Zirconia is unique in its polymorphic crystalline makeup, reported to be sensitive to manufacturing and handling processes, and there is debate about which processing method is least harmful to the final product. Currently, zirconia restorations are manufactured by either soft or hard-milling processes, with the manufacturer of each claiming advantages over the other. Chipping of the veneering porcelain is reported as a common problem and has been labelled as its main clinical setback. The objective of this systematic review is to report on the clinical success of zirconia-based restorations fabricated by both milling processes, in regard to framework fractures and veneering porcelain chipping. A comprehensive review of the literature was completed for in vivo trials on zirconia restorations in MEDLINE and PubMed between 1950 and 2009. A manual hand search of relevant dental journals was also completed. Seventeen clinical trials involving zirconia-based restorations were found, 13 were conducted on fixed partial dentures, two on single crowns and two on zirconia implant abutments, of which 11 were based on soft-milled zirconia and six on hard-milled zirconia. Chipping of the veneering porcelain was a common occurrence, and framework fracture was only observed in soft-milled zirconia. Based on the limited number of short-term in vivo studies, zirconia appears to be suitable for the fabrication of single crowns, and fixed partial dentures and implant abutments providing strict protocols during the manufacturing and delivery process are adhered to. Further long-term prospective studies are necessary to establish the best manufacturing process for zirconia-based restorations.

Additive manufacturing or 3D printing is becoming an alternative to subtractive manufacturing or milling in the area of computer-aided manufacturing. Research on material for use in additive manufacturing is ongoing, and a wide variety of materials are being used or developed for use in dentistry. Some materials, however, such as cobalt chromium, still lack sufficient research to allow definite conclusions about the suitability of their use in clinical dental practice. Despite this, due to the wide variety of machines that use additive manufacturing, there is much more flexibility in the build material and geometry when building structures compared with subtractive manufacturing. Overall additive manufacturing produces little material waste and is energy efficient when compared to subtractive manufacturing, due to passivity and the additive layering nature of the build process. Such features make the technique suitable to be used with fabricating structures out of hard to handle materials such as cobalt chromium. The main limitations of this technology include the appearance of steps due to layering of material and difficulty in fabricating certain material generally used in dentistry for use in 3D printing such as ceramics. The current pace of technological development, however, promises exciting possibilities.

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The aim of this pilot study was to evaluate the feasibility of a new tooth preparation assessment software, Preppr, as an educational tool for dental students in achieving optimal parameters for a crown preparation. In February 2015, 30 dental students in their fourth year in a five-year undergraduate dental curriculum in New Zealand were randomly selected from a pool of volunteers (N=40) out of the total class of 85. The participants were placed into one of three groups of ten students each: Group A, the control group, received only written and pictorial instructions; Group B received tutor evaluation and feedback; and Group C performed self-directed learning with the aid of Preppr. Each student was asked to prepare an all-ceramic crown on the lower first molar typodont within three hours and to repeat the exercise three times over the next four weeks. The exercise stipulated a 1 mm finish line dimension and total convergence angles (TOC) between 10 and 20 degrees. Fulfillment of these parameters was taken as an acceptable preparation. The results showed that Group C had the highest percentage of students who achieved minimum finish line dimensions and acceptable TOC angles. Those students also achieved the stipulated requirements earlier than the other groups. This study's findings provide promising data on the feasibility of using Preppr as a self-directed educational tool for students training to prepare dental crowns.

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