Effects of Accelerated Artificial Daylight Aging on Bending Strength and Bonding of Glass Fibers in Fiber‐Embedded Maxillofacial Silicone Prostheses

Purpose: The purpose of this study was to test the effect of different periods of accelerated artificial daylight aging on bond strength of glass fiber bundles embedded into maxillofacial silicone elastomer and on bending strength of the glass fiber bundles. Methods and Materials: Forty specimens were fabricated by embedding resin‐impregnated fiber bundles (1.5‐mm diameter, 20‐mm long) into maxillofacial silicone elastomer. Specimens were randomly allocated into four groups, and each group was subjected to different periods of accelerated daylight aging as follows (in hours); 0, 200, 400, and 600. The aging cycle included continuous exposure to quartz‐filtered visible daylight (irradiance 760 W/m²) under an alternating weathering cycle (wet for 18 minutes, dry for 102 minutes). Pull‐out tests were performed to evaluate bond strength between fiber bundles and silicone using a universal testing machine at 1 mm/min crosshead speed. Also a three‐point bending test was performed to evaluate bending strength of the fiber bundles. One‐way ANOVA and Bonferroni post hoc tests were carried out to detect statistical significance (p < 0.05). Results: Mean (SD) values of maximum pull‐out forces (in N) for groups 1 to 4 were: 13.63 (7.45), 19.67 (1.37), 13.58 (2.61), and 10.37 (2.52). Group 2 exhibited the highest pull‐out force that was statistically significant when compared to the other groups. Maximum bending strengths of fiber bundles were in the range of 917.72 MPa to 1124.06 MPa. Bending strength significantly increased after 200 and 400 hours of aging only. Conclusions: After 200 hours of exposure to artificial daylight and moisture conditions, bond strength between glass fibers and heat‐cured silicones is optimal, and the bending strength of the glass fiber bundles is enhanced.     

Porosity and Color of Maxillofacial Silicone Elastomer

Purpose: Prosthesis color production and stability as a result of pore entrapment during mixing has not been investigated for maxillofacial silicone prostheses. The purpose of this study was to investigate pore numbers and percentages of a maxillofacial silicone elastomer mixed by two different techniques, using X‐ray microfocus computerized tomography (Micro‐CT), and to investigate the effect of porosity on color reproducibility and stability after two different aging conditions. Materials and Methods: Sixty‐four disk‐shaped specimens were prepared (8‐mm diameter, 3‐mm thick) by mixing TechSil S25 silicone elastomer (Technovent, Leeds, UK) following two techniques: manual mixing (n = 32) and mechanical mixing under vacuum (n = 32). Half the specimens in each group were intrinsically pigmented, and the other half remained unpigmented. Pore numbers, volumes, and percentages were calculated using the Micro‐CT, and then specimens of each subgroup were stored in simulated sebum for 6 months (n = 8), and exposed to accelerated daylight aging for 360 hours (n = 8). Color change (ΔE) was measured at the start and end of conditioning. Pore numbers and percentages were analyzed using one‐way Analysis of Variance (ANOVA) and Dunnett’s‐T3 post‐hoc tests (p < 0.05). Independent t‐test was used to detect differences (p < 0.05) in ΔE between manually and mechanically mixed specimens, in both unpigmented and pigmented states and to detect differences (p < 0.05) in ΔE before and after conditioning within each mixing method. Results: Mechanical mixing under vacuum reduced the number and percentage of pores in comparison to manual mixing, within pigmented and unpigmented silicone specimens (p < 0.05). Perceptible ΔE between manual and mechanical mixing techniques were 5.93 and 5.18 for both unpigmented and pigmented specimens, respectively. Under sebum storage, manually mixed unpigmented specimens showed lower ΔE (p < 0.05) than those that were mechanically mixed; however, pigmented silicone specimens showed the same ΔE (p > 0.05). After light aging, mixing method had no effect on ΔE of unpigmented specimens (p > 0.05). Furthermore, mechanically mixed pigmented specimens showed lower ΔE (p < 0.05). Conclusions: Within silicone elastomers (whether pigmented or unpigmented), mechanical mixing under vacuum reduced pore numbers and percentages in comparison to manual mixing. For selected skin shade, pores affected the resultant color of prosthesis (color reproducibility). Additionally, silicone pores affected silicone color stability upon service. Clinical significance: In fabricating maxillofacial prostheses, mechanically mixing silicone under vacuum produces pore‐free prostheses, tending to enhance their color production and stability.     

Effect of Extraoral Aging Conditions on Color Stability of Maxillofacial Silicone Elastomer

Purpose: Maxillofacial prostheses require enhancement or replacement due to deterioration in their color during service. The purpose of this study was to investigate color stability of pigmented and nonpigmented maxillofacial silicone elastomer exposed to different human and environmental aging conditions. Material and Methods: One hundred and twelve disk‐shaped silicone (TechSil S25, Technovent, Leeds, UK) specimens were prepared and equally divided into pigmented (using intrinsic rose‐pink skin shade, P409, Principality Medical, Newport, UK) and nonpigmented categories of seven groups (n = 16; 8 pigmented and 8 nonpigmented): dark storage (control) (group 1), sebum solution storage (group 2), acidic perspiration storage (group 3), light aging (group 4), natural outdoor weathering (group 5), silicone‐cleaning solution (group 6), and mixed conditioning of sebum storage and light aging (group 7). Conditioning periods (groups) were 6 months (groups 1, 2, 3, 5), 360 hours (groups 4, 7), and 30 hours (group 6). Color change (ΔE) was measured at the start and end of conditioning. In addition, for groups 1, 2, and 4, ΔE was measured at fixed intervals of 30 days, 15 days, and 30 hours, respectively. Data were analyzed with one‐way analysis of variance (ANOVA), Dunnett’s‐T3 post hoc, and independent t‐tests (p < 0.05). Linear regression was implemented to investigate ΔE with time for groups 1, 2, and 4. Results: Six of the seven treatment conditions induced perceivable color change (ΔE > 3). Within the nonpigmented category, specimens stored in the dark for 6 months (group 1) exhibited high ΔE (6.17), which was greater (p < 0.05) than that produced by silicone‐cleaning solution for 30 hours (group 6) (ΔE = 2.08). Within the pigmented category, light aging (group 4), outdoor (group 5), and mixed (group 7) conditionings induced greatest color changes (ΔE = 8.26, 8.30, 9.89, respectively) (p < 0.05); however, there was a strong positive linear function of log‐time after dark storage (group 1) and light aging (group 4). Conclusions: There is inherent color instability of nonpigmented silicone elastomer, which adds to the overall color change of silicone prostheses. Storing silicone elastomer in simulated sebum under light aging induced the greatest color changes. Overall, the color stability of TechSil S25 maxillofacial heat‐temperature‐vulcanizing (HTV) silicone elastomer was unacceptable (ΔE > 3.0, range from 3.48 to 9.89 for pigmented and 3.89 to 10.78 for nonpigmented) when subjected to six of the seven extraoral aging conditionings used in this study. Inherent color instability of nonpigmented facial silicone elastomers primarily contributes to the color degradation of extraoral facial prostheses. Sebaceous skin secretions along with daylight radiation cause the greatest perceivable color change to the silicone and pigment used in this study.     

Bond Strength of Soft Liners to Fiber‐Reinforced Denture‐Base Resin

Purpose: This study evaluated bond strengths of four soft liners to fiber‐reinforced (FR) and unreinforced poly methyl(methacrylate) (PMMA) denture‐base resin. Materials and Methods: The autopolymerized denture‐base resin Palapress Vario (Heraus Kulzer GmbH, Hanau, Germany) was used as the substrate (15 × 15 × 5 mm³). The test group consisted of substrates reinforced with porous PMMA preimpregnated unidirectional glass fibers (Stick [StickTech, Turku, Finland]) (PMMA + FR group), and the control group was unreinforced acrylic resin (PMMA group) (n = 80 per group). One of four soft liners (Ufi Gel SC [Voco, Cuxhaven, Germany], Sofreliner Tough [Tokuyama Dental Corporation, Tokyo, Japan], Vertex SoftSil 25 [Vertex‐Dental B.V., Zeist, The Netherlands], and Eversoft [Dentsply Austenal, York, PA]) was placed and cured between two substrates using a polyethylene ring (10 mm inner radius, 3 mm height). Tensile bond strength tests (crosshead speed = 10 mm/min) were performed, and the results were analyzed using analysis of variance followed by Tukey's test (p= 0.05). Fracture surfaces were categorized as adhesive or cohesive‐mixed modes, and failure types were statistically analyzed using chi‐square test. Results: FR did not affect the bond strength results significantly (p > 0.05) except for Ufi Gel SC. Significant differences in bond strength were found among the reline materials (p < 0.001). FR specimens showed a significantly higher number of cohesive‐mixed fractures compared to unreinforced specimens (p < 0.05), except for plasticized acrylic‐based reline material (Eversoft [Dentsply Austenal]), which showed fewer cohesive‐mixed failures with FR. Conclusions: The choice of appropriate reline material system with FR acrylic resin is important for the soft liner/denture‐base polymer bond. Glass FR did not have a decreasing effect on the bond strength, except for Ufi Gel SC.     

Bond Strength of Resin Cements to Co‐Cr and Ni‐Cr Metal Alloys Using Adhesive Primers

Purpose: The aim of this study was to evaluate the effectiveness of adhesive primers (APs) applied to Co‐Cr and Ni‐Cr metal alloys on the bond strength of resin cements to alloys. Materials and Methods: Eight cementing systems were evaluated, consisting of four resin cements (Bistite II DC, LinkMax, Panavia F 2.0, RelyX Unicem) with or without their respective APs (Metaltite, Metal Primer II, Alloy Primer, Ceramic Primer). The two types of dental alloys (Co‐Cr, Ni‐Cr) were cast in plate specimens (10 × 5 × 1 mm³) from resin patterns. After casting, the plates were sandblasted with aluminum oxide (100 μm) and randomly divided into eight groups (n = 6). Each surface to be bonded was treated with one of eight cementing systems. Three resin cement cylinders (0.5 mm high, 0.75 mm diameter) were built on each bonded metal alloy surface, using a Tygon tubing mold. After water storage for 24 hours, specimens were subjected to micro‐shear testing. Data were statistically analyzed by two‐way ANOVA and Tukey's studentized range test. Results: The application of Metal Primer II resulted in a significantly higher bond strength for LinkMax resin cement when applied in both metal alloys. In general, the cementing systems had higher bond strengths in Co‐Cr alloy than in Ni‐Cr. Conclusions: The use of AP between alloy metal surfaces and resin cements did not increase the bond strength for most cementing systems evaluated.     

Bond strength of Resilon/Epiphany compared with Gutta‐percha and sealers Sealer 26 and Endo Fill

This study evaluated the bond strength of Epiphany^®/Resilon^® to radicular dentine and compared this with that of Gutta‐percha sealed with Sealer 26 and also Endo Fill^®. Fifteen human single‐rooted extracted teeth had their root canals enlarged to a standardised internal root diameter of 1.5 mm. The teeth were filled with: Group I – Epiphany^®/Resilon^® cones, Group II – Sealer 26^®/Gutta‐percha cones and Group III – Endo Fill^®/Gutta‐percha cones. Three 2 mm‐thick cross‐sections were obtained from each tooth (n = 15) and subjected to a push‐out test (1 mm min^?1). The means (MPa) were compared using anova /Newman–Keuls tests. There were no statistically significant differences between Group I and Group II (P > 0.01). Both bond strengths were significantly higher than that of Group III (P < 0.01). The root canal filling system Epiphany^®/Resilon^® showed bond strength similar to that of Sealer 26^®/Gutta‐percha. Both were higher than that of Endo Fill^®/Gutta‐percha.     

The Effect of Recasting on Bond Strength between Porcelain and Base‐Metal Alloys

Purpose: Long‐term success of metal ceramic restorations depends on metal ceramic bond strength. The purpose of this study was to determine whether recasting of base‐metal alloys has any effect on metal ceramic bond strength. Materials and Methods: Super Cast and Verabond base‐metal alloys were used to cast 260 wax patterns. The alloy specimens were equally divided into five groups and cast as: group A 0.0%, B 25%, C 50%, D 75%, and E 100% once‐cast alloy. Each group was divided into two subgroups: the first group was cast with Super Cast and the second with Verabond. In each subgroup half of the cast alloys were veneered with Vita VMK 68 and the others with Ceramco 3. Results: Recasting decreased bond strength (p < 0.006) when used for 50% once‐cast alloy. Group E with 100% new Super Cast alloy veneered with Vita VMK 68 porcelain had the highest bond strength (30.75 ± 9.58 MPa), and group B including 25% new and 75% recast Super Cast alloy veneered with the same porcelain had the lowest bond strength (21.72 ± 5.19 MPa). Conclusions: By adding over 50% once‐cast alloy in base‐metal alloys, metal‐ceramic bond strength decreases significantly.