The windowed partial denture: an elastomeric retention technique in patients with lone-standing teeth

A removable prosthesis relies greatly on active retention, through the use of clasping elements into undercuts and the utilization of guide planes. Retention is defined as the resistance of displacement along the path of insertion. Various factors influence a patient's satisfaction with a removable prosthesis, which include technical quality of the prosthesis, patient's previous denture-wearing experience and patient-related factors such as the denture-bearing area.¹

In challenging patients with an unfavourable pattern of tooth loss, medically compromised or oncology patients with altered anatomy, alternative techniques may be required to retain a prosthesis since conventional methods of retention may be inadequate. The responsible clinician is often presented with a dilemma when lone-standing teeth are present. The decision whether to extract or retain any lone-standing teeth in a patient who requires rehabilitation with a removable partial denture needs to be carefully assessed. Various factors ought to be considered during treatment planning:

If lone-standing teeth are left in situ, the retention of the removable appliance can be adversely affected owing to the lack of sufficient peripheral seal. This poses more of a problem in the maxillary arch. The extraction of over-erupted or drifted teeth may allow the placement of denture teeth in a more favourable position without compromising on the retention or the occlusal stability of the prosthesis. This, however, may lead to further bone resorption and eliminates any possibility of providing active retention. In certain patients, who receive intravenous bisphosphonates or radiotherapy to the head and neck region, the extraction of teeth is not advised due to the increased risk of developing ONJ.^2,3

Windowed Partial Denture (WPD) design essentially encircles the tooth using an elastomeric material, thus utilizing the undercut around lone-standing teeth to retain the removable prosthesis (Figure 1). This design also allows for the maintenance of the peripheral seal.⁴

The WPD design can be used in patients:

The WPD also has the added psychological benefit to the patient since it delays the total loss of teeth. The elastomeric retention rings that form around the abutment teeth effectively increase the retentive capabilities of the prosthesis. Construction of the windowed partial denture warrants standard prosthodontic techniques; a high quality secondary impression is crucial to ensure good clinical outcome. Several factors need to be considered to judge the longevity of abutment teeth, which include the periodontal support, degree of undercut and location in the dental arch. Abutment teeth with reduced periodontal support can still be used as a transitional appliance, which can easily be modified at a later date should the tooth be lost. Surveying the abutment teeth on the master cast is essential as it will allow the study of the path of insertion/withdrawal, identify the presence/absence of any suitable undercuts, and finally locate where the rigid acrylic denture base will contact the abutment tooth at the level of the survey line to prevent ‘gum stripping’, whilst confining the elastomeric material to the undercut area to provide active retention (Figure 2). The lone-standing tooth may be overerupted and adjustment could be required to allow optimal tooth positioning and a favourable occlusal plane. The technician may need to cut the impression tray away from the master cast (rather than pulling it off) in order to prevent fracture of the lone-standing tooth from the working model.

Elastomeric materials used in WPD construction should exhibit viscoelastic behaviour, which maximizes retention by utilizing the available undercut and reduces any harmful torquing forces on abutment teeth. They are classified into either acrylic resins or silicone-based liners, depending on their chemical composition. They can be further subdivided into autopolymerizing (cold-cured) or heat-cured liners. Resin-based liners exhibit high bond strengths to acrylic denture bases and good viscoelastic behaviour.⁵ However, they do contain a soluble plasticizer component that leaches out over time, which consequently increases the surface roughness and porosity of the liner. The dimensionally unstable material can discolour and degrade over time, and has the potential to harbour pathogenic bacteria.⁶ Silicone-based liners are similar in composition to silicone impression materials. They can be divided into addition-cured or condensation-cured materials, where addition-cured materials are considered more stable due to the absence of by-products. Owing to their low water absorption and solubility of the components, they are more dimensionally stable and durable when compared to their resin-based counterparts.⁷

Molloplast-B® (DETAX GmbH & Co), a heat-cured, silicone-based elastomer used in this case, is considered one of the most durable elastomeric materials present. It also possesses antibacterial and antifungal properties against Staphylococcus aureus and Candida albicans.⁸

As with any removable appliance, excellent plaque control needs to be instigated in order to reduce the risk of caries and periodontal disease. The WPD design can cause plaque accumulation as total gingival coverage occurs. Patients should be advised to brush remaining teeth twice a day and the appliance should not be worn during sleep. The use of a single tufted brush may prove to be beneficial in optimizing plaque control around the lone-standing teeth. Patients are encouraged to attend for regular reviews in order to detect any deterioration in oral health. Elastomeric materials can potentially fail by partial/complete delamination from the underlying heat-cured acrylic overtime.⁹ Therefore it is important to monitor any signs of deterioration of the elastomeric material, which will compromise the peripheral seal and retention of the appliance. For ease of repair, chairside addition-cured soft lining materials, such as Sofreliner Tough® (Tokuyama Dental Corporation Inc) could be used, which can last up to 24 months.¹⁰

In conclusion, the WPD technique can overcome many of the drawbacks of maintaining lone-standing teeth as it combines the advantages of complete dentures by the maintenance of the peripheral seal and the use of active retention associated with partial dentures. This is particularly important in individuals that have compromised denture-bearing areas and poor neuromuscular control. It may also provide some psychological benefit to the patient. Clinicians should consider this as an alternative to extraction of the lone-standing tooth.