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ABSTRACT:
It's been more than a century that third molar has drown the attention of orthodontists . This attention has been based on two main reasons:
1- The probable role of third molar in making late crowding or Recrowding of lower anterior teeth
2- Evaluation of causative factors and prediction of impaction of this tooth and orthodontic 's help to prevention of problems caused by this tooth.
This study is about :
1- The comparsion of axial inclination and Space for eruption of lower third molar in 3 skeletal groups CL 1,CL II,CL III .
2- The probable affect of body length, ant-post position,vertical position of lower jaw in position of third molar.
This study just has been done on lower third molar . 12 patient with skeletal cl I ,12 patient with skeletal cl III, 12 patient with skeletal CL II have been chosen ,that in all of them SNA angle was in the normal range . The age of these patients have been 17 ,18 years old. They had not crowding ,spacing , supernumerary missing ,extraction in lower jaw. They had all permanent teeth in lower jaw specially third molar .All of them had Lat. cephalogram, O.P.G. and study model. From cephalogram ,SNA ,SNB,ANB ,Facial angle „Mand .plane angle, GoFn -SN,Gonial angle , Y axis angle ,also the distances Go-Pog , Ar -Pog, Ramus width have been measured . (as descriptive variables).
Also distance of Xi point to distal of lower second molar and distance of ant. border of ramus to the distal of lower first molar from cephalogram and axial inclination of third molar in relation to second molar in both left and right from O.P.G. have been measured. (as answer variables).
Statistical relationship of descriptive variables with answer variables were evaluated and the following results have been noted:
1- The axial inclination of third molar in 3 skeletal groups had not significant differences.
2- The distance of Xi point to the distal of lower second molar only in cl I group had significant difference while that in other groups had not significant differences.
3-The body Length of lower jaw (Go-pog) has been reconized as most important factor in impaction of third molar. In other words as the distance decreases ,probability of impation increases.
4. The ant .post. position of lower jaw had not significant relationship with impaction of third molar.
5- The vertical position of lower jaw had opposite relationship with impaction. In other words as the amount of skeletal deep bite increases the probability of impaction also increases.

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