It’s unfortunately to find so many M2s were extracted in clinic. The authors analyzed 908 M2s extracted from 800 patients to reveal who’s M2s were removed, why were M2s removed, what degree were M2 loss related with their neighboring M3s and how were the M3s handled. The results showed that patients with the presence of M3s, no matter with I-M3s or E-M3s, may loss their neighboring M2s earlier than those with the absence of M3s; the prevalence of M2 loss due to caries and/or periodontal diseases in the distal surface (which were closely related to their neighboring M3s) was much higher in quadrants with M3s than that of those without M3s; nearly 2/3 I-M3s and 1/3 E-M3s were also removed during the neighboring M2 extraction surgeries.
Tooth loss can not only affects the daily quality of life of patients, but is also associated with a variety of systemic diseases, even with patients’ longevity [1, 2]. Therefore, clarifying the patterns of tooth loss may help dentists protect teeth more efficiently. Caries and periodontal diseases are the two major causes of tooth loss reported in previous studies [21, 22], and they were consistent with the main reasons for M2 loss in this study (Fig. 1). Meanwhile, what makes the M2 different from other teeth is the presence of neighboring M3s, which is the most frequently impacted tooth . M3 related diseases includes pericoronitis, caries, cysts, tumors, as well as the destruction of adjacent M2s, etc. [24, 25, 26]. The influence of M3s on adjacent M2s is widely concerned. Studies have confirmed that the presence of M3s is a risk factor for adjacent M2s [8, 9, 10]. Thus, it’s not rigorous to separate M2s with neighboring M3s when tracing the pattern of M2 loss.
In this study, the main reason for M2 extraction was caries and its sequelae and the next was the periodontal diseases, however, the distribution of the indications were different in different groups. In the I-M3 group, approximately 1/2 M2s were extracted for caries and its sequelae, and 1/3 M2s were removed due to periodontal diseases (Table 3). However, when M3s were erupted, the percentage of periodontal diseases were much higher and it almost caught up with the percentage of caries and its sequelae. In recent years, several studies have explored the negative effect of erupted M3s and found that the risk of periodontal destruction of M2s may increase 1.44–6.79 folds when erupted M3 present [8, 9, 20, 27]. In this study, the high percentage of periodontal diseases in the E-M3 group also remained clinicians to focus not only on the caries but also on the periodontal diseases.
There are too many factors affect a M2’s life span [6, 19], that’s why to date there is no direct evidence revealing the relationship of M2 loss with their neighboring M3s. It’s generally believed that the negative effect of M3s mainly occurred in the distal M2s [9, 13, 20], thus the prevalence of M2 loss due to diseases in the distal surface may reflect the influence of M3s on the adjacent M2s. On the basis of this assumption, we compared the incidence of distal diseases in extracted M2s adjacent to different status of M3s. In this study, when M2s were divided into different groups according to the status of neighboring M3s, the findings were interesting. Results (Table 3) showed that the prevalence of M2 loss due to diseases in the distal surface was higher in M2s with presence of adjacent M3s (14.4%) than those without M3s (1.8%), especially when M3s were impacted (44.1%). Although the rate of M2 extraction due to distal diseases (3.5%) was lower than that of the I-M3 group, the passive effect of erupted M3s on their neighboring M2s still needs attention. In a 25-year corhort study, scientists reported that during the follow-up, 14.6–39.1% M2s adjacent to impacted M3s and 3.8% M2s neighbored with erupted M3s were removed, while none of the M2s with the absence of M3s were extracted . Combined the two studies, the authors concluded that the presence of M3s may lead to the loss of M2s. Besides, in this study, the diseases in 4/5 M2s were too severe to confirm where they mainly occurred (Table 3), which means diseases in some of these M2s may originated from the distal surface. What’s more, most of caries or periodontal diseases that occurred only in distal M2s can be treated, thus our study may underestimated the negative effect of M3s.
Although the presence of M3s is generally believed to increase the residual periodontal pockets and caries risks of M2s [24, 26, 28], the decision on preventive extraction of asymptomatic M3s is hard for dentists and patients [17, 29]. Even in most studies, the preventive extraction of M3s can be benefit for the health of neighboring M2s [11, 13, 14], in many clinical situations, asymptomatic M3s are indeed more likely retained until irreversible damages happen [15, 16]. Such evidence were also found in this study (Table 4). Nearly 2/3 I-M3s and 1/3 E-M3s were removed during the neighboring M2 extraction surgeries, and among them, 81.5% I-M3s and only 18.8% E-M3s were prophylactically extracted even when their M2s were heavily damaged (Fig. 2). For patients with low compliance and high risks, early intervention like prophylactical extraction of M3s may be more beneficial for the health of adjacent M2s than reserved until severe diseases occur. It's time for a change.
Preventive extraction of M3s may be the most thoroughly way to protect M2s. However, M3 extraction is neither risk-free nor cost-free, especially for some of E-M3s whom still have occlusal function in mouth. Therefore, the prophylactic removal of all asymptomatic M3s is unreasonable, and it is important to screen out high-risk patients and M2s. Studies have found that men, old age and other factors can increase the risk of M2 destruction [10, 27, 30]. Although the characteristics of the population included in this study could not verify the risk factors of M2s, the mean age of patients were old, the percentage of males were higher than that of females (Table 1).
Many studies have confirmed that age is significantly associated with tooth loss [3, 4]. In our study, patients’ mean age was 54.1 years and it was similar to that of the studies of all types of permanent tooth loss [5, 31]. Compared with M2s without adjacent M3s, the mean age of M2 extraction was younger when M3 present (Table 2). Especially in M2s with neighboring impacted M3s, the average age of M2 removal had been advanced for a whole decade. The authors speculate that the presence of M3s had accelerated the progression of diseases in neighboring M2s and this hypothesis can be explained by other studies. Compared with M2s without neighboring M3s, the retained impacted M3s have been confirmed to increase the risk of M2 pathology by 2.16–4.88 folds .
As far as the authors know, this was the first time to investigate the relationship of M2 extraction and status of neighboring M3s, and provided a specific view on the effect of M3s. Due to the large number of outpatient patients, it was difficult to examine every patient who met the criteria at the same time, meanwhile, the electronic record system is relatively complete and follows the unified standards, so we conducted the retrospective cross-sectional study. Besides, imaging data were collected to ensure the accuracy of data. It was concluded that the presence of M3s may advanced the age of M2 extraction. Long-term retention of impacted M3s and erupted M3s may cause great harm to adjacent teeth.
There were also some limitations. It was hard to judge whether the missing M3s were congenital absent ones or those M3s had been removed before this investigation. If so, the influence of M3s would be underestimated. Second, when an M2 suffers severe periodontal disease, it’s easy to fall off itself, so the actual percentage of M2 loss due to periodontal diseases may be higher than that in this study.
Previous studies found that the presence of M3s can increase risk of pathology in adjacent M2s [32, 33]. In this study, the mean age and reasons for M2 extraction were different in the three groups, only indicating that M3s may be related with M2 loss, and we still don’t know the scale and degree of that influence. The results reminds clinicians to pay more attention to the negative influence of M3s, no matter the impacted M3s or erupted M3s, and to conduct timely clinical interventions before irreversible damages emerge in neighboring M2s.