Objective:To observe and compare the clinical effects of electrosurgical retraction and secondary gingival retraction in fixed prosthodontics.Methods:68 teeth of 36 patients undergoing crown and bridge restoration were randomly distributed into 2 groups.Secondary gingival retraction and electrosurgical retraction were adopted to perform gingival retraction,take impression in sequence and make master cast respectively.Then,the time of gingival retraction and shoulder of preformed teeth,gingival condition and master cast were evaluated after gingival retraction.Results:25 of 33 teeth performed with secondary gingival retraction reached satisfactory level,while 8 teeth were unsatisfactory,with the satisfactory rate of 75.8%.28 of 35 teeth performed with electrosurgical retraction were satisfactory,but 7 teeth were not satisfactory,with the satisfactory rate of 80.0%.Time of gingival retraction by this two methods showed significant statistical difference,while the effects have no statistical difference.Conclusion:Electrosurgical retraction is a relatively good way of gingival retraction and worthy of clinical application because of easy and simple handling,easy exposure of clear dividing line between soft and hard tissue,satisfactory effects of gingival retraction,and quick recovery after surgery.

OBJECTIVETo investigate the effects of different tooth preparations on three-dimensional adaption of computer aided design and computer aided manufacturing (CAD/CAM) crowns based on the reverse engineering.

METHODSThe tooth preparation model of the left maxillary first molar was scanned to build five different tooth preparations using the NX Imageware 13.2 software. The resin cores (n=8) were designed with the exocad software. The scanning data of the inner surface of each resin core were used to analyze the three-dimensional adaption with the Geomagic Qualify 12 software.

RESULTSAccording to the color-coded deviation images, the deviation of the heavy chamfer shoulder was the smallest and the most uniform, whereas the other groups with sharp lines showed large deviation. After statistical analysis, the total deviation of the heavy chamfer shoulder, 135° shoulder, feather shoulder, 90° shoulder with lipped margins, and sharp marginal ridges preparation were (16.88±2.83), (26.88±3.61), (53.56±4.30), (51.38±4.46), and (47.19±4.62) μm. A statistical significance was observed between the heavy chamfer and 135° shoulders (P<0.05). The other groups fitted poorly, without statistical significance between each group (P>0.05).

CONCLUSIONThree-dimensional analysis using a computer is a preferable method to study the three-dimensional adaption of crowns. The heavy chamfer shoulder and round line preparation are clinically recommended. However, feather shoulder, 90° shoulder with lipped margins, and sharp marginal ridge preparation should be avoided.

Composite Resins ; Computer-Aided Design ; Crowns ; Dental Marginal Adaptation ; Dental Prosthesis Design ; Humans ; Molar ; Tooth Preparation ; Tooth Preparation, Prosthodontic

BACKGROUND:In recent years,additive manufacturing(also known as 3D printing)has gradually become the mainstream method for producing titanium alloy brackets for removable partial dentures.Heat treatment,as an important method to improve the mechanical properties of 3D printed titanium alloys,has become a current hot topic of attention.OBJECTIVE:To summarize the main heat treatment technologies currently applied to 3D printed titanium alloy specimens(including annealing,solution aging,hot isostatic pressing,and other heat treatments)and their effects on the mechanical properties and microstructure of 3D printed titanium alloy specimens,providing a theoretical basis for improving the heat treatment technology of removable partial denture titanium alloy supports.METHODS:A computer search was conducted on research materials related to 3D printed titanium alloy heat treatment in databases such as CNKI,PubMed,and ScienceDirect.The search period was from 2012 to 2023.A total of 61 articles were selected based on inclusion and exclusion criteria.RESULTS AND CONCLUSION:(1)Using conventional annealing techniques to treat 3D printed titanium alloy specimens,keeping them at 500-900 ℃ for 2-4 hours,can effectively increase the elongation of 3D printed titanium alloy specimens.(2)Compared to conventional annealing techniques,solid solution aging treatment is more complex,and the titanium alloy specimens after solid solution aging treatment exhibit outstanding yield strength and better corrosion resistance.However,the 3D printed titanium alloy specimens after solid solution aging treatment have no advantage in terms of ductility.(3)Hot isostatic pressing treatment can reduce the internal defects of 3D printed titanium alloy specimens,significantly increase the elongation of 3D printed titanium alloy specimens,and increase their fatigue life.(4)Rapid heat treatment can significantly improve the elongation of 3D printed titanium alloy specimens,and the speed is faster.In terms of elongation improvement and heat treatment efficiency,it has more advantages than conventional annealing in the past.(5)The improvement of elongation of 3D printed titanium alloy specimens by cyclic heat treatment exceeds that of conventional annealing.Cyclic heat treatment can significantly improve the grain structure of 3D printed titanium alloy specimens,but the heat treatment time is too long and the efficiency is low.

Objective To evaluate the accuracy of selective laser melting of cobalt-chromium(CoCr)clasps with different build angles and support densities.Methods The 3D software constructed a sample clasp(reference object)with an undercut depth of 0.50 mm,which was then imported to a metal 3D printer to form CoCr clasps at different build angles(0°,45°,and 90°)and support densities(0.50,0.70,0.90,and 1.10 mm).A model scanner was used to obtain scans of all clasps(test objects);the test objects were aligned with the reference object and test subjects were aligned with each other through a best-fit alignment method using Geomagic software to obtain root mean square error(RMSE)values for the accuracy of the clasps.The data were statistically analyzed using one-way analysis of variance or non-parametric rank sum test(α=0.05).Results In terms of trueness,the different build angle groups with support density of 0.50 mm were ranked by RMSE values as follows:90°group>0°group>45°group.For the different build angle groups with support densities of 0.70,0.90,and 1.10 mm,the RMSE values ranked as follows:45°group>90°group>0°group.In terms of precision,the different build angle groups with support density of 0.50 mm were ranked by RMSE values as follows:90°group>0°group>45°group.For support density of 0.70 mm,the ranking was 90°group>45°group>0°group,while for support densities of 0.90 and 1.10 mm,the ranking was 45°group>90°group>0°group.Conclusion The accuracy of CoCr clasps varies with the build angle and support density.However,clasps produced at a build angle of 0°and a support density of 0.90 mm exhibit higher accuracy and are recommended for use in clinical and technical laboratories.