The Effect of Third Molars on the Mandibular Anterior Crowding Relapse—A Systematic Review

The present systematic review updates the evidence on wisdom teeth contributing to lower incisor crowding following orthodontic treatment. Relevant literature was searched on online databases, namely Pubmed, Scopus, and Web of Science, up to December 2022. Eligibility criteria were formulated using the PICOS approach and PRISMA guidelines. Eligible research included original clinical studies involving patients previously being treated orthodontically with permanent dentition at the end of treatment, regardless of sex or age. The initial search yielded 605 citations. After considering eligibility criteria and removing duplicates, only 10 articles met the criteria for inclusion. The risk of bias of eligible studies was evaluated using the Cochrane Handbook for Systematic Reviews and Interventions tool. The majority were highly biased, mainly regarding allocation concealment, group similarity, and assessment blinding. The vast majority did not report statistically significant associations between the presence of third molars and crowding relapse. However, a minor effect has been suggested. Seemingly, there is no clear connection between mandibular third molars and incisor crowding after orthodontic treatment. The present review did not find adequate evidence to advocate preventative removal of the third molars for reasons of occlusal stability.


Introduction
In most cases, wisdom teeth do not participate in active orthodontic treatment. However, they have an impact in treatment planning, their fate being a matter of concern. Indeed, up to two thirds of surveyed orthodontists and oral surgeons believed that unerupted third molars can generate an anteriorly directed force component that culminates in mandibular incisor crowding [1]. However, Tufekçi et al. (2009) [2] reported that the majority of orthodontists in Sweden and the US believed that the erupting mandibular wisdom teeth rarely cause crowding despite their potential for generating an anteriorly oriented force. Nevertheless, significantly more American orthodontists may recommend prophylactic removal of third molars in comparison with their European colleagues. Surgeons were significantly more likely to recommend third molar extraction to prevent undesirable anterior crowding [3]. Niedzielska (2005) [4] claimed that in the case of insufficient space for the third molars to erupt, forces may be applied on adjacent teeth causing crowding, but in the case of The reviewing process and the article selection are presented in Figure 1. In total, 605 records were initially retrieved. Among them, 341 proved duplicates and were excluded, as did 224 more after evaluating the titles and abstracts. For reasons such as the type of study, the outcome of interest, and the absence of previous orthodontic treatment, a further 30 records were excluded. Eventually, 10 full-text articles were deemed appropriate for inclusion in the systematic review [15][16][17][19][20][21][22][23]25,27].  Table 1 presents features of the included studies. The vast majority had used the Little Irregularity Index as means of measurement and plaster models for assessment.   Table 1 presents features of the included studies. The vast majority had used the Little Irregularity Index as means of measurement and plaster models for assessment.

Within Studies Risk of Bias
Details regarding the risk of bias quality assessment are presented in Table 2. Eight studies were assessed for high risk of bias [15][16][17]19,20,23,25,27] and two for low risk of bias [21,22]. The majority of studies were highly biased, mainly regarding allocation concealment, group similarity, caregiver blinding, and assessment blinding.  [20] H

Discussion
Human dentoalveolar structure undergoes substantial change during growth [34,35]. The process appears more pronounced until the onset of adulthood [36], but it does not cease throughout aging, and is an adaptation to environmental conditions [37][38][39][40][41]. Until the age of the late mixed dentition, the changes may become notable because facial appearance is affected by anterior tooth malalignment [42][43][44]. Consequently, anterior dental crowding is a common reason for seeking orthodontic treatment [45][46][47]. Surprisingly, though, potential relapse of tooth crowding may not prove a major disappointment for the patient, despite the orthodontist's likely embarrassment [48,49].
In Orthodontics, relapse is defined as any unfavourable change in tooth position after orthodontic treatment that is not consistent with the corrected malocclusion [50], and the mandibular incisor imbrication is a common manifestation [19]. The eventuality of anterior crowding recurring worries orthodontists, and is inevitably observed in many treated cases [51]. In particular, it tends to happen after debonding in patients lacking retention during periodontal fiber healing [52][53][54][55]. Therefore, it is up to the orthodontist to prevent such an occurrence through planning, best practice, application of retention, and an appropriate recall regimen [56]. Nevertheless, post-treatment maintenance of mandibular arch alignment remains challenging for the clinician [57,58]. Research has tried to fully clarify the process regulating longitudinal changes in incisor arrangement, albeit with questionable credibility [8,14]. Angle (1907) [59] asserted that attaining occlusal stability could preserve the orthodontic therapeutic outcome [9]. This is in agreement with the finding of Kahl-Nieke et al. (1995) [27] that a perfect molar relationship may contribute to maxillary incisor alignment. However, recurrence of lower arch crowding is observed clinically quite often, even in cases with great dental intercuspation [60]. So, clinicians still attempt to preclude anterior relapse by over-correction, supra-crestal fibrotomy, and long-term retention [50,54,57]. The connection of the third molar with post-treatment relapse of malocclusion, particularly in the anterior dental arch segment, remains widely controversial and unresolved [9]. Allegedly, the erupting wisdom tooth has the potential to generate an anterior force component to be transmitted along the dental arch, concentrating in the area of the canines and incisors to result in tooth malalignment [3].
Several factors have been investigated in the search for the etiology of tooth crowding relapse, which happens after the end of active orthodontic treatment. Overall, it has been suggested that crowding in the lower anterior region and third molar impaction are both the sequela of inadequate growth [61][62][63]. In cases of restricted mandibular anterior growth and inefficient remodelling, enough space may not be generated for the mandibular incisors to move forward without getting crowded [64,65]. The effect of the tension by the neighboring soft tissues (lips, cheeks, tongue) has also been highlighted [66,67]. In addition, a mesial migration of the posterior dentition might have an effect [68] and the anteriorly oriented occlusal force component might play a role [24], while the initial orthodontic condition [27] and subsequent manipulations, (Little, 1999) the tooth dimensions [27], and the function of the periodontal tissues [57] should all not be disregarded. Lastly, various evolutionary factors [69], the gender [70,71], and the race [70,72,73] of the patient may be of importance.
Robinson (1859) [74] claimed that late lower incisor crowding is caused by the erupting mandibular permanent third molar [16]. The indictment of wisdom teeth was reaffirmed in 1917, when Dewey commented that the mandibular third molar may create space for its eruption by pushing the more anteriorly positioned teeth to move forward, potentially ending up crowded [30]. Ever since, a bulk of research has attempted to ascertain or refute the statement. Studies vary in their conclusions, several finding little relationship between third molars and late anterior tooth crowding, whereas some suggest associations of varying degree. The issue is still regarded as controversial and unresolved on the basis of emerging research, which is popular and intriguing to the dental society [14,27]. Disappointingly, Shanley (1962) [75] found insignificant differences between groups with bilaterally erupted, impacted or developmentally absent third molars, and concluded that the third molar has little influence on late anterior crowding. On the other hand, Vego (1962) [76] reported conflicting observations in a research project based on study models, without previous orthodontic treatment. He found that patients with missing wisdom teeth developed less statistically significant crowding in comparison to those with a complete dentition. So, it was concluded that erupting third molars can produce a force to approximate the teeth. However, he reported late crowding also in cases with congenitally missing wisdom teeth and, so, he suggested that mandibular malalignment might be multifactorial. It is noteworthy that Zachrisson (2005) [77] appeared to accept that lower incisor crowding during the post-orthodontic period is probably impacted by a variety of events.
The third molar is special, featuring considerable variability in the timing of formation and eruption, its course of eruption and final position, and its morphology. Erupting third molars continually change their angular positions and show important pre-eruptive rotational movement [78]. Calcification may become evident by the age of 7 years [79], commonly emerging in the mouth after the age of 17-20 years [80], competing for the highest rate of impaction and congenital absence [81,82].
In day-to-day practice, after active orthodontic treatment, the goal is to preserve the achieved therapeutic outcome. The protocol of retention should be specially planned for each patient and be carefully applied by the clinician in order to avoid unwanted occlusal relapse. In the literature, various factors have been implicated for crowding relapse, such as residual growth, sex, extractions or not, periodontal status, and dental arch expansion. The mandibular incisor crowding relapse remains unresolved and its association with mandibular third molars is still a matter of debate. Motivated by evidence-based dentistry, the current systematic review aimed to study the contribution of mandibular wisdom teeth in the re-emergence of incisor crowding.
In the present review, only Kahl-Nieke et al. (1995) [27] found a statistically significant association between wisdom teeth and lower incisor irregularity. However, the effect was considered minor and of questionable clinical value.
In 1970, Fastlicht [15] claimed that incisor crowding constitutes a normal procedure of adjustment that might be detected notwithstanding the patient's orthodontic status. The blame was put on a number of developmental factors, namely, the sex, the age, the increased antero-posterior skull dimension, the discrepancy between dental dimension and arch length, the pronounced overbite and diminished intercanine distance, the muscular function, and even incomplete mechanotherapy. In particular, he observed that the degree of crowding increased along with mesiodistal incisor dimensions. It was also observed that crowding of the lower incisors appeared more outstanding in male patients, who measured larger maxillary and mandibular incisors in comparison with females. Overall, no connection emerged between third molars and anterior crowding in both sexes, although differences tended to be greater in women. Further, less crowding was evidenced among orthodontically treated individuals. As a result, it was suggested that orthodontic treatment might prove beneficial for occlusal stability. The age of the patients was positively related to incisor crowding, and maxillary teeth showed less irregularity.
According to Kaplan (1974) [16], most of the orthodontically treated patients may eventually end up with lower front tooth relapse. This did not correlate statistically significantly with erupted, impacted, or even congenitally missing third molars. In particular, no major changes of the dental arch length, the mandibular incisor position, or lower tooth inclination were registered throughout the period following orthodontic treatment. Furthermore, dimensional alterations of intermolar and intercanine distances happened independently of the third molar status, and they were not significant. It was of no surprise that the hypothesis of third molars applying pressure on mesial dental units could not be reasonably supported. Ades (1990) [17] alleged that mandibular anterior crowding tends to increase by aging, while dental arch length and intercanine distance usually diminish. Post-retention records displayed only insignificant differences among the subgroups in which third molars presented congenital absence, impaction, normal eruption, or had been extracted regarding mandibular incisor crowding after considering the growth pattern. Interestingly, most investigated cases presented rather lower incisor crowding, a finding that has not been correlated to the third molars.
Van der Schoot et al. (1997) [20] claimed that the existence or absence of upper or lower third molars does not affect the long-term occurrence of the arch length variance and the anterior tooth irregularity. Indeed, when third molars were congenitally missing, the arch length discrepancy was significantly lower at the premolar area. Therefore, the presence of third molars had no noteworthy clinical connection to the developing late crowding.
In a prospective study, Harradine et al. (1998) [21], investigated the effects of early third molar removal on late mandibular anterior crowding after the completion of retention period, and they concluded that removal of these teeth cannot be supported with robust evidence. These findings are in agreement with Linquist and Thilander (1982) [29], who investigated unilateral third molar extraction, and Vego (1962) [76], who reported on third molar agenesis. In both studies, no clinically significant effect of the wisdom teeth was found on incisor crowding, which is in agreement with the retrospective study of Ades et al. (1990) [17] involving patients who had previously been treated orthodontically. Nevertheless, these suggestions disagree with the retrospective study by Schwarze (1973) [83], concluding that wisdom tooth removal may have a benefit in alleviating later incisor crowding or improving upper arch irregularity, and also with the findings of Richardson (1996) [84] on the role of distal pressure displacing the incisors. However, there is agreement with Southard et al. (1991) [24] who believed that such a force is not significantly affected by third molar removal. Little (1999) [19] stated that third molars' presence or absence is not directly linked with post-retention stability or relapse, because it seemed that patients with congenital absence of mandibular third molars did not differ significantly from those having the teeth. The pilot study by Al-Balkhi (2004) [23] supported the notion that the mandibular third molars may not be major contributors in the re-appearance of incisor crowding in the case of absent intimate interproximal contacts. Nevertheless, a larger sample might be more appropriate to verify the conclusion.
Okazaki (2010) [25] alleged that increasing the total interproximal force may serve as an indication for relapse in lower incisor crowding and, so, clinicians need to be vigilant for relapse in the lower anterior segment for at least a semester following the delivery of retention when facing severe anterior incisor crowding.
The evidence from the present systemic review cannot justify third molar extraction with the aim to prevent post-retention incisor crowding deterioration. Contrarily, extraction of the third molar may be considered in case of existing pathology, such as nerve irritation, periodontal inflammation, or increased caries risk. This is in line with the most recent past investigation by Cotrin et al. (2019) [22], who were also opposed to the recommendation for extracting the third molars to prevent potential relapse of anterior mandibular crowding because they found no connection between mandibular third molars and incisor maladjustment.

Strengths and Limitations
The present systematic review was constructed on established guidelines, already described in the Materials section. The reviewing process was meticulous up to December 2022, including all potentially eligible reports.
Limitations of this systematic review might be related to the nature of the eligible articles and the features of the data. The heterogeneity of protocols in the included research and the increased risk of bias in the majority of them discouraged the realization of further meta-analysis.

Recommendations for Future Research
Treatment outcome stability appears important for clinicians and patients. To achieve the goal of properly functional occlusion, patients have invested time, patience, and resources. Thus, anterior occlusal relapse constitutes an unwanted occurrence. Further randomized clinical studies, in accordance with ethical codes, are required to exclude such potentially disappointing results.

Conclusions
Considering the findings of all the above studies, we assert that there is no proven connection between mandibular wisdom teeth and lower anterior crowding relapse after orthodontic treatment. In the present systematic review, several factors likely to participate in the deterioration of tooth alignment after the actual orthodontic treatment were discussed.
Supplementary Materials: The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/dj11050131/s1, Table S1: Eligibility criteria for the present systematic review; Table S2: Strategy for database search (up to December 2022).

Data Availability Statement:
The data presented in this study are available in the included studies of this systematic review.

Conflicts of Interest:
The authors declare that there is no conflict of interest.