Introduction

Partial edentulism is a growing issue.1,2 In 2009, 6% of adults in the UK were fully edentulous and 40% of dentate adults in the UK had fewer than 27 natural teeth.3 There were 14% who had fewer than 21 natural teeth, which is believed to be the minimum requirement for a functional dentition.3 In the UK there is an ageing population, who are retaining their natural teeth for longer,3,4 combined with a 'heavy-metal generation' who have many heavily-restored teeth.5 It may not be possible to maintain these heavily-restored teeth long term. Therefore dentists are likely to become more frequently involved in the management of the partially dentate patient.

Partial edentulism problems

Functional

Missing teeth can lead to reduced masticatory efficiency.2 Loss of teeth may also cause issues with speech. Dental problems can include over-eruption, drifting and loss of space.6 It has been suggested that a loss of posterior teeth may result in an unfavourable distribution of occlusal loads, occlusal interferences and periodontal breakdown.6 A systematic review by Van't Spijker et al.7 found no evidence suggesting that loss of posterior support leads to increased attrition.

Aesthetic

The degree of impact on appearance depends on both the site and number of missing teeth, along with the attitude of the patient. There is a social stigma that comes with a loss of visible teeth, especially anteriorly.2 Decreased occlusal face height and a lack of lip support can also change facial features.2

Psycho-social

Dental health is of great concern to many patients and losing teeth can have a detrimental impact on a person's self-esteem.4

Treatment options

Some patients are accepting of their edentulous spaces and do not want or need restorations. Others have active disease and poor oral hygiene, which precludes them from some treatment options. Those who are suitable and desire replacement often have multiple options.

Fixed prostheses (bridges) tend to be limited to short spans, and patients with acceptable abutments and suitable occlusal schemes. Conventional bridgework requires extensive preparation of abutment teeth and evidence suggests that 29.2% of these teeth will lose vitality after ten years.8 Resin-bonded bridges (RBBs) offer a less destructive, reversible alternative, with minimal or no tooth preparation.9 RBBs have higher failure rates than conventional bridges, but tend to fail less catastrophically.10 Metal-framed RBBs have an estimated survival rate of 80.8% at five years.9

Implants can be used to retain a single crown or larger prostheses. These are limited to patients with a clinical presentation appropriate for implant placement. Certain patient factors are associated with poorer outcomes with implants including smoking, poorly-controlled diabetes, bisphosphonate use and radiotherapy to the jaws.11 Patients need to be willing to undergo a surgical procedure and be able to meet the costs of treatment. Walton and MacEntee's study found that 36% of edentulous patients refused implants to retain lower dentures, even when offered at no financial cost to the patient.12

Orthodontic space closure may be a possibility but spaces are often unsuitable for this method due to their size, position and asymmetry.

Few contraindications exist for removable partial dentures (RPDs); these include intolerance of major connectors and patients with psychiatric disorders or repeated loss of consciousness. RPDs carry a risk of increasing plaque accumulation and therefore increase the risk of caries and periodontal disease.13,14 If designed appropriately, conventional RPDs can be an effective way to restore large and/or multiple, bilateral edentulous spaces at a reasonable cost.1,14 It is recognised that some patients may struggle to tolerate the bulk and wide coverage of such an appliance. In these cases, where a single bounded saddle exists and implants and bridges are neither suitable nor acceptable options for the patient, an alternative prosthetic option may be a unilateral RPD.

The unilateral RPD

The potential advantages of a small unilateral design over a conventional RPD include:

  • Avoidance of palatal coverage and the need for a major connector.13,15 Lesser impact on speech, gag reflex and generally more tolerable

  • Lower biological cost – fewer surfaces for plaque accumulation, candida colonisation and fewer natural teeth recruited as abutments

  • Decreased bulk

  • Does not feel like a conventional denture. Potential for less stigma and improved self-confidence.

The disadvantages and limitations of unilateral RPDs include:

  • Rarely suitable for restoring masticatory function, as they may transmit damaging lateral forces to abutment teeth and oral tissues if placed in occlusal function during excursive movements. This is due to the lack of cross-arch stabilisation, which may also lead to easy displacement13

  • Restricted to bounded saddles

  • Complex designs require more maintenance and can restrict patients to certain practices, laboratories and hospitals

  • Require good manual dexterity to take in and out of the mouth13

  • May require preparation of adjacent teeth to provide guide planes, rest seats and undercuts in order to ensure good retention and stability

  • Risk of inhalation and swallowing.13,16

Smaller denture designs are more likely to be swallowed or inhaled.16 These events can have serious consequences and therefore other prosthetic options must be appropriately considered and discounted before a unilateral RPD is provided. Justifications for choosing this prosthesis over others must be reasonable and recorded, as otherwise an adverse incident involving the denture could leave the dentist undefendable.16 The enhanced stability provided by a bilateral RPD, due to cross-arch bracing,2 means this design should be prioritised over a unilateral design where possible to minimise the impact of functional forces on the oral tissues and reduce the risk of inhalation or swallowing.

When to provide a unilateral RPD

There is currently a paucity of literature relating to the provision of unilateral RPDs and many clinicians do not recommend their use. Davenport et al.17 asked a group of prosthodontists and found that 50% agreed with the statement 'bounded edentulous areas should not be restored with a unilateral denture'. The authors believe these prostheses provide a viable option for the replacement of missing teeth, but only in specific, appropriate situations. They are potentially advantageous for single, shorter-span, bounded saddles,13,15,17 but it is not always necessary to restore these bounded saddles and the patient should also consider the benefits of not doing so. Eighty-eight percent of the prosthodontists consulted by Davenport et al.17 disagreed with the statement that 'bounded edentulous saddles should always be restored'.

Unilateral RPDs are a useful alternative when abutments are unsuitable or the span is too long for a bridge, implant-retained restorations are not an option and a conventional RPD is not acceptable for the patient. The patient's concern must be primarily aesthetic and space must be available to allow artificial teeth to be placed out of occlusal contact during excursive movements. The lack of cross-arch stabilisation means unilateral RPDs should be limited to areas where occlusal forces are lesser. Flatter alveolar ridges and more compressible mucosa would be potential contraindications, as these would limit stability further,18 even in a well-designed, tooth-supported prosthesis. The patient must have good manual dexterity and no contraindicating medical or social history for a unilateral RPD. Contraindications would include a history of psychiatric conditions, repeated loss of consciousness or alcohol and drug intoxication; these factors appear to increase the risk of inhalation or swallowing of foreign bodies.16 Accidental ingestion or inhalation of foreign bodies is more common in the very young or the elderly,19 therefore unilateral RPDs are best avoided in these age groups. These events are generally uncommon however.16

It is the authors' opinion that maxillary unilateral RPDs are more successful than mandibular unilateral RPDs. Patients tend to prioritise restoring maxillary spaces and, due to the functional limitations of these appliances, the authors would not recommend their use in the lower arch.

Design and material options

The three main base material options are:

  • Acrylic

  • Cobalt-Chrome (Co-Cr)

  • Nylon-based (flexible).

Acrylic unilateral RPDs gain their support from the mucosa. They can, however, be constructed to gain additional tooth support by incorporating pre-formed (stainless steel) occlusal rests.14 Stainless steel clasps may also be added to improve retention.

Cobalt-Chrome designs are tooth-borne and may be single-part or sectional. A sectional denture is composed of two or more parts, each utilising different paths of insertion.1 Sectional dentures have the advantage of combining intra-orally to engage opposing undercuts and aid retention via a wedging effect.1,20 Locking components, such as bolts, can keep the parts together and increase the security of a sectional denture.17 Retention for both types of Co-Cr design is typically gained from clasps that engage the undercut surfaces of teeth.14 These may be cast in Co-Cr as part of the metal substructure or added later if other materials are used such as gold, stainless steel or polyoxymethylene. Precision attachments are an alternative to clasps for achieving direct retention, but their use is not commonplace.21 Precision attachments are two-part connectors; one part is connected to the tooth, root or implant and the other to a prosthesis.22 Defined according to the tooth- or implant-connected element, precision attachments may be intra-coronal (embedded in a restoration), extra-coronal (extending outside the contour of the tooth) or radicular (connected to a root preparation).21 Examples include rod and tube designs, magnets and stud-type attachments.

Nylon-based RPDs are mucosa-borne. They obtain their retention via clasps which are an extension of the denture base material.23 The nylon resin allows the denture to be flexible, with enough elasticity to be manipulated into the edentulous space.

Table 1 summarises the benefits and drawbacks of each option.

Table 1 Advantages and disadvantages of the various types of unilateral RPD

Clinical cases

The following two cases exemplify the use of unilateral RPDs to restore a single edentulous bounded saddle. Both patients were treated in the Charles Clifford Dental Hospital, Sheffield Teaching Hospitals NHS Foundation Trust.

Case 1

This 57-year-old female was unhappy with the appearance of the edentulous area of teeth 23 and 24 and requested replacement. Traumatic failure of a fixed-fixed bridge 22 to 24 led to sectioning of this bridge and the extraction of tooth 24. Subsequently the 22 became non-vital and was successfully root-treated. The patient reported a history of unsuccessful partial dentures before the placement of this bridge.

Following stabilisation of her periodontal condition, the patient had excellent plaque control. Her medical history was not a contraindication to any prosthesis.

On clinical examination, the patient had a porcelain-fused-to-metal (PFM) crown tooth 22 (previous bridge retainer through which endodontic access had been gained) and the tooth 25 was un-restored (Figs 1a and 1b). The patient had group function on lateral excursions and anterior guidance in protrusion.

Figure 1
figure 1

(a & b) Case 1: Pre-treatment views showing the edentulous space of teeth 23 and 24; (c) Case 1: Milled PFM crown cemented on tooth 22 (designed specifically to aid support, retention and stability of the unilateral RPD provided); (d) Case 1: The Co-Cr unilateral RPD provided; (e-g) Case 1: Post-treatment views showing the unilateral RPD in situ

The options for restoring the edentulous space were discussed with the patient at length. An implant-retained bridge was proposed but financial considerations made this option unfeasible. Conventional RPDs were rejected as the patient had previously been unable to tolerate any palatal coverage. A bridge was deemed unsuitable due to the length of span, the un-restored tooth 25 and root-treated tooth 22. A Co-Cr unilateral RPD was the other option and this was the treatment of choice.

The compromised tooth 22 crown was replaced with a milled PFM crown to be integrated with the RPD design (Fig. 1c). The tooth 25 had a mesial rest seat and palatal guide plane prepared, along with the addition of composite buccally to provide an undercut for engaging a retentive clasp. The use of composite restorations to enhance RPD retention has been suggested by Davenport et al.27

The tooth 22 crown was designed with a palatal rest seat and guide plane parallel to the mesial surface of tooth 25, in order to provide a single, altered path of insertion. A precision attachment on the tooth 22 crown was considered to avoid using an anterior clasp, but there was insufficient inter-occlusal space to allow for housing an intra-coronal attachment. Additionally, an extra-coronal attachment would have been irritating and clearly visible when the patient was not wearing the RPD. One benefit of using a removable appliance is that it is reversible, therefore the tooth 22 crown was designed considering when the patient was and was not wearing the RPD.

The unilateral Co-Cr RPD was designed with an occlusally-approaching clasp, mesial rest and palatal reciprocating plate on tooth 25 (Fig. 1d). The design included a palatal plate and rest to engage the milled aspect of the tooth 22 crown and a gingivally-approaching I-bar clasp anteriorly. As the tip would be visible in the smile-line, this clasp was fabricated in gold at the patient's request. A tooth-coloured, polyoxymethylene I-bar was considered for aesthetics but the patient preferred the appearance of gold; polyoxymethylene clasps tend to be bulkier.29

The casts were mounted on a semi-adjustable articulator and this was used throughout the fabrication of the crown and RPD. The patient was very satisfied with the final result (Figs 1e–g).

Case 2

This 41-year-old female had a history of poor tolerance of conventional RPDs with palatal coverage. The edentulous space of teeth 14, 15 and 16 was of aesthetic concern to the patient.

The patient had good plaque control, a moderately-restored dentition and was partially edentulous (Fig. 2a). Her medical history was not a contraindication to any forms of treatment to replace these missing teeth. The bounded saddle in the upper right quadrant was too long to predict success with fixed bridgework and preparation of the un-restored tooth 13 would have been destructive. The patient was not receptive to provision of an implant-retained prosthesis. The treatment of choice was a Co-Cr unilateral RPD.

Figure 2
figure 2

(a) Case 2: Pre-treatment view showing the edentulous space of teeth 14, 15 and 16; (b-d) Case 2: Post-treatment views showing the Co-Cr unilateral RPD in situ and the integrated restorative elements; (e) Case 2: The Co-Cr unilateral RPD provided; (f) Case 2: Post-treatment extra-oral view with the unilateral RPD in situ

The tooth 17 was heavily-restored with composite and amalgam. These restorations were replaced with a composite core build-up and a full gold crown. The crown was designed with sufficient buccal undercut to engage a clasp tip. A composite addition was also made labially to tooth 13 to aid retention via a gingivally-approaching clasp, which was provided in Co-Cr due to the shallow undercut available and need for a stiff clasp to maximise retention (Figs 2b–d). An anterior precision attachment may have been more aesthetic, but it would likely have involved irreversible preparation of the virgin tooth 13.

A Co-Cr unilateral RPD was provided bearing two denture teeth. This was designed with an occlusally-approaching clasp, reciprocating arm and occlusal rest on tooth 17, along with a cingulum rest and gingivally-approaching clasp on tooth 13 (Fig. 2e). The patient was delighted with the final result (Fig. 2f).

Recommendations for success

All other prosthetic options must be considered and deemed unsuitable before a unilateral RPD is offered. Even then, a critical element when deciding if a unilateral RPD is appropriate is to consider the individual patient. Suitable candidates are well-motivated and have good oral hygiene.13 Good manual dexterity is also necessary,1,13 especially if considering a sectional design. Evidence suggests that very young or elderly patients and those with a history of psychiatric conditions, repeated loss of consciousness or alcohol and drug intoxication are more likely to swallow or inhale foreign bodies.16 Therefore it is best to avoid unilateral RPDs for such patients.

The authors would only recommend Cobalt-Chrome unilateral RPDs. These are tooth-supported and the metal framework is rigid and strong, providing stability and resistance to deformation. The metalwork allows adequate strength to be obtained from a thin cross-section of material; this reduces bulk and the need for wide gingival coverage, which could cause difficulties with plaque control and soft tissue trauma. Effective, direct retention is essential and retentive components must be designed carefully. Abutment teeth should have good periodontal support, sufficient clinical crown length and adequate undercut.17 Davenport et al.17 suggest that conventional clasps, magnets and attachments relying on frictional retention may be insufficiently reliable for unilateral RPDs. The authors have found that clasps can be reliable if correctly designed; guide planes and reciprocating elements are essential to ensure effective retention.18 Stability should be maximised by incorporating bracing components and extending the framework and flanges to cover as much vertical height of the alveolar ridge as possible. Effective retention and stability reduces the risk of inhalation and swallowing and the radio-opaque nature of the Co-Cr would make the appliance easily identifiable should this problem occur.

Deciding on a single-part or sectional design depends on the angulations of abutment teeth. If opposing undercuts are significant then the potential for utilising these for retention makes a sectional design more attractive.1 However, if opposing undercuts are less significant, it may be more prudent to modify the teeth to provide parallel guide planes and use a single-part RPD. If the abutment teeth have no opposing undercuts, a single-part RPD would be more appropriate. Designs should be kept as simple as possible to promote continued oral health, with clearance of gingival margins where practical.28

The patient must be made aware of the need for regular follow-up and maintenance of such appliances. It is essential that any faults in the prosthesis are identified and corrected early, as retentive elements may lose some of their efficiency over time.29 A loss of retention in a unilateral RPD could lead to inhalation or swallowing and cause serious health problems including asphyxiation, tissue perforation and infections.16 For this reason, the dentist and dental technician must be highly competent in partial denture design and fabrication before providing unilateral RPDs. For example, if a clasp is designed incorrectly and the proportional limit of the material exceeded during insertion and removal, permanent clasp distortion and a loss of retention could occur.29

Unilateral RPDs can be a safe, effective and predictable option for replacing missing teeth, but careful case selection, appropriate design and high-quality fabrication are essentials for success.