Introduction
The purposes of this study were to determine (1) how masticatory performance changes with age, (2) whether masticatory performance differs between the sexes, and (3) whether patterns of masticatory performance differ among subjects with various types of malocclusion.
Methods
A total of 450 children and adolescents (244 boys, 206 girls) were assigned to 4 age cohorts (ages 6, 9, 12, and 15 years) and followed for 3 consecutive years. The subjects were selected based on having about equal numbers of boys and girls, and about equal numbers of subjects with normal occlusion and Class I and Class II malocclusions. Masticatory performance was assessed by using the artificial food CutterSil (Heraeus Kulze, South Bend, Ind). The peer assessment rating index was used to quantify the severity of the malocclusions.
Results
Median particle size (MPS) decreased significantly from 6 to 17 years of age. There were no statistically significant differences in MPS between the 3 occlusal groups, but there were significant sex differences, with girls having smaller MPS than boys. Multilevel analysis showed greater decreases in MPS between 6 and 9 years, and after 12 years of age, than between 9 and 12 years of age. There were no significant correlations between MPS and the weighted peer assessment rating index. MPS showed significant intercorrelations between measures of MPS obtained at years 1, 2, and 3, with correlations tending to be highest for the oldest age cohort.
Conclusions
Masticatory performance improves with age, and the changes appear to be influenced by the loss of the deciduous teeth during the late mixed dentition phase of dental development. Although there are limited sex differences in masticatory performance among subjects 6 to 17 years of age, mild forms of Class I and Class II malocclusions have little or no effect on masticatory performance.
The first step in the digestive process is the breakdown of food (mastication). This process produces greater surface areas, which improve digestion and facilitate gastric emptying. It has been suggested that masticatory performance provides the most direct and perhaps best overall measure of masticatory function. Although both artificial and natural foods have been used to evaluate masticatory performance, artificial foods better fulfill the requirements of ideal foods. Various factors have been shown to influence masticatory performance, including the number of occluding pairs of teeth, areas of occlusal contact and near contact, and body size.
Age is a well-established determinant of masticatory performance; various cross-sectional studies have reported improvements of masticatory performance with increasing age. Shiere and Manly, who were among the first to evaluate this relationship, showed that 14-years-olds have better masticatory efficiency than 6-years-olds. More recently, Julien et al found that young women had better masticatory performance than 6- to 8-year-old girls; Henrikson et al found better performance in 15-year-old girls than in 11-year-olds. Most recently, Toro et al found the poorest masticatory performance among 6-year-olds, and increasingly better performance at 9, 12, and 15 years of age. Currently, no longitudinal data are available describing the temporal changes and variability in performance that occur.
The relationship between masticatory performance and sex remains controversial. Shiere and Manly and Toro et al found no significant sex differences in masticatory efficiency and performance in subjects 6 to 15 years of age. Maki et al also did not find sex differences for 7- and 8-year-olds, but showed that 9-year-old boys had significantly better masticatory efficiency than 9-year-old girls. The adult findings are also controversial. Julien et al found that 22- to 35-year-old men were better able to break down artificial foods than 22- to 29-year-old women, whereas Fontijn-Tekamp et al detected no adult sex differences in masticatory performance.
Although it is well established that masticatory performance is affected by occlusion, differences among subjects with various types of malocclusion remain controversial. Shiere and Manly found no differences in masticatory performance between Class I and Class II subjects; this might have been related to the rather crude methods they used to quantify efficiency. Henrikson et al and English et al found better masticatory performance for children with normal occlusion than in those with Class II malocclusion. In contrast, Toro et al reported that Colombian children with normal occlusion had marginally, but significantly, better masticatory performance than children with Class I malocclusions. However, they found no differences between children with normal and Class II malocclusions; this, they suggested, might have been due to the low severity level of the subjects’ malocclusions.
To date, there have been no longitudinal studies evaluating the relationship between masticatory performance and malocclusion. These studies are important because they provide direct information concerning within-subject changes and, more importantly, the variability of those changes. Such reference data make it possible to more accurately evaluate how treatment might affect masticatory performance. Longitudinal data also make it possible to evaluate how actual changes in performance are related to changes in malocclusion.
The goal of this study was to provide mixed longitudinal reference data for Colombian children and adolescents. The purposes were to determine (1) how the rate of improvement in masticatory performance changes with age, (2) whether performance differs between boys and girls, (3) whether performance is related to the type or severity of malocclusion, and (4) the consistency of the level of performance within subjects during growth.
Material and methods
For this study, we screened 2954 children who attended 3 private schools in different areas of Medellín, Colombia. The sample, which was self-selected based on their willingness to participate, included similar numbers of boys and girls representing 4 age cohorts: age-group cohort 6, between 5.5 and 6.5 years of age; age-group cohort 9, between 8.5 and 9.5 years; age-group cohort 12, between 11.5 and 12.5 years; and age-group cohort 15, between 14.5 and 15.5 years. The subjects were also selected based on their occlusal status, by using the following criteria.
Normal occlusion: Class I molar relationship with less than 3 mm of crowding, less than 3 mm overjet, and overbite covering less than one third of the mandibular incisors.
Class I malocclusion: Class I molar relationship with more than 3 mm of crowding, more than 3 mm of overjet, and overbite covering more than one third of the mandibular incisors.
Class II malocclusion: at least one half cusp Class II molar relationship.
Less than 0.5% of the subjects with normal occlusion, less than 1% of the subjects with Class I malocclusion, and 4.5% of the subjects with Class II malocclusion had more than 1 tooth in buccal crossbite. Subjects were rejected if they had congenitally missing teeth, signs or symptoms of temporomandibular dysfunction, previous orthodontic treatment, and any teeth with more than two thirds of their occlusal surface restored. A total of 450 children and adolescents (244 boys, 206 girls) met the criteria and were included in the study ( Table I ). Informed consent was obtained from all subjects and their parents. This study was approved by ethics committee of Corporacion para estudios de la salud (CES) University, Medellín, Colombia.
| Age-group cohort |
Sex | Normal occlusion (n) |
Class I malocclusion (n) |
Class II malocclusion (n) |
Total (n) |
|---|---|---|---|---|---|
| 6 | Male | 33 | 18 | 16 | 67 |
| Female | 26 | 12 | 16 | 54 | |
| 9 | Male | 18 | 14 | 29 | 61 |
| Female | 11 | 16 | 27 | 54 | |
| 12 | Male | 13 | 18 | 23 | 54 |
| Female | 19 | 17 | 13 | 49 | |
| 15 | Male | 20 | 21 | 21 | 62 |
| Female | 18 | 19 | 12 | 49 | |
| Total | 158 | 135 | 157 | 450 | |
Annual measurements were made over 3 consecutive years. At the first evaluation (year 1), the cohorts were 6, 9, 12, and 15 years of age; at the second evaluation (year 2), the same cohorts were evaluated at 7, 10, 13, and 16 years of age; at the last evaluation (year 3), the cohorts were 8, 11, 14, and 17 years of age ( Table I ). Approximately 22% and 14% of the original sample (year 1) were lost during the second and third evaluations, respectively ( Table II ). Some children changed schools, some did not attend school on the day of the measurements, some had begun orthodontic treatment, and others did not want to continue with the study.
| Age-group cohort | Sex | Year 1 (n) | Year 2 (n) | Year 3 (n) |
|---|---|---|---|---|
| 6 | Male | 67 | 56 | 48 |
| Female | 54 | 43 | 37 | |
| 9 | Male | 61 | 47 | 41 |
| Female | 54 | 38 | 32 | |
| 12 | Male | 54 | 43 | 32 |
| Female | 49 | 41 | 28 | |
| 15 | Male | 62 | 45 | 40 |
| Female | 49 | 38 | 28 | |
| Total | 450 | 351 | 286 |
Alginate impressions (New Stetic, Medellín, Colombia) were taken from all subjects, and stone models (Whip Mix, Louisville, Ky) were produced. The peer assessment rating (PAR) index was computed for each subject by using the stone models. According to the methods described by Richmond et al, the PAR index was based on the sum of 11 weighted components of malocclusion, including maxillary right, anterior, and left segments; mandibular right, anterior, and left segments; right buccal occlusion; overjet; overbite; centerline; and left buccal occlusion.
CutterSil (Heraeus Kulze, South Bend, Ind), a condensation silicone impression material, was used to measure masticatory performance. It was mixed according to the manufacturer’s instructions and shaped into round tablets, 5 mm thick and 20 mm in diameter, by using an acrylic plastic template. After hardening for 15 minutes, the tablets were removed from the template and allowed to set for at least 1 hour. The hardness of each tablet was measured by using a type A durometer (model 306L, PTC Instruments, Los Angeles, Calif) and maintained between 474 and 512 load g (62-65 in durometer units). Each tablet was then cut into quarters.
Each subject placed 3 of the quarter-sized tablets onto the tongue and was instructed to chew naturally for 20 chews. The investigator (L.M.B.) counted the number of chews. The subjects expectorated the chewed particles into a paper filter and rinsed with water until all particles were removed from the mouth. The rinsings were also collected in the filter. This procedure was repeated 5 times until approximately 10 g of CutterSil had been chewed and expectorated into the filter. The subjects were instructed to rest for 40 seconds between trials to prevent fatigue.
The chewed sample was dried in an oven for 1 hour at 80°C and then separated in a series of 7 sieves with mesh sizes of 5.6, 4.0, 2.8, 2.0, 0.85, 0.425, and 0.25 mm, and stacked on a mechanical shaker for 2 minutes. Once the sample was separated, the content of each sieve was weighed to the nearest 0.01 g. Cumulative weight percentages (defined by the amount of the sample that could pass through each successive sieve) were calculated for each subject. From these percentages, the median particle size (MPS) and the broadness of the particle distribution were estimated simultaneously by fitting the Rosin-Rammler equation
to each subject’s data, where Q w is the weight percentage of particles with a diameter smaller than x (for each of the 7 sieve apertures), x50 represents the MPS as the aperture of a theoretical sieve through which 50% of the weight can pass, and b is a unitless measure that describes the broadness of the distribution of the particles.
Statistical analysis
The skewness and kurtosis statistics showed normal distributions for MPS. Annual and biannual rates of change were computed by dividing the changes in MPS over time by the changes in age over time. Changes over time were statistically evaluated by using paired t tests. Sex differences were determined with Student t tests. The effects of malocclusion were assessed by using analysis of variance (ANOVA), followed by Bonferroni post-hoc tests.
The data were also modeled over time by using MLWin statistical software (Centre for Multilevel Modelling, Institute of Education, London, United Kingdom). The fixed part of each polynomial model described the changes over time and evaluated sex differences. To increase the numeric stability of the linear models, 10 years was subtracted from each subject age. Centering the values in this way reduced the size of individual terms when they were carried to a higher power in the models. The random part of the model partitioned variation into 2 levels, with subjects at the higher level and age, nested within subjects, at the lower level. Estimates were derived by using iterative generalized least squares.
Results
MPS decreased from approximately 4.2 mm 2 at 6 years to 3.1 mm 2 at 17 years of age ( Table III ). Although there were no statistically significant differences in MPS between the 3 occlusal groups, there were significant sex differences. Girls had significantly smaller MPS than did boys at 12 and 16 years of age. The multilevel analysis also showed significant sex differences; boys followed a sixth order polynomial between 6 and 17 years of age, whereas girls followed a fourth order polynomial ( Fig 1 ).
MP S males = 3.684 + ( .0428 ∗ age ) + ( .00852 ∗ ag e 2 ) − ( .0134 ∗ ag e 3 ) + ( .000388 ∗ ag e 4 ) + ( .000359 ∗ ag e 5 ) − ( .0000303 ∗ ag e 6 )
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