When we complete a resin restoration, we hope that it will last, be attractive, wear well, retain its polish, maintain shade, reject extrinsic stain, and not fracture (Figs. 2 & 3) Resins today number in the hundreds. While most are clearly superior to the handful available at the beginning of the adhesive era, we remain substantially in the dark about how to choose among them.
Eventually, as clinicians, we distill a daily suite of resins from this field of hundreds, choosing, for practical reasons, only a small subset. Our simplest clinical portfolio often entails a posterior resin with high flexural strength and masking capacity, and an anterior resin with lower strength but better blending capacity and higher gloss. (Fig. 4) To this, most practitioners add a flowable with good handling and high radiopacity.
A bulk fill may round out the resin armamentarium, or a dual-cure (DC) flowable/bulk fill for light-starved applications such as deep proximal boxes, post-luting, small core buildups, crown repair, and interim treatment.
A small spectrum of shades in these resins completes our suite, workhorses we rely on from pedodontic (Fig. 5) to senior cases, from the smallest simple interventions to grand Herodontic restorations (Fig. 3) and full mouth restorations. (Fig. 6)
Hundreds of other resins are omitted from our palette.
Where do we obtain the information to make this cull? In the Age of Data, it seems an irony that when reaching for resins, we do not have comparative data at our fingertips. There are dental journals that publish valuable data, but nowhere is it comprehensive.1,2,3
Twenty years ago, this author compiled a spreadsheet of 40 composite parameters for 40 resins from copious correspondence to manufacturers, as a basis for recommendations to study club members. This involved 1600 potential data points, a very laborious task, which is now obsolete, because those resins have been superseded. The work on today’s offerings remains undone. For everyday practice, what rational basis is readily available to guide one’s final selections from the resin lottery?
Factually, we do know that resins differ widely in their parameters. For example, flexural strength varies as much as 400%, polymerization contraction 250%, particle size 1000%, elastic modulus (also called flexural modulus) 700%. Cure remains invisible but a lack of curability can be a serious drawback as seen when comparing two similar resins. (Tables 1 & 2).
Table 1: Curing Time for Gingival wall Increment
Table 2: Curing Time for Gingival wall Increment
Clearly, with such a spread of variables, much might be gained by fully understanding the dimensions of the resin one is using. From clinical experience, practitioners can anecdotally tie outcomes to these parameters. But that is not science. True scientific method is founded on objective measurement. Sadly, correlations with measured values and clinical performance have not been a research focus.
Within each resin formulation, these 40 parameters, measurable and distinct, are under a manufacturer’s control. From a trial sample we can assess external factors: handling, esthetics, opacity, polish, and ease of finishing. However, the deeper aspects of formulation can only be guessed at. “Try it Doctor, you will love it…” is offered at point of sale.
The clinical questions are, “Which parameters matter the most and what do they predict?”
As a long-term study club mentor, I am routinely asked what resin I use. My summary opinion is: “The best posterior results are obtained using a resin with a flexural strength exceeding 150 MPa ( from a 3-Point test, not a 4-point test), a flexural modulus approaching or exceeding average dentin (10 to 12 GPa), with a curability under 20 Joules, a finish exceeding 95 Gloss Units (95% of incident light is reflected back to the human eye), over 80% filled by weight, with a largest particle size under 30 microns, less than 2% polymerization contraction, high molecular weight polymer, low-slump, adaptable viscosity, lack of stickiness when heated, shade stability, free from extrinsic stain acquisition or intrinsic shade change, and higher than medium opacity to hide sclerotic or amalgam-stained dentin.”
I am aware that perhaps half of this short-list of variables may be unavailable to most dental practitioners and incomprehensible to many even if it was.
Try to imagine how our practices could be different if a “Resin QR Code” (Fig. 1) accompanied each product, and a matching App was available for its interpretation. It would provide a measuring stick to inform practitioners, in advance of trial or purchase, the composition of each resin.
Consider this: manufacturers know these factors, why can’t we know them too? Their product, which is brought to market, is what they consider their best blend of interactive properties. But are they best for the clinical treatment we have at hand? Are there weaknesses we need to compensate for in clinical placement? If we acquire greater depth of understanding, dialing in the properties to the clinical need, can we reliably expand our successful range of resin dentistry?
A universe of trial and error, blind product loyalty, and mysterious outcomes could be swept away, along with the painful process of maintaining credibility while explaining to a patient why treatment is failing. (Fig. 7)
BREAKING DOWN THE VARIABLES
Resin properties can be organized into two categories:
- Those that can be perceived by the operator
- Those that cannot
1: VISIBLE PROPERTIES
Essentially visible factors, accessible through use, include:
- Shade fidelity at placement: is A3 actually A3?
- Blending/translucency – also called “metamerism” (Fig. 8)
- Opacity to hide dark tooth structure (Fig. 9)
- Extrinsic stain acquisition
- Polishability gloss (GU) as a percent of reflected incident light after polish (100=mylar,50= matte) relevant to appearance and to Class V tissue acceptance (Fig. 10)
- Viscosity = firmness/softness to condensation
- Tendency to cohere vs granulate in placement
- Stickiness/pullback to placement instruments
- Response to chairside heating (Fig. 11)
- Thickness of oxygen inhibited layer
- Tendency to dull finishing carbides: an overhead cost factor when burs dull too quickly
- Tendency to “load” or clog burs
2: INVISIBLE PROPERTIES
Invisible factors are far more numerous than the preceding and are critical for clinical durability. These are available only as data and cannot be perceived with human eyes and hands.
- Flexural strength: while dentin lies between 200-250MPa; resins are 100-187. The numbers in this range represents an industry standard, derived from a 3-point test. A 4-point test exists but delivers a different numerical value
o This drives suitability for light/medium/heavy functional load, and survival of marginal ridges (author’s opinion=AO) from 45 years of practice and study
- Compressive strength. Almost all resins exceed 350 MPa, including most flowables. Amalgam lies between 350-500MPa. Therefore, not clinically discriminatory (AO)
- Flexibility: elastic or flexural modulus in GPa: dentin is 12-16, enamel is 80. Paste resins are usually 9GPa or higher, flowables are generally 4 to 9 GPa
o High modulus, 9-18 GPa are suitable for posterior application (AO)
o Low modulus, 5 to 9 GPa, indicates suitability for abfractions (AO)
- Polymerization contraction, ranging from
o Under 2% in the best paste resins
o Over 5% in flowables. Contraction interfaces with flexural modulus to predict post-operative crazing and white line formation (AO)
- Hardness , reported in three different scales; Barcol, Knoop, and Brinell
- Wear = microns per year, average; enamel is 15-25. Average wear is only revealed through clinical trial research or laboratory testing through accelerated aging protocols
- Wear is critical to the durability of a full mouth composite reconstruction of a bruxing patient/ (Figs. 12 & 13)
- Abrasiveness against denture teeth, which correlates with largest particle size (AO)
- Point load tolerance. Relevent to occlusions with sharp cuspal morphology(AO). (Fig. 14)
- Thermal expansion: the closer to tooth structure, the better, to reduce cyclic strain to bond interface
- Radiopacity: as mm. of aluminum equivalent. Commonly understood
- Resin molecular weight. Higher is better (AO)
- Water absorption. Less is better (AO)
- Solubility: clearly, we wish our restorative materials to be as insoluble as possible, not only to avoid their dissolution but also to avoid transport of constituents into the human body
- Durability to chemical attack, for example from alcohol, ketones and other organic food constituents
- Biocompatibility of organic compounds
o Apoptosis, a measure of decreased intracellular respiration, for example, only available through studies of immortal cell cultures4
- Particle loading by weight and volume
o Largest particle
o Smallest particle
o Plucking resistance
o Particle surface coating and particle fillers (silica, zirconium, barium glass, yrbettium trifluoride, etc. This information guides the need for primers in composite repairs.
o Biocompatibility of fillers as an occupational exposure and patient health factor
- Microbubble inclusions at manufacture; this may vary from 0.5% to over 2%. This is much higher than what one would expect and differs widely from product to product5.
- Curability for each shade, expressed as Joule requirement to 80% at the gingival floor, usually based on hardness, relative to 100% at the occlusal surface (Table 1 & Table 2)
o Catalyst system for each shade
o Optimal curing wavelength
o Rapidity of cure at recommended exposure. Gradual polymerization may be better for some applications, and may relate to crazing of thin residual tooth structure if flexural modulus is high (AO)
o Depth of cure, in mm. at recommended exposure
o Distance limit for curing tip- to- resin photopolymerization. This guides the need to resort to Dual Cure (DC) or Self-Cure (SC) resins in deeper proximal boxes, for example
- Polymerization contraction. Initial and subsequent after 5 seconds, 10 seconds, and 5 minutes. Manufacturers can quote values from early-stage contraction to gain market advantage (Table 3)
- Extent of cure of DC materials in DC mode alone and not light cured after placement.
3: SHELF LIFE
Finally, for effective practice management and overhead control we need to be aware of product deterioration, such as:
- Degradation variables, such as catalyst expiry if refrigerated or not
- Absorption of atmospheric moisture
- Deterioration from chairside heating
These factors form a pyramid of measurable variables that have a bearing on the likelihood of clinical success, such as Class II fracture in a patient who bruxes (relating to flexural strength (AO)), or Class V enamel margin survival in a clencher who abfracts his/her CEJs (relating to flexural modulus (AO)).
They enable predictive power for a patient whose resin-restored lower natural dentition occludes against an acrylic complete denture (largest particle size). They establish limits to our expectations when creating a five-surface shoed-cusp Herodontic resin masterpiece (curability, adaptability, voids and flexural strength). They determine difficulty of placement, feasible increment size, ease of finishing, ultimate gloss, blend or opacity of shade, speed of placement, and ultimately patient satisfaction, and practice profitability. They enlighten reverse-engineering composite failures: breakage, premature wear, lost color fidelity, poor polish. With data we may judge if it was initial resin inadequacy or was it improper placement. (Fig. 15)?
RESEARCH COMBINED WITH PARAMETERS WILL PREDICT CLINICAL OUTCOMES:
Until we have data, the profession cannot research relevance to clinical outcomes. These correlations are neither clear nor consensual. Take the case of two clinically similar resins; one has a flexural strength of 110 MPa, the second 220 MPa? What applications require the higher flexural strength? Conversely, what does high compressive strength predict when it so similar to amalgam?
What is the relationship to overall resin technique? Can superior resins forgive flawed restorations such as pinpoint contacts and high occlusion? Can benign contraction overcome white line due to improper margin design in preparation? Will a resin with an exceptionally low wear rate still provide a satisfactory clinical service life if poorly cured and thin? Will high flexural strength override failure to remove stress risers created by sharp internal form? Does resin heating or the lack thereof determine if resins deliver as well clinically as under laboratory conditions?
Currently, answers to these sorts of questions are the anecdotal outcome of clinical experience.
ISSUING A CHALLENGE TO THE PROFESSION:
Frankly, at the moment we suffer profession-wide design deficiencies. We don’t have the data, we don’t know what to predict if we did have it, and preparations are not standardized.
The definitive textbook of composite method has yet to be written. We are trained by dental faculties who develop an in-house consensus. This is by no means standard across the continent, either in form or in instrumentation. Even so fundamental a factor as preparation design differs widely between universities. We train in isolated domains.
Historical principles from the stolid universe of GV Black methodology have morphed into a fluid continuum of dissimilar adhesive concepts. Preparation design has become dependent on where we trained and where we practice, not a universal basis.
This quasi-scientific approach achieves no standard of care, a major problem when regulatory bodies need to re-train practitioners whose composite resin outcomes are the subject of complaints. Profession-wide surveys continue to indicate problems, such as reported in Clinicians Report in June 2018, highlighting recurrent decay seen by 43% of clinicians within 2 years of resin placement.1 A repeat survey, four years later, in February 20221 similarly found that wear, fracture and recurrent caries were frequently seen. Four years have gone by, no progress. We must admit that all is not well in the composite resin world.
We require a better roadmap through the largely invisible world of resins, both for the individual practitioner and for the profession. At the moment, we listen to sales reps, consult with a colleague down the hall, read a review of 5 good points in a resin’s formulation, overlook the other 35 parameters, and then proceed by trial and error. Everything composite resin that fails obviously needed a crown, not a better resin or a better engineered preparation design and more appropriate resin.
We need to pull up our collective bootstraps; a framework akin to a QR code with an accompanying interpretive App is needed to intelligently guide our resin selection.
Without data, clinical research cannot establish correlations to clinical outcomes. Without baseline correlations, science cannot be brought to bear on preparation design.
The public wants to believe we are capable of sound, credible, and reproducible treatment, meeting a defined standard of care. Taking control of the Wild West of resins is a critical first step. From this we will derive the ability to correlate clinical outcomes. Then and only then can we rationalize, test, and standardize preparation forms.
There is hard and detailed work ahead if we wish to bring science to resin treatment and optimum service to patients. QR Code, Gentlemen, Please Start Your Engines.
Oral Health welcomes this original article.
- CLINICIAN’S REPORT https://www.cliniciansreport.org/
- DENTAL ADVISOR https://www.dentaladvisor.com/
- REALITY RATINGS AND REVIEWS https://realityratings.com/
- J Biomed Mater Res. 1998 Sep 5;41(3):474-80.Cytotoxicity of 35 dental resin composite monomers/additives in permanent 3T3 and three human primary fibroblast cultures.Geurtsen W, Lehmann F, Spahl W, Leyhausen G.
- Reality Yearbook 2012, Reality Publishing ppg .296 to 399
About the Author
Peter Walford is a longtime mentor of live and virtual study clubs in composite technique and adhesive prosthodontics. He practices on a gulf island in British Columbia, builds racing sailboats, and operates a small organic farm with his wife. firstname.lastname@example.org