# Mandibular Second Molar Impaction-Part II: Etiology and Role of the Third Molar
## 1 Introduction  
The process of non-orthodontic movement of a tooth, occurring from the time of its initial formation to its occupying place in a functional occlusal table, can be divided and described as pre- and post-eruptive. For the purpose of this work, the term \"eruption\" shall mean the tooth's penetration of the alveolar mucosa, or the stage when some part of a tooth can be seen by a naked eye. This is also termed the emergence of a tooth, however this later term, although very well defined by various anthropologists [1], is not routinely used. Consequently, various disturbances in the tooth's progression from its place of initial formation to the so-called \" anatomically correct position in the dental arch\" can happen pre- and post-eruptively. Better terms, and by the anthropologists preferred ones, would be the pre- and post-emergence phases of tooth eruption. Eruption disturbances can occur in any stage by an obstacle in the eruptive path, such as a supernumerary tooth or odontoma, ectopic position of the tooth germ, and failure of the eruption mechanism [2].  
This paper will address both of these phases, with a particular emphasis on the pre-eruptive disturbances.  
The cessation of eruption of a normally developed tooth germ before emergence without identification of a physical barrier in the eruption path is defined as primary retention [3]. The post-emergence cessation of eruption of a tooth without a physical barrier in the path of its eruption is considered secondary retention [4]. The cessation of eruption of a tooth caused by a physical barrier in the path of eruption, as well as a lack of space and the abnormal anatomical position of the tooth, is termed impaction. In a case of a clinicallyobserved or a post-emergence mandibular molar impaction, often times a tooth presents with an obvious mesial angulation. Similarly, the great majority of pre-emergence impacted mandibular second molars (MM2), typically detected radiographically, have been reported with a marked mesial angulation [5].  
MM2 impaction is usually associated with crowding and arch length deficiency in the posterior region [6; 7]. In addition, it may also be a result of an ectopic germ position or an ectopic eruption path. It is often diagnosed as a secondary finding during examinations for pediatric or orthodontic diagnosis and treatment planning. It is rarely the main reason for referral to an orthodontic clinic. It is a relatively rare dental anomaly with a reported incidence of 0.03% to 0.6% in the general population [8], and 1-3% in orthodontic patients [5; 6]. MM2 impactions can be classified as mesio-angular, disto-angular, vertical, or horizontal. The mesio-angular position has been reported for most of the MM2 impactions [5; 9]. Unilateral impaction of the MM2s is more common than bilateral. They have been reported more often in males than in females and with a right side predominance [6].  
Various treatment options have been suggested in the literature, depending on the position and the depth of the impacted tooth in the bone, including surgical exposure and orthodontic uprighting [9; 10], surgical repositioning with or without removal of the third molar [11; 12], extraction of the second molar with substitution with the third molar [11], and early prophylactic removal of the third molar germ when it presents an obstacle to the erupting second molar [13]. The optimal time for treatment of MM2 impaction cases coincides with the expected time of the MM2 eruption, from 11 to 14 years.  
This paper evaluates radiographic characteristics of the MM2 and MM3 and the possible association in the process of their impaction. It discusses the early diagnosis and progression in the process of MM2 impaction, including the possible role of the third molars in this process.