Dental displacement in complete dentures influenced by different flask types and polymerization cycles.

This study evaluated the dental displacement in dentures included in different flasks and processed by the Australian cycle modified or by microwave. Metal pins were placed on the teeth as reference for measurements: a) Incisal edge of maxillary and mandibular central incisors, buccal cusp of maxillary and mandibular vestibular first premolars, and mesiobuccal maxillary and mandibular cusps of second molars; b) Incisor to incisor, premolar to premolar and molar to molar; c) Left incisor to left molar and right incisor to right molar, and d) Vertical. Dentures were divided according to flasks (n=10): G1- Metal; G2- HH bimaxillary metal; G3-Microwaves; G4- HH bimaxillary microwaves. For G1 and G2, polymerization at 65ºC/30 min, flame quenched for 30 min, heating at 65ºC/30 min, boiling water for 1 h. For G3 and G4, microwave (20 min/140 W and 5 min/560 W). Comparator microscope with digital camera and analytical unit assessed the measurements before and after denture polymerization. The final distance was made subtracting the distance before the denture processing from the distance of the dental displacement after processing. The value of the difference was submitted to 2-way ANOVA, considering the flask type and denture type. Maxillary denture showed greater displacement when compared to mandibular denture. Flask types and interaction with denture types showed similar difference between before and after polymerization. In conclusion, displacement promoted by flask types in dentures cured by polymerization cycles promoted similar effect on the distance between teeth. Greater value for distance between teeth occurred for maxillary denture.

Dental displacement in complete dentures influenced by different flask types and polymerization cycles

Abstract This study evaluated the dental displacement in dentures included in different flasks and processed by the Australian cycle modified or by microwave. Metal pins were placed on the teeth as reference for measurements: a) Incisal edge of maxillary and mandibular central incisors, buccal cusp of maxillary and mandibular vestibular first premolars, and mesiobuccal maxillary and mandibular cusps of second molars; b) Incisor to incisor, premolar to premolar and molar to molar; c) Left incisor to left molar and right incisor to right molar, and d) Vertical. Dentures were divided according to flasks (n=10): G1- Metal; G2- HH bimaxillary metal; G3-Microwaves; G4- HH bimaxillary microwaves. For G1 and G2, polymerization at 65ºC/30 min, flame quenched for 30 min, heating at 65ºC/30 min, boiling water for 1 h. For G3 and G4, microwave (20 min/140 W and 5 min/560 W). Comparator microscope with digital camera and analytical unit assessed the measurements before and after denture polymerization. The final distance was made subtracting the distance before the denture processing from the distance of the dental displacement after processing. The value of the difference was submitted to 2-way ANOVA, considering the flask type and denture type. Maxillary denture showed greater displacement when compared to mandibular denture. Flask types and interaction with denture types showed similar difference between before and after polymerization. In conclusion, displacement promoted by flask types in dentures cured by polymerization cycles promoted similar effect on the distance between teeth. Greater value for distance between teeth occurred for maxillary denture.

Resumo O objetivo neste estudo foi avaliar o deslocamento dental em próteses totais incluídas em diferentes tipos de muflas e polimerizadas pelo ciclo australiano ou por micro ondas. Pinos metálicos foram colocados nos dentes como pontos de referência para as medidas: a) Borda incisal dos incisivos centrais maxilares e mandibulares, cúspide vestibular dos primeiros pré molares maxilares e mandibulares e cúspides mésiovestibulares dos segundos molares maxilares e mandibulares; b) Distâncias látero-laterais incisivo a incisivo, pré-molar a pré-molar e molar a molar; c) Distâncias horizontais anteroposterior incisivo esquerdo a molar esquerdo e incisivo direito a molar direito, e d) Vertical. As próteses foram separadas de acordo com o tipo de mufla (n=20): G1- Metálica; G2-HH bi-maxilar metálica; G3- Micro-ondas; G4- HH bi-maxilar micro ondas. Ciclo de polimerização para G1 e G2, água a 65ºC por 30 minutos, aquecimento interrompido por 30 minutos, reaquecimento a 65ºC por 30 minutos, seguido de água fervente por 1 hora. Para G3 e G4, micro ondas (20 minutos a 140 W e 5 minutos a 560 W). As medidas foram avaliadas com microscópio comparador linear. A distância final entre os dentes foi obtida subtraindo o valor da distância antes do processamento da prótese do valor da distância resultante do deslocamento dentário após o processamento. A diferença obtida entre essas distâncias foi submetida à ANOVA de 2 fatores, considerando como variáveis tipo de mufla e tipo de prótese. A prótese maxilar apresentou maior deslocamento dental quando comparada à mandibular. Os tipos de muflas e a interação com os tipos de próteses mostraram movimentos dentais similares antes e depois da polimerização. Em conclusão, o deslocamento dental promovido por diferentes tipos de frascos em próteses curadas por diferentes ciclos de polimerização promoveu efeito semelhante na alteração da distância entre os dentes. Maior valor para a mudança de distância entre os dentes ocorreu na prótese maxilar.

Can ceramic veneer spark erosion and mechanical cycling affect the accuracy of milled complete-arch frameworks supported by 6 implants?

STATEMENT OF PROBLEM: Milling is a well-established method for manufacturing prosthetic frameworks. However, information about the influence of ceramic veneer and spark erosion on the accuracy of the all-on-six complete-arch fixed frameworks manufactured from different materials is lacking. PURPOSE: The purpose of this in vitro study was to compare the accuracy of milled complete-arch fixed frameworks with zirconia, cobalt-chromium, and titanium at different steps of their manufacturing process and the influence of mechanical cycling. MATERIAL AND METHODS: Fifteen milled complete-arch fixed frameworks, supported by 6 implants, were made in zirconia, cobalt-chromium, and titanium (n=5). The fit was measured by the single-screw test protocol. Stress was measured by photoelastic analysis. The loosening torque was evaluated by tightening the screws, retightening them after 10 minutes, and then evaluating the loosening torque 24 hours later. Thereafter, all frameworks received ceramic veneer, and the previous tests were repeated. Cobalt-chromium and titanium frameworks received spark erosion after ceramic veneer, and all analyses were repeated. Before and after mechanical cycling, loosening torque was evaluated. The results were subjected to 2-way repeated-measures ANOVA and the Bonferroni test (α=.05). RESULTS: Titanium presented higher fit values than zirconia (P=.037) and similar to cobalt-chromium frameworks (P>.05) at baseline. After ceramic veneer, higher fit levels were observed for zirconia (P=.001) and cobalt-chromium (P=.008). Titanium showed higher stress values (P<.05) regardless of time. Baseline for all materials presented lower stress values (P<.05). Higher loosening torque values were found for the titanium group at baseline (P<.001) and after ceramic veneer (P<.001). Spark erosion improved fit and loosening torque values only for cobalt-chromium (P<.05). Mechanical cycling did not influence the loosening torque (P>.05). CONCLUSIONS: Titanium milled complete-arch fixed frameworks presented poorer fit values than zirconia, although the loosening torque at baseline was higher. Ceramic veneer increased the fit levels for zirconia and cobalt-chromium, decreased the loosening torque values for cobalt-chromium, and enhanced stress levels. Spark erosion can be a reliable technique to improve fit and loosening torque for cobalt-chromium frameworks. Mechanical cycling did not decrease loosening torque.

Is one dental mini-implant biomechanically appropriate for the retention of a mandibular overdenture? A comparison with Morse taper and external hexagon platforms.

STATEMENT OF PROBLEM: Limited information is available to clinicians on the use of dental mini-implants (MI) as opposed to standard-diameter implants (SDIs) for the stabilization of implant-retained mandibular overdentures (MOs). PURPOSE: The purpose of this in vitro and finite element analysis study was to analyze and compare the biomechanical behavior of MOs with either 1 or 2 implants with external hexagon (EH), Morse taper (MT) SDIs, and MIs. MATERIAL AND METHODS: Thirty photoelastic models (n=30) of each group (n=5) of SDIs (EH-1, EH-2, MT-1, MT-2) and MI (MI-1, MI-2) were fabricated for posterior, peri-implant, and total maximum shear stress evaluation by quantitative photoelastic analysis. One specimen of each group was further used to create the 6 computational models to be analyzed by finite element analysis. The maximum von Mises values and stress maps were plotted for each ductile component. Two types of load were applied to the overdenture: a150-N load bilaterally and simultaneously on the first molar and a 100-N load on the incisal edge of the central incisors at a 30-degree angle. The data were subjected to the 2-way ANOVA test and the Tukey honestly significant difference test (α=.05). RESULTS: The EH-2 and MT-2 showed the lowest posterior (P<.001) and total (P<.05) mean shear stress values. For peri-implant shear stress, no difference was found among all groups (P>.05). Regardless of the loading area, the MI-1 and MI-2 groups showed the lowest von Mises stress values. However, for implant housing, the MI-1 group, under incisor loading, presented greater stress, followed by MT-1, EH-1, EH-2, MI-2, and MT-2. The attachment was the most overloaded structure, with high values under incisor loading, especially for the groups with 2 implants (MT-2, EH-2) as compared with the other models. CONCLUSIONS: Biomechanically, regardless of the implant number, MI is a promising rehabilitation method with similar peri-implant shear stress and lower von Mises stress on the implant when compared with SDIs for MOs.

3D metal printing in dentistry: An in vitro biomechanical comparative study of two additive manufacturing technologies for full-arch implant-supported prostheses.

The use of 3D technologies is progressing in the dental field. However, little is known about the biomechanical behavior of the additive manufacturing of full-arch fixed dental prostheses (FAFDPs) for the establishment of clinical protocols. We investigated the influence of three CAD/CAM technologies: milling (control), Selective Laser Melting (SLM) and Electron Beam Melting (EBM) for FAFDP manufacturing. Also, the effects of ceramic veneer and spark erosion on marginal misfits of FAFDPs, the stability of prosthetic screws, strain and stress on the implant-supported system, as well as the effect of chewing simulation on screw stability were evaluated. Fifteen Ti-6Al-4V alloy FAFDPs were obtained by means of CAD/CAM systems: milling, SLM and EBM (n = 5/group). The marginal misfit was analyzed according to the single-screw test protocol. Screw stability was analyzed by screw-loosening torque. Strain-gauge analysis investigated the strain on the mini-abutment analog, and photoelastic analysis investigated the stress on the peri-implant region. Subsequently, all frameworks underwent ceramic veneer and spark erosion procedures. Marginal misfit, screw-loosening and strain and stress analyses were assessed after each evaluation time: initial, ceramic veneer and spark erosion. Finally, all prostheses were subjected to 106 mechanical cycles (2 Hz/150 N), and screw-loosening was re-evaluated. Data were subjected to two-way ANOVA for repeated measures, and the Bonferroni test as a post hoc technique (α = 0.05). At the initial time, the milling group presented the lowest marginal misfit (p < 0.001). Ceramic veneer did not alter marginal misfit for all groups (p > 0.05); spark erosion decreased the misfit values for the SLM and EBM groups (p < 0.05). Evaluation time did not alter screw-loosening values for all groups (p = 0.191), although the milling group presented the highest screw-loosening values (p < 0.05). Ceramic veneer and spark erosion reduced strain in the components regardless of the manufacturing technology used (p < 0.05). The milling group presented the lowest stress values regardless of evaluation time (p = 0.001), and lower stress values were found after spark erosion regardless of the manufacturing group (p = 0.016). In conclusion, although milled frameworks exhibited the best biomechanical behavior, frameworks manufactured by additive technologies presented acceptable values of screw-loosening torque, strain and stress. Ceramic veneer did not negatively interfere in the biomechanical tests of the study, and clinically acceptable marginal misfit was achieved after spark erosion. Therefore, such 3D printing technologies seem to be feasible for the manufacturing of full-arch implant-supported frameworks.

Biomechanical behavior of CAD/CAM cobalt-chromium and zirconia full-arch fixed prostheses.

PURPOSE: To verify the influence of computer-aided design/computer-aided manufacturing (CAD/CAM) implant-supported prostheses manufactured with cobalt-chromium (Co-Cr) and zirconia (Zr), and whether ceramic application, spark erosion, and simulation of masticatory cycles modify biomechanical parameters (marginal fit, screw-loosening torque, and strain) on the implant-supported system. MATERIALS AND METHODS: Ten full-arch fixed frameworks were manufactured by a CAD/CAM milling system with Co-Cr and Zr (n=5/group). The marginal fit between the abutment and frameworks was measured as stated by single-screw test. Screw-loosening torque evaluated screw stability, and strain analysis was explored on the implant-supported system. All analyses were performed at 3 distinct times: after framework manufacturing; after ceramic application in both materials' frameworks; and after the spark erosion in Co-Cr frameworks. Afterward, stability analysis was re-evaluated after 106 mechanical cycles (2 Hz/150-N) for both materials. Statistical analyses were performed by Kruskal-Wallis and Dunn tests (α=.05). RESULTS: No difference between the two materials was found for marginal fit, screw-loosening torque, and strain after framework manufacturing (P>.05). Ceramic application did not affect the variables (P>.05). Spark erosion optimized marginal fit and strain medians for Co-Cr frameworks (P<.05). Screw-loosening torque was significantly reduced by masticatory simulation (P<.05) regardless of the framework materials. CONCLUSION: Co-Cr and Zr frameworks presented similar biomechanical behavior. Ceramic application had no effect on the biomechanical behavior of either material. Spark erosion was an effective technique to improve Co-Cr biomechanical behavior on the implant-supported system. Screw-loosening torque was reduced for both materials after masticatory simulation.

Dimensional precision of implant-supported frameworks fabricated by 3D printing.

STATEMENT OF PROBLEM: Selective laser melting (SLM) is a promising additive technology for clinical practice, but data on dimensional precision assessed by marginal fit combined with stress and strain investigations of implant-supported fixed partial dentures (FPDs) are lacking. PURPOSE: The purpose of this in vitro study was to verify whether the SLM additive manufacturing technology provides better dimensional precision for 3-unit FPD frameworks than subtractive manufacturing with soft metal block (SMB) milling and the standard casting technique. MATERIAL AND METHODS: Thirty 3-unit implant-supported FPDs with Co-Cr frameworks were made by the casting, SMB milling, and SLM methods (n=10). The marginal fit between the framework and the implant abutment was evaluated with photoelastic (PH) and strain gauge (SG) models. Stress and strain in the implant-supported system were measured by quantitative PH and SG analyses after prosthetic screw tightening. Data were subjected to the Kruskal-Wallis test, Mann-Whitney U test, and Spearman correlation test (α=.05). RESULTS: The framework manufacturing method affected the marginal fit (P<.001), stress, and strain values (P<.05). The SLM group showed the best mean ±standard deviation marginal fit (µm) (PH model: 8.4 ±3.2; SG model: 6.9 ±2.1) in comparison with SMB milling (PH model: 42.3 ±15.7; SG model: 41.3 ±15.3) and casting (PH model: 43.5 ±27.8; SG model: 41.3 ±24.6) (P<.05). SLM showed lower mean ±standard deviation stress and strain values (60.3 ±11.6 MPa; 91.4 ±11.1 µstrain) than casting (225.5 ±142.8 MPa; 226.95 ±55.4 µstrain) and SMB milling (218.6 ±101.7 MPa; 289.7 ±89.3 µstrain) (P<.05). A positive correlation was observed between fit and stress or strain for all groups (P<.05). CONCLUSIONS: Three-unit FPD frameworks made using the SLM technology showed better dimensional precision than those obtained with the casting or SMB milling methods.

Biomechanical behavior of 2-implant- and single-implant-retained mandibular overdentures with conventional or mini implants.

STATEMENT OF PROBLEM: The use of single or mini dental implants to retain mandibular overdentures is still questionable. PURPOSE: The purpose of this finite element analysis (FEA) study was to investigate the biomechanical behavior of 2- and single-implant-retained mandibular overdentures with conventional or mini implants. MATERIAL AND METHODS: Four 3-dimensional (3D) finite element models were constructed with the following designs of mandibular overdentures: 2 (group 2-C) and single (group 1-C) conventional external hexagon implants with ball or O-ring attachment and 2 (group 2-M) and single (group 1-M) 1-piece mini implants. A 150-N axial load was applied bilaterally and simultaneously on the first molar. Overdenture displacement, von Mises equivalent stress (implants and/or prosthetic components), and maximum principal stresses (peri-implant bone) were recorded numerically and then color-coded and compared among the groups. RESULTS: The overdenture displacement (in mm) was higher for the 1-M (0.16) and 2-M (0.17) groups when compared with 1-C (0.09) and 2-C (0.08). Irrespective of the type of implant, the single-implant groups presented higher values of stress (in MPa) on the implants than did the 2-implant groups (1-C=52.53; 1-M=2.95; 2-C=34.66; 2-M=2.37), ball attachment (1-C=201.33; 2-C=159.06), housing or O-ring (1-C=125.01; 1-M=1.96; 2-C=88.84; 2-M=1.27), and peri-implant cortical bone (1-C=19.37; 1-M=1.47; 2-C=15.70; 2-M=1.06). The mini implant overdentures presented lower stress values on the implants, housing or O-ring, and peri-implant bone than did the conventional implant overdentures, regardless of the number of implants. CONCLUSIONS: The 2-implant-retained overdentures exhibited lower stresses than the single- implant-retained overdentures, irrespective of the type of implant. The mini implants demonstrated higher overdenture displacement and lower stresses than did conventional implant overdentures for single- and 2-implant-retained overdentures.

Influence of spark erosion on the fit of screw-retained Co-Cr fixed complete denture frameworks veneered with different materials.

STATEMENT OF PROBLEM: Spark erosion is a fit corrective technology that can be used even after the veneering material has been applied. The framework does not require sectioning, thus preserving its mechanical resistance. However, the spark erosion effect on veneered Co-Cr fixed complete denture (FCD) frameworks has not been investigated. PURPOSE: The purpose of this in vitro study was to evaluate whether spark erosion is effective in improving marginal fit on screw-retained Co-Cr FCD frameworks veneered with different materials. A comparison between ceramic applications and simulated ceramic firing cycles was also investigated. MATERIAL AND METHODS: Forty FCD frameworks were fabricated with a Co-Cr alloy. Four groups (n=10) were obtained according to the veneer material used on frameworks: HR (heat-polymerized resin); LR (light-polymerized resin); C (ceramic); and SC (simulated ceramic firing cycle). The spark erosion process was conducted for all groups. The marginal fit was analyzed according to the single-screw test protocol, and the measurements were performed at 3 evaluation times: initial, after veneer material application, and after spark erosion process. The results were submitted to a 2-way repeated measures ANOVA and the Tukey honest significant differences test (α=.05). RESULTS: Poorer marginal fit (in micrometers) was noted after veneer material application, where the HR and C groups presented the worst values (HR: 170; LR: 72; C: 165; SC: 86; P<.05). The spark erosion process was effective in improving the fit for all groups (HR: 109; LR: 52; C: 110; SC: 60; P<.05). CONCLUSIONS: Spark erosion improved the fit of Co-Cr FCD frameworks veneered with different materials. An actual ceramic application should be used to assess distortions generated by veneer material application instead of using only simulated ceramic firing cycles.

Photoelastic and finite element stress analysis reliability for implant-supported system stress investigation

Aim: To compare the reliability between photoelastic and finite element (FE) analyses by evaluating the effect of different marginal misfit levels on the stresses generated on two different implant-supported systems using conventional and short implants. Methods: Two photoelastic models were obtained: model C with two conventional implants (4.1×11 mm); and model S with a conventional and a short implant (5×6 mm). Three-unit CoCr frameworks were fabricated simulating a superior first pre-molar (P) to first molar (M) fixed dental prosthesis. Different levels of misfit (µm) were selected based on the misfit average of 10 frameworks obtained by the single-screw test protocol: low (<20), medium (>20 and <40) and high (>40). Stress levels and distribution were measured by photoelastic analysis. A similar situation of the in vitro assay was designed and simulated by the in silico analysis. Maximum and minimum principal strain were recorded numerically and color-coded for the models. Von Mises Stress was obtained for the metallic components. Results: Photoelasticity and FE analyses showed similar tendency where the increase of misfit generates higher stress levels despite of the implant design. The short implant showed lower von Mises stress values; however, it presented stresses around its full length for the in vitro and in silico analysis. Also, model S showed higher µstrain values for all simulated misfit levels. The type of implant did not affect the stresses around pillar P. Conclusions: Photoelasticity and FEA are reliable methodologies presenting similarity for the investigation of the biomechanical behavior of implant-supported rehabilitations

The role of welding techniques in the biomechanical behavior of implant-supported prostheses.

This in vitro study investigated the role of welding techniques of implant-supported prostheses in the 2D and 3D marginal misfits of prosthetic frameworks, strain induced on the mini abutment, and detorque of prosthetic screws. The correlations between the analyzed variables were also investigated. Frameworks were cast in commercially pure titanium (cp-Ti). A marginal misfit of 200µm was simulated in the working models (control group) (n=20). The 2D marginal misfit was analyzed according to the single-screw test protocol using a precision optical microscope. The 3D marginal misfit was performed by X-ray microtomography. Strain gauge analysis was performed to investigate the strain induced on the mini abutment. A digital torque meter was used for analysis of the detorque and the mean value was calculated for each framework. Afterwards, the frameworks were divided into two experimental groups (n=10): Laser (L) and TIG (T). The welding techniques were performed according to the following parameters: L (390V/9ms); T (36A/60ms). The L and T groups were reevaluated according to the marginal misfit, strain, and detorque. The results were submitted to one-way ANOVA followed by Tukey's HSD test and Person correlation analysis (α=0.05). Welding techniques statistically reduced the 2D and 3D marginal misfits of prosthetic frameworks (p<0.001), the strain induced on the mini abutment replicas (p=0.006), and improved the screw torque maintenance (p<0.001). Similar behavior was noted between L and T groups for all dependent variables (p>0.05). Positive correlations were observed between 2D and 3D marginal misfit reading methods (r=0.943, p<0.0001) and between misfit and strain (2D r=0.844, p<0.0001 and 3D r=0.864, p<0.0001). Negative correlation was observed between misfit and detorque (2D r=-0.823, p=0.003 and 3D r=-0.811, p=0.005). In conclusion, the welding techniques improved the biomechanical behavior of the implant-supported system. TIG can be an acceptable and affordable technique to reduce the misfit of 3-unit Ti frameworks.

Marginal fit and photoelastic stress analysis of CAD-CAM and overcast 3-unit implant-supported frameworks.

STATEMENT OF PROBLEM: Several studies have shown the superiority of computer-assisted design and computer-assisted manufacturing (CAD-CAM) technology compared with conventional casting. However, an advanced technology exists for casting procedures (the overcasting technique), which may serve as an acceptable and affordable alternative to CAD-CAM technology for fabricating 3-unit implant-supported fixed dental prostheses (FDPs). PURPOSE: The purpose of this in vitro study was to evaluate, using quantitative photoelastic analysis, the effect of the prosthetic framework fabrication method (CAD-CAM and overcasting) on the marginal fit and stress transmitted to implants. The correlation between marginal fit and stress was also investigated. MATERIAL AND METHODS: Three-unit implant-supported FDP frameworks were made using the CAD-CAM (n=10) and overcasting (n=10) methods. The frameworks were waxed to simulate a mandibular first premolar (PM region) to first molar (M region) FDP using overcast mini-abutment cylinders. The wax patterns were overcast (overcast experimental group) or scanned to obtain the frameworks (CAD-CAM control group). All frameworks were fabricated from cobalt-chromium (CoCr) alloy. The marginal fit was analyzed according to the single-screw test protocol, obtaining an average value for each region (M and PM) and each framework. The frameworks were tightened for the photoelastic model with standardized 10-Ncm torque. Stress was measured by quantitative photoelastic analysis. The results were submitted to the Student t test, 2-way ANOVA, and Pearson correlation test (α=.05). RESULTS: The framework fabrication method (FM) and evaluation site (ES; M and PM regions) did not affect the marginal fit values (P=.559 for FM and P=.065 for ES) and stress (P=.685 for FM and P=.468 for ES) in the implant-supported system. Positive correlations between marginal fit and stress were observed (CAD-CAM: r=0.922; P<.001; overcast: r=0.908; P<.001). CONCLUSIONS: CAD-CAM and overcasting methods present similar marginal fit and stress values for 3-unit FDP frameworks. The decreased marginal fit of frameworks induces greater stress in the implant-supported system.