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The Ugly Facts on Dental Amalgam by Dr. Michael Wahl

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by Michael J. Wahl, DDS

“The great tragedy of science: the slaying of a beautiful hypothesis by an ugly fact.” Thomas Henry Huxley

Dental amalgam has been maligned as a filling material, both because of its clinical properties and its alleged effects on systemic health, but reports of its demise have been premature. As of 2008, 57% of dentists were placing amalgam (March 2008 Dental Products Report Survey). A large 2011 survey showed that dentists in the United States placed more dental amalgam than composite for first-time restorations in posterior teeth.1

Still, amalgam use has certainly declined over the last two decades, in part because of improved alternative filling materials and techniques and declining rates of caries in children and young adults. When the automobile replaced the horse and buggy as the primary mode of transportation, there were few if any emotional attacks on horses, and no one called for a ban on horse and buggies – they were simply replaced with automobiles. Similarly, when composite resins were commercially introduced, they fairly quickly replaced silicate cements as the material of choice for anterior fillings. As was the case with automobiles replacing horses, there were few if any emotional attacks on silicate cements, and no one called for a ban on silicate cements – they were simply replaced with better materials.

Compare this to dental amalgam. Never has a material been so vituperatively savaged as dental amalgam. And it’s not just the dental amalgam itself, but also the dentists who place amalgam. Maroon described anyone still using amalgam as “a fool....AMALGAM SUCKS!”2 (Using reasoning like this, there must be a lot of “fools” in our profession since dental amalgam may be the most commonly used restorative for posterior fillings.) Dickerson called amalgam “...a disgusting restoration.”3 He was probably not just referring to the restoration’s color. Harper stated that amalgam is “...an inferior restoration that I would not place in the mouths of my family or friends, much less into patients’ mouths.”4 Lowe stated that direct posterior composites’ “clinical superiority over amalgam in class I situations has been well documented...”5 No documentation was provided. Lutz et al. stated, “Adhesive cavity preparations for Class I, III, IV, and V restorations are achieved, and their clinical superiority over the conventional amalgam restorations is well documented.”6

Dickerson referred to the fact that dental amalgam is the most common restoration as not just unfortunate, but even criminal: “What is even more of a crime is that the most common restoration today is the same as it was 100 years ago. Where is the progress in our profession? What other industry has not had a significant advancement in materials used in the last 100 years?”7

Many dentists proudly assert that they practice “mercury-free” dentistry – they place no amalgam restorations. To dentists who do place amalgam, it would seem ludicrous to assert that there have not been significant advances in dental amalgam materials and techniques in the last 100 years, but “mercury-free” dentists who have not placed any amalgams in many years may be unaware of them. A few of these advances include amalgam bonding, new preparation techniques including slot preparations and the elimination of sharp line angles and point angles, preweighed dosages, encapsulated and self-activating dispensing techniques, and more. I remember using hatchets, hoes, and chisels to help create sharp line and point angles when I went to dental school in the 1980s, but I have never used them since! Regardless, the fact that a material or technique is over 100 years old does not make it a “crime.” Airplanes, aspirin, automatic dishwashers, automobiles, contact lenses, electric light, telephones, toilet paper, trains, mechanical dental chairs, and postage stamps are all over 100 years old and in common use today. Even modern composite resins can be traced to the discovery of acrylic acid over 150 years ago and the discovery of methacrylate esters and their polymers over 100 years ago.8



Cusp Fracture
Amalgam restorations are often criticized because they allegedly contribute to cusp fractures: Erickson has stated, “[T]he cuspal fracture characteristic of amalgam is well-known and observed almost daily in every general dental practice.”9 Referring to the likelihood of cusp fractures, Davis stated that Class II amalgam restorations are “time bombs [that] may threaten not only specific teeth, but possibly an entire dentition.”10 It is understandable (but arguable) to allege that a filling material may play a role in a future cusp fracture of that tooth, but how the “time bomb” would threaten an entire dentition is not explained and no examples of “amalgam-induced edentulousness” are provided. DiTolla asked, “[W]hy would I plant this amalgam ‘crown seed’ and then wait for the tooth to break[?]”11

Dickerson asserted, “Many studies have shown that, after seven years, 50 percent of the teeth filled with amalgam have fractured.”3 Although no studies were cited in the article, Dickerson later cited just two (personal communication, February 3, 2000), both of which were on endodontically-treated teeth with large MOD restorations.12 In the first study, there were only 181 MOD (probably unbonded) amalgam restorations and 40 resins. The teeth with resin had a much lower incidence of cusp fractures, but the author stated that the results of the study “should be cautiously interpreted, especially since the number of resin-restored teeth was rather small.” In two similar but much larger studies published by the same author two years later, there were much different results: in 1584 endodontically-treated teeth with MOD amalgam restorations, 34% (532) had cusp fractures after 20 years13 and in 190 endodontically-treated teeth with MOD composite, 28% had cusp fracture after 10 years.14 The incidence of cusp fracture after 20 years in (presumably unbonded) amalgam-restored teeth was thus about the same as the incidence of cusp fracture in composite resin-restored teeth after only 10 years. From these studies, it is clear that the cusp fracture incidence in endodontically-treated teeth restored with MOD amalgam restorations is more favorable than that of endodontically-treated teeth with resin composite restorations.

The other study cited by Dickerson showed that the incidence of cusp fractures in endodontically-treated teeth with MOD amalgam was much greater after 1979 than before 1975.15 Anti-amalgamists have asserted that the greater incidence of cusp fracture coincided with the introduction of high-copper amalgams around 1975. Another possibility was also cited by the authors, and this explanation seems more likely: the common use of Gates Glidden burs and straight-line access to endodontic apices, along with larger access openings began in 1979, and these lead to weaker teeth.

But enough about cusp fractures in endodontically-treated teeth – those teeth should be restored with crowns or some other type of cuspal coverage anyway.16,17 When evaluating cuspal fractures, it is more appropriate to examine amalgam-restored vital teeth under more common clinical conditions. In a long-term 1989 study of about 600 amalgam-restored teeth, there was less than a 1.5% cusp fracture rate after 5 years.18 In a 1993 study of 1415 class II amalgam-restored teeth, only 1.8% had a fractured cusp after 10 years.19 After 15 years, a 1996 study of 1214 class II amalgam-restored teeth showed only 3.8% failed because of enamel fracture only and only 5.0% failed because of enamel fracture only or a combination of enamel fracture, isthmus fracture, and/or caries.20

Amalgam bonding increases fracture resistance and decreases cuspal deflection21-24 at least as much as composite bonding does25 and can allow for smaller preparations and restorations, which last longer and are less likely to be associated with cusp fracture.26-31 Consider that these studies were before amalgam bonding and conservative preparation techniques were in common use, but the amalgam restorations still had a low incidence of cusp fracture.

Many studies on cusp fracture in amalgam- and composite-restored teeth are also inherently biased against amalgam-restored teeth because they don’t take restoration size into account.32 Dentists have long been advised to use composite only for smaller restorations but amalgam for larger ones.33 Larger restorations generally do not last as long as smaller restorations, and larger restorations are more likely to be associated with cusp fractures.28,29 In spite of restoration size bias, amalgam restorations compare favorably to composite restorations in cusp fracture incidence and longevity. Even tying one hand behind its back, (unbonded, larger) amalgam restorations perform well versus (bonded, smaller) composite restorations.

In assailing the cusp fractures associated with amalgam-restored teeth, DiTolla stated, “I thought back over my first 6 years in practice and realized that 99 percent of the teeth that required crowns all had silver fillings.”11 DiTolla’s assertion is a classic case of the logical fallacy of “base rate neglect,” in which he focuses only on the irrelevant (the number of teeth requiring crowns) and ignores the relevant base rate (the incidence of amalgam-restored teeth requiring crowns including both those that required crowns and those that did not).34

In a 2004 study we published in JADA, we examined every posterior tooth with an amalgam or composite resin restoration in 1902 consecutive patients. There were a total of 10,689 posterior teeth with amalgam or composite resin, 10,082 with amalgam and 787 with composite resin. Of these, there was a 1.88% cusp fracture rate in amalgam-filled teeth and 2.29% cusp fractures in composite-filled teeth.35 The cusp fracture rate was actually slightly higher in composite-filled teeth than in amalgam filled teeth although the difference was not statistically significant. So why does it seem like amalgam is more likely to be associated with cusp fracture than composite resin? Our study is probably typical of many dental practices – there were 13 times more amalgam-restored teeth than composite-restored teeth in our “snapshot” of patients’ posterior dentition. It therefore follows that there are far more amalgam-restored teeth with cusp fractures than composite resin-restored teeth even though the incidence of fracture was roughly the same. It is common that posterior teeth requiring crowns have been amalgam-restored because it is common that posterior teeth have been amalgam-restored. Another factor may be that dentists spend very little time examining successful restorations, and tend to focus on examining and restoring failed restorations.36

We are told that the amalgam “filling expands and contracts at a rate greater than that of the tooth and that’s why the patient’s MB cusp broke off or there is a marginal fracture, etc.”11 It is thus alleged that dental amalgam expands and contracts to temperature changes more than composite resin does, and that’s what contributes to tooth fractures in amalgam restored teeth. Actually, the opposite is true: the expansion and contraction to temperature changes are greater with resin composite than with amalgam. The coefficient of thermal expansion of composite is greater than that of amalgam.37-41

Fortunately, expansion and contraction of restorative materials is not an important clinical issue. Extreme temperature changes occur only fleetingly in vivo.42 When cold soda or hot coffee contacts teeth, it is usually only fleetingly. The liquid is usually not held against the teeth for very long (if at all) before it is swallowed so thermal expansion is not clinically important.43 Unlike in the mouth, thermal expansion and contraction is an important issue in a concrete sidewalk or the steel of an automobile, where there may intense heat and cold for hours and days at a time. When it comes to likelihood of cusp fractures, far more important than thermal expansion and contraction are issues like tooth preparation size, diet, and masticatory habits.

As mentioned above, teeth filled with composite resin suffer cusp fractures also: In a 1995 study of 1360 composite resin restorations, almost 9% were replaced due to cusp fracture.44 Many dentists consider gold restorations to be the “holy grail” of dentistry, but in a 1993 study of 745 gold restorations, 36% of 139 cast gold restorations that were replaced were replaced due to tooth fracture.45 There is very little evidence that amalgam restorations are associated with high rates of cusp fracture. There is a breach in the argument that amalgam restorations are associated with high rates of cusp fracture.

Amalgam and Recurrent Caries
Amalgam restorations have been criticized because of the alleged high incidence of recurrent caries associated with them. Van Dyke stated, “Almost every amalgam I remove has decay or structural fractures of the tooth.”46 Dickerson has stated, “Over 40 percent of the amalgams deemed in good clinical shape had caries under them.”3 Although no studies were cited in the article, in a personal communication (February 3, 2000), Dickerson cited two studies of extracted teeth without clinical evidence of caries sectioned and examined microscopically, one on 17 teeth47 and the other on 30 teeth.48 The “caries” in these studies was not clinically detectable, but radiographs were not taken. The caries was detected only after the teeth were extracted, sectioned, and examined microscopically in histologic section – a clinical impossibility! There was no way to detect, locate, or remove the caries clinically. Even then, in the Kidd and O’Hara study,48 the authors noted that “demineralization was slight, no lesion having progressed to cavitation” and hypothesized that the microscopic caries was arrested. Kidd has also noted that many “recurrent” caries are actually residual caries left at the time of preparation, 49 which would have nothing to do with the filling material but would be a factor of the preparation technique.

There were at best only sketchy dental histories provided, and there was no radiographic examination. The studies were published before amalgam bonding came into common use so the amalgams in the studies were probably unbonded. Amalgam bonding decreases microleakage50 as much as bonding does in composites,51 which decreases recurrent caries.52 Finally, there were no nonamalgam controls in these studies. What was not cited by Dickerson was a comparative study on microscopic caries that showed higher rates in composite resin restorations than in amalgam restorations.53

These studies of microscopic caries on 47 histologically-sectioned teeth do not offer good evidence that amalgam restorations are prone to recurrent caries. Instead, let us look at clinical studies to determine the incidence of recurrent caries in amalgam and resin composite restorations.

Recurrent caries has been studied extensively in clinical restorations in thousands and thousands of teeth, and it is true that recurrent caries is the most common cause of amalgam failure.54-60 However, recurrent caries is also the most common cause of resin failure44,55,56,58-60,61-67 and even cast gold restoration failure.45 In fact, recurrent caries is the most common cause of restoration failure in every kind of restoration.54,68

A 1993 study of 1415 class II amalgams showed only 16 (~1.1%) had recurrent caries after 10 years.19 At 10 years, none of 35 class II amalgams had recurrent decay.69 After 14 years, less than 5% had recurrent decay.70 In a 2000 randomized, prospective study of 1748 restorations in children over seven years, the composite restorations had an incidence of recurrent caries 3.5 times greater than that of the amalgam restorations.71 In a 2007 cross-sectional radiographic study of 557 amalgam and 93 resin composite interproximal restorations, there was an 8% rate of secondary caries in the amalgam restorations versus a 43% rate in the resin composite restorations.63 In a 1993 five-year prospective study of small class II restorations of primary caries: composite restorations had a higher incidence of recurrent caries than amalgam.72 Composites have higher levels of cariogenic bacteria at margins than amalgam restorations.73,74 It is clear that there are holes in the argument that amalgam restorations are associated with high levels of recurrent caries.

Amalgam Bonding
Many dentists assert that amalgam cannot be bonded to dentin. Freedman has stated, “Composites are bonded to dentin and enamel, recreating the monobloc of the original undecayed tooth. Amalgam simply fills a cavity, and may act as a wedge during mastication....Teeth are prepared more conservatively for composite restorations…. The preparation for amalgams requires extensions for retention and prevention, implying the loss of healthy tooth structure.”75 Vasserman stated that amalgam “has forced us to extend cavity preparations to accommodate the material rather than have the materials accommodate the preparation.”76

Some more recent comments on the Dentaltown message boards show that many dentists still believe that amalgam cannot be bonded: “amalgam needs significant bulk... it needs 90 degree cavosurface margins... and it needs mechanical undercuts. I agree they can be done conservatively... but those requirements still exist. Resin I can make thin... with any margin I want... and no need for undercuts. Coach DDS …. The preperation [sic] for an amalgam is by my standards today... extreme and excessive. What's the point of the restoration lasting 30 years if it predisposes the tooth to breaking in 2? That's why I'm glad I can do resins well... because I like saving teeth... not destroying half of them out in order to hold an MODL amalgam.” [ Accessed October 6, 2011.] It is possible, indeed probable, that dentists with such beliefs are proudly “amalgam-free” – they place no amalgams. As a result, they may not be aware of many of the latest amalgam techniques and materials, including amalgam bonding.

Although some assert that amalgam cannot be bonded to teeth, scientific evidence belies such assertions. There have even been successful amalgam pit and fissure sealants! (Please note that this author does not advocate using amalgam pit and fissure sealants.) In a pit and fissure sealant study comparing bonded amalgam sealants and resin sealants, there was no difference between the two types of sealants at 6 months, 1 year, 2 years, and 5 years.77 Some amalgam sealants lasted 5 years. If that doesn’t prove to even the most fervent anti-amalgamist that amalgam can be bonded to teeth, then nothing will. The potential benefits of amalgam bonding are similar to the potential benefits of resin composite bonding and include decreased microleakage, decreased incidence of recurrent caries, decreased postoperative sensitivity, increased fracture resistance of the tooth, decreased cuspal deflection, conservation of tooth structure, and increased retention.78 As a result of amalgam bonding, tooth preparation for amalgam restorations can be done exactly the same as for composite resin restorations.

 Many comparative studies have shown better bonding and less leakage in amalgam restorations than in composite resin restorations.79 Amalgam bonding is effective and can help ensure minimal cavity preparation techniques.26,80 One study has questioned the efficacy of amalgam bonding,81-83 but this study included amalgam bonding agents other than 4-META HPA. Amalgam restorations bonded with 4-META HPA (Amalgambond) powder adhesive are generally superior to those bonded with other types of bonding materials.84 Amalgam bonding with 4-META HPA has been shown to be equal to pin retention in large complex restorations.85

When comparing longevity of amalgam and composite restorations, it is not clear how important bond strength to dentin is. That is, beyond a certain (but as yet unknown) minimum level, it is unclear how important additional bond strength is.86 But for those who think that amalgam and composite bond strength is important, there have been studies showing amalgam bond strengths as high as 27 MPa87,88 and 33 MPa,89 compared to 23 to 25 MPa typically reported in composite restorations.90 In a 1999 comparative study, the bond strength to dentin was 33.0 MPa in the amalgam versus 26.4 MPa in composite.91 There is obviously a gap in the argument that amalgam cannot be bonded to dentin.

Amalgam Longevity
Dickerson stated, “We can all find amalgam that has lasted a long time, but statistically, that is the exception, not the rule.”92 This assertion is not backed up by scientific studies. In the 1960s, “extension for prevention” was state of the art. Sharp line and point angles were created using hoes and hatchets – and hoes had nothing to do with low women. There was no amalgam bonding. Large-scale studies (published in the 1970s) on amalgams probably placed in the 1950s or 1960s (before amalgam bonding) showed the median life of an amalgam filling to be about 10 years.93,94 Since then, virtually every large study has shown that amalgam longevity is significantly greater than composite resin longevity.55,58,63,71,95-103 It should be noted that as early as 1995104 and 1998,105 some authors had advised dentists to stop using amalgam, yet virtually every large study since then has shown greater longevity for amalgam restorations than for composite restorations.

There are very few studies in which resin composite restorations have shown better longevity than dental amalgam restorations. In two studies by the same authors,61,106 amalgam and composite restorations were placed by the same operator and followed over many years of private practice. The resin composite restorations had a better survival rate than the amalgam restorations. This operator placed virtually all amalgams for posterior teeth in the first decade of his practice from 1983 to 1993. After a two-year transition period, he switched to virtually all posterior composite restorations thereafter. Any restorations placed during the transition period were not included in the study. In other words, virtually all the amalgams were placed by an inexperienced operator (0 to 10 years of private practice) whereas virtually all the composites were placed by (the same) much more experienced operator (13 to 20 years of private practice). These studies do not provide evidence that composite resin restorations last longer than amalgam restorations. Restoration longevity is directly related to operator experience,102 which was significantly greater with the composite resin restorations. In addition, since they were placed between 1983 and 1993, the amalgam restorations were probably unbonded, but the composite resin restorations (which were placed between 1996 and 2003) were bonded. Amalgam bonding has been shown to decrease microleakage, cuspal deflection cusp fracture, recurrent caries, increase retention, and allow for smaller restorations. These could have yielded greater longevity for the amalgam restorations. There are still those who argue that amalgam does not have greater longevity than composite restorations, but this argument is getting old. There is overwhelming evidence that amalgam restorations typically have greater longevity than composite restorations.

Amalgam and Systemic Effects
Mercury occurs in the air, food, and water,107,108 and it is the main ingredient of dental amalgam restorations – about 50% Hg before mixing,109 but less after condensation. Although mercury is tightly bound in set amalgam restorations, tiny amounts of mercury are released. In 1985, Vimy and Lorscheider claimed that patients with amalgam fillings were exposed to an average daily dose of 20 µg Hg and those with 12 surfaces of amalgam as much as 29 µg Hg.110,111 It was later shown that Vimy and Lorscheider grossly overestimated that daily dose of mercury, mostly by misusing the mercury detector in their research, and their studies were widely discredited by several independent researchers.112-114 Using Vimy and Lorscheider’s own data, the independent researchers recalculated average daily doses were 1.7 µg Hg,112 1.7 µg Hg,114 and 1.3 µg Hg.115 Since then, Halbach has calculated an integrated daily dose of 3 µg Hg for an average number of fillings and only 7.4 µg for a patient with a high amalgam load.116 These levels are well below the World Health Organization’s tolerable level for daily mercury intake of 2.0 µg/kg body weight or about 68 µg Hg for a 150 lb adult.117

Vimy and Lorscheider have estimated that as much as 15 µg Hg per day can be released from a single occlusal amalgam filling.105 The average weight of a one-surface amalgam is estimated at 310 mg118 so the mercury content would be about 155 mg. If this restoration actually releases 15 µg Hg per day, then the entire mercury content would be depleted in 28 years. Having lost 27 mg of mercury, or nearly 20% of its main component, the restoration would probably implode within 5 years if 15 µg Hg were lost daily. Even the most rabid anti-amalgamists would admit that amalgam restorations do not typically implode within 5 years, or even within 20 years so it is highly unlikely that 15 µg Hg per day could be released from an occlusal amalgam restoration.

In a 1998 study of 1127 healthy males, Kingman et al. estimated there was only a 1 µg Hg/L increase in urinary Hg for every 10-surface increase in amalgam fillings.119 A similar 2005 study of 1626 women, Dye et al. estimated a 1.8 µg Hg/L increase in urinary Hg for every 10-surface increase in amalgam fillings.120 A 1995 study determined that a single amalgam filling releases only 0.03 µg/day of mercury so a patient would require 2,740 amalgam fillings in order to reach the mercury exposure threshold limit value of 83.30 µg /day considered dangerous for occupational exposure in the United States.121 Obviously, it is impossible for a patient to have anywhere near that many amalgam fillings. The World Health Association has stated that urine mercury levels are raised more from eating seafood once a week than from dental amalgam exposure.122

Mercury from dental amalgam accumulates in body tissues, especially the kidneys and brain,123,124 and there is a correlation between the number of amalgam surfaces and the amount of mercury accumulation in tissues.125 Some imply that this accumulation proves that mercury from dental amalgam causes systemic health effects,126 but the question is not whether mercury from dental amalgam can accumulate in body tissues (it can and does), but rather whether this accumulation can cause systemic health problems. There have been numerous studies attempting to show that dental amalgam causes systemic health problems, but there has never been a credible scientific study showing that mercury from dental amalgam causes systemic health problems in humans, other than occasional reports of allergic reactions.

A 2001 University of Calgary study showed disruption of “the membrane structural integrity of neurites and the growth cones of identified neurons” after exposure to mercury.”127 Anti-amalgamists asserted that this study “should remove all doubt regarding the role that dental mercury from amalgam fillings plays in the development of Alzheimer’s disease.”128 It’s curious that this study would be the one to remove all doubt since it was done on the tissue of dead snails. In fact, partly because there has never been a snail (either dead or alive) diagnosed with Alzheimer’s Disease, it is questionable whether this study has any relevance at all to the issue of amalgam restorations and Alzheimer’s Disease – let alone whether it should “remove all doubt” about the relationship. Another study cited by anti-amalgamists is a 1997 study from the Universities of Calgary and Kentucky purportedly showing that some rats exposed to high levels of mercury showed brain lesions similar to those in patients with Alzheimer’s Disease.129 But the rats were exposed to 100 times the level of mercury that patients with 25 surfaces of amalgam restorations would typically inhale even under stimulated conditions so its relevance is also questionable at best.

Well-controlled human studies have failed to show any relation between mercury exposure from dental amalgam restorations and Alzheimer’s Disease. A relatively homogenous group of Roman Catholic nuns was studied for the performance on a battery of neuropsychological tests, including one from the Consortium to Establish a Registry for Alzheimer’s Disease. There was no correlation with the presence, absence, or number of surfaces of amalgam.130 It is a result of studies like these that the Alzheimer’s Association has concluded, “According to the best available scientific evidence, there is no relationship between silver dental fillings and Alzheimer's.”131

In a 2006 randomized study of more than 500 patients in the Journal of the American Medical Association, there were no differences in neuropsychological or renal effects in children with amalgam versus composite restorations.132 In another 2006 randomized study of more than 500 patients, also published in the Journal of the American Medical Association, there was no difference in neurobehavioral effects in children with amalgam versus composite restorations.133 Clarkson et al., writing in the New England Journal of Medicine, concluded, “Patients who have questions about the potential relation between mercury and degenerative diseases can be assured that the available evidence shows no connection….There is no clear evidence supporting the removal of amalgams [for health reasons].”134

A 2011 study compared subjective health complaints in patients who had all their amalgam restorations replaced with composite versus patients who had no amalgam restorations replaced.135 After 3 years, those who had their amalgam restorations replaced had significant reductions in self-reported health complaints. This study does not provide scientific evidence that amalgam restorations cause health problems or that their removal improves health although it may show a beneficial psychological effect to amalgam removal. The health complaints were subjective and self-reported by the patients without any preoperative or postoperative medical examination (other than preoperative and postoperative mercury levels in the treatment group), and there was no information on the number of surfaces of amalgam restorations and whether there was any matching of the number of surfaces with the control group.

A 2011 study of 587 mother-child pairs concluded, “This study's results provide no support for the hypothesis that prenatal Hg(0) exposure arising from maternal dental amalgam restorations results in neurobehavioral consequences in the child.”136 In a 2008 randomized, prospective study of 507 children, those restored with amalgam restorations were compared with those restored with composite restorations.137 Children restored with amalgam restorations had a relatively high average of between 8 and 12 surfaces of amalgam, but there were no significant differences in neurological findings in the amalgam versus the composite group. The National Multiple Sclerosis Society states, “There is no scientific evidence to connect the development or worsening of MS with dental fillings containing mercury, and therefore no reason to have those fillings removed.”138

The Food and Drug Administration (FDA) “considers dental amalgam fillings safe for adults and children ages 6 and above….FDA concludes that the existing data support a finding that infants are not at risk for adverse health effects from the breast milk of women exposed to mercury vapor from dental amalgam. The estimated daily dose of mercury vapor in children under age 6 with dental amalgams is also expected to be at or below levels that the EPA and the Centers for Disease Control and Prevention (CDC) consider safe.”139

Extensive reviews on dental amalgam restorations have failed to show any credible evidence of systemic health effects (other than rare cases of allergy) from dental amalgam.122,140,141 The Life Sciences Research Office (LSRO), composed of representatives from the National Institutes of Dental and Craniofacial Research (NIDCR) of the National Institutes of Health, the Centers for Devices and Radiological Health of the U.S. Food and Drug Administration, the Centers for Disease Control and Prevention, and the Office of the Chief Dental Officer of the Public Health Service, conducted an extensive review of the literature on dental amalgam’s health effects in 2004 and concluded, “The current data are insufficient to support an association between mercury release from dental amalgam and the various complaints that have been attributed to this restoration material.”142,143 The American Dental Association has concluded that amalgam remains a safe and effective filling material.144

There are many dentists who assert that dental amalgam should not be used because its main ingredient mercury is “toxic” or “poisonous.” Mosby’s Medical Dictionary defines toxic as “pertaining to a poison.”145 Poison is defined as “any substance that impairs health or destroys life when ingested, inhaled, or absorbed by the body in relatively small amounts. Some toxicologists suggest that, depending on the dose, all substances are poisons.”146

Composite Resin and Toxicity
Many dentists advertise themselves as “mercury-free,” implying that while mercury is toxic, other dental materials like composite resins are non-toxic. It is true that mercury is toxic – it can impair health depending on its dose. But the same is true with any material, including composite resin.147 There are between 14 and 22 potentially hazardous compounds released from composite resin restorations, including 14 to 22 separate potentially hazardous compounds, including DL-camphorquinone; 4-dimethylaminobenzoic acid ethyl ester (DMABEE), drometrizole; 1,7,7-trimethylbicyclo[2,2,1]heptane, 2,2-dimethoxy[1,2] diphenyletanone (DMBZ); ethyleneglycol dimethacrylate (EGDMA); and triethyleneglycol dimethacrylate (TEGDMA).148 Geurtsen et al. identified 35 dental resin composite monomers/additives of commercial composite resins of which 9 were shown to be severely or moderately cytotoxic components.149 Various composite restorative materials implanted into animals have been shown to cause inflammatory responses, including an increase in lymphocytic infiltration as well as fibroblasts and epithelioid cells.150 Some composite resin components are cytotoxic (causing damage or destruction of cells).151-154

Dentin bonding agents and their components are mutagenic (causing mutations in new generations).155-159 Resin composite components “are hazardous in that they all cause significant toxicity in direct contact with fibroblasts.”160 They have been shown to cause immunosuppression or immunostimulation,161 to inhibit DNA162 or RNA163 synthesis, and to suppress the mitochondrial activity of macrophages.164 Composite resin restorative material has been shown to be more cytotoxic than amalgam in a comparative in vitro study.165 The blue light used to cure composite resin restorations may cause retinal damage.166-168

There is no “nontoxic” material for filling teeth. Mackert stated, “A frequent claim by the anti-amalgamists is that ‘no research has shown that dental amalgam is safe,’ yet the same charge can be leveled against composites and other dental materials. No material can ever be judged ‘safe’ with any kind of finality, because new biological evaluation techniques are always being developed, and previously unanticipated adverse effects are continually being discovered for all materials.”169

There have been many cases of allergy to composite resins and their ingredients.170-174 Resin composite effects include hand eczema and skin symptoms in dentists,175 Vinyl, latex, and modified latex gloves are permeable to composite ingredients.176 Resin monomers can encourage the growth of cariogenic microorganisms.177

Even water can be fatal in certain doses. According to the Centers for Disease Control, there were nearly 4,000 fatal drownings in the United States in 2007.178 There have been many cases of fatal water intoxication reported in the medical literature.179,180 It is doubtful that any “mercury-free” dentists eschew water in their practices, even though water is toxic. Those who describe composite resins, glass ionomers, and sealants to patients as “mercury-free” would be intellectually honest if at the same time, they described amalgam restorations as “triethyleneglycol dimethacrylate-free” or “formaldehyde-free” since composite resins, glass ionomers, and sealants have been shown to release formaldehyde, a possible carcinogen.181-185 Water is toxic in high doses, but necessary for life in lower doses. Similarly, warfarin sodium, fluoride, and yes mercury in dental amalgam, are among substances that are toxic in high doses but necessary for health in lower doses in some patients. The argument that mercury from dental amalgam causes systemic health effects is downright poisonous!

Components of composite resin:149
  • BEA Benzyl alcohol
  • BEMA Benzyl methacrylate
  • BHT 2,6-Di-t-butyl-4-methyl phenol
  • Bis-EMA Ethoxylated bisphenol-A-di-methacrylate
  • Bis-GMA Bowen monomer, isopropyliden-bis (2-hydroxy-3-(4-phenoxy)-propylmethacrylate
  • Bis-MA Bisphenol-A-dimethacrylate
  • BL Benzil
  • BME Benzoic-acid-methylester
  • BPE Benzoic-acid-phenylester
  • CA Camphoric acid anhydride
  • CQ CamphoroquinoneDBPA Dibenzoyl-peroxide
  • DCHA Dicyclo-hexylamine
  • DCHP Dicyclo-hexyl-phthalate
  • DDMA 1,10-Decane-diol-dimethacrylate
  • DEAE Diethyl-amino-ethanol
  • DEGDMA Diethyleneglycol-di-methacrylate
  • DHEPT Dihydroxy-ethyl-p-toluidine
  • DICH 1,6-Diisocyanato-hexane
  • DIPA 2,6-Diisopropyl-aniline
  • DMABEE 4-Dimethylaminobenzoic acid ethyl ester
  • DMAPE 2-(4-Dimethyl-aminophenyl)ethanol
  • DMBZ Dimethoxybenzoine
  • DMDDA Dimethyl-dodecane-amine
  • DMPT Dimethyl-p-toluidine
  • DMTDA N,N-Dimethyltetradecylamine
  • EGDMA Ethyleneglycol-di-methacrylate
  • HEMA 2-Hydroxy-ethyl-methacrylate
  • HMBP 2-Hydroxy-4methoxy-benzophenone
  • TEG Triethylenglycol
  • TEGDMA Triethylenglycol-di-methacrylate
  • THA Trihexylamine
  • TPP Triphenylphosphine
  • TPSb Triphenylstibane
  • UDMA Urethane-di-methacrylate


Conclusion
Dentists sometimes look for solutions to problems without realizing that the problems weren’t what they seemed or that the solution was right in front of them the whole time. Such is the case with dental amalgam. Thomas Henry Huxley could have been speaking about the “crucifixion” of dental amalgam when he said, “The great tragedy of science: the slaying of a beautiful hypothesis by an ugly fact.”

Dental amalgam certainly has a big drawback – it’s ugly, but according to exhaustive research published in the medical and dental literature, the other “problems” with amalgam are not what they seem. Dental amalgam restorations are associated with excellent longevity and low rates of cusp fractures and recurrent caries, and dental amalgam restorations have not been linked to systemic health problems other than rare reports of allergic reactions. While composite resin restorations are more esthetic and are certainly an acceptable alternative, they generally do not compare favorably in other respects. The “solution” may not necessarily be to switch to composite resins but rather to continue using dental amalgam, not composite resin. It is a wonderful time to practice dentistry and to have so many choices for restorations, including both amalgam and composites. Pardon the pun, but for clinical efficacy and safety, dental amalgam is still the “gold standard” – or the “silver/mercury standard” – to which other direct filling materials including composite resin should be compared.

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Author's Bio
Dr. Michael J. Wahl graduated from Case Western Reserve University with his DDS degree in 1985. He has owned a general dentistry practice – Wahl Family Dentistry – in Wilmington, Delaware, since 1996. He works as an assistant attending dentist at Christiana Care Health System and as an evaluator for Clinical Research Associates. Wahl has written numerous academic articles for publication and speaks nationally and internationally. To contact Dr. Wahl, e-mail wahldentistry@aol.com.
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