Acrylic dentures: fill the gap. part 2. indirect retention, major connectors, review of the design and case study

Indirect retention

Indirect retention can be defined as the effect achieved by one or more indirect retainers of a removable partial denture that reduces the tendency for a denture base to move in an occlusal direction or in a rotational path about the fulcrum line.¹ Therefore, the denture design will need to include at least two clasps for this concept to be considered.

Indirect retention can be provided by a clasp on the same side of the clasp axis as the denture saddle in question, or by tooth/retained root support on the opposite side of the clasp axis. It may also be achieved through coverage of a large edentulous free end saddle area (Figure 1).

In a Kennedy Class 1 situation for the mandible, designing the denture with a wrought metal lingual bar, to decrease tissue coverage, will result in a denture lacking support and provide no indirect retention. Without indirect retention the denture will rotate, forcing the major connector into the mucosa and leading to pain and discomfort of the affected area. When occurring around natural standing teeth it causes compression around the gingival margins, resulting in tissue damage. This type of denture has the unglamourous term of being a ‘gum stripper’.² An alternative solution is to design the major connector so that it ends just above the survey line.²

The trade-off between tissue coverage to abide by the principles of design and periodontal considerations are discussed below.

Upper mucosal dentures are generally well supported due to their palatal coverage so indirect retention is not as much of an issue unless deciding to keep the palate uncovered. This may introduce rotational difficulties that can be prevented by considering indirect retention and extending the acrylic onto the palatal surfaces of the appropriate remaining teeth or to increase palatal coverage after all.

Major connector

The main role of the major connector is to link all the components of an acrylic denture together. It can also provide support, indirect retention and in many cases (some sort of) border seal. In the upper arch, an acrylic connector is much lighter in weight than a cobalt-chromium (Co-Cr) connector, which helps with retention. However, the thickness of material can feel bulky, which may be a drawback for some patients. Usual design characteristics such as a post dam for the upper denture must always be included. It is good practice for the clinician to carve the post dam on the master cast or provide clear instructions to the laboratory.

A lower acrylic connector that feels bulky may be less well tolerated. Where this is a problem, or greater strength is required, a Co-Cr connector could be used. The principles of the overall denture will remain as a mucosal-borne design.

Review design

When reviewing the finished design of the denture, consideration should be given to the periodontal tissues. All dentures will increase plaque accumulation around teeth when compared to no prosthesis. Often a compromise will be made between trying to achieve the ideal denture design, patient's wishes (including their quality of life) and the possible effect on their periodontal tissues. It is fundamental that the clinician delivering any prosthesis reinforces the need to maintain excellent oral and denture hygiene to minimize periodontal damage. Ideally the denture should be designed to avoid as many of the natural teeth as possible, with at least 3-mm clearance between the gingival margins of the teeth and the major connector.² For mandibular acrylic dentures, often this cannot be achieved without affecting the strength and rigidity of the connector. Therefore, a compromise is accepted and inevitably, mandibular acrylic connectors will be relatively large and bulky.

A case study illustrates how to put the denture design principles discussed in this series of articles into practice.

Case study: an acrylic maxillary partial denture

A male patient in his 40s requested a new partial upper denture because his current prosthesis lacked retention. The patient had undergone periodontal therapy, which had stabilized his periodontal condition; however, the UL6 remained grade 2 mobile with 10 mm of recession distally. The original denture had no clasps. The opposing arch was fully dentate. Figures 2 and 3 show the final denture and master cast.

Advances in denture base materials

Conventionally, acrylic dentures are made of polymethylmethacrylate (PMMA). This material has adequate mechanical properties, but low impact strength and fatigue resistance, which can result in the prosthesis fracturing in areas of very high occlusal load or if the denture is dropped.³·⁴ A more recent alternative to PMMA is the use of flexible thermoplastic resin materials. One example of this is a polyamidebased denture, such as Valplast (Valplast International Corporation, NY, USA) (Figure 4).

Polyamide dentures have become a popular alternative denture base for partial dentures for aesthetic reasons, mainly due to the absence of metal clasps. Flexible dentures can be extended closely around the natural teeth in order to use the undercuts of abutment teeth and increase retention. They also tend to be of reduced thickness, which patients may find more comfortable.⁵

However, the coverage of Valplast dentures around the abutment teeth and surrounding vestibular and buccal gingiva can lead to inflammation around the mucosa, which may be further exacerbated if the prosthesis is entirely mucosa supported. The lack of rigidity of flexible dentures makes gaining adequate support more difficult. Without this, there is a risk of rotation and sinking of the denture base into the tissues, which is thought to increase ridge resorption. For this reason, the available literature considers that polyamide dentures are contraindicated when many teeth are missing with no vertical stops present.^5,6

Other disadvantages are increased water sorption compared to PMMA, which can lead to degradation and discolouration over time.⁷ A consequence of this is greater plaque accumulation and the potential for candidal growth because the surface becomes roughened with wear. Additionally, the polyamide denture base is not chemically bonded to the artificial teeth. This means that the teeth rely on mechanical retention for their incorporation into the denture. There is a risk that the teeth may be lost if overloaded or there is a fracture.⁵ Polyamide dentures are also not easy to polish because any adjustment exposes the fibres in the material. Specialized polishing and laboratory techniques are required.

Ultimately, the use of flexible dentures could be useful in certain clinical situations, such as a few missing anterior teeth or posterior teeth in a bounded saddle, but it is important to be aware of the limitations.

Computer-aided design and computer-aided manufacturing (CAD-CAM) has been reported on in the literature for complete acrylic dentures and the manufacturing of metal frameworks. However, a limited number of clinical studies is available, with the literature consisting mainly of case reports.⁸ Partial acrylic denture construction using this technology has even received less attention. This does not mean that it is not applicable to partial acrylic dentures. A case report from France discusses the manufacturing of a successful immediate removable partial acrylic denture using CAD-CAM.⁹ An explanation for why this technology may not have been used widely for acrylic dentures so far might be that these are often considered of a temporary nature, and not worth more expensive manufacturing costs.

Similarly for printed dentures, the literature related to partial acrylic dentures is very limited. Commercial labs advertise printed Valplast dentures; however, examples for acrylic partial dentures are difficult to find. Subtractive milling techniques seem to be dominant for non-metal-based partial dentures at the time of writing.

Summary

Given that acrylic partial dentures are routinely prescribed in primary and secondary care settings, it is important that we devote the necessary time and attention when considering their design. In truth, there are many factors to be considered when designing dentures, and compromises may need to be made because there is no single universal design that can be applied to all partially dentate patients. Therefore, we must follow a system of design to maximize support and retention, taking into consideration bracing, indirect retention and reciprocation. Part of the design process is to review how these design elements come together and how changes in design can limit further detriment to the patients overall oral health and hygiene. Case selection, awareness of patient factors and patient tolerance is essential. In the Part 3 of this series, examples of common partially dentate situations will be discussed and the design of the appropriate acrylic prostheses provided.