Resin-retained bridges: ten tips for success and an update on all-ceramic designs

1. Abutment tooth selection

There are numerous considerations for deciding on which abutment tooth to use. However, the initial priority should be to ensure the periodontal and endodontic health of the proposed tooth.3

As RRB retention relies on bond strength, it is important to consider the quality and quantity of the enamel available for bonding. Bond strength could be significantly affected by hypomineralized enamel, or where tooth surface loss has resulted in exposed dentine. It is therefore crucial to consider this when treatment planning.3

Assessment of occlusion is also important in planning RBBs. Resin bonding results in high compressive strength, but poor tensile and shear strength. High occlusal forces on the pontic, particularly guiding forces, therefore, make prosthesis debond much more likely.3

Where occlusion is unfavourable, several options exist. The occlusion could be modified to improve the occlusal scheme; this can be achieved by composite bonding to adjacent teeth or selective intra-enamel tooth adjustments.11 Alternatively, consideration could be given to use of alternative prostheses such as conventional bridgework.

Accommodating a connector of the required cross-sectional area can also pose an aesthetic challenge. This is commonly the case when replacing a lateral incisor, as the mesial bulbosity of the canine can result in a connector that is inadequate in size, encroaches upon the gingival tissues resulting in metal shine-through (Figure 1). In such circumstances, the introduction of axial guide planes on abutment teeth may be considered, to increase the height and rigidity of the connector. Alternatively, composite bonding can be used to alter the morphology of abutment teeth, thus modifying the line angles of abutment teeth.

In patients who have been wearing orthodontic appliances, gingival hyperplasia can be frequently encountered. In such patients, the surface area for retainer coverage may be increased by crown lengthening, via conventional means or electrosurgery. This may provide a limited increase in the surface area for bonding to enamel by exposing more of the tooth structure coronal to the cemento-enamel junction. Any further crown lengthening beyond this would expose dentine, and thus may only provide a more limited area for bonding.3

2. Replacement of restorations

Adhesive resin cements, when used with any restoration, require a large surface area to achieve predictable long-term success. The presence of restorations in the bonding area is associated with an increased risk of bonding failure.12 It is reported that improved restoration retention is seen when bonding to an unrestored and unprepared tooth.12 Nonetheless, the presence of an intracoronal plastic restoration should not preclude the use of a tooth as an abutment for an RRB. It is reported that bridge survival improves significantly when bonding to a new resin composite restoration.12 Therefore, restorations should be replaced with composite where appropriate, prior to the cementation of RRBs.

Consideration should be made as to whether to finish metal framework margins on sound tooth tissue, or incorporate the framework into intracoronal restorations. While extending the framework into intra-coronal restorations may significantly improve retention, it also increases complexity and difficulties when dealing with failures.12

3. Bridge design

While fixed–fixed 3-unit conventional bridges are reported to have a better 5-year survival compared to cantilever conventional bridges,13 single cantilever RRBs are generally deemed more successful than a fixed–fixed RRB design.12 In fixed–fixed designs, there is a reported increased risk of cement failure due to heterogeneity of functional loading forces.4 This loss of cementation of one of the retainers, also known as ‘silent-debond’, allows for microleakage, which increases the risk of caries formation beneath the failed retainer.

Despite this, there may be an advantage for fixed–fixed design when there are concerns over the stability of abutment tooth position. For example, the use of both central incisors as abutments to replace lateral incisors following orthodontic treatment has been proposed to enable fixed orthodontic retention.12 It is important to note that, where orthodontic treatment has been carried out, removable retainers should be provided by an appropriate clinician following bridge cementation.

Additionally, fixed–fixed bridge designs may be required when considering the replacement of multiple anterior teeth. In such circumstances, it is essential to consider the rigidity of the framework to minimize the risk of debond and optimize prosthesis survival. Close long-term monitoring is also an essential proviso when treatment planning a fixed–fixed RRB, and this should be incorporated into the consent process.

When designing an all-ceramic RRB, it is important to consider the differences between them and their metal-winged counterparts. All ceramic RRBs have an increased risk of fracture when they undergo flexural forces, necessitating a greater connector thickness to improve their rigidity and flexural strength. This thickness, however, can affect both the aesthetics and ability to clean restorations. Ibbetson highlights that, when considering metal frameworks, rigidity is maintained by a minimum connector cross-sectional area of 2 mm² Contrastingly, 3M (Minnesota, USA) recommends a minimum connector cross-sectional area of 7 mm² in anterior segments and 9 mm² posteriorly when using zirconia.11^,14 This subsequently impacts on connector height, which can have implications for aesthetics in the anterior region. Achieving this minimum connector area can be challenging given normal tooth proportions, and framework designs often result in palatally/lingually positioned connectors to accommodate the required space. However, one should be aware that the patient's occlusal scheme may not always allow such connector positioning. The authors recommend a close working relationship between the laboratory technician and the clinician when designing such prostheses; this commonly necessitates a review of digital wax-ups of bridgework prior to framework milling.

4. Tooth preparation

Minimal or no preparation should be carried out in the provision of metal RRBs. In contrast to this, preparation of abutment teeth is commonly advised for zirconia RRBs.15Figure 2 shows the preparation used by Kern in their longitudinal study,15 which is also recommended by 3M (Minnesota, USA) for their monolithic zirconia Lava Plus system. While preparation with a minimum depth of 0.5 mm is recommended, it is worth noting the incidence of dentine exposure during 0.5-mm preparations.16 Preparation of abutment teeth into dentine is associated with a two-fold increase in bridge failure,12 which can be attributable to the reduced bond strength obtained from dentine (Figure 3).3 It is also worth noting that, while minimally invasive resin-bonded bridges can be used as provisional restorations during implant treatment, the more extensive preparation design required for zirconia RBBs would preclude their use in such cases.

5. Pontic size

When replacing anterior teeth with RRBs, the aesthetics of the pontic is paramount. Tooth proportions are integral to optimize aesthetic outcomes. Many different proportions have been discussed in the literature over the years, the earliest being the ‘golden proportion’ in 1973.19 The ‘golden percentage’ (Figure 4) is a more recent calculation for tooth proportions. It has been found to more accurately demonstrate proportions in the general population and, therefore, is more relevant when used in dental aesthetics.19

While geometric proportions can assist in deciphering anterior dental aesthetics, many other factors may impact on patients' perception of an aesthetic smile. This includes patients' personality, wishes, ethnicity and socio-economic background.19 For these reasons, a diagnostic wax-up can be invaluable in ensuring a patient is happy with the pontic size and positioning (Figure 5).

6. Shade matching

The success of restorations is often governed by the patient's satisfaction with the aesthetics. The restoration shade has been found to be the primary cause for patient dissatisfaction (Figure 6).21 Thus, albeit a challenging process, shade matching between the natural teeth and pontic is crucial. In the placement of RRBs, this process is further complicated by the potential greying and loss of translucency of the abutment tooth (Figure 7).

Detailed shade mapping of the cervical, mid and incisal third of adjacent teeth will help the ceramist achieve optimum aesthetics. Shade selection should be carried out when teeth are hydrated and in natural lighting at the beginning of the appointment.3 The use of shade tabs is standard procedure when selecting a shade; however, the laboratory ceramics used may differ, therefore, communication is paramount.22 Digital photographs with, and without, polarizing filters can be used as an adjunct to shade tabs as an invaluable tool for dental shade matching.23 Mapping of enamel characteristics and levels of translucency should be noted, along with a description of the surface texture (Figure 8).24

With metal RRBs, greying of the abutment tooth can be avoided with the use of opaque cements; however, it should be noted that opaque cements will compromise the translucency of the abutment tooth. In cases of high aesthetic demand, avoiding extending the retainer within 2 mm of the incisal edge, where enamel becomes more translucent, can help minimize this.3 In cases where this is unavoidable, it may be appropriate to place composite labially to disguise the greying or use a grey tint within the pontic to mimic the abutment shade. Alternatively, one could also consider an all-ceramic design of RRB, if appropriate (Figure 9).

7. Pontic design

Pontic design can be overlooked by clinicians who may leave this important aspect to the dental technician. A ridge lap pontic can provide satisfactory aesthetics, particularly in cases with hard and soft tissue deficiencies. This design, however, is not cleanable, and leads to plaque accumulation and mucosal ulceration. It is therefore not recommended.25 A modified ridge lap pontic allows reasonable aesthetics and facilitates hygiene; however, an ovate pontic creates a better emergence profile in aesthetically critical areas (Figure 10).

An ovate pontic has increased mucosal contact, with light pressure applied to the underlying mucosa to improve the emergence form of the pontic from gingival tissues (Figure 11). When used as an immediate tooth replacement option, this pontic design can support and maintain papillae tissues, helping to prevent the occurrence of ‘black triangles’.25

8. Pontic site preparation

In conjunction with an ovate pontic, modifications to the soft tissues at the pontic site can be made to create a more realistic emergence profile. This can be carried out by defining the pontic site with a high-speed bur, or electrosurgery, prior to taking the definitive impression. At least 1 mm of mucosa should be maintained over the underlying alveolar bone, and this can be assessed using a periodontal probe. Patients awaiting bridges in the aesthetic zone may have an existing partial denture or orthodontic retainer. After pontic site preparation has been performed, composite or cold-cure acrylic can be added chairside to these removable prostheses, to aid mucosal contouring and prevent soft tissue rebound (Figure 12).25

9. Laboratory prescription

Framework thickness and connector dimensions need to be considered to ensure adequate rigidity and resistance to dislodgement. Where a base metal framework is to be employed, this should be of 0.7-mm minimum thickness,3 while zirconia allows for a 0.5-mm minimal thickness.14

A 180-degree wing wraparound is favourable for the retainer; however, this needs to be balanced with the demand for aesthetics.3 In some cases, particularly with ceramic framework RRBs, the connector may also need to extend further palatally/lingually to achieve adequate size.

A locating tag/seating lug can be requested to help locate metal retainers correctly, especially where no preparation has been carried out (Figure 1). These extend over the incisal edge to help resist cervical displacement, and can then be removed following cementation.3 While seating lugs are beneficial for clinicians, they can make it difficult for patients to assess aesthetics prior to cementation. Table 1 shows the information required for a laboratory prescription.

10. Cementation

If patients are compliant, adequate moisture control can be achieved in the anterior region using cotton wool and saliva ejectors; however, in posterior regions, a split dam technique can be used.3 The use of rubber dam is an issue of contention. It was found that bridges cemented under rubber dam were almost twice as likely to fail, although it is thought this may have been related to dental students' reduced clinical experience.12

It is recommended that metal wings are sandblasted prior to cementation. This is best performed after ‘trying-in’ the prosthesis to eradicate any saliva contamination. This can be performed with chairside air abrasion units, for those who do not have on-site laboratory facilities.

The try-in can be performed by digitally securing the RRB with the aid of a seating lug. If further assessments are desired, such as assessing abutment opaqueness, occlusion, pontic aesthetics and speech, one can use the base paste of a dual-cure cement (eg Panavia, Karrary Company Ltd, Osaka, Japan) as a try-in cement.

Sand blasting increases the surface area and tus the micromechanical properties of the retainer. Aluminium oxide at 250-μm grit size has recently been shown in in vitro studies to facilitate highest bond strengths, but traditionally has also been used at 50 μm.26 After sandblasting, it is recommended to clean the wing of any contaminant. This can be performed with 99% isopropanol.

It is key that operators use a dual-cured resin cement to enable setting beneath an opaque wing with the appropriate adhesive properties required for an RRB. Panavia 21 is the traditional cement of choice, demonstrating prolonged high bond strengths. It is important to ensure the manufacturer's instructions are followed throughout the bonding process to ensure optimal outcomes. The high bond strength is partially attributable to its inclusion of 10-methacryloloxydecyl dihydrogen phosphate (MDP), which exhibits effective bonding to both hydroxyapatite and metal alloys.27 MDP is no longer under patent and is found in many contemporary resin cement formulations.28

When cementing an all-ceramic RRB, different luting protocols are required. While bonding to metal and glass ceramics, such as lithium disilicate, is predictable, the same cannot always be said for oxide ceramics such as zirconia. Unlike lithium disilicate, zirconia is densely sintered and does not have a glass phase, meaning that it is extremely difficult, to the point of impracticality, to etch zirconia. Zirconia tends to be favoured as an all-ceramic RRB material over glass ceramics because of its mechanical properties; however, due to the difficulties in bonding, it requires specific abutment preparation, and is significantly less minimally invasive than its metal counterpart. Example regimens of zirconia etching include:

None of these regimens is feasible for the general practitioner to perform, even with on-site laboratory support. Thus, different approaches are required for bonding zirconia. Kern described creating micromechanical retention via air abrasion with 50-μm aluminium oxide at 0.1 MPa pressure.15 Zirconia primers are also recommended in the bonding protocol and should contain silane and MDP. These are now available from a range of dental manufacturers. Specifically, 3M ESPE advise the following chairside protocol to prepare their Lava Plus Zirconia fitting surface for cementation:

All ceramic RRBs should then be cemented with a dual-cure resin cement in a moisture-controlled environment, as per metal winged RRBs. It is clear that, while20 all-ceramic RRBs may be able to deliver superior aesthetic results, the abutment preparation and cementation process are significantly more taxing for the clinician when compared to metal RRBs.

Conclusion

RRBs can be an effective means of tooth replacement. Their conservative and minimally invasive nature make them an advantageous alternative to traditional bridgework. Owing to the minimal clinical time required to complete an RRB, the complexity of their planning can often be overlooked, which can lead to poor results. Within the cases demonstrated above, we have highlighted the possible aesthetic improvements that can be made with simple changes, such as soft tissue preparation and composite additions to adjacent teeth.

With ceramic RRBs gaining popularity, education on their use, along with the differences in bonding principles is imperative. It is important to recognize that, while RRBs are generally minimally invasive, ceramic RRBs do require more extensive preparation, which can come at a biological cost. It is noteworthy that minimal preparations frequently lead to dentine exposure, of which the operator may be unaware. The reader should be prepared to consider this when treatment planning. This update on ceramic RRBs has highlighted some tips for the general practitioner to improve the aesthetics and longevity of their RRBs.