Basil Al-Amleh

Clinical trials in zirconia: a systematic review

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.

Influence of veneering porcelain thickness and cooling rate on residual stresses in zirconia molar crowns

OBJECTIVE The aim of this study was to investigate the influence of increasing veneering porcelain thickness in clinically representative zirconia molar crowns on the residual stresses under fast and slow cooling protocols. METHODS Six veneered zirconia copings (Procera, Nobel Biocare AB, Gothenburg, Sweden) based on a mandibular molar form, were divided into 3 groups with flattened cusp heights that were 1mm, 2mm, or 3mm. Half the samples were fast cooled during final glazing; the other half were slow cooled. Vickers indentation technique was used to determine surface residual stresses. Normality distribution within each sample was done using Kolmogorov-Smirnov & Shapiro-Wilk tests, and one-way ANOVA tests used to test for significance between various cusp heights within each group. Independent t-tests used to evaluate significance between each cusp height group with regards to cooling. RESULTS Compressive stresses were recorded with fast cooling, while tensile stresses with slow cooling. The highest residual compressive stresses were recorded on the fast cooled 1mm cusps which was significantly higher than the 2 and 3mm fast cooled crowns (P<0.05). There was a significant linear trend for residual stress to decrease as veneering porcelain thickness increased in the fast cooled group (P<0.05). No significant differences were found between the various cusp heights during slow cooling (P≥0.05). SIGNIFICANCE Cooling rate and geometric influences in a crown anatomy have substantially different effects on residual stress profiles with increasing veneering porcelain thickness compared to the basic flat plate model.

Additive Technology: Update on Current Materials and Applications in Dentistry.

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.

Clinical tooth preparations and associated measuring methods: A systematic review

STATEMENT OF PROBLEM The geometries of tooth preparations are important features that aid in the retention and resistance of cemented complete crowns. The clinically relevant values and the methods used to measure these are not clear. PURPOSE The purpose of this systematic review was to retrieve, organize, and critically appraise studies measuring clinical tooth preparation parameters, specifically the methodology used to measure the preparation geometry. MATERIAL AND METHODS A database search was performed in Scopus, PubMed, and ScienceDirect with an additional hand search on December 5, 2013. The articles were screened for inclusion and exclusion criteria and information regarding the total occlusal convergence (TOC) angle, margin design, and associated measuring methods were extracted. The values and associated measuring methods were tabulated. RESULTS A total of 1006 publications were initially retrieved. After removing duplicates and filtering by using exclusion and inclusion criteria, 983 articles were excluded. Twenty-three articles reported clinical tooth preparation values. Twenty articles reported the TOC, 4 articles reported margin designs, 4 articles reported margin angles, and 3 articles reported the abutment height of preparations. A variety of methods were used to measure these parameters. CONCLUSIONS TOC values seem to be the most important preparation parameter. Recommended TOC values have increased over the past 4 decades from an unachievable 2- to 5-degree taper to a more realistic 10 to 22 degrees. Recommended values are more likely to be achieved under experimental conditions if crown preparations are performed outside of the mouth. We recommend that a standardized measurement method based on the cross sections of crown preparations and standardized reporting be developed for future studies analyzing preparation geometry.

Reporting numeric values of complete crowns. Part 1: Clinical preparation parameters

STATEMENT OF PROBLEM An implemented objective measuring system for measuring clinical tooth preparations does not exist. PURPOSE The purpose of this study was to compare clinically achieved tooth preparations for ceramic crowns by general dentists with the recommended values in the literature with an objective measuring method. MATERIAL AND METHODS Two hundred thirty-six stone dies prepared for anterior and posterior complete ceramic crown restorations (IPS e.max Press; Ivoclar Vivadent) were collected from dental laboratories. The dies were scanned and analyzed using the coordinate geometry method. Cross-sectioned images were captured, and the average total occlusal convergence angle, margin width, and abutment height for each preparation was measured and presented with associated 95% confidence intervals. RESULTS The average total occlusal convergence angles for each tooth type was above the recommended values reported in the literature. The average margin widths (0.40 to 0.83 mm) were below the minimum recommended values (1 to 1.5 mm). The tallest preparations were maxillary canines (5.25 mm), while the shortest preparations were mandibular molars (1.87 mm). CONCLUSIONS Complete crown preparations produced in general practice do not achieve the recommended values found in the literature. However, these recommended values are not based on clinical trials, and the effects of observed shortfalls on the clinical longevity of these restorations are not predictable.

Coordinate geometry method for capturing and evaluating crown preparation geometry

STATEMENT OF PROBLEM A validated universal method requiring no human input is needed to capture and evaluate preparation geometries in a manner that can be used to see the correlation of different parameters. PURPOSE The purpose of this study was to present a method of capturing and evaluating crown preparation geometry. MATERIAL AND METHODS One manually machined acrylic resin block and 9 randomly selected preparations for ceramic complete crowns prepared by general dentists were selected and prepared. The specimens were scanned (3D scanner; Nobel Biocare), and buccolingual and mesiodistal cross section images were collected. The images were imported into digitizing software (Engauge Digitizer 4.1) to convert the outlines into x and y coordinates. Six points were chosen by using a set of algorithms, and the resulting parameters were calculated. RESULTS The acrylic resin block was milled with a 12 degree total occlusal convergence (TOC) instrument producing a 12.83 degree TOC. For the other specimens, average TOC values ranged from 18 degrees to 52 degrees. The mean average margin width was 0.70 mm, and the mean average base dimension was 6.23 mm. The surface area/volume ratio, resistance length, and limiting taper were also calculated. CONCLUSIONS The method described provides a basis for accurately evaluating preparation geometry without human input.

Convergence angles and margin widths of tooth preparations by New Zealand dental students

STATEMENT OF PROBLEM Fundamental crown preparation principles are taught and are highly regarded in dental education. Whether tooth preparations made by dental students match these principles is unclear. PURPOSE The purpose of this retrospective study was to report the total occlusal convergence (TOC) and margin widths of crown preparations clinically prepared by New Zealand predoctoral dental students between 2013 and mid-2015. MATERIAL AND METHODS A total of 371 stereolithography files of tooth preparations for metal ceramic crowns prepared by predoctoral dental students were extracted from the Technical Services Laboratory database at the University of Otago. The files were put through the preparation measuring software Preppr, with outputs being TOC angles in faciolingual and mesiodistal cross sections and margin widths in facial, lingual, distal, and mesial aspects. Means, standard deviations, 95% confidence intervals, and distributions using box and whisker graphs were calculated and presented. RESULTS The majority of TOC angles fell within an acceptable range of 10 to 20 degrees; however, the angles ranged from undercuts (<0 degrees) to >60 degrees. The majority of margin widths were between 0.5 and 1 mm, while the maximum was approximately 2 mm and the minimum was 0 mm. CONCLUSIONS Predoctoral dental students in New Zealand are able to produce literature-recommended TOC angles and margin widths for metal ceramic crowns; however, further attention and training are needed for excessive tooth preparations, mainly in the form of large TOC angles.