The reasons for tooth impaction might include a several factors such as position and size of adjacent teeth, dense overlying bone, excessive soft tissue or a genetic abnormality including abnormal eruption path, dental arch length and space in which to erupt (Jojic & Perovic, 1990; Alling et al., 1993; Hupp et al., 2008). Generally speaking these factors are subdivided into a two groups as local and general factors. The most common impacted teeth are mandibular and maxillary third molars, followed by the maxillary canines and mandibular premolars (Jojic & Perovic, 1990; Hupp et al., 2008). Third molars have inadequate space for eruption, thus they are the last teeth to erupt. New data suggests that 72,7% of the world population has at least one impacted tooth (usually lower third molar), and it is more frequently in female than the male patients (Ahlqwist & Grondahl, 1991; Alling et al., 1993,).
Although indications for removal of impacted teeth vary from orthodontics, prosthodontics, pathologic and prophylactic, one of the reasons that impacted teeth should be removed, is their influence on the adjacent teeth with development of the caries lesions.
Caries is mentioned as one of the common pathological features associated with extracted mandibular third molars (Battaineh et al.,2002; Lysell & Rohlin, 1988; Punwutikom et al., 1999). This is a reason why in this section the emphasis will be on these teeth.
There is an opinion that the tooth position and inclination play a main roles in caries development process (Knutsson et al.,1996). For better understanding this relationship it is necessary to know a classification of impacted lower third molars. The most common used classification is by Winter in which third molars are classificated by their long axis angulation with respect to the long axis of adjacent second molars. Mesioangular position is the most seen type of third molar impaction comprising 43% of all third molar impactions, characterized by mesial direction of the third molar's long axis toward to the second molar with convergency angle of >30 (Kan et al., 2002). In vertical position, the long axis of impacted tooth runs parallel to the long axis of the second molar comprising 38%. Distoangular position including 6% of the cases is characterized by distally or posteriorly angled long axis of the tooth away from the second molar. If the long axis of the impacted tooth is perpendicular to the second molar comprising 3% of all cases, this position is known as the horizontal (Kan et al., 2002; Hupp et al., 2008). However, atypic positions in which impacted teeth are angled in buccal, linqual, palatal or buccolinqual directions are also recorded (Jojic & Perovic, 1990; Hupp et al,. 2008). .
The second also in use classification is by Pell and Gregory, in which are described three positions of the third molars: depending of the relation of tooth to ramus and second molar subtypes (Type A), relative depth of the third molar in bone (Type B) tooth on same level with occlusal plane and position of long axis of the impacted tooth in relation to the second molar as taken from the Winter classification (Type C). (Kan et al., 2002; Hupp et al., 2008).
The practice suggests that horizontal and mesioangular positions are more critical to adjacent second molar, because impacted teeth in these positions may impige and resorb a distal surface and root of the second molars (Knutsson et al.,1996).
2.1 Winter classification of impacted lower third molars
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