Q: When casting a full palate with 3/10" thick stippled pattern, is it possible for the investment to expand and stop the alloy from fully casting?
A: In theory, if the mold were heated to an extremely high temperature, the expansion could cause the top of the mold cavity to touch the bottom. However, if the investment were to survive, the casting would be useless because over-expansion would make the fit unacceptable.
If you're having trouble casting a full base, the real problem is probably the spruing technique. The most common mistake technicians make when casting full bases is trimming the refractory cast so that the posterior ridges are parallel to the base. Instead, the cast should be trimmed so that the posterior ridges form at least a 30-degree angle to the base. This allows the molten alloy to enter the mold cavity in a smooth flow rather than at a 90-degree angle which causes a great deal of turbulence. Turbulence causes a breakdown of the refractory cast, traps gases in the casting and results in a casting with porosity or even holes.
I use three 8-gauge sprues, placing the first sprue in the anterior papilla area. The other two sprues are placed in the cuspid area. To prevent porosity in the palate, I place a vent in the center of the palate and another at the mid-line of the posterior border. I've found that broom straw makes a good vent; it's about 18 to 20 gauge and rigid and strong enough to withstand investment procedures.
It's also a good idea to review some of the basic principles of spruing:
All sprues should be curved in the direction in which the molten alloy should flow. The curved sprue also minimizes the shrinkage between the casting and the bulk of alloy in the button during solidification.
Sprues should be attached to the heavier areas of the casting. This will ensure a dense casting.
The connection of the sprue to the casting should be smaller than the sprue itself. This is accomplished by tapering the end of the sprue before sealing it to the casting.
Sprues should be placed in such a way as to ensure a smooth flow of the molten alloy into the mold cavity.
Avoid sharp edges or flash at the sprue connections.
Sprues should be no longer than necessary.
Normal castings require 8-gauge sprues. Use heavier or lighter sprues as indicated on special cases. Metal dummies or extra thick saddle areas require reservoirs.
Answered by Milton C. Pokladnick, CDT, February 1998
Q: Is a reconstruction done in centric relation or centric occlusion?
A: There is much misunderstanding about centric relation and occlusion; the best place to start is with definitions from the Academy of Prosthodontics' Glossary of Prosthodontic Terms:
Centric relation is the maxillomandibular relationship in which the condyles articulate with the thinnest avascular portion of their respective disks with the complex in the anterior-superior position against the shapes of the articular eminences. This position is independent of tooth contact and is clinically discernible when the mandible is directed superiorly and anteriorly. It is restricted to a purely rotary movement about the transverse horizontal axis and is the position that is established to produce an oral prosthesis.
Centric occlusion is the occlusion of opposing teeth when the mandible is in centric relation. This may or may not coincide with the maximum intercuspation position.
In ideal occlusion, centric relation and centric occlusion are the same; however, that is rarely the case in the mouth of a removable partial denture patient. When the two relations do not coincide we either (1) accept centric occlusion as presented, perhaps with minor tooth equilibration or (2) modify the occlusion through restorations and equilibration so that the centric relation and centric occlusion coincide.
Answered by Milton C. Pokladnick, CDT, February 1998
Q: What are the reasons a framework would rock?
A: I don't know if you mean the framework rocks on the model or in the mouth, so let's start with the following areas to check on the model. In order to protect the master model, I pre-fit my frameworks to a duplicate of the master cast.
Be sure you're following the manufacturer's directions to ensure accurate refractory models and proper casting times and temperature.
Check the rests. They should not be overextended beyond the margin of the rest prep.
Be sure there are no bubbles under the rest. A very small discrepancy will cause the frame not to seat properly on the model and especially in the mouth.
Make sure the clasp is not impinging on adjacent teeth.
Make sure the rest and indirect retainer are seated, then check the tissue stop. If you're not careful when knocking the casting free of the investment, it's possible to bend the retention area slightly, leaving the tissue stop off the model.
Consider your finishing and polishing techniques. Excessive pressure or heat caused by friction from the finishing compounds can cause distortion.
In the mouth, the same procedures are followed, except you should compare the tissue to the model for any obvious discrepancies. In addition, check the frame to make sure it is not impinging on tissue that may have been displaced while the impression was taken.
Answered by Milton C. Pokladnick, CDT, February 1998
Q: What causes a distal extension hinge to freeze upon casting?
A: Neatness is the first thing to consider; even a very small flash of wax can cause a hinge not to open. Assuming the waxup and spruing are clean, what else could cause the problem? In order to cast metal against metal and then have the two pieces separate, we depend on an oxide formation on the piece we are casting against. The more oxide you can get to form, the looser the hinge will be. In order to increase the oxide layer, leave the mold in the furnace longer.
However, even if you have a suitable oxide layer before casting, it's still possible to get a frozen hinge if the oxide layer breaks down during casting. This can be caused by overly hot metal or high centrifugal force as the alloy is being cast into the mold. In that case:
Decrease casting temperature by 50 degrees.
Wind the casting arm one less turn or, if using an electromatic casting machine, decrease acceleration.
Leave the ring in the furnace longer.
Make one change in technique at a time then, if necessary, combine steps.
Answered by Milton C. Pokladnick, CDT, February 1998
Q: How can we convince our dentist-clients to use temporary wrought wire partials to maintain the position of the teeth (like C&B) when they order cast partials? This is our biggest cause of remakes.
A: You have hit upon a problem that has perplexed technicians for ages, and one that costs laboratories, our dentist-clients and their patients thousands of dollars. It is especially important to make provisional partials when constructing immediate partials, or partials that include isolated teeth used for clasping. The best time to sell this to your client is before he has quoted the case price to the patient. We offer a service to our clients whereby we design the casting on a study model and provide a breakdown of the cost of the casting, setup and processing, teeth and--if needed--a provisional appliance.
It is also good to remember that nothing helps build customer relations better than conversation. Take the time to talk to your clients, explain what this service could do for their practices, and explain how costly remakes are for their offices, not only in dollars but also in patient confidence.
Answered by Milton C. Pokladnick, CDT, January 1999
Q: Resiliency vs. rigidity--when choosing maxillary or mandible attachments how do you determine which is important and why?
A: First, let's look at the two basic types of attachments: intracoronal and extracoronal. Intracoronal attachments (within the crown) connect the partial denture at a point inside the abutment crown contour and are usually rigid. Extracoronal attachments (outside the crown) connect outside the abutment crown and most are resilient.
Intracoronal designs don't have projecting surfaces that may accumulate plaque and irritate tissue. The point of connection is deep within the abutment so that the occlusal forces are directed closer to the root's axis. This would be the attachment of choice for a restoration that is totally tooth-borne, and always the attachment of choice for anterior replacements. If a rigid attachment were to be used on an abutment for a free-end saddle, you would want to double-abut and splint the crowns together.
Advocates of extracoronal attachments prefer that the point of connection be outside the crown because the tooth is easier to prepare. You can use the extracoronal attachment where the mesio-distal dimension of the abutment is far too small (or the pulp chamber too large to permit an intracoronal attachment). These attachments also provide a stress-breaking feature.
Rigid attachments transfer a significant portion of the occlusal load to the abutment tooth, while resilient attachments permit a slight vertical movement or hinging action as the patient chews. The vertical movement brings the saddle into heavier contact with the mucosa, so the tissue bears much more of the occlusal load. The rotation will even further reduce the load on the abutment. This would be the type of attachment to use on free-end saddles; it should never be used as abutments to replace anterior teeth.
Precision attachments have been prescribed to retain partial dentures since early in the century. Attachments not only look better than clasps, but they can spare abutments by redirecting unwanted forces, and by spreading the occlusal forces between the saddle tissue and the tooth's periodontal ligaments.
More and more dentists are prescribing attachment-retained overdentures in place of complete dentures. Instead of extracting all of the remaining teeth, a couple are retained, usually--but not always--the cuspids. The dentist removes the crown of the tooth to create a favorable crown/root ratio. This is accomplished by reducing the occlusal height of the tooth down to the level of the tissue surrounding it. (Once the excessive leverage is reduced, the root begins to stabilize.) He performs endodontic therapy and designs a removable prosthesis that connects to these crownless abutments with attachments. Here, a resilient attachment would be the attachment of choice. This type of restoration allows the patient proprioception (natural stiumli to the mastication muscles) through his retained roots, which provides a more natural feel of chewing.
Answered by Milton C. Pokladnick, CDT, January 1999
Q: I have a dentist-client who takes alginate impressions for his removable partial frameworks. Is alginate accurate enough to use for RPDs?
A: The simplest answer is yes, but--it's always the "but" that complicates the issue--the alginate must be handled properly to provide an accurate impression. There is an old saying that when using alginate, "don't walk, run" to pour the impression. It's not really that bad, however, here are some tips to share with your clients to ensure good impressions and casts.
Always follow the manufacturer's instructions, especially the water-to-powder ratio.
Unless otherwise stated, use room temperature water to ensure consistent setting time. Tap water changes temperature with the seasons (i.e., colder in winter, warmer in summer). The water temperature can change the setting time of the alginate; the warmer the water the faster the alginate sets.
Have the patient sit up in the chair when taking the impression. This allows the tray to be more accurately positioned in the mouth so it won't impinge on the teeth or tissue. It also helps keep impression material from flowing out of the tray and down the throat.
Some systems have two types of alginate--one has a thin body and is applied with a syringe onto the occlusal surface and the other has a heavy body and is applied in a tray. The heavy body causes the lighter to capture every detail of the teeth and tissue. If a single-system alginate is used, it helps to spread a little alginate with your fingertip across the occlusal surface and then apply the tray of alginate over the arch. This eliminates occlusal bubbles.
The impressions should be poured within five minutes of removal from the mouth. In fact, the opposing arch impression should be taken first and poured second; the impression of the arch to be restored should be taken second and poured first. While the dentist is waiting to pour the impressions, they should be disinfected and covered with a damp towel to prevent them from drying out.
Most manufacturers recommend using a high strength lab stone rather than die stone. Die stones have a different crystalline structure and special additives that may react with some alginates.
Recommend that your client check for defects before pouring the impressions. It is cheaper to retake the impression while the patient is still in the office rather than have to call him back later.
After the cast has been separated from the impression, advise your client to check it carefully. If he sees incisal edges of anteriors that are thin and sharp, or posteriors that have taken on a trapezoidal shape with sharp cusp tips, it is an indication that the impression has dried out and become useless.
Answered by Milton C. Pokladnick, CDT, January 1999
Q: What are the advantages of using a gold wrought wire clasp on a removable denture?
A: The advantages of using a gold wrought clasp are flexibility, adjustability and appearance. However, to fully answer this question we must first look at what is expected from a clasp. A clasp has several parts that function together. Naturally, we expect it to have a resilient arm for retention, a non-resilient arm for reciprocation and a rest for support.
Flexibility. In the resilient arm, the greater the average diameter of the clasp arm, the less flexible it will be. Since all cast clasps are essentially half round in shape (one flat side and one rounded side), they may flex away from the tooth. This flexure is limited and is most satisfactorily used on tooth-borne appliances. Flexibility may exist in any form, but the only universally flexible shape is round, which is practically impossible to cast.
It is a given that a gold casting must have more bulk to equal the strength of a chrome casting. However, since the gold clasp was formed by being drawn into a wire, its arm is stronger than a chrome cast clasp arm. Therefore, the gold wrought wire clasp may be used in smaller diameters to provide greater flexibility without fatigue.
In a partial with a free-end extension saddle, the forces of mastication will try to displace the appliance in several directions, so we must design a clasp that will function in several directions. We must also utilize a clasp arm that will function properly. The wrought gold clasp solves this problem because it is round, able to flex in all directions and will not place too much stress on the abutment tooth.
Adjustability is another important factor to consider in a clasp. As stated before, cast clasps can usually be adjusted in or out in relation to the cross-section of the clasp. A wrought clasp, however, may be adjusted in any direction due to its round shape.
Some notes of caution:
Be careful not to nick or dent a wrought clasp when bending or adjusting. The nicks and dents will cause a weak spot in the clasp, which could lead to breakage.
Keep in mind that if you solder the gold wrought clasp to the frame, you may anneal (or soften) the wire if it is not handled properly. Always allow the clasp to bench cool. Quenching will anneal the alloy from which the clasp is made. To keep this from happening, cure the clasp to the frame with a little self-polymerizing acrylic.
Appearance. The final advantage of a gold wrought clasp is the ability to hide the clasp or make it less conspicuous. Since the clasp is round, it reflects light in many directions and since it's more flexible, it may be placed deeper into the undercut, making it less visible.
Answered by Milton C. Pokladnick, CDT, November/December 1999
Q: When making tooth additions on partials, what technique works best: soldering, welding or laser welding?
A: The choice of technique depends on the patient's needs, including time constraints and how much money he wants to spend.
Laser welding is the best method of adding to a partial. Welding is the joining or uniting of separate pieces of similar or dissimilar metal with heat causing the parts to become melted or fused together. Upon cooling or solidifying, a metallic bond is formed. The source of heat may be electric (as in arc welding), gas (as in torch welding) or laser. The laser (which is short for Light Amplification by Stimulated Emission of Radiation) is a device that amplifies electromagnetic energy into a nearly nondivergent beam of bright light that is capable of producing intense heat in a very small area. However, the cost of a laser-welding machine may be prohibitive to many laboratories.
Welding with a torch is the next best way of repairing or adding to a partial framework. The only problem is that torch welding produces a great amount of heat, which may necessitate the removal of acrylic saddles first to prevent burning them up. This may greatly increase the time and cost of the repair.
Soldering is the easiest and most cost-effective way of adding to or repairing a frame. Soldering is the process of joining together pieces of metal through the use of an alloy, which has a lower melting point, usually at least 180 oF below the fusion temperature of the parts being joined. However, in soldering you only accomplish a mechanical bond, which is not as strong as a metallic one.
Answered by Milton C. Pokladnick, CDT, November/December 1999
Q: Is it possible to achieve a casting that weighs about 15 dwts without porosity? If so, how?
A: The same basic concept holds true whether you're asking about partial framework construction or crown and bridge. This is the basic rule: first chill the molten alloy in the pattern you are casting, then chill the sprues and lastly the button.
When spruing a cast partial with large dummies, I use this technique:
Sprue the lingual bar or horseshoe as always, then attach the sprues to each of the dummies. The sprues should be eight to six gauges with reservoirs twice the diameter of the sprue. Between the reservoir and the dummy the sprue should be about 3/8" in length and tapered at the end joining the dummy.
Attach a 20-gauge vent to the opposite side of the dummy to help eliminate the gases that build up as the molten alloy rushes in to fill the pattern.
When casting the partial you want the same setup as for the bridge. The button should be in the center of the ring with a minimal amount of investment over the top of the pattern. You can control this by making the size of the refractory model proportionate to the size of the casting ring you use:
Before investing, mark the outside of the ring at the height of the pattern, then trim the excess down after the investment has set. Always make sure to use sufficient amounts of alloy when making the casting and let the casting arm spin a sufficient length of time to ensure that the button has solidified.
Answered by Milton C. Pokladnick, CDT, November/December 1999
Q: How do you retain porcelain denture teeth if the lugs have to be ground off?
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?
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?
A: 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.
Answered by Karen M. Crace, CDT, May 2001
Q: How can I achieve a functional mucus seal in an acrylic partial?
A: 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 relin ing 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 me dium 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 manu facturer 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 oF and 210 oF 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: What to consider when designing distal extension removable partial dentures.
A: Using Dr. Edward Kennedy's classification, distal extension removable partial dentures fall under two distinct categories:
Class 1 with bilateral edentulous areas located posterior to the remaining dentition, and
Class 2 with only one edentulous area located posterior to the remaining dentition. Any additional edentulous areas that fall outside of these definitions are called modifications.
Both classifications are tooth-tissue supported and before choosing a designyou should first analyze movement resulting from mastication. Movement precedes torque, and movement--if not counteracted horizontally as well as vertically--can destroy the remaining dentition and oral structures. We cannot control the forces of mastication any more than we can control what the patient eats.
The first step begins with a study of the edentulous areas in order to determine how they react and relate to one another during function and how that action will translate throughout the prosthesis.
For example, let's look at a Kennedy Class 1 situation and see what happens during mastication. Vertical and horizontal forces--as well as all the lateral forces in between--are being applied to both denture base areas but not necessarily at the same time or to the same degree. This results in rotation or movement around fixed points on the RPD itself. If a downward force is applied only to the right distal extension base--as we see with individuals who chew predominately on their right side--the initial axis of rotation actually lies between the most distal rest on the right side and the distal border on the left distal extension base. If a dislodging force is applied to the right base, the axis is the same. But, if the force is severe, the rotation moves to a line drawn between the most distal rests on both sides. Therefore, the goal in planning designs for Class 1 and Class 2 RPDs is to visualize potential movement and the resulting rotational paths, and then plan countermeasures to achieve stabilization in all planes, thus preserving as much of the remaining dentition and oral structures as possible.
Ultimately, the responsibility for stabilization falls solely on the practitioner. Even if we use the appropriate clasp designs, indirect retainers, horizontal and vertical bracings and achieve rigidity with the major connector, the prosthesis may fail if the practitioner doesn't provide for maximum denture base coverage up to the physiological tolerance of the patient, or if he fails to use secondary impression techniques.
Answered by Greg Curd, CDT, September 2002
Q: How to determine the best major connector to use on a maxillary RPD.
A: In covering the palate with a major connector, you should first look at what not to cover with metal. In doing so, the space available for the major connector is naturally revealed. The technique I generally use--first described by Dr. L. Blatterfein in 1953--allows you to choose a major connector that covers a minimum amount of palatal space. Here's the technique:
Outline the denture bearing areas that you know the acrylic bases will cover on the cast.
Outline the areas that, with few exceptions, you do not want to cover with metal. This includes the gingival tissue areas to within 5 or 6mm of the remaining teeth, tori, hard areas along the median palatine suture and the soft palate posterior to the vibrating line.
After completing these two steps, you will see the available area left for placement of the major connector. You can now decide on the shape of the major connector itself. This decision draws on many factors pertinent to each case. In general, the primary concerns are rigidity, patient comfort, as well as the location, condition and extent of the denture-bearing areas themselves.
Answered by Greg Curd, CDT, September 2002
Q: How to downsize bulky RPD frameworks without compromising overall strength.
A: When designing and constructing RPDs, you have to find a balance between providing sufficient rigidity, support and bracing in order to avoid traumatizing the remaining oral structures. Your goal is to offer a comfortable restoration that causes the least amount of harm to the patient. With thoughtful planning and waxing, you can achieve this goal without excessive bulk. Here are a few tips:
Structural design. Take a simple palatal strap using a pattern of .5mm thickness. A cross-section reveals a flat design. Strengthen the middle of the pattern with a small half-round piece of wax to achieve a middle thickness of about .7mm, tapering out to either side. A cross-section now reveals a v-shape design, which is structurally superior in strength to a flat pattern. This allows you to thin the width of patterns without compromising the overall strength or adding noticeable bulk.
Waxing for patient comfort. Avoid crossing over convex surfaces; stay in the valleys and embrasures. When meeting a convex surface, taper the metal into that surface. If you need to cross a convex surface, do so in the shortest amount of distance and avoid the crest of the surface as much as possible. Also, try to avoid covering the rugae since this is noticeable to the patient's tongue. When designing mandibular frameworks, ask the dentist to use a simple periodontal probe to quickly measure the distance between the lingual gingival margins and the floor of the mouth with the tongue elevated. By defining the available space, you ensure the lowest possible placement of the bulk contained in the chosen connector.
Replacing missing natural teeth and tissue in relationship to the metal framework is perhaps the greatest cause of bulkiness in today's RPD work. When fabricating an RPD, the following method not only reduces overall bulkiness, but also enhances the phonetic qualities and the natural feel of the prosthesis:
Arrange the teeth and wax in the missing tissue on a mounted duplicate of the master model first, keeping in mind that the patient may need to approve the arrangement before you can proceed. This also allows you to work with the dentist-client to address the occlusal and esthetic issue before constructing the framework.
Using occlusal putty indexes, transfer the tooth placement to the master model during the survey and design step and eventually to the investment model for waxing. Knowing the denture tooth and missing tissue positions before constructing the framework allows you to develop a more natural contour to the prosthesis.
When waxing, place the external finish lines in close proximity to the palatal and/or lingual surfaces developed from the above steps.
By keeping these factors in mind you'll be well on your way to providing a natural, comfortable prosthesis for the patient.
Answered by Greg Curd, CDT, September 2002
Greg Curd, CDT, discussses major connectors.
The major connector is viewed as the foundation upon which all other removable partial denture (RPD) components originate. In his book, Removable Partial Prosthodontics, William McCracken defines a major connector as "the unit of the partial denture that connects the parts of the prosthesis located on one side of the arch with those on the opposite side of the arch."
A successful major connector must be designed for sufficient rigidity, which in turn permits functional stresses to be effectively distributed throughout the remaining supporting hard and soft tissue elements. A flexible major connector, on the other hand, often localizes trauma to either the abutment teeth, the supporting soft tissues, or both.
Major connectors should be positioned in relationship to the remaining oral structures in order to avoid impingement during insertion, removal and the normal course of function. In all cases, the major connector of choice is the one least noticeable and least damaging to the patient, and most consistent with that particular patient's restorative requirements.
Here is an overview of the most common mandibular and maxillary major connectors:
Mandibular major connectors Lingual Bars run superiorly to the remaining dentition and should be pear shape in cross-section, tapering superiorly into the soft tissue, slightly relieved relative to tissue conditions, and rounded inferiorly in order to avoid tissue impingement. A minimum of 7 to 8mm must exist between the gingival margins and the functional floor of the mouth (this includes a minimum of 3 to 4mm between the gingival margins and the superior border of the bar, plus roughly 4mm for the average height of the bar). If less space exists, you should choose a different type of connector. For patient comfort, the bar should ideally be located as far inferiorly as possible.
Sublingual Bar is a more esoteric approach of running the lingual bar on the floor of the mouth and is useful when space does not permit a lingual bar without gingival impingement and a lingual plate is not desired. For this technique, you must rotate the bar 90 o relative to the slope of the lingual tissue surfaces, and impress the functional floor of the mouth in order to avoid tissue impingement.
Lingual Plate is a continuation of the lingual bar, in plate form, up and onto the cingulum surfaces of the teeth resulting in a scalloped appearance. The lingual bar portion can be thinned down considerably without sacrificing rigidity.
Plates are often used as indirect retainers to address rotational forces on unilateral and bilateral cases, as splints on mobile teeth, as a means to prevent eruption of mandibular anterior teeth opposing a full maxillary denture, and where further anterior tooth loss is anticipated. Lingual plates only work under these conditions if proper rest seats are developed first. Without them, the lingual plate acts as an inclined plane operating against the inclined plane formed by the lingual surfaces of the teeth. A sliding action and possible damage can also occur.
The patient must be properly instructed on maintenance issues since posterior settling in bilateral cases--and to a lesser extent, unilateral cases--often opens the superior edge of the plate, resulting in food entrapment and gingival impingement.
Labial Bar is a rare but interesting major connector used when the remaining mandibular teeth exhibit a severe lingual tilt. Under these conditions, use of a lingual connector would leave too great a space between the bar and the tissues, resulting in excessive food entrapment and tongue irritation. The labial bar uses the same spacing and placement requirements as lingual bars, but runs on the labial side of the remaining dentition. Due to the extra length, the bar should be formed slightly larger to maintain adequate rigidity.
Cingulum Straps run solely on the cingulum surface of the anterior and, depending on design, the lingual walls of any remaining posterior teeth. With this design, it's difficult to strike a balance between rigidity and patient comfort. However, its useful when high lingual frenum attachments or other conditions superior to the remaining anterior teeth limit design choices.
Kennedy Bar is a combination of a lingual bar and a small cingulum strap and leaves the gingival tissue area exposed. The main purpose of this design is to allow for more natural massaging of the gingival tissues and avoid possible gingival impingement. However, in practice, it's often irritating to the patient's tongue and a potential food trap.
Maxillary major connectors Full Palatal Coverage takes full advantage of the entire palatal area and is obviously the strongest, most stable choice for maxillary connectors. This is the connector of choice in situations where a combination of some or all of the anterior teeth remain, abutment support is weak and ridge resorption is severe.
Full coverage can be completely metal or a combination of metal and acrylic. Complete metal is preferable for patient comfort and sensation, but obviously difficult if any palatal adjustments are necessary, or a future reline is anticipated. For these reasons, you must provide proper palatal relief in order to functionally balance the metal on the hard and soft portions of the palate.
The Single Palatal Strap is arguably the best maxillary major connector in terms of patient comfort, since the design avoids coverage of the anterior and posterior areas of the palate. The width ranges from narrow to broad depending on tooth replacement requirements. Individual anterior replacements can often be cantilevered off a palatal strap, but extensive anterior replacement limits the use of this design. Relief is required along an elevated or hard median suture line and you must provide adequate rigidity to narrow straps.
The Anterior-Posterior Palatal (A-P) Strap is one of the most widely used maxillary major connectors. Since the anterior and posterior straps fall into two different planes, this design is inherently rigid. The A-P strap is often seen as the best choice for extensive anterior replacements, or for inoperable tori that do not extend posterior to the junction of the hard and soft palate. However, the anterior strap generally covers the rugae and the placement of the posterior strap may prove uncomfortable for the patient. The A-P strap also exhibits the longest metal-to-tissue open palate margin of all the maxillary designs, which can result in food entrapment on bilateral and unilateral cases.
The U-Shape Palatal Strap is perhaps the poorest choice of all maxillary connectors due to its inherent lack of rigidity and coverage of the rugae area. The connector also fails to take advantage of a stable vertical stop by not crossing over the hard palate in the roof of the mouth. The anterior portion of the connector is often placed anterior to the most anterior rest seats. On bilateral designs--and to a lesser degree on unilateral designs--this forward placement eventually causes a lifting of the anterior portion of the major connector resulting in food entrapment. The posterior portions of the connector are also prone to lift and food entrapment. This connector is best used when an inoperable torus extends to the junction of the hard and soft palate. As with all maxillary major connectors, the metal should stay at least 6mm away from the gingival tissues.
by Greg Curd, CDT, April 2003
