Introduction

Several scientific articles on third molars begin with the statement that the third molar is the most frequently extracted tooth [1,2,3,4,5,6,7]. Also, it is often stated that extraction of this tooth is the most common surgical procedure in dentistry. References are typically not given for these statements. Clinically, it is obvious that third molar extraction is the most frequent type of treatment at oral surgical units and in young adults. However, this topic has not been commonly analyzed using population-level data on all teeth on the grounds of the most frequently extracted or missing tooth. The lack of scientific evidence is mainly due to two reasons; third molars are often ignored in clinical oral examinations of large populations [8, 9] and panoramic radiographs are rarely obtained in population studies [8,9,10,11].

According to clinical oral examinations of several distinguished population studies, mandibular incisors and canines are the most long-lasting teeth [8,9,10,11,12]. More detailed information about individual teeth is available on the prevalence of third molars based on two larger studies with panoramic radiographs. In a nationwide survey of Finnish adults, all four third molars are missing in the age group 30‒39 years in 37.5% of individuals and in persons older than 70 years in 80.8% [13]. A similar trend in missing third molars has been reported in a Swedish radiographic study [14]. However, there are no population-level studies on missing teeth based on radiological findings and evaluating the entire dentition.

When assessing the rates of extracted or missing teeth at population level, congenital absence of teeth must be considered. In a meta-analysis on the agenesis of third molars, the worldwide rate for at least one congenitally missing third molar is 22.6% of individuals and for all four third molars 3.4% [15]. According to another meta-analysis on teeth other than third molars, prevalence of agenesis of one or more teeth varies between 2.2% and 7.7% depending on the continent, race, and sex [16]. Of single teeth, the mandibular second premolar is the most frequent absent tooth at 2.9–3.2%, followed by upper lateral incisor at 1.6–1.8% and upper second premolar at 1.4–1.6%. In individuals with tooth agenesis, one or two absent teeth is far more common (82.9%) than four missing teeth (6.0%) [16].

The aim of this study was to examine which tooth is most frequently missing from panoramic radiographs when evaluating the entire dentition of a nationally representative adult population. From the analysis of missing teeth, it is possible to conclude the most frequently extracted tooth. Based on earlier prevalence studies, the hypothesis was that the third molar is the most frequently extracted tooth.

Materials and methods

Study design

This study was part of the Health 2000 Survey (BRIF8901, Bioresource Research Impact Factor), which was a cross-sectional study organized in 2000 − 2001 by the Finnish Institute for Health and Welfare (THL) [17, 18]. The main sample of the nationally representative survey consisted of 8028 adults aged 30 years and older. The present analysis was based on panoramic radiographs, which were obtained after the clinical oral examination.

The research protocol included a two-stage stratified cluster sampling strategy, calibration of radiologists, diagnostic quality control in the examination of radiographs, and missing data analysis [12, 18, 19]. The sampling design covered all five university hospital regions as stratums and 80 health center districts as clusters from which the target persons were selected using systematic random sampling [17, 18]. The radiologists were trained beforehand by examining 50 radiographs, whereby diagnostic criteria were specified [12]. When examining the radiographs, the same radiologist re-examined an earlier image taken a day before or earlier, for every 30th radiograph [12]. The participants in the clinical oral examination did not differ by sex but were younger than the nonparticipants [19].

From the starting sample, 6335 individuals participated in the clinical oral examination, and digital panoramic radiographs were obtained from 6115 participants. All radiographs were evaluated by specialists in oral radiology. Due to insufficient quality of radiographs, 14 participants were excluded. Data on 19 participants were not available for the present examination due to withdrawal from the study, and thus, the final sample comprised 6082 participants.

Study variables

Data on participants’ age and sex were retrieved from the Population Register of Finland. The location and condition of each tooth were recorded from the radiographs as follows: missing, impacted, root remains, retained root in bone, dental implant, carious tooth, restored or sound tooth. Number of each participant’s missing teeth, including implants, was calculated from the data.

The primary outcome was the frequency of missing teeth by tooth type. Explanatory variables were age, sex, and the jaw (maxilla or mandible).

Statistical analysis

Differences in frequencies between categorical variables were tested with χ2 test. Analyses were performed with SPSS Statistics, version 29 (IBM Corporation, Armonk, NY, USA). Age of participants was grouped as 30‒39, 40‒49, 50‒59, 60‒69, 70‒79, 80‒89, and ≥ 90 years. Teeth were categorized by tooth types as central incisors, lateral incisors, canines, first premolars, second premolars, first molars, second molars, and third molars. Numbers of missing teeth per participant were calculated, and comparisons were made between the upper and lower jaw. Differences between sexes were assessed. Rates of missing teeth by tooth types were analyzed according to age groups separately for the maxilla and the mandible.

A binomial logistic regression was performed on each tooth type in the maxilla and in the mandible to ascertain the associations of age and sex of participants with the likelihood of missing teeth. Odds ratios (ORs) and their 95% confidence intervals (CIs) were reported. Finally, an estimation of truly missing teeth was made based on percentages of congenital agenesis of teeth according to information from the literature [15, 16].

Ethical considerations

This study followed the Declaration of Helsinki on medical research protocols and ethics. All participants signed a written informed consent before the clinical and radiographic examination. For clinical examinations, ethical approval was received from two committees: the Ethics Committee of the National Public Health Institute (KTL1999_6167) and the Ethics Committee of Epidemiology and National Health in the Hospital District of Helsinki and Uusimaa (TET3/9/310,500). For radiography, a safety license was granted by the Radiation and Nuclear Safety Authority of Finland (4969/L1/00). Permission for the present investigation was obtained from the THL Biobank (THLBB2023_13).

Results

Of the 6082 included participants with panoramic radiographs, 46% were men and 54% women (Table 1). The mean age of participants was 53 years (SD 14.6; median 51; range 30‒97 years). Full dentition with 32 teeth occurred in 41% (n = 2469) of participants, and 12% (n = 739) were fully edentulous.

Table 1 Description of the 6082 participants classified according to age and sex
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In the analysis of all participants and all tooth types, the third molar was the most frequently missing and the canine the least frequently (Table 2). As regards all four teeth missing, significant differences at P < 0.001 were observed in frequencies between the third molar and each of the other tooth types.

Table 2 Distribution of missing teeth by tooth type in 6082 persons
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Teeth were missing more often in the maxilla than in the mandible in all tooth types, except for the first and second molars, where the mandible prevailed (P < 0.001 for all tooth types). In both the maxilla and the mandible, the third molar showed the highest rates of missing teeth, 79% and 71%, respectively.

Each of the eight tooth types were missing more often in women than in men (P < 0.001) (Table 3). In both sexes, the third molar was the most frequently missing tooth type. The greatest proportional difference between the sexes occurred in third molars; all four were missing in 60% of women and in 44% of men (χ2 = 165.14; df = 1; P < 0.001).

Table 3 Differences between sexes in numbers of missing teeth by tooth type in 6082 persons
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Occurrence of missing teeth according to age group is presented in Fig. 1 (maxilla) and 2 (mandible). In both jaws, the third molar was missing most often and the canine or incisors least frequently. A significant difference at P < 0.01 was found in all comparisons between the frequencies of the third molar and each of the other tooth types, except in the two oldest age groups.

Fig. 1
figure 1

Percentage curves of missing teeth by tooth type in the maxilla according to age group (N = 6082 persons). Percentage values are shown only for the third molar and canine (highest and lowest curves). Asterisk (*) shows comparisons where a statistically significant difference in missing teeth was not found between the third molar and all other tooth types

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Fig. 2
figure 2

Percentage curves of missing teeth by tooth type in the mandible according to age group (N = 6082 persons). Percentage values are shown for the third molar and canine. Two asterisks (*) show comparisons where statistically significant differences in missing teeth were not found between the third molar and some other tooth types

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In the logistic regression analysis, a significant covariate (P < 0.001) for all tooth types to explain the absence of teeth was age; the higher the age the more likely a tooth was missing. Regarding sex, both in the maxilla and the mandible, third molars were twice as likely to be missing in women than in men (OR for the maxilla 2.0; 95% CI 1.75‒2.40 and the mandible 1.8; 95% CI 1.59‒2.09). The prevailing sex was reversed for the upper first molar, as it was 1.3 times (95% CI 1.18‒1.52) more likely missing in men than in women. The logistic regression models for each tooth type were significant at P < 0.001 (Omnibus tests; df = 2), and correctly classified 73‒87% of cases.

Considering the rates of congenital agenesis of individual teeth, the third molar remained the most frequently missing tooth (Table 4). The second most commonly missing tooth type was the first molar in the mandible. The sequence of other teeth is present in Table 4.

Table 4 Comparison between present results on rates of missing teeth and congenital agenesis of individual teeth derived from meta-analyses in the literature [15, 16]. Final column shows the rank of the most commonly missing teeth
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Discussion

The purpose of the study was to establish the authenticity of the often-mentioned statement that the third molar is the most frequently extracted tooth. The findings confirmed the hypothesis; among all tooth types, the third molar was the most frequently extracted tooth. However, this preponderance began to taper off after the age of 80 years. In consideration of the rates of congenital agenesis of teeth, the third molar remained the most extracted tooth.

Based on this large nationwide population data with panoramic radiographs, the most important finding was that the third molar was the most extracted tooth. This finding has not been shown previously in a large population-based sample. The sample of the Health 2000 Survey was collected using a two-stage stratified cluster sampling strategy to properly represent the main demographic distribution of the Finnish population [12, 17,18,19]. Also, the rate of participation in the survey was very high; of the initial sample of 8028 adults, 79% participated in the clinical oral examination and 76% in radiography [12, 17,18,19]. Thus, the sample was suitable for the purpose of the study.

Another interesting finding was that there was an age limit for the third molar being the most frequently extracted tooth type; in the maxilla, it was 90 years for all tooth types, and in the mandible, the limit was 80 years for the first molar and 90 years for the other three posterior teeth. No age limit emerged for anterior teeth in the mandible. The age limit is a new finding. This limit is explained by the progressive loss of other teeth catching up with the level of the third molar. Tooth extractions in a general population typically accumulate in young adults and in persons older than 75 years [20]; especially the third molar prevails in young adults [2, 21, 22]. However, the present cross-sectional analysis showed that the preponderance of third molars among all missing teeth occurred until the age of 80 years.

In the bivariate analysis, each of the eight tooth types was missing more often in women than in men. Particularly the third molar was more than twice as likely to be missing in women than in men. This preponderance of women with missing third molars has also been shown earlier [14, 22]. Women are ahead of men regarding congenital agenesis of teeth, as well [15, 16]. According to the referred meta-analyses, congenital agenesis of teeth other than third molars is 1.37 times higher in women than in men [16], and at least one missing third molar is 14% more likely to occur in women than in men [15]. This finding may also suggest that women visit the dentist more regularly, and thus, more often undergo extractions.

A comparison of the present findings with those in the literature is difficult, as large population studies with panoramic radiographs are relatively rare. However, a US study with a large number of insured patients, states that third molars are the most extracted permanent teeth [21]. In more restricted samples, such as dental specialties, the third molar may not be the most frequently extracted tooth, for example, in orthodontics, it is not uncommon to extract premolars [23]. In general, studies on presence of teeth do not even include third molars, nevertheless, they show that teeth are lost along with increasing age and posterior teeth more frequently than anterior teeth [8,9,10,11, 24]. Similar illustrations of tooth loss by age group as in the present study are also reported from Greece for some teeth, but not for third molars [25].

The data used for the present study were cross-sectional. The oldest participants were born in the 1900s and the youngest in the 1960s. Appreciation of dentition, dental treatment strategies, and willingness to extract teeth have changed over these decades [8,9,10]. This is also evident in Figs. 1 and 2, where the youngest cohorts retained more teeth than older ones. An updated study on the most frequently extracted tooth is justified with current material to examine whether the preponderance of the third molar continues to hold up. It is anticipated that, along with improved dental health, extraction of teeth other than third molars will diminish.

The main limitation of this study was that the data were collected more than 20 years ago. Even so, the present analysis was carried out on this unique data with panoramic radiographs, as so far, no population-level studies on the most frequently extracted tooth have been published. Another limitation was the small number of remaining teeth in participants older than 90 years, which rendered most of the χ2 tests in that age group insignificant. Moreover, the data did not include information on congenital agenesis of individual teeth. However, the analysis took into account data on agenesis published in two meta-analyses [15, 16]. As the rates of agenesis are so low (0‒3.2%, third molars 22%), and the rates of missing teeth in the present study so high (16‒50%, third molars 53‒92%), the utilization of information from meta-analyses was acceptable. In addition, both meta-analyses on agenesis of teeth included a study also from Finland [26]. In that local study, the agenesis rate of third molars, 20.81%, falls within the 95% CI (18.4‒25.1) of Europeans in the first meta-analysis [15], which supports the comparability of values.

The main finding of the study is expected to be of use to clinicians and policy makers, but also to researchers writing forewords to their articles on third molars. The third molar, as the most common target for extraction, but also the most common tooth associated with malpractice claims [27], calls for skills, adequate equipment, and other resources for a successful extraction. Findings of the present study are applicable to populations in which impacted third molars cause inconvenience to individuals and are often extracted ahead of other tooth types.

In conclusion, based on this nationally representative sample of adults with panoramic radiographs, the most frequently extracted tooth was the third molar. Even considering the congenital agenesis of individual teeth, the third molar remained the most extracted tooth up to the age of 80 years.