Evaluation of Push out Bond Strength of A Dual-Cure Self-Adhesive Resin-Cement with Fiber Post Systems and Dentine

Madiha Pirvani¹                             BDS, MSc

Shoaib Khan^{2                                             BDS, MSc}

Sofia Malik^{3                                     BDS, MSc}

INTRODUCTION

ndodontically treated (ET) teeth are known to have a higher fracture rate than vital teeth. This happens because the tooth structure is lost as a result of caries and then access preparation for root canal treatment (RCT)^1,[1]. To improve the prognosis of ET teeth, they should be restored to enhance their structural integrity^[2].  In this regard post plays an important role in providing retention to a core which supports the coronal restoration. The decision to place post  depends on the amount of  remaining natural tooth substance after the removal of caries and endodontic treatment in order to provide adequate retention to the core buildup and to support the restoration⁴.

There are different types of posts available according to their mode of retention including active and passive posts which can be further subdivided⁵. Various materials have been used to fabricate posts such as metals, ceramics or fiber-reinforced composites. Metallic posts exhibit good physical properties but due to their disadvantages like high incidence of root fracture due to wedging effect, corrosion and nickel sensitivity, metal-free post and core system such as ceramic and fiber reinforced posts has been introduced^4,6.

A number of fiber posts have been introduced during the past decade which are pre fabricated and can be cemented to the root canals. They have similar elastic modulus as that of dentine decreasing the incidence of root fracture, aesthetic acceptable, easy to remove in case of retreatment, do not corrode and are able to be cemented with a luting agent to avoid friction between the post and the root dentine walls^7-10.

Glass fiber posts were introduced in 1992 to overcome the aesthetically unpleasant appearance of carbon fiber posts¹. They can be made of either Electrical glass (E-Glass), Silica Glass (S-Glass) or Quartz fibers¹¹.They are white or translucent and are ideal for situations of higher cosmetic demand. Some major advantages associated with glass fibers include distribution of stress over a broad surface area¹², increased light transmission within the root and overlying gingiva¹³, value of elastic modulus less than carbon fiber posts¹¹,  thus transmitting a very small fraction of forces to the root dentine walls¹⁴.

Composite resin luting cement is the most suitable adhesive to be used with fiber-reinforced posts¹⁵. Fiberreinforced composites (FRCs) have shown improved mechanical properties than the conventional composite resin material¹⁶. It consists of difunctional monomer bisGMA, diluents and quartz or silica fillers. The surfaces of the fillers are treated with a coupling agent to improve their bonding to the matrix thus exhibit better mechanical properties¹⁷. The resin cement bonds to the glass fiber posts either by interdiffusion of the resin monomer into the polymer structure of the posts or by free radical polymerization^18,19. In enamel micromechanical retention is achieved by etching the enamel surface with acid²⁰. Whereas in dentine the retention is achieved by the flow of resin in the dentinal tubules that have been exposed during the procedures of cutting and smear layer removal, resulting in the formation of resin tags²¹. The application of adhesives is a multi-step procedure which is time consuming, complex and technique sensitive²². Therefore, self-etching adhesives have been introduced in which the polymerizable acidic monomers etch and prime the enamel and dentine thus eliminating the need for pretreatment of the teeth²³.

Although resin cements are predominantly used with fiber posts, Glass ionomer cement (GIC) may offer several advantages over resin cements for luting fiber posts. GIC is inherently adhesive to tooth without the need of pre-conditioning²⁴. GIC has enjoyed a fair amount of success as a luting cement and is widely used for a variety of indications²⁵.

Literature is replete with studies on different types of resin cements and their interaction with fiber posts in extracted teeth. Few studies as available to date that compare the bonding efficacy of resin cement and GIC with fiber posts. Therefore; the aim of the study was to compare the bond strengths of two luting cements with two types of fiber posts system and with dentine using a push-out test setup.

METHODOLOGY

Specimen Preparation

Forty intact, extracted, non-carious human single rooted teeth were selected. The teeth were cleaned and stored in alcohol. Then the crowns of all the teeth were cut approximately at the level of cemento-enamel junction (CEJ) with a water- cooled diamond blade on a precision slicing machine (Microslice 2, Malvern Instruments, Malvern, England). Post spaces were made in each root with the help of drills supplied by 3M, UK. These drills were color coded according to their sizes (yellow drill: 1.3mm; red drill: 1.6mm & blue drill: 1.9mm). The drills were fixed in a bench drill machine and a straight post space was made in each root with continuous supply of water acting as a coolant. The root canal in each tooth was rinsed with distilled water and dried gently. Total 40 roots were prepared and were assigned to four equal groups (A1, A2, B1 & B2) according to the luting cement and fiber posts used to fill the post space as shown in table 1.

Two luting agents were used, glass ionomer luting cement (AquaCem®, Dentsply, DeTrey, Germany) and a dual-curing self-adhesive universal resin cement (RelyX™ Unicem, 3M ESPE, USA). Two types of glass fiber posts were used, one with resin impregnated nonpolymerized E-glass fibers (Everstick® POST, Stick Tech, Turku, Finland) and the other containing pretensioned glass fibers embedded in a resin matrix (RelyX™ Fiber Post, 3M ESPE, St Paul, USA).The composition of all the materials used in the study is given in Table 2.

The Everstick® POST were first cut according to the length of the root canal, then coated with a thin layer of a light cured adhesive (3M ESPE Adper™ Scotchbond™, Multipurpose Adhesive, USA), placed in the post space and then light cured for 5 seconds. The posts were then taken out of the post space. The other fiber posts; the RelyX™ Fiber Post did not need any kind of pretreatment with the adhesive.

3M ESPE RelyX™ Unicem was manually mixed according to the manufacturer’s instructions and then both type of fiber posts in groups A2 and B2 were coated with the cement. The cement coated posts were inserted directly into the post space with the application of a slight pressure. Then for the cement to set, it was light cured for 20 seconds. AquaCem® Dentsply, was also manually mixed according to the manufacturer’s instructions and then coated onto the fiber posts of groups A1 and B1, the posts were inserted into the post space with slight pressure and left so that the cement can set by self-curing. Each post cemented root was sectioned horizontally into three slices of 2mm thickness with the help of water cooled diamond blade on a precision slicing machine (Microslice 2, Malvern Instruments, Malvern, England).

Bond Strength Assessment

To assess the bond strength between the luting cement and the fiber posts, push-out test was selected. Each section was placed on a horizontal stainless steel platform with a central circular perforation. The plunger tip is a metal rod 1mm thick and is used to apply a vertical static load to only the post without stressing the surrounding root canal walls. To avoid any limitation to the  post movement during load application due to root canal taper the load was applied in an apical aspect of the root slice in an apical-coronal direction so that the post could be pushed towards the larger part of the root slice. Force was applied by a testing machine (J J Instruments T30K, tensile testing machine from Lloyd Instruments, UK) at a crosshead speed of 1mm.min-1 till the post was extruded out of the root section manifesting bond failure. The bond strength (MPa) was calculated by dividing the load at failure (Newton) with bonding surface area (mm²).

MPa =    N/2 X prh

Where p = 3.14, r is the radius of the post segment (mm), and h is the thickness of the post segment (mm). A digital caliper was used to measure the thickness of each section and the range was in between 1.9- 2.2 mm.

SEM Analysis

Two cross sections of root from each sample were taken for SEM analysis. Each cross section was cut in the middle with water- cooled diamond blade on a precision slicing machine (Microslice 2, Malvern Instruments, Malvern, England). These halves and the post extruded from them after the push-out test were mounted with carbon cement on the stainless steel platforms. After drying the samples were then sputtercoated with gold. They were kept covered until they were analyzed by using a field emission scanning electron microscope JSM- 6300F (JEOL, Tokyo, Japan).

Statistical Analysis

The differences between the bond strengths of the two luting cements used with two types of fiber posts were assessed using a statistical software package (SPSS® version 22 for Windows). A non-parametric test, kruskul wallis was used for comparing the medians of the four study groups. Tukey’s post hoc analysis was used to determine the group with statistically significant values. Level of significance was kept at 0.05. A p value < 0.05 was considered as significant.

RESULTS

The mean push-out bond strength and standard deviation (SD) achieved after the extrusion of both types of posts from the root segments in each group are listed

in Table 3.  Multiple comparisons with kruskul wallis test and tukey’s post hoc analysis demonstrated that the bond strength was significantly influenced by the interaction between the type of luting cement and fiber post used (Table no 4). The mean bond strengths of RelyX™ Fiber Posts cemented with AquaCem® and RelyX™ Unicem were not statistically significant (p=0.696). Whereas the mean bond strength of Everstick glass fiber posts cemented with AquaCem or RelyX™

Unicem was lower than RelyX™ Fiber Posts cemented

with either cement. This difference was statistically significant. (p<0.05) (Table 4).

Regarding SEM analysis, there was significant difference in the interaction between the luting cement and dentine after the push-out test. Etched dentine and pieces of the AquaCem®  attached to the tooth structure were clearly visible whereas very little amount of AquaCem® could be detected attached to RelyX™ Fiber posts. The interactions of RelyX™ Unicem with the dentine also illustrated formation of resin tags but penetration of the IPN polymer structure of Evertsick® POST by RelyX™ Unicem was also evident as a considerable amount of cement could be seen with the RelyX™ Fiber Post after the push out test.

DISCUSSION

In this study bonding strength of two types of luting cements (AquaCem® and RelyX™ Unicem) with the two types of fiber posts (Everstick® POST and RelyX™ Fiber Posts) and dentine was evaluated. The factors affecting the bond strength are degree of micromechanical interlocking as well as chemical adhesion between the root canal dentine, the luting agent and the posts. The two basic modes of adhesion of the resin materials are  resin monomer diffusion into the polymer phase of the substrate and free radical polymerization of the bonding resin²⁶. It was also concluded by a study conducted by Sperling et al 1994 and Mannocci et al 2005, that there are two requirements for the resin based materials to undergo interdiffusion i.e. availability of a polymer substrate that is not cross linked and has partially or totally linear polymer structure such as the IPN polymer structure and a close match between the solubility parameters of the solvent so the linear or IPN polymer structure can be dissolved by monomers of the resin^18,19.

The push-out test results for model A1 and A2 showed a significant difference between the bond strengths of the two luting cements used with Everstick® POST (p<0.005). The RelyX™ Unicem cement exhibited much higher bond strength with the Everstick® POST (1.621 MPa; SD±0.686) than the AquaCem (0.613 MPa; SD±1.496). This could be due to the fact that the monomers of RelyX™ Unicem cement penetrated into the linear phase of the IPN polymer structure of everstick posts which is polymethylmethacrylate (PMMA), whereas there process of interdiffusion  is not possible in AquaCem  and its setting reaction is primarily an acid base reaction which allows it to bond  to the tooth substance chemically. This occurs by the  chelation of the carboxyl groups in the acid  present in AquaCem with the calcium and phosphate ions present in the apatite of enamel and dentine and thus had no effect on the polymer structure of the posts^27,28.

The results showed that there was no significant difference between the bonding strengths of both the luting cements when used with RelyX™ Fiber Posts (Model B1 and B2).The results obtained were quiet expected as there is a basic structural difference between the two fiber posts. The Everstick® POST contain a semi-IPN polymer structure while the RelyX™ Fiber Posts have an already cured and cross-linked epoxy resin matrix leaving  little if any reactivity required  for free radical polymerization bonding  therefore there was no actual chemical bonding  between the resin cement and the posts²⁹. This structural difference results in different modes of adhesion of the luting cement to the posts. The semi-IPN makes interdiffusion mechanism possible in Everstick® POST. But, in the RelyX™ Fiber Posts neither interdiffusion nor free radical polymerization was possible. So in that case where resin based cements could not penetrate into the posts polymer structure their bond strength was almost same as that of AquaCem®. These results can be further supported by comparing them with the results obtained from a study carried out by Mannocci et al (2005) in which the penetration of two different bonding resins applied on glass reinforced composite root canal posts was compared, one containing an interpenetrating polymer network (IPN) and the other containing a cross-linked polymer matrix. The posts with IPN were EverStick® POST and the ones with cross linked polymer matrix were C Post Millennium. These posts were immersed in bonding resins (Scotchbond Multi Purpose Plus, 3 M, St Paul, MN, USA and Stick Resin, Stick Tech Ltd). The confocal microscopic results showed that the degree of penetration of resin into the EverStick® POST was remarkably higher than in the C Post Millenium¹⁸.

The similar bond strengths of the luting cements in models B1 and B2, could also be due to the fact that the RelyX™ Fiber Posts are tapered in shape i.e. they are cylindrical coronally and conical apically. This configuration resembles the anatomical structure of the root. Additionally, the post holes were made by the drills which were provided by the manufacturer according to the size and shape of the RelyX™ Fiber Posts so these posts fit in the post spaces quite well. On the contrary, the Everstick® POST are not tapered and sometimes two or three posts were used to pack the post space. Therefore, it can be hypothesized that RelyX™ Fiber Posts were fixed quiet tightly and gained much of the retention by their shape in the canals and hence both the cements showed higher bond strength with these posts as compared to Everstick® POST.

In the present study there was no pretreatment done on the root canal dentine before the application of both the cements. The pretreatment such as acid etching could have generated different results because acid etching removes the smear layer from the root canal dentine and smear plugs are formed by the resin cement resulting in more efficient micromechanical retention³⁰.

SEM Analysis

The SEM results revealed the interaction of the two luting cements with the dentine after the push-out test. The AquaCem® showed bonding to dentine by chelation and little if any cement was observed to be attached with the Everstick® POST after the test concluding that the bond failure occurred between the post and the cement.

This result confirms that the lower bond strength values obtained from the combination of AquaCem® and Everstick® POST was because of the fact that the cement made a chemical bond with dentine and did not show any affinity towards the posts’ polymer structure. The same mode of adhesion occurred in both the cases i.e. AquaCem® with Everstick® POST and AquaCem® with RelyX™ Fiber Post. But the latter provided higher bond strength values because the RelyX™ Fiber Posts were tapered in shape and fitted quiet well into the prepared post spaces as these post spaces were made with the drills which were supplied by the manufacturer according to the size of the posts.

The RelyX™ Unicem SEM images show etching of the dentine and hence resin tags formation, but it is also clear from the images that this resin cement penetrated into the IPN polymer structure of Everstick® POST and made a bond with them. In the case of RelyX™ Fiber Posts the cement could have reacted with the polymer network to make a bond with these posts too. Therefore a considerable amount of cement was seen attached to the Everstick® POST and RelyX™ Fiber Posts after the push-out test. This indicates that adhesive failure of the cement occurred in these cases and the bond strength values obtained were therefore higher.

CONCLUSIONS

Within the limitations of the study it can be concluded the type of luting cements and posts significantly affected the bond strength. RelyX™ Unicem was more effective and demonstrated the highest bond strength values with both type of fiber post systems used in the study compared to AquaCem®. AquaCem® presented similar bond strength values as the RelyX™ Unicem with RelyX™ Fiber Post (p =0.696). The study also highlighted that bonding mechanism to root canal dentine was dependant on the type of luting cement rather than the glass fiber posts used in the study. With AquaCem®, the mode of failure as observed by SEM, was predominantly at the interface of glass fiber post and root canal dentine, whereas the mode of failure associated with RelyX™ Unicem was predominantly adhesive in nature.

ACKNOWLEDGEMENTS

“Based on a dissertation submitted to School of Engineering and Materials, Queen Mary University of London, London, United Kingdom, in partial fulfillment of Master of Science (MSc) in Dental Materials degree.”

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1. Assistant Professor, Department of Science of Dental Materials, Dr. Ishrat-ul- Ebad Khan Institute of Oral Health Sciences, Dow University of Health Sciences, Karachi, Pakistan.
2. Assistant Professor, Head of the Department, Department of Science of Dental Materials, Ziauddin College of Dentistry, Ziauddin University, Karachi, Pakistan.
3. Assistant Professor, Department of Science of Dental Materials, Dow International Dental College, Dow University of Health Sciences, Karachi, Pakistan.

Corresponding author: “Dr Madiha Pirvani ”  < madihapirvani@hotmail.com  >