Soft Denture Liner Adhesion to Conventional and CAD/CAM Processed Poly(Methyl Methacrylate) Acrylic Denture Resins-An In-Vitro Study.

This study aimed to evaluate the airborne-particle abrasion surface treatment effects on the tensile bond strength (TBS) between resilient denture liner and CAD/CAM or conventional heat polymerized poly (methyl methacrylate) (PMMA) acrylic denture resins. A total of 48 dumbbell-shaped specimens (70 mm in total length, and 12 mm and 7 mm in diameter at the thickest and thinnest section, respectively) were prepared from CAD/CAM and conventional acrylic resins. Before relining with denture liner, 12 specimens from each material were surface-treated by 110 µm Al2O3 airborne-particle abrasion, and the remaining specimens served as control (no treatment). Following relining, all the specimens were aged by thermal cycling (1000 cycles, 5-55 °C). The TBS of denture liner to acrylic denture resins was tested in a universal testing apparatus at a 5 mm/min crosshead speed. The debonded surfaces were visually examined for the failure modes. ANOVA and multiple comparisons posthoc analysis tests were applied to determine the significant difference in TBS between the study groups (α = 0.05). A significant difference in TBS was observed between the control and surface treated groups (p < 0.001) for both acrylic resins materials. However, there was no statistically significant difference in bond strength between the acrylic resins materials (p = 0.739). Surface treatment with airborne-particle abrasion demonstrated increased TBS of the soft denture liners to acrylic resins. The TBS of conventional and CAD/CAM acrylic resins to soft denture liners were not considerably different.

Color stability of 3D-printed denture resins: effect of aging, mechanical brushing and immersion in staining medium.

PURPOSE: This in-vitro study evaluated and compared the color stability of 3D-printed and conventional heat-polymerized acrylic resins following aging, mechanical brushing, and immersion in staining medium. MATERIALS AND METHODS: Forty disc-shaped specimens (10 mm in diameter and 3 mm thick) were prepared from two 3D-printed [DentaBASE (DB) and Denture 3D+ (D3D)] and one conventional polymethylmethacrylate (PMMA) denture materials. The specimens were thermo-cycled, subjected to mechanical brushing, and were immersed in either coffee, lemon juice, coke, or artificial saliva (AS) to simulate one and two years of oral use. Color measurements of the specimens were recorded by a spectrophotometer at baseline (T0), and after one (T1) and two years (T2) of simulation. The color changes (∆E) were determined and also quantified according to the National Bureau of Standards (NBS) units. Descriptive statistics, followed by factorial ANOVA and Bonferroni post-hoc test (α=.05), were applied for data analysis. RESULTS: The independent factors, namely material, staining medium, and immersion time, and interaction among these factors significantly influenced ∆E (P<.009). Irrespective of the materials, treatments, and time, the highest and the lowest mean ∆Es were observed for PMMA in lemon juice (4.58 ± 1.30) and DB in AS (0.41 ± 0.18), respectively. Regarding the material type, PMMA demonstrated the highest mean ∆E (2.31 ± 1.37), followed by D3D (1.67 ± 0.66), and DB (0.85 ± 0.52), and the difference in ∆E between the materials were statistically significant (P<.001). All the specimens demonstrated a decreased color changes at T2 compared to T1, and this difference in mean ∆E was statistically significant (P<.001). CONCLUSION: The color changes of 3D-printed denture resins were low compared to conventional heat polymerized PMMA. All the tested materials, irrespective of the staining medium used, demonstrated a significant decrease in ∆E values over time.

Effect of different maintenance time of torque application on detorque values of abutment screws in full-arch implant-supported fixed prostheses.

BACKGROUND: The effect of different maintenance time of torque application and screw loosening in full-arch implant-supported prosthesis remains uninvestigated. PURPOSE: To examine the effect of different maintenance time of torque application on detorque values of implant abutment screw in full-arch implant-supported fixed complete denture. MATERIALS AND METHODS: Passively fitting framework supported by four implants stabilized on resin model torqued to 35 N-cm and maintained for different times; instant torque application (protocol A), 10 seconds (protocol B), and 30 seconds (protocol C) were used. Detorque values were recorded during removal of the screws. Comparison between mean torque and detorque values were made using paired sample t-test. The mean removal torque values of each protocol were compared using three-way analysis of variance (ANOVA). RESULTS: The mean removal torque values were lower than the applied torque for all the protocols. The highest mean removal torque value was found in the immediate protocol (A) (24.44 ± 1.7), followed by the 30 seconds protocol (C) (23.37 ± 1.75), and then by the 10 seconds protocol (B) (23.35 ± 1.6). All these differences were found to be statistically significant between torque and detorque values (P = .001). However, the differences among detorque values were not statistically significant (P > .05). CONCLUSION: The application of 35 N-cm for different maintenance time of torque application on implant abutment screw did not appear to affect the detorque value in a multiple implant-supported fixed prosthesis. Maintaining the torque for prolonged time (10 seconds or 30 seconds) was not significantly associated with higher preload than instant torque application in full-arch implant-supported prosthesis.

The Effect of Torque Application Technique on Screw Preload of Implant-Supported Prostheses.

PURPOSE: To examine the effect of different torque application techniques (torqued, retorqued once, and retorqued twice) on the removal torque of implant-supported fixed complete dental prostheses. MATERIALS AND METHODS: Four Nobel Biocare implants (4.3 × 13 + 3; 13-mm thread height + 3-mm collar height) were stabilized temporarily inside four holes made in an acrylic mandibular master model. A metal framework was constructed, casted, and finished using a standardized technique. A passively fitting framework was achieved by removing the implants from the acrylic master model and hand-screwing them to the metal framework. Then, the whole assembly was restabilized in the acrylic master model. The torque experiment consisted of three protocols: (1) torquing screws to 35 Ncm a single time; (2) torquing the screws to 35 Ncm and then immediately retorquing the same screws to the same value; and (3) torquing the same screws to 35 Ncm three consecutive times. Removal torque was recorded for each implant using a digital torque meter. RESULTS: The highest torque value was recorded for the retorqued-once application technique (29.5 ± 1.5 Ncm); next was the torqued technique (27.9 ± 0.7 Ncm); and, last was the retorqued-twice technique (27.2 ± 1.6 Ncm). The Games-Howell post hoc test showed that the retorqued-once application technique resulted in significantly higher torque values than the torqued and retorqued-twice torque application techniques (P ≤ .05). CONCLUSION: Retightening abutment screws once after the initial torquing could enhance the removal torque of the screw. Care must be taken when retorquing the screws more than once, as this may inversely affect the removal torque.

Effect of Single Off-Axis Implant Placement on Abutment Screw Stability Under Lateral Loading.

PURPOSE: The aim of this study was to evaluate the effect of off-axis implant placement in relation to the prosthetic crown on abutment screw loosening under different directions of twisting moments. MATERIALS AND METHODS: Twenty-one implant assemblies were divided equally into three groups (n = 7). Each assembly consisted of an internal-octagon implant measuring 4.1 × 12 mm (standard plus implants) with corresponding 5.5-mm synocta abutments (Straumann) mounted in an epoxy resin-glass fiber composite. Group A had a straight implant configuration in relation to the prosthetic crown and was subjected to clockwise twisting moment (control). The other two groups, B and C, with off-axis implant placement were subjected to clockwise and counterclockwise moments, respectively. A lateral load of 80 N was applied to the specimens for 1 × 10(6) cycles at 90 degrees to the long axis of the implant. The loading point was 4 mm from the center of the implant in group A and 6 mm in groups B and C. The reverse torque value (RTV) of the abutment screw was measured before and after loading. Data were analyzed using a paired samples t test and one-way analysis of variance (ANOVA) at a significance level of α = .05. RESULTS: An increase in postloading RTV was found relative to preloading RTV in all groups, but was only significant in group A (P < .05). The mean reverse torque difference (RTD) was higher in group A (3.17 ± 1.04 Ncm), followed by groups B (1.03 ± 1.41 Ncm) and C (0.43 ± 1.09 Ncm). A significant difference in RTD was noted between group A and the remaining groups (P < .05). However, no significant difference was found between groups B and C (P > .05). CONCLUSION: Placement of an implant in an off-axis relation to the prosthetic crown resulted in significantly lower reverse torque values compared with straight implant configuration. However, the extent of reduction in Ncm is small and is considered clinically insignificant in the studied implant system. Furthermore, no significant difference was found in RTV between clockwise and counterclockwise twisting moments.

The effects of two torque values on the screw preload of implant-supported prostheses with passive fit or misfit.

PURPOSE: To determine the effect of increased torque of the abutment screw and retorquing after 10 minutes on implant-supported fixed prostheses. MATERIALS AND METHODS: Two strain gauges (SGs) were attached to four implants stabilized on an acrylic resin mandible. Four implant-supported frameworks were constructed to represent passive fit (PF) and different amounts of misfit (MF1, MF2, and MF3). Vertical misfit was measured using a traveling microscope. Each framework was torqued to 35 Ncm (the manufacturer's recommendation) and 40 Ncm, and the preload was recorded immediately and again after retorquing 10 minutes later (torque stage). RESULTS: The smallest gap was observed under the PF framework. Three-way analysis of variance revealed significant effects of the framework, torque value, and torque stage on preload. The PF showed the highest mean preload under both torque values. An independent-sample t test between the torque values revealed a statistically significant difference only for MF1 and MF2. A dependent-sample t test of the torque stage revealed a statistically significant difference at a torque value of 35 Ncm under the PF and MF3 frameworks. CONCLUSION: Increasing the torque value beyond the manufacturer's recommended amount and retorquing of the screws at 10 minutes after the initial torque did not necessarily lead to a significant increase in preload in full-arch implant-supported fixed prostheses, particularly under non-passively fitting frameworks.