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 percent 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-year-olds 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 entire study period, so no patients were lost to follow-up.
The risk of experiencing TMD after third-molar removal has been acknowledged3,4 but not previously quantified, and we hope this information will be of value to our colleagues. The absolute risk is not great and, as Dr. Dodson notes, it is similar in magnitude to the risk of experiencing inferior alveolar or lingual nerve damage. However, due to the frequency of third-molar removal, the population-attributable risk is relatively large and should be taken into consideration when recommending prophylactic third-molar removal.
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