Many dentists believe that dental amalgam commonly causes teeth to fracture , necessitating more extensive treatment. Erickson stated, "the cuspal fracture characteristic of amalgam is well-known and observed almost daily in every general dental practice." 
Referring to the likelihood of fractured cusps in teeth with Class II amalgams, Davis and Nesbitt called Class II amalgam restorations "time bombs" that "may threaten not only specific teeth, but possibly an entire dentition."  DiTolla asked, "[W]hy would I plant this amalgam 'crown seed' and then wait for the tooth to break[?]"  Van Dyke stated, "Almost every amalgam I remove has decay or structural fractures of the tooth."  Dickerson stated, "Many studies have shown that, after seven years, 50 percent of the teeth filled with amalgam have fractured." No studies were cited, but in a personal communication [February 3, 2000], Dr Dickerson mentioned two retrospective studies on endodontically treated teeth [6,7]
Close examination of these and other studies reveals that they do not support the assertion that amalgam restorations commonly cause cusp fracture. The first study cited by Dickerson (personal communication, February 3, 2000), published in 1988 by Hansen, retrospectively compared the incidence of cusp fractures in endodontically treated premolars restored without cuspal coverage, 181 with MOD amalgam restorations and 40 with MOD resin restorations . The teeth restored with resin had a much lower incidence of cusp fractures.
The study has serious limitations, however. Hansen pointed out that the results of his study "should be cautiously interpreted, especially since the number of resin-restored teeth was rather small." [6 ] He also asserted that the optimal restoration for endodontically treated posterior teeth is not the intracoronal restorations in this study, but rather restorations with cuspal coverage, because endodontic treatment can weaken teeth. Amalgam bonding has been shown to increase fracture resistance [8-11] and decrease cuspal deflection .. Although the resins in this study were enamel bonded, the study was published before amalgam bonding was prevalent; the preparations were therefore probably larger than commonly used for bonded amalgam restorations, further weakening the teeth.
Hansen et al  and Hansen and Asmussen  later published the results of two much larger retrospective on amalgam and on resin composite, respectively. Of 1,584 endodontically treated posterior teeth restored with amalgam, 532 (34%) had cuspal fractures after 20 years . Of 190 endodontically treated posterior teeth restored with enamel-bonded resin composite, 28% had cusp fracture after 10 years [13 ]. Although the composites were enamel bonded and the amalgams were presumably unbonded, the percentage of amalgams without cuspal fracture after 20 years (66%) was about the same as the percentage of composites without cusp fracture after only ten years (72%).
The other study cited by Dickerson (personal communication, February 3, 2000) compared cuspal fracture in endodontically treated teeth restored with amalgam before 1975 with cuspal fracture in those restored after 1979. Using data from the previously discussed study , Hansen and Asmussen  showed that teeth restored after 1979 were much more likely to fracture than were those restored before 1975. Anti-amalgamists claim that the higher fracture rate is most likely caused by the expansion and low creep of modern high-copper amalgams, not widely used before 1975. A more likely explanation, also mentioned by the authors, is the use of Gates-Glidden burs for straight-line access to the root apex in endodontics, which were not used commonly until after 1979. These burs and wider access openings can weaken teeth, and the authors speculated that this is an equally plausible explanation for the higher fracture rate after 1979 .
Long-term studies not limited to endodontically treated teeth have shown that tooth fracture is relatively uncommon in teeth with amalgam. A long-term study of about 600 amalgam restorations showed a cuspal fracture rate of less than a 1.5% after 5 years . A later study of 1,415 class II amalgams showed that, after 10 years, only 120 restorations (8.5%) failed at all . Only 25 (1.8%) of the 1,415 teeth had a fractured cusp after 10 years. After 15 years, 1,213 Class II amalgam restorations were available for examination , and only 46 (3.8%) failed because of enamel fracture alone. A total of 61 (5.0%) failed because of an enamel fracture only or a combination of enamel fracture, isthmus fracture, and/or caries.
Di Tolla explains to his patients 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."  Although temperature changes can cause amalgam to expand or contract, these same changes can also cause resin composites to expand or contract  . In fact, temperature changes may affect composites more than amalgams since the coefficient of thermal expansion of composite [18-20] is generally higher than that of amalgam [21,22] . Regardless, extreme temperature changes occur only fleetingly in vivo . Neither hot coffee nor cold soda contacts the teeth very long, let alone any part of the mouth, before being swallowed. The use of presently employed thermocycling regimes is therefore questionable . The aforementioned studies indicate that it is unlikely that these fleeting contacts are enough to cause many teeth with amalgam restorations to fracture.
Teeth with composite restorations can be associated with fractured cusps also. Browning and Dennison showed that, of 1,360 replaced composite restorations, 8.7% were replaced because of tooth fracture [325. Indeed, even cast gold restorations, long considered the functional ideal restoration in dentistry, are frequently replaced because of tooth fracture .
One reason that some studies of posterior resin composites show low incidences of cusp fracture is that dentists have long been advised to use composites only in small cavity preparations , but otherwise to use amalgam. Smaller restorations generally last longer and make teeth less likely to fracture than do larger restorations [28,29]. If larger cavity preparations are necessary, dentists have been advised to use amalgam. There may be an inherent bias in favor of composites in many of the cross-sectional or comparative studies that did not take restoration size into account: Dentists may limit posterior resin composite restorations to small preparations and place amalgam in larger preparations.
Di Tolla 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."  It may be true that many, if not most, teeth needing first-time crowns have amalgam restorations, but this does not mean that most teeth with amalgam restorations fracture or need crowns. Resin composites have not been recommended for moderate to large posterior restorations. Amalgams have historically been placed in already weaker teeth. Most presently functioning posterior restorations are amalgam. The number and age of amalgam restorations functioning in people's mouths in the population as a whole probably far exceeds the number and age of posterior resin composite restorations. It would only make sense that a similar proportion of teeth with amalgam restorations may eventually need more extensive restorations.
Dentists spend a disproportionate amount of time on extensive restorative procedures such as crowns, much more so than on examining well-functioning restorations. Practicing dentists spend only about 10% of their time on diagnostic services, but about 90% of their time on other services . As a result, the perception that a huge number of teeth with amalgam break may be different than the reality that only a few teeth with amalgam break.
Dr. Wahl practices dentistry in Wilmington, Delaware. This article was originally published as Wahl MJ. Amalgam -- resurrection and redemption. Part 2: the medical mythology of anti-amalgam. Quintessence International 2001; 32:696-710, 2001. It is reproduced here with the kind permission of the publisher. The author thanks Drs. J. Rodway Mackert, Ivar A. Mjör, and Fred Eichmiller for reading the manuscript and offering several helpful suggestions.