The overall topic of NIOM’s presentation at the Norwegian Dental Association’s annual meeting and Swedental in 2015 was Materials for indirect restorations; both metals and all-ceramic materials. A brief summary of the presentations is given below.
Metal–ceramic restorations still much used
Professor Nils Roar Gjerdet & professor Jon E. Dahl
Information from a major dental laboratory in Norway indicates that metal–ceramic restorations still are in extensive use, even if all-ceramic restorations are becoming more common than before. There has been a strong shift towards use of base-metal alloys − usually cobalt–chromium alloys − as compared with noble-metal alloys for crowns and bridges. Moreover, the production technologies for metal restorations have moved away from the traditional casting to computerized design and manufacture using milling (subtractive technology) or 3D printing in the form of selective laser melting (additive technology) (Figure 1).
So far, there is limited information on the performance of cobalt-chromium alloys processed by different techniques. It is indicated that mechanical properties are influenced by the production method, a topic that should be further investigated. Cobalt and chromium are potential sensitizers. There is, however, a lack of data regarding the incidence and prevalence of clinical reactions of allergic nature.
Prosthetic materials, the right material of choice for each clinical situation
Guest researcher, associate professor Marit Øilo
The success of all-ceramic restorations depends on more than just the material. Design of preparation, design of restoration, pre-treatment and cementation are only a few of the factors that can affect the clinical long-term function of a restoration. All-ceramic restorations require a preparation design of the chamfer type in order to avoid fracture starting in the crown margin. The depth of this depends on the type of restoration.
Monolayered (monolithic) crowns require less space than bi-layered crowns. Monolithic crowns means crowns made only of one layer of material. Veneers for adhesive cementation are one such type. Monolithic crowns of zirconia are another. Monolithic zirconia crowns are not as aesthetic as the veneers, but may be a strong and durable alternative to metal crowns in the lateral segments, especially for patients with grinding or clenching habits. These patients often experience a lot of chipping and wear with all other types of restorations. It is important to consider all factors, such as aesthetics, loss of tooth substance, retention and bite force, before deciding which material is best suited for the patient in question.
Research on all-ceramic materials at NIOM and future prospects
Senior engineer Ketil Kvam
Research at NIOM has focused on zirconia materials and evidenced very low solubility even in an aggressive test solution which is meant as an accelerated time-in-saliva test. Surface failures like micro cracks are more likely the cause of strength reduction than tetragonal to monolithic transformation, the so-called ageing phenomenon. Novel etching techniques for zirconia surfaces to improve bond strength of crowns/bridges – inside to resin cements, outside to ceramic veneering with limited phase transformation or ageing – has been developed at NIOM (Figure 2).
The widespread skepticism to all-ceramic dental solutions has been reduced considerably over the last 20 years, in particular for crowns and bridges, but for implants there is still doubt. Yet they will probably gradually be gaining ground. 3D printers for direct sintering of ceramic powder to individualized dental products are also future prospects.
New products and material combinations are and will be introduced for CAD/CAM, both zirconia-based advanced crystalline materials, glass-ceramics with high strength and ceramic particle-filled polymers. ADA has sometimes wrongly classified the latter as resin nano-ceramics which really are composites with limited strength.