Resin-retained bridges: considerations to maximize enamel bonding and aesthetics for abutment teeth with compromised contour

What options are available?

When considering treatment options for these patients, there are multiple choices available. These include removable prostheses, conventional bridgework, resin-retained bridgework (RRBs) or dental implants. Patients commonly opt for fixed-tooth replacement, and such treatment has been facilitated by developments in both adhesive dentistry and dental implants.

Although single implant-supported crown survival rates are high, at around 89% at 10 years, even dental implants cannot be considered a lifelong replacement option.³ In addition, patient-related factors may not always make dental implant placement possible. These are sometimes anatomical, or related to the patient's medical history, but may also be age-related. Dental implants should not be placed until growth is complete, the age of which will vary between individuals.⁴ As hypodontia patients often require restorative intervention at a young age, other options may therefore be considered initially.

Conventional bridgework has the benefits of being able to optimise abutment tooth shape, provide better shade control and close residual spacing. Furthermore, they demonstrate high success rates of 87% at 10 years.⁵ Despite these benefits, conventional techniques are much more invasive in terms of tooth tissue removal, a commodity that is limited if microdontia is present. In addition, the literature suggests 29.2% of conventional bridge abutments demonstrate signs of pulpal necrosis at 10 years.⁶ These failures are also likely to be more catastrophic than if resin-retained bridges are used.

Minimal damage to abutment teeth, potentially high aesthetic results, retrievability (particularly if implant treatment is considered in the future) and success rates quoted as 87.7% at 5 years, ensure that RRBs are an attractive, conservative treatment option to practitioners.⁷

Where RRBs are deemed the treatment modality of choice, careful design considerations are required, especially where microdont teeth are required to perform as bridge abutments. Given the reduced surface area, technical design is key in maximizing retainer bond to available enamel. Similarly, if composite resin is required for residual space closure and/or recontour of the abutment tooth, it is also important to ensure that this does not encroach onto the enamel that would otherwise be used for bonding of the retainer wing.⁸

The traditional approach to a patient presenting with these issues would often be to undertake composite resin build-ups of the abutment teeth first (ideally via a diagnostic wax-up). This would then be followed by impressions for the RRB and fit of the prosthesis at a separate appointment, termed a two-stage approach. This method has the advantage that it is technically less demanding in terms of fabrication and fit of the RRB. However, the wing retainer is mostly likely to be bonded to composite resin rather than enamel, which will yield lower bond strength and thus compromise long-term outcome.⁸ Additionally, aesthetics may be compromised by shine-through of the metal wing underlying the composite resin.

An alternative method of overcoming these challenges is to undertake the concurrent (same visit) addition of composite resin. This is placed directly onto the abutment tooth and against part of the metal retainer of the RRB at the time of cementation. The advantages and disadvantages of this technique are given in Table 1. This is considered a one-stage approach and is demonstrated in the two cases presented in this series.

Case 1

This 19-year-old patient presented to the prosthodontic department following previous assessment within the hypodontia clinic, and a course of fixed orthodontic treatment. His main dissatisfaction concerned the shape and spacing of his teeth, and the aesthetic implications of this. He was medically fit and well, and a non-smoker. Key findings following the intra-oral examination included severe hypodontia (missing UR542, UL542, LL12 and LR1), midline diastema/residual spacing post-orthodontics, microdont canines, barrel shaped maxillary central incisors and deficiencies in alveolar bone volume (Table 2). The pre-operative intra-oral appearance is shown in Figure 2, while Figure 3 shows the post-operative view.

Following comprehensive intra-oral examination of the hard and soft tissues as well as the occlusal scheme, the various replacement options were outlined to the patient in detail, including their risks and benefits. In particular, the alveolar bone deficiencies and the implications of this regarding implant therapy were discussed. The patient expressed the desire for a fixed replacement option, but did not wish to undergo potentially extensive/multiple surgical procedures. Thus, tooth replacement via resin-retained bridgework was agreed rather than implant therapy. Owing to the need to close the midline diastema and optimise the shape of the adjacent teeth, cementation of the RRBs and composite resin build-ups of the diminutive abutment teeth using a one-stage technique were planned.

Diagnostic wax-ups were undertaken on the definitive casts, which had been articulated on a semi adjustable articulator (Denar mark II, Whip Mix Corporation, KY, USA) using a facebow and retruded contact position record. This was then tried in the patient's mouth using a chairside silicone putty stent and Integrity (Dentsply, NC, USA) provisional crown and bridge material. This allowed confirmation of tooth position and aesthetics to allow both the clinician and patient insight into the final outcome. Adjustment to the wax-up was then made as required prior to this being cut back in the lab to produce the metal framework. Opaque ceramic was then baked onto the areas where composite was to be added. This was followed by a try-in of the metal frameworks to assess fit and design prior to progressing to a bisque try-in to finalize shape and shade and then definitive bridge cementation and composite resin build-up.

For cementation, the tooth surfaces and retainer wings were prepared in line with accepted protocols, as shown in Table 3.⁹ This involved rubber dam isolation, air abrasion of both the tooth and retainer wing surfaces, etching of the enamel, alloy priming of the metal, and hydrofluoric acid etch and silanation of the opaque porcelain prior to cementation. For this case, Panavia F2.0 (Kuraray Noritake Dental Inc, Japan) dual-cured adhesive resin cement was used due to its ability to adhesively bond to enamel, the metal retainer surface as well as the treated porcelain.¹⁰ Additionally, it is methacryloyloxydecyl dihydrogen phosphate-(MDP) based, which has been demonstrated to provide good adhesive stability.¹¹

Once cemented, composite resin was then added to the exposed fitting surface of the retainers as well as bonded to the abutments, as shown in Figure 4. Figure 3 illustrates the post-operative result, which satisfied the patient's need for an aesthetic, fixed replacement option. This also potentially optimises prosthesis longevity compared to the conventional two-step method because this ensures bonding to all available enamel.

Case 2

This 21-year-old patient presented to the prosthodontic department following cleft lip and palate management by maxillofacial, orthodontic and plastic surgery colleagues. As part of her previous treatment, the upper left central and lateral incisors were extracted, maxillary advancement surgery was undertaken and fixed orthodontic therapy initiated to optimise the remaining spaces. The patient's main concerns were in relation to the poor aesthetics of her missing teeth and the mobility of her remaining maxillary central incisor. Key findings following the intra-oral examination are presented in Table 4 with the pre-operative appearance following fixed appliance removal shown in Figure 5.

Prior to orthodontic debond, an extensive examination and Kesling set-up were used to assess different treatment options, and it was decided to accept the final tooth position given the limitations in achieving further useful tooth movement. The final treatment plan was to extract the remaining maxillary central incisor due to its poor prognosis, provide prosthetic replacements for the missing central incisors and mask the canines as lateral incisors.

Owing to the severe limitations of bone volume in the site, and subsequent need for extensive augmentation, implant placement was discounted by the patient. Additionally, conventional bridge placement was deemed too destructive. As there was a need for tooth replacement and concurrent optimization of abutment shape, a one-stage technique was used. Figure 6 shows the post-operative appearance with Figure 7 demonstrating the treatment stages. Once the new incisal plane had been established, the patient's plastic surgeon was able to undertake a final lip revision.

Discussion

Conventionally, composite resin build-ups are undertaken prior to a definitive impression being taken for RRB fabrication. When using this approach, traditional teaching advocates bonding the composite resin restoration to a large area of enamel to improve their longevity. This will be likely to leave minimal enamel surface area for bonding the subsequent retainer wing. Aboush and Jenkins demonstrated tensile bond strength was greater to enamel than to even freshly cured composite resin, with a comparison of 28Mpa to 25Mpa, respectively.⁸ In addition, when the bonding area was half enamel and half restorative material, the bond strength achieved was only comparable to the less favourable of the two adherents.⁸ It is therefore evident that by undertaking the composite addition first, the enamel available for bridge cementation will be reduced resulting in a weaker bond.

This theory is proven in practice with King et al in 2015 demonstrating a 46.2% failure rate for resin-retained bridges cemented onto teeth with old restorations compared to 24.1% for new restorations and 16.7% if there were no restorations.¹² Even if enamel coverage of the initial restoration is kept minimal, multiple factors will affect the timeframe between the build-up and RRB fit appointments. It is not easy to establish how ‘new’ the restorations can be considered by the time the cementation stage for the bridge is reached. These restorations are likely to become waterlogged and lose residual monomer while the prostheses are being fabricated, reducing the bond strength further. All of these issues are overcome with the one-stage technique discussed in this article.

When the framework is being produced, it is essential that basic principles are followed. This includes confirming that the nickel–chromium retainer has a thickness of at least 0.7 mm for anterior RRBs to ensure adequate rigidity and to reduce the chance of bond failure due to flexure.^13,14 Rigidity will also be increased by ensuring maximum connector heights and extending the retainer wings to the incisal edge. Full palatal coverage should be undertaken wherever possible when fabricating the RRB to aid with seating at cementation and to ensure maximum contact for enamel bonding. For cases where this one-stage technique is used, this is even more important because full extension will provide support and guidance for the addition of the composite resin.

A downside of using resin-retained bridges and composite over dental implants or fixed bridgework is the risk of metal shine through. To mitigate this risk, opaque porcelain can be fired onto the retainer wing where the composite is to be added. Although this reduces grey discolouration, this does add extra steps to the retainer wing preparation. Owing to the one-stage nature of this technique, there are multiple mediums in contact with different surface characteristics. It is therefore essential that an appropriate sequence is used in line with established treatment protocols to ensure optimum bond strength.⁹

It could be argued that by undertaking the composite addition prior to fabrication of the resin-retained bridgework, aesthetics can be modified and adjusted prior to the more rigid step of bridge fabrication. To mitigate this risk, a provisional try-in of the wax-up can be undertaken. This allows modification intra-orally to the provisional material (for example Integrity, Dentsply), which can be impressed and sent to the laboratory for guidance. This ensures clinician and patient satisfaction prior to final fabrication.

Conclusion

In cases where both composite resin additions (to abutment teeth) and resin-retained bridge placement are indicated, a one-stage placement technique provides many advantages over the two-stage technique. This method is minimally invasive, optimises the available enamel surface area for bonding and thus, should improve the longevity of the restoration while also producing an aesthetic outcome.