Two year clinical evaluation of a low-shrink resin composite material in UK general dental practices

Abstract

Objective

A novel resin composite system, Filtek Silorane (3M ESPE) with reduced polymerization shrinkage has recently been introduced. The resin contains an oxygen-containing ring molecule (‘oxirane’) and cures via a cationic ring-opening reaction rather than a linear chain reaction associated with conventional methacrylates and results in a volumetric shrinkage of ∼1%. The purpose of this study was to review the literature on a recently introduced resin composite material, Filtek Silorane, and evaluate the clinical outcome of restorations formed in this material.

Methods

Filtek Silorane restorations were placed where indicated in loadbearing situations in the posterior teeth of patients attending five UK dental practices. These were evaluated, after two years, using modified USPHS criteria.

Results

A total of 100 restorations, of mean age 25.7 months, in 64 patients, were examined, comprised of 30 Class I and 70 Class II. All restorations were found to be present and intact, there was no secondary caries. Ninety-seven per cent of the restorations were rated optimal for anatomic form, 84% were rated optimal for marginal integrity, 77% were rated optimal for marginal discoloration, 99% were rated optimal for color match, and 93%% of the restorations were rated optimal for surface quality. No restoration was awarded a “fail” grade. No staining of the restoration surfaces was recorded and no patients complained of post-operative sensitivity.

Significance

It is concluded that, within the limitations of the study, the two year assessment of 100 restorations placed in Filtek Silorane has indicated satisfactory clinical performance.

Introduction and literature review

Practice-based research

A majority of research into the effectiveness of dental materials is carried out in dental hospitals or other academic institutions, rather than in general dental practice, even though the latter is where the majority of dental treatment is performed, worldwide. Reasons for this divergence include the potential cost, given that practices are geared to the efficient treatment of patients rather than research and a perception that the training of general practitioners in research methods may be incomplete. However, there are many reasons why dental practice increasingly should become the prime location for clinical dental research. Dental practice is the real world, better representing the day-to-day handling, placement and service life of resin composites. The importance of practice-based research has been emphasized by Mandel, who considered that “research is not only the silent partner in dental practice, it is the very scaffolding on which we build and sustain a practice” . An advantage for the practitioner is the benefit of being involved in something not normally within the daily routine of practice, and that patients have been found to approve of practitioner involvement in research, with the practice and practitioner’s professional image being enhanced .

The performance of a restorative material by one operator is necessarily subjective, but when practitioners band together to form a group in order to evaluate new materials in dental practice, the results are likely to be more objective. All of this is possible when practitioner-based research groups are teamed with the expertise available in academic institutions. Perhaps the most well known group of practice-based researchers is the C linical R esearch A ssociates (CRA) founded by Gordon Christensen in Utah, USA over thirty years ago. A UK-based group of practice-based researchers is the PREP ( P roduct R esearch and E valuation by P ractitioners) Panel. This group was established in 1993 with 6 general dental practitioners, and has grown to contain 33 dental practitioners located across the UK and one in mainland Europe. It has completed over 50 projects—“handling” evaluations of materials and techniques, and more recently, clinical evaluations ( n = 9) of restorations at one year and up to five years. This paper describes the early performance of a novel resin composite restorative material, Filtek Silorane (3M ESPE, Seefeld, Germany), when placed in loadbearing situations in posterior teeth of patients attending the practices of five members of the PREP Panel.

Resin composite restorations and polymerization contraction stresses

The majority of conventional resin composite restorative materials shrink up to 3% on polymerization, resulting in stresses at the (bonded) restoration margin, or within the restorative material itself , with the clinical result of these stresses being :

  • Internal microcracks within the bulk of the material.

  • Separation of the bonding agent from the cavity wall, with resultant marginal leakage and post-operative sensitivity.

  • Enamel microcracks, with a resultant white line adjacent to, or at a distance from the restoration.

  • Deformation of tooth, also leading to pain post-operatively, generally when the patient bites on a cusp.

Shrinkage stress is not an intrinsic material property and the magnitude of the stresses depends on a number of factors, including properties that are intrinsic to the material, such as:

  • volumetric shrinkage,

  • the modulus of elasticity,

  • the degree of cure (polymer conversion),

  • the coefficient of thermal expansion,

  • silanization characteristics at the resin-filler interface,

and clinically oriented factors such as:

  • the rate of cure and polymerization kinetics,

  • the configuration of the cavity into which the restoration is placed,

  • compliance of the remaining tooth structure.

In this respect, it has recently been demonstrated that it is in larger, rather than smaller, Class I cavities that the effect of the so-called configuration factor may be most relevant .

A number of clinical techniques have been suggested to reduce or overcome the effect of polymerization contraction stresses. Table 1 presents some of the techniques which have been advocated for minimizing stress. The benefits of certain techniques such as “soft-start” or “ramped” curing, or the use of flowable resin composites is debated in the literature. The former method may lead to decreased structural integrity and, depending on material formulations, the latter may increase polymerization shrinkage compared with conventional techniques.

Table 1
Clinical techniques which have been suggested to reduce or overcome the effect of polymerization contraction stresses.
• Incremental placement, with one increment touching only one wall of the cavity, and, limiting the size of the increments
• Ramped curing, in which the curing light does not reach its maximum intensity for up to 20 s
• Pulse activation, in which the resin composite material is cured for 5 s and then left for up to 5 min
• Use of macro-fillers to reduce resin volume: however, this has not been shown to improve clinical effectiveness
• Placement of a flowable composite base layer which has been shown to reduce microleakage at the gingival margin in Class II cavities in a number of in vitro experiments
• Use of a chemically cured composite or glass ionomer base

It could also be considered that some or all of these additional stages lead to increased technique sensitivity during placement of resin composite restorations, and indeed, that these stages, which are designed to reduce polymerization contraction stress, could be a source of operator stress! The use of a resin with reduced polymerization shrinkage, with a net volumetric shrinkage of nil could therefore be an advantage to the clinician.

Resin composite modification and reduced shrinkage stress

The magnitude of polymerization shrinkage stress generated at the tooth-restoration interface and extent of any gap formation is a multifactorial process. It might be considered that commercial resin composites with lower elastic modulus (i.e. “flowables”) do not necessarily reduce the magnitude of shrinkage stress since the volume or viscosity of the resin component is reduced and volumetric shrinkage will increase. Likewise, resin composites with lower volumetric shrinkage generally exhibit higher shrinkage stress since materials with high filler loads exhibit increased elastic modulus and an increased change in stiffness on cure. Accordingly, low-shrinking materials do not necessarily provide lower contraction stress. However, materials that exhibit reduced shrinkage using alternative resin chemistries rather than increasing filler loads may reduce stress values of constrained composites.

Bisphenol glycidyl methacrylate (BisGMA) has been used as a resin in dental composite restoratives since its development and introduction by Bowen in 1958 , However, this is a viscous resin which would be unworkable as a dental restorative when filler particles are added, and, accordingly, it is necessary to add a diluent resin to the material to allow the manufacture of a resin composite material which is readily handled by dental healthcare workers . This diluent resin is, in many materials, triethylene glycol dimethacrylate (TEGDMA). Its polymerization contraction is circa 5%, thus increasing the overall polymerization contraction of the resin composite material to which it is added. Manufacturers have obviated the use of TEGDMA, in materials introduced in the late 1990s, by substituting BisGMA in part with less viscous resins such as urethane dimethacrylate (UDMA) and bisphenol ethoxylated methacrylate (BisEMA), thereby reducing the polymerization contraction from ∼3% to 2%. In this respect, improved filler particle morphologies, which improve packing and reduce shrinkage, may also play a part.

The significant decrease in use of TEGDMA in commercial materials has played a role in reducing shrinkage stress and cuspal deflection of wide MOD cavities . A similar reduction in cuspal movement was demonstrated when an Ormocer material (Admira: Voco, Cuxhaven, Germany) with a polymerizaton contraction of 2% was used . However, the resins used in the above materials are based upon methacrylate chemistry and it would appear impossible to reduce the polymerization shrinkage of these materials to much less than the values stated above because of the inherent nature of the resins and polymerization reaction involved, although a dimer acid based material, in which phase separation purportedly reduces shrinkage without decreasing polymer conversion, has recently been reported .

The use of alternative chemistries has been at the forefront of research and development for dental resin composites for many years. Researchers have investigated the use spiro-orthocarbonate resins which expand on polymerization . However, poor reactivity and decreased mechanical properties have precluded their viability as a commercial material. Moreover, it might be argued that a net zero shrinkage or even expansion would be more detrimental than <3% shrinkage of methacrylates which allows for water uptake during service. The use of thio-lene resins may provide a suitable replacement for conventional resins and have been subject of modern resin research. The thio-lene chemistry offers a “step-growth” rather than the “chain-growth” curing characteristic associated with methacrylates. This has been reported to provide more control of the curing process and reduce polymerization shrinkage stress .

Filtek Silorane™

Filtek Silorane (3M ESPE Dental Products, Seefeld, Germany) (hitherto, in this paper, termed Silorane) has recently been marketed and is based on an innovative resin matrix . The epoxy-based resin, which contains an oxygen-containing ring molecule (‘oxirane’), cures via a cationic ring-opening reaction rather than a linear chain reaction associated with conventional methacrylates and results in a volumetric shrinkage of ∼1%, which may reduce the deleterious effects of shrinkage stress at the tooth-restoration interface. In this respect, work by Watts in 2007 has demonstrated substantially reduced polymerization shrinkage stress in comparison to other resin composite restorative materials . The incorporation of a siloxane molecule (hence the term “ silo (xane)(oxi) rane ”) has resulted in a material with comparable material properties , and increased hydrolytic stability , compared with conventional materials.

Because of the comparatively recent introduction to dentistry of Silorane, there is not a large volume of research into its properties and performance. However, the results of a number of experiments may be considered to be of interest, particularly those published in the peer reviewed literature.

Mutagenic effects

Silorane has been found to have no mutagenic effects when analyzed for the formation of chromosomal aberrations and the induction of gene mutations in mammalian cells .

Marginal adaptation

In a laboratory experiment examining the setting characteristics of commercial composites using a bonded disk method, degree of conversion and cavity adaptation, Silorane exhibited superior properties compared with two dimethacrylate-based materials (Ceram × Mono [Dentsply De Trey, Konstanz, Germany] and Clearfil Majesty [Kuraray, Japan]), in terms of shrinkage strain and marginal adaptation . The authors added that “the setting shrinkage characteristics of resin composites affects their marginal adaptation with dentin and that shrinkage strain rate and time at maximum strain rate were found to be more important than total volumetric shrinkage in predicting the adaptation in dentin cavities”.

Cusp deflection

The results of in vitro cusp deflection and microleakage of maxillary premolars restored with novel low-shrink dental composites indicated reduced cusp deflection when compared with two conventional materials . Those results concur with those obtained by Bouillaguet and colleagues who showed that cusp movement during polymerization of Silorane induced the lowest tooth deformation when tested against four conventional resin composite materials . Microleakage was also found to be reduced when a low shrink resin, Hermes (a low shrinkage Silorane prototype) was used .

Physical properties

The physical properties of Silorane, in comparison with four conventional resin composite materials and a giomer, have recently been investigated by Lien and Vandewalle . Their results indicated a lower compressive strength and microhardness for Silorane, but a relatively higher flexural strength/modulus and higher fracture toughness. Silorane was shown to have the lowest polymerization shrinkage, confirming the original testing by Weinmann and co-workers. Their results indicated a 0.94% and 0.99% volumetric shrinkage, respectively, when using the bonded disc and Archimedes method . The physical properties of Silorane have also been tested by Ilie and Hickel, whose results indicated that these were comparable to other clinically successful methacrylate-based composite materials . These workers also noted that there was no difference in degree of cure at depths of 2 mm and 6 mm.

Shrinkage stress

Shrinkage stress has been found to be reduced for Silorane by Ernst and co-workers when tested against ten conventional resin composite materials.

Elution

No substances were found to elute from Silorane in water at 72 h, although Silorane monomers and an initiator component were found to elute into a solution of 75 vol% ethanol, although the authors, Kopperud and colleagues, stated that this may not have represented a clinically relevant situation . These data confirm the earlier work of Eick et al. whose results indicated the stability of Silorane in all the aqueous fluids in which it was tested.

Water sorption and solubility

When tested against two conventional methacrylate-based resins (Z100 and Z250), in water sorption and solubility testing, Silorane showed a lower solubility compared with the methacrylate resins . It has also been shown that the hydrophobicity of the siloxane groups improves the stability of Silorane in biological fluids .

Oxygen inhibited layer

A study by Tezvergil-Mutluay and co-workers suggested that no oxygen inhibited layer (OIL) existed at the surface of a freshly cured sample of Silorane, and incremental bond strengths between successive layers of Silorane were slightly lower than conventional dimethacrylate composites. In addition, these workers demonstrated that the shear bond strength between successive layers of Silorane composite showed a decrease in shear bond strength and an increase in the percentage of adhesive failures when the time of placement between successive layers increased, timing being “fresh”, 20 s and 5 min. The finding with regard to OIL differs from the results of a study by Shawkat and co-workers in which the incremental bond strengths of a range of experimental and commercial resin composite materials were tested, with the results indicating a range of oxygen inhibited layer (OIL) thicknesses from 19.2 to 13.8 μm for dimethacrylate-based composites and 9.0 μm for Silorane. No material exhibited a measureable OIL when cured in nitrogen. The authors concluded that the bond strength between successive layers of Silorane should be no different to conventional methacrylate materials .

Bonding to dentin

Bonding to dentin using the Silorane adhesive system (SAS), when investigated by Santini and Miletic , was found to produce a hybrid layer of similar thickness as a methacrylate-based adhesive (Excite [Ivoclar Vivadent]) and thicker than the one-step adhesives (G Bond, [GC] and AdheSE [Ivoclar Vivadent]) that it was tested against. This presence of an interaction zone has also been confirmed by the work of Mine and colleagues who also found that the two bottles in the Silorane adhesive system, SSA-Primer and SSA-Bond, were distinguishable as two distinct layers. In this respect, it has been postulated that the high viscosity second layer of adhesive might act as an elastic buffer . In addition, Van Ende and colleagues examined the stress at the adhesive interface with differing configuration factors, with their results indicating that cavity configuration did affect the micro-tensile bond strength of the Silorane adhesive system and considered that an incremental layering technique was still required for placement of Silorane restorations. The authors considered that factors other than polymerization shrinkage influenced the micro-tensile bond strength.

Further work is obviously indicated to test other features of Silorane, such as polymerization exotherm, the properties of the quartz glass filler, wear resistance and hydrophobicity. However, given the generally favorable in vitro testing of Silorane, it would appear appropriate to test its clinical effectiveness. It was therefore the aim of this study to assess the performance of Silorane restorations placed in load-bearing situations in patients attending five UK dental practices.

Methods

Ethical standards

The study was conducted in accordance with the Declaration of Helsinki (1964) as revised in Venice in 1983. Multicentre Research Ethics Committee approval was obtained from Southampton and South West Hampshire Research Ethics Committee (REC Ref: 08/H502/93) prior to commencing the study, as too was an additional ethical requirement (peculiar to the UK) for each practice, Site Specific Assessment. Informed written consent was obtained from all patients prior to registration for participation in the evaluation. Implicit in giving informed written consent was the right of patients to withdraw from the study at any time.

Selection of clinicians

Five members of the PREP Panel, who had previous experience in clinical evaluations of dental materials, were asked if they would be prepared to evaluate the performance of restorations placed in Filtek Silorane. Their practices were located in Abingdon, England, Birmingham, England (2), Burton-on Trent, England and Coleraine, N. Ireland. Each practice was asked to recruit sufficient patients to provide a minimum of 20 restorations per center. Sequential patients attending these practices, who required a Class I or II restoration and fulfilled the inclusion criteria, were informed about the study and were given a Patient Information Leaflet (PIL) describing their potential involvement. They were given two weeks to decide whether they were happy to be involved. Patients paid the “normal” amount for their restoration(s) but were reimbursed for attending for the clinical evaluation(s) of their restorations. The participating dentists were also reimbursed for the surgery time.

Operative procedures for Silorane

The practitioners were asked to place the Silorane restorations in situations where it was indicated (i.e. for the restoration of cavities in posterior teeth) and as described in the manufacturers’ instructions. Inclusion and exclusion criteria are shown in Table 2 .

Table 2
Inclusion and exclusion criteria.
To be considered appropriate for inclusion in the study a patient must:
• Have been over 18 years of age
• Have a molar supported permanent dentition free of any clinically significant occlusal interferences
• Have well maintained dentitions free of any active, untreated periodontal disease
• Have a maximum of three Filtek Silorane™ restorations
• Be a regular dental attender who agrees to return for assessments.
Patients will be excluded from participating in the study if:
• There was a history of any adverse reaction to clinical materials of the type to be used in the study
• There was evidence of occlusal parafunction and/or pathological tooth wear
• They are pregnant or have medical and/or dental histories which could possibly complicate their attendance for the assessment of the restorations and/or influence the behavior and performance of the restorations in clinical service
• They were irregular dental attenders
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Two year clinical evaluation of a low-shrink resin composite material in UK general dental practices
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