The present study revealed that among patients with partially erupted lower 3Ms, advanced ages, and greater angulations between the 3M and 2M were independent factors that contributed to the presence of a blurry bone aspect, horizontal bone loss, or vertical bone loss in the alveolar bone crest between 3 and 2M. In addition, older age was positively associated with radiolucency in the distal aspect of lower 3M values.
Although there is evidence that 2Ms adjacent to 3Ms may show periodontal problems, such as gingival inflammation2,4, periodontal pocket3, and alveolar bone resorption3,5, to the authors’ knowledge, this is the first paper to demonstrate an association between age and alveolar bone crest status. The radiographic examination assessing alveolar bone crest levels is a very useful adjunct diagnostic to the clinical periodontal examination12. Periodontal diseases have some radiographic signs that usually start as a blurry bone aspect (“fuzziness”) or a lamina dura discontinuity in the alveolar bone crest region12. Then, the destruction will extend to the interdental septa and end up in a height reduction of the bone crest12. In this sense, the bone crest alteration should be considered the initial process of periodontal disease.
According to the statistical results, after controlling for confounding factors, for each year increased in the individual’s age, the chance of alveolar bone crest alteration increases by 15%. Therefore, age should be considered an important predictor for bone crest alterations and, consequently, an important reason to consider prophylactic third molar removal1. On the other hand, sufficient space in the dental arch must be considered for decision-making on prophylactic 3M removal once this variable can predict the 3M full eruption15.
The decision of lower 3M prophylactic removal is still a topic of wide discussion. Dental surgeons often have difficulties in the decision-making: remove the 3M as a prophylactic approach or wait until the development of associated pathologies as a reason for the removal? Some studies elucidate reasons to indicate 3M removal, such as caries, periodontitis, and pain1,2,5,16,17. A current systematic review concluded that the evidence comparing the prophylactic removal of impacted lower 3M with its retention is very limited18. Although sufficient evidence is lacking, the authors suggest that prophylactic removal may be the most cost-effective strategy18. However, these results are related to impacted teeth and systematic reviews evaluating the effectiveness of removing partially erupted teeth were not found. Thus, the lack of reliable evidence hinders the decision-making process. In this sense, this study elucidates new findings based on periapical radiographs’ analysis of the alveolar bone crest in the 3M region. The findings of this study are an additional consideration for both surgeons and patients when deciding whether to prophylactically extract or retain 3Ms.
When performing radiologic investigation in an asymptomatic 3M region, at least three radiographic signs can determine hard tissue alterations and indicate the necessity of surgical removal of the tooth: marginal bone loss in the distal aspect of adjacent 2M; the increased radiolucent area surrounding the 3M crown; and bone resorption in the adjacent 2M8,19. Greater angulations between 3 and 2M promote greater distances between these teeth and lead to the occurrence of food impaction20. Some studies reported an increased local inflammation and bone resorption related to this impacted food21,22, which may be observed clinically9. The present study showed that for each increased degree in the 3M angulation, keeping constant the other variables, the chance of comprising the bone crest increases by 3%. Some studies have reported that most pathological changes presented in panoramic radiographs were associated with mesioangular and horizontally impacted 3Ms, considering Winter’s classification8,19, which corroborates the results of this study. Few studies in this field describe the 3M angulation as a continuous variable. However, the literature suggests the advisability of using an objective measurement method to minimize the error introduced by observer interpretation23.
Specifically, pathological changes in radiographs as pericoronal radiolucency of more than 2.0 mm in the distal aspect, is a generally accepted reason for the extraction of impacted lower third molars24. The present study showed that only the age variable was significantly associated with radiolucency between the distal aspect of the lower 3M crown and mandibular ramus in the final multivariable regression model. Thus, older patients presented wider radiolucency in the distal aspect of lower 3M crown when compared to younger ages. According to previous studies25,26, the earlier diagnosed the necessity to remove a 3M that causes periodontal alterations distal to adjacent 2M, the greater will be the chance of reducing the local inflammatory activity. A study showed that this inflammatory activity may decrease from 77 before the surgery to 23% after 3M removal in young patients25. Thus, earlier radiographic examinations as a complement to clinical exam in periodontal conditions will help guide clinical assertive recommendations, especially considering asymptomatic teeth.
Cone-beam computed tomography is the gold standard radiographic method to analyze third molars and associated structures19, however, it has a high cost. The two most used methods to evaluate the regional bony anatomy when planning a surgery is the panoramic and periapical radiographs12. However, a study compared the measurements on panoramic and periapical radiographic methods with the real measurements of the 3M region and, although no statistically significant difference was found, distortions on panoramic exams were slightly greater than on periapical27,28. Another study suggested a full-mouth survey of periapical radiographs as the gold-standard for periodontal diagnosis25. Considering the radiation exposure, a single panoramic radiograph may sufficiently represent the periodontal status of an individual. However, as the interest of the present study was to evaluate only the lower 3M region, it can be suggested that the present evidence, which is based on periapical radiographs, is sufficient to indicate the presence or absence of periodontal pathologies with considerable accuracy without exceeding radiation exposure.
In addition, considering the ALARA principle (“as low as reasonably achievable”), where it should be avoided to expose a patient even to a small dose of radiation if receiving that dose has no benefits29, it is not justifiable to use a panoramic radiography to analyze only one lower 3M. Some undesired events may occasionally occur, such as with those patients that cannot tolerate the periapical X-ray film and thus it may be necessary to retake the exam. In these cases, the three main protective measures of the ALARA principle (time, distance, and shielding) should be followed precisely to avoid unnecessary X-ray exposure29. Further, X-ray positioners are also a great alternative to avoid mistakes when taking a periapical radiography.
Although periapical radiograph is not the best imaging exam to verify pathological conditions related to the 3M region20, it is a largely used method in clinical practice due to its low cost, easiness, and necessary low amount of equipment27,28. In addition, in the institution where this study was conducted, periapical radiographs are the required preoperative exam because they can be executed in the university with no costs to the patients. Periapical radiographs were the images available to conduct this study and investigate the target conditions. In future studies, the periodontal diagnosis must consider the association between radiographic and clinical findings.
Some limitations of the present study should be highlighted. The prevalence rate of periodontal pockets in the distal aspect of 2Ms adjacent to 3Ms used to calculate the sample size, considered patients from the United States, and it may be not representative of the population included in this study. Different techniques and executors may be adopted to perform the radiographic exams resulting in possible differences between time of X-ray exposure and incidence direction. Moreover, to precisely assess possible anatomic alterations caused by 3Ms, it would be necessary a random sample of a generalized population, but there is a limitation to perform X-ray exams without consistent indication, for ethical reasons. Thus, for patients with partially erupted 3Ms, it is recommended radiographic and clinical monitoring to verify the necessity of prophylactic or resolutive removal of these teeth. Finally, this is a cross-sectional study based only on periapical radiographs. To further elucidate other conditions associated with the risk of wider radiolucency and bone crest involvement, longitudinal studies must be conducted. No temporal relationships among these variables can be determined regarding our findings. Furthermore, additional studies are required to develop predictive equations to estimate the periodontal performance when the 3M is present.
In conclusion, older patients, and greater angulations between the 3M and the adjacent 2M are more likely to develop alveolar bone crest alterations in this region. Also, age is associated with wider radiolucency in the distal aspect of lower 3Ms crown. It is suggested that these characteristics should be considered when evaluating a 3M for extraction because of future implications. The proposed null hypothesis was rejected, and association between the presence of a partially erupted lower 3M and alterations in the local bone structures was confirmed by this study.
