Acrylic dentures: fill the gap. Part 3: the most common acrylic denture cases

Acrylic partial dentures are commonly prescribed by dental practitioners to replace missing teeth. The first two articles of this series considered the system of design that is followed when planning acrylic mucosal-borne dentures. This final article demonstrates how to apply this in practice, with a review of the most common partially dentate cases seen and treated with acrylic dentures at the Birmingham Dental Hospital.

Owing to routine work being suspended during the COVID-19 crisis, ongoing denture cases at Birmingham dental hospital were reviewed and assessed. Acrylic partial dentures were prescribed for 314 partially dentate arches. Of these cases, 63% replaced teeth in a free-end saddle situation. Trends were noted in the design for distinct groups of partial denture cases and, for the purpose of this article, have been categorized as shown in Table 1.

Maxillary arch

Replacement of one or two anterior teeth

Acrylic dentures that replace one or a limited number of anterior teeth are often provided as a temporary solution to replace teeth lost following extraction or trauma. They are usually fitted immediately as an aesthetic replacement following tooth loss, and used as an interim measure to allow for a period of healing prior to bridge/implant placement or the construction of a more permanent denture.¹

Because the anterior saddle area is small, and the force applied to the artificial tooth/teeth during function is limited, soft tissue support is usually adequate and tooth support not always necessary. The main challenge associated with these types of dentures is in achieving sufficient retention because tight posterior interdental contacts and a fully dentate opposing arch may complicate clasp design. Another difficulty is in obtaining sufficient indirect retention as clasping posterior teeth on either side of the arch forms a clasp axis, around which the denture could rotate. Common design options include the following.

Denture without clasps

In this type of denture, retention is derived from the buccal undercut of the anterior edentulous ridge, friction between the acrylic tooth/teeth and neighbouring teeth, friction between the major connector and the palatal surfaces of the adjacent teeth and adhesion/cohesive forces between the denture base and the denture-bearing area. Because this denture type uses a planned path of insertion, casts will need to be surveyed and tilted so that the path of insertion will allow the denture flange to sit in the buccal undercut of the ridge. This will provide resistance to the path of displacement vertically. In addition, the denture will need to be well adapted to the soft tissues and a posterior pindam, which is considered to be a narrower form of postdam, should be included in the design. Maximizing palatal coverage will provide better support and retention, which can be improved further by using the undercuts on the palatal surfaces of the natural teeth. This may involve requesting the lab technician to limit/reduce blocking out these undercuts. Ideally the denture base should cover the teeth just above the palatal survey lines (Figure 1).² This design is not suggested in immediate denture cases, as the amount of ridge resorption that will occur following tooth extraction can be unpredictable, leading to early retention loss.³ Acrylic coverage of the major connector onto and around the palatal surfaces of the natural dentition has the potential to be detrimental to periodontal health if the patient's oral hygiene is not satisfactory. However, there may be certain situations where a short-term compromise in periodontal health may be considered in favour of denture retention and stability.

Spoon denture

A spoon denture (Figure 2) is designed at least 8 mm clear of the gingival margins and retained mainly through adhesive forces between the denture base and mucosa, the tongue and tight contact between the artificial tooth and adjacent teeth. To aid this tight contact, the tooth contact point on the plaster model can be very lightly abraded using fine sandpaper. Palatal extension of the connector helps to provide support and bracing, hence, a high-vaulted palate will be more anatomically favourable in comparison to a shallow palate. For best results, the posterior border of the denture should lie past the mesial surfaces of the second molars. Smaller spoon dentures with less palatal coverage will be less retentive. Additionally, smaller dentures could pose an aspiration risk.³

Modified spoon denture

A modified spoon denture is a traditional spoon denture with clasps added to increase retention. C-clasps (Figure 3) are more commonly used, but Adam's cribs (Figure 4) can also be used in patients with wider interdental spaces. However, there is potential for them to deform, so their use tends to be limited to short-term options only. Another alternative is the use of embrasure clasps, although they have the disadvantage of being plaque retentive. Clasps will be reciprocated by extending acrylic onto the palatal surfaces of the clasped teeth. Adding an additional tooth unit on the opposite side of the clasp axis of the anterior saddle will help provide indirect retention (Figure 3). The acrylic of the major connector is kept away from the gingival margins of the natural teeth where possible to limit increased plaque accumulation in these areas.

An Every design denture

An Every design denture (named after RG Every who first described it) (Figure 5) is useful when there are bounded saddles in an anterior Kennedy class 3 or Kennedy class 4 situation. The aims of the design are to minimize the risk of exacerbating periodontal disease or caries while gaining retention through the application of principles employed in complete denture construction.⁴ The design should include:

• Accurate shape of the polished surfaces to assist muscular forces
• Appropriate extension of denture base onto the palate to provide sufficient support
• Point contact between artificial denture teeth and natural dentition with wide embrasures
• Distal wire stabilizers contacting distal surface of last standing tooth to prevent distal movement of abutment teeth
• Acrylic of major connector kept at least 3 mm free from gingival margins palatally
• Free-occlusion, meaning no occlusal interferences

Flexible dentures

As discussed in Part 2 of this series, polyamide-based flexible dentures (eg Valplast; Figure 6) can be used to replace a few missing anterior teeth. Owing to their small size, they provide limited support, which may traumatize the surrounding mucosal tissues around the abutment teeth. There is also a risk of inhalation with small dentures.

Limited number of teeth remaining

Limited number of anterior teeth remaining

The main challenge faced in designing dentures for patients with a few remaining anterior teeth is achieving sufficient retention because incisors often lack appropriate buccal undercuts. However, simple tooth modification can be helpful to produce a more favourable undercut depth, and commonly this is created by composite addition. Clasping anterior teeth with wrought clasps can compromise aesthetics. Some patients may find the use of an acetal resin circumferential clasp (Figures 7 and 8) more aesthetically acceptable. Acetal resin (polyoxymethylene) is a strong but flexible material. To provide sufficient retention when engaging undercuts, the clasps need to be greater in cross-sectional diameter than metal-based clasps to prevent excessive clasp flexure.⁵ A bulkier clasp will have implications on aesthetics and plaque accumulation.

Another option is to reduce the weight of these dentures by decreasing the bulk of the denture acrylic as much as possible. Limited palatal coverage can also be considered to reduce the forces of displacement acting on the denture (Figure 8). Denture fixative will likely be a helpful adjunct, and this can be discussed with the patient at the start of treatment so that expectations are appropriately managed.

Limited number of posterior teeth remaining

The design of a denture for a patient with one or two, spaced posterior teeth remaining is another difficult situation. It is important to rely on complete denture principles and modify these to allow for successful treatment. Accurate impressions are essential to record the full denture-bearing area, which will optimise support. However, obtaining a full border seal is not possible even with optimal flange extension because of the presence of the remaining teeth, and retention is likely to be suboptimal with a conventional partial denture design.

An alternative approach is the Cusil design (Figure 9). Denture acrylic is wrapped around the remaining tooth/teeth above the survey line and a soft resilient lining is attached to the fit surface of the denture underneath this point of maximum bulbosity around the entire tooth circumference.⁶ This allows a better border seal to be maintained, and retention is increased through frictional resistance. As the lining material is flexible, the denture can still be removed and inserted without difficulty. In order to use this approach, a high survey line is needed around the remaining tooth/teeth, and they should not be excessively tilted.

Where the use of a Cusil design is not possible, retention can be obtained by clasping the remaining teeth (Figure 10). It is likely in these cases that patients will need to rely on denture fixative to augment retention. Often, patients will use too much adhesive, which will prevent contact between the denture base and mucosa, so it is important to advise on how to use their fixative appropriately, and that they follow the manufacturers guidance for use.

Multiple bounded saddles

Usually when there are multiple bounded saddles a Cobalt–Chromium (Co-Cr) denture is the preferred choice. However, when the remaining teeth have a dubious prognosis and planning for failure is indicated, or when patients' preferences are taken into consideration, an acrylic denture could be an alternative. Often these cases are relatively more straightforward to design because indirect retention is less of a concern. It is worth assessing primary models to identify/create guide surfaces to increase retention and prevent rotation. Depending on the distribution of the remaining teeth, the acrylic connector can either be kept close to the survey lines on the palatal aspect of the teeth (to provide reciprocation and possibly some retention) or kept 3-mm free if periodontal health is a concern.

Large anterior saddle areas

A frequent finding of the large anterior saddle cases being treated in our prosthetics department was the presence of anterior retained roots (Figure 11), which lend themselves to being used as overdentures in these cases.

An advantage of retaining pathology-free anterior roots is that they can provide tooth-support for the denture and can act as an indirect retainer, helping to counteract rotational movement. However, the roots will maintain the volume of the bone which will limit the space available for acrylic teeth. There may also be difficulties in extending the buccal flange owing to the buccal undercut of the anterior ridge.

In cases where there are no anterior retained roots, the large edentulous space provides sufficient support. This can be increased by extending the denture base up to the distal aspect of the last remaining natural tooth of the arch. Retention can be gained by clasps around the remaining abutment teeth, and if there is insufficient undercut, this can be modified and created. Distal stabilizers can also be added as discussed for the Every denture.

A potential problem that may be encountered is rotation of the saddle away from the anterior edentulous space. This can be reduced by extending the major connector into the embrasure spaces above the survey line (Figure 12), but this could have a detrimental effect on periodontal health especially if oral hygiene is poor. Another solution would be to engage the buccal undercut of the anterior ridge, clasp the most anterior abutment teeth or by addition of occlusal units posteriorly as previously discussed.

Mandibular arch

Free-end saddle with limited number of remaining teeth

In free-end saddle situations with a limited number of anterior teeth (Figure 13), it is likely that support will mostly be derived from the mucosa. Covering the remaining lingual surfaces of the anterior teeth with acrylic above the survey line will add an element of tooth support and provide indirect retention. If possible, clasping the two teeth next to the free end saddle areas will enhance retention. Guide planes could be considered when an additional anterior bounded saddle is present, but often, this will affect the aesthetics.

Optimal impressions will allow for an accurate record of the functional depth and width of the sulcus so that flanges can be appropriately extended. This will help provide bracing, as the denture cannot solely rely on the remaining dentition if there are only a few remaining teeth. Lingual plates are used as a major connector and extending the major connector up to the lingual aspects of the clasped abutment teeth provides the required reciprocation.

For partially dentate arches with poor quality abutment teeth, it is important to treatment plan the remaining teeth in respect to their importance in the denture design. Decisions are made on whether extraction may be more suitable with the aim of trying to future-proof the prosthesis because extraction of poor teeth following denture completion will result in bone resorption and deterioration of fit. Furthermore, trying to preserve at least one long-rooted tooth either side of the midline will help improve stability of the denture.

In a situation where only one canine remains (Figure 14), a Cusil approach can be used, although it may not be the most aesthetic option. For canines where there has been significant recession or more extensive undercuts, this option will be less suitable. The denture will also be quite bulky in this area to allow for sufficient strength of the denture acrylic and space for the soft liner. A simple wrought clasp around the tooth is an alternative, but often there is a tendency for the denture to rotate around a single clasped tooth. Extension of the lingual flanges into the retromolar fossae may help counteract this to some degree and prevent excessive lateral movement.

For canines with limited undercuts, a partial ‘wrap-around’ with denture acrylic can also prevent rotation and would be more aesthetic than a Cusil approach. However, the close adaptation to the tooth surfaces combined with the movement of the saddle could compromise the long-term survival of the abutment tooth. Alternatively, retained canines may be root filled and used as overdenture abutments or extracted.

Unilateral free-end saddle

Unilateral saddle areas can be best treated by clasping the most posterior teeth of the dentate segment and at least one other clasp crossing over into a tooth embrasure (Figure 15). Extending the denture appropriately will also maximize the support available.

Bracing will be provided by the remaining teeth and clasps. Indirect retention will be an issue, but the presence of multiple clasps will limit rotational movement. Also, for these cases, denture fixative is a useful adjunct.

Anterior saddles

A denture replacing a few anterior lower incisors is another common situation. Issues with patient tolerance could arise as a frequent complaint is that the denture will feel bulky on the lingual aspect and are easily displaced by the tongue. The acrylic major connector will need to be of a certain thickness to provide sufficient strength. If bulkiness of the denture is an issue a Co-Cr denture may be a better alternative.

If an acrylic prosthesis is planned, using or creating parallel guide planes is extremely useful as well as to clasp the last standing teeth (Figure 16). If the tooth adjacent to the anterior saddle area is an incisor or bulbous canine then preparation of a guide surface to enhance retention is difficult to achieve without changing the appearance of the tooth, thereby compromising the overall aesthetics. Another option to increase retention could be to not fully block out the undercuts and have acrylic collets extending into the interdental areas. This may create a risk to periodontal health if a patient does not have optimal oral hygiene.

To overcome rotation of the denture into the anterior saddle area during function, the acrylic should cover the palatal aspect of the canines up to and above the survey line thus providing indirect retention.

Conclusion

Acrylic partial dentures can be useful in restoring function and aesthetics of the partially dentate patient. Clinical care and attention needs to be given to the design of the prosthesis in order to prevent damage to the hard and soft tissues, and to achieve an optimal result. When greater numbers of teeth are lost, there will be more reliance on mucosal support, and this could also limit overall retention and stability of the prosthesis, which has been discussed for several partially dentate situations. Therefore, any successful treatment plan will need to ensure that the patients' expectations are realistic and appropriately managed.