Effect of Thermocycling on Shear Bond Strength of PEEK-A Comparative Study of Resin Luting Cements: An In-Vitro Study.

Introduction: Thermocycling is an in-vitro process; it may affect the bonding strength between tooth and resin which is depending upon the adhesive system. Aim and
Objectives: To evaluate and compare the thermocycling effect on shear bond strength of RelyX Unicem and G-CEM Linkace to polyether ether ketone (PEEK) surface. Materials and
Methods: A total of 40 PEEK disk-shaped specimens were fabricated with dimensions of 10 × 3 mm and randomly allocated into two groups. Group A was cemented with Rely X Unicem material and Group B was cemented with G-CEM Linkace. About 10 specimens from each group were thermocycled 500 times at 5°C and 55°C. By applying force at the speed of 1 mm/min using a universal testing machine, shear bond strength was measured.
Results: The mean bond strength was compared using paired t test. There was a significant difference even before and after thermocycling.
Conclusion: In this experiment, Shear bond strength (SBS) of G-CEM Linkace showed more even before and after thermocycling when compared to RelyX. Also bond strengths of two cements decreased after thermocycling. Copyright:

INTRODUCTION

Polyether ether ketone (PEEK) is a new polymer with excellent mechanical properties.[1] Thermocycling test is used for screening new materials and also comparing different adhesive systems.[2] This study evaluated the effect of thermocycling treatment on bond strength of PEEK and resin cements.

MATERIALS AND METHODS

CAD-CAM technique was used for the fabrication of specimens. Disk-shaped PEEK specimens were obtained by milling procedure. Alumina air blasting with 50 μm alumina particles was done for 10 s to enhance the bonding strength. RelyX Unicem (3M ESPE) and G-CEM LinkAce adhesive cements were used. A total of 40 specimens were divided into two groups, each with 20 specimens. A cylindrical putty mold index of dimensions 5 × 4 mm was used to guide the application of resin cements. Group A was cemented with RelyX Unicem and self-cured for 2 min, then light cured for 20 s. Group B was cemented with G-CEM LinkAce, which was light cured for 20 s then allowed it to self-cure for 4 min. Ethics committee permission is obtained by 24-07-2017T.

Thermocycling

The 10 specimens from Group A and Group B were subjected to thermocycling in two different thermal baths with temperatures at 5°C and 55°C. Distilled water is used to maintain the temperatures of thermal baths. Each sample was exposed to thermocycler for a period of 20 s at 5°C and 55°C with 5-s interval between each cycle. A total of 500 cycles were carried out for each sample.

Testing shear bond strength

Universal testing machine (UTM) was used to test shear bond strength. Each specimen was tightened in a metal holder in a UTM. Load was applied parallel to the long axis of the specimen at the interface of cement-disk at a speed of 1 mm/min until the resin cement cylinder gets separated from the PEEK disk. Bond strength was measured by dividing maximum load applied to the cross-sectional area. Scanning electron microscope was used to observe the surface of the specimens to compare the failure.

Scanning electron microscopy

Scanning electron microscope was used to observe the fractured interface at 5.00 kV x 500 SE at 100 m and 5.00 kV x 150 SE at 300 m magnification to find out the type of failure and these failures were classified into three categories:

Adhesive failure at the resin/PEEK material interface,

Combined adhesive/cohesive failure,

Cohesive failure within the resin cements.

The specimens were obtained by microscopic observation and these specimens were treated by argon deposition (HPC-IS, Vacuum Device, Mito, Japan) for 30 s and was then observed with a scanning electron microscope operated at 15 kV. The specimens were held in the holder of an X-ray diffractometer with CuKα radiation which is equipped with a Ni foil filter to clear CuKβ radiation. The XRD was operated at 30 kV of tube voltage and 15 mA of tube current, with a scanning speed of 2.0°/min.

RESULTS

The paired t-test was used for comparison between two groups. All P values (P < 0.05) were considered as statistically significant. Comparison of shear bond strength between RelyX control and RelyX test was done using paired t test in which the mean of the control group was 3.91 and standard deviation was 0.38. The mean of the test group was 1.87 and standard deviation was 0.43. There was significant difference in mean shear bond strength of RelyX group (P < 0.0001) [Table 1]. Comparison of shear bond strength between G-CEM control and G-CEM test was done using paired t test in which the mean bond strength and standard deviation of G-CEM control and G-CEM test was found to be 5.07, 0.95 and 4.01, 0.68, respectively [Table 2]. Comparison of shear bond strength between RelyX and G-CEM after thermocycling represent a significant difference in shear bond strength (P < 0.005). G-CEM linkage showed maximum SBS after thermocycling and RelyX unicem showed minimum [Table 3]. Graphical bar representation depicts that RelyX test showed a maximum loss of SBS and G-CEM test group showing minimum loss of SBS [Graph 1]. SEM images of RelyXUnicem and G-CEM LinkAce specimens showed adhesive type of failure [Figures 1 and 2].

Table 1

Comparison of shear bond strength using paired t test

Procedures	n	Minimum	Maximum	Mean	SD	P	Percentile of shear bond strength
RelyX control	10	3.31	4.43	3.91	0.38	<0.0001	52.2% Reduction
RelyX test	10	1.02	2.34	1.87	0.43

The mean of the control group was 3.91 and standard deviation 0.38. The mean of the test group was 1.87 and standard deviation 0.43. There was significant difference in mean shear bond strength of RelyX group. (P<0.0001)

Table 2

Comparison of shear bond strength using paired t test

Procedures	n	Minimum	Maximum	Mean	SD	P	Percentile of shear bond strength
G-CEM control	10	3.40	5.98	5.07	0.95	0.0001	20.9% reduction
G-CEM test	10	2.65	4.78	4.01	0.68

The mean bond strength of G-CEM control was 5.07 and standard deviation 0.95. The mean bond strength of G-CEM test was 4.01 and standard deviation 0.68. There was significant difference in shear bond strength of G-CEM group.

Table 3

Comparison between shear bond strength of two cements after thermocycling using paired t test

Procedures	Groups	n	Minimum	Maximum	Mean	SD	P	Percentile of shear bond strength
Test	RelyX	10	1.02	2.34	1.87	0.43	<0.0001	53.4% increased
	G-CEM	10	2.65	4.78	4.01	0.68

There was significant difference in shear bond strength (P<0.005). G-CEM linkage showed maximum SBS after thermocycling and RelyX unicem showed minimum.

Graph 1

Graphical representation showing mean loss of shear bond strength after thermocycling. Graphical bar diagram representation depicts that RelyX test showed maximum loss of SBS and G-CEM test group showing minimum loss of SBS

Figure 1

SEM images of specimens cemented by RelyXUnicem resin cement. This figure shows adhesive type of failure

Figure 2

SEM images of specimens cemented with G-CEM LinkAce resin cement. This figure shows adhesive type of failure

DISCUSSION

Shear bond strength test is a evaluation procedure which is used to test the adhesion of the dental cement.[3] Thermal cycling is an in-vitro method to increase the aging of restorative materials Amaral et al.[4] 2007. Rosentritt M et al.[5] (2005) stated that the tensile strength of PEEK is equivalent to bone, enamel, and dentin, which makes it more acceptable.

CAD CAM milling is used for fabrication of PEEK prosthesis, injection molding, or pressing.[6] Bidra et al.[7] (2013) reported the advantages of CAD-CAM technique such as decreased number of appointments, decreased duration of prosthesis manipulation, easy manufacturing of trial dentures using stored digital data, and highly precise, shape, or quality. In this study, CAD CAM technique was used for the fabrication of PEEK samples, in which a steel die of dimensions 1 cm × 3 mm was sprayed with titanium dioxide and scanned, which is in conformity with the study done by Merk et al.[8] (2016). To enhance the mechanical bond strength, sandblasting is an effective method to change surface morphology of materials. Zhou L et al.[9] (2014).

RelyX Unicem and GC G-CEM LinkAce used in this study are dual-cure self-adhesive universal resin cements. Samples were divided into two groups with Group I cemented with RelyX Unicem (20 samples were divided into each half control and test groups) and Group II cemented with G-CEM LinkAce (20 samples were divided into each half control and test groups). Cylindrical putty mold index of 5 mm in diameter and 4 mm in height was used to guide the application of resin cements in accordance with Jung lee et al.[6] (2015). The technique, as described by Darius Raudy et al.[10] (2015), is adopted in our study to ensure a precise fit of the resin cements to PEEK sample. Each half from group (A&B) was subjected to thermocycling for 20 s at 5°C and 55°C with a 5-s interval between each cycle. Each sample was subjected to 500 cycles. ISO TR 11405 (1994) – Number of cycles: 500 cycles Temperature: 5–55° Dwell time ≥20 s.[5]

Specimens were held in a metal holder in a UTM. Load was applied parallel to the long axis of the specimen through a wedge at the PEEK resin cement interface. The maximum load at failure was recorded by the system's software. Shear bond strengths [MPa] was obtained by dividing the failure load [N] with the bonding area in conformity with the study done by S.T. Rasmussen et al.[11] (1996) and Jung Jun Lee et al.[6] (2015). Reduction in shear bond strength is more in RelyX Unicem (52.2%) than G-CEM LinKace (20.9%). G-CEM LinkAce showed highest shear bond strength even before and after thermal cycling.

The mode of failure analyzed by scanning electron microscope was found to be adhesive failure between the adhesive and adherent. These types of failure could be due to lack of adhesion between the bonded tooth surface and the resin cement, which is in conformity with study done by Jong-Eun Kim et al.[12] (2016).

CONCLUSION

G-CEM LinkAce has shown highest shear bond strength even before and after thermocycling.

The disparity in the mean shear bond strength before and after thermocycling was less for G-CEM Linkace when compared to the RelyX.

Adhesive types of failures were observed.

Statistical analysis

Data was analyzed by Microsoft excel and graph pad prism software. Data was summarized by Mean ± SD for continuous data. The comparison between two groups was done by paired t-test for continuous data. All P values (P < 0.05) were considered as statistically significant.

For the characterization of mode of failure, the test specimens and tooth specimens were evaluated by SEM.

SEM images of specimens cemented by RelyX Unicem resin cement.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.