Resin-based composite performance: Are there some things we can’t predict?

Abstract

Objective

The objective of this manuscript is to address the following questions: Why do direct dental composite restorative materials fail clinically? What tests may be appropriate for predicting clinical performance? Does in vitro testing correlate with clinical performance?

Methods

The literature relating to the clinical and laboratory performance of dental composite restorative materials was reviewed. The main reasons for failure and replacement of dental composite restorations provided the guidance for identifying specific material’s properties that were likely to have the greatest impact on clinical outcomes.

Results

There are few examples of studies showing correlation between laboratory tests of physical or mechanical properties and clinical performance of dental composites. Evidence does exist to relate clinical wear to flexure strength, fracture toughness and degree of conversion of the polymer matrix. There is evidence relating marginal breakdown to fracture toughness. Despite the fact that little confirmatory evidence exists, there is the expectation that clinical fracture and wear relates to resistance to fatigue. Only minimal evidence exists to correlate marginal quality and bond strength in the laboratory with clinical performance of bonded dental composites.

Significance

The use of clinical trials to evaluate new dental composite formulations for their performance is expensive and time consuming, and it would be ideal to be able to predict clinical outcomes based on a single or multiple laboratory tests. However, though certain correlations exist, the overall clinical success of dental composites is multi-factorial and therefore is unlikely to be predicted accurately by even a battery of in vitro test methods.

Introduction

The use of dental composite as a posterior restorative is increasing worldwide, though its use in relation to dental amalgam varies widely from country to country . An extensive database from the Washington Dental Service in the U.S. reveals that composite use for posterior teeth surpassed that of amalgam in 1999 . As until recently there were actually very few restrictions on the use of amalgam in countries around the world , it is likely that the main reasons for this change in usage is the desire for tooth-colored fillings and the increasing familiarity and comfort of the practitioner with the use of composites.

The title of this manuscript suggests that there are some aspects to the clinical performance of dental composite that we can’t predict. This implies that there are things that we can predict. This is a very optimistic outlook, as will become apparent through this manuscript. Despite substantial effort by the manufacturing and research communities, the ability to predict clinical performance of virtually all dental materials has not advanced significantly. Certain correlations between specific properties and clinical performance have been identified over the years and have been used to guide materials selection and placement, but the science remains largely empirical.

Current research and development efforts are aimed at modifications of the resin and/or filler component to produce formulations with reduced curing contraction, and more importantly, reduced curing contraction stress. This effort is a direct response to the sensitive and somewhat complex placement techniques required for these materials and the fact that recurrent decay has consistently been the primary reason given for replacement of dental composite restorations in private practice, with fracture being the second most important reason . While some have suggested that replacement is primarily a response to fracture during the first five years of service, and for secondary caries after that , other data does not support this trend . Little progress has been made on increasing the fracture resistance of dental composites. In fact, on the whole, the most popular current formulations are significantly weaker and less fracture resistant than those sold in the 1970s and 1980s, before the major push to minimize particle size occurred.

There are well over one hundred published studies evaluating the clinical performance of dental composites. Undoubtedly nearly as many more clinical studies have been conducted but have not been documented in the peer-reviewed literature. Despite this extensive investment in time, energy, and money to study these materials in the ultimate proving ground, it is still not possible to identify a precise level of required properties or characteristics that ensure the clinical success of a new formulation. Thus, new materials are subjected to a battery of test modalities in order to describe their physical characteristics, predominantly in comparison to other products already on the market for which some indication of clinical success exists. In this regard, the development of dental composites is only to a limited extent based on sound engineering design principles, and therefore many questions surround the release of a new product line. These questions must typically be answered through clinical evaluation, which either delays acceptance of the product, or allows immediate use with some substantial degree of discomfort and uncertainty. A rational approach to this dilemma would seem to be to analyze the primary reasons for the clinical failure or success of dental composite restorations, and then to identify testing methods that address these most important aspects of the material’s performance. An excellent review article in this area has been published by Sarrett .

The objective of this manuscript is to address the following questions: Why do direct dental composite restorative materials fail clinically? What tests may be appropriate for predicting clinical performance? Does in vitro testing correlate with clinical performance? The discussion primarily concentrates on composite placement in stress bearing areas, as the use of composites in other sites is generally considered to be without serious problem or concern aside from esthetics and marginal sealing. While the bonding and sealing of composites remains a major issue, it is equally critical for anterior and posterior composites. Further, these aspects of the restoration relate largely to the adhesive material, and have been addressed by others .

Why do direct dental composite restorative materials fail clinically?

In order to address the issue of predicting clinical outcomes through laboratory tests, it is necessary to have a firm understanding of the actual clinical performance through controlled studies or evaluations . This in itself is a significant problem. Clinical data, even from controlled studies, is typically plagued by high variability. A good example of this was pointed out years ago when trying to correlate data generated on the wear of dental composites using an in vitro testing device with clinical wear reported in the literature. In an attempt to calculate “average” wear results for developing a correlation, the wide range of results for similar materials from different studies precluded making a meaningful attempt . Instead, developing a similar ranking of materials was considered to be a more reliable method for relating the laboratory to the clinical situation. Wear is a particularly difficult characteristic to assess clinically. It may be produced in a variety of ways, and is very dependent upon the oral conditions, but generally must be of significant amount to be recognizable. Studies have shown that relatively simple assessments of casts produced from impressions are not very accurate in most cases . These methods cannot quantitate the wear of an entire surface, and are typically confined to semi-quantitate assessments at margins, when the bulk of wear may be occurring elsewhere on the restoration. More sophisticated techniques require more expensive equipment and time and are therefore less readily available .

This limitation is common for all types of clinical assessments, in part due to the difficulties in developing clear and specific diagnoses of “failures,” as well as to differences in clinical opinion . Marginal staining around a dental composite may cause the restoration to be identified as a failure due to compromised esthetics, but the composite itself is largely unaffected and remains serviceable. How is it possible to predict this in anything other than a clinical situation? Likewise, marginal fracture of composite restorations on occlusal surfaces possibly may be related to inherent deficiencies in the properties of the composite that were revealed under the conditions of cyclic loading. However, this “failure” may also be due to deficiencies in the bonding, or placement procedure, either of which predisposed an otherwise acceptable material to premature degradation. In any case, the underlying assumption is that the clinical judgment that differentiates “failure” from “acceptable” is capable of being standardized across multiple evaluators and sites. This is likely incorrect. Despite the laudable past and more recent efforts of investigators to develop well-defined criteria for clinical assessment of dental restorations , clinical opinion is still left to interpretation and affects the accuracy, and precision, of the evaluation. Thus, we are left with an imperfect system and it is not surprising that there are so few instances in which a reasonable prediction of clinical performance is generated from laboratory testing. However, clinical studies do provide guidance on the primary reasons for failure, and therefore which properties may be most applicable for testing.

Clinical studies show increasing evidence for the successful long-term performance (10+ years) of dental composite in small to moderate-sized restorations . A 17 year study of four UV-cured conventional composites reported excellent outcomes, and a 17 year and 22 year study of two visible light-cured hybrid composites placed in class I and II cavities under rubber dam showed overall success rates of 75% and 64%, respectively. The main reasons for failure in these studies were wear and secondary caries and fracture . An evaluation of clinical outcomes from an extensive insurance database for approximately 200,000 amalgams and 100,000 composites showed survival rates of 94% and 93%, respectively at 7 years . While this difference seemed minimal, statistical evaluation for this large dataset showed about a 16% greater chance of failure for composite than amalgam. A nearly double rate of failure for composite compared to amalgam as a posterior restorative has been reported based on reviews of controlled clinical studies . More recently however, Opdam et al. have shown equivalent or even higher success rates for posterior composites vs. amalgam, except perhaps in situations where the patient had a high caries status.

Results from cross-sectional studies from private practices vary showing an equivalent failure rate for composite and amalgam over three years to nearly twice the mean longevity for amalgam over 12–14 years . A recent study reporting on the 5-year success of over 700 posterior composites placed by dental students showed a failure rate of 14%, with class II restorations failing at nearly three times the rate of class I restorations . The predominant cause of failure was caries/marginal openings and restoration fracture. Further evidence for the performance of composites in larger cavities, with many restorations involving cusp replacement, has shown survival rates at 11 years of 73% and 70% , with success being better in premolars than in molars. The predominant reason for failure in both studies was fracture of the restoration. Another study of the survival of extensive composite restorations (where a crown would have been the first choice), showed a median survival of 7.8 years for composite, which was less than that for amalgam (12.8 years) and metallic crowns (more than 14.6 years), with the primary reason for failure again being composite fracture followed by secondary caries .

The clinical studies cited suggest that dental composite can be expected to provide excellent service for small to moderate-sized restorations, and perhaps adequate service for large restorations. However, performance seems to be better in premolars than in molars, and the most common reasons for failure are fracture and secondary caries. Wear seems to continue to be a concern, but perhaps only for patients with heavy occlusal patterns, such as bruxing and clenching.

Why do direct dental composite restorative materials fail clinically?

In order to address the issue of predicting clinical outcomes through laboratory tests, it is necessary to have a firm understanding of the actual clinical performance through controlled studies or evaluations . This in itself is a significant problem. Clinical data, even from controlled studies, is typically plagued by high variability. A good example of this was pointed out years ago when trying to correlate data generated on the wear of dental composites using an in vitro testing device with clinical wear reported in the literature. In an attempt to calculate “average” wear results for developing a correlation, the wide range of results for similar materials from different studies precluded making a meaningful attempt . Instead, developing a similar ranking of materials was considered to be a more reliable method for relating the laboratory to the clinical situation. Wear is a particularly difficult characteristic to assess clinically. It may be produced in a variety of ways, and is very dependent upon the oral conditions, but generally must be of significant amount to be recognizable. Studies have shown that relatively simple assessments of casts produced from impressions are not very accurate in most cases . These methods cannot quantitate the wear of an entire surface, and are typically confined to semi-quantitate assessments at margins, when the bulk of wear may be occurring elsewhere on the restoration. More sophisticated techniques require more expensive equipment and time and are therefore less readily available .

This limitation is common for all types of clinical assessments, in part due to the difficulties in developing clear and specific diagnoses of “failures,” as well as to differences in clinical opinion . Marginal staining around a dental composite may cause the restoration to be identified as a failure due to compromised esthetics, but the composite itself is largely unaffected and remains serviceable. How is it possible to predict this in anything other than a clinical situation? Likewise, marginal fracture of composite restorations on occlusal surfaces possibly may be related to inherent deficiencies in the properties of the composite that were revealed under the conditions of cyclic loading. However, this “failure” may also be due to deficiencies in the bonding, or placement procedure, either of which predisposed an otherwise acceptable material to premature degradation. In any case, the underlying assumption is that the clinical judgment that differentiates “failure” from “acceptable” is capable of being standardized across multiple evaluators and sites. This is likely incorrect. Despite the laudable past and more recent efforts of investigators to develop well-defined criteria for clinical assessment of dental restorations , clinical opinion is still left to interpretation and affects the accuracy, and precision, of the evaluation. Thus, we are left with an imperfect system and it is not surprising that there are so few instances in which a reasonable prediction of clinical performance is generated from laboratory testing. However, clinical studies do provide guidance on the primary reasons for failure, and therefore which properties may be most applicable for testing.

Clinical studies show increasing evidence for the successful long-term performance (10+ years) of dental composite in small to moderate-sized restorations . A 17 year study of four UV-cured conventional composites reported excellent outcomes, and a 17 year and 22 year study of two visible light-cured hybrid composites placed in class I and II cavities under rubber dam showed overall success rates of 75% and 64%, respectively. The main reasons for failure in these studies were wear and secondary caries and fracture . An evaluation of clinical outcomes from an extensive insurance database for approximately 200,000 amalgams and 100,000 composites showed survival rates of 94% and 93%, respectively at 7 years . While this difference seemed minimal, statistical evaluation for this large dataset showed about a 16% greater chance of failure for composite than amalgam. A nearly double rate of failure for composite compared to amalgam as a posterior restorative has been reported based on reviews of controlled clinical studies . More recently however, Opdam et al. have shown equivalent or even higher success rates for posterior composites vs. amalgam, except perhaps in situations where the patient had a high caries status.

Results from cross-sectional studies from private practices vary showing an equivalent failure rate for composite and amalgam over three years to nearly twice the mean longevity for amalgam over 12–14 years . A recent study reporting on the 5-year success of over 700 posterior composites placed by dental students showed a failure rate of 14%, with class II restorations failing at nearly three times the rate of class I restorations . The predominant cause of failure was caries/marginal openings and restoration fracture. Further evidence for the performance of composites in larger cavities, with many restorations involving cusp replacement, has shown survival rates at 11 years of 73% and 70% , with success being better in premolars than in molars. The predominant reason for failure in both studies was fracture of the restoration. Another study of the survival of extensive composite restorations (where a crown would have been the first choice), showed a median survival of 7.8 years for composite, which was less than that for amalgam (12.8 years) and metallic crowns (more than 14.6 years), with the primary reason for failure again being composite fracture followed by secondary caries .

The clinical studies cited suggest that dental composite can be expected to provide excellent service for small to moderate-sized restorations, and perhaps adequate service for large restorations. However, performance seems to be better in premolars than in molars, and the most common reasons for failure are fracture and secondary caries. Wear seems to continue to be a concern, but perhaps only for patients with heavy occlusal patterns, such as bruxing and clenching.

What tests may be appropriate for predicting performance?

An obvious question to be addressed is “What controls the clinical performance of dental composite restoratives?” Three separate factors may be considered to answer this question. First, the mechanical properties, as well as wear resistance, must play a significant role. These properties are highly dependent upon the material’s formulation as defined by the manufacturer, but are also heavily influenced by the extent of the curing reaction and the care in placement, both of which are controlled by the clinician. While it is true that specific design criteria have been elusive, few would deny that there is a threshold for every mechanical property below which premature failure would be very likely. The best evidence for this is the fact that certain materials, such as glass ionomer and zinc oxide eugenol, can typically only be used as temporary restoratives in stress bearing situations in permanent teeth due to a high propensity for fracture and wear under occlusal loads . Further evidence is the fact that clinical studies have shown fracture of the restorative material to be either the primary or secondary reason for failure of dental composite restorations placed in stress bearing situations.

Second, the dimensional characteristics that determine marginal adaptation are of critical importance, as evidenced by the fact that secondary caries is a primary reason for composite restoration replacement. Included in this consideration is the volumetric shrinkage of the material during polymerization and the subsequent swelling that may occur due to solvent uptake in the oral environment. These properties are highly dependent upon the material’s formulation as defined by the manufacturer, but may also be affected by the extent of curing. The placement technique, entirely under the control of the clinician, is also critical to the overall adaptation, retention and sealing of the material within the preparation. Finally, the preparation design plays a key role in success, including its geometry (i.e. quantity and quality of the remaining tooth structure), location in the arch, and relation to opposing and adjacent teeth. These factors are for the most part under the control of the clinician, though they often may be steered from the ideal by the dictates of the particular case.

Mechanical properties and wear

The mechanical properties of consideration for use in evaluating composites and predicting clinical success are highlighted in Table 1 . The table presents the property of interest, what physically it relates to or tests, and what in the author’s opinion its possible relation may be to the clinical performance of dental composites.

Table 1
Potentially important mechanical properties.
Property What it relates to Possible relation to clinical performance
Compressive strength Fracture in compression Intuitively important, but most restorations likely fail in tension or shear (easy to test)
Elastic modulus Elastic deformation under force Intuitively important to maintain form, especially under high forces (easy to test)
Fatigue resistance Repeated mechanical stressing Important – likely many failures due to fatigue (difficult to test)
Flexure strength Fracture – includes tensile and compressive components Important – has tensile aspect and failures likely due to tensile stresses; some correlation with wear (easy to test)
Fracture toughness Chipping and bulk fracture Important – many failures due to fracture; not geometry dependent (more difficult to test)
Hardness Resistance to indentation (and wear?) Intuitively important, but only limited correlation with wear (easy to test)
Tensile strength Fracture in tension Important – failures likely due to tensile stresses (difficult to test)
Wear Volume loss due to abrasion and opposing contact stresses Important – abrasive and occlusal contact stresses (difficult to test)
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Nov 28, 2017 | Posted by in Dental Materials | Comments Off on Resin-based composite performance: Are there some things we can’t predict?
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