# Effect of Genetic and Environmental Factors on the Impaction of Lower Third Molars
## 2 Materials and Methods
### Selection and Evaluation of Digital Panoramic Radiographs  
All the panoramic radiograms were performed by the same machine (Carestream Kodak 9300) for all study participants using standard protocols of the patient positioning to avoid distortion on panoramic radiographs. Patients were oriented according to their mid-sagittal line, Frankfort plane and the canine-corner base of the nose alignment line. In addition, each panoramic radiogram was evaluated for the detection of geometric distortion. [PERSON] et al. found that the magnification in the lower first molar region was the most stable; thus, this tooth was used for the control of geometric distortions [20].  
Consequently, in the recent research, all panoramic radiograms were checked by the same two expert operators comparing the magnification coefficients in the lower first molar region on the right and left sides. If this coefficient was the same on both sides, then the panoramic radiogram was accepted without geometric distortion, as a good quality panoramic radiogram. Other radiograms were excluded from the study. The analysis of panoramic radiograms was performed using Dolphin Imaging program 11.8. The measurements on the plaster casts were performed by the use of a digital caliper with the tips sharpened to a point and 0.01 mm accuracy (Dentaurum, Ispringen, Germany).  
The impaction of the lower third molar was determined by the evaluation of each panoramic radiogram for the angulations and eruption levels of the lower molar and for the eruption space for the LM3. The positions of these teeth were determined using mandibular plane (MND), occlusal plane (OKL) and horizontal plane (HP) (Figure 1).  
In addition, the eruption levels of lower molars were investigated according to the mandibular and occlusal planes (Figure 2).  
The distances between occlusal surfaces of the third and second molars (DLR8/LR7, DLL8/LL7) were measured from the mesial cusps of the lower third molars tangentially to the lines, which extended over the occlusal surfaces of the second molars in the right and left side. All measurements were performed using Dolphin Imaging program 11.8.  
Figure 1: Description of the variables for estimating the positions of the lower third molars. LR8—longitudinal axis of the lower right third molar is traced from the midocclusal point to the midpoint of the root bifurcation, and the same on the opposite side with LL8; LR7—longitudinal axis of the lower right second molar is traced from the midocclusal point to the midpoint of the root bifurcation, and the same with LL7; LR6—longitudinal axis of the lower right first molar is traced from the midocclusal point to the midpoint of the root bifurcation, and the same with LL6; MND-R—right mandibular plane is drawn through most inferior border of the mandible, and the same with MND-L; CCL-LR—lower right occlusal plane is drawn through the incisional edge of the lower right lateral incisor to the distal cusp of the lower right second molar, and the same with OCL-LL; HP—horizontal plane is traced through the most superior parts of the right and left condyles.  
Figure 2: Description of the variables for estimating eruption levels of the lower third molars. MND-R/BIFLR6—the distance from the MND-R plane to the bifurcation of the right lower first molar, and the same with MND-L/BIFLR6; MND-R/BIFLR7—the distance from the MND-R plane to the bifurcation of the right lower second molar, and the same with MND-L/BIFLL7; MND-R/6-7—the distance from the MND-R plane to the edge of the alveolus ridge between the lower right second and first molars, and the same with MND-L/6-7; BIFLR8/OCL-R—the distance from the OCL-R plane to the bifurcation of the right lower third molar, and the same with BIFLL8/OCL-L.