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In recent years, technological advances have facilitated the development of CAD/CAM and decreased the inaccuracies of conventional techniques.1 Digital systems have been integrated into the fabrication of inlays, onlays, crowns, fixed partial dentures, implant abutments/prostheses, and maxillofacial prostheses.2 However, removable denture design has employed the same traditional procedures for more than 50 years despite the many risks of errors. Digital removable denture design software is relatively new. This article will explain the traditional removable denture device process and compare it with this new digital option, which has proven to be relatively efficient and facilitates the standardization of clinical results.
More than 36 million Americans are edentulous, and about 90% of edentulous people have dentures.3 Dentures are also a common and popular restorative dentistry treatment for people who have just partial tooth loss. One goal of dental reconstruction is to regain durable function and esthetics that are as similar to natural teeth as possible. For an edentulous patient, a variety of treatment options can be considered, including mucosa-supported removable dental prostheses (RDP), implant-retained RDP, fixed detachable prostheses, and metal-ceramic fixed dental prostheses. Each treatment option has unique advantages and limitations with respect to space, implant distribution, and implant number.4-7
Patient management plays an important role in the process of creating removable prosthetics. From the beginning, during the patient consultation, communication is crucial. It is important to listen to what motivates the patient. For example, is the patient more concerned with function or esthetics? The answer may help determine which material to use. Concerning denture quality, patients are generally more satisfied with customized skeletal dentures for retention, masticatory efficiency, and esthetics because they prefer not to see classic wire hooks.8
It is also important to discuss alternative treatment plans and options with a patient, such as All-on-4® (Nobel Biocare, nobelbiocare.com) or an overdenture. The main disadvantage of these treatments is cost. For older patients, partial edentation therapy with partial dentures is often required.9 Many elderly patients live on a fixed income, so a large expense may not fit within their budget. Conservative treatments such as dental implants are expensive. Consequently, their use is limited in lower socioeconomic groups, in whom the highest rates of tooth loss occur.10-12 Conventional removable prosthodontic treatments, therefore, continue to outnumber implant tooth replacements in general practice.13
Partial Denture Components
The important steps in making partial dentures involve forming rest seats, picking the correct type of partial allocation, and taking a good impression. A partial denture consists of a handful of parts. There are major and minor connectors, denture bases, artificial teeth, and direct and indirect retainers. A major connector is the main body of a denture, which provides rigidity and unites all the parts of the denture. A minor connector joins together all other parts of a denture to the major connector. A denture base is the part that is adapted to the soft tissues in the region of missing teeth and is used to support the artificial teeth. Artificial teeth replace the missing teeth. Direct retainers, often clasps, are the parts of the denture that prevent it from falling out of the mouth. Indirect retainers prevent the denture from shifting during eating or talking. Rests are indirect retainers.
Rests
A rest seat is a small projection of metal that rests on a chewing surface of a natural tooth from the main frame of the body of the denture. A rest seat has many functions. It is important to place a rest seat because it prevents partials from rocking, shifting, or rotating when eating and talking. It also prevents open bite. Furthermore, it helps maintain a level of clasp engagement with tooth surfaces. Placement of a rest requires that a small amount of enamel is removed from the tooth structure using a small dental bur. This creates a space so that the metal projection from the denture rests on the tooth. This way the patient should not feel a high bite when biting the teeth together. Rests are important because without them, a denture base would rest entirely on an edentulous ridge, causing all the load created during chewing to fall on the bone within the edentulous ridge. That type of denture support is called mucosal support and may result in a faster rate of resorption of the edentulous ridge, which is ultimately detrimental to the ridge. A tooth support is when a rest is placed on a tooth on each side of the gap so that the load of mastication is shared between the two teeth. A tooth and mucosal support combined are used when a rest can only be placed on one tooth; then the load is shared between the natural tooth and the ridge.
Placing a rest seat is normally a quick procedure. There are occlusal rests, cingulum rests, and incisal rests. Occlusal rests are placed on one or both sides of a molar or premolar. Cingulum rests are placed on an upper incisor or a canine, resting on the cingulum of the tooth. Incisal rests are placed on lower incisors, where preparation of a cingulum rest may penetrate beyond the thin enamel. A cingulum rest is the least esthetic type of rest because it can be seen from the front.
To optimize partial denture design, retention and function, the author always places rest seats on abutment teeth. The author also takes a closer examination of abutment teeth for wear. If they are too worn down, the author recommends crowns before the impression to restore the tooth to its ideal anatomy.
Frameworks
Acrylic resin framework removable partial dentures (RPDs) are far more common than cast metal framework RPDs.14 Long-term studies on acrylic resin framework RPDs are lacking, but they continue to be used frequently.15,16 Because they lack the strength and established design principles of cast metal framework RPDs, alternative RPD frameworks have been shown in some studies to have reduced longevity and significant periodontal consequences.17
A metal framework works best when esthetics are not the patient's primary concern. The advantages to a metal framework for RPDs are the ability to adjust the clasps if the partial is too loose or too tight, the lower costs, and the assistance from plates and clasps in creating retention. Disadvantages can include a metallic taste and discomfort. Flexible partials are a more esthetically pleasing alternative. The advantages include the cosmetics, lack of metallic taste, and how thin and flexible they are. The disadvantages include cost, that the plate does not create retention, that not everyone is a candidate, and that they can be hard to adjust. Combination partials are good for retention and esthetics, although some patients still complain about a metallic taste. A unilateral partial can be used to reserve space for a bridge or implant later.
From the author's experience with digitally fabricated frameworks versus conventionally fabricated frameworks, digitally fabricated frameworks tend to have a tighter and more consistent fit from patient to patient. With conventionally fabricated frameworks, the author sometimes faces challenges with partials not fitting in the patient's mouth even though they fit perfectly in the model. This situation can occur because of poor impression techniques, impression distortions, and improper pour-up of the models.
The Traditional Denture Experience
In the traditional denture creation process, the first visit is critical because that is when perhaps the most crucial step is taken: the impression. A well-made impression will save a dentist from complications later. The consistency of the mixture used must be precise (Figure 1). The mixture must be given enough time to set. After the impression is taken, the mixture should be poured immediately. The second visit is when the most time will likely be spent with the patient. This is when the patient's vertical dimension of occlusion (VDO), vertical dimension of rest (VDR), midline, and smile line are measured (Figure 2), in addition to a bite registration being taken.
During the third visit, the artificial teeth are tried in and occlusion checked. Occlusion is one of the most important factors affecting denture stability (Figure 3). Vertical forces on the denture base lying over the resilient tissue will produce lever forces on the denture. Vertical forces acting outside the ridge crest will produce tipping of the denture. The clinician should ensure the fit is comfortable and tight. The fourth visit is when the denture is delivered (Figure 4). Any final adjustments are then made, and the dentures are polished. Reline protocol dictates that patients should come in a few times after the denture delivery. Reline can be performed in-office or sent to a laboratory.
Digitally Designed Dentures
Advancements in CAD/CAM technology can now be used for the fabrication of removable dentures, including milling and rapid prototyping.18CAD/CAM technology refers to digital design and manufacture. CAD software recognizes the geometry of an object, and CAM software is used for the manufacture. CAD/CAM has been used for several years for the fabrication of inlays, onlays, crowns, fixed partial dentures, implant abutments/prostheses, and maxillofacial prostheses.19 Now it can be used to make fixed restorations and removable dentures as well.20-22 During CAD, impressions of the edentulous maxilla and mandible or existing dentures are subjected to laser scanning.23 Unlike the traditional method where a wax pattern is made on a definitive cast, a digital RPD is designed on a computer. Additionally, cone-beam computed tomography, a special type of x-ray equipment used when regular dental or facial x-rays are not sufficient, can be used for the modification of previous dentures.20
Digitally designed dentures can be used in lieu of conventional denture design with impressions, but intraoral digital impressions can also be taken when traditional impressions are not an option. For example, digitally designed dentures are a possibility for patients with severe gag reflexes who cannot tolerate a traditional impression. Digital imagery for dentures provides better accuracy, fewer chairside adjustments, comfort for the patient, and overhead savings (Figure 5 and Figure 6). As with any new technique, there is a learning curve for the dental practice to be comfortable and successful with this process. Many intraoral scanners are on the market to help with digital dentures.
The commercial manufacturers of removable complete dentures with CAD/CAM employ a gadget for transferring the maxillomandibular relation (MMR) to a digital articulator and finalizing the dentures completely with CAD/CAM. There are two options for MMR transfer during complete prosthesis fabrication using CAD/CAM: 1) the MMR can be transferred using conventional impression and transfer techniques or 2) one of the system kits on the market can be used, such as the AvaDent® (AvaDent Digital Dental Solutions, avadent.com) system kit or the Dentca™ (Dentca CAD/CAM Denture, dentca.com) system kit.24,25
As few as two clinical appointments are required for the manufacturing of removable complete dentures using these system kits. In the first appointment, impressions are recorded using special trays provided in the system, the jaw relation is recorded using an anatomical measuring device, and the occlusal vertical dimension is determined using conventional methods. Then the centric relation is recorded and teeth are selected. The last step of the first appointment is the delivery of the final impression to the manufacturer. The manufacturer will define and mark the denture borders using the system's computer software. Then the teeth will be virtually set and the prosthesis base milled from traditional denture resin material. A trial denture can be prepared if requested. In the second clinical appointment, the dentures are delivered and adjustments are made as needed, often very similarly to conventional prosthesis delivery.25
There are also just a few steps for the manufacturing of partial prosthesis frameworks with CAD/CAM. First, dental casts are prepared using a conventional impression method or digital impression. For conventional impression techniques, casts are then scanned using a digital scanner. The path of insertion of the RPDs is defined digitally, and then the shape of the components of the framework is designed 3-dimensionally by dentists or laboratory technicians. Based on this design, a 3D pattern is printed using rapid prototyping, with a resin that can be eliminated. The pattern subsequently undergoes investment, elimination, and casting, thereby completing the fabrication of a digital RPD framework.26,27Finally, the digitally designed metal RPD frameworks are produced.28 A digital denture fabrication method has various advantages over conventional methods. In particular, the scan data can help determine the best path of insertion and removal and can control the undercut amount through digital computation in a more straightforward and accurate way than conventional surveying. The author finds that Kennedy Classification I and II are the most difficult cases. With no posterior teeth for support, the partial tends to lift, causing a food trap and discomfort for patients.
The use of digital impressions and design for removable prosthodontics is still in its relative infancy. The future of this technology is very bright. As with any new technology, the early adopters will have frustration as the software for design evolves.
Conclusion
By simplifying the denture-making process, digital denture design is increasing efficiencies for dentists, laboratories, and patients. Although several digital denture design systems are now available on the market, their use by dental practitioners and technicians remains marginal.29,30Too few clinical studies have evaluated the accuracy of a digitally generated RPD intraorally. Although the learning curve is steep, clinicians and laboratory technicians would benefit from adopting this digital practice into their repertoire; it is likely to become the standard of care for RPDs.
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