|Year : 2015 | Volume
| Issue : 1 | Page : 3-7
Rely-X unicem self-adhesive universal resin cement
Saleh Zidan1, Syed Wali Peeran2, Karthikeyan Ramalingam3
1 Department of Dental Materials, Faculty of Dental Materials, Sebha University, Sebha, Libya
2 Department of Periodontology and Oral Implantology, Sebha University, Sebha, Libya
3 Department of Oral Pathology and Microbiology, Faculty of Dentistry, Sebha University, Sebha, Libya
|Date of Web Publication||21-Jan-2015|
Department of Oral Pathology and Microbiology, Faculty of Dentistry, Sebha University, Sebha
Source of Support: None, Conflict of Interest: None
There is an expansive usage of adhesives in all branches of dentistry. Newer adhesives can be more beneficial to the patients. This review attempts to highlight the important features of adhesive cements in general and emphasize the applications of Rely-X Unicem cement.
Keywords: Rely-X Unicem, self-adhesive cements, universal resin cements
|How to cite this article:|
Zidan S, Peeran SW, Ramalingam K. Rely-X unicem self-adhesive universal resin cement. Dent Med Res 2015;3:3-7
| Introduction|| |
The use of adhesives in the field of dentistry has grown extensively. Beside the quantitative growth, the products have gone through the process of significant qualitative enhancements. New and different types of adhesive materials are introduced that could offer many advantages that will deliver large advantages to both patients and dentists.
The capability of adhesive resin cements to link to both tooth structure and restoration is a valuable quality. "The integration produces reinforcement of both structures and reduces microleakage at the restoration-tooth interface, postoperative, marginal staining and recurrent caries." 
Rely-X self-adhesive cement is an all in one adhesive in which these three bonding processes come together into a single step. 
Rely-X Unicem Self-adhesive Universal Resin Cement ( Rely-X Unicem), from 3M ESPE, United States provides a significant step in this direction. The overall advantages of Rely-X Unicem are that it combines the advantages of conventional cements and modern composite cements. Both Rely-X Unicem and glass ionomer cements release fluoride ions into the tooth substance. As a result of its good self-adhesive bonding quality there is no need for costly and lengthy procedures with Rely-X Unicem, and this provides a lot more comfort to the patient and is far more efficient to the dentist. The disadvantage of Rely-X Unicem is that it is a relatively new product and therefore lacks a long period of clinical experience. 
| Development of adhesives|| |
Adhesives were introduced to dentistry in the 1950's by studies of bonding enamel and dentin.  The acrylic resins filled with silica particles were adhered to the surface enamel that had been etched with acid. In early years, the major problem of resin-based adhesives was their hydrolysis. The hydrolysis leads to a lack of bond between materials and tooth surfaces, as well as degradation of the bridge or dental filling that on its turn could cause caries. The improvement of adhesive techniques to bond materials to teeth is highly desirable in dentistry because dental applications require excellent adhesive qualities. 
In terms of clinical uses, these can be defined into two categories: The first system contains a phosphoric acid etching for enamel and a bottle of adhesive resin. The second system contains a self-etching primer which combines the etching and priming in one procedure, so called self-adhesive resin cement. 
Self-adhesive resin cement (Rely-X Unicem; 3M ESPE) requires one step, in contrast to other etching adhesive systems, which involve a multi-step application technique and are, therefore, more time-consuming. "The self-adhesive resin cement is based on a new monomer, filler, and initiation technology. The manufacturer purports that the organic matrix consists of newly developed multifunctional phosphoric acid methacrylates. The phosphoric acidic methacrylates can react with the basic fillers in the luting cement and the hydroxyapatite of the hard tooth tissue." 
| Principles of adhesion|| |
Adhesion is the procedure of building adhesive joints between two surfaces. The first surface is called adherent 1 and the second surface is called adherent 2. The material between them is called adhesive material. Thus, the joining of the two adherents through an adhesive material will produce two interfaces. Such two-interface joints are the most common type used in dentistry. 
Anusavice defined dental adhesion as "when two substances are brought, or are attracted to, molecules of other, the molecules of one substance adhere, or are attracted to, molecules of the other substance." Anusavice refers to adhesion as a situation in which unlike molecules are attracted, and cohesion when molecules of the same kind are attracted. He refers to the material or film used to cause adhesion as the adhesive and to the material to which it is applied as the adherent. 
The most chemical adhesion between two phases is based on chemical bonding. If this bond is very strong across an interface, it will increase the attachment that gives good adhesion. 
Chemical bonds are divided into interatomic primary bonds and interatomic secondary bonds "the forces that hold atoms together are called cohesive forces. These interatomic bonds may be classified as primary or secondary. The strength of these bonds and their ability to reform
after breakage determine the physical properties of a material." There are three different types of primary atomic bonds: Ionic, covalent and metallic. For interatomic secondary bonds, there are two types of hydrogen bonding and secondary bonding. 
Mechanical adhesion relies on mechanical interlocking of two phases that include microscopic attachments. That can be seen when resin bonds to etched enamel. Stresses occur in porcelain crowns around the metal core. 
However, if the thermal expansion coefficient is different between adhesive and substrate, the difference in temperature will produce stresses in the bond.  Diffusion bonding results when one phase penetrates by diffusion into the surface of the second phase and forms a "hybrid" layer, which is a composite of the two materials. 
Adhesive materials have been used to bond the metal surface of crowns and bridges with teeth surfaces in order to create the best retention between them. It is difficult to place a crown or bridge without adhesive materials. 
Orthodontic brackets and bands have been bonded to tooth structures using adhesive materials. To bond orthodontic brackets on the tooth surface, the acid etching technique has to be used. This technique was discovered over 40 ago by Buonocore and involves the cure of the tooth surface with phosphoric acid for the brackets to bond [Table 1]. 
|Table 1: Comparison between different types of luting cements with their right applications and weaknesses (from 3M ESPE 2005) |
Click here to view
Rely-X Unicem has been created as a result of the need for adhesive luting cement that has wide application in dentistry for cementation of crowns, bridges, indirect restorative and orthodontic. Rely-X Unicem has better mechanical properties and excellent adhesion and esthetics when compared to other cements. A study by 3M ESPE (2002) has shown that Rely-X Unicem tolerates the moisture and releases fluoride ions, as well as the process of cure requires only one step, accordingly there is no need for etching and removal of the smear layer. 
The 3M ESPE has reported that "the existence of at least two phosphoric acid groups and a minimum of two c = c double bond units per molecule are typical for the monomers in Rely-X Unicem and this supplies high reactivity as well as the matrix' high degree of cross linking. This leads to excellent mechanical properties and an adhesive bond without pretreatment that has long period stability (3M ESPE Technical Product Profile 2002). 
The new initiator system had to meet other requirements: Effectiveness in light and self-curing, moisture tolerance, resistant to a wide pH range and a high degree of cross-linking of the monomer matrix. Achieving excellent bonds by Rely-X Self Adhesive on tooth surfaces, involves a setting reaction that is a radical polymerization reaction. From this reaction, 3M ESPE has pointed out that we can obtain two advantages, which are the release of fluoride and the intelligent switch to a hydrophobic matrix (3M ESPE Technical Product Profile 2002). 
| Applications|| |
The use of Rely-X Unicem in operative and prosthodontic applications without etching the enamel or dentin was evaluated by Bishara et al.  In a study by De Munck et al. it has been reported that resin based adhesive luting materials are widely used for the fixation of inlays and onlays, crowns, bridges, posts and veneers. They were all resin cements that relied on the use of an etch adhesive with multi-step application technique, which is complex and slightly technique sensitive. However, recently a new technique of resin-based cement has been marketed that incorporates the use of adhesive and cement in one single application. 
The first self-adhesive universal resin cement designed for universal application is Rely-X Unicem. It is presented in capsules that can be used for adhesive cementation of indirect ceramic, composite and metal based restoration as well as crowns and bridges (3M ESPE Technical Product Profile 2002). 
| Properties of rely-x|| |
A study by Piwowarczyk et al. has evaluated microleakage in full cast crown restoration bonded with Rely-X Unicem Self Adhesive Resin Cement in enamel and dentin. The smallest degree of micro leakage that was noted can be compared with that of other adhesive cements, such as Rely-X ARC and Panavia F. The study of Rely-X Unicem Self-Adhesive cement concluded that it can be used for long term cementation for many applications as well as offer good adhesion. 
In a study by Behr et al., it has been reported that the marginal adaptation of ceramic crowns on dentin using Rely-X Self-Adhesive resin cement, without pretreatment and one step compared with other adhesive systems, is good.  The evaluation of marginal quality after the stress test in the mastication simulator exhibited excellent results in marginal quality and density by Rely-X Unicem compared to other adhesive systems.
Another study by 3M ESPE (2003) has shown a superb marginal integrity after constant load. Rely-X Unicem is easy to apply in tooth structures, like ceramic inlays. Ceramic inlays have shown good quality of marginal seal and abrasion characteristics during thermocycling and mechanical load. 
The fit of a precision indirect restoration is highly dependent on the thickness of the cement film. When the thickness is too high, tight fitting restorations may suffer from open margins and misalignment as a result of not being able to seat well enough. A low film thickness is desirable for tight fitting indirect restorations, as it allows them to seat completely. In case of light cured restorations under indirect restorations and for endodontic posts, extra layers of the adhesive should not be applied and pooling should be avoided in the areas that could potentially be of influence to the fit. 
Properties of water absorption and expansion of Rely-X Unicem cement can lead to clinical problems with indirect restorations, like fixed crowns and bridges that should have long-term stability without failure. A study has reported that Rely-X Unicem cement has a low expansion which can bear for long time stability. 
Radiopacity of Rely-X Unicem Self-Adhesive cement is important for dentists to diagnose decay and to notice open gingival margins at the cervical of the tooth as well as determining the existence of a gap between tooth and cement. It is significant that adhesive cements have greater radiopacity than dentin, due to the fact that it is hard to discover a cement line radiographically when the material is not significantly more radiopaque than dentin. In some situations where the margin is set in a difficult access area, of which the marginal fit is less than perfect, optimal radiopacity helps to ensure complete removal of excess material. The testing of Rely-X Unicem Self-Adhesive Cement has shown that the radiopacity is the same as human enamel. 
Bonding agents may cause local reactions in dentists and dental assistants, who, therefore, must protect themselves from critical materials by wearing gloves and immediately replacing them in case of contamination. Furthermore, they have to use volume evacuation where the materials are being used. All bottles must be conserved closed, or unit dose systems must be used. Patients should also be protected while bonding operations.  In a study that evaluated the cytotoxic effect of Rely-X Unicem on pulp it was found that with Rely-X Unicem the survival rate of pulp cells on dentin slices of 200 μm is almost 80%. On dentin slices of 500 μm, the survival rate is about 95%. With Rely-X Unicem, occurrences of postoperative sensitivity are close to nothing. Out of 4829 independently monitored cases, only 5 (0.1%) showed short-term sensitivity. Compared with other materials, the overall conclusion is that Rely-X Unicem has a good biocompatibility (3M ESPE November, 2003). 
The mechanical properties are important in terms of adhesive cement that can stand for a long time without fractures. Rely-X Unicem self-adhesive cement has superb mechanical properties and establishes increased retentive competency. In a study by Attar et al. it was reported that according to ISO standard 4049 Rely-X Unicem has a high flexural strength and high modulus of elasticity. 
Self-adhesive cements like Rely-X Unicem have brought the bonding process of, for example, orthodontic brackets, back to a one-step procedure. Many other applications have also been simplified as a result of recently developed self-adhesive cements. Due to a range of desirable properties, Rely-X Unicem has become a popular dental material in many dental procedures. Among these properties are a high compressive strength, although not as strong as resin cements, a relatively high flexural strength and durability in terms of adhesion and strength. 
When used in operative and prosthodontic applications without the etching of enamel, Rely-X Unicem has shown contradictory results. Another study showed that Rely-X Unicem's and Panavia F's tensile strength bond was only similar when the tooth surface was acid-etched before Rely-X Unicem was applied. 
The available data on Rely-X Unicem use as an orthodontic bracket adhesive is quite minimal. In one study it was shown that after storing the teeth for 24 h at 37C, the shear bond strength produced by Rely-X Unicem in orthodontic bracket bonding was significantly lower than in the cements that used three-step acid-etch procedures. Despite this, Rely-X Unicem's shear bond strength was still acceptable for use as an orthodontic bracket adhesive. 
In an experiment, Rely-X Unicem showed a significantly lower μ Tensile Bond Shear (TBS) to enamel than the control cement, Panavia F. No difference was observed in dentin bonding. When applied on acid-etched enamel, Rely-X Unicem's μTBS significantly increased to a level comparable to that of Panavia F when bonded to (nonetched) enamel. When applied on acid-etched dentin, the acid-etching significantly lowered Rely-X Unicem's μTBS and Panavia F showed a significantly higher μTBS than Rely-X Unicem. 
In vitro studies have been performed on Rely-X Unicem on its properties of adhesion to restorative materials, marginal adaptation, and microleakage. , In comparison with Rely-X unicem, Hattar et al. have reported that the highest strength was observed for SmartCem2 (14.18 Mpa). 
Rely-X Unicem did not have a significant decrease of mechanical stability after thermal loading. The hydrophilic nature and water sorption due to low-pH values in the final cured cements leads to a higher decrease in flexural strength in most self-adhesive cements. 
| Conclusion|| |
Most of the internal and independent data suggest that Rely-X Unicem has better characteristics for dispensing, handling and overall ease of use with clinical excellence and reliability. It had reduced the incidence of microleakage and very low debonding rates in various dental applications.
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