precise localization of the iac is important and successful implant placement or orthognathic surgery depends on the exact determination of the location of the inferior alveolar nerve ( ian ) . permanent or temporary ian damage can cause numbness of the lower lip and chin in orthognathic surgery [ 
 1 
 ] .
pressure on this nerve during implant placement is one of the common causes of treatment failure and postoperative pain . on some radiographs
, the iac has a cortical boundary , but in others , the canal may be mistaken for bone marrow especially in osteoporotic patients [ 
 2 
 ] .
in addition , the anatomical variation of the ian may be a factor that leads to the failure of block injections [ 
 3 
 ] .
recently , by use of cbct , high anatomical variability of this nerve was discovered [ 
 4 
 ] .
although the morphology and the position of the canal vary in different ethnic groups and in different types of jaws , these variations are ignored in many cases and cause problems in dental treatments [ 
 5 
 ] .
a detailed understanding of the factors affecting the anatomical variations of canal shape and morphology can minimize this problem .
therefore , in this study , we investigated the relationship between the ga size and course of the iac in human dry mandibles using cbct .
the results may be useful for more accurate localization of the iac on radiographs before dental treatment .
in this in vitro study , we obtained dry mandibles from the anatomy department of mashhad university of medical sciences .
samples with no anomaly or bone defects were assessed , and those in primary or mixed dentition were excluded [ 
 3 
 , 
 6 
  
 8 
 ] . a total of 25 dry adult human mandibles and 6 left and 5 right hemi mandibles ( a total of 61 hemi mandibles ) of unknown gender or origin were studied . to increase the accuracy and for easy tracing of the canal , a 0.5 mm diameter orthodontic wire was inserted into the iac before imaging [ 
 7 
 ] .
the cbct scans were obtained ( promax 3d , planmeca , helsinki , finland ) with 8080 mm field of view .
as the image field did not cover the entire mandibular bone , the stitch option of cbct was used , and an oral and maxillofacial radiologist , using romexis software 2.4.2.r ( planmeca inc .
intra - observer error was calculated by re - measuring a random sample of 30 images after an interval of two weeks .
both readings were then analysed by paired t - test . in the sagittal view of each ramus ,
line b was traced tangential to the most prominent point on the posterior border of the ramus and condyle .
the angle between lines b and d was measured as ga ( 
 fig . 1 
 ) [ 
 9 
  
 14 
 ] .
gonial angle measurements the scans were divided into the following two ga groups : low angle ( 125 ) and high angle ( > 125 ) [ 
 15 
 ] . to evaluate the shape and position of the canal in coronal views
, the distance between the mandibular foramen and the mental foramen was divided into three equal segments and the central cuts ( c1 , c2 , and c3 ) were measured in each segment [ 
 16 
 , 
 17 
 ] . in order to evaluate the canal shape , the superior - inferior and buccolingual diameters of the canal
were measured in each slice . to evaluate the buccolingual position of the iac , we measured the distances from the center of the iac to the tangential lines with buccal and lingual borders and to the lowest point of the inferior border ( fig .
2 ) [ 
 17 
 , 
 18 
 ] . in each slice ,
the distance from the buccal plate to the lingual plate was measured as buccolingual width of the mandibular body [ 
 16 
 , 
 19 
 ] .
consequently , in order to define the buccolingual position of the canal in each section , the ratio of the distances between the center of the iac and the buccal and lingual plates to the width of mandible in each section was calculated [ 
 16 
 ] .
evaluation of the buccolingual position of the iac by calculating the distance from the center of iac to buccal , lingual , and inferior borders in c1 , c2 , and c3 slices to investigate the total length of iac , the nerve was traced in coronal views . by using the serial measurements option ,
the total length of the nerve was exactly measured on the sagittal view ( fig .
3 ) . to investigate the canal course on the sagittal view , the nerve path was classified into three types of a , b , and c [ 
 7 
 , 
 17 
 ] . in type
a , the canal course had a straight path and was positioned at the same level of the mental foramen . in type b ,
the canal had a curved path , and in type c , the canal had a forward path and then ran superiorly to reach the mental foramen with a sharp ascent ( fig .
to evaluate the canal course on the axial view , the nerve path was traced and classified according to the mental foramen angle .
if the canal course made an acute angle with mental foramen (  90 ) , it was defined as type a1 ; otherwise , it was type a2 ( 

 fig . 5 

 ) [ 
 7 
 ] .
different types of canal course on the sagittal view type a : the canal course has a straight path at the same level as the mental foramen .
type c : the canal has a forward path and then runs up to reach the mental foramen with a sharp ascent . 

the canal course forms an acute angle with the mental foramen on the right side ( a1 type ) and an obtuse angle on the left side ( a2 type ) 
  statistical analysis was done using pasw version 18 ( spss inc . ,
paired - sample t - test was used for right and left symmetry and calculation of intra - observer agreement between the two readings .
pearson s correlation coefficient was used to examine the correlation between the right and left gas .
also , independent - samples t - test was used for quantitative comparison between different canal types on different views .
pearson s chi - square test was applied for evaluation of the correlation of canal course in the sagittal plane and ga . a p - value less than 0.05
intra - observer reproducibility for all variables was calculated by re - examining 30 scans .
the differences between measurements in the first and second readings were not significant ( p=0.160.35 ) .
the mean size of ga was 121.87.05 at the right side and 123.86.32 at the left side ( table 1 ) .
the results showed that there was a statistically significant correlation in the ga size at both sides ( p=0.000 , r=0.9 ) ; however , a significant difference was noted between them ( p=0.03 ) .
the mean  standard deviation of canal length at the right and left sides were 63.035.48 mm and 62.75.51 mm respectively .
there was no significant correlation in the mean total canal length in the right and left sides .
no statistically significant difference was found between the two sides in terms of canal lengths ( p=0.53 ) .
descriptive findings for gas sd : standard deviation after evaluating different sagittal views of the mandibular canal , it was obvious that the most common type of iac course on the sagittal view was type b ( 53.8% ) , followed by type c ( 26.2% ) and type a ( 20% ) . in 70% of the cases , there was no right - left symmetry in the course of the canal on the sagittal view .
in addition , it was clear that there was no significant difference between the canal length on the sagittal view , according to canal course ( p=0.59 ) .
the samples were grouped according to ga size as high angle ( > 125 ) and low angle ( 125 ) .
the gas were smaller than 125 and larger than 125 in 60.7% and 39.3% of samples , respectively .
as shown in table 2 , there was a significant relationship between different types of canal courses on the sagittal view and the different ga groups ( p=0.04 ) .
therefore , in the low ga group , type b was the more common canal course ( 73.5% ) , while type a was dominant in the high ga group ( 66.7% ) as shown in table 2 .
cross tabulation between different canal course types and ga groups on the sagittal view . after evaluating the canal course on the axial view
, type a1 was found to be more common ( 73.43% ) than type a2 ( 26.56% ) . according to the data in 10.34% of the cases , the canal type was different at both sides of the mandible .
there was no statistically significant relationship between the canal course on the axial view and ga group ( p>0.05 ) .
the total canal length was 63.255.4 mm in type a1 and 60.744.7 mm in type a2 .
no significant difference existed in the canal length in different types of canal course on the axial view ( p=0.09 ) .
evaluation of the iac course on the coronal view in segments c1 , c2 , and c3 showed that on the right side , the canal was closer to the lingual cortex in 86% of the cases in c1 , 100% in c2 , and 90% in c3 ; on the left side the corresponding percentages were 77% , 93% , and 93% of the cases , respectively . according to our results ,
the canal is positioned lingually throughout its path up to the mental foramen , and then reaches the mental foramen with an acute or an obtuse angle .
analysis of the diameters of iac showed that the greatest mean diameter in the superior - inferior and buccolingual dimensions was in c1 and the smallest was in c2 .
in this study , the gender of the individuals from whom the dried human mandibles were obtained was unknown .
many previous studies , including those conducted by de oliveira - santos et al , [ 
 4 
 ] ozturk et al , [ 
 7 
 ] kisser et al , [ 
 8 
 ] liu et al , [ 
 17 
 ] apinhasmit et al , [ 
 20 
 ] angel et al , [ 
 21 
 ] and raustia and salonen [ 
 22 
 ] demonstrated that the position of some anatomic landmarks , such as iac and ga , is not related to gender or age .
the mean size of ga in this study was similar to that found in earlier studies 
 [ 
 9 
  
 13 
 , 
 23 ] 
 .
the mean ga of the right side was 2 smaller than that in the left side , and this difference was statistically significant .
the difference in the mean size of the right and left ga was mentioned in the study by raustia and salonen [ 
 22 
 ] , who stated that the right ga was significantly smaller than the left one .
when we analysed the canal shape in the coronal sections , we found that ovoid shape was more common than other shapes .
similar to ozturk et al , [ 
 7 
 ] we found that type b was the most common type of canal course on the sagittal view , followed by types c and a. liu et al .
[ 
 17 
 ] classified the canal course into four groups ( types 1 , 2 , 3 , and 4 ) using panoramic radiography [ 
 17 
 ] .
the shape of the canal in type 1 was similar to type a in the current study and the study carried out by ozturk et al [ 
 7 
 ] .
types 2 and 3 , which had a catenary like path , were similar to type b , and type 4 was similar to type c. in the study by liu et al , [ 
 17 
 ] the frequency sum of types 2 and 3 was greater than that of type b , but the frequency of types 1 and 4 was lower than that of types a and c [ 
 17 
 ] .
the difference in the prevalence rate of iac course shape in the study by liu et al , [ 
 17 
 ] compared to that observed by ozturk et al , [ 
 7 
 ] and in the current study , may be due to the difference in the type of imaging system and methodology of the studies , and may also be because of the difference in ga [ 
 7 
 , 
 17 
 ] .
there was no difference in the total canal length between types a and b ; however , there was a high prevalence ( 66.7% ) of type a in the high - angle group , while type b was dominant ( 73.5% ) in the low - angle group .
it is analogous to a situation in which you have two identical ropes ; if you tie the ends of one of the ropes , its path will be bent , whereas the other rope would have a more direct route .
there was no difference in the canal length observed in our study and that found by liu et al [ 
 17 
 ] .
although the method to obtain the total canal length used by liu et al . was via the tracing cord with a 5 mm arch , while we calculated the length using a specific type of software ; the average length found in both studies was very similar [ 
 17 
 ] .
introduced four types of canal course on the axial view , and two of these ( types b and c ) were introduced for the first time [ 
 7 
 ] .
similar to liu et al , [ 
 17 
 ] we did not observe these types in the current study , but type a1 was the most commonly found , which is in accordance with the results of ozturk et al [ 
 7 
 ] . in the current study ,
the iac was located near the lingual plate throughout its path , and then travelled to the buccal plate with a sharp ascent near the mental foramen in the majority of samples .
ozturk et al , [ 
 7 
 ] hwang et al , [ 
 19 
 ] som and curtain [ 
 24 
 ] , kim et al , [ 
 25 
 ] and fabian [ 
 26 
 ] also found that the iac is close to the lingual plate , and then runs to the buccal plate near the mental foramen [ 
 7 
 , 
 19 
 , 
 24 
  

hwang et al . showed that the iac is close to the lingual plate in the posterior two thirds of the mandible and runs towards the buccal side in the anterior third [ 
 19 
 ] . in our study , the entire canal course was of a1 and a2 types on the axial view , which shows that the canal is in the lingual plate , then in front of the mental foramen , it reaches with a sharp turn to the buccal side [ 
 7 
 , 
 19 
 ] .
the canal was located almost 1 cm above the inferior border of the mandible in the second premolar and the first molar region , which appears to provide the greatest superior - inferior dimension for implant placement .
showed that the least distance to the lower border was almost 1 cm in the first molar region [ 
 7 
 ] . 
 
 according to the results of the current study , the iac has the greatest diameter in the ramus region ( behind the third molar ) and the smallest diameter in the molar region .
de oliveira - santos et al . studied canal diameter in the first molar region and their conclusion was in accordance with our findings [ 
 4 
 ] .
no other study has calculated iac diameter on the coronal view . in this study we were not able to identify the sex of subjects whose dry mandibles were evaluated ; thus further studies on human mandibles are suggested taking into account the sex and age parameters .
we concluded that : 
 1-the ga size has a correlation with the iac course . in subjects with small gas
but in cases with large gas , the canal is more straight and at the same level as the mental foramen.2-the alveolar crest has the maximum distance from the iac in the second premolar and first molar region . 
 
the ga size has a correlation with the iac course . in subjects with small gas
but in cases with large gas , the canal is more straight and at the same level as the mental foramen .
the alveolar crest has the maximum distance from the iac in the second premolar and first molar region .
since the iac is located in the lingual side of the mandible , the safe zone for implant placement and pre - prosthetic surgery is believed to be the buccal side .
in addition , the greatest height of bone can be obtained in the second premolar and first molar regions .
 objectives : accurate localization of the inferior alveolar canal ( iac ) is extremely important in some dental treatments . 
 anatomical variation of the canal means that it can be difficult to locate . 
 the purpose of this study was to assess the relationship of the gonial angle ( ga ) size and iac position using cone beam computed tomography ( cbct).materials and methods : in this in vitro study , 61 dry adult human hemi - mandibles were used . 
 the cbct scans were taken of all samples and ga was measured on all cbct scans . 
 the samples were divided into two groups of low angle ( 125 ) and high angle ( > 125 ) . 
 the canal dimensions , length and course were evaluated . on the sagittal view , 
 the iac path was classified as type a , b or c. on the axial view , canal course was defined as a1 or a2 according to the mental foramen angle.results:the average ga size was 121.87.05 at the right side and 123.86.32 at the left side . on the sagittal view 
 , there was a significant correlation between the ga size and the canal course ( p=0.04 ) . 
 in the high - angle group , type a was dominant ; whereas in the low - angle group , type b was more common . on the axial view of iac course , type a1 was more common ( 73.43%).conclusion : the results showed that ga size was associated with iac course . in cases with a larger ga , the canal runs in a more straightforward path , and at the same level as the mental foramen . 