Resin-bonded bridges – the problem or the solution? part 1: assessment and design

How long can RBBs last?

Resin-bonded bridges (RBBs) have been used to replace teeth in short edentulous spans with increasing success since the 1970s.¹ A systematic review by Pjetursson et al reported a survival rate of 87.7% at 5 years,² deeming them an acceptable and minimally invasive (MI) method of restoring modest-sized spaces in the dental arch. It has often been considered that they are an under-utilized restoration modality in general dental practice due to a perceived high rate of clinical failure, which may be associated with incorrect design and execution.³

A recent prospective study of 771 adhesive bridges by King et al found that most failures of RBBs occurred within the first four years, and that very few failed thereafter, with an estimated survival rate of 80.4% at 10 years.⁴ In this single-centre study, because the point of failure was recorded as the first de-bond, the overall survival in clinical service may have been greater where bridges had been re-bonded successfully. An evidence-informed summary of key papers assessing RBBs has been given in Table 1.^4,5,6,7,8,9

Careful case selection, appropriate design and attention to operative detail are key factors for the clinical longevity of RBBs and will therefore be covered in this paper.

When can RBBs be used?

RBBs have an important role to play in restorative dentistry, with their indications extending beyond the replacement of lateral incisors. They are a MI way of replacing missing teeth compared to conventional bridgework¹⁰ or implants, usually not requiring local anaesthetic, making the procedure suitable for patients who are needle phobic or do not wish to go through lengthy surgical treatment. RBBs are an option where there may be a lack of 3-dimensional space or bone for implant placement. The benefits of shorter appointments and associated cost, compared to conventional bridgework and implant-supported restorations, are also favoured by patients.

However, as with any form of treatment, the use of RBBs is not without limitations and success is associated with appropriate case selection and planning. As with the provision of any restoration, the patient must be well motivated with good oral hygiene and primary dental disease well controlled. The indications and contra-indications are summarized in Table 2.

The loss of a tooth can have unwanted changes and effects in the mouth that may complicate the provision of a prosthetic replacement, such as tilting of adjacent teeth or over-eruption of the opposing dentition.¹¹ Therefore, the edentulous space should be assessed for adequate space for an aesthetic pontic and sufficient connector height, not only considering the one arch in isolation, but in both static and dynamic occlusion. A lack of vertical space may require an increase in the occlusal vertical dimension (OVD), which will be discussed in this paper. In cases where the mesio-distal dimension of an edentulous space is smaller than ideal, a RBB may be feasible if aesthetics allow, but implant placement may not be practical.

Post-orthodontic fixed retention can also be incorporated into adhesive bridge design with some success.⁴ Since the use of RBBs lends itself particularly to the restoration of missing teeth in mild or moderate hypodontia, this is frequently useful in the management of cases where pre-restorative orthodontics has been carried out to create appropriate spaces for replacement of the missing teeth. In Figure 1, the central incisor teeth have been used as abutments with the framework linked to prevent orthodontic relapse. Similarly, where canines have been de-rotated, these may be incorporated in a fixed-fixed design to replace lateral incisors, which would control the position of the canines and prevent orthodontic relapse. An alternative would be to use a cantilever design of an adhesive bridge and provide a separate form of orthodontic retention, such as a vacuum-formed Essix retainer, but this is reliant on patient compliance with regards to retainer wear and is therefore arguably less predictable.

Consideration of parafunctional habits is important, as fixed prosthodontic work may be more likely to fail where heavy loads are placed on the teeth. The provision of RBBs in bruxists, therefore, requires careful occlusal assessment and planning, as described later in this paper, and consideration may be given to protecting restorative work through the use of an occlusal splint.^12,13

Selecting suitable abutment teeth

There are multiple factors to consider in the selection of appropriate abutment teeth for RBBs. These are outlined below and also summarized in a flowchart (Figure 2).

Angulation and position

An unfavourably tilted or bulbous abutment tooth crown anatomy may reduce the height of the connector between the pontic and retainer wing, causing an increase in flexibility of the framework. In such cases, consideration should be given to a minimal proximal surface preparation of the abutment to reduce the bulbosity, thereby maximizing connector height to increase rigidity, and allowing the maximum surface area to be incorporated into the design (Figure 3). This also limits the path of insertion, which assists in retention by reducing forces on the cement lute. As well as all these advantages, proximal surface preparation can also reduce the size of embrasure spaces to eliminate black triangles in anterior regions. As with all forms of cantilever bridges, mesial cantilever designs are preferred over distal cantilever bridges. This is due to the increased biomechanical levering forces around the abutment, which acts as a fulcrum.

Preparing abutment teeth

Over the years, some literature has suggested that a degree of coronal preparation (such as vertical grooves and preparation of rest seats) of abutment teeth can increase resistance form and thereby longevity of RBBs.^9,20,21 However, there is conflicting evidence in relation to this as some studies suggest no benefit of abutment preparation on the overall success of RBBs, and that significant preparation is associated with an increased risk of failure.^4,5,6,22 The biological cost of tooth preparation, however minimal, is significant enough to negate the requirement for preparation as there are undeniable advantages of the MI approach, whilst the preservation of enamel for adhesion is important for RBB success. The non-operative approach avoids the risk of exposing dentine,²³ which is less ideal as an adhesive substrate and more prone to Caries Associated with Restorations/Sealants (CARS, previously known as secondary or recurrent caries) if the bond fails. There are also complications associated with tooth preparation, such as the need for the preparation to be sufficiently accurate with parallel retention slots/grooves in order for them to be usable or confer any advantage with respect to limiting the path of insertion. The preparation must then be recorded accurately and transferred to the dental laboratory via the master impression and working cast, and for the laboratory to conform the retainer manufacture exactly to the preparation. The loss of detail at each stage may adversely affect the final fit of the RBB and negate any perceived advantage.

One suggested advantage of tooth preparation is that it can create space for the retainer, removing the need for occlusal adjustment of the opposing dentition. However, preparation for this purpose is not advised at the impression stage as it is difficult to provisionalize predictably, risking loss of the created space in the interim between impression and fit appointments due to dento-alveolar compensation and over-eruption.

However, minimal preparation of the abutment teeth to create a guide plane within enamel by removing the natural bulbosity of the clinical crown can assist positive seating of the RBB and increase resistance form by limiting the path of insertion as well as providing the room for an increased connector height (Figures 3 and 4). This is particularly suited for fixed-fixed RBB designs, where the terminal guide planes must be parallel.

Designing the RBB framework

Degree of coverage

The literature suggests that the best outcome for adhesive bridges can be achieved where maximum enamel coverage is incorporated in the retainer design, and a 180-degree wraparound of the abutment teeth is often recommended.^6,24 Incorporating the palatal/lingual cusps and occlusal surfaces of posterior teeth within the retainer wing coverage increases the surface area of enamel for bonding. This also offers some protection against shear forces that would be applied to the cement lute in the axial portions of the retainer wings during function and increases rigidity of the framework, protecting it from dislodging forces.

In anterior cases, extending the retainer onto the incisal edge is advocated as this can assist with location of the retainer, provides a degree of resistance against axial shear forces, and increases the area of bonding further (Figure 5).⁶ The effect on aesthetics is minimal as when the patient smiles with his/her teeth apart, the retainer extension becomes less visible against the dark background of the oral cavity (Figure 5a). The aesthetic requirements of the patient should also be assessed and good communication maintained throughout the planning and consent stages to ensure that the patient has understood the potential impact that a visible metal wing retainer may have on his/her smile against the perceived advantages associated with this approach. The rationale for maximum palatal coverage and wrapping on to the incisal edge should be made clear to the patient, but in the authors' experience this is often not a problem for the patient. Potential aesthetic failure of RBBs is sometimes described, due to the grey shine through of the metal retainer at the translucent incisal edge, but this can be eliminated predictably by using opaque luting cement.

The use of an incisal locating tag has been described in cases where retainer location may be difficult, but this can result in inaccurate seating of the retainer, resulting in an increased thickness of luting cement.²⁵ In addition to this, the locating tag must be removed at the fit appointment, which theoretically may compromise the cement lute owing to vibration and heating of the immature material immediately after polymerization. The authors' preference lies with extension of the retainer on to the incisal edge, for the advantages already described.

In addition to the aforementioned advantages of maximum coverage of the framework in RBB designs, there is an additional benefit of full coverage in fixed-fixed RBBs. Stopping short of the incisal edge is contra-indicated in fixed-fixed cases where protrusive and excursive forces beyond the retainer may result in differential movement of the abutment tooth, in effect pushing the tooth away from the framework when two retainers are used. Therefore, the retainer should be full coverage to prevent such a mode of failure in fixed-fixed cases where the excursive guidance may otherwise drive the teeth away from the framework.

Using RBBs to replace missing posterior teeth

Due to an ageing population and a change in the pattern of tooth loss, there is likely to be an increase in the number of partially dentate patients in the population. Although accepting a gap is sometimes appropriate, this may lead to unwanted complications such as tilting of adjacent teeth and over-eruption of unopposed teeth, which has been reported to occur in up to 83.9% of cases.¹¹ This may complicate the future restoration of the edentulous space, and may introduce occlusal interferences. Therefore there may be an increasingly significant role that adhesive bridges can play in the replacement of posterior teeth in a MI way in order to maintain tooth position.

There is sometimes cause for concern with using RBBs in the first molar region with regards to design and success associated with the large mesio-distal dimensions of the edentulous space. One option is to replace a first molar with two premolar-sized pontics on cantilevered bridges, using the second molar and the second premolar as abutment teeth. By using two smaller adhesive cantilever bridges to replace one large unit, there is more favourable stress distribution through the framework and a greater surface area of the combined two abutment teeth can be utilized. The alternative option is to cantilever mesially from the second molar tooth, with generous occlusal and lingual/palatal coverage (Figure 6). In most cases the authors prefer a cantilever design in this situation, however, a fixed-fixed design may be accepted to match the increased occlusal demands posteriorly or where there is concern regarding the risk of tilting of the molar abutment (Figure 7).

The use of RBBs to replace posterior teeth is less predictable than anterior teeth due to increased occlusal demands.^4,5 There is little published evidence assessing the factors associated with success for the replacement of molar teeth with RBBs. However, applying the general principles described previously, the rigidity of the framework, degree of coverage and the occlusion are likely to be important factors for consideration. The extension of the retainer onto the occlusal surface of posterior teeth (Figures 6 and 7) is advised because this incorporates more enamel for bonding and increases the rigidity of the framework. Furthermore, the cement lute is protected from shear forces that could dislodge a bridge that is retained by a retainer wing limited to the axial walls only because the occlusal load is directed onto the occlusal extension, and therefore the cement is loaded in compression.

Occlusion and RBBs

Pre-operative assessment should include examination of the patient's occlusion in the inter-cuspal position (ICP) and in dynamic excursions to ensure that there is enough room for a functional and aesthetic replacement. Ideally, the occlusion should be organized so that the pontic is in light contact in ICP to control the axial position of the opposing tooth, but not involved in guidance, where possible. If guidance on the pontic cannot be avoided, guidance should be shared with the natural teeth, especially where a cantilever design is used. The ICP contact should be kept away from the margin of the retainer. This may be achieved by minimal adjustment of the opposing dentition. It is prudent to warn the patient about this prior to bonding, to reassure the patient that the adjustment is planned rather than a necessity to compensate for a poorly fitting bridge. The use of a facebow transfer to allow casts to be mounted on an articulator, although not always essential for RBB provision, may aid in planning a predictable occlusal scheme in larger span or multiple bridges (Figure 8).

If pre-operative assessment reveals limited space available for the metal framework, an increase in the OVD can be planned. The bridge can be bonded at an increased vertical dimension, to achieve what is commonly referred to as the ‘Dahl effect’. This should result in relative axial movement as a result of alveolar compensation of the teeth brought out of contact in this way, such that the occlusal contacts re-establish in the weeks or months following bridge placement.³⁰ The technique was initially described using an anterior bite plane to create space anteriorly after a number of months. The mechanism described for the creation of space in an anterior ‘Dahl appliance’ was found to be through a combination of intrusion of anterior teeth (40%), and eruption of unopposed posterior teeth (60%).³¹ The use of this technique specifically for RBBs has also been described by Briggs et al and is part of the standard protocol for achieving interocclusal space for RBB restorations in our secondary care unit.³²

The patient should be pre-warned about occlusal changes and the possibility of transient lisping, and how chewing may feel awkward at first. A review of the literature suggests that the Dahl technique has a high predictability of success (94–100%) and is generally well tolerated by patients.

The average amount of time for contacts to re-establish is 6 months, therefore a longer period of follow-up is advised where this technique is used.³³

RBB pontic design

The pontic should achieve a passive contact with the tissues and allow adequate hygiene by the patient. The two most common pontic designs for bridgework are the modified ridge-lap and the ovate pontic. The potential benefit of ovate over the modified ridge lap is its potentially improved emergence profile and aesthetics. However, the use of an ovate pontic is more technically demanding as it requires planning and possibly the creation of room for the convex portion of the pontic. Where the soft tissue in the pontic bearing area is thick, convex and not compressible, this may be achieved with the use of electrosurgery or, where this is not available, the use of an oval-shaped, diamond bur in a high-speed air turbine handpiece has been described.³⁴ Electrosurgery has distinct advantages for this as it also arrests haemorrhage and allows the master impression to be taken immediately after soft tissue re-contouring and is therefore preferred by the authors.

Summary

As with all aspects of restorative dentistry, careful case selection and attention to detail are critical for the successful provision of RBBs. This paper has covered common challenges that practitioners face during planning and designing of RBBs, and hopes to have emphasized the key, evidence-based principles of design that are important for their longevity. The operative techniques involved in RBB provision, including effective laboratory communication, will be covered in Part 2.