New Approaches to Removable Prosthetics
Inside Dental Technology delivers updates on digital workflows, materials, lab techniques, and innovation in dental technology through expert articles and videos.
October 1, 2019
Volume 10, Issue 10, October 2019
Page(s): p. 36
Valerie McMillan, DDS, MS
Denture fabrication has been a much-neglected dental service, with only a few dedicated fans, but that could change soon. Digitally fabricated dentures are quickly becoming reality and they promise many advantages, even a coolness factor. While digital techniques transformed fixed dental prostheses years ago, some feel the tipping point of removable prosthetics is now. We are on the edge of an exciting time in dentures, not only for the clinician but also laboratories and patients. Digital techniques have been shown to be time efficient and equally effective compared to conventional techniques, and they are preferred by providers.1 In addition, an update to these early studies shows that outcomes are positive. Significantly reduced clinical time, improved retention, and the ability to digitally archive were cited as the main advantages.2 The classic five-appointment technique has been reduced to needing only four or three appointments in described digital techniques. Even a two-appointment approach has been used successfully in a clinical research setting. At the 1-year follow-up visit, clinical and patient-centered outcomes were favorable for dentures delivered in only two visits.3 However, two important difficulties in digital denture techniques have become apparent: The communication required between the clinician and laboratory is different than with conventional techniques, and an increased amount of clinician time is required for them to aid in the design.3Additionally, try-in procedures are much different with digital techniques and some report difficulty using digital try-ins. Lastly, the cost of digitally fabricated dentures is a factor, and the difficulty of integrating the technology is a hurdle for many laboratories. However, work to overcome these challenges is ongoing because reducing clinical time and improving outcomes is so compelling.
Advancing Techniques
How can new techniques suddenly reduce the number of appointments to as little as two? Is it just hype, or is something else happening? These accelerated denture workflows that reduce clinical time are greatly assisted by digital tools but also are part of an overall trend toward simplification. Simplified denture techniques have been validated prior to the advent of digital denture methods. A randomized clinical trial in 2005 found that simplified techniques were as effective as more complex traditional ones.4 This sentiment was repeated in a review by Gunnar Carlsson in 2008, when he discussed research showing no difference in outcomes regarding many of the popular concepts in removable prosthodontics such as balanced occlusion, facebow, and impression techniques and materials.5A Cochrane Review in 2018 on the topic of impression materials and techniques for complete dentures showed that no difference could be observed between any of them.6
Moreover, some digital tools can help greatly with denture fabrication. When placing teeth for immediate denture designs, the technician could simply select or deselect the reference of the previous tooth position; import facial or lip scans and overlay into the design; and reference an overlaid photograph or even a digital smile design the patient already viewed. If a patient has a current denture that they like, the technician can simply place in new teeth that match, providing a very close copy with a simple reline impression. With the digital process, the patient can be sent home with a try-in; then they can chew with it, spend time in front of the mirror, and even ask their friends how it looks. Of course, there is always the patient who asks one or more of the once-dreaded questions: Could the teeth be a different color, a bit smaller, or shifted a little to the left? No longer does the doctor need to fear these requests, as these are all very simple and may not even require another try-in. Any changes needed can be easily incorporated into the final design. Similarly, it is not the end of the world if a patient is not happy with their final denture upon placement, such as if the shade, the fit, or the vertical is not quite right. This does not mean the process must be started over completely. All the records have been kept. For the shade, click a few buttons. For the fit, do a reline, send for scanning, and get new dentures back. For the vertical, simply estimate and make a few adjustments.
Very soon, the cost to make a printed denture will be so inexpensive that try-ins may be a thing of the past. Make the denture; try it in; and if it is no good, adjust and print it again. This means that the relevant skills for making dentures are changing. Forget how to shape a wax rim and instead learn how to evaluate a current denture for all its mistakes. Be able to evaluate a patient's denture and say definitively, for example, that the vertical needs to go down 2 mm, the incisal edge should move superiorly by 1.5 mm, the midline must shift right 2 mm, and teeth Nos. 5 and 12 need to be moved facially.
Manufacturing Methods and Materials
As the process of making dentures has changed with digital techniques, so have the materials. While the first digital dentures were milled from prepolymerized pucks of PMMA with bonded manufactured denture teeth, now there are many additional techniques and innovations. A dental laboratory wanting to produce digital dentures has the most important task of evaluating the different options. Although some of these products have been compared in laboratory studies, few have been evaluated in clinical studies. At this point, reports on long-term clinical performance are anecdotal.
Occasionally, it is proposed that due to the low cost of producing some types of digital dentures, patients may not need a specific prosthetic for longer than 1 or 2 years, since at that point the laboratory can simply make another. While this may work for some patients, it could be a disaster for others, as transitioning a patient from their fully adjusted and comfortable prosthesis to a new one is not easy.
Consider the most common pros and cons of making digital dentures through different manufacturing methods.
1. Method/product: Milled denture base
Pros:
• Comparable strength and esthetics to a conventional gold standard prosthesis.
• Improved fit is possible due to lack of polymerization shrinkage.
• Known and established long-term clinically used material (PMMA).
Cons:
• The time needed for milling means limited daily production.
• The machines are costly when considering how many units per day they can produce.
• High overall cost to manufacture the denture.
2. Method/product: Printed denture base
Pro:
• Low overall cost due to high number of units a machine can produce in a day.
Cons:
• Strength and esthetic properties vary greatly. Higher impact and improved-esthetics materials are becoming available.
• The material is not always suitable for use with existing reline and repair products.
• The printing process is not always as accurate as other methods; distortion can occur at several steps without
careful manufacturing.
• Critics may argue that the material has not been used long enough to safely justify large-scale adoption.
3. Method/product: Use of premanufactured teeth
Pros:
• Long-term clinical use.
• Established record of good wear resistance, strength, and high esthetics.
Cons:
• Premanufactured teeth are made from locked moulds, so they cannot be adapted using software to scale or morph.
• They often need adjustment to prevent penetration into the ridge. This can be easily solved by reducing them by milling prior to bonding or after bonding into a milled base, but is much harder to solve when the base is printed.
• Premanufactured teeth may have undercuts and manufacturing irregularities making them more difficult to place.
• Critics state that the process of using a separate material to bond denture teeth into any base is less predictable than the conventional process.
4. Method/product: Milling the teeth
Pros:
• Unlocked moulds allow the ability to scale and morph the teeth and easily adapt the occlusion.
• Materials are well-known and trusted.
• Minimal thickness is needed.
• Segments of multiple teeth may reinforce the arch.
• Esthetics are good, although not as ideal as premanufactured teeth.
• Milled teeth can be incorporated into a milled base either by luting in the segments of teeth or in the process of creating a custom puck.
Cons:
• Difficulty reproducing fine detail due to tool limits, which means additional handwork may be needed.
• Time needed to mill and hand-finish means that the overall cost to produce may be high.
5. Method/product: Printing the teeth
Pros:
• Fast manufacture and low cost.
• High detail is possible, which means that placing printed teeth into a base requires minimal work.
• Unlocked moulds.
Cons:
• Unknown materials without a long clinical record.
• Users suspect possible poor wear resistance; and there are some reports of stain accumulation and fractures.
• Critics are cautious of using printed teeth in final dentures and feel they are more appropriate for interim use.
6. Method/product: Hybrid conventional and digital process. Some laboratories have digitized the records process and set-ups, generating printed try-ins that, once approved, are invested and used to conventionally process.
Pros:
• Utilize existing technicians and equipment to process dentures without using mills.
• Take advantage of some digital processes and maintain digital records for reproducibility.
• Positives of using premanufactured teeth.
Cons:
• Negatives of using premanufactured teeth.
• Accuracy of printed bases is under question.
• Conventional process drawbacks, such as polymerization shrinkage.
Dental technology is advancing quickly toward an ideal selection of materials and methods for digital dentures. The ideal process would offer low manufacturing cost per unit, the same quality as conventional dentures, and the capacity to handle the necessary volume. When the digital denture revolution occurs, I predict we will be able to make better dentures for more people, using human skill and machine capabilities together for optimal results (Figure 8).
Denture prosthetics is entering an exciting time with many possibilities. Dental providers and technicians have many options going forward.
About the Author
Valerie McMillan, DDS, MS, operates a Private Practice in Dayton, Ohio.
