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Age and Third Molar Extraction as Risk Factors for Temporomandibular Disorder
1 Department of Orthodontics, Box 357446, Correspondence: * corresponding author, ghuang{at}u.washington.edu
This study investigated third molar removal as a risk factor for temporomandibular disorder (TMD) in all age groups. We compared 2217 Kaiser Permanente Northwest health plan enrollees with a history of third molar extraction with 2217 age-and gender-matched enrollees with radiographic confirmation of no lifetime third molar removal. Common Dental Terminology codes were used to identify information on third molar removal, and International Classification of Disease codes were used to identify TMD. Relative risks were calculated overall, and by each decade of life, in univariate and multivariate analyses. The incidence of TMD in subjects with and without third molar removal were 7 and 5 per thousand person-years, respectively. Third molar removal among subjects of all ages resulted in a statistically insignificant increased relative risk for TMD (1.4, 95% confidence interval (CI): 0.9–2.2). The relative risk was slightly higher in those under 21, but was also not statistically significant (1.6, CI: 0.8–3.1).
Key Words: third molar extraction • age • TMD • risk factors
In a recent retrospective cohort study, third molar removal was found to be associated with a significantly increased incidence of temporomandibular disorder (TMD) (Huang and Rue, 2006). That study utilized dental insurance claims data for ascertainment of third molar removal and subsequent TMD status, and was restricted to a population of 15- to 20-year-olds. This is a period when many third molars are unerupted or partially erupted. It is not uncommon, however, for third molars also to be extracted in older individuals, after these teeth have had a continued opportunity to erupt. No literature is available on the relationship between age at time of third molar extraction and subsequent risk for TMD. Thus, this study investigated third molar removal as a risk factor for TMD among all age groups.
The participants in this population-based cohort study were drawn from enrollees of Kaiser Permanente Northwest (KPNW). KPNW is a group-model Health Maintenance Organization that provides complete medical and dental services to a population of about 200,000 individuals in Southwest Washington and Northwest Oregon. Since its inception, KPNW has used a unique health record number, enabling researchers to link data on all health care that an individual has received under the Kaiser system. Third molar extraction is covered under the dental program, and TMD care is covered under both medical and dental programs. This study was approved by the Institutional Review Board at the University of Washington, as well as by the KPNW Institutional Review Board.
Exposed Cohort
Unexposed Cohort
Statistical Analyses
Anesthesia was categorized as local anesthesia only, local anesthesia with nitrous oxide, intravenous sedation, and general anesthesia, based on CDT codes. Arch of extraction was coded as maxillary only, mandibular only, or both. All statistical analyses were performed with SAS version 9.1 (Cary, NC, USA). A significance level of P < 0.05 was established for all tests.
The selection of the sample is outlined in the Fig.  We confirmed 2296 matched pairs, 79 of which were excluded due to TMD outcomes prior to the date of extraction, leaving a total of 2217 pairs for analysis. The distribution of TMD diagnoses is provided in Table 1. The largest group consisted of unspecified temporomandibular joint (TMJ) disorders, followed by arthralgia, articular disc disorders, and other specified TMJ disorders. While the exposed persons had proportionately more arthralgia, articular disc disorders, and specified disorders, these differences were not statistically significant.
Of the 2217 matched pairs, about 50% of the participants fell in the 10- to 20-year-old age bracket, 25% in the 21 to 30 age bracket, and progressively fewer in each subsequent decade. About 64% of all participants were male, and 36% were female. However, of the 77 participants with TMD diagnoses, 50 were female. Thus, female gender was associated with a significantly increased relative risk of TMD (Table 2 ). The annual incidence of TMD in the exposed and unexposed cohorts was 0.7% and 0.5%, respectively. The annual incidence was much higher in females (1.1%) compared with males (0.3%). Dental utilization was 0.3 prophylaxis visits/year in both cohorts.
For the entire sample, the relative risk for TMD in individuals who had third molars removed compared with that in individuals who did not was 1.4 (Table 2  ). Although the point estimate was elevated, this increase was not statistically significant. When stratified by age, the point estimates for the relative risks were 1.6 in the 15- to 20-year-olds, 1.1 in the 21-to 30-year-olds, 1.2 in the 31- to 40-year-olds, and 2.5 in the participants over 40. However, none of these relative risks reached statistical significance. The estimates were minimally affected by adjustment for utilization.
Among participants with third molar extraction, clear patterns emerged for parameters related to the removal of teeth with regard to age (Table 3
Third molar removal has been implicated as a risk factor for TMD in several descriptive TMD papers (Greene et al., 1969; Butler et al., 1975; Pullinger et al., 1988; Huang et al., 2002). Additionally, three studies specifically investigated the relationship between third molar removal and TMD. Raustia compared 22 university students before and after third molar removal, and found an increased severity of TMJ dysfunction 3 mos after third molar removal (Raustia and Oikarinen, 1991). Threlfall conducted a case-control study, utilizing 220 TMD cases and 1100 age- and gender-matched control individuals. He reported an odds ratio of 1.28 (95% CI, 0.96–1.71) in a mostly older population (Threlfall et al., 2005). Finally, Huang reported a significantly elevated risk of 1.6 for TMD after third molar removal in a sample of 35,000 15- to 20-year-olds (Huang and Rue, 2006). The current study did not find significantly elevated risks for TMD subsequent to third molar extraction. However, the point estimate for individuals under the age of 21 does suggest a relationship, especially in light of the identical risk estimate from Huang’s prior work in a completely independent sample (Huang and Rue, 2006). In both studies, the relative risk of 1.6 for teens and young adults after third molar removal was only moderately increased, and the overall incidence of TMD under the age of 21 appeared to be relatively low (around 1%). Although TMD is not common in this age group, the high frequency of third molar removal results in a population-attributable risk of 23%, indicating that almost a quarter of all TMD cases in this age group might be related to third molar extraction. The patterns related to third molar removal in younger persons may partially explain the elevated risk, since they were more likely to have all 4 third molars removed at one time, deeper levels of anesthesia, and more severely impacted third molars. Oral surgeons often advocate third molar removal during the teens, citing advantages such as incompletely formed roots, better healing, and less morbidity (Bruce et al., 1980; Osborn et al., 1985; de Boer et al., 1995; Phillips et al., 2003). However, these studies typically obtain participants from oral surgery practices, and difficult extractions among older individuals may be over-represented in these populations. Studies that do not account for this factor are likely to overestimate the impact of age (Phillips et al., 2003). In fact, others, using prospective designs or multivariate analyses, have reported that the risk of complications is not increased greatly in older age groups (Bui et al., 2003; Haug et al., 2005). The non-significant relative risks of 1.1 and 1.2 for persons in their third and fourth decades of life are similar to the odds ratio of 1.3 reported in a prior study (Threlfall et al., 2005). The consistency of these estimates would suggest that third molar removal is not likely to be a large risk factor for TMD in older populations. This might be explained by the simpler extraction patterns observed in older persons. While this study did not find a significant relationship between severity of impaction and risk for TMD, a tendency for this to occur was observed. Severity of impaction has also been described as being associated with increased risk for other complications. For instance, alveolar osteitis, secondary infection, and dysesthesia were all more common when impacted teeth were removed (10%, 10%, and 1.2%, respectively), compared with erupted teeth (2%, 0.2%, and 0.2%, respectively) (Osborn et al., 1985). It has been reported that third molars can continue to erupt in the 3rd and 4th decades of life, and this eruption is difficult to predict (Hattab, 1997; Kruger et al., 2001; Venta et al., 2004; Nance et al., 2006). On the basis of this information, simply delaying removal of asymptomatic third molars until the mid- to late 20s might reduce extraction-related morbidity. Obviously, the decision to extract third molars is complex, and can involve many factors, such as potential health benefits, risks and complications, family preferences, professional recommendations, and insurance coverage. While three cost-effectiveness analyses have concluded that retention of asymptomatic third molars is the most cost-effective approach with the least disability (Tulloch et al., 1990; Edwards et al., 1999; Song et al., 2000), third molars continue to be the most commonly extracted teeth in teenagers and young adults. A clear inverse relationship was seen between age and severity of impaction, with 74% of the extractions done prior to the age of 21 reported as full bony impactions, compared with about 5% after the age of 40. These age-related extraction patterns could be influenced by several factors. Continued eruption of third molars is one explanation. Additionally, more severe impactions might be preferentially referred earlier in life, resulting in an increased prevalence of third molar extractions with bony impactions in teenagers. Also, it is possible that care-seeking behavior differs over time. The current study raises interesting questions about age and risk for complications, since it included persons who had extractions from all dental providers. The patterns we found for severity of impaction by age were quite different from those reported in the studies that enrolled individuals from only oral surgery practices, and these relationships merit further investigation. A strength of this study was its ability to verify that comparison individuals had no lifetime history of third molar removal, thus preventing misclassification of the exposure. The central chart-housing facility in which all Kaiser dental charts are maintained made this verification process feasible. Only about 20% of the potential comparison individuals could be confirmed as having no lifetime third molar removal. Although this figure may seem low, from 40 to 50% of individuals in insured populations have third molars removed by the age of 20 (Eklund and Pittman, 2001; Huang and Rue, 2006), and the rate of third molar agenesis has been reported to be between 9 and 35% (Thompson et al., 1974). Other strengths of this study were its population-based cohort design, the age- and gender-matched sample, largely equal access to dental and TMD care by both cohorts, and the unbiased ascertainment of exposure and outcome information. Additionally, individuals who had extractions from all dental providers were included. One limitation was the relatively low incidence of TMD, which affected the power of the statistical analyses. Our initial estimates for TMD rates were 3% and 4.5% in the unexposed and exposed groups, respectively, which would have resulted in 80% power. Post hoc calculations indicated that the power of our study to detect a statistically significant difference was only 34%. The results of this study should be generalizable to other similarly insured populations in the US. Based on the results of this study, extraction patterns exhibit considerable change over the decades of life. The risk for TMD after third molar removal was not statistically elevated overall, or for each decade of life. However, analysis of these data suggests that the risk may be greater for persons under the age of 21, at which time the factors associated with third molar removal indicate that it is usually a more invasive procedure.
This study was supported by NIDCR grant #DE14609-01. The authors thank Drs. Linda LeResche, Alex White, Gerardo Maupome, Kari Borgen, and David Covell. We also thank Mr. Paul Cheek and the Kaiser Permanente Center for Health Research for their assistance with this project. Received for publication February 12, 2007. Revision received October 12, 2007. Accepted for publication December 14, 2007.
Journal of Dental Research, Vol. 87, No. 3,
283-287 (2008)
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