Dental Update - Acrylic dentures: fill the gap. Part 1. overview, support, retention, reciprocation and bracing

Abstract

Many dental students find the principles of partial denture design difficult to learn. It is also recognized that dentists in general practice within the UK do not always provide sufficient design specification on their laboratory prescription. In that regard, it seems that confusion about how to come up with a suitable denture design persists after graduation. The aim of this series of articles is therefore to review the principles of denture design as applied to mucosal-borne dentures. Part 1 focuses on support, retention, reciprocation and bracing.

CPD/Clinical Relevance: This article reviews the principles of design in relation to mucosal-borne partial dentures.

Article

Acrylic partial dentures are the most common type of removable dentures provided in both general and private practice.^1,2 However, there are relatively few published articles that cover acrylic partial denture design.

Denture design is part of the undergraduate curriculum and qualified dentists are presumed to be competent to ‘Assess the need for, design, prescribe and provide biomechanically sound partial and complete dentures’ for their patients.³ While undergraduate training may have provided some basic principles regarding denture design, it is impossible to cover all the possible variations, including number, shape, angulation and distribution of teeth, occlusion, patient preference, soft tissue interference and various other confounding factors.

This variation may be a reason that students find the principles of partial denture design difficult to learn. Often this confusion persists after graduation; it is recognized that dentists in general practice within the UK do not always provide sufficient design specification on their laboratory prescription.^2,4 One explanation is that dentists are not confident in applying their knowledge of denture design principles to their partially dentate patients. Perhaps, because undergraduate courses devote much of their time to teaching design principles for cobalt–chromium-based partial dentures rather than acrylic-based partial dentures.

This series of articles reviews the principles of denture design as applied to to acrylic dentures, also referred to as mucosal-borne dentures. Part 1 revisits the design principles relating to support, retention, bracing and reciprocation. Part 2 discusses indirect retention and major connectors, and Part 3 reviews these principles using case studies.

Background

As a result of the COVID-19 crisis in the UK in 2020 and the subsequently published guidance from the Chief Dental Officer of England, all routine, non-urgent dental care was ‘stopped and deferred until advised otherwise’. This meant that all prosthetic denture work at Birmingham dental hospital was temporarily put on hold in line with the guidance.⁵ This allowed for a review of the denture cases being treated up to March 2020. In total, 314 partially dentate arches were identified as having been prescribed partial acrylic dentures – 63% of which were for free-end saddles. Cases where the type of denture being constructed was not clear were excluded from the figures. Examples of this were primary casts with no indication whether acrylic or cobalt–chromium (Co-Cr) dentures would be constructed, oncology cases, complex overdenture cases where the final treatment plan was unclear or preliminary jaw registrations with no definitive treatment plan.

Where possible, data were collected using the Kennedy classification, although the limitation of this classification is that it lacks detail. An example would be a patient with a large anterior saddle area crossing the midline that falls under a class 4, whereas a similar patient with one additional bounded saddle of a single tooth unit would be classified under a class 3 modification 1. It was therefore felt that the Kennedy classification did not provide a good framework to meaningfully group the partially dentate arches and it was decided not to include the data in this article.

In the dental literature, it is recommended that mucosal-borne partial dentures are prescribed as transitional dentures, or for patients with large edentulous spaces.⁶ From the cases reviewed, it was evident that there might be additional partially dentate situations that lend themselves to partial mucosal-borne acrylic dentures. It is also accepted that if a Co-Cr baseplate is used for strength, but the denture itself does not rely on tooth support, then it is classified as a mucosal-borne partial denture. Previous literature has also shown that maxillary partial acrylic dentures tend to be more successful than mandibular acrylic dentures because there is a larger area available for mucosal support.⁶

Out of the 314 partial dentate cases reviewed, 59% of these were prescribed for the maxilla, which would confirm this statement.

General design principles for acrylic partial dentures

It is good clinical practice to articulate preliminary casts where indicated, and to survey the casts with a dental surveyor. Indications for articulating casts include an anticipated complex occlusal arrangement, cases where the strength of acrylic may be compromised or for other occlusal problems or where a lack of space is identified on examination. It is also recommended to articulate casts when there have been difficulties in previously provided acrylic dentures. An extra visit to undertake the occlusal registration will be required in such cases. If there are enough teeth to provide three stable points of contact (one anterior and two posterior) then hand articulation is generally sufficient.

Acrylic partial dentures are designed according to the following accepted procedural steps often called a ‘system of design.’ They are equally employed for Co-Cr and acrylic dentures:

Outline the saddle area(s);

Support;

Retention;

Bracing and reciprocation;

Indirect retention;

Connector;

Review design.

Support

Support is defined as resistance to forces directed toward the basal tissue or underlying structures.⁷ It can be provided by teeth, the mucosa or both.

Tooth support

Although acrylic partial dentures tend to be designed without occlusal rests, tooth support could still be considered. The acrylic should be designed just above the survey line and partially cover the neck/crown of the natural teeth. This will transfer forces, directed towards the denture-bearing area, to the roots of those teeth.

Tooth support can also be provided by retained roots, which when loaded will transmit these forces to the periodontal ligament. In some situations, such as long free-end saddle areas this may not always be favourable. A retained root close behind the last standing abutment tooth could lead to unwanted pivoting/tipping over the retained root, resulting in possible trauma and discomfort (Figure 1). A possible solution to this would be to cover the retained root on the master cast with tinfoil before processing. This requires close liaison with the laboratory. Alternatively the tooth could be extracted.

Figure 1. Retained roots in region UL3/UL4, which could cause unwanted pivoting for the free end saddle area upper left.

For edentulous spaces with well-distributed retained roots of favourable shape, these roots will be able to take most of the load generated during function (Figure 2). Treatment planning is essential, because retained roots also retain alveolar bone around them. This can create soft tissue undercuts that may not allow an acrylic flange and lead to difficulty with aesthetics, particularly labially in the upper arch when smiling. Compromises within treatment plans may need to be considered.

Figure 2. Anterior retained roots that are able to take the load for an anterior saddle area.

Mucosa support

For patients with a limited number of abutment teeth or unfavourable distribution of the abutment teeth, the masticatory load applied to the denture will be transferred to the mucosa. Therefore, it is essential for the clinician to capture as much of the denture bearing area as possible in their impression, including the relevant anatomy and functional sulcus width and depth.

Patient acceptance of palatal coverage can be improved when uncovering an area palatal to the upper anterior teeth and incorporating this into the design (Figure 3). In the mandible, uncovering the area lingual to the lower incisors is perceived as more comfortable. However, this is difficult to achieve with acrylic dentures as the strength of the major connector becomes compromised.⁸

Figure 3. Degree of comfort for palatal coverage. (a) Quite comfortable; (b) better than plate (a); (c) very much more comfortable than plates (a) and (b); (d) Better than plate (c); (e) best of all upper plates.

Retention

Retention may be defined as resistance against movement of the saddle away from the denture bearing area. Retention may be provided by direct retainers, for example clasps, guide surfaces, precision attachments and implants. The options most used in general dental practices are as follows.

Clasps

Wrought clasps are routinely used for partial acrylic dentures as opposed to cast clasps due to their malleability, are of uniform thickness and have a round cross section. They can be cut off a spool of wrought wire at the desired length (Figure 4) and manually adjusted by the technician to the master cast. It must be noted that clasp adaptation to the tooth surface is not as accurate when compared to cast clasps.

Other disadvantages are the potential for distortion by the patient during handling of the denture and deformation during function. This is common and more likely with increased length or inappropriate shape of the clasp. When the wire is directed gingivally in a linear direction towards the undercut, only the tip of the clasp arm is in contact with the bulbosity of the tooth. This type of contact can cause distortion with repeated insertion and removal. Ideally, the clasp arm should cross the tooth bulbosity into the desired depth of undercut and then curve in an occlusal direction. This allows a more significant length of the clasp arm to contact the tooth during insertion or removal.⁹ Deformation will result in loss of retention and could irritate the soft tissue or increase food retention.

Wrought clasps do, however, have many advantages. They are more flexible, enabling them to engage deeper undercuts. In addition, they are more easily adjusted and suffer less fatigue failure compared to cast clasps. The reduced tooth contact also leads to reduced plaque accumulation around the clasp.¹⁰

For clasps to function well, they must engage the appropriate undercut. Studies have shown that wrought clasps can effectively engage undercuts between 0.25 and 0.5 mm depending on the length and alloy of the clasp.¹¹ The clasp should be at least 7 mm in length.¹² Without having surveyed the cast, it will be difficult to determine where the appropriate undercuts lie. Therefore, it is critical to have surveyed casts available when designing acrylic partial dentures. There should be no compromises and acrylic dentures should receive the same attention to detail as partial Co-Cr dentures (Figure 5). In addition, studies suggest that wrought wire clasps are preferred for retention when the undercut occurs in the mesial third of the buccal surface of an abutment molar tooth.⁹ They are also recommended for occlusally approaching retentive clasps on a premolar or canine, which is worth considering when designing a denture.¹²

Figure 5. The tips of the clasps are not engaging any undercuts. This can be prevented by surveying the primary cast before designing the denture. In this case the undercut lies on the lingual aspect of the clasped teeth.

Guide planes

Given that acrylic dentures are often designed around a reduced number of abutment teeth, the use of guide planes can help improve retention when clasps alone are not sufficient or possible. Guide surfaces are two or more parallel vertical surfaces of abutment teeth shaped to the path of placement and removal. By increasing the contact area between the parallel surfaces of the abutment teeth and denture, frictional resistance to displacement is increased.¹³ Ideally, they are prepared parallel to the long axes of abutment teeth and preferably on more than two teeth. Distributing them over the dental arch will increase their efficiency. The recommended dimensions of these guide surfaces are half the width between the tips of the buccal and lingual cusps. Vertically, they are two-thirds of the length of the enamel crown portion of the tooth.¹⁰

Soft tissues

The fit surface of an acrylic denture is not polished when processed, as opposed to the fit surface of a metal framework. This achieves a much tighter fit in relation to the denture bearing area, which allows for some cohesion/adhesion to take place between saliva and the denture. Another way of gaining additional retention is to engage soft tissue undercuts, for example by using an acrylic ‘lip’ engaging the undercut above the canine prominences.⁸

Bracing and reciprocation

Bracing is the resistance against horizontal displacing forces on saddles caused by soft tissues, for example the cheeks or masticatory forces. This is an issue for mucosal-borne saddles and especially for free-end saddles, which are both common in acrylic partial dentures. Bracing will depend on the shape of the edentulous ridge and on the number and distribution of the abutment teeth. This unfavourable movement may also be partially reduced by appropriately extending the flanges of the saddle, although this will depend greatly on the accuracy of the master impression.

Reciprocation is defined as resistance against horizontal displacing forces on abutment teeth caused by clasps when they are removed from an undercut. Acrylic dentures rely mainly on the acrylic base for reciprocation. Due to adaptation issues and deformation, posterior clasps should not be reciprocated by another wrought clasp arm unless a ring clasp is used (Figure 6). This makes the design of reciprocation straightforward as clasped teeth will have acrylic on the opposite side to provide adequate reciprocation (Figure 7).

Figure 6. Unfavourable reciprocation by wrought wire arm as the wire is susceptible to deformation.

Figure 7. Reciprocation by acrylic of major connector.

Summary

In conclusion, it is important that the design of acrylic dentures receives the same due consideration as might be given when prescribing partial cobalt–chromium dentures. Following a system of design will enhance the quality of the dentures provided for patients and optimize function. In this paper the design principles for mucosal-borne dentures were discussed focusing on support, retention, bracing and reciprocation.

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References

Acrylic dentures: fill the gap. Part 1. overview, support, retention, reciprocation and bracing

Authors

Wouter Leyssen

Jasmeet Heran

AD Walmsley