mouth opening is a clinical parameter , which we encounter routinely in our daily practice . a known normal range of mouth opening is necessary to enable the clinician conduct a thorough oral examination conveniently . a reduced mouth opening capacity ( moc ) may be one of the first clinical signs of pathological changes and traumatic conditions in the masticatory system . early recognition of decreased or limited mouth opening is necessary for a prompt and efficient approach to diagnosis and to plan out the treatment options judiciously . to make a diagnosis of decreased mmo , it is essential to establish what constitutes the normal mmo for children of different age . research has shown that the measurement of mouth opening varies significantly with age , gender , and race . numerous studies across the world have characterized the mmo in their adult as well as pediatric population . however , very little data exist on normal mmo in the indian pediatric population . in a growing population , furthermore , most existing datasets are of limited value because they do not cover the entire age range . despite these limitations , the existing normal values indicate a significance of growth on the mmo and a wide range of normalcy within a certain age category . in india , it is fairly common to report the mouth opening in terms of finger breath in our daily practice . understandably , this is semi - quantitative measurement subject to many variations . hence , establishing numerical value will be of significant value . the present study aims to establish age- and height - related percentiles for the mmo of healthy children and adolescents which may serve as a basis for various studies and will have the clinical utility in treatment and diagnosis of diseases directly or indirectly affecting the mouth opening . the present study was carried out in the pediatric outpatient clinic in a tertiary care center in western india . a total of 985 children , 560 males and 425 females , in the age range of 518 years were studied . the children were asked to open their mouth maximally unassisted till no further opening was possible . mmo was measured when the children rested their heads against a firm wall surface in an upright position . the distance from the incisal edge of the upper incisor teeth to the incisal edge of the lower incisor teeth was measured using a calibrated fiber ruler , and the findings were recorded in millimeters [ figure 1 ] . three readings were recorded for each individual , and their average was recorded as the final reading . to control for inter - examiner and intra - examiner reliability , the age , sex , height , and weight of all the children were recorded . method of measuring interincisal distance with fiber ruler the study included all the children between the ages of 518 years of age attending pediatric outpatient clinic , having all the incisors and able to understand and cooperate with the investigators . exclusion criteria for the study were children with ( 1 ) habit of smoking , tobacco , or betel nut chewing , ( 2 ) no natural front teeth , ( 3 ) previous maxillofacial trauma , ( 4 ) oral malignancies , ( 5 ) angular cheilosis or oral ulcers affecting mouth opening , ( 6 ) chronic systemic disease , and ( 7 ) pain while opening mouth . this study was approved by the research ethics committee of the institute . as children involved in the study were minors , weight and height for each child were obtained using a standard calibrated anthropometric scale , with precision of 100 g and 1 mm , respectively . mean mmo values were analyzed according to age , height , and weight ranges , which were established appropriately for statistical analysis . hence , the sample was divided according to age and height : 13 age groups ( between 5 and 18 years ) and 10 height groups of 10 cm each ( 8695 cm ; 96105 cm ; 106115 cm ; 116125 cm ; 126135 cm ; 136145 cm ; 146155 cm ; 156165 cm ; 166175 cm ; 176185 cm ) were assigned . continuous variables are reported as mean  standard deviation . to determine correlation of mouth opening with age and height , a linear regression analysis was performed and statistical significance was tested by the application of t - test and f - test . the statistical analysis has been performed using open source software r version 3.0.1 . of r foundation , vienna , austria . mean mmo values were analyzed according to age , height , and weight ranges , which were established appropriately for statistical analysis . hence , the sample was divided according to age and height : 13 age groups ( between 5 and 18 years ) and 10 height groups of 10 cm each ( 8695 cm ; 96105 cm ; 106115 cm ; 116125 cm ; 126135 cm ; 136145 cm ; 146155 cm ; 156165 cm ; 166175 cm ; 176185 cm ) were assigned . continuous variables are reported as mean  standard deviation . to determine correlation of mouth opening with age and height , a linear regression analysis was performed and statistical significance was tested by the application of t - test and f - test . the statistical analysis has been performed using open source software r version 3.0.1 . of r foundation , vienna , austria . our study included a total of 985 children with 560 males and 425 females among them . the median age for the entire sample is 10.5 years , and median height was 130 cm for girls and 130139.99 cm for boys . the mean mmo was 43.5  5.19 for girls and 44.24  5.84 cm for boys . table 1 presents the number of children in each age group with mean moc values ( mm ) for indian children . table 2 presents the mean moc values ( mm ) for the indian children in each height group . the scattered diagram relation of moc with age and height was plotted [ figure 2a and b ] . number of individuals in each age group for boys and girls with mouth opening capacity values ( mm ) variation in mouth opening with height ( a and b ) the scattered diagram showing relation of mouth opening capacity with age and height diagrammatic representation of mean mouth opening capacity value according to the height there was a significant difference between the moc of male and female in all age groups . a linear relationship was observed between the mouth opening and age but more significantly with height in entire sample , with correlation coefficient of 0.63 , which showed that moc increases as the age and height increases , and this relation was found to be statistically significant ( p < 0.0001 ) . as the height reflects the skeletal growth , we also found that the mmo is more significantly associated with height than age . the dataset was entered into the r program for the creation of age - related percentiles . best results were achieved using the following settings : l = 1 , i.e. , l constant , set to l = 0.65 by the program , m = 5 , s = 2 for girls , and l = 1 , set to 0.5 by the program , m = 7 , s = 2 for boys . table 3 depicts four linear regression models evaluating impact of age ( model 1 ) , height ( model 2 ) , age and height ( model 3 ) , and age , height , and gender ( model 4 ) on moc . in isolation , age and height significantly impact mouth opening . however , impact of height on mouth opening is more significant than that of age . further , when age and height were considered together , it was only height that had influenced moc and not age . norms  could be designated for age with gender and height with gender [ tables 47 ] . age - related percentiles for moc in boys and girls are given in figure 4a and b. height - related percentiles for moc in boys and girls are given in figure 5a and 5b . linear regression models for mouth opening capacity age - related percentiles of mouth opening capacity for males ( in mm ) age - related percentiles of mouth opening capacity for females ( in mm ) height - related percentiles of mouth opening capacity for males ( in mm ) height - related percentiles of mouth opening capacity for females ( in mm ) ( a and b ) age - related percentiles for mouth opening capacity in boys and girls ( a and b ) height - related percentiles for mouth opening capacity in boys and girls the dataset was entered into the r program for the creation of age - related percentiles . best results were achieved using the following settings : l = 1 , i.e. , l constant , set to l = 0.65 by the program , m = 5 , s = 2 for girls , and l = 1 , set to 0.5 by the program , m = 7 , s = 2 for boys . table 3 depicts four linear regression models evaluating impact of age ( model 1 ) , height ( model 2 ) , age and height ( model 3 ) , and age , height , and gender ( model 4 ) on moc . in isolation , age and height significantly impact mouth opening . however , impact of height on mouth opening is more significant than that of age . further , when age and height were considered together , it was only height that had influenced moc and not age . norms  could be designated for age with gender and height with gender [ tables 47 ] . age - related percentiles for moc in boys and girls are given in figure 4a and b. height - related percentiles for moc in boys and girls are given in figure 5a and 5b . linear regression models for mouth opening capacity age - related percentiles of mouth opening capacity for males ( in mm ) age - related percentiles of mouth opening capacity for females ( in mm ) height - related percentiles of mouth opening capacity for males ( in mm ) height - related percentiles of mouth opening capacity for females ( in mm ) ( a and b ) age - related percentiles for mouth opening capacity in boys and girls ( a and b ) height - related percentiles for mouth opening capacity in boys and girls patients with temporomandibular joint diseases , craniofacial syndromes , maxillofacial trauma , oral malignancies and those who have been treated for these conditions often have complaint of restricted mouth opening . as with any other disease or condition , the aim of treatment of disorders affecting mouth opening is to restore the mouth opening to its normal value , hence the relevance of establishing normal values . a large number of methods have been described in the literature to measure the mouth opening . the measurement most often used to assess mouth opening is the interincisal distance attained during active opening by the subject . this method underestimates the movement of the mandible as it does not include the overbite . however , as pointed by mezitis et al . , the functional opening of the mouth is more important . sufficient mouth opening to allow normal social function is clinically important for the patients and adequate access to the oral cavity for clinicians , which in effect is the interincisal distance without consideration for the overbite . hence , the interincisal distance has been used as a measurement of mmo in this study . an advantage of the incisal edge distance is that the measuring point is relatively more consistent , permanent , and more easily determined . wood and branco examined three methods of measuring interincisal distance and concluded that direct measurement using a ruler was the most accurate . described how the mmo decreases in order of forward , natural , and retracted head positions . in the present study , the mmo was measured when the subjects rested their heads against a firm wall surface in an upright position to eliminate the possible influence of different head positions . mouth opening is influenced by a number of factors in children as they are in the period of skeletal growth . the age , weight , height , gender , and race affect the mouth opening . rothenberg noted a significant relationship between the maximum vertical opening of the mouth and age , among 189 caucasian children , aged 414 years . the present study also reported a gradual increase in mmo in different age groups of children . a few studies have observed a gender difference in mmo among children . in the present study , a statistically significant difference was observed in between boys and girls in various age groups . the study also revealed a definitive correlation between mmo and height as well as age . rothenberg also observed a positive correlation between mmo values in relation to weight and height in children aged 414 years of age group . the observed gradual increase in mmo with increasing age , height , and body weight , as found in the present study , is due to changes in the temporomandibular joint apparatus , facial morphology , muscle development , and growth of cranial base and mandible , particularly in length . although age has the significant influence on the mmo values , a definite and stronger correlation exists between mmo and height . children with poor nutrition often have stunted growth , and their skeletal growth is hampered . in such cases , measurement of mmo will not correspond to the established norm for the age since the height is unaccounted for , in these malnourished children from lower socioeconomic group . in such cases , to measure height by age can give a false - positive value of reduced mouth opening . this study is therefore providing the normal percentile for different height groups for the first time . these data are important because a high number of complex surgical procedures are routinely performed in this part of the world without a proper reference value for established norms . assessment of mouth opening is an important part of clinical examination for the physician , surgeon , or clinician involved in the treatment of head and neck disorders and dental disorders . to diagnose an abnormality , knowledge of normal values is very important . reference to international data is obviously not justified as the mouth opening is clearly varied among different ethnicities . to the best of our knowledge , this data sample of 985 unselected children covers the entire age range of indian children where unassisted mouth opening can be measured . we believe that these percentiles will therefore be of great importance for future studies as well as clinical assessment of indian children with affections of the masticatory system .
background and aim : maximal mouth opening ( mmo ) is used as a marker of masticatory pathology . 
 however , mmo among children varies considerably with their age , height , sex , and race . while accurate percentile of normal mouth opening and relationship with anthropometric measurement are not precisely defined for the indian population , we designed prospective , observational study to define the percentiles for normal mmo in our children.methods:a total of 985 children , 560 males and 425 females , in the age range of 518 years attending the pediatric clinic in a tertiary care center in western india were studied . 
 in addition to the basic demographic data , mmo was measured in these children . 
 the children were asked to open their mouth maximally until no further opening was possible . 
 the distance from the incisal edge of the upper incisor teeth to the incisal edge of the lower incisor teeth was measured using a calibrated fiber ruler . 
 statistical analysis was performed to assess the impact of other anthropometric measures such as age , gender , and height on mmo.observations:the mean mmo for males was 44.24 ( 5.84 ) mm and for females was 43.5 ( 5.19 ) mm . 
 age- and height - related percentiles were created for girls and boys separately , showing the 5th , 10th , 25th , 50th , 75th , 90th , and 95th percentiles from 5 through 18 years of age with 86185 cm height.conclusion:the mmo percentile range for different age and height groups is established for the normal children . 
 the mouth opening seems to increase with the age and especially with the height as per the skeletal growth . 
 height affects mouth opening more than the age .