Investigation of dynamic occlusal contacts when food particles are being pulverized during chewing is of interest for many researchers and clinicians. However, measurement of dynamic occlusal contacts during chewing is difficult, and differences between children and adults have not been established. The purpose of this study is to test the hypothesis that dynamic occlusal contacts of children differ from those of adult females.
and methods: Thirteen healthy children (4–6 years of age; mean age 5 years, 7 months) and thirteen adult females (18–26 years of age; mean age 20 years, 7 months) with normal occlusion participated in this study. Occlusal contact area (OCA) was estimated with a developed measurement system combining 3-D digitization of tooth shape with 3-D tracking of mandibular movements (1) during the closing stroke, (2) at the maximum closing position (MCP), and (3) during the opening stroke. OCA at static maximum intercuspation (ICP) was also estimated.
At the MCP, the children’s OCA was less than 76.4% of the contact area seen at the ICP. The timing of maximum OCA in children was shifted more towards the opening stroke compared with adults, and the OCA remained greater during opening in children than adults. The occurrence of the MCP was less stable in children than in adults, both between subjects and within subjects.
We conclude that both the amount of OCA and the pattern of occlusal contacts during the occlusal phase of chewing completely differ between children and adult females.
Stomatognathic function involves the coordinated movement of hard tissues (i.e., the teeth, temporomandibular joints and mandible) by the muscles of mastication, the muscles of facial expression and the neural mechanisms that control them. One previous study reported that occlusal contact area (OCA) was an important factor for better chewing performance . However, there were few reports on factors that might contribute to occlusal contacts during mastication in children with primary dentition, because the reproducibility in children is lower than in adults during chewing . Japanese elementary school children (7-12 y) who liked hard foods such as cabbage and celery had higher bite forces and greater OCA measured with the Dental Prescale® than those who disliked these foods . The authors suggested that a positive attitude towards harder food items might contribute to healthy development of the masticatory apparatus.
A number of studies have examined how occlusal contacts between the upper and lower teeth during chewing, especially the occlusal phase, change in some critical stages of food breakdown. The greatest number of occlusal contacts occurs at the static intercuspal position (ICP) in adults , and the jaw closing muscles are capable of exerting maximum masticatory force at this position . Moreover, a strong positive correlation was found between the degree of intercuspal contact and the force of chewing strokes . Greater OCA, measured with bite registration materials, was related to better masticatory performance . Subjects with larger areas of contact and near contact were better able to break down foods . Miura et al. showed a significant correlation between the total contact area and the mastication score . Nakata et al. showed that the OCA of young adult patients with prognathism was smaller than that of adults with normal occlusion, and that OCA changes after orthodontics combined with surgical treatment . Although the methods described above are relatively easy to apply and evaluate in clinical use, they can only evaluate OCA in some static positions ; none can reliably evaluate the dynamic change of the occlusal contacts during chewing.
In a previous study, we showed that the estimated length of the occlusal glide during closing was significantly shorter in children than in adult females, contrary to their respective glide lengths during opening . Moreover the range of lateral excursion during chewing in children was smaller than adult females , suggesting that children with a developing nervous system might have difficulty controlling asymmetric muscle activity . Although these obtained results indicate that the chewing pattern in children is completely different from adults, the measurement of occlusal contacts during chewing is difficult, and clear differences in OCA between preschool children and adults have not been established. The purpose of this study was to test the null-hypothesis that the occlusal contacts during chewing in children were not different from those in adult females.
Subjects and methods
Thirteen healthy children with primary dentition (none had cavities or fillings including sealant: 4–6 years of age; mean age 5 years, 7 months) and thirteen adult females with permanent dentitions (none had third molars or large fillings, including crowns or fixed partial dentures: 18–26 years of age; mean age 20 years, 7 months) with normal occlusion participated in this study. Only adult females were selected to exclude variability in chewing because of gender differences . On the other hand, children had no gender differences in this study. For a subject to be included, they had to meet the following criteria: (1) no pain during temporomandibular joint and muscle palpation, (2) no joint sounds, (3) a maximum opening greater than 30 mm in children and 40 mm in adults, and (4) deviations or deflections less than 2.0 mm . All experiments were approved by the Ethics Committee for Human Research at Kyushu University, and all our experiments complied with its ethical policies and regulations. Prior to entering this study, informed consent was obtained from all subjects.
Each subject was given a stick of chewing gum (New TRIDENT, Warnar Lambart, 1.6 g). Gum was chewed until soft before beginning the recordings for almost 20 s. Starting from the ICP, each chewing sequence was recorded over a period of 20–30 s. Subjects chose their own pace for chewing. In adults, it averaged 0.67 s/cycle, and in children averaged 0.61 s/cycle in this study. Chewing gum was chosen because it formed a more consistent bolus than real food, producing a more consistent masticatory pattern over many cycles .
Details about the measuring system have been reported previously , however, a brief description follows. Silicon-based registrations (Exafine, GC Co., Inc., Tokyo, Japan) were taken and poured immediately in dental stone (New Fujirock White, GC Co., Inc., Tokyo, Japan). Morphologic data from the resulting dental models were measured using an automatic 3-D digitizer ( Fig. 1 A, B) (TRISTATION 400FE: NIKON INSTEC, Co., Inc., Tokyo, Japan). This contact-type digitizer records the three-dimensional coordinates of the surfaces of the dental models at 0.2 mm intervals ( Fig. 1 C). Once all coordinates (about 40,000 points on each dentition) were recorded, a mathematical data mesh was constructed with intervals of 0.2 mm ( Fig. 1 D).
Chewing movement was measured using an optoelectronic analysis system (TRI-MET, Tokyo-Shizaisha, Tokyo, Japan) with 6 degrees-of-freedom ( Fig. 1 E, F). The accuracy of this optoelectronic instrument in bench tests is better than 0.19 mm . Each subject was asked to maintain their static intercuspal position with a slight force between the teeth for two seconds of recording of ICP.
Data from both the TRIMET and TRISTATION were transformed to a newly defined coordinate system with its origin at the mesial tip of the lower left central incisor at the ICP. The x-y plane (horizontal plane) extended from the origin to the tips of the right and left distal cusps of the lower first molars.
A special computer program automatically identified each maximum opening position of the lower incisor and the two maximum closing positions (MCP) on either side of the maximum opening. Based on these maximums the entire chewing sequence was divided into a series of chewing cycles, and ten cycles from the middle of the sequence were selected for analysis. Each selected occlusal phase of a cycle was divided into its closing and following opening strokes at the MCP. The last 3.0 mm (3-D straight-line distance) of the closing stroke and the first 3.0 mm of the following opening stroke were defined as boundaries for the analysis. The three-dimensional straight-line distance traveled by the lower incisal point (IP) was defined as the “IP Distance” ( Fig. 1 G). This region includes the expected length of the occlusal glide and has often been used for functional analyses .
Because subjects were instructed to chew normally, the working side could change from one occlusal phase to the next. To compare opening and closing strokes regardless of chewing side, the movements were standardized by inverting the y-axis (right-left) coordinates when the closing stroke was located on the right side. This transformation put the working side of all chewing cycles on the left side for the purposes of analysis.
The IP Distance for both the closing and opening strokes were divided into 0.1 mm intervals, creating a total of 61 mandibular positions, including the MCP and 30 positions each for the closing and opening strokes ( Fig. 1 G). At each of these positions, distances from all points on the mandibular model to all points on the maxillary model were calculated, and the minimum distance for each point on the mandibular model was identified ( Fig. 2 A). On the basis of previous studies, in which silicon-based registration materials were used , actual contacts were defined as the area of contact with a thickness at or below 50um, and near contacts were defined as those with a thickness greater than 0.05 mm but less than 0.35 mm . Our previous results suggest that a clearance of less than 0.2 mm corresponds to occlusal contacts occurring in this system . Therefore, the OCA of each tooth on the mandible was calculated as the sum of all areas with 0.2 mm or less distance between tooth surfaces ( Fig. 2 B). The morphological data (mathematical data mesh) from each model consisted of points that were 0.2 mm apart on the horizontal plane. If the surface of the dentition had been flat, each point would represent an area of 0.04 mm 2 . However, the occlusal surface is curved, and each point in the model had a different vertical value. Therefore, the vertical values of surrounding points were considered when calculating the area for each point. When the clearance of a mesh point on the mandible was less than 0.2 mm, the covered area of this point on the mandible was calculated and stored. The sum of these stored values for all points on the mandible was the total OCA at each position of the mandible.