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Letters: Examining amalgam prep

September 07, 2015 I am writing in response to Dr. Stephen H. Chronister's letter ("Amalgam Challenge," July 13 ADA News). Dr. Chronister states that in his clinical practice he replaces many failed amalgams due to tooth fracture, and that the literature is replete with research for the past 40 years demonstrating that the setting expansion of amalgams created outward forces on tooth structure which will eventually lead to fracture.  

He also challenges every reader to do a thorough literature search on amalgams and tooth fracture. I performed a search and could not find any articles condemning amalgam for excessive expansion and fracturing teeth. What I found in the literature was that there is no significant difference in the prevalence of cusp fracture rates in amalgam restored teeth versus composite-restored teeth in subjects, and in older patients there was a higher cusp fracture rate with composite-restored teeth. The cusp fracture rate for amalgam overall was 1.88 percent versus composite at 2.29 percent.1 In another study, annual failure rates in posterior stress-bearing areas were 0-7 percent for amalgam restorations and 0-9 percent for direct composites.2 Bernardo performed a randomized clinical trial and concluded that amalgam restorations performed better than did composite restorations and the difference in performance was accentuated in large restorations where more than three surfaces were involved.3  

ISO 1559 sets the standard for the dimensional change of amalgam at -.1 to +.2 percent.

Direct composites have a dimensional change of 1-3 percent and can undergo dimensional change due to water sorption. Amalgams, if not properly placed by the operator, can lead to higher expansion rates than those set by ISO. Mercuroscopic expansion from a poorly condensed amalgam may lead to additional expansion, and if good isolation (rubber dam) is not obtained, moisture contamination of zinc-containing amalgam can lead to additional expansion. These are not necessarily material issues —they are technique problems created by the operator.  

Finally, the observation of fractured cusps in amalgam restorations may also be explained by poor, overly aggressive preparation design. A major problem with tooth fracture occurs when the isthmus width increases.4,5 Many of us were taught extension for prevention principles and to use sharp line angles in our preparation design for amalgam restorations. These flawed principles, along with overzealous undercuts for retention, are the most likely culprits for the observation of fractured cusps, not setting expansion of the amalgam.

William J. Dunn, D.D.S.
San Antonio

1. Wahl MJ, Schmitt MM, Overton DA, Gordon MK. Prevalence of cusp fractures in teeth restored with amalgam and with resin-based composite.  JADA. 2004;135(8):1127-1132.
2. Hickel R, Manhart J. Longevity of restorations in posterior teeth and reasons for failure.  J Adhes Dent. 2001;3(1):45-64.
3. Bernardo M, Luis H, Martin HD, et al. Survival and reasons for failure of amalgam versus composite posterior restorations placed in a randomized clinical trial.  JADA. 2007;138(6):775-783.
4. Osborne JW, Gale EN. Relationship of restoration width, tooth position, and alloy to fracture at the margins of 13- to 14-year-old amalgams.  J Dent Res. 1990; Sep;69(9):1599-601.
5. Blaser PK, Lund MR, Cochran MA, Potter RH.  Effect of designs of Class 2 preparations on resistance of teeth to fracture.  Oper Dent. 1983;8(1):6-10.