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1

Hardness investigation of conventional, bulk fill and flowable dental composites

Georgiev G., Dikova T.

Journal of Achievements in Materials and Manufacturing Engineering

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2021

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Vol. 109, nr 2

68--77

EN

Purpose: of the present paper is to investigate the micro-hardness of three types of resin-based composites – conventional, bulk fill and flowable. Design/methodology/approach: Cylindrical specimens with a diameter of 5 mm and thicknesses of 2, 3 and 4 mm were made from each composite. They were light cured for 20, 40 and 60 s with light intensity of 600, 1000 or 1500 mW/cm2. The Vickers micro-hardness was measured on the top and bottom surface of the specimens. Findings: The highest micro-hardness was measured in bulk fill composite, followed by conventional and the lowest was measured in the flowable one. Increasing the light intensity leads to increase of the micro-hardness on both surfaces of the three composites. The increase of the irradiation time results in increase of the micro-hardness mainly on the bottom surface of the composites. The change of the layer thickness influences the conventional and the flowable composites and almost does not affect the hardness of the bulk fill composite. Research limitations/implications: The limitations of this study concerns to the values of the light intensity, which are defined by the light curing unit (LCU) used. There are many LCUs on the market; consequently, constant investigations of dental composites micro-hardness are needed. Practical implications: The investigation of the micro-hardness of the three types of composites in different modes would be very helpful for clinicians to obtain successful polymerization of composite restorations in their everyday practice. Originality/value: The micro-hardness of three types resin-based dental composites – conventional, bulk fill and flowable is investigated and compared in varying of three mode parameters – light intensity, curing time and layer thickness.

2

Study the precision of fixed partial dentures of Co-Cr alloys cast over 3D printed prototypes

Dikova T., Dzhendov D., Katreva I., Tonchev T.

Archives of Materials Science and Engineering

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2018

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Vol. 90, nr 1

25--32

EN

Purpose: of this paper is to investigate the accuracy of Co-Cr dental bridges, manufactured using 3D printed cast patterns. Design/methodology/approach: Four-unit dental bridges are fabricated from the alloys i-Alloy and Biosil-f by lost-wax process. The polymeric cast patterns are 3D printed with different layer’s thickness (13 pm, 35 pm and 50 pm). Two 3D printers are used: stereolithographic “Rapidshape D30” and ink-jet “Solidscape 66+”. The geometrical and fitting accuracy as well as the surface roughness are investigated. Findings: It is established that Co-Cr bridges, casted from 3D printed patterns with 50 pm layer thickness, characterize with the largest dimensions - 3.30%-9.14% larger than those of the base model. Decreasing the layer thickness leads to dimensional reduction. The dimensions of the bridges, casted on patterns with 13 pm layer thickness, are 0.17%-2.86% smaller compared to the primary model. The average roughness deviation Ra of the surface of Co-Cr bridges, manufactured using 3D printed patterns, is 3-4 times higher in comparison to the bridge-base model. The greater the layer thickness of the patterns, the higher Ra of the bridges. The silicone replica test shows 0.1-0.2 mm irregular gap between the bridge retainers and abutments of the cast patterns and Co-Cr bridges. Research limitations/implications: Highly precise prosthetic constructions, casted from 3D printed patterns, can be produced only if the specific features of the 3D printed objects are taken in consideration. Practical implications: Present research has shown that the lower the thickness of the printed layer of cast patterns, the higher the dimensional accuracy and the lower the surface roughness. Originality/value: The findings in this study will help specialist in dental clinics and laboratories to choose the right equipment and optimal technological regimes for production of cast patterns with high accuracy and low surface roughness for casting of precise dental constructions.

3

Prosthetic treatment protocol with fixed dental constructions made on 3D printed cast patterns

Dzhendov D., Katreva I., Dikova T.

Archives of Materials Science and Engineering

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2018

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Vol. 90, nr 1

33--40

EN

Purpose: of the present paper is to develop prosthetic treatment protocol for fixed partial dentures made of 3D printed cast patterns. Design/methodology/approach: The clinical and laboratory protocols for manufacturing of fixed prosthetic constructions upon 3D cast patterns are developed on the basis of the literature review and our previous experimental investigations. Comparison between the conventional technique and innovative approach is made. Findings: The terms "semi-digital treatment plan" and "fully digital treatment plan" are defined according to the way of obtaining data for the virtual 3D model and the production method of the fixed prostheses. A classification of treatment protocols with non-removable partial dentures produced by additive technology is developed. Protocols for "semi" and "fully" digitized treatment plans with fixed partial dentures made by casting with 3D printed models are created. Research limitations/implications: Implementation of the fully digitized protocol for manufacturing of fixed prosthetic constructions via 3D printed prototypes requires specific equipment in the dental office and dental technician laboratory - intraoral scanner and CAD/ CAM system with 3D printing machine. Practical implications: Establishing of systematic clinical and laboratory protocols helps dental specialists to implement the innovative working approach in their practice with no risk of neglecting or omitting of some important procedures which increases the quality and long lasting effect of the dental constructions. Originality/value: Following the developed protocols reduces the role of the subjective factor in production technology of fixed prosthetic constructions while saving labour and time.

4

Development of treatment protocol with selective laser melted fixed partial dentures

Dzhendov D., Katreva I., Dikova T.

Archives of Materials Science and Engineering

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2018

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Vol. 90, nr 2

68--73

EN

Purpose: of the present paper is to offer treatment protocol with fixed partial dentures, produced by selective laser melting, including clinical and laboratory parts. Design/methodology/approach: The treatment protocols with selective laser melted fixed partial dentures was developed on the basis of literature survey and our previous research about accuracy and mechanical properties of dental bridges, manufactured by additive technologies. Findings: The treatment protocol with fixed partial dentures, produced by selective laser melting, consisting of clinical and laboratory parts, was developed. The treatment procedures with FPD made by SLM were classified as semi-digital when working with extraoral scanner and fully-digital - with intraoral scanner. Research limitations/implications: The introduction of the proposed treatment protocol into the clinical and laboratory practice would lead to a systematic approach and working optimization for prosthodontists and dental technicians when using selective laser melting. Practical implications: Due to the elimination of multiple manual manipulations and technological operations, treatment protocols with FPD, produced by SLM, ensure higher accuracy and quality of the constructions and shorter time for their manufacturing compared to the conventional procedure. Originality/value: The developed clinical and laboratory protocols for the treatment and manufacturing of FPD through SLM clearly show the benefits of the new technology in dentistry and dental technician field.

5

Accuracy of polymeric dental bridges manufactured by stereolythography

Dikova T, Dzhendov D., Katreva I., Pavlova D.

Archives of Materials Science and Engineering

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2016

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Vol. 78, nr 1

29--36

EN

Purpose: Purpose of this paper is to investigate the accuracy of dental bridges produced via digital light projection stereolithography process. Design/methodology/approach: 3D printer Rapidshape D30 was used for manufacturing of two groups of samples – temporary four-part bridges and cast patterns for permanent bridges. The temporary bridges were made of NextDent C+B polymer, while the cast patterns - of NextDent Cast. The samples were manufactured with different layer’s thickness (0.035 mm and 0.050 mm). The geometrical and adjusting accuracy were investigated by measuring of the samples’ dimensions and silicone probe, while the surface roughness was studied out by profile meter and optical microscopy. Findings: It was established that the dimensions of the temporary bridges and the cast patterns, printed with layer thickness 50 µm, are larger than that of the virtual 3D model with 0.1-0.3 mm. Decreasing the layer thickness to 35 µm leads to 0.29%-1.10% smaller sizes of dental bridges and cast patterns in comparison to that of the virtual 3D model. The average roughness deviation Ra of the 3D printed temporary bridges and cast patterns is larger than that of the initial model. As the surface roughness depends on the layer’s thickness, the samples with 0.035 mm layer characterize with lower Ra values. The silicone probe shows that the temporary bridges as well as the cast patterns need additional adjusting in the dental office or corrections during design of the virtual 3D model and 3D printing process in the dental laboratory. Practical implications: The stereolithography as part of CAD/CAM manufacturing process characterizes with high accuracy as a whole. But present study reveals that additional adjusting or preliminary corrections of the design of 3D printing process are needed for dental constructions produced by SLA. Originality/value: The geometrical and fitting accuracy as well as the surface roughness of dental bridges, produced by stereolithography were evaluated. The data, shown in the present study, will help dentists and dental technicians to precise the manufacturing regimes for production of dental constructions with high accuracy.

6

Microstructure and hardness of fixed dental prostheses manufactured by additive technologies

Dikova T., Dzhendov D., Simov M.

Journal of Achievements in Materials and Manufacturing Engineering

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2015

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Vol. 71, nr 2

60--69

EN

Purpose: The additive technologies characterize with building of one layer at a time from a powder or liquid that is bonded by means of melting, fusing or polymerization. The methods, mostly used in dentistry, include selective laser sintering, selective laser melting and 3D printing. The aim of the present paper is to investigate the microstructure and hardness of fixed dental prostheses produced by three different technologies. Design/methodology/approach: Four-part dental bridges were manufactured of Co-Cr alloy by standard lost-wax process, casting of 3D printed wax models and Selective Laser Melting (SLM). The microstructure was investigated by optical microscopy and SEM. EDX and EPMA analyses and Vickers microhardness measurements was done. Findings: It was established that the microstructure of cast samples is dense, inhomogeneous, consisting of large grains with dendrite morphology, while the microstructure of the SLM bridges is porous. Pores, elongated along the direction of the melted layers were observed. The microhardness investigations showed highest average hardness of the samples, produced by SLM (356HV-407HV), followed by the hardness of the samples, cast by 3D printed models (327HV-343HV) and these, manufactured by standard lost-wax process (251HV-274HV). The measurements along depth of the samples showed nearly even microhardness distribution in the bridges, produced by SLM, and fluctuations of the microhardness values along the depth of the cast bridges due to the inhomogeneous microstructure. Research limitations/implications: As the additive technologies for production of dental restorations from wax, polymers and metal alloys are developed last years, additional investigations are needed for development of more precise technological regimes. Originality/value: The comparison between the microstructure and hardness of dental prostheses made by lost-wax process and SLM reveals the peculiarities of the constructions produced by new technology.