Position of Impacted Mandibular Third Molar in different Skeletal Facial Types

Introduction: Variation in growth pattern in different facial forms affect the space available for tooth eruption, therefore the mandibular third molar impaction has correlation with different skeletal facial types. Objective: To determine the position of impacted mandibular third molar in different skeletal facial types in Nepalese samples. Materials & Method: This cross sectional study was carried out on pre-treatment lateral cephalogram and orthopantomogram records of 207 orthodontic patients visiting Kantipur Dental College and Hospital. Age range of the sample was 16-33 years. Various facial forms were determined with the help of mandibular plane angle (SN-GoGn); for which Nemoceph software was used. Beta angle was used to determine the angle of impaction of mandibular third molar. Impaction pattern was classified as per Pell & Gregory and Winter’s system. SPSS version 20 was used for statistical analysis. Result: Overall rate of mandibular third molar impaction was 63.77% (11.11% unilateral and 52.66% bilateral impaction). The most common type of impaction bilaterally was mesioangular based on Winter’s classification, Class II Position B based on Pell & Gregory’s classification. Higher rate of mandibular third molar impaction was found in dolichofacial type of face (49.2%). Conclusion: Decreased mandibular third molar impactions are seen in brachyfacial type compared to mesofacial and dolichofacial types.


MATERIALS AND METHOD
Impaction was also classified according to Pell & Gregory's classification system based on the amount of space available between distal surface of second molar and anterior border of ramus. 9

Pell & Gregory's Classification
Class I Sufficient amount of space for accommodation of the mesio-distal diameter of crown of the third molar

Class II
Space between the ramus and distal side of second molar is less than the mesiodistal diameter of the third molar

Class III
All or most of the third molar is located within the ramus

Level of impaction was classified as Position A, B and C;
which is related to the occlusal height as compared to the adjacent second molar. 9

Level of impaction
Position A At the level of adjacent tooth

Position B
Between occlusal and cervical margins of the adjacent tooth

Position C Below the cervical margin
The findings were recorded using a data collection sheet.Kruskal-Wallis test was done to check the relationship of impacted mandibular third molar with different skeletal facial types.Level of significance was set at p<0.05.Regarding the angle of impaction, dolichofacial type of face had highest frequency of mesioangular impactions, whereas mesofacial and brachyfacial types had maximum of distoangular impactions (Table 1).Regarding the class of impaction, Class I and Class II impactions were found more in dolichofacial type and Class III in mesofacial type (Table 2).Concerning the position of impaction, the frequency of Position C was found to be more in mesofacial type, while Position A and B were more in dolichofacial type (Table 3).

DISCUSSION
According to Andreasen et al impaction of mandibular third molar varies from 18-32% in different populations. 10lela and Vitol reported the most frequently impacted teeth as third molar (90%) with the higher prevalence in mandible (60%) compared to maxilla (30%). 11In this study, the frequency of mandibular third molar impaction was found to be 63.8%.

Short mandibular length provides insufficient space
for third molar to erupt causing the impaction. 3,12oz et al demonstrated shorter length of mandible in dolichofacial patients predisposing to impaction. 13creased occurrence of mandibular third molar impaction was seen in dolichofacial type of face (49.2%) compared to mesofacial (40.2%) and brachyfacial (10.6%).This result is consistent with the previous studies by Bashir et al and Eroz et al . 6,13sioangular impaction was the most common type of impaction (46.9% on right and 47.3% on left) and distoangular was the least common type bilaterally (1.4%).This finding is in accordance to the study by Sokri et al. 7 Breik and Grubber showed that over 80% of the mandibular third molar impaction in all facial types was in mesioangular position. 14Viela and Vitol found that vertical position was most prevalent, followed by mesioangular position contrary to findings by Sandhu et al 11,15 and Venta et al 16 where mesioangular position was the most prevalent.
Limitation of our study was the sample size and sample type.As orthodontic patients are more likely to suffer malocclusion and potential crowding, they are also likely to possess higher occurrence of mandibular third molar impaction compared to general population sample.As the study was carried out in a limited sample, the findings cannot be generalized.

CONCLUSION
Greater horizontal facial growth pattern in brachyfacial subjects provides increased space for the full eruption of mandibular third molars.Decreased mandibular third molar impactions are seen in brachyfacial facial type as compared to mesofacial and dolichofacial.There is significant difference between facial form and presence or absence of mandibular third molar impaction.

A
cross-sectional study was performed on 207 pretreatment lateral cephalogram and orthopantomogram records of patients visiting Department of Orthodontics, Kantipur Dental College and Hospital.The study was conducted during September 2016 to January 2017 after receiving ethical approval from Institutional Review Committee.Inclusion criteria for the study were: complete patient records, pretreatment radiographs showing complete dentition, presence of mandibular third molars with at least two-third of root formation completed, and good quality radiographs.Patients with pathological conditions related to mandibular second and third molars such as extensive caries, cystic lesions were excluded.All the radiographs were evaluated by a single examiner.Nemoceph software (Dental Studio NX) was used to trace the lateral cephalograms whereas orthopantomograms were traced manually.The angle between Sella-Nasion line and Gonion-Gnathion line (mandibular plane) was measured using cepholometric protractor; which was termed as SN-GoGn angle or mandibular plane angle (Figure 1).This angle was used to determine different skeletal facial types based on Rickett's classification as shown in the following table.

Graph 1 :Graph 2 :
Occurrence of impacted mandibular third molar in different skeletal facial types (%) Angulations of third molar impaction on right and left sides (%) Sapkota MR, Bhatta S, Shrestha S, Shrestha RM : Position of Impacted Mandibular Third Molar in different Skeletal Facial Types