goal—quality oral care service for the public. Clinical practice provides useful observations and forms behavior changes and opinions for practitioners to the betterment of their patients. Scientific projects augment and support practice. I welcome a discussion with you regarding your thoughts on the subject of “when is a full crown indicated?” Gordon J. Christensen, DDS, MSD, PhD Provo, Utah

Editor’s note: Most of the available space in JADA is devoted to scientific research, with an emphasis on clinical research in recognition that the majority of our readers are dentists engaged in the daily practice of dentistry and are seeking information that will help them improve their practices. JADA also devotes space each month to a “Perspectives” section in which contributors within the profession are invited to express their views on topics of current interest in dentistry. These pieces are clearly labeled as the opinions of the authors and, as noted, are set aside in their own Perspectives section to distinguish them from the rest of The Journal. Furthermore, Dr. Gordon Christensen’s monthly reports carry the heading “Observations,” an additional signal to the reader that the author is expressing a personal opinion, based on his years of experience in dentistry. JADA’s Perspectives section exists to add interest to The Journal and to trigger debate. It would appear to be succeeding on both counts. THIRD MOLARS AND TMD

This letter is in regard to the November JADA study by Dr. 574

JADA, Vol. 138

Greg Huang and Ms. Tessa Rue, “Third-Molar Extraction As a Risk Factor for Temporomandibular Disorder.” While we would like to commend the authors for addressing an important problem in dentistry, we have some concerns about the conclusions put forth in the article. Our first concern is in regard to the estimate of the risk of developing TMD following thirdmolar (M3) extraction. The authors report that patients who have their M3s removed are 60 percent more likely to experience TMD than those who don’t have their M3s removed, after controlling for sex and dental care utilization. The overall frequency of TMD in the sample was 1.1 percent. Using the adjusted relative risk, this translates into adjusted frequencies of 0.4 percent and 0.7 percent in the non-M3 extraction and M3-extraction groups, respectively, a risk increase of 0.3 percent. This risk increase is of the same magnitude as those reported for inferior alveolar and lingual nerve injuries, which are known complications for which the benefit of M3 removal exceeds the risk. In addition to the discordance between the authors’ conclusions and the clinically relevant magnitude of the reported effect, there is the question of whether the choice of analysis was appropriate. Generally, rigorous survival analysis is appropriate for “time to event” cohort data. One might argue against using a survival analysis in this study, as the “time to event” is not necessarily something akin to the development of a disease that has a latent period (for example, cancer).

The mechanism proposed for relating TMD to M3 removal may be trauma associated with the extraction or maintaining an open mouth position for the duration of the procedure. It is difficult to make a convincing argument that the study population is continually at risk of developing TMD at time points significantly after the extraction. If the proposed mechanism is injury associated with the extraction, the risk for developing TMD should be inversely related to duration of time after extraction. For example, patients are more likely to develop TMD within a month after the extraction, rather than 24 months after the extraction. Another dimension to consider is, during the “at risk” period, some of the subjects had other dental procedures completed, which could have caused TMD in a similar manner as proposed for M3 extraction, or possibly exacerbated a condition that initially was caused by the M3 extraction to the extent that treatment was subsequently required. While the authors controlled for “other dental procedures” in their analyses, this may not adequately adjust for procedures associated with extended operating time. Thus, perhaps the exposure should be “duration of mouth opening for a procedure,” rather than M3 extraction. If the onset of TMJ symptoms is related to prolonged mouth opening rather than the trauma of extraction itself, then, generally speaking, any evaluation between M3 extraction and TMD will be overestimated without controlling for the true intermediate variable “prolonged mouth opening.” Finally, there is the issue of

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selection bias. The authors reported only 13 percent of the subjects met the inclusion criteria for the study. Given this small proportion of patients who were included, nothing can be said about the remaining 87 percent. Though the sample size is large, the large number of patients lost to follow-up weakens the conclusions of the study. Thomas B. Dodson, DMD, MPH Attending Oral and Maxillofacial Surgeon and Director Center for Applied Clinical Investigation Massachusetts General Hospital and Associate Professor of Oral and Maxillofacial Surgery Department of Oral and Maxillofacial Surgery Boston

Srinivas M. Susarla, BA DMD Candidate

Sung-Kiang Chuang, DMD, MD, DMSc Associate Professor of Oral and Maxillofacial Surgery

R. Bruce Donoff, DMD, MD Distinguished Walter Guralnick Professor of Oral and Maxillofacial Surgery and Dean Harvard School of Dental Medicine Boston

Authors’ response: We would like to thank Dr. Dodson and colleagues for their comments, and we appreciate this opportunity to clarify the points they raise. First, we agree that the incidence of TMD in this population is low. The overall rate of 1.1 percent is consistent with our data, but their proposed rates of 0.4 percent and 0.7 percent are not. Rather, the corresponding rates should be 1.35 percent in the exposed and 0.85 percent in the unexposed. These rates allow us to calculate an average risk of 1.1 percent, a risk difference of 0.5 per-

cent and a relative risk of 1.6 percent (indicating an increased risk of 60 percent in the exposed). Even this is a simplification, as the relative risk was statistically adjusted for utilization and sex. The comment that benefit exceeds risk is easily stated, but more difficult to justify. Tulloch and colleagues’1 article on the cost-effectiveness of third-molar removal states that prophylactic extraction results in more overall morbidity than extracting only third molars that develop pathology. Additionally, other countries have adopted policies recommending against the prophylactic removal of third molars.2 Our decision to utilize a survival analysis was made after consultation with two senior biostatisticians and an epidemiologist at the University of Washington. We know of no other design that would have allowed us to utilize the data in a more efficient or appropriate manner. Dr. Dodson suggests that the risk for TMD after third-molar removal should diminish quickly following the procedure. However, there are several arguments against this hypothesis. First, there is usually a period of discomfort, swelling and limited function due to the surgical procedure, and any TMD is likely to be masked during this time. As more normal function returns, TMD symptoms may need to be relatively severe or persistent before these teenaged subjects are prompted to seek treatment. Additionally, once traumatized, the TMJ, like other joints, may be susceptible to exacerbations and re-injury. Thus, we do not believe that the extended

period of increased risk argues against a relationship between third-molar removal and TMD, but rather that the documentation of TMD in these subjects occurred over several years. Controlling for “duration of mouth opening” was not possible in our large, retrospective study, as it cannot be directly ascertained from insurance records. Thus, we chose a surrogate variable, dental utilization, which we felt would generally represent both duration and magnitude of opening. In fact, adjustment for dental utilization did reduce the relative risk. The theory that prolonged opening is causal for TMD is interesting and certainly could be investigated in prospective studies. However, oral surgery textbooks report uncontrolled forces and inadequate support of the mandible as the reasons for temporomandibular joint injury during exodontia.3,4 It is true that we only reported on 13 percent of the 15-yearolds in the Washington Dental Service system during the study period. This was the result of the application of a single criterion—that the subjects had five years of continuous coverage. Any “selection bias” introduced at this stage would not affect the internal validity of the comparison of the 35,000 subjects who were chosen for the study. The important question is whether the results of these analyses are generalizable to the remaining 87 percent. We can think of no strong arguments that the included and excluded populations would not share similar dental experiences during the periods that they had insurance coverage. Additionally, all selected subjects were followed for the en-

JADA, Vol. 138 http://jada.ada.org Copyright ©2007 American Dental Association. All rights reserved.

May 2007



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