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Mastication is the first phase of the digestive process. It serves to reduce the size ofsolid food and to mix it with saliva so that it can be swallowed. The degree to which food is reduced in size depends on various factors, such as the masticatory force generated by the jaw muscles, the number and form of the (pre-)molars, the neuro-muscular control of the movements of the mandible and the quantity and composition of the saliva. If the mastication is to operate efficiently, form and function have to be properly balanced. Disturbances in function and abnormalities inform can lead to complaints and are often accompanied by a diminished fragmentation of the food. By investigating the fundamental mechanisms of the functional process, abnormalities can clinically be better understood.

Changing the occlusal vertical dimension is a common procedure in restorative dentistry, during treatment of patients with cranio-mandibular disorders, and during orthodontic and orthognathic treatment. The treatment may alter the length of the main jaw elevator muscles and the position of the mandibular head in the fossa temporalis. These changes may influence the bite forces that are generated during chewing and thus may affect the masticatory function. We measured the objective masticatory function, defined as masticatory performance, by determining an individual's capacity to pulverize a test food. The immediate influence of the increase in the occlusal vertical dimension on the masticatory performance was determined using three anatomical maxillary splints in a group of seven dentate subjects. The splints gave an increase in the occlusal vertical dimension of 2, 4 and 6 mm, respectively. Before we started the experiments the subjects practiced chewing with the splints during about 5 min. No significant differences were observed in masticatory performance among the conditions without and with the three splints. Thus, an increase in the occlusal vertical dimension up to 6 mm did not have a significant effect on the masticatory performance. Maxillary splints may be used to study the effect of occlusal factors on the chewing process by manipulating tooth shape and occlusal area of the splint.

Missing teeth are often replaced by fixed or removable prosthodontic appliances to improve masticatory function. However, there have been few studies directly determining the influence of prosthodontic treatment on masticatory function. Our aim was to obtain information on the objective and subjective masticatory function of partially edentulous subjects before and after prosthetic restoration. In this way, the improvement of the masticatory function as a result of the treatment could be quantified. The objective and subjective masticatory function of a group of 15 subjects was determined before and after replacement of an average of 3.1 post-canine teeth. The results were compared with data from a control group of 26 subjects having a complete dentition. The total number of occluding post-canine teeth increased as a result of the prosthodontic treatment, yielding a significantly improved objective masticatory function. The average masticatory performance was found to approach the level of the control group if all occlusal units of the longest posterior side were replaced. Subjects with an incomplete dentition tended to chew predominantly on the side of the longest posterior arch. The average subjective masticatory function improved as a result of the treatment. The appreciation of the masticatory function after treatment was equal to that of the control group, despite an average of 2.7 missing teeth. However, no correlation was found between the additional number of occlusal units and the change in subjective masticatory performance. Rehabilitation of post-canine teeth restores some objective masticatory function and leads to an increased appreciation of the masticatory function, although no correlation was found between the changes in objective and subjective masticatory function.

The masticatory performance of a group of subjects missing on average 5.7 postcanine teeth and of a control group with complete dentitions were compared. There were significant differences in performance between the two groups. Within the patient group the number of occluding units significantly influenced performance. The average number of chewing cycles needed to prepare the food for swallowing was significantly larger for the patient than for the control group. Despite this, subjects in the patient group swallowed, on average, significantly larger food particles. Correlations between chewing performance and the number of chews up to swallowing within each of the two groups did not reach significance. However, combining the data from the two groups and thus increasing the range of chewing performances yielded a highly significant correlation. Subjects with reduced performance needed more chewing cycles before swallowing the food. Significant correlations were also found between chewing performance and the size of the swallowed food; subjects with poor performance swallowed larger particles. The numbers of chewing cycles made to swallow a natural (peanut) and an artificial (Optosil) test food were highly correlated, indicating that both foods are suitable for testing chewing performance.

The masticatory performance of subjects can objectively be determined by describing the particle size distribution of a comminuted test food (cubes Optosil). After a fixed number of chewing strokes the particles are collected and sorted with a stack of sieves. The size distribution can be characterized with two parameters; the median particle size and the broadness of the distribution. The rate of reduction of the median size is used to characterize the chewing performance. The chewing process can be described as the result of two processes: selection and breakage. The parameters of these two processes can be derived from the chewing result using a mathematical model. The differences in chewing performance between subjects with a natural dentition and subjects with a complete denture occurs already in the first 20 chewing strokes. Experimental results indicate that especially the selection chance is smaller in denture wearers.

The force-deformation characteristics of two artificial test foods (Optosil and Optocal) for measurements of food comminution during mastication were investigated in a bite simulator and compared with those of carrots and peanuts. The influence of cusp geometry was evaluated by use of a flat plate and three cusp forms. The forces at the yield point were lower for Optocal than for Optosil artificial test food. The forces needed for Optocal overlapped those needed for carrots and peanuts. The natural foods showed more variation in the force and percentage of deformation at the yield point than the artificial foods. The artificial foods reflected the differences in cusp form better than did the natural foods. The use of artificial foods fulfills a need for standardization and warrants consideration in studies of mastication.

This study investigated the relationships between the ability of 38 patients with complete dentures to comminute a tough artificial test food and their answers to questions about the chewing experience. Also, clinical assessments were made of denture quality and oral conditions. Significant but weak relationships were found between the ability to comminute the test food and the subjective chewing experience as well as the degree of resorption of the mandibular residual ridge. Masticatory ability is conceivably determined by many other factors as well. Results indicate that dentists cannot rely on asking denture wearers about chewing problems and clinical responses with respect to oral conditions and denture quality for predicting those patients' masticatory abilities. The ability of wearers of complete dentures to comminute tough foods should be determined individually by a series of chewing tests.

A method for quantifying the comminution of an artificial test food (Optosil) was evaluated with respect to its suitability for measurements of masticatory performance in complete-denture wearers. Reference was made to subjects with natural dentitions. The description of particle size distributions generated by complete-denture wearers by a Rosin-Rammler equation was subject to limitations, due to the presence of large proportions of almost-intact particles, which had hardly been damaged or broken during chewing. This finding might be explained by: (i) the relatively high fracture strength of Optosil as compared with natural foods; and (ii) the limitations of complete-denture wearers in exerting bite forces. The particle size distributions obtained after repeated measurements and described by linear interpolation of data points were reproducible. In comparison with young adults with natural dentitions, the denture-wearers needed approximately seven times more chewing strokes to achieve an equivalent reduction in particle size.

Eight subjects participated in chewing experiments to examine how the jaw gape in consecutive chewing cycles depends on the dimensions and the volume of the food bolus. Different volumes of chewing-gum and a silicone rubber (Optosil) were used. The initial particle size of Optosil was also varied. With the aid of a sieving procedure, the size distribution of Optosil particles was determined after different numbers of chewing cycles. The maximum size of a chewing-gum bolus along three orthogonal axes was determined after a random number of chewing cycles for all the volumes of chewing-gum offered. The jaw gape was measured by means of an optical motion analysis system and calibrated at the level of the first molars. The maximum jaw gape and the gape at the first fall in mandibular velocity after the onset of jaw-closing were determined for different chewing cycles from the position signal and its time derivative. For each volume of chewing gum offered to the subject, the average of the maximum gapes attained in the subsequent cycles of jaw movement was related to the characteristic height of a bolus of that volume; maximum velocity of closing appeared to occur while the antagonistic teeth were already penetrating the gum bolus. The increase in the maximum jaw gape, the gape at the velocity maximum, and the height of the gum bolus, as a function of the volume of chewing-gum, could be well-described by a power function of the volume. The exponent of the power function for both jaw gapes was only slightly smaller than the exponent obtained for the bolus height.(ABSTRACT TRUNCATED AT 250 WORDS)

Food comminution in man was studied in terms of intra-oral selection and breakage processes, with a silicone rubber used as test food, aided by a sieving procedure and simultaneous form- and color-labeling of particles. The subjects chewed on mixtures of particles with a specific form (either cubes or half-cubes). In addition, seven particle sizes, within a range of 1.2 to 8.0 mm, each had a specific color. Damaged cubes or half-cubes revealed their chance of being selected for breakage. The color of the fragments indicated their original size, and thus the breakage distribution of selected particles. The selection chance increased as a power function of particle size. The degree of fragmentation was, in general, maximal for a particle size of about 4 mm. Our double-labeling technique permits a detailed unraveling of anatomical and physiological variables in the chewing process.

Chewing performance was quantified by determining the particle-size distribution of comminuted food as a function of the number of chewing strokes. The rate of food breakdown was taken to be the result of a combined selection and breakage process; this was quantified in a mathematical model. A linear operation on the particle-size distribution described the changes in this distribution that resulted from an additional chewing stroke. Detailed information was obtained from eight subjects on the selection and breakdown of food particles of different sizes. There were considerable inter-individual differences in the selection chances for small particles. The mathematical method facilitates study of the influence of dental morphology and muscle-related factors on the comminution of food particles.

The masticatory performance of an individual may be quantified by describing the size distribution of particles of a comminuted test food as a function of the number of chewing strokes. A standardized sieving method and a standardized artificial test food (Optosil) were used to obtain reproducible results. Some measurements were performed using peanuts. The distribution of particle sizes of the comminuted food was adequately described by a Rosin-Rammler distribution function for the seven participating individuals which characterizes the size distribution by the median particle size (x50) and the broadness of the distribution (b). The broadness variable b hardly depends on the number of chewing strokes, and the differences in b between the individuals are small. Considerable differences in the median particle sizes at a given number of chewing strokes were observed between the subjects, reflecting differences in efficiency of comminution of a test food. For all subjects, the median particle size decreased as a function of the number of chewing strokes N according to the relation, x50 = c X N-d. Variables c and d characterized the efficiency of comminution by the subjects.