# Mandibular Second Molar Impaction-Part II: Etiology and Role of the Third Molar
## 4 Discussion
It is therefore suggested that, in the presented cases, an arch length deficiency in the mandibular posterior segment is not the major contributing factor in causing second molar impaction. We repeatedly observed that the MM2 region had more than adequate arch length for the second molar, yet the tooth became impacted. A similar situation has been reported where the second molar became impacted on the side where a larger space was available [10; 22]. In addition, previous reports [7; 23] demonstrated mesial inclination among most of the mandibular second molars, suggesting it mainly attributed to their abnormal eruption pathway. This is contrary to several reports, supported by the presented radiographic evidence, that the majority of MM2 impactions are associated with crowding and arch length deficiency in the posterior region of the dental arch [6]. It has also been suggested that excess space between the first molar root and the developing second molar crown may allow the later to incline more mesially and become impacted. This is probably due to the initial mesial angulation of the developing tooth bud [9]. Another explanation has been suggested by which the MM2 may change its angulation due to differential, asymmetrical root growth, with a decrease in the mesial root length and increased length of the distal root (Figure 5). This was demonstrated in an 11-year-old girl, where the mandibular left second molar was horizontally impacted with a shorter mesial root than the distal one (Figure 6). A significantly shorter mesial root has been previously reported in mesially angulated impacted MM2s, whereas a distally angulated MM2 appears to be associated with a shorter distal root [5]. Most studies in the literature report on MM2 impaction in adolescent individuals presenting the impacted MM2 in association with the presence of the third molar. To the best of our knowledge, this is the first report presenting the early stages of the MM2 impaction process. The following longitudinal panoramic radiographs demonstrate an unequal differential growth of the MM2 roots, where the mesial root is shorter than the  
Figure 5: A horizontally impacted left MM2 with shorter mesial than distal root. distal root. As a result, the mesial root serves as a pivot for the MM2 which gradually tips and becomes mesially angulated, often resulting in its impaction distal of the first molar. The panoramic radiograph of an 8-year-old boy in his early mixed dentition is a representative of the developing crowns of the MM2s in normally erupting positions with no signs of the third molar buds (Figure 7A). It is reasonable to predict that their eruption would be uneventful. A panoramic radiograph taken one year later, at the age of nine years, shows the right permanent second molar mesially inclined with its mesial root shorter than the distal. At that stage, the early developing third molar bud can be identified at the anterior rim of the ramus behind and away from the already mesially angulated second molar when comparing this with the contralateral view, where a small space exists between the second molar and the adjacent developing third molar bud (Figure 7B). In a one year interval, at the age of 10 years, the panoramic radiograph revealed the MM2 impacted in a more horizontal position the more clearly showed its shorter mesial root with the developing third molar bud still away from the second molar (Figure 7C). The panoramic radiograph taken two years later at the age of 12 years demonstrates a horizontally impacted right second molar with a shorter mesial root and the third molar crown now mesially inclined close to the second molar's distal root (Figure 7D). Figure 6: Panoramic radiograph of an 11-year-old girl with a horizontally impacted left MM2. Note its shorter mesial root and the developing MM3 crown on top of the MM2 longer distal root.