REVIEW

 

A simplified and evidence-informed approach to designing removable partial dentures. Part 5. Design examples for Kennedy classifications

 

 

C Peter Owen

Professor Emeritus, Faculty of Health Sciences, University of the Witwatersrand, Johannesburg, South Africa ORCID: 0000-0002-9565-801

Correspondence

 

 

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SUMMARY

For many decades the literature has regularly reported that there is a discrepancy between what is taught in dental school and what is practised, especially in the field of removable partial dentures. Not only that, but for more than 60 years reports from around the world have shown that, usually, the majority of clinicians abdicate their responsibility to design a removable partial denture (RPD) and instead leave this to the dental technician, who has no knowledge of the clinical condition of the patient and works only from a cast. Most patients around the world who require RPDs to improve aesthetics and chewing can only afford a removable prosthesis simply because the majority are poor. But RPDs can improve these aspects and contribute to an improved quality of life.

Keywords:Removable partial denture, design, support, retention, acrylic-based, framework-based

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The purpose of this series of articles is to derive the basic, evidence-informed principles of partial denture design and to suggest a simplified explanation and application of those principles in the hope that clinicians will increasingly take responsibility for the design of partial dentures. Part 1 summarised studies revealing what can only be described as the malpractice of abdication of responsibility for design by clinicians, and then explained the evidence-informed basic principles of design; Part 2 looked at the biomechanical basis of those principles in terms of support; Part 3 did the same for the biomechanical basis of retention; Part 4 provided a simple seven-step approach to design, applied to an example of an acrylic resin-based and a metal framework-based denture for the same partially edentulous arch; and, Anally, this part will provide examples of designs for RPDs that have been successfully worn by patients, for each of the Kennedy Classifications of partially dentate arches. Much of this is referenced from an electronic book on the Fundamental of removable partial dentures.¹

Introduction to Part 5

The purpose of these design examples is not to be prescriptive about how a design should look, such as for a particular Kennedy classification. No clinical details have been given, so that some designs are only possible because of favourable occlusions: many times modifications have to be made, for example when an overbite exists. So, these designs are more to provide a suggestion for how a prescription to a dental laboratory might look, and how much information can be conveyed by means of a simple drawing. Although these designs have been produced with a computer drawing package, simple hand drawings are easier to produce and, with practice, clinicians can become very adept at making them quickly; or at least conveying what they need to the dental technician. For the metalbased designs shown here, the darker shading represents the polished portions of the base, and the lighter shaded areas the sand-blasted portions which carry the acrylic base with the replacement teeth. Any cingulum rest seats beneath acrylic or metal are shaded darker. Designs will be given for each main Kennedy classification, with examples using both acrylic and metal bases. All are real-life examples, taken from dentures that are being successfully worn - and used - by patients.¹ They do not preclude other appropriate designs, but it should be noted that if sensible and biological principles of design are adhered to, it turns out that there will, in fact, be limited options available for each clinical situation.

 

KENNEDY CLASS I

Mandibular

All mandibular distal extension base dentures require indirect retention as well as active retention in the form of clasps on the abutment teeth, as explained in Part 3 of this series.² A recent paper also found a positive association between masticatory performance and the presence of indirect retention.³ There is no evidence that a mesial rest imparts any advantage, and as also explained in Part 3² there is no evidence for the use of the RPI clasp system.

For acrylic bases, a lingual plate is unavoidable. It is possible to purchase stainless steel lingual bars for these cases, but they are unstable and the acrylic over the saddles must be quite bulky; even so, crazing and fractures are common. They are therefore not recommended. The lingual plate will rest against the inclined planes of the incisors, so it is necessary to place cingulum rests to prevent any untoward forward movement of the incisors. It is recommended that good cingulum rest seats be prepared on the canines and smaller ones on the incisors (as there is less enamel on these teeth). The rests on the canines also provide indirect retention. All this will apply also to a metal lingual plate or dental bar as a major connector. The rules for the possible framework-based major connectors are given in the box.

 

 

Whatever the base material used, the evidence is that these dentures are not easy to use, but there is little evidence to explain why, and the few follow-up surveys have shown contrasting results. After 8-9 years, one study found thai 24% of mandibular Class I dentures were no longer being worn;⁴ whereas another study found that there was a 7% loss over 10 years and a 31% loss over 20 years (but the observation period was from 3 to 36 years).⁵ There is, however, some correlation between some features of these dentures when having been worn successfully, and which were important in preserving health, and those are:⁶

• the presence of positive occlusal and/or cingulum rests

• the presence of guide plane/guiding surface contact on the abutment teeth

• extension of the distal extension base onto the retromolar pad and over the buccal shelf to the buccal vestibule

• placing rests on 2 or more teeth adjacent to each distal extension.

It is important to bear these features in mind, especially that of the denture base over the distal extension, as so many encountered are underextended. It is useful to think of the denture base that extends from the abutment tooth posteriorly as being the equivalent of what a complete denture would be in that same area.

 

DESIGN EXAMPLES

Mandibular

Figures 1-2 show typical examples of acrylic-based and framework-based mandibular distal extension partial dentures.

If there is insufficient room for a lingual bar, then an alternative design well accepted by patients (again, if there is sufficient room) is the Kennedy bar, as in Figure 3.

Maxillary

There are differing opinions, not often expressed in the literature, and certainly without evidence, as to whether indirect retention is required in maxillary distal extension bases. The issue is whether you believe the distal part of the denture will fall down either under gravity or during chewing.

The answer may lie in what happens in a complete denture. Patients who wear and use complete dentures successfully appear to use their tongue quite differently: the posterior part rises to contact the posterior half of the maxillary denture, and the mandibular denture is controlled by the tongue sitting comfortably behind and in contact with the mandibular incisors. This has been shown to increase the retention and stability of the mandibular complete denture.⁷ It is reasonable to assume that the same will apply to distal extension bases. In general, then, the (albeit anecdotal, experiential) advice is that indirect retention is unlikely to be required in a maxillary distal extension base denture.

Design examples

Figure 4 shows a typical design for a maxillary Class I acrylic-based denture. There is a cingulum rest on the 13 which will be continuous with the guide plane. The clasps will, unfortunately, be visible if the patient has a high smile line. Where sufficient guide planes can be made, and the guiding surfaces are refined in the mouth, clasps may not be necessary, as shown in Figure 5.

The metal-based denture shown in Figure 6 does have indirect retention by extending onto the 14 and the 23.

 

KENNEDY CLASS II

Mandibular

The principles elucidated above apply to unilateral distal extension bases as well, but the design requires cross-arch stabilisation as well as retention, which makes acrylic-based dentures a lot bulkier than their framework-based counterparts. When there is a modification space, that makes the design a little easier.

Design examples Mandibular

Figure 7 shows a typical design for a Kennedy Class II acrylic-based denture and Figure 8 shows a design when there is no modification space in the opposing arch. Figure 9 shows a typical metal-based design and Figure 10 shows a variation to reduce gingival coverage.

Maxillary

Once again, the same principles apply, as illustrated in Figures 11-13.

 

KENNDEY CLASS III

The design for these dentures should be comparatively easy and can be kept simple, though they are often overly complicated. Depending on the number of edentulous spaces, and the size and length of the guide planes, many Class III dentures can rely on guide plane retention without the need for clasps.

Mandibular

As with all acrylic-based dentures, covering many gingival margins is unavoidable. Posterior spaces are often difficult because a lone standing distal molar is often left, and tends to tip mesially (because of the space) and often lingually (under the influence of the masseter). Therefore, there is usually no buccal undercut region and so these teeth can seldom be clasped; but they must of course receive a rest and will require slightly more modification than usual to create a guide plane.

Design examples

Figure 14 shows a typical design for an acrylic-based denture and Figure 15 illustrates that with enough guide planes, clasps may not be necessary. Figure 16 shows a typical framework-based design.

Maxillary

As with the mandibular designs, the maxillary Class III denture designs will depend entirely on the form and number of edentulous spaces. The more spaces there are, the more obvious the design will be because of the design principles such as the need for support on either side, the need for guide planes etc.

Design examples

Single edentulous space examples have been chosen here because they illustrate the need to consider where support should be placed, and the need for active retention (Figures 17-19).

 

KENNEDY CLASS IV

Mandibular

As discussed in Part 3² the design of these dentures depends entirely on whether the clinician believes that the predominant displacing action is a rotation of the anterior part downwards and forwards from incising or upwards from chewing sticky foods. Once again there is only anecdotal and experiential evidence, and that advice is that these dentures tend to tip up posteriorly, not anteriorly, and hence the use of distal undercuts for the clasps. The designs are therefore straightforward, and once again the difference between an acrylic base and a framework base is the gingival coverage.

Design examples

Mandibular

Figures 20 and 21 show typical designs for acrylic-based and framework-based dentures.

Maxillary

The rotation of these dentures is always considered as downwards from the anterior generally because of the poor guide planes anteriorly. But when the guiding surfaces on the denture are developed in the mouth, the retention of the anterior segment greatly improves. The retention is also improved by using the mesial undercuts of the distal most appropriate tooth (Figures 22-23).

 

DISCUSSION AND CONCLUSION TO THE SERIES

There are, of course, many more aspects to the design and use and making of removable partial dentures, but these papers were intended to try to simplify many of the somewhat confusing aspects of partial denture design to be found in the literature, and deliberately concentrated on the basic forms of acrylic-based and framework-based dentures, with no other support such as from implants. These are the partial dentures that are available to most patients worldwide. There is a paucity of evidence for any specific design features, and in fact there is evidence to the contrary for some long-held beliefs such as that torquing forces and clasp assemblies result in the loss of teeth.

There are many factors that contribute to the increased rate of tooth loss among partial denture wearers, but if simple and hygienic designs are used, the denture itself is unlikely to contribute. Although it has been shown that the amount of bone support prior to receiving an RPD was not a risk factor,⁸ this has been contradicted by other studies investigating the crown-root ratio of abutment teeth;^9,10 all such studies, though, point to what are really multiple biological factors such as occlusal support, root canal treatment and alveolar bone density. There is little doubt, though, that the greatest single contributor to tooth loss in RPD wearers is insufficient plaque control, and it should be a sine qua non that no prosthesis should be placed in a patient who cannot control biofilm through their own oral hygiene measures as well as with professional help.

There is also little doubt that RPDs contribute to patients' quality of life, especially when they fulfil the needs for aesthetics and improved chewing, and even in the presence of periodontal disease.¹¹ As discussed in Part I¹² there may be some association between chewing and cognition and a recent study suggested that the use of dental prostheses might be a protective factor for cognitive decline.¹³ It has even been suggested that the use of RPDs may have long-term benefits in reducing mortality, although it was acknowledged that further research is necessary.¹⁴

The majority of partially edentate people in the world will not be able to afford a fixed or implant-supported prosthesis merely because the majority of people in the current world order are poor. The removable partial denture will continue to have a place in the therapeutic regimen and it is hoped that this series of papers will encourage better designed, non-iatrogenic prostheses for everyone who needs them.

 

REFERENCES

1. Owen CP 2020. Fundamentals of removable partial dentures, 5^th Edition. Electronic book available at www.appropriatech.com        [ Links ]

2. Owen CP. A simplified approach to removable partial dentures. Part 3. The biomechanical basis of retention. SADJ. 202?        [ Links ]

3. Yoshimoto T, Hasegawa Y Maria MTS, Marito P Salazar S, Hori K, et al. Effect of mandibular bilateral distal extension denture design on masticatory performance. J Prosthodont Res. 2023 4. doi: 10.2186/jpr.JPR_D_22_00135. Epub ahead of print        [ Links ]

4. Chandler JA, Brudvik JS. Clinical evaluation of patients eight to nine years after placement of removable partial dentures. J Prosthet Dent. 1984;51(6):736-43. doi: 10.1016/0022-3913(84)90366-4        [ Links ]

5. Yoshino K, Ito K, Kuroda M, Sugihara N. Survival Rate of Removable Partial Dentures with Mandibular Bilateral Free End Saddle: A Retrospective Study. Bull Tokyo Dent Coll. 2021;62(4):205-14. doi: 10.2209/tdcpublication.2020-0061        [ Links ]

6. Frank RP, Brudvik JS, Leroux B, Milgrom P, Hawkins N. Relationship between the standards of removable partial denture construction, clinical acceptability, and patient satisfaction. J Prosthet Dent. 2000;83(5):521-7. doi: 10.1016/s0022-3913(00)70008-4.         [ Links ]

7. Lee JH, Chen JH, Lee HE, Chang HP, Chen HS, Yang YH, Chou TM. Improved denture retention in patients with retracted tongues. J Am Dent Assoc. 2009;140(8):987-91. doi: 10.14219/jada.archive.2009.0308        [ Links ]

8. Nisser J, Kisch J, Chrcanovic BR. Risk Factor Assessment for Survival of Removable Partial Dentures and Their Abutment Teeth: A Retrospective Analysis. Int J Prosthodont. 2022;35(5):598-608. doi: 10.11607/ljp.7457        [ Links ]

9. Tada S, Ikebe K, Matsuda K, Maeda Y. Multifactorial risk assessment for survival of abutments of removable partial dentures based on practice-based longitudinal study. J Dent. 2013;41(12):1175-80. doi: 10.1016/j.jdent.2013.07.018        [ Links ]

10. Watanabe C, Wada J, Mizutani K, Nagayama T, Uchida H, Shibata Y Wakabayashi N. Radiographic predictive factors for 10-year survival of removable partial denture abutment teeth: Alveolar bone level and density. J Prosthodont Res. 2023;67(3):437- 43. doi: 10.2186/jpr.JPR_D_22_00034        [ Links ]

11. Gotfredsen K, Rimborg S, Stavropoulos A. Efficacy and risks of removable partial prosthesis in periodontitis patients: A systematic review. J Clin Periodontal. 2022;49 Suppl 24:167-81. doi: 10.1111/jcpe.13519        [ Links ]

12. Owen CP. A simplified approach to removable partial dentures. Part 1. Evidence- informed design principles. SADJ. 202?        [ Links ]

13. Bof de Andrade F, de Oliveira C, de Oliveira Duarte YA, Sabbah W, Bernabé E. Tooth loss, dental prostheses use and cognitive performance in older Brazilian adults: The SABE cohort study. Geriatr Gerontol Int. 2021;21(12):1093-98. doi: 10.1111/ggi.14290        [ Links ]

14. Bashir NZ, Bernabé E. Removable partial dentures and mortality among partially edentulous adults. J Dent. 2022;126:104304. doi: 10.1016/j.jdent.2022.         [ Links ]

 

 

Correspondence:
Name: Prof CP Owen Tel: +27 83 679 2205
Email: peter.owen@wits.ac.za

 

 

Running title
A simplified approach to designing RPDs
Conflict of interest
None