In oral surgeries, whether prophylactic extractions of asymmetric M3Ms were indicated had become a hot issue. Oral therapies should be carefully considered, especially during COVID–19 epidemic period, due to the aerosol pathway was found as a potential route of transmission. To solve this problem, the first question we need to answer is: what are the characteristics of lower wisdom teeth that have higher risk to cause pathologies? The pathologies caused by lower wisdom teeth could be categorized into soft tissue disease and hard tissue disease. Soft tissue disease mainly refers to pericoronitis, and hard tissue disease mainly refers to M2M distal caries . Distal periodontal pathology of M2M has both soft and hard tissue defects, which further leads to the mobility of M2M increased and distal root absorption. Therefore, pericoronitis, M2Ms caries and distal M2M periodontal disease were evaluated as the pathologies in this study. The “soft tissue” or ‘bony” impaction was the briefest tooth classification. In this study, 91.67% of the teeth were soft tissue impacted. It was in accordance with the understanding that the loose gap between tooth and soft tissue would be an easy approach to accumulate the bacterial biofilms1,13. Based on Pell & Gregory’s classification, 72.22%, 20.83% and 6.94% of the teeth were in position A, B and C. It suggested teeth in less deep positions were more likely to cause pathologies, which was in accordance with the other studies [13,14,15]. The explanation of it might be due to that teeth in high positions would be, closer to oral environment. Based on the winter’s classification, more than half of the M3Ms were vertically angulated. It might relate to the high rate prevalence of pericoronitis in this study, and the third molar in vertical position was the one with the greatest chance of presenting pericoronitis as literatures reported . The high rate prevalence of pericoronitis might interfere the evaluation of the association of M3Ms with the other pathologies, thus we could not conclude the impaction features of M3Ms which were most susceptible to all pathological changes.
In different age groups, the types of pathologies varied. In this study, the age of the patients ranged from 18 to 63. By evaluating the association of the age groups and the pathologies, we found: (1) pericoronitis was the major pathology in all age groups; (2) with age increased, the propensities of M2M distal caries and periodontal pathology increased. Similarly, there was a study showed that symptomatic M3Ms in patients over 50 were more related to M2M distal caries . M2M distal caries and periodontal disease appeared in older patients indicated that these two pathologies occurred after a long period of time when M3Ms in inadequate positions and angulations. Therefore, the risks of M2M distal caries and periodontal disease could be less concerned, especially in the COVID–19 epidemic.
The patients in different age groups had different concerns of pathologies, therefore, it was justified to compare M3Ms features with one specific disease. Pericoronitis is an inflammation that occurs in the soft tissues around an erupting tooth [13,15]. In this study, soft tissue impacted and vertically angulated teeth were more associated with the occurrence of pericoronitis, due to the occlusal M3M faces which had grooves and fissures in contact with the pericoronary hoods. It was in accordance with the published research . We also found around 70% of the mesio-angular and horizontal impacted teeth caused pericoronitis. The most convincing explanation is that, cementoenamel junction distance increases of M2M and M3M, which accumulates food in the interdental space. In addition, it is difficulty to brush and floss due to the lack of ideal interproximal contact point, which enhances the food and bacterial accumulations. In this study, there was no difference among the teeth in position A, B and C which had pericoronitis. In literature, there was limited evidence that pericoronitis was related to teeth position based on the Pell & Gregory classification, whereas teeth in position A had a relative greater risk than in position B . It might be interpreted due to the small number of samples. And similar to our study, the data in the literature were collected in the hospital, and the patients included always had symptomatic M3Ms, which might be another explanation.
Impaction depth and angulation of the M3M were associated with distal caries in the M2M . In this study, mesio-angular impacted teeth in position A had greater risk of M2Ms distal caries, which was in accordance with the other studies . A previous research showed that, M2Ms developed more distal caries with mesial angulations from 43° to 73° . And for the CEJ distance between the distal M2M and the mesial M3M ranged from 6 to 15 mm, distal caries in M2Ms occurred more frequently . This was also related to the inappropriate contact point between the two teeth which increased the difficulty of daily oral hygiene.
Periodontal pathology of the M2M is related to the impacted depth and inclination of M3M. In this study, we found deeper impacted teeth with mesial and horizontal angulations were more likely to cause adjacent periodontal pathologies. Progression of periodontal probing depth was reported widely in the literature [20,21,22,23], but only a few researches considered the impaction patterns. There was a study suggested that the prevalence and the incidence of periodontal pathology of M2Ms varied significantly depending on whether M3Ms were absent, erupted, soft tissue impacted, or bony impacted1. And second molars adjacent to soft tissue impacted third molars had significantly greater chance than that for second molars adjacent to any other category of third molars . However, the inclinations of the M3Ms were not discussed. The periodontal pathologies might lead to external root resorptions (ERRs) of the second molars [24,25]. The previous studies showed ERRs of the second molars were associated with mesio-angular and horizontally impacted M3Ms [26,27,28,29]. In our study, we found the adjacent periodontal disease also related to this type of M3Ms, suggesting the potential relationship between periodontal pathology and ERR.
Uncurable root caries and severe looseness of M2Ms were the worst outcomes in this study. Among them, most of them were soft tissue impacted in position A&B and were mesially and horizontally angulated. The average age in those patients was 44 years old. The severe outcomes suggested the necessity of the regular oral examinations. Though risks of M2M root caries and periodontal disease might be less concerned in the COVID–19 epidemic because they are more likely to be occurred after a long time, they should be considered when the epidemic situation is ended.
In summary, soft tissue impacted and vertically angulated M3Ms were more likely to cause pericoronitis; mesially angulated M3Ms with less deep positions (position A) were more likely to generate M2M distal caries; mesially and horizontally angulated M3Ms with relative deep positions were more often to engender M2Ms distal periodontal pathologies. Thus, extractions of soft tissue impacted teeth in vertical angulations should be considered. While removals of mesially and horizontally angulated or bony impacted teeth could be delayed due to the pathologies caused by them occurred after a relative long time. The surgical procedures are more complicated and the surgical time is usually longer for teeth in horizontal angulation and bony impaction could be another reason.