# Mandibular Second Molar Impaction-Part II: Etiology and Role of the Third Molar
## 4 Discussion  
An impacted molar remains unerupted in the alveolar bone or in the body of the mandible (corpus mandibular) beyond the time at which it normally erupts. Crowding and arch length deficiency in the posterior region of the mandibular arch have been suggested as important local factors causing MM2 impaction [6; 14]. Our findings, however, do not fully support that theory, as will be explained later in the discussion. Tooth size may be affected by genetic factors that control dental morphogenesis [15]. In addition, the tooth size in ethnic Chinese people has been reported to be larger than in Caucasians [16; 17]. This may partially explain the higher prevalence of MM2 impactions in the Chinese population. The presence of third molars in 97.4% of our cases was in agreement with a previous study reporting that all patients with arrested MM2 had the germ of the third molar [18]. Paradoxically, according to [PERSON] and [PERSON] [19], the prevalence of developing third molars in the general population is 63.4% to 77.5%, which is significantly less than our study's observations. This may suggest a different role of the developing MM3 in MM2 impactions. While the presence of MM3 may contribute to the space shortage for the MM2, equally, or even more importantly, it may contribute to the arrested eruption of the MM2, often leading to its impaction. This data suggests that the third molar might be involved in MM2 impaction. However, the mechanism of such an effect is still unknown. It has been previously reported that the second molar eruption may be unpredictable and, for unknown reasons, its inclination changes during its eruption, becomes mesially inclined and resulting in impaction [9]. This has been observed in an 8-year-old boy with excess space between his fully erupted first molar and the developing crown of the second molar. However, 3.5 years later, the tooth became mesially angulated and almost horizontally impacted, while the bud of the third molar had just began its development [9]. A similar phenomenon occurred even where the third molar was missing, as seen in the panoramic radiograph of an 11-year-old girl with a mesially angulated right MM2 (Figure 2). Similarly, in cases where the developing third molar crown appears to be far distal to the right second molar, as was present in an 11-year-old boy (Figure 3) and in a 10-year-old girl, where excess space appeared between the bilaterally developing buds of the third molars and the mesially angulated and impacted MM2s (Figure 4). \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Ethnicity**} & \multicolumn{1}{c}{**No. of Patients**} & \multirow{2}{*}{**Male**} & \multirow{2}{*}{**Female**} & \multirow{2}{*}{**Unilateral**} & \multirow{2}{*}{**Bilateral**} & **Total No. of** \\  & & & & & & **Impacted MM2** \\ \hline Chinese-Americans & 151 & 73 & 78 & 90 & 61 & 212 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Distribution of MM2 impactions. Figure 4: Bilateral mandibular second molar impaction showing excess space between the second molars and the developing buds of the third molars. Figure 3: Mandibular right second molar mesially angulated and impacted with excess space between the second molar and the third molar developing crown. Figure 2: Fully developed mandibular right second molar mesially angulated and impacted with missing third molar behind it. It appears that, at least in these and the similar cases, radiographic evidence-based treatment suggests insignificant association between developmental paths of the MM2 and MM3. In an abnormal eruption pathway of the second molar, the role of an ectopic position of the third molar is still questionable. According to previous reports, the third molar position should be considered as a contributory factor and not the primary cause for second molar impaction [20; 21]. Instead, we suggest that the mesial angulation of the second molar may provide a better clue or indication of the MM2 eruptive outcome.