the size and form of the dental arches vary among individuals according to tooth size , tooth position , pattern of craniofacial growth and by several genetic and environmental factors .
survey of arch size could help clinicians in the selection of stock trays , the size of artificial teeth , and the overall forms of artificial dental arch at the wax trial stage are amenable to modification by the dental surgeon and in orthodontic treatment .
as orthodontics has advanced as a specialty and the number of adults seeking orthodontic care has increased , an understanding of the changes that normally take place in adult craniofacial structures becomes critical .
a systematic and well - organized dental care program for any target population in a community requires some basic information , such as the prevalence of the condition . in the more developed parts of the world , where
the specialty of orthodontics has been established , adequate baseline information is available . despite efforts in the arab world during the last decades to make health systems more equitable
access to dental healthcare is still far from adequate , especially in poor communities . in yemeni population
hence , this study has been designed to provide a baseline data on the mandibular dimensions of yemeni adults with normal normal dento - skeletal relationship , aged ( 18 - 25 ) years .
ethical approval was obtained from both the ethics committee of sanaa university and the faculty of dentistry at the university of sanaa .
a brief outline of the study was explained to all participants and consent was obtained prior to participation .
the sample of this study consists of 214 adults aged between 18 and 25 years , among whom 113 were males and 101 females , selected from clinical examination of 765 yemeni adults , comprised of 387 males and 378 females .
all subjects considered for the sample have the following dental and skeletal features 
 full complement of normal shaped permanent teeth ( excluding third molars ) with no heavy fillings , congenital missing teeth , retained deciduous , and supernumerary teeth.class i molar and canine relationships and class i skeletal relationship , decided visually by using the two - finger technique.normal vertical and horizontal dental relationships ( normal overjet and overbite).no previous orthodontic , orthopedic , facial surgical treatments and no history of bad oral habits such as thumb sucking or mouth breathing.well-aligned arches with less than 3 mm of spacing or crowding in either arch
. 
  full complement of normal shaped permanent teeth ( excluding third molars ) with no heavy fillings , congenital missing teeth , retained deciduous , and supernumerary teeth .
class i molar and canine relationships and class i skeletal relationship , decided visually by using the two - finger technique . normal vertical and horizontal dental relationships ( normal overjet and overbite ) .
no previous orthodontic , orthopedic , facial surgical treatments and no history of bad oral habits such as thumb sucking or mouth breathing .
well - aligned arches with less than 3 mm of spacing or crowding in either arch .
all the individuals examined under natural light with interchangeable plane mouth mirrors . during this examination , each individual was seated on an ordinary chair with his head being positioned so that the frankfort horizontal plane is parallel to the floor .
the selected individuals were subjected to a thorough clinical examination to reassure the fulfillment of the required sample specifications .
certain selected tooth - related points visible in an occlusal view were marked bilaterally with a sharp pencil in the mandibular study casts .
great care was taken to ensure that the landmarks were accurately located on the study casts .
measurements were taken from 214 mandibular dental casts , which were made of dental stone , with the base , made of plaster of paris .
dental arch dimension measurements were carried out using the modified sliding caliper gauge , which is accurate up to 0.02 mm .
a - inter - canine distance : the linear distance from cusp tip of one canine to the cusp tip of the other .
dental model measurements , inter - canine width ( a ) , inter - first molar width ( b ) , inter - second molar width ( c ) , anterior arch length ( d ) , molar arch length ( e ) , total arch length ( f ) b - inter - first molar distance : the distance from the mesiobuccal cusp tip of one first permanent molar , to the mesiobuccal cusp tip of the other .
c - inter - second molar distance : the distance between the disto - buccal cusp tips of one second permanent molar , to the disto - buccal cusp tip of the other .
d - anterior arch length : the vertical distance from the incisal point to the inter - canine distance at the cusp tip .
e - molar arch length : the vertical distance from the incisal point perpendicular to a line joining the mesiolingual cusp tip of first permanent molars .
f - total arch length : the vertical distance from the incisal point to the midpoint of a line joining the disto - buccal cusp tip of the second permanent molars .
descriptive statistics were performed for the calculation of the mean , standard deviation , minimum , maximum , range and coefficient of variation ( cv ) .
the t - test was applied to test the level of significance between the mean for males and females for all mandibular arch dimensions . the spearman 's correlation coefficient ( r ) was also calculated between the measurement of arch widths and lengths in female and males .
a - inter - canine distance : the linear distance from cusp tip of one canine to the cusp tip of the other .
dental model measurements , inter - canine width ( a ) , inter - first molar width ( b ) , inter - second molar width ( c ) , anterior arch length ( d ) , molar arch length ( e ) , total arch length ( f ) b - inter - first molar distance : the distance from the mesiobuccal cusp tip of one first permanent molar , to the mesiobuccal cusp tip of the other .
c - inter - second molar distance : the distance between the disto - buccal cusp tips of one second permanent molar , to the disto - buccal cusp tip of the other .
d - anterior arch length : the vertical distance from the incisal point to the inter - canine distance at the cusp tip .
e - molar arch length : the vertical distance from the incisal point perpendicular to a line joining the mesiolingual cusp tip of first permanent molars .
f - total arch length : the vertical distance from the incisal point to the midpoint of a line joining the disto - buccal cusp tip of the second permanent molars .
descriptive statistics were performed for the calculation of the mean , standard deviation , minimum , maximum , range and coefficient of variation ( cv ) . the t - test was applied to test the level of significance between the mean for males and females for all mandibular arch dimensions . the spearman 's correlation coefficient ( r )
was also calculated between the measurement of arch widths and lengths in female and males .
the means and standard deviations ( mean  sd ) of the mandibular dimensions are shown in table 1 . the minimum and maximum values were recorded and expressed by the range .
it can be noticed that the inter - second molar distances have the widest range while the lowest difference existing in the anterior arch length .
mean and standard deviation ( sd ) of mandibular arch widths ( mm ) and lengths for the sample table 2 depicts the mandibular arch dimensions according to gender .
overall , the male group displayed greater transverse and sagittal mandibular dimensions ; however , this was only statistically significant for measurements of inter - first and second molar distances and anterior arch length ( p < 0.05 ) .
mandibular arch dimensions ( mm ) according to gender the correlation coefficient was calculated between arch widths and lengths ; some of them were highly significant , positive and direct relationship was noted , others showed moderate , weak or negative relationships [ table 3 ] .
relationship between the mandibular widths and lengths shown with the corresponding spearman correlation coeficients ( r ) statistically significant correlations between the inter - canine distance and mandibular arch lengths were noted , but the r values were not greater than 0.5 .
in the present study , the measurements taken for the dimensions of the mandibular arch , confirmed the accepted view that male dental arches are greater than that of females . in most studies , the arch dimensions depended on the gender of the subjects , with smaller values in females .
this may be attributed to the smaller bony ridge and alveolar process of females , the average weakness of musculature in females that play an important role in facial breadth measurements , width and height of the dental arch and the later growth period in males than females .
the findings of this study showed statistically significant difference between males and females for some measurements , this confirms those results previously published by many investigators . while it contradicted with other investigators .
kuntz reported that both sexes had nearly similar inter - canine distance and ismail et al . reported that females had larger widths than males , but the difference was not statistically significant
the largest difference between males and females existed in the inter - second molar distance , which might be attributed to the differences in arch form in the sample . in this study
, it seems to indicate also that male group had greater mandibular dimensions in the sagittal direction than female group , this difference was statistically significant in the anterior arch length , which agree with the finding of raberin et al . and borgan . on the other hand , this difference was statistically insignificant in the molar arch length and total arch length , which does not agree with the finding of raberin et al . and borgan .
the difference with the other studies may be attributed to different ethnic groups , sample sizes and environmental factors .
the craniofacial and dental dimentions represent a highly complex interaction between numerous genetic and environmental factors .
overall , the correlation coefficient values were less than 0.5 indicating that the relationship is at best indicating a moderate linear relationship .
studying the descriptive analysis for the mandibular arch width revealed that the strongest relathinships were observed between the inter - canine distance and total arch length , molar arch length and anterior arch length .
it could be also seen that the cv values for all measurements were nearly close to each other , with the anterior arch length distances showing the higher cv than the others .
the anterior arch length values share the inter - canine distance in the contribution of determining all types of the dental arch forms .
thus , it is not unexpected for the anterior arch length to have the highest coefficient of variation among other dimensions .
the craniofacial and dental dimentions represent a highly complex interaction between numerous genetic and environmental factors .
overall , the correlation coefficient values were less than 0.5 indicating that the relationship is at best indicating a moderate linear relationship .
studying the descriptive analysis for the mandibular arch width revealed that the strongest relathinships were observed between the inter - canine distance and total arch length , molar arch length and anterior arch length .
it could be also seen that the cv values for all measurements were nearly close to each other , with the anterior arch length distances showing the higher cv than the others .
the anterior arch length values share the inter - canine distance in the contribution of determining all types of the dental arch forms .
thus , it is not unexpected for the anterior arch length to have the highest coefficient of variation among other dimensions .
based on this study it could be said that among studied mandibular dimensions in subjects with normal dento - skeletal relationship , the inter - canine distance showed the strongest linear relationship with the mandibular arch sizes .
 objective : to assess the relationship between mandibular arch length and widths in a sample of yemeni subjects aged ( 18 - 25 ) years.materials and methods : the investigation involved clinical examination of ( 765 ) adults ; only 214 ( 101 females , 113 males ) out of the total sample were selected to fulfill the criteria for the study sample ( normal dento - skeletal relationship ) . 
 study models were constructed and evaluated to measure mandibular arch dimensions . the spearman 's correlation coefficient ( r ) 
 was calculated between the measurements of arch widths and lengths.results:overall , the male group demonstrated greater transverse and sagittal mandibular dimensions ; however , this was only statistically significant for measurements of inter - first and second molar distances and anterior arch length ( p < 0.05 ) . 
 relatively stronger linear relationships were observed between the inter - canine distance and mandibular arch lengths ( p < 0.05 , spearman 's r ranged between 0.17 to 0.50).conclusion : among studied mandibular dimensions in subjects with normal dento - skeletal relationship , only the inter - canine distance demonstrated a week to moderate linear relationship with the mandibular arch lengths . 