner manufacturer's directions.
After processing, devest, equilibrate then finish the silicone soft liner on a high-speed lathe using a series of grinding sleeves and stones. Finally, after polishing the denture base portion, apply a silicone sealant to all exposed surfaces of the soft liner.
Auto polymerizing silicone soft lining material is an excellent choice for relining distal extension partial dentures. This type of liner is also well suited for applying soft material over bars, copings and into isolated gaskets. While there are several processing methods, using a simple reline jig seems to produce efficient and consistent results. The finishing technique is the same as described above for heat-cured silicones.
Answered by Karen M. Crace, CDT, March 2003
Q: Sometimes my self-curing acrylics don't cure; they're jelly-like after removing them from the curing pot. What am I doing wrong?
A: There are several potential causes of your curing problem: expired shelf life or improperly stored monomer, insufficient time or water temperature in the pressure pot, or over-saturation of polymer when applying monomer.
If the expiration date on the label of your bottle of monomer is current and the bottle is being stored according to the manufacturer's instructions, try the following:
Dispense monomer into a dappen dish to avoid contamination of the entire bottle.
Discard unused monomer from the dish after each repair.
Check the temperature of the water being used in the pressure vessel. It should be between 112 degrees F and 116 degrees F.
Allow at least 15-20 minutes in the pressure pot for complete curing.
Most importantly, avoid over-saturating the polymer with monomer. When using a paintbrush technique, use an absorbent paper to blot a bit of the monomer from the brush before picking up the polymer. If using a "salt and pepper" method, sprinkle an additional amount of polymer onto the surface of the repair before placing it into the pressure pot.
Answered by Karen M. Crace, CDT, March 2003
Q: I've been getting dentures returned to the lab because teeth are coming off within three months of processing. What could be the issue?
A: This is a fairly common problem, so you're not alone. This phenomenon was exacerbated a decade or so ago with the development of highly cross-linked and more durable plastic resin teeth. When processing to these types of teeth, cleanliness becomes even more critical and the monomer needs more time to "attack" them and form a good bond.
A 1998 study in the Journal of Prosthetic Dentistry, "Failure Load of Acrylic Resin Denture Teeth Bonded to High Impact Acrylic Resins," concluded that "bonding failures of cross-linked teeth to resins was significantly influenced by modifications in the ridge lap before processing." The highest bond strength was achieved by removing the "glaze" from the underside of the tooth and applying a small amount of monomer just prior to packing or injecting the acrylic. (The use of
