The Effect Of Non-impacted Third Molars On The Periodontal Condition Of Adjacent Second Molars

Background:Third molars（M3s）are associated with a variety of diseases,including pericoronitis,cysts,tumors,pathologies of adjacent teeth,etc.^[1-3]Due to the important role of second molars（M2s）in oral function,the effect of M3s on adjacent M2s has been widely concerned.For a long time,plenty of researches focused on the impacted M3s^[4,5].However,in recent years,an increasing number of studies have found that the non-impacted third molars（N-M3s）may also be a negative factor for their adjacent M2s^[6-8].The harmfulness,risks and clinical treatment strategies of N-M3s need further research.Previous studies indicate that the risks of periodontal pathologies in M2s are significantly increased when N-M3 exists^[6-9].However,clinicians have not paid enough attention to the negative effects of N-M3s.In addition,there are few studies focused on N-M3s.At the same time,due to the differences in sample size,study methods and other factors,results among these studies varied widely^[7,8,10].Hence,further researches are urgently required based on the previous findings.The ultimate purpose of research is to provide evidence for the clinical decision making of M3s.In clinic,attitudes to M3s are generally divided into two groups:reservation and preventive extraction.On the one hand,studies found that diseases usually occurred before the onset of symptoms,thus asymptomatic M3s should be removed as early as possible to prevent irreversible damages^[11,12].On the other hand,some researchers believe that the removal of M3 is not riskless or cost-free,and regular inspection and maintenance may be a more prudent approach^[13].Currently,clinical decisions regarding asymptomatic M3s（especially N-M3s）are inconclusive,mainly due to the lack of strong evidence for the prognosis of M2s after long-term retention or removal of neighboring N-M3s.In order to solve the problems mentioned above,we conducted a series of studies.Firstly,the risks of periodontal diseases in M2s with neighboring N-M3s were analyzed.We further explored the periodontal prognosis of M2s after neighboring N-M3 extraction.The authors hope these findings to arouse more clinicians to pay attention to the negative influences of N-M3s,as well as to assist in the clinical decision making of N-M3s.Objective:This study aimed to investigate the effect of N-M3s on their neighboring M2s and to study effect of N-M3 removal on adjacent M2s.Methods:1.The effect of N-M3s on the periodontal health of adjacent M2sThe samples were obtained from subjects who visited the Third Affiliated Hospital of AFMU from April 2018 to October 2018.Demographic information of participants（gender,age,level of education,smoking status,etc.）were recorded.Periodontal examination were conducted in quadrants which met the criteria（M3 was absent or non-impacted）.At tooth level,periodontal indexes were:the average pocket probing depth（PD）,at least one probing site with a≥5 mm pocket（PD5+）,at least three probing sites were bleeding on probing（BOP3+）and the average plaque index（PLI）.With confounding factors（e.g.gender and age）balanced,the periodontal indexes of M2s were compared in groups with the presence or absence of adjacent N-M3s;the periodontal risks of M2s with neighboring N-M3s were also analyzed.Furthermore,subjects without M3 and with an N-M3 in the contralateral side of the same jaw were analyzed in a paired subgroup.In addition,periodontal indicators in the mesial and distal surface of M2s were compared.2.The effect of N-M3 removal on the periodontal health of adjacent M2sDemographic data of the patients and periodontal indexes of included quadrants（M3was non-impacted or had been extracted for more than 6 months prior to this study）were collected.With confounding factors balanced,the periodontal indexes of M2s were compared in groups with retained or removed N-M3s.Besides,subjects with a N-M3 and a N-M3 which had been removed in the contralateral side of the same jaw were analyzed in a paired subgroup.Furthermore,Patients with at least one N-M3 removed after baseline examination were included in this study.Periodontal examination was conducted at 3 and6 months respectively after N-M3 removal.Finally,the changes of periodontal indexes of M2s before and after N-M3 extraction were compared.Results:1.The effect of N-M3s on the periodontal health of adjacent M2sA total of 1301 quadrants were included;489 M3s were absent（controlled group）and812 M3s were non-impacted.Compared with the controlled group,when N-M3s were present,PD of adjacent M2s was 0.23mm deeper（p<0.001）,and the risks of PD5+and BOP3+were 2.52（p<0.001）and 1.65（p<0.001）folds increased.Taking PD5+as the primary periodontal outcome,other factors that increased the PD5+risk of M2s were male sex,aging and mandibular M2s.When the N-M3s present,PD in the distal surface of M2s was 0.49mm deeper than that in the mesial surface（p<0.001）,and the prevalence of PD5+and BOP+（at least one probing sites with BOP）was higher than that in the mesial surface（p<0.001,p<0.001）.However,when the N-M3s absent,there was no significant difference in all periodontal indicators between the mesial and distal surfaces of M2s.Furthermore,126 participants were enrolled into the subgroup of bilaterally matched quadrants,and the results showed that PD,PD5+and BOP3+of adjacent M2s of N-M3s was significantly higher than that of absent M3s.2.The effect of N-M3 removal on the periodontal health of adjacent M2sA total of 1035 molars were included,of which 812 molars with an N-M3 and 223quadrants without N-M3（N-M3 had been removed for more than 6 months）.Compared to M2s with the presence of N-M3s,M2s without neighboring N-M3s exhibited a 0.27-mm reduction of PD（p<0.001）,a 0.38-fold reduced risk of PD5+（p<0.001）and a 0.54-fold reduced risk of BOP3+（p=0.001）.Furthermore,71 participants were enrolled into a comparison of bilaterally matched quadrants.The results showed that the mean PD（p<0.001）,prevalence of PD5+（p<0.005）and BOP3+（p=0.008）of M2s with removed N-M3s were lower than those of retained N-M3s.Next,28 patients and 41 quadrants were enrolled in the cohort study.Results showed that 6 months after neighboring N-M3extraction,the PD（p<0.001）of M2s decreased by 0.31 mm,while their incidence of PD5+（p=0.004）decreased 21.9%when compared to the parameters detected before tooth extraction.Conclusion:N-M3s have a negative effect on the periodontal condition of adjacent M2s.Removing N-M3s was associated with an improved periodontal condition in neighboring M2s.