Australian Dental Journal
The official journal of the Australian Dental Association
Australian Dental Journal 2013; 58: 448–453 doi: 10.1111/adj.12122
Effect of G-Coat Plus on the mechanical properties of glass-ionomer cements R Bagheri,* NA Taha,† MR Azar,* MF Burrow‡ *Department of Dental Materials, Biomaterial Research Centre, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran. †Department of Conservative Dentistry, Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan. ‡Oral Diagnosis and Polyclinics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, Hong Kong SAR.
ABSTRACT Background: Although various mechanical properties of tooth-coloured materials have been described, little data have been published on the effect of ageing and G-Coat Plus on the hardness and strength of the glass-ionomer cements (GICs). Methods: Specimens were prepared from one polyacid-modified resin composite (PAMRC; Freedom, SDI), one resinmodified glass-ionomer cement; (RM-GIC; Fuji II LC, GC), and one conventional glass-ionomer cement; (GIC; Fuji IX, GC). GIC and RM-GIC were tested both with and without applying G-Coat Plus (GC). Specimens were conditioned in 37 °C distilled water for either 24 hours, four and eight weeks. Half the specimens were subjected to a shear punch test using a universal testing machine; the remaining half was subjected to Vickers Hardness test. Results: Data analysis showed that the hardness and shear punch values were material dependent. The hardness and shear punch of the PAMRC was the highest and GIC the lowest. Applying the G-Coat Plus was associated with a significant decrease in the hardness of the materials but increase in the shear punch strength after four and eight weeks. Conclusions: The mechanical properties of the restorative materials were affected by applying G-Coat Plus and distilled water immersion over time. The PAMRC was significantly stronger and harder than the RM-GIC or GIC. Keywords: G-Coat Plus, glass-ionomer cements, mechanical properties. Abbreviations and acronyms: ANOVA = analysis of variance; GIC = glass-ionomer cement; PAMRC = polyacid-modified resin composite; RM-GIC = resin-modified glass-ionomer cement; VHN = Vickers Hardness Number. (Accepted for publication 5 March 2013.)
INTRODUCTION Conventional glass-ionomer cements (GICs) are formed by an acid-base neutralization reaction between an aqueous polyalkenoic acid and an aluminosilicate glass powder, which results in a relatively brittle material compared to resin composite. Since the end of the 1980s, more developed GICs such as resin-modified glass-ionomer cements (RM-GICs) have become available. Stronger and less brittle hybrid materials have been produced by the addition of water-soluble polymers to create a light-curing GIC formulation.1 The aim of introducing RM-GIC was to maintain the desirable properties of GIC and overcome the disadvantages such as moisture sensitivity and poor early mechanical strength.2 In their simple form, RM-GICs are water-hardening cements with the addition of a hydrophilic monomer, 2-hydroxyethyl methacrylate (HEMA).3 A recently introduced tooth-coloured restorative material that claimed to have the benefits of the GICs 448
and resin composite filling materials is the polyacidmodified resin composite (PAMRC), commonly called ‘compomer’. PAMRCs were introduced in approximately 1994.4 Commercially, the term ‘compomer’ (composite-ionomer) is widely used to reflect its resin composite and glass-ionomer derivation. PAMRCs combine glass polyalkenoic components with polymerizable resin constituents such as dimethacrylates. PAMRCs do not have an auto-setting acid-base reaction as in GICs but rely on resin polymerization to create a ‘set’ functional restorative material.2 When selecting a material to restore teeth, one of the main considerations is its mechanical properties. A restorative material used to replace missing tooth structure needs to be strong enough to withstand the forces associated with mastication and other possible loading. Hardness and shear punch strength are two tests that can be used to evaluate the mechanical properties of a filling material. The material hardness can be defined as its resistance to surface indentation.5,6 The Vickers Hardness test is a method used © 2013 Australian Dental Association
Mechanical properties of restorative materials for brittle materials6 in which a pyramidal indentation is made using a specified load and application time, the resultant hardness number being independent of the applied load.7 The shear punch test has been used to determine properties of clinical significance such as occlusal or incisal forces that occur during mastication.8 It has been used as a simple, reliable technique for assessing the mechanical properties of resin-based materials.9 Both hardness and shear punch strength of tooth-coloured materials have been evaluated to predict their durability, and a relationship between these mechanical properties to material filler content, filler size and silane coupling agent has also been demonstrated.10,11 The objectives of the present study were to place GIC, RM-GIC and PAMRC in distilled water for up to eight weeks at 37 °C and determine: (1) the resultant surface hardness and shear punch strength; (2) the effect of ageing on the surface hardness and shear punch strength; and (3) the effect of a recently introduced resin surface coating (G-Coat Plus) on the hardness and shear punch strength of the GIC and RM-GIC. The null hypotheses are that there is no difference among the materials; that the ageing does not affect mechanical properties; and that the surface coating does not affect the mechanical properties of GICs.
were cured according to the manufacturers’ instructions on each side using an LED curing light with a wavelength range of 440–480 nm at an output of 1500 mW/cm2 (Radii plus LED, SDI, Bayswater, VIC, Australia). Specimens were removed from the mould and excess material around the mould was removed by manual gentle wet grinding both sides of the specimens in a circular motion with a sequence of 1000-, 1500-, 2000-grit silicon carbide papers. Each specimen was washed in an ultrasonic bath between each grinding. Specimens were randomly divided into five groups for each test (Tables 2 and 3); each group was Table 2. Mean shear punch strength (MPa) and standard deviations () of the materials following times interval (n = 6) Materials Freedom Fuji II LC Fuji II LC+ Fuji IX Fuji IX+
24 hours aA
47.8 45.5 aA 40.0 bA 31.7 cA 25.0 aA
(4.0) (7.5) (6.6) (2.3) (1.1)
4 weeks aA
47.2 40.3 cC 59.3 bA 35.9 aA 42.56 aA
(2.2) (2.3) (7.6) (3.8) (3.0)
8 weeks aB
111.6 80.6 cB 95.2 dB 59.9 bB 76.2 bB
(5.1) (6.5) (5.0) (6.2) (3.4)
–Determines control group (not coated by G-Coat Plus). +Determines treatment group (coated by G-Coat Plus). Means with the same upper-case letter in each row are not significantly different (p > 0.05). Means with the same lower-case letter in each column are not significantly different (p > 0.05).
MATERIALS AND METHODS Specimen preparation The materials used in the study are listed in Table 1. For Freedom a total of 9 and for the GIC and RM-GIC, a total of 36 disc-shaped specimens for hardness testing (10.0 mm diameter 9 1.5 mm thick) were prepared. For shear punch testing, 18 and 72 disc-shaped specimens (10.0 mm diameter 9 0.7 mm thick) were prepared for Freedom, and the GIC and RM-GIC respectively (n = 6). All materials were placed in the appropriate plastic mould and pressed between two plastic matrix strips and glass slabs under hand pressure to extrude excess material. The glass slabs were removed and the light-cured materials
Table 3. Mean Vickers Hardness Number (VHN; MPa) and standard deviations () of the materials following times interval (n = 3 discs 3 5 indentation = 15) Materials Freedom Fuji II LC Fuji II LC+ Fuji IX Fuji IX+
24 hours aA bA cB dA dA
87.6 49.3 27.8 15.5 15.4
(3.5) (2.4) (1.5) (0.5) (0.5)
4 weeks aB
66.3 40.4 bA 43 cB 11.8 cB 7.3
bB
(3.5) (4.7) (1.7) (0.7) (0.3)
8 weeks aB
77.4 49.1 bB 28.4 cA 13.2 cB 8.2
aA
(6.1) (2.4) (1.0) (1.7) (1.8)
–Determines control group (not coated by G-Coat Plus). +Determines treatment group (coated by G-Coat Plus). Means with the same upper-case letter in each row are not significantly different (p > 0.05). Means with the same lower-case letter in each column are not significantly different (p > 0.05).
Table 1. Materials Name
Manufacturer
Material type
Freedom
SDI, Vic, Australia
Polyacid-modified resin composite
Fuji II LC Fuji IX
GC Corporation, Tokyo, Japan GC Corporation, Tokyo, Japan GC Corporation, Tokyo, Japan
Resin-modified glass-ionomer cement Self-cure (conventional) glass-ionomer cement Nanofilled self- adhesive light- cured protective coating
G-Coat Plus
© 2013 Australian Dental Association
Filler/resin type
Batch#
strontium fluoroaluminium silicate/urethane dimethacrylate based Aluminium-fluoro-silicateglass/Poly-HEMA
121314
Aluminium-fluoro-silicate glass
1110191
1202221
449
R Bagheri et al. subdivided to three groups and conditioned in distilled water at 37 °C for 24 hours, four and eight weeks. G-Coat Plus was applied on the specimens in the coated group by placing a thin coat of G-Coat Plus on the top surface of the specimen with a microbrush, then gently air blown for 5 seconds and lightcured for 20 seconds according to the manufacturer’s recommendation. Specimens were stored, and tested after 24 hours, four and eight weeks immersion in distilled water at 37 °C. The water was changed weekly for each of the time periods. Measurements of Vickers Hardness and shear punch strength were carried out as described below. Shear punch strength The thickness of each specimen was measured with a digital micrometer, positioned in the shear punch jig (Fig. 1) and held in place by gently tightening the restraining screw. The shear punch jig was aligned to the loading axis of the universal testing machine (Zwick/Roll Z020, Zwick GmbH & Co, Germany). A flat-ended 3.2-mm diameter stainless steel rod was used to punch out a disc through the centre of each specimen at a crosshead speed of 1 mm/min, and the maximum load recorded. Shear punch strength (MPa)
was calculated using the following formula: load (N) punch circumference (mm) specimen thickness (mm)
Vickers Hardness Each disc was subjected to five indentations with 35 lm apart across the specimen surface by applying a load of 300 g for 15 seconds using a Digital Hardness Tester (Buehler, Chicago, USA) (n = 3 discs 9 5 indentation = 15). The Vickers Hardness Number (VHN) was determined by dividing the load (kgf) by the surface area (mm2), and the resulting value converted to MPa multiply by 9.807 (MPa = Kgf 9 9.8/m2 9 10 6). Data analysis To evaluate the interaction between material and immersion time as well as the effect of G-Coat Plus in each of the two tests, two-way analysis variance (ANOVA) was carried out. To determine inter-material differences for each test, data were analysed using one-way ANOVA and Tukey’s test at a significance level of 0.05. A Pearson Correlation test was also conducted to determine if a relationship could be observed between hardness and shear punch strength. RESULTS Shear punch strength The means and standard deviations are shown in Table 2 and Fig. 2. The base line strength for the PAMRC was slightly greater than the RM-GIC, but significantly greater than the GIC (p < 0.05). All materials showed a significantly higher shear punch value after eight weeks immersion in distilled water. G-Coat Plus coated specimens showed an increase in
24 hours
Shear Punch Strength (MPa)
120
4 weeks
8 weeks
100 80 60 40 20 0 Freedom
Fig. 1 A schematic representation of the shear punch jig. 450
Fuji II LC -
Fuji II LC +
Fuji IX -
Fuji IX +
Fig. 2 Shear punch strength (MPa) versus time interval for all materials in distilled water. © 2013 Australian Dental Association
Mechanical properties of restorative materials shear punch strength for the RM-GIC and GIC (p < 0.05) after four and eight weeks immersion respectively compared to the non-coated specimens. The Pearson Correlation test showed over each test period (24 hours, four and eight weeks) no correlation could be determined between shear punch strength and hardness. The correlations were 0.297, 0.175 and 0.23 for each of the time periods respectively. Irrespective of time, the correlation was calculated at 0.052. Vickers Hardness The means and standard deviations are shown in Table 3 and Fig. 3. With respect to the material tested, regardless of time, the hardness of the PAMRC was the highest and GIC the lowest. There was a significant difference between all materials at baseline VHN (p < 0.05). Ageing in distilled water for most of the materials showed significantly lower VHN after four weeks but then an increase after eight weeks that remained lower than at baseline. Tukey’s test showed G-Coat Plus exhibited a significant decrease in the VHN of the RM-GIC after 24 hours and eight weeks (p < 0.05), while it showed significant increase after four weeks immersion compared to that of 24 hours. DISCUSSION Roydhouse8 introduced the shear punch test as a practical and reliable test for comparing dental cements. The shear punch test was confirmed by Nomoto et al.,11 as a suitable method for standard specification testing across a broad range of restorative materials. Although flexural, compressive and diametral tests are the most commonly used, differences in specimen quality and stress concentration during loading are common problems when comparing inter-laboratory test results. In contrast to flexural, diametral and compression testing, the shear punch test is not
24 hours
90
4 weeks
8 weeks
Fuji II LC +
Fuji IX -
Vickers hardness Number (MPa)
80 70 60 50 40 30 20 10 0 Freedom
Fuji II LC -
Fuji IX +
Fig. 3 Vickers Hardness Number (MPa) versus time interval for all materials in distilled water. © 2013 Australian Dental Association
particularly technique sensitive in terms of the quality of the circumference edges of the disc. Therefore, simplicity of specimen preparation has been mentioned as the main advantage of the shear punch test over the other tests.11,12 In the current study, specimens were polished to obtain flat surfaces for uniform stress distribution around the punch circumference. The modified shear punch test jig (Fig. 1) introduced by Nomoto et al.11 was used in the present study. In this method, the specimen was restrained during shear punch testing by a screw clamp over the top of the specimen, which has been advocated for the prevention of specimen flexure during application of the force from the punch.11 In this study, in contrast to the effect of ageing on the hardness of the tested materials, storage in distilled water was associated with an increase in shear punch strength (Fig. 2). A statistically significant increase in shear punch strength was observed for all materials as the time interval increased with the greatest increase observed after eight weeks immersion in distilled water (Table 2). The shear punch value for the specimens with the coating agent was more than the uncoated group. Coated specimens showed a significant increase after four weeks immersion in distilled water and even more after eight weeks. It is a possibility that the coating agent exerted some local control on the setting of the materials after four and eight weeks or reduced the effect of surface porosity and crack propagation. The rank order of increasing shear punch strength in this study (GIC
The official journal of the Australian Dental Association
Australian Dental Journal 2013; 58: 448–453 doi: 10.1111/adj.12122
Effect of G-Coat Plus on the mechanical properties of glass-ionomer cements R Bagheri,* NA Taha,† MR Azar,* MF Burrow‡ *Department of Dental Materials, Biomaterial Research Centre, Faculty of Dentistry, Shiraz University of Medical Sciences, Shiraz, Iran. †Department of Conservative Dentistry, Faculty of Dentistry, Jordan University of Science and Technology, Irbid, Jordan. ‡Oral Diagnosis and Polyclinics, Faculty of Dentistry, The University of Hong Kong, Prince Philip Dental Hospital, Hong Kong SAR.
ABSTRACT Background: Although various mechanical properties of tooth-coloured materials have been described, little data have been published on the effect of ageing and G-Coat Plus on the hardness and strength of the glass-ionomer cements (GICs). Methods: Specimens were prepared from one polyacid-modified resin composite (PAMRC; Freedom, SDI), one resinmodified glass-ionomer cement; (RM-GIC; Fuji II LC, GC), and one conventional glass-ionomer cement; (GIC; Fuji IX, GC). GIC and RM-GIC were tested both with and without applying G-Coat Plus (GC). Specimens were conditioned in 37 °C distilled water for either 24 hours, four and eight weeks. Half the specimens were subjected to a shear punch test using a universal testing machine; the remaining half was subjected to Vickers Hardness test. Results: Data analysis showed that the hardness and shear punch values were material dependent. The hardness and shear punch of the PAMRC was the highest and GIC the lowest. Applying the G-Coat Plus was associated with a significant decrease in the hardness of the materials but increase in the shear punch strength after four and eight weeks. Conclusions: The mechanical properties of the restorative materials were affected by applying G-Coat Plus and distilled water immersion over time. The PAMRC was significantly stronger and harder than the RM-GIC or GIC. Keywords: G-Coat Plus, glass-ionomer cements, mechanical properties. Abbreviations and acronyms: ANOVA = analysis of variance; GIC = glass-ionomer cement; PAMRC = polyacid-modified resin composite; RM-GIC = resin-modified glass-ionomer cement; VHN = Vickers Hardness Number. (Accepted for publication 5 March 2013.)
INTRODUCTION Conventional glass-ionomer cements (GICs) are formed by an acid-base neutralization reaction between an aqueous polyalkenoic acid and an aluminosilicate glass powder, which results in a relatively brittle material compared to resin composite. Since the end of the 1980s, more developed GICs such as resin-modified glass-ionomer cements (RM-GICs) have become available. Stronger and less brittle hybrid materials have been produced by the addition of water-soluble polymers to create a light-curing GIC formulation.1 The aim of introducing RM-GIC was to maintain the desirable properties of GIC and overcome the disadvantages such as moisture sensitivity and poor early mechanical strength.2 In their simple form, RM-GICs are water-hardening cements with the addition of a hydrophilic monomer, 2-hydroxyethyl methacrylate (HEMA).3 A recently introduced tooth-coloured restorative material that claimed to have the benefits of the GICs 448
and resin composite filling materials is the polyacidmodified resin composite (PAMRC), commonly called ‘compomer’. PAMRCs were introduced in approximately 1994.4 Commercially, the term ‘compomer’ (composite-ionomer) is widely used to reflect its resin composite and glass-ionomer derivation. PAMRCs combine glass polyalkenoic components with polymerizable resin constituents such as dimethacrylates. PAMRCs do not have an auto-setting acid-base reaction as in GICs but rely on resin polymerization to create a ‘set’ functional restorative material.2 When selecting a material to restore teeth, one of the main considerations is its mechanical properties. A restorative material used to replace missing tooth structure needs to be strong enough to withstand the forces associated with mastication and other possible loading. Hardness and shear punch strength are two tests that can be used to evaluate the mechanical properties of a filling material. The material hardness can be defined as its resistance to surface indentation.5,6 The Vickers Hardness test is a method used © 2013 Australian Dental Association
Mechanical properties of restorative materials for brittle materials6 in which a pyramidal indentation is made using a specified load and application time, the resultant hardness number being independent of the applied load.7 The shear punch test has been used to determine properties of clinical significance such as occlusal or incisal forces that occur during mastication.8 It has been used as a simple, reliable technique for assessing the mechanical properties of resin-based materials.9 Both hardness and shear punch strength of tooth-coloured materials have been evaluated to predict their durability, and a relationship between these mechanical properties to material filler content, filler size and silane coupling agent has also been demonstrated.10,11 The objectives of the present study were to place GIC, RM-GIC and PAMRC in distilled water for up to eight weeks at 37 °C and determine: (1) the resultant surface hardness and shear punch strength; (2) the effect of ageing on the surface hardness and shear punch strength; and (3) the effect of a recently introduced resin surface coating (G-Coat Plus) on the hardness and shear punch strength of the GIC and RM-GIC. The null hypotheses are that there is no difference among the materials; that the ageing does not affect mechanical properties; and that the surface coating does not affect the mechanical properties of GICs.
were cured according to the manufacturers’ instructions on each side using an LED curing light with a wavelength range of 440–480 nm at an output of 1500 mW/cm2 (Radii plus LED, SDI, Bayswater, VIC, Australia). Specimens were removed from the mould and excess material around the mould was removed by manual gentle wet grinding both sides of the specimens in a circular motion with a sequence of 1000-, 1500-, 2000-grit silicon carbide papers. Each specimen was washed in an ultrasonic bath between each grinding. Specimens were randomly divided into five groups for each test (Tables 2 and 3); each group was Table 2. Mean shear punch strength (MPa) and standard deviations () of the materials following times interval (n = 6) Materials Freedom Fuji II LC Fuji II LC+ Fuji IX Fuji IX+
24 hours aA
47.8 45.5 aA 40.0 bA 31.7 cA 25.0 aA
(4.0) (7.5) (6.6) (2.3) (1.1)
4 weeks aA
47.2 40.3 cC 59.3 bA 35.9 aA 42.56 aA
(2.2) (2.3) (7.6) (3.8) (3.0)
8 weeks aB
111.6 80.6 cB 95.2 dB 59.9 bB 76.2 bB
(5.1) (6.5) (5.0) (6.2) (3.4)
–Determines control group (not coated by G-Coat Plus). +Determines treatment group (coated by G-Coat Plus). Means with the same upper-case letter in each row are not significantly different (p > 0.05). Means with the same lower-case letter in each column are not significantly different (p > 0.05).
MATERIALS AND METHODS Specimen preparation The materials used in the study are listed in Table 1. For Freedom a total of 9 and for the GIC and RM-GIC, a total of 36 disc-shaped specimens for hardness testing (10.0 mm diameter 9 1.5 mm thick) were prepared. For shear punch testing, 18 and 72 disc-shaped specimens (10.0 mm diameter 9 0.7 mm thick) were prepared for Freedom, and the GIC and RM-GIC respectively (n = 6). All materials were placed in the appropriate plastic mould and pressed between two plastic matrix strips and glass slabs under hand pressure to extrude excess material. The glass slabs were removed and the light-cured materials
Table 3. Mean Vickers Hardness Number (VHN; MPa) and standard deviations () of the materials following times interval (n = 3 discs 3 5 indentation = 15) Materials Freedom Fuji II LC Fuji II LC+ Fuji IX Fuji IX+
24 hours aA bA cB dA dA
87.6 49.3 27.8 15.5 15.4
(3.5) (2.4) (1.5) (0.5) (0.5)
4 weeks aB
66.3 40.4 bA 43 cB 11.8 cB 7.3
bB
(3.5) (4.7) (1.7) (0.7) (0.3)
8 weeks aB
77.4 49.1 bB 28.4 cA 13.2 cB 8.2
aA
(6.1) (2.4) (1.0) (1.7) (1.8)
–Determines control group (not coated by G-Coat Plus). +Determines treatment group (coated by G-Coat Plus). Means with the same upper-case letter in each row are not significantly different (p > 0.05). Means with the same lower-case letter in each column are not significantly different (p > 0.05).
Table 1. Materials Name
Manufacturer
Material type
Freedom
SDI, Vic, Australia
Polyacid-modified resin composite
Fuji II LC Fuji IX
GC Corporation, Tokyo, Japan GC Corporation, Tokyo, Japan GC Corporation, Tokyo, Japan
Resin-modified glass-ionomer cement Self-cure (conventional) glass-ionomer cement Nanofilled self- adhesive light- cured protective coating
G-Coat Plus
© 2013 Australian Dental Association
Filler/resin type
Batch#
strontium fluoroaluminium silicate/urethane dimethacrylate based Aluminium-fluoro-silicateglass/Poly-HEMA
121314
Aluminium-fluoro-silicate glass
1110191
1202221
449
R Bagheri et al. subdivided to three groups and conditioned in distilled water at 37 °C for 24 hours, four and eight weeks. G-Coat Plus was applied on the specimens in the coated group by placing a thin coat of G-Coat Plus on the top surface of the specimen with a microbrush, then gently air blown for 5 seconds and lightcured for 20 seconds according to the manufacturer’s recommendation. Specimens were stored, and tested after 24 hours, four and eight weeks immersion in distilled water at 37 °C. The water was changed weekly for each of the time periods. Measurements of Vickers Hardness and shear punch strength were carried out as described below. Shear punch strength The thickness of each specimen was measured with a digital micrometer, positioned in the shear punch jig (Fig. 1) and held in place by gently tightening the restraining screw. The shear punch jig was aligned to the loading axis of the universal testing machine (Zwick/Roll Z020, Zwick GmbH & Co, Germany). A flat-ended 3.2-mm diameter stainless steel rod was used to punch out a disc through the centre of each specimen at a crosshead speed of 1 mm/min, and the maximum load recorded. Shear punch strength (MPa)
was calculated using the following formula: load (N) punch circumference (mm) specimen thickness (mm)
Vickers Hardness Each disc was subjected to five indentations with 35 lm apart across the specimen surface by applying a load of 300 g for 15 seconds using a Digital Hardness Tester (Buehler, Chicago, USA) (n = 3 discs 9 5 indentation = 15). The Vickers Hardness Number (VHN) was determined by dividing the load (kgf) by the surface area (mm2), and the resulting value converted to MPa multiply by 9.807 (MPa = Kgf 9 9.8/m2 9 10 6). Data analysis To evaluate the interaction between material and immersion time as well as the effect of G-Coat Plus in each of the two tests, two-way analysis variance (ANOVA) was carried out. To determine inter-material differences for each test, data were analysed using one-way ANOVA and Tukey’s test at a significance level of 0.05. A Pearson Correlation test was also conducted to determine if a relationship could be observed between hardness and shear punch strength. RESULTS Shear punch strength The means and standard deviations are shown in Table 2 and Fig. 2. The base line strength for the PAMRC was slightly greater than the RM-GIC, but significantly greater than the GIC (p < 0.05). All materials showed a significantly higher shear punch value after eight weeks immersion in distilled water. G-Coat Plus coated specimens showed an increase in
24 hours
Shear Punch Strength (MPa)
120
4 weeks
8 weeks
100 80 60 40 20 0 Freedom
Fig. 1 A schematic representation of the shear punch jig. 450
Fuji II LC -
Fuji II LC +
Fuji IX -
Fuji IX +
Fig. 2 Shear punch strength (MPa) versus time interval for all materials in distilled water. © 2013 Australian Dental Association
Mechanical properties of restorative materials shear punch strength for the RM-GIC and GIC (p < 0.05) after four and eight weeks immersion respectively compared to the non-coated specimens. The Pearson Correlation test showed over each test period (24 hours, four and eight weeks) no correlation could be determined between shear punch strength and hardness. The correlations were 0.297, 0.175 and 0.23 for each of the time periods respectively. Irrespective of time, the correlation was calculated at 0.052. Vickers Hardness The means and standard deviations are shown in Table 3 and Fig. 3. With respect to the material tested, regardless of time, the hardness of the PAMRC was the highest and GIC the lowest. There was a significant difference between all materials at baseline VHN (p < 0.05). Ageing in distilled water for most of the materials showed significantly lower VHN after four weeks but then an increase after eight weeks that remained lower than at baseline. Tukey’s test showed G-Coat Plus exhibited a significant decrease in the VHN of the RM-GIC after 24 hours and eight weeks (p < 0.05), while it showed significant increase after four weeks immersion compared to that of 24 hours. DISCUSSION Roydhouse8 introduced the shear punch test as a practical and reliable test for comparing dental cements. The shear punch test was confirmed by Nomoto et al.,11 as a suitable method for standard specification testing across a broad range of restorative materials. Although flexural, compressive and diametral tests are the most commonly used, differences in specimen quality and stress concentration during loading are common problems when comparing inter-laboratory test results. In contrast to flexural, diametral and compression testing, the shear punch test is not
24 hours
90
4 weeks
8 weeks
Fuji II LC +
Fuji IX -
Vickers hardness Number (MPa)
80 70 60 50 40 30 20 10 0 Freedom
Fuji II LC -
Fuji IX +
Fig. 3 Vickers Hardness Number (MPa) versus time interval for all materials in distilled water. © 2013 Australian Dental Association
particularly technique sensitive in terms of the quality of the circumference edges of the disc. Therefore, simplicity of specimen preparation has been mentioned as the main advantage of the shear punch test over the other tests.11,12 In the current study, specimens were polished to obtain flat surfaces for uniform stress distribution around the punch circumference. The modified shear punch test jig (Fig. 1) introduced by Nomoto et al.11 was used in the present study. In this method, the specimen was restrained during shear punch testing by a screw clamp over the top of the specimen, which has been advocated for the prevention of specimen flexure during application of the force from the punch.11 In this study, in contrast to the effect of ageing on the hardness of the tested materials, storage in distilled water was associated with an increase in shear punch strength (Fig. 2). A statistically significant increase in shear punch strength was observed for all materials as the time interval increased with the greatest increase observed after eight weeks immersion in distilled water (Table 2). The shear punch value for the specimens with the coating agent was more than the uncoated group. Coated specimens showed a significant increase after four weeks immersion in distilled water and even more after eight weeks. It is a possibility that the coating agent exerted some local control on the setting of the materials after four and eight weeks or reduced the effect of surface porosity and crack propagation. The rank order of increasing shear punch strength in this study (GIC
