Beuer F, Schweiger J, Edelhoff D. Digital dentistry: an overview of recent developments for CAD/CAM generated restorations. Br Dent J. 2008; 204:505-511 https://doi.org/10.1038/sj.bdj.2008.350
Vojdani M, Giti R. Polyamide as a denture base material: a literature review. J Dent (Shiraz). 2015; 16:1-9
Singh K, Aeran H, Kumar N, Gupta N. Flexible thermoplastic denture base materials for aesthetical removable partial denture framework. J Clin Diagn Res. 2013; 7:2372-2373 https://doi.org/10.7860/JCDR/2013/5020.3527
Zoidis P, Papathanasiou I, Polyzois G. The use of a modified poly-ether-ether-ketone (PEEK) as an alternative framework material for removable dental prostheses. A clinical report. J Prosthodont. 2016; 25:580-584 https://doi.org/10.1111/jopr.12325
Fueki K, Ohkubo C, Yatabe M Clinical application of removable partial dentures using thermoplastic resin-part I: definition and indication of non-metal clasp dentures. J Prosthodont Res. 2014; 58:3-10 https://doi.org/10.1016/j.jpor.2013.12.002
Aly Sadek S, Dehis WM, Hassan H. Comparative study clarifying the most suitable material to be used as partial denture clasps. Open Access Maced J Med Sci. 2018; 6:1111-1119 https://doi.org/10.3889/oamjms.2018.226
Bathala L, Majeti V, Rachuri N The role of polyether ether ketone (Peek) in dentistry – a review. J Med Life. 2019; 12:5-9 https://doi.org/10.25122/jml-2019-0003
Tannous F, Steiner M, Shahin R, Kern M. Retentive forces and fatigue resistance of thermoplastic resin clasps. Dent Mater. 2012; 28:273-278 https://doi.org/10.1016/j.dental.2011.10.016
Skirbutis G, Dzingutė A, Masiliūnaitė V PEEK polymer's properties and its use in prosthodontics. A review. Stomatologija. 2018; 20:54-58
Muhsin S, Wood D, Johnson A Effects of novel polyetheretherketone (PEEK) clasp design on retentive force at different tooth undercuts. J Oral Dent Res. 2018; 5:13-25
Ali Z, Baker S, Sereno N, Martin N. A pilot randomized controlled crossover trial comparing early OHRQoL outcomes of cobalt-chromium versus peek removable partial denture frameworks. Int J Prosthodont. 2020; 33:386-392 https://doi.org/10.11607/ijp.6604
Marie A, Keeling A, Hyde TP Deformation and retentive force following in vitro cyclic fatigue of cobalt-chrome and aryl ketone polymer (AKP) clasps. Dent Mater. 2019; 35:e113-e121 https://doi.org/10.1016/j.dental.2019.02.028
Nattress B, Touloumi F, Thalji G OHRQoL comparison between cobalt chrome versus polymer removable partial dentures. J Dent Res. 2020; 98:(Spec Iss A)
Zlatarić DK, Celebić A, Valentić-Peruzović M. The effect of removable partial dentures on periodontal health of abutment and non-abutment teeth. J Periodontol. 2002; 73:137-144 https://doi.org/10.1902/jop.2002.73.2.137
Owall B, Budtz-Jörgensen E, Davenport J Removable partial denture design: a need to focus on hygienic principles?. Int J Prosthodont. 2002; 15:371-378
Solvay Dental 360. Biofilm study review: in vitro biofilm formation studies on polymer coupons. 2020. http://www.solvaydental360.com (accessed September 2020)
Brum RS, Labes LG, Volpato CÂM Strategies to reduce biofilm formation in PEEK materials applied to implant dentistry – a comprehensive review. Antibiotics (Basel). 2020; 9
An update on indirect prosthodontic materials and their manufacturing techniques David Gray Olivia Barraclough Zaid Ali Brian Nattress Dental Update 2024 48:8, 707-709.
Authors
DavidGray
Associate Dentist, The Broadway Dental Practice, Catford, London, SE6 4SN and Specialty Doctor in Prosthodontics, Eastman Dental Hospital, 47-49 Huntley Street, London WC1E 6DG, UK
Innovations in the fabrication of removable partial dentures depend not only on the development of new materials, but also on the availability of manufacturing techniques that can be applied to a dental environment. Many of these new materials have limited clinical evaluations, hence it can be difficult for the general dental practitioner to confidently determine which materials to use. The introduction of any new material into clinical practice often requires practitioners to go through a learning curve to make the most of the material and employ it most appropriately. This article provides an update on the materials available for removable partial dentures and discusses the advantages and disadvantages to enable the GDP to make an evidence-based decision.
CPD/Clinical Relevance: It is important that clinicians are aware of the alternative materials to conventional acrylic and cobalt chrome.
Article
Dentistry, as with most fields in healthcare, is inundated with regular advances and developments. The development of dental materials, a cornerstone of prosthetic dentistry, is an area with a rapidly evolving landscape driven by advances in materials science and industrial innovations.1 The introduction of a new material into clinical practice is often driven by the need to address a particular clinical concern or overcome a weakness with another material. Historically, the choice for a removable prosthetic framework or veneering material has been limited. The introduction of new dental materials makes it difficult for dental practitioners to truly appraise these and decide which would be worth adopting into their clinical practice. This is no less true with removable partial dentures, where advances in both material science and manufacturing processes have allowed dentistry to broaden the range of material options and workflows available.
This article addresses some of the questions that are likely to arise when clinicians are faced with a new denture material proffered for the provision of partial dentures. The aim is to inform readers about some of the novel materials that they are likely to encounter, along with their respective advantages and disadvantages, and to explain how the introduction of novel manufacturing techniques and digital prosthetic workflows have allowed the introduction of these new materials. Readers will note the cited references rarely refer to large scale clinical studies. The authors would therefore caution that the presented evidence is based on a narrative review of the best available evidence. Indeed, as the availability of higher quality clinical research in this area develops, so too will the evidence-based clinical recommendations evolve.
Novel removable prosthodontic workflows
There are two broad components to denture framework fabrication using a digital workflow – computer-aided design (CAD) and computer-aided manufacture (CAM) (Figure 1).
Figure 1. The digital workflow for the construction of partial denture frameworks.
Digital design for partial denture frameworks
There are various types of software available for designing the removable partial denture frameworks, such as ExoCAD (exocad GmbH, Darmstadt, Germany) partials and Dental Wings (Straumann Group, Montreal, Canada). Once the designer, almost exclusively a dental technician, has mastered the software, digital design confers a level of speed, accuracy and reproducibility that is difficult to attain with the conventional workflow.
The first stage of digital design is data acquisition. A digital impression is obtained, whether through intra-oral scanning of the soft and hard tissues using an appropriate intra-oral scanner (such as 3Shape Trios, 3Shape A/S, Copenhagen) or Primescan (Dentsply Sirona Inc, PA, USA), or through the scanning of a conventional impression or master cast to create a digital model using .stl data, which is then uploaded onto a computer to allow for design (Valplast International Corporation, NY, USA) (Figure 2).
Figure 2. Scanning of a master cast to produce a digital model
As with metal-based partial dentures, when planning polymer partial dentures, casts require articulating and surveying before designing the prosthesis. This can be completed digitally, which is accurate,2 predictable and can be completed more quickly than in a conventional process (Figure 3). The software enables identification of suitable undercuts (both position and depth), guide-planes, and determination of the path of insertion. The process needs to take into account the individual properties of the polymer being used, and CAD allows for the various components of the denture framework to be designed to pre-set dimensions and parameters. The denture is designed using the same prosthodontic principles adopted in designing a metal partial denture, taking into account saddles, support, retention, bracing and reciprocation, connectors and indirect retention.3 While not quantified at present, it is thought that the degree of precision, which may be conferred by technology, will improve accuracy through a reduction in human error that may arise throughout the denture construction process. As with any partial denture design, communication between the laboratory and clinician in the process of verifying a design is imperative to a successful outcome. Once the framework has been designed, this can be converted into a 3D pdf file and emailed to the clinician for verification before progressing to the manufacturing phase.
Figure 3. . (a–d) CAD in the construction of a CAM framework.
Novel manufacturing techniques
Traditional partial denture frameworks are manufactured using the lost wax casting technique (Figure 4). A wax pattern is fabricated onto the master cast to replicate the desired framework design. A moulding form is prepared by attaching a sprue and sprue cone onto the wax pattern. The wax pattern and sprue are embedded in investment, and once the wax is burned away, a defined space exists into which the metal alloy is cast. The produced framework is completed by hand, by cutting off the sprue, finishing and polishing.
Figure 4. The conventional lost-wax technique for metal framework fabrication.
This is a time-consuming and technique-sensitive process, and failure at any point in the process will require a complete restart. The digital workflow aims to improve accuracy, efficiency and reproducibility. Anecdotally, it is significantly faster for an individual to design a denture digitally then to conventionally lay a wax pattern. Furthermore, the digital workflow has enabled the production of denture bases using a variety of novel materials, including the high-performance polymers discussed later in this paper, all of which require the use of a CAD-CAM pathway.
Manufacturing techniques can be subcategorized into reductive or additive manufacturing. Reductive manufacturing is the process by which material is cut away from a solid prefabricated block. Conversely, additive manufacturing, or 3D printing, is the process of fabricating the framework through the sequential deposition of material.
In dentistry, the most commonly used reductive manufacturing technique is milling. In this technique, CAM software converts the computer-generated design into a series of instructions for the milling machine to follow. A bur, which can be moved in three-to-five axes, cuts away material from a core block, and in turn, the end product is fabricated. Milling is already commonplace in fixed prosthodontics; however, its use in the construction of removable prosthesis frameworks has previously proven challenging. When it was first adopted with metal frameworks, the intricacies of the design were are often not compatible with the cutting tools available, and the hardness of the material rapidly blunted the milling tools. However, the advent of novel polymers for frameworks has allowed the technique to be adopted successfully as the primary means of framework construction.
Additive manufacturing, or 3D printing, is better suited to the complexities of removable prosthodontics where chrome and titanium are used and is less transferable to polymers. It has been used successfully for the printing of complete denture prostheses, but has not been routinely taken up for partial denture frameworks. Other forms of additive manufacturing are selective laser melting (SLM) or selective laser sintering (SLS), which are additive processes used in the fabrication of metal framework materials.
Novel materials
Polyamide (nylon)
Polyamides are the most well-known and popular polymer alternatives to traditional acrylic dentures, being manufactured under trading names such as Valplast (Valplast International Corporation, NY, USA). They were introduced to the denture market in the 1950s, making them an older-generation alternative to polymethylmethacrylate (PMMA),4 and have been widely used in general practice. At the time, polyamides were marketed for the increased aesthetics and greater comfort conferred by their flexibility. Because polyamides are semi-crystalline, they have high heat resistance, impact strength, toughness and resistance to fracture, but are also elastic.5,6
The elasticity of polyamides allows extensions from the denture base to engage in undercuts that would previously have been redundant in conventional acrylic dentures.7 This flexibility makes these dentures an ideal material in cases of severe soft/hard tissue undercuts, but a suboptimal material choice in cases of flat or flabby ridges, where a more rigid prosthesis is required.8
However, the shortfalls of polyamides are not insignificant. By design, polyamide dentures are tissue-borne because they cannot use rest seats for tooth support. While this allows for minimal/no tooth preparation, without using support from the teeth, these dentures increase the risk of alveolar resorption, especially in Kennedy Class I and II cases.8 Further, the necessity for a thicker denture base allows less opportunity for relief from the gingival tissues and thus has implications for hygiene.6 They may, however, be used with some success in patients with only a few missing teeth where the dentures carry no significant functional loading.9
Polyamides also have other disadvantages. Their surface roughness may increase microbial colonization and biofilm formation. Colour deterioration occurs over time, with a concomitant loss of aesthetics compared to when the dentures were newly provided (Figure 5), and the structure of polyamides makes relining, rebasing and repairing these dentures almost impossible.6,7
Figure 5. 7-year-old Valplast denture showing significant deterioration of aesthetics. (Image courtesy of Katie Davis.)
Acetal resin (polyoxymethylene, POM)
One of the early alternatives to conventional acrylic resins, acetal resin, was introduced as a removable partial denture clasp material in the 1980s (Figures 6, 7).10 Being of the same polymer group as nylon, it shares many of its desirable characteristics. For use as a clasp material, its strength, elasticity and fracture resistance are particularly valuable.11 Furthermore, acetal resins are not as porous as nylon, making plaque accumulation and discolouration less prevalent.12
Figure 6. Acetal resin clasps on a cobalt-chrome maxillary partial denture. (Images courtesy of Finlay Sutton.)Figure 7.
(a–d) Comparative photographs showing the relative size, position and aesthetics of metal versus acetal resin clasps. (Images courtesy of Finlay Sutton.)
Clinically, the flexibility of acetal resin allows clasps to engage in deeper undercuts than cobalt–chromium (CoCr), while exerting fewer stresses on abutment teeth. This makes it of particular use where bone loss is present on the abutment clasping tooth.11 However, while the flexibility has many advantages, it does have downsides. To be sufficiently retentive, the clasps need a wide cross-sectional area, which may increase plaque accumulation.11
Polyacryletherketones (PAEK) polymers
Polyacryletherketones (PAEK) are a large group of polymers frequently used in medicine.8 Two polymers within this group, polyetheretherketone (PEEK) and polyetherketoneketone (PEKK), are currently used in dentistry. While they have been adopted in the construction of a range of dental products, such as implants, implant abutments and fixed prosthodontics, their use in removable prosthetics is a more recent development (Figure 8).13
Figure 8.
(a) PEEK disc pre- and post-milling. (b) PEEK partial denture. (Image courtesy of Juvora Ltd.)
As with the previously discussed materials, PAEKs have elastic properties allowing their clasps to engage in deeper undercuts (0.5 mm in the anterior region and 0.5–0.75 mm in the posterior region).14 The clasps, however, have reduced retentive forces in thin section and, therefore, require increased bulk to improve retention. As mentioned for acetal resin use, there are concerns about their compatibility with gingival health.15 The required framework dimensions can be found in Figure 9.16
Figure 9. The dimensions required for a PEEK denture framework. (Image courtesy of Juvora Ltd.)
Unlike polyamides, PAEKs have a reduced degree of discolouration.17 They can also be used as a tooth-supported prosthesis, reducing load to edentulous saddles and enabling their use in patients with a range of partially edentate patterns. The design principles for PAEK allow for its tooth support to originate either from conventional rest seats (Figure 8) or by the placement of a clasp that extends above and below the survey line (Figure 10), using support from its extension above the survey line.18
Figure 10. Survey line image.
While further research is required, research on PAEK dentures and oral health-related quality of life has found outcomes are at least as good as that for CoCr frameworks and thus, its use is promising.19 PAEKs are an innovative material that would be beneficial to patients with metal hyposensitivities or who dislike the taste, weight or aesthetic of metal denture frameworks.
Aryl ketone polymer (AKP)
The final higher performance polymer discussed in this article is AKP (Figure 11). The mechanical and physical properties of AKP enable the production of dentures that are more comfortable than rigid dentures made of PMMA or CoCr.20 AKP has sufficient strength and rigidity to be used for an implant-retained prosthesis, and being monomer free, it is a safe alternative for patients allergic to PMMA.
Figure 11. A maxillary AKP framework denture.
One marked advantage of the material's use is its retentive longevity. The repeated flexure of a CoCr clasp leads to work hardening, distortion, and in some cases, fracture. Conversely, while an AKP clasp generates lower retentive forces than CoCr, it remains consistent and stable over time.21
However, at present, the frameworks are manufactured from a monochrome block conforming to a limited number of standard tooth colours. As a result, it is not possible to have components of the framework matching the colour of the soft tissues while at the same time having tooth-coloured clasps (Figure 12).
Figure 12. Relative appearances of a mandibular partial denture constructed in CoCr and AKP.
Being one of the newer materials on the market, research into AKP is underway, with an early study suggesting an improved quality of life compared with conventional chrome framework dentures.22
Discussion
There are many driving forces for the progression of material science. In the case of dentures, two significant drivers are the desire to improve patient comfort and the increasing demand for aesthetic dentistry. Attempts to develop materials to meet these goals have been made for several decades; however, despite initial successes, the first generation of polymers had many shortfalls. One example is that while the aesthetics of polyamide dentures were initially excellent, their surface roughness led to their appearance markedly diminishing over time. Another example is that polyamide's flexibility meant that patients found them to be comfortable; however, their designs often contravened many of the prosthodontic principles that facilitate dental and gingival health. Thus, polyamide dentures may have had a detrimental impact on periodontal tissues.23 From a clinical perspective, these materials were less than ideal.
Ongoing material development has led to a newer generation of high-performance polymers that has largely superseded nylon and acetyl resin. These new polymers were developed to bridge the gap between PMMA and flexible dentures, while addressing the shortcomings that emerged with the older generation of polymers. They have sought to do this primarily through improving the properties in relation to function and design.
The newer generation of polymers addresses one of the significant disadvantages of nylon – their inability to add teeth or clasps after production. To do so, however, these require mechanical retention, whether in the form of embedding a clasp in acrylic (Figure 13) or perforating the framework for a tooth addition (Figure 14).
Figure 13. Adding an AKP clasp to a denture. (a) The clasp is designed digitally. (b, c) After manufacture, the clasp is embedded into acrylic in the existing prosthesis.Figure 14. Tooth additions, while possible with AKP, still rely on mechanical retention.
Another shortfall overcome is the ability novel polymers gave for dentures to be tooth-supported rather than mucosal borne. Previous flexible dentures infringed on ideal denture design principles, and this tissue borne design led to an increased risk of alveolar resorption.6
While the ability to create accurate, tooth-borne dentures will allow many of the drawbacks of mucosal borne dentures to be addressed, there are still material hurdles that make it not always possible to adhere to hygienic principles, as is proffered in the Scandinavian denture design, that is gaining traction in the UK.24 As per many of the earlier images (such as Figures 12 and 14), the novel polymers are thicker, and therefore allowed less opportunity for adequate gingival relief6 than would be with a CoCr or titanium-based denture framework. While this is perhaps not of as great clinical significance given the reduced biofilm compared to CoCr,25 it is nonetheless something that manufacturers will no doubt wish to address as the materials develop further.
Another shortfall with the older generation, in particular polyamide dentures, was the increased surface roughness. This roughness made the dentures difficult to successfully polish, and therefore prone to colour deterioration.6,7 It also impacted soft tissue health, as higher surface roughness results in increased microbial and biofilm colonization. The novel polymers discussed in this article have reduced surface roughness in comparison to the older generation of flexible polymers. While not yet supported well in the literature, there is emerging evidence that some of these novel materials, such as AKP and PEEK, have reduced biofilm in comparison to classic denture polymers, approaching levels seen with implant abutments, however, may require surface treatment in order to optimize this feature.25,26
Finally, an issue that remains to be successfully addressed is aesthetics. Currently, the novel polymers are milled from a single, monochromatic block, conforming to a limited number of colours. Thus, the framework colour may either match the soft tissues or allow tooth-coloured clasps, but not both. Post-manufacture, the frameworks may be altered, with manufacturers such as GC having developed staining materials to enable customization of colour (GC Optiglaze, GC Corporation, Japan). These allow the application of a thin coating layer (25–50 μm), with a claim of high wear resistance and discolouration resistance.
The aesthetic issue is, however, a double-edged sword: where the materials fall short with regards to colour and their thickness/bulk, they outperform metals in their ability for clasps to be shorter and less visible (Figure 15). The increased flexibility of the polymers allows a greater depth of undercut to be engaged (0.5 mm vs 0.25 mm in chromes), and as such, the clinician can therefore design shorter, more discrete clasps that are less noticeable anteriorly.
Figure 15. The shorter clasp assembly possible with an AKP framework.
Conclusion
Polymers adopted for denture frameworks have advanced considerably since they were introduced many years ago. First generation polymers were highly aesthetic and comfortable for the patient, but did not stand the test of time. Within a few years of fit, it became apparent that discolouration, an inability to create hygienically designed appliances, and an inability to add or repair, meant that clinicians reverted to conventional denture materials, while the material scientists strove to advance newer materials to meet the market void.
The novel denture materials have addressed many of these shortcomings, and show promise. They are comparatively priced to CoCr dentures (albeit that there is of course a range of costs in laboratories), and there is increasing evidence of the comfort that they provide. However, there is still the need for more extensive clinical trials of these materials to prove their long-term efficacy.
