# Effect of Genetic and Environmental Factors on the Impaction of Lower Third Molars
## 4 Discussion  
The hypothesis of this study that genetic factors played a key role in the lower third molar position and eruption space using a genetic model of twins was supported. One of the considerations of such study designs is the estimation of the age of the study group. There was no doubt that during adolescence, the lower third molars were changing their position; thus, it was necessary to collect data with the patients after the growth spurt. It was found that the formation of the pulp chamber and radicular bifurcation of the mandibular third molar was an indicator of the deceleration of the growth spurt. Consequently, the twins with at least CVM stage 5 were chosen for the study sample [23]. First, this study showed that the space for the eruption of the lower third molar was highly affected by genetic factors, and this matched with the findings of [PERSON] et al. with regard to the eruption space for the upper third molars [24]. The differences of eruption space in dizygotic and monozygotic twins were not statistically significant (\(p\) values varied from 0.14 to 0.17 in dizygotic twins and from 0.19 to 0.23 in monozygotic twins). The correlations between siblings varied from 0.53 to 0.60 in dizygotic twins but were much higher in monozygotic twins (it ranged from 0.89 to 0.91). The best fitting model for space for the upper third molar eruption was AE, where the effect of additive genetic factors was a slightly lower and reached 61% (instead of 69% in the current study). However, the impact of the common environment for the space of upper third molars was higher by 8 percent and reached 39%. This difference might be due to the stronger genetic determination of the shape of the mandible. Previous studies suggested that the horizontal linear variables of the lower jaw seemed to be more influenced by genetic factors than the vertical ones [17]. The same tendency was seen even in the agenesis of M3 where additive genetic factors had an even up to 19% higher effect on the agenesis of lower third molars in comparison to the agenesis of upper third molars [25; 26]. In addition, while analyzing the position of these teeth, the angulations of M3s did not have statistically significant differences in either dizygotic or monozygotic twins (\(p\) values varied from 0.1 to 0.99). The correlations between siblings varied from 0.28 to 0.98 and were higher in monozygotic twins. [PERSON] in the non-twin sample found that until the age of 15 years, the lower third molars had a tendency to incline mesial approximately 11 degrees without any clear reason [27]. According to the recent study, this may be most affected by genetic factors. The findings of our research showed that the additive genetic effect on the position of the lower third molars was almost the same as on the position of the upper third molars and varied from 74 to 76 percent. In addition, these findings are consistent with those of many others, indicating that most of the variation of dental development in humans, such as teeth crown size or mineralization of the germs of third molars, had a strong genetic component [2; 11; 12; 28]. Considering the findings of our study with regard to the position of lower third molars and their eruption space, which were the most important factors for the impaction of lower third molars, it may be concluded that the retention of lower wisdom teeth was mostly affected by additive genetic factors with a small part of the effect from the common and specific environments. This is a very promising area in biotechnology, especially now, when we have a modern tool--DNA scissors. Finally, this study found that the effect of environmental factors on the impaction of lower third molars ranged from 12% to 34%; thus, this effect should not be negated. These findings correspond with the results of [PERSON], where the specific environment ranged from 8% to 29% for permanent teeth sizes (excluding third molars) [13].