Effect of polyhedral silsesquioxane (POSS) on the flexural strength and color of interim materials Tamer A. Hamza, BDS, MSD, PhD,a William M. Johnston, PhD,b and Scott R. Schricker, PhDc Faculty of Dental Medicine, Al-Azhar University, Cairo Egypt; College of Dentistry, The Ohio State University, Columbus, Ohio Statement of problem. Polyhedral silsesquioxane (POSS) nanofillers can reinforce interim materials. However, the interaction between the POSS and the brand and its effect on color are unknown. Purpose. The purpose of this study was to determine the effect on the flexural strength, color, and color stability of 4 commercially available interim materials modified with 1 wt% POSS. Material and methods. Four commercial interim resin materials were used in this study: Jet, Trim, Temphase, and Protemp. The flexural strength of the interim materials was measured with and without 1 wt% POSS with an Instron universal testing machine. The color of the interim material and the material with 1 wt% POSS were evaluated, followed by the evaluation of the color change after coffee staining. The data were analyzed with a 2-way analysis of variance (ANOVA) followed by the Tukey standardized range test (HSD) to determine statistical significance (a¼.05). Results. POSS enhanced the flexural strength of Protemp and Trim but had no effect on Temphase and Jet. POSS had a significant affect on the color (L*, a*, b*), but only 2 pairwise differences (in L* Trim versus Trim POSS and in a* Temphase versus Temphase POSS) were detected. The effect of POSS on color change after coffee staining was minimal. POSS only had a significant effect on DL, and only 1 pairwise difference was detected (Temphase versus Temphase POSS). The collapsed means revealed significant differences in color change among brands after coffee staining. Conclusions. The reinforcement effect of POSS on flexural strength depended on the brand, suggesting that the chemistry of the resin affects the ability of POSS to modify mechanical properties. POSS affected the initial color of the interim materials but had little effect on the color change after coffee staining. (J Prosthet Dent 2014;-:---)
Clinical Implications Changes to the flexural strength, color, and color stability of interim materials can affect their clinical performance. Polyhedral silsesquioxane (POSS) is used in commercial dental materials and can reinforce interim materials. Interim restorations are used in fixed prosthodontic treatment to protect the tooth until the definitive restoration is ready to be cemented in the patient’s mouth. Although these materials have a limited use, interim restorations satisfy important esthetic and mechanical functions for the patient and the dentist. Therefore, an optimal interim a
restoration must satisfy many interrelated demands: biologic, mechanical, and esthetic.1 In general, materials for dental applications need to meet demanding requirements, particularly in terms of mechanical performance and chemical resistance. Since the 1930s, different types of materials for interim restoration have been
introduced, and their performance has significantly improved. The earliest example of a resin used for this purpose was Biolon, a heat-polymerized acrylic resin introduced in 1937. Subsequently, autopolymerizing poly(methyl methacrylate) (PMMA) resins were introduced in 1947, resulting in such products as Jet (Lang Dental Manufacturing Co).
Assistant Professor and Head, Crowns and Fixed Prosthetic Department, Faculty of Dental Medicine, Al-Azhar University. Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University. c Associate Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University. b
Hamza et al
2
Volume
This study consisted of 2 parts. The first part attempted to determine whether POSS reinforces the flexural strength of 4 types of interim restorations: an autopolymerizing PMMA resin (Jet; Lang Dental Manufacturing
O
H 3C O Si O
O R
Si
R Si
O
R
O Si
R
O Si
A
Si
O
O
Si
O Si O
O Si
O
O
R
O
Si
R
R
Si
B
R
O
CH3
O
O
O Si
O
CH2CH2CH2OCC=CH2
Si
O R
CH3
H3C
O
O
R
CH3
R
O
Si
O
CH2CH2CH2OCC=CH2
Si
R
-
MATERIAL AND METHODS
CH3 R
Issue
The effect of adding 1% by weight of POSS on the flexural strength and color characteristics of 4 commercial interim restoration materials is described in this in vitro study. The choice of 1 wt% was based on previous work suggesting that optimal POSS formulations for methacrylate based dental materials are in the range of 1 to 2 wt%. Concentrations higher than 2 wt% are comparable to the control, and at concentrations of 10 wt% and higher, POSS degrades the physical properties.19,20,25,26 The optimal concentration depends on the property measured, but for flexural strength measurements, 1 wt% is a reasonable selection. A trimethacrylate POSS (Fig. 1B) was added to the interim restoration materials, and the flexural strength, initial color, and color change after staining was measured. The null hypothesis was that the flexural strength, the inherent color, and the color stability of interim restoration materials would not be affected by adding POSS.
of a 1.5 nm silicon-oxygen core with pendant organic groups bound to the 8 silicon atoms. The organic functionalities of POSS may include the methacrylate, vinyl, epoxy, and silane groups, all of which vary widely in their architecture.15-18 Many homogeneous mixtures of POSS molecules with thermosetting polymeric resins are known. Methacrylate functionalized POSS with 1, 2, or 3 methacrylate functionalities are known and are compatible with methacrylate resins, dental restorative resins, and denture base resins. Functionalized POSS have been used to improve the properties of several dental polymer systems. For example, composite resins and interim materials modified with methacrylated POSS have demonstrated improved properties.19-25 The addition of POSS to elastomers and adhesives can improve their mechanical properties.26,27 Previous work on interim materials demonstrated that POSS will have an effect on fracture toughness that ranges from minimally positive to detrimental. However, only 1 interim material was studied, and other relevant mechanical properties such as flexural strength were not measured. In addition, the effect of POSS on the color and color stability of a methacrylate based dental material has not been reported.
Further developments included vinyl poly(ethyl methacrylate) materials introduced in the 1960s and represented by Trim (Harry J. Bosworth Co) and composite resin products introduced in the 1980s represented by Protemp (3M ESPE).2,3 For patients with bruxism or those requiring the use of interim restorations for long periods, a strong interim restoration is of great importance. Fractured interim restorations may require unscheduled appointments for repair and result in patient dissatisfaction. Because of clinical problems resulting from the mechanical failure of interim fixed dental prostheses, many strategies have been used to reinforce them. These include the use of metal wire and fibers such as carbon fibers, plasma-treated polyethylene fibers, and glass fibers.4-8 Additionally, fixed dental prostheses are exposed to many foods that cause staining, for example, coffee, tea, and wine. Therefore, maintaining or improving the color stability of interim restorations is important to preserve esthetics.9-14 The color of the material is also important when matching the natural tooth color of individual patients. Polyhedral silsesquioxanes (POSS) have been used as a nanoscale reinforcing agent in many different types of polymeric materials. POSS belong to a class of discrete nanoscale organicinorganic hybrids (Fig. 1A) composed
-
Si
Si
O
CH2CH2CH2OCC=CH2
R Si
O
H3C
CH3
CH3
R
1 A, Generic structure of POSS molecule. R is organic group. B, Trimethacrylate POSS used.
The Journal of Prosthetic Dentistry
Hamza et al
-
2014
Co), a vinyl poly(ethyl methacrylate) (Trim; Harry J. Bosworth Co), and 2 composite resins (Temphase; Kerr and Protemp Plus; 3M ESPE). Subsequently, the effect of POSS on color and color stability was measured. The rectangular flexural strength specimens were made according to known standards (ISO 14077).28 A split, stainless steel mold as previously described6 was used to form rectangular specimens of dimensions 2225 mm. Jet and Trim specimens were fabricated with the 2:1 polymer to monomer ratio recommended by the manufacturers. Briefly, the polymer and monomer were mixed in a glass jar at room temperature with a stainless steel spatula. Upon reaching the dough stage, the mixture was slowly packed into the mold to avoid trapping air. The mold was then covered with a glass slab and any excess resin was removed. The specimens were allowed to polymerize for 15 minutes at room temperature. The specimens were then removed from the mold, the flash was removed with a razor blade, and the specimen visually examined for voids. The Temphase and Protemp were supplied in an automixing cartridge but otherwise were prepared in the manner described above. Reinforced specimens were formed in a similar manner by adding 1% (wt/wt) of POSS material to all fabricated specimens. The interim resin specimens were finished to the desired dimensions with 400- and 600-grit abrasive paper and verified with a caliper (CD-60 CS; Mitutoyo). The specimens were then stored in deionized water at 37 C for 24 hours. The flexural strength of the fabricated specimens was determined with a universal testing machine (Instron 4204 with the Partner v.8.4a software package; Instron Corp). Each specimen (2225 mm) was positioned on a 3-point bending fixture machined from steel with a 20 mm span. The specimens were tested at a crosshead speed of 0.5 mm/min with a 1 kN load cell.6 The peak force (P) in newtons
Hamza et al
3 from each specimen, taken from the stress-strain curve, was used to calculate the flexural strength in MPa as follows:
FS ¼
3PL ; 2bd2
where FS is the flexural strength in MPa, P is the maximum applied load (N), L is the supporting length (mm), b is the breadth of the test specimens (mm), and d is the depth of the test specimen (mm). The mean values and standard deviations (SDs) for each group were calculated. To measure the effect of POSS reinforcement on color and color stability, circular specimens for each group were made as previously described,29 polished to maintain a smooth surface with rubber cups, and then numbered for reference after staining. A colorimeter (Model CR-200; Minolta Corp) was used to determine the CIE L*, a* and b* values. Subsequently, all of the specimens were placed in coffee solution consisting of 2 tablespoons of coffee in 1 cup of water at room temperature for 1 week. After this time, the specimens were rinsed and dried, another set of measurements was obtained, and the color difference and DE for the individual specimens were calculated. Flexural strength data, color parameters before staining, and individual and total color changes of each type of provisional material were analyzed with a 2-way analysis of variance (ANOVA), followed by the Tukey standardized range test (HSD) (a¼.05) to determine
Table II.
statistical significance. A Ryan-EinotGabriel-Welsh Q (REGWQ) post hoc test was used to determine statistical significance among the mean flexural strength values. Values with the same superscript letter were considered statistically identical. A power analysis was not performed to determine sample size. The techniques described in this study have been used previously by the authors, and the sample sizes were based on that previous work.
RESULTS The results shown in Table I and the corresponding 2-way ANOVA analysis in Table II demonstrate that adding 1 wt% POSS resulted in a significant
Table I. Mean (SD) flexural strength and REGWQ rankings Material
Flexural Strength (MPa)
ProtempþPOSS
86.2 (11.3)a
Protemp
61.0 (8.3)b
TemphaseþPOSS
60.1 (3.1)b
Temphase
62.3 (8.5)b
JetþPOSS
54.1 (2.5)b
Jet
52.9 (7.6)b
TrimþPOSS
32.2 (2.6)c
Trim
16.3 (3.5)d
REGWQ, Ryan-Einot-Gabriel-Welsh Q test to determine statistical significance between mean flexural strength values; POSS, polyhedral silsesquioxane. Values with same superscript letter are considered statistically identical.
Two-way ANOVA for flexural strength measurements
Source
Degree of Freedom
Sum of Squares
Mean Square
F
P
Type
7
15422.86
2203.27
48.91
Clinical Implications Changes to the flexural strength, color, and color stability of interim materials can affect their clinical performance. Polyhedral silsesquioxane (POSS) is used in commercial dental materials and can reinforce interim materials. Interim restorations are used in fixed prosthodontic treatment to protect the tooth until the definitive restoration is ready to be cemented in the patient’s mouth. Although these materials have a limited use, interim restorations satisfy important esthetic and mechanical functions for the patient and the dentist. Therefore, an optimal interim a
restoration must satisfy many interrelated demands: biologic, mechanical, and esthetic.1 In general, materials for dental applications need to meet demanding requirements, particularly in terms of mechanical performance and chemical resistance. Since the 1930s, different types of materials for interim restoration have been
introduced, and their performance has significantly improved. The earliest example of a resin used for this purpose was Biolon, a heat-polymerized acrylic resin introduced in 1937. Subsequently, autopolymerizing poly(methyl methacrylate) (PMMA) resins were introduced in 1947, resulting in such products as Jet (Lang Dental Manufacturing Co).
Assistant Professor and Head, Crowns and Fixed Prosthetic Department, Faculty of Dental Medicine, Al-Azhar University. Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University. c Associate Professor, Division of Restorative and Prosthetic Dentistry, The Ohio State University. b
Hamza et al
2
Volume
This study consisted of 2 parts. The first part attempted to determine whether POSS reinforces the flexural strength of 4 types of interim restorations: an autopolymerizing PMMA resin (Jet; Lang Dental Manufacturing
O
H 3C O Si O
O R
Si
R Si
O
R
O Si
R
O Si
A
Si
O
O
Si
O Si O
O Si
O
O
R
O
Si
R
R
Si
B
R
O
CH3
O
O
O Si
O
CH2CH2CH2OCC=CH2
Si
O R
CH3
H3C
O
O
R
CH3
R
O
Si
O
CH2CH2CH2OCC=CH2
Si
R
-
MATERIAL AND METHODS
CH3 R
Issue
The effect of adding 1% by weight of POSS on the flexural strength and color characteristics of 4 commercial interim restoration materials is described in this in vitro study. The choice of 1 wt% was based on previous work suggesting that optimal POSS formulations for methacrylate based dental materials are in the range of 1 to 2 wt%. Concentrations higher than 2 wt% are comparable to the control, and at concentrations of 10 wt% and higher, POSS degrades the physical properties.19,20,25,26 The optimal concentration depends on the property measured, but for flexural strength measurements, 1 wt% is a reasonable selection. A trimethacrylate POSS (Fig. 1B) was added to the interim restoration materials, and the flexural strength, initial color, and color change after staining was measured. The null hypothesis was that the flexural strength, the inherent color, and the color stability of interim restoration materials would not be affected by adding POSS.
of a 1.5 nm silicon-oxygen core with pendant organic groups bound to the 8 silicon atoms. The organic functionalities of POSS may include the methacrylate, vinyl, epoxy, and silane groups, all of which vary widely in their architecture.15-18 Many homogeneous mixtures of POSS molecules with thermosetting polymeric resins are known. Methacrylate functionalized POSS with 1, 2, or 3 methacrylate functionalities are known and are compatible with methacrylate resins, dental restorative resins, and denture base resins. Functionalized POSS have been used to improve the properties of several dental polymer systems. For example, composite resins and interim materials modified with methacrylated POSS have demonstrated improved properties.19-25 The addition of POSS to elastomers and adhesives can improve their mechanical properties.26,27 Previous work on interim materials demonstrated that POSS will have an effect on fracture toughness that ranges from minimally positive to detrimental. However, only 1 interim material was studied, and other relevant mechanical properties such as flexural strength were not measured. In addition, the effect of POSS on the color and color stability of a methacrylate based dental material has not been reported.
Further developments included vinyl poly(ethyl methacrylate) materials introduced in the 1960s and represented by Trim (Harry J. Bosworth Co) and composite resin products introduced in the 1980s represented by Protemp (3M ESPE).2,3 For patients with bruxism or those requiring the use of interim restorations for long periods, a strong interim restoration is of great importance. Fractured interim restorations may require unscheduled appointments for repair and result in patient dissatisfaction. Because of clinical problems resulting from the mechanical failure of interim fixed dental prostheses, many strategies have been used to reinforce them. These include the use of metal wire and fibers such as carbon fibers, plasma-treated polyethylene fibers, and glass fibers.4-8 Additionally, fixed dental prostheses are exposed to many foods that cause staining, for example, coffee, tea, and wine. Therefore, maintaining or improving the color stability of interim restorations is important to preserve esthetics.9-14 The color of the material is also important when matching the natural tooth color of individual patients. Polyhedral silsesquioxanes (POSS) have been used as a nanoscale reinforcing agent in many different types of polymeric materials. POSS belong to a class of discrete nanoscale organicinorganic hybrids (Fig. 1A) composed
-
Si
Si
O
CH2CH2CH2OCC=CH2
R Si
O
H3C
CH3
CH3
R
1 A, Generic structure of POSS molecule. R is organic group. B, Trimethacrylate POSS used.
The Journal of Prosthetic Dentistry
Hamza et al
-
2014
Co), a vinyl poly(ethyl methacrylate) (Trim; Harry J. Bosworth Co), and 2 composite resins (Temphase; Kerr and Protemp Plus; 3M ESPE). Subsequently, the effect of POSS on color and color stability was measured. The rectangular flexural strength specimens were made according to known standards (ISO 14077).28 A split, stainless steel mold as previously described6 was used to form rectangular specimens of dimensions 2225 mm. Jet and Trim specimens were fabricated with the 2:1 polymer to monomer ratio recommended by the manufacturers. Briefly, the polymer and monomer were mixed in a glass jar at room temperature with a stainless steel spatula. Upon reaching the dough stage, the mixture was slowly packed into the mold to avoid trapping air. The mold was then covered with a glass slab and any excess resin was removed. The specimens were allowed to polymerize for 15 minutes at room temperature. The specimens were then removed from the mold, the flash was removed with a razor blade, and the specimen visually examined for voids. The Temphase and Protemp were supplied in an automixing cartridge but otherwise were prepared in the manner described above. Reinforced specimens were formed in a similar manner by adding 1% (wt/wt) of POSS material to all fabricated specimens. The interim resin specimens were finished to the desired dimensions with 400- and 600-grit abrasive paper and verified with a caliper (CD-60 CS; Mitutoyo). The specimens were then stored in deionized water at 37 C for 24 hours. The flexural strength of the fabricated specimens was determined with a universal testing machine (Instron 4204 with the Partner v.8.4a software package; Instron Corp). Each specimen (2225 mm) was positioned on a 3-point bending fixture machined from steel with a 20 mm span. The specimens were tested at a crosshead speed of 0.5 mm/min with a 1 kN load cell.6 The peak force (P) in newtons
Hamza et al
3 from each specimen, taken from the stress-strain curve, was used to calculate the flexural strength in MPa as follows:
FS ¼
3PL ; 2bd2
where FS is the flexural strength in MPa, P is the maximum applied load (N), L is the supporting length (mm), b is the breadth of the test specimens (mm), and d is the depth of the test specimen (mm). The mean values and standard deviations (SDs) for each group were calculated. To measure the effect of POSS reinforcement on color and color stability, circular specimens for each group were made as previously described,29 polished to maintain a smooth surface with rubber cups, and then numbered for reference after staining. A colorimeter (Model CR-200; Minolta Corp) was used to determine the CIE L*, a* and b* values. Subsequently, all of the specimens were placed in coffee solution consisting of 2 tablespoons of coffee in 1 cup of water at room temperature for 1 week. After this time, the specimens were rinsed and dried, another set of measurements was obtained, and the color difference and DE for the individual specimens were calculated. Flexural strength data, color parameters before staining, and individual and total color changes of each type of provisional material were analyzed with a 2-way analysis of variance (ANOVA), followed by the Tukey standardized range test (HSD) (a¼.05) to determine
Table II.
statistical significance. A Ryan-EinotGabriel-Welsh Q (REGWQ) post hoc test was used to determine statistical significance among the mean flexural strength values. Values with the same superscript letter were considered statistically identical. A power analysis was not performed to determine sample size. The techniques described in this study have been used previously by the authors, and the sample sizes were based on that previous work.
RESULTS The results shown in Table I and the corresponding 2-way ANOVA analysis in Table II demonstrate that adding 1 wt% POSS resulted in a significant
Table I. Mean (SD) flexural strength and REGWQ rankings Material
Flexural Strength (MPa)
ProtempþPOSS
86.2 (11.3)a
Protemp
61.0 (8.3)b
TemphaseþPOSS
60.1 (3.1)b
Temphase
62.3 (8.5)b
JetþPOSS
54.1 (2.5)b
Jet
52.9 (7.6)b
TrimþPOSS
32.2 (2.6)c
Trim
16.3 (3.5)d
REGWQ, Ryan-Einot-Gabriel-Welsh Q test to determine statistical significance between mean flexural strength values; POSS, polyhedral silsesquioxane. Values with same superscript letter are considered statistically identical.
Two-way ANOVA for flexural strength measurements
Source
Degree of Freedom
Sum of Squares
Mean Square
F
P
Type
7
15422.86
2203.27
48.91
