Improve adhesive bridgework success. By Paul Tipton

An analysis of the literature regarding resin-bonded prostheses is more straightforward than that concerning conventional fixed partial dentures.

Most resin-bonded prostheses are single-tooth replacements and reported results are not as influenced by large prostheses as they are with conventional replacements.

The same criteria of bridge design apply equally to adhesive bridgework as conventional bridgework, such as retentional requirements, occlusion and hygiene, etc.

Failure

Dislodgement is the overriding cause of failure of resin-bonded replacements (Al-Shammery, 1989) and if double abutments are used, this can lead to rapid caries (Figure 1). Results suggest that overall long-term retention may be unpredictable failures ranged from 10 per cent over 11 years (Barrack, 1993) to 54 per cent over 11 months (Hansson, 1994).

Although combined long-term retention rates of resin-bonded prostheses appear problematic in many reports, certain selected variables show greatly improved success rates.

Restorations placed on tooth preparations having definite guide planes, proximal grooves and occlusal rests performed better than those with little or no preparation (Creugers, 1991).

The original concept of reversible resin-bonded fixed prostheses must be reconsidered if predictable retention is to be achieved. Current design parameters require that retainers more closely resemble traditional partial coverage restorations retained by resin cement (Burgess, 1989).

Luting agents and metal surface preparation techniques appear to have significant effects on retention rates. Panavia EX (Morita, Japan) coupled with aluminium oxide abraded retainers showed improved retention (Hussey, 1991). Cement fatigue as noted by Wood and Thompson (1993) or cement washout as defined by Boyer (1993) might best explain unexpected long-term failure after short-term success.

Success variables

Occlusal forces, enamel surface area and isolation feasibility are factors that probably contribute to divergent success rates in specific locations in the arch.

Maxillary anterior prostheses demonstrate the highest retention rates (Verzijden, 1994). Prostheses placed later in experimental periods usually fared better than those initially seated, indicating that operator skill and experience are influential variables (Creugers, 1991).

Cantilever Design

Chang (1991) has reported increased failure rates when the number of pontics or retainers in an adhesive bridge were increased.

This would suggest that adhesive bridgework is ideally suited to cantilevered bridges where there are small spans and only one pontic and abutment.

Designs for adhesive anterior cantilevered bridgework are broadly the same as for conventional anterior cantilevers, the contradictions being abutment teeth that present a reduced enamel surface available for bonding (Tay, 1988).

The retentional demands on the adhesive wing in a cantilever situation are not as great as in a fixed-fixed situation and give better success rates (Figures 2 and 3).

Therefore, tooth preparation may not be as critical in the upper jaw where the cement is under compression as opposed to the lower jaw where, because the cement lute is under tension, tooth preparation may be more critical.

Posterior bridgework

In posterior bridgework, the functional and retentional demands are greater as there is an increased chewing force posteriorly (Lundgren,1986), thus preparation techniques for maximising the surface area are increasingly important with the use of guide planes, wrap-arounds and grooves essential.

This can lead to an increase in the amount of metal shown and decrease in the degree of acceptance by patients because of poor aesthetics.

The stress on the cement lute may be too great for fixed-fixed designs unless occlusal coverage is also used.

More usually, a fixed-moveable design is used, allowing for unprepared occlusal surfaces. The attachment, however, is extra-coronal as opposed to the conventional intra-coronal.

Framework design

Northanston (1980) stated that the metal framework should be thick enough to prevent flexing of the metal wings but no thinner than 0.3mm.

Al-Shammery (1983) found that nickel-chromium beryllium alloys had a higher bond strength to composite than purely nickel-chromium alloys, but that there were problems with the carcinogenic nature of beryllium.

Ideally, for a definitive restoration, the metal framework should not be perforated for retention, but the retention should come from sandblasting the framework with aluminium oxide.

The size of the aluminium oxide appears to play a part with 110 micron and 250 micron grit size being better than 50 microns (Wiltshire,1986).

Preparation

Tooth preparation is often required for adhesive retainers, especially when the bridge will have a fixed-fixed design with the accompanying increased retentional demands this brings to the abutment teeth.

This is usually in the form of intra-enamel guide planes, finish lines, grooves and steps (Figures 4 to 11).

The goal is to produce a retentive preparation with resistance form so that as much stress as possible is removed from the cement lute.

One groove placed partially into dentine also increases resistance form and rigidity by increasing macro-retention (Flood, 1989).

Tooth preparation also exposes a subsurface layer of enamel that is more reactive and allows better bonding (Schneider, 1981). The incorporation of two proximal grooves (in dentine), wrap-around and occlusal rest seat can give the same retention to the adhesive wing as a three-quarter crown (Burgess, 1989).

Occlusion

It has been shown previously that increasing the number of abutments in an adhesive leads to an increased failure rate.

In an attempt to make these types of adhesive bridges more aesthetic by eliminating the graying effect of the incisal edge, there has been the tendency to cut back the metal framework from the incisal edge.

This has led to a better aesthetic outcome, but also a reduction in the retention of the retainer, as retention is proportional to the surface area available for bonding.

In upper anterior fixed-fixed adhesive bridgework, during excursive movements the initial anterior guidance is usually taken by metal wings and then onto the natural upper teeth in the incisal edge position.

When the framework has been cut back for aesthetics, this movement from metal wing to tooth results in increased stress on the adhesion of the metal wing to the tooth and, in the long-term, can lead to breakdown of the bond and the tooth “walking out” of the bridge.

This is often the cause of failure of this style of bridgework and can be overcome by a change of design. If the bridgework is made of a cantilever design, then the stress on the bond during lateral or protrusive movements would be taken up by the periodontal ligament of the tooth and the tooth would move accordingly, thus dissipating the stress.

Alternatively, if a fixed-fixed design is required, the metal framework should be taken up to the incisal edge not allowing any tooth-to-tooth contact in excursive movements.

Shade matching becomes difficult in this instance and either an opaque resin cement should be used or shade taking should be completed while the metal framework is tried in the mouth. This allows the darker incisal edges of the abutment teeth to be reproduced on the pontic.

Conclusions

Adhesive bridgework has a very unpredictable success rate. If, however, attention is paid to case selection involving missing single anterior teeth, occlusion and tooth preparation, success rates can be increased and predictable.

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About the author

Dr Paul Tipton is an internationally acclaimed prosthodontist who has worked in private practice for more than 30 years. He lectures for Tipton Training Ltd, one of the UK and Ireland’s leading private dental training academies and is the author of more than 100 scientific articles for the dental press.