Chapter 18 Minimal preparation (resin retained) bridges
Minimal preparation bridges have also been referred to in the literature as resin retained bridges, adhesive bridges and resin-bonded bridges. In this text the term minimal preparation (resin retained) bridge will be used. These are bridges that require minimal preparation of tooth tissue and rely on adhesive cements for their primary retention, unlike conventional bridges that require significant tooth reduction to make retentively shaped preparations which provide the primary retention and then rely on a cement lute for secondary retention.
Minimal preparation (resin retained) bridges are therefore used when the potential abutment teeth are unrestored or minimally restored and there is a need to reduce unnecessary tooth preparation. Consider the 22-year-old patient in Figure 18.1: preparation of the vital upper right central incisor tooth for a metal–ceramic retainer has been destructive of tooth tissue as this tooth only had a small fractured incisal edge. A cantilevered minimal preparation (resin retained) bridge using the central incisor as an abutment tooth would have been much more conservative. However, in this patient both lateral incisors are missing. Conventional bridge retainer preparation of the non-vital upper left central incisor tooth has been necessary not only to retain a pontic but also to disguise the appearance of the discoloured tooth. Furthermore, this patient had orthodontic treatment to close a midline diastema and since this treatment tends to be unstable, the conventional retainers were linked to also act as an orthodontic retainer.
This technique can be used for a tooth that needs to be extracted. Once extracted, the root is resected and then the crown is cemented to the adjacent tooth/teeth using composite resin cement. At the resected root end the root canal should be opened and restored with either a glass ionomer or composite resin to prevent food and plaque stagnating within the tooth. To bond the extracted tooth the corresponding approximal surfaces are acid etched, bond applied and then composite used to cement the crown in place. Figure 18.2 illustrates diagrammatically how this procedure is carried out and Figure 18.3 shows an example where this has been done. In the author’s opinion this technique has been remarkably successful for the patient in Figure 18.3 as this ‘temporary’ measure had been carried out many years previously.
Figure 18.2 Diagrammatic representation of the patient seen in Figure 18.3. The teeth are periodontally involved and the lower right central incisor requires extraction. Following extraction the root is resected (top right), the root canal is cleaned from the resected end and the canal restored with composite resin or glass ionomer (bottom left). The tooth is then bonded to the adjacent tooth with composite resin cement (bottom right).
If a patient presents with a missing tooth, which is either lost or not reusable, a replacement tooth can be made from composite resin using a crown former or simply an appropriately sized acrylic denture tooth can be used. Additional retention can be obtained, if required, by embedding a contoured orthodontic wire, wire mesh or fibre-reinforced ribbon in composite to the lingual surface of the extracted tooth and the tooth or teeth to which it is being bonded.
A Rochette bridge consists of a pontic connected to a metal wing retainer which covers the lingual surface of the abutment tooth. Within the wing, countersunk holes are cut which allow retentive composite plugs or rivets to form when the bridge is cemented in situ (Figure 18.4). When Rochette bridges were originally described, they were designed to act as permanent fixed prosthodontic restorations (Figure 18.5). However, to achieve composite plugs of adequate strength they had to be of a significant diameter and depth, the latter necessitating a thick wing which caused problems in relation to the occlusion, especially when placed in the upper anterior region. In addition, occlusal stresses were concentrated on a limited number of composite plugs, predisposing to them to fracture and, as such, retention rates were often poor in the long term.
Today, Rochette bridges can still be considered as an immediate provisional tooth replacement when ease of removal is a benefit. For example, if a patient was unable or unwilling to wear a removable prosthesis and an upper central incisor required extraction, its immediate replacement with a cantilever Rochette bridge, using the contralateral central incisor as an abutment, could be considered. The tooth could be extracted, rubber dam applied and the Rochette bridge cemented without any blood contamination from the socket.
Provided the bridge is cemented in place with a non-adhesive, dual or chemically cured composite resin luting cement, such as Nexus luting cement (Kerr/SDS), removal is achieved by simply removing the composite from the holes within the wing. It is important to note that if the permanent tooth replacement is to be a minimal preparation (resin-bonded) bridge using the same abutment tooth, the superficial enamel will be impregnated with resin and this will also have to be removed, otherwise the subsequent bond for the permanent bridge will be compromised. Therefore, this technique is preferred when either a different tooth is to be used as an abutment for the definitive bridge or if the final replacement is an implant which is not reliant on a tooth for support and retention.
Rochette bridges suffered from a number of problems, namely potential degradation of the exposed composite (wear) and areas of stress concentration within the countersunk holes and composite rivets. In an attempt to retain a solid wing with reduced thickness and provide an improved retention for minimal preparation (resin retained) bridges Thompson and Livaditis in 1982 described a method of electrolytically etching the fit surface of non-precious alloys. The acid, its concentration and etching time were different for different alloys. In each case, however, an etch pattern was produced that allowed micromechanical retention of the bridge when a chemically cured non-adhesive resin was used for cementation. This bridge became known as the ‘Maryland bridge’. The Maryland bridge technique required acid baths in the laboratory, and was a technique-sensitive procedure. Attempts were made to use chairside acid etch gels for the metal frame; however, the introduction of adhesive resin cements in the late 1980s superseded their further development.
Unlike resin luting cements, adhesive resin luting cements have the ability to bond to metal in addition to etched tooth tissue. Such cements include those that contain 4-META (4-methacryloxyethyl trimellitate anhydride) or phosphate groups (e.g. Panavia) which allow a chemical bond to form with metal. Superior bonding of these cements occurs with surface-abraded non-precious metal alloys such as cobalt chromium or nickel chromium. Whilst the latter allows a more rigid casting to be formed, there are problems associated with allergies (to nickel) for laboratory workers, dentists and patients. Precious metal alloys have therefore been used despite earlier reports of inferior bond strengths to precious metals.