We thank Dr. Smith for sharing his thoughtful observations about tooth fractures in his practice. We have four comments concerning his letter.
First, we commend Dr. Smith for being a careful observer of his practice experiences relative to this issue. His observations raise some important, clinically relevant questions that are ideally suited to be studied as part of a major new initiative of the National Institute of Dental and Craniofacial Research (NIDCR), which will conduct practice-based research in a network of dental practices.
NIDCR is reviewing proposals for such a network this fall, with the goal of funding one or more networks in 2005. Inquiring practitioners, such as Dr. Smith, will be central in identifying issues that can be addressed by such a research network, as well as in participating in studies mounted by the network. Perhaps the prevention and management of tooth fracture will be a topic addressed by one of the NIDCR-funded networks.
Second, with respect to Dr. Smith’s questions regarding the relative risk of cusp fracture associated with amalgam and composite restorations, it is important to note that our study did not analyze the type of restorative material present at the time of fracture, since virtually all restorations in the case and control groups were amalgam restorations.
However, Dr. Michael Wahl and colleagues’ article, "Prevalence of Cusp Fractures in Teeth Restored With Amalgam and With Resin-Based Composite," in the August issue of JADA, presents information that disagrees with Dr. Smith’s observations. But these authors also provide an explanation for why the perception of more frequent fractures associated with amalgam restorations may arise. They illustrate that it is necessary to take into account the base rates for the two types of restorative materials—that is, the frequencies with which amalgam and composite restorations are present in the mouth.
Third, Dr. Smith’s observations concerning the relative frequency of fractures associated with amalgam and composite may have an explanation related to time. If Dr. Smith originally placed principally amalgam restorations, and then the relative proportion of amalgam to composite restorations gradually changed, the amalgam restorations in Dr. Smith’s practice are likely to be older than the composite restorations. As our study suggested indirectly, restoration age may be a risk factor for fracture.
Fourth, although more tenuous, an explanation for Dr. Smith’s observation of an association between use of rubber dam for amalgam restorations and subsequent cusp fracture may be related to the composition of the amalgam alloy he used. Moisture contamination of zinc-containing alloys can result in expansion that could increase the risk for fracture.
We make these comments to partially illustrate the difficulties in deciding what is "truth" on the basis of a single study or a single set of personal observations. Both Drs. Smith and Wahl and their colleagues are to be congratulated for exploring a current issue in clinical dentistry. One can hope that more attention, and more careful examination, are devoted to these questions in the future.
