PRACTICE BRITISH DENTAL JOURNAL VOLUME 192 NO. 4 FEBRUARY 23 2002 199 Crowns and extra-coronal restorations: Materials selection R. W. Wassell 1 A. W. G. Walls 2 and J. G. Steele 3 Materials selection is the second in the series on crowns and other extra-coronal restorations. Some of us are less than inspired by dental materials science. Nevertheless, many of the things that concern us clinically with crowns and their alternatives are based on material properties. We worry about the strength of the restoration, how well it fits and its aesthetics. We also worry about wear, occlusal control and biocompatibility. Not least of our concerns are dental laboratory charges, which inevitably have to be passed on to the patient. 1* Senior Lecturer in Restorative Dentistry, 2 Professor of Restorative Dentistry, 3 Senior Lecturer in Restorative Dentistry, Department of Restorative Dentistry, The Dental School, Framlington Place, Newcastle upon Tyne NE2 4BW *Correspondence to: R. W. Wassell E-mail: r.w.wassell@ncl.ac.uk Refereed Paper © British Dental Journal 2002; 192: 199-211 ● Familiarisation with the broad spectrum of contemporary materials allowing better-informed decisions to be made ● Discovering the factors that influence choice of alloys for indirect metallic restorations ● Knowing the pitfalls of selecting a cheap alloy for cast post and core construction ● Familiarisation with the different types of all ceramic crown and which ones can be used posteriorly ● An awareness of the many different types of metal copings for ceramo-metal restorations ● Consideration of the use of composite crowns for specific clinical situations IN BRIEF An understanding of the materials available, their application and limitations will not only help with selection but will allow more effective communication with patients and laboratories. Promotion of such understanding allowing bet- ter-informed decisions to be made is in essence the aim of this article. There are now more materials available than ever before for indirect restorations. These fall under the broad headings of all metal, ceramics, metal-ceramics and resin composites. Cements and cementation, either conventional or adhesive will be considered in later articles. Manufacturers’ details are shown at the end of the article. ALL METAL RESTORATIONS All metal extra-coronal restorations include crowns, onlays and shims (onlays relying soley on adhesive retention). Cast posts and cores can cause particular difficulties and are considered separately at the end of this sec- tion. In the UK as in other countries we are aware that economic pressures are driving the adoption of less expensive alloys. These alloys contain either low concentrations of noble metals or are comprised entirely of base metals. In this section we consider the various types of alloys and the factors which influence their selection including: • Cost • Castability and handling • Physical properties • Resin bonding • Corrosion and tarnish • Biocompatibility Gold and palladium alloys In the 1950s making an alloy selection for a cast metal restoration would simply have entailed choosing an ADA specification high gold alloy, all of which had a greater than 75% gold and platinum content. The soft type I alloy contained most noble metals (83%) with the harder alloys (types II, III and IV) containing an increasing amount of silver and copper. Type I alloy was used for small inlays, type II for larger inlays, type III for crowns and bridges, and type IV for partial denture frameworks. In the early 1970s the gold price increased four fold resulting in the introduction of less expensive alloys of lower Karat. 1 Palladium, which had been used since the 1930s to produce a cheaper white gold (Au 30%, Pa 10–35%, Ag 35-60%, Cu 6–25%), became the noble con- stituent of many of the new precious alloys. These alloys include: • Silver-palladium • Palladium-silver-gold • Gold-silver-copper-palladium (with a gold content greater than 40%) • Palladium-copper • Palladium-tin Palladium has a strong whitening effect which means that most of these alloys will have a silvery appearance unless the gold content is greater than 40% and the palladium less than 6%. 2 Unfortunately, both palladium and silver absorb oxygen when molten, which can result in porous castings especially if casting buttons are reused. Also, reducing the gold content lowers the specific gravity (density), which may make casting less reliable than with high gold alloys. This is because less dense alloys have 2 CROWNS AND EXTRA-CORONAL RESTORATIONS: 1. Changing patterns and the need for quality 2. Materials considerations 3. Pre-operative assessment 4. Endodontic considerations 5. Jaw registration and articulator selection 6. Aesthetic control 7. Cores for teeth with vital pulps 8. Preparations for full veneer crowns 9. Provisional restorations 10. Impression materials and technique 11. Try-in and cementation of crowns 12. Porcelain veneers 13. Resin bonded metal restorations