Yang Yang,Yi Tian,Li-Juan Sun,Hong-Lei Qu,Zhi-Bang Li,Bei-Min Tian,Fa-Ming Chen
The impact of Anatomic Features of Asymptomatic Third Molars on the Pathologies of Adjacent Second Molars: A Cross-sectional Analysis
Anatomic characteristic,Pathology,Second molar,Third molar
Background We aimed to comprehensively examine how the anatomic characteristics of asymptomatic third molars (As-M3s) affect distal pathologies of adjacent second molars (Ad-M2s). Materials and methods Patients with at least 1 quadrant having intact As-M3s and first and second molars were enrolled. Distal pathologies of Ad-M2s, including caries, pocket depth of 4 mm or more (PD4+), and alveolar bone loss of 3 mm or more (ABL3+), were analysed based on As-M3 status (absent/impacted/nonimpacted). Especially within nonimpacted M3s (N-M3s), the effects of regions (maxillary vs mandibular) and dental intervals (narrow vs wide) on Ad-M2 pathologies were further compared. Results A total of 248 patients with their 805 quadrants were finally included in this cross-sectional study. The impacted and nonimpacted As-M3s elevated the risk of any distal pathology (caries, PD4+, or ABL3+) of Ad-M2s vs M3 absence with odds ratios (ORs) of 8.33 and 3.27, respectively. Within N-M3s, mandibular regions increased the odds of PD4+ (OR, 1.96); wide dental intervals increased the odds of ABL3+ (OR, 3.01). However, maxillary regions and narrow dental intervals contributed to more severe bone loss in Ad-M2s with ABL3+. Conclusions The presence of As-M3 is a risk factor for Ad-M2 pathologies irrespective of impaction status. Within N-M3s, Ad-M2 pathologies are significantly influenced by anatomic characteristics such as regions and dental intervals.
The impact of Anatomic Features of Asymptomatic Third Molars on the Pathologies of Adjacent Second Molars: A Cross-sectional Analysis

We aimed to comprehensively examine how the anatomic characteristics of asymptomatic third molars (As-M3s) affect distal pathologies of adjacent second molars (Ad-M2s).

Patients with at least 1 quadrant having intact As-M3s and first and second molars were enrolled. Distal pathologies of Ad-M2s, including caries, pocket depth of 4 mm or more (PD4+), and alveolar bone loss of 3 mm or more (ABL3+), were analysed based on As-M3 status (absent/impacted/nonimpacted). Especially within nonimpacted M3s (N-M3s), the effects of regions (maxillary vs mandibular) and dental intervals (narrow vs wide) on Ad-M2 pathologies were further compared.

A total of 248 patients with their 805 quadrants were finally included in this cross-sectional study. The impacted and nonimpacted As-M3s elevated the risk of any distal pathology (caries, PD4+, or ABL3+) of Ad-M2s vs M3 absence with odds ratios (ORs) of 8.33 and 3.27, respectively. Within N-M3s, mandibular regions increased the odds of PD4+ (OR, 1.96); wide dental intervals increased the odds of ABL3+ (OR, 3.01). However, maxillary regions and narrow dental intervals contributed to more severe bone loss in Ad-M2s with ABL3+.

The presence of As-M3 is a risk factor for Ad-M2 pathologies irrespective of impaction status. Within N-M3s, Ad-M2 pathologies are significantly influenced by anatomic characteristics such as regions and dental intervals.

The presence of third molars (M3s) has been reported to contribute to many oral diseases, especially involving pathologies on their adjacent second molars (Ad-M2s).1, 2, 3 Removal of symptomatic M3s is widely accepted, but making clinical decisions on asymptomatic M3s (As-M3s) is amongst the most controversial clinical issues.4, 5, 6 The focus on the controversy is that some As-M3s lead to poor outcomes (e.g., pericoronitis, periodontitis, caries) with aging, whilst others may remain silent for life. The crucial solution for this controversy is to distinguish high-risk As-M3s, and the impact of As-M3s on Ad-M2 health can provide a valuable reference. In recent years, the relationship between the As-M3 impaction status and Ad-M2 pathologies has attracted increasing attention. In a clinical survey of a large sample, when impacted and nonimpacted As-M3s were present, the incidence of deep periodontal pockets around Ad-M2s was 42.9% and 33.9%, respectively; the odds of periodontal pathology were elevated by 3.20-fold and 1.67-fold, respectively, vs M3 absence. Another longitudinal study over 25 years also suggested that the risk of any distal pathology (caries or/and periodontitis) of Ad-M2s was elevated with erupted, soft-impacted, and bony-impacted As-M3 presence by 1.74-fold, 4.88-fold, and 2.16-fold, respectively, compared to those with M3 absence. These studies suggest that the risk of Ad-M2 pathologies changes with the As-M3 impaction status, and the influence of nonimpacted third molars (N-M3s), which have been ignored for a long time, comes to the view of researchers.8, 9, 10 Except for the impaction status of As-M3s, other anatomic characteristics are needed to identify high-risk As-M3s. Abundant literature has explored the anatomic characteristics of impacted third molars (I-M3s), including region (maxillary or mandibular), angulation (Winters’ classification), height (P&G classification), and covered tissue (soft tissue or bony), which are influencing factors for Ad-M2 pathologies.,11, 12, 13 However, which anatomic characteristics of N-M3s contribute to Ad-M2 pathologies is still unknown. The plaque control and bone density in N-M3 regions exhibit differences between maxillary and mandibular; hence, N-M3 regions may have an impact on Ad-M2 pathologies., Additionally, the dental interval width has been identified to be related to the development of periodontal pathologies in other teeth., Therefore, we hypothesise that the regions and dental intervals of N-M3s would be important influencing factors for Ad-M2 pathologies. Our primary objective in this study was to assess the occurrence of distal pathologies of Ad-M2s associated with As-M3s using clinical and radiographic examinations. Second, we aimed to compare the incidence of distal pathologies of Ad-M2s between different anatomic characteristics of As-M3s, including the impaction status of As-M3s and the regions and dental intervals of N-M3s.

Data were consecutively collected from patients seeking routine dental care from October 2020 to September 2021, at the School of Stomatology, Fourth Military Medical University. All patients aged 18 years or older with at least 1 quadrant that had visible As-M3s with intact first and second molars were eligible for inclusion. Patients with the following conditions were excluded: (a) systemic diseases such as hypertension, diabetes, and cancer; (b) pregnancy; (c) orthodontic experience; (d) periodontal therapy within 6 months; (e) anti-inflammatory drug use within 3 months; or (f) refusal to provide sociodemographic information or take orthopantomograms (OPGs). The sociodemographic characteristics of all included patients were collected with a structured questionnaire, which included age, sex, education level (senior high school or below; college graduate or above) and smoking status. This cross-sectional study was conducted in accordance with the Helsinki Declaration of Human Studies, and the research protocol was ethically approved by the Ethics Committee and Institutional Review Board, School of Stomatology, Fourth Military Medical University (IRB-REV-2017034). Informed consent was obtained from all participants included in the study to participate and use their data and radiographic images for publication.

All quadrants in included patients were further screened according to OPGs, and only the quadrants having intact first and second molars, either with visible As-M3s or without M3s, were finally included. As-M3 status was categorised as absent M3s, I-M3s, and N-M3s. Absent M3s included congenitally missing and later removed M3s. I-M3s were defined as M3s that failed to erupt to the occlusal plane, and N-M3s were defined as M3s that erupted to the occlusal plane, with a 0 to 10 angle between the long axis of M2s and M3s. The sample size was estimated by the z statistic with calculations based on the study of Qu et al. The results showed that at least 640 quadrants with As-M3s were sufficient to estimate the prevalence of pathologies on A-M2s (parameters: significance level α = 0.05, δ = 10%, expected prevalence = 38.5%).

All included quadrants were subjected to clinical and radiographic examinations (Orthopantomograph, OC100D-1-4-1, Instrumentarium, Tuusula, Finland). Periodontal data were recorded with a manual periodontal probe (CP15UNC; Hu-Friedy), and the scale nearest to the gingival margin was recorded as the probing depth (PD). Two M2 sites (distobuccal and distolingual) were measured, and a mean PD greater than 4 mm was recorded as PD4+. Distal caries of M2s were confirmed by the combination of clinical examinations and radiographic assessments. Any low-density shadow in the distal surface of M2s was diagnosed as caries, and filled distal caries were also identified as caries. Distal alveolar bone loss (ABL) of M2s was deemed present if the distances from the alveolar bone to the cementoenamel junction (CEJ) were 3 mm or more (ABL3+) in OPG examinations. In addition, the dental intervals of N-M3s and overall periodontal status were further identified by OPG examinations (Supplemental Figure 1). The horizontal distance at the proximal CEJ between N-M3s and their Ad-M2s is the dental interval. A horizontal width of 2 mm or greater was defined as the wide group and that less than 2 mm was defined as the narrow group (Supplemental Figure 1a). The overall periodontal status was evaluated by measuring the remaining bone proportion in root length (Supplemental Figure 1b). When the mean value of all teeth (mesial and distal) except M3s was less than 80%, the patients were diagnosed with periodontitis. All clinical and radiographic examinations were conducted by one experienced periodontist (Y.Y.). The intraexaminer reproducibility of OPG analysis was 92%.

Data were imported into SPSS software (version 22) and analysed. Distal pathologies of Ad-M2s included caries, PD4+, and ABL3+. The relationship of anatomic characteristics of As-M3s and distal pathologies of Ad-M2s was analysed at quadrant level, which means the percentage of diseased Ad-M2s was calculated using the number of quadrants with different As-M3 status as denominators. The differences in distal caries, PD4+, ABL3+, and any one of the 3 pathologies of Ad-M2s amongst different As-M3 statuses were compared. In particular, the N-M3s were subdivided according to regions (maxillary or mandibular) and dental intervals (narrow or wide), and their effects on distal pathologies of Ad-M2s were further compared. All analyses were conducted using generalised estimating equation logistic regression models to correct correlation data at the patient level, and other confounding factors, including sex, age, education level, smoking status, and periodontal status, were adjusted. All bivariate associations were explained using the odds ratio (OR) and 95% confidence interval (95% CI) throughout the statistical analyses. All tests were 2-sided with a significance level of .05.

A total of 296 patients met the inclusion criteria and were invited to enroll; 43 of them were filtered by exclusion criteria. The main reason for exclusion was systemic diseases (65.1%), followed by periodontal treatment history. Additionally, OPGs from 5 patients were excluded for poor quality. Finally, 248 patients were included in this study, and their 992 quadrants were further screened and classified according to the anatomic characteristics of As-M3s. Based on the bias of the screening criteria, 187 quadrants were excluded for having symptomatic or completely impacted M3s and broken or lost first/second molars. In those enrolled 805 quadrants, the number of quadrants with N-M3s (n = 518) was greater than that with I-M3s (n = 137). Within N-M3s, slightly more than half of them were in the maxillary (n = 276), but when referring to dental intervals, close to three-quarters of them had narrow dental intervals (n = 383; Supplemental Figure 2). The demographic characteristics of the included patients (40.9 ± 12.3 years) and the distribution of enrolled quadrants (1–4) at patient level are presented in Table 1.

Table 2 reveals the influence of As-M3 status on the incidence of each distal pathology of Ad-M2s. The results suggested that I-M3 presence significantly increased the odds of distal caries, PD4+, and ABL3+ of Ad-M2s vs M3 absence by 2.61-fold, 9.75-fold, and 8.75-fold, respectively. Different from I-M3s, there was no significant difference between quadrants with or without N-M3s regarding the incidence of distal caries of Ad-M2s, but the odds of distal PD4+ and ABL3+ of Ad-M2s was significantly elevated with N-M3 presence by 3.39-fold and 2.95-fold, respectively. The incidence of any one of the 3 distal pathologies of Ad-M2s amongst different As-M3 impaction statuses was further analysed, as shown in Table 3. In all variables, overall periodontitis displayed the most prominent impact on Ad-M2 pathologies, with an increased OR of 10.70, and As-M3 status was the second contributing factor. Compared with M3 absence, both the presence of I-M3s and N-M3s significantly elevated the odds of Ad-M2 pathologies (OR, 3.27 and 8.33 for N-M3s and I-M3s, respectively). Additionally, age older than 35 years also contributed to a greater risk of Ad-M2 pathologies, with an OR of 1.59.

Table 4 indicates the difference in each distal pathology of Ad-M2s between different N-M3 regions (maxillary vs mandible) and dental intervals (narrow vs wide). When N-M3s were located in the mandible, their Ad-M2s were at significantly higher risk for distal PD4+ (OR, 1.96). Although the risks of distal caries and ABL3+ of Ad-M2s were reduced in the mandible, the difference was not statistically significant. When N-M3s had wide dental intervals, the risks of all 3 distal pathologies of Ad-M2s were elevated, but only the difference in the occurrence of distal ABL3+ was significant (OR, 3.01). Representative radiographic images for this ABL3+ trend are shown in Supplemental Figure 3. However, we found a contrary trend in the impact of N-M3 regions and dental intervals on the severity of Ad-M2 distal bone loss. Further analyses within N-M3s were conducted using the proportion of alveolar bone loss of 6 mm or more (ABL6+) in Ad-M2s with ABL3+ (Table 5). The data were analysed using the chi-square test for limited data. Intriguingly, the results identified that when compared with mandibular regions, maxillary regions significantly increased the ABL6+ proportion in Ad-M2s with ABL3+ (OR, 1.84). Meanwhile, the ABL6+ proportion was significantly elevated with narrow dental intervals vs that with wide dental intervals (OR, 3.86). Representative radiographic images for this ABL6+ trend are shown in Supplemental Figure 4.

There has been no conclusion about the prophylactic removal of As-M3s until now due to a lack of sufficient and reliable evidence. Retaining asymptomatic and disease-free M3s and monitoring with active surveillance are more acceptable methods for both patients and clinicians.,, However, abundant literature has identified that even As-M3s can induce Ad-M2 pathologies, and M2 pathologies associated with M3s may lead to increased expenses and poor prognoses for patients.22, 23, 24 This study aimed to elucidate the role of anatomic characteristics of As-M3s in Ad-M2 pathologies with a comprehensive approach. We found that the impaction status of As-M3s and the regions and dental intervals of N-M3s were influencing factors for distal caries and periodontitis of Ad-M2s. Irrespective of impaction status, the presence of As-M3s increased the odds of distal pathologies (caries and/or periodontitis) of their Ad-M2s, even after controlling for periodontal status, age, and other confounding factors (Table 3). Periodontal status was measured by the OPG method, which has been verified as a reliable tool for periodontitis screening under the 2018 periodontitis case definition. This finding is also in accord with the observations of Nunn et al and Li et al, who found that the presence of N-M3s threatened Ad-M2 health., Moreover, we noticed that N-M3s accounted for three-quarters of As-M3s. Although the odds of Ad-M2 pathologies related to I-M3s were higher than those related to N-M3s, the total number of diseased M2s associated with N-M3s was greater than that associated with I-M3s. This means that M2 pathologies associated with N-M3s might pose a heavy burden on health care resources in the general population with As-M3s. To distinguish high-risk N-M3s more quickly and easily in clinical practice, understanding the relationship of N-M3 anatomic characteristics and Ad-M2 pathologies is crucial. Therefore, we conducted the following analysis on N-M3s. Within the N-M3 regions, we found that N-M3s in the mandible were more than twice as likely to contribute to distal PD4+ of Ad-M2s than those in the maxilla (Table 4). We hypothesise that plaque elimination is more difficult in mandibular N-M3 regions, which may partly explain the difference in distal PD4+ of Ad-M2s between N-M3 regions. However, once alveolar bone loss began, M2s with N-M3s in the maxilla were prone to have more severe bone loss than those in the mandible (Table 5). The possible reason for this trend is that the bone density at the molar area is lower in the maxilla than in the mandible. Within the dental intervals, N-M3s with wide dental intervals increased the odds of distal ABL3+ of Ad-M2s by 3 times vs that with narrow dental intervals (Table 4). The space below the contact area results in food impaction, which contributes to plaque accumulation and causes later periodontal pathologies. Our finding is consistent with an earlier observation, which showed that central papilla recession was more associated with a wide dental interval of more than 2 mm. Moreover, Vermylen et al revealed that when patients have bilateral root proximity, a distance between interdental spaces less than 1 mm, they had 3.6 times higher risk of experiencing bone loss greater than one-third of root length at least at one site. Although with different criteria for ABL, their result also agreed with our observations, which showed that narrow dental interval (<2 mm) N-M3s were related to a higher proportion of ABL6+ in Ad-M2s with ABL3+ (Table 5). On the one hand, narrow dental intervals could hamper oral hygiene measures and mechanical debridement delivery at the site. On the other hand, alveola bone is thinner in narrow dental intervals than in wide dental intervals. Therefore, narrow dental intervals contributed to more severe bone loss in the disturbed situation. In this study, we compared the difference in Ad-M2 pathologies between different anatomic characteristics of As-M3s, especially involving the regions and dental intervals of N-M3s, which have not been explored. Our results will help clinicians make As-M3 clinical decisions for individuals based on the anatomic characteristics of As-M3s. In addition to As-M3 status, we explored the relationship of other factors (eg, overall periodontal status) and Ad-M2 pathologies using generalised estimating equation logistic regression models. However, our study also has limitations. The reference group included both congenitally missing M3s and later removed M3s. Such conditions mean that our analyses led to an underestimated risk of Ad-M2 pathologies associated with As-M3 presence. Moreover, we used OPGs instead of periapical radiographs to measure the dental-interval width. Periapical radiographs were difficult to conduct in M3 regions due to limited space. Cone-beam computed tomography could be used to conduct more precise measurements. Although the measurement of OPGs leads to bias, we hope this study could draw more attention to how N-M3s affect their Ad-M2 pathologies. Our study provides evidence for more active intervention for As-M3s. When As-M3s are impacted, prophylactic removal is recommended to decrease the risk of Ad-M2 pathologies under appropriate conditions. When the As-M3s are nonimpacted, the situation would be different by N-M3 regions and dental intervals. If the N-M3s were in the mandible or had wide dental intervals, mechanical cleaning methods, including toothbrushing and dental floss use, should be emphasised for patients to prevent periodontal pathologies in the N-M3 regions. However, if M2s have periodontal pathologies associated with maxillary or narrow dental interval N-M3s, clinicians should understand that the periodontal pathologies would develop rapidly and eventually lead to M2 loss. Early therapeutic removal may be a better clinical choice. Prevention and early treatment are crucial, and we hope our study provides new perspectives regarding As-M3 decisions for clinicians. In the near future, we plan to further explore more clinical parameters associated with As-M3 pathologies (eg, root resorption, odontogenic cyst, occlusal trauma) and the impact of other As-M3 anatomic characteristics (eg, contact area) on Ad-M2 pathologies in subsequent studies.

Within As-M3s, both I-M3s and N-M3s contributed to higher odds of distal pathologies of their Ad-M2s compared with M3 absence, even after controlling for periodontal status and other factors. Although the odds associated with N-M3s were lower than those associated with I-M3s, the number of diseased A-M2s was greater with N-M3s than with I-M3s. Within N-M3s, regions and dental intervals significantly influenced the occurrence and development of distal periodontitis of Ad-M2s. There is a close relationship between the anatomic characteristics of As-M3s and distal pathologies of Ad-M2s, and clinical decisions on As-M3s should be made based on the relationship.

Yang Yang and Yi Tian contributed equally to this work.

None disclosed.