# Mandibular Second Molar Impaction-Part II: Etiology and Role of the Third Molar
## 4 Discussion
Figure 7: _Cont._  
Figure 7: **(A)** Panoramic radiograph of an 8-year-old boy presents the developing crowns of the mandibular second molars in the normally erupting position with no signs of the third molar buds. **(B)** Panoramic radiograph of the same boy at the age of nine years, showing the right second molar mesially inclined with a shorter mesial root than the distal. 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. **(C)** At the age of 10 years, the panoramic radiograph reveals the MM2 impacted in a more horizontal position with its shorter mesial root, and the developing third molar bud still far away from the second molar. **(D)** At the age of 12 years, the panoramic radiograph demonstrates a horizontally impacted right second molar with shorter mesial root and the third molar crown now mesially inclined close to the second molar distal root. Figure 7A-D have previously been published in the article \"Rootless eruption of a mandibular permanent canine\". AJO-DO 2011; 139:563-566. Based on the available recent information, it is not clear what causes the differential root development in certain MM2s [24]. The tooth development undergoes a serious of complex reciprocal interactions between the ectodermally derived dental epithelium and the underlying mesenchymal cells during tooth morphogenesis. It has been reported that the Wnt/betta-catenin signaling process controls the root odontogenesis and cementogenesis during the tooth root development [25; 26]. Disruption of the Wntless leads to the inhibition of odontoblast maturation and root elongation, thus resulting in short roots [27]. Experiments on the mandibular first molars in mice have demonstrated that deficient Wnt/betta-catenin signaling effectively blocked odontoblasts and cementoblasts differentiation during the root development, resulting in their arrested development [28]. An association between root anomalies and eruption disorders of MM2s has also been reported [18]. Why this phenomenon affects the mesial root more than the distal root of the MM2 is not yet clear and should be further investigated. This may also support our theory [5; 9] that MM2 mesial angulation occurs early, before the third molar bud is developed and even when the third molar is congentially missing, as presented in the panoramic radiograph of a 12-year-old girl with a mesio-angular impacted right MM2 having a shorter mesial root (Figure 2). This was also demonstrated in a 10-year-old girl with normal erupting-positioned MM2s (Figure 8A), which four years later, at the age of 14 years, presented her right MM2 as mesially inclined, while the third molars were congentially missing (Figure 8B). That is, the differential MM2 root development and its mesial angulation could probably be the major cause, while the posterior crowding might have been a secondary contributing factor, occurring later and causing the MM2 to become impacted. It has been shown that patients with congentially missing third molars have fewer problems of crowding or impacting of the MM2 during treatment. Thus, early removal of the third molar relieves the congestion at the end of the lower arch. An early prophylactic approach for interceptive removal of the third molar germ has been suggested. Such extraction of the mandibular third molar germ, before or during orthodontic treatment, could be beneficial because it may facilitate the eruption of the second molar [15]. In 1934, [PERSON] [29] described the enucleation technique at the bud stage of the third molar at an early age (7-9 years) as a simple and relatively atraumatic procedure. This is contrasted by the often difficult extraction of deeply impacted third molars in adults. Similarly, in 1970, [PERSON] [30] revisited the enucleation of third molar buds as a prophylactic measure in young children, using electrosurgery.