J.H. Abbink

Oral physiology and mastication.

Mastication is a sensory-motor activity aimed at the preparation of food for swallowing. It is a complex process involving activities of the facial, the elevator and suprahyoidal muscles, and the tongue. These activities result in patterns of rhythmic mandibular movements, food manipulation and the crushing of food between the teeth. Saliva facilitates mastication, moistens the food particles, makes a bolus, and assists swallowing. The movement of the jaw, and thus the neuromuscular control of chewing, plays an important role in the comminution of the food. Characteristics of the food, e.g. water and fat percentage and hardness, are known to influence the masticatory process. Food hardness is sensed during mastication and affects masticatory force, jaw muscle activity, and mandibular jaw movements. When we chew for instance a crispy food, the jaw decelerates and accelerates as a result of resistance and breakage of food particles. The characteristic breakage behaviour of food is essential for the sensory sensation. This study presents a short review of the influence of oral physiology characteristics and food characteristics on the masticatory process.

Swallowing threshold and masticatory performance in dentate adults

A variety of both natural and artificial foods are commonly used for the evaluation of masticatory function. We compared swallowing thresholds of three natural foods (peanuts, cheese and carrots) to those of a standardized artificial test food (Optocal Plus) and examined the relationship between masticatory performance and the swallowing threshold. Eighty-seven healthy dentate subjects participated (25 men and 62 women, aged 42.0+/-12.1 years). We evaluated the dental state, registered the number of chewing strokes used before swallowing, analyzed the chewed particles and determined median particle sizes (X50) for Optocal Plus after 15 chewing strokes and at the moment of swallowing. The results show that the number of strokes used before swallowing each natural food linearly increased with volume (P<0.001), and that carrots required more strokes than peanuts and cheese (P<0.001). The number of chewing strokes used before swallowing Optocal Plus was comparable to the number used for carrots. Masticatory performance was significantly influenced by dental state, but not by age or gender. Significant correlations were observed for: (1) the number of chewing strokes used before swallowing natural foods and Optocal Plus; (2) the median particle sizes after 15 strokes and before swallowing; (3) the number of chewing strokes before swallowing and the corresponding median particle size. However, median particle sizes as obtained after 15 strokes did not correlate with the number of strokes used before swallowing (r=0.02). Thus, bad chewers did not necessarily chew longer before swallowing than good chewers. As a consequence bad chewers would, on average, swallow larger food particles.

Bite force and electromyograpy during maximum unilateral and bilateral clenching

Maximum voluntary bite force has often been studied as an indicator of the functional state of the masticatory system. Bilateral, as well as unilateral, methods have been used to determine bite force. Only a few studies have compared the outcomes of both methods. The aim of this study was to measure bite force and jaw-muscle activity during bilateral as well as unilateral maximum clenching in a large number of healthy subjects, so that the results could be compared. In a group of 81 dentate subjects we observed an average bilateral bite force of 569 N. The average unilateral bite force was significantly lower, being 430 N (right) and 429 N (left). Masseter and anterior temporal muscle activities were also significantly lower during unilateral clenching as compared with bilateral clenching. The masseter muscles showed no difference in activity between the ipsilateral side and the contralateral side during unilateral clenching. In contrast, the activity of the anterior temporal muscle on the ipsilateral side was significantly higher than on the contralateral side. Thus, the change in the forces acting on the jaw during unilateral clenching compared with bilateral clenching leads to a different response in the temporal muscles than in the masseter muscles.

Mixing ability test compared with a comminution test in persons with normal and compromised masticatory performance

A mastication test was needed with a material that forms a bolus and is soft enough to be chewed by persons with compromised oral function, in particular patients confronted with oral cancer. We therefore developed a wax-mixing ability test and compared it with a comminution test using Optocal as test food. We hypothesized that the mixing ability test would be better at differentiating between groups of persons with compromised masticatory performance than the comminution test. Sixty healthy subjects were recruited in three groups of 20, matched for age and gender: a group with natural dentition; a group with full dentures; and a group with maxillary denture and implant-supported mandibular overdenture. The mixing ability test was found to discriminate better between the two full-denture groups than the comminution test.

A comparison between data analysis methods concerning particle size distributions obtained by mastication in man

Three mixtures of chewed food particles (coarse, medium and fine) were quantified by measuring the particle sizes with an optical scanning device. The particle sizes were described by three different particle-size distributions: a cumulative volume, a volume and a number distribution. The median particle size was determined from each of the distribution functions. Suitability for characterizing a mixture of chewed food particles was tested, showing that the median particle size as obtained from a cumulative volume (or weight) distribution should be preferred. It was shown to be the most sensitive measure for characterizing mixtures of chewed food.

Comparing masticatory performance and mixing ability

Masticatory performance has often been measured by determining an individuals capacity to comminute a test food. Another method to determine masticatory performance, which is now widely used, evaluates the ability to mix and knead a food bolus. Two-coloured chewing gum and paraffin wax have been used as test foods for the quantification of the mixing ability. The aim of our study was to compare the results obtained with the comminution of an artificial test food and the results obtained from mixing of a two-coloured chewing gum. The degree of mixing of the colours of the chewing gum was quantified with an optical method. Twenty young subjects with a natural dentition (average age 24 years) and twenty elderly subjects, mostly with complete dentures (average age 72 years), participated in the study. Significant differences in masticatory performance between the two groups were detected with both methods. However, the comminution test was better in discriminating the masticatory performance of the two groups. The mixing ability test with the two-coloured chewing gum proofed to be a good method to determine masticatory function in subjects with a compromised masticatory performance (elderly subjects). However, the method appeared to be less suitable for subjects with a good masticatory performance (young subjects).

Modulation of the Mandibular Stretch Reflex Sensitivity During Various Phases of Rhythmic Open-Close Movements in Humans

The muscle spindles of the jaw elevator muscles provide positive feedback to the alpha motoneurons. It is generally assumed that the feedback is modulated during chewing so that counterproductive forces of the jaw elevator muscles can be avoided during jaw opening. Our aim was to investigate the modulation of the muscle spindle input to the alpha motoneurons during various phases of open-close movements in man. To that end, subjects made rhythmic open-close movements at their natural chewing frequency. A force impulse (5 N, 10 ms), eliciting a jaw-jerk reflex, was unexpectedly applied. The impulse was applied to the mandible at 8 different phases during an open-close cycle, but only 1 impulse per cycle. Jaw movement and surface EMG of the masseter and temporal muscles on both sides were recorded during 3 cycles without an impulse and 3 succeeding cycles with an impulse. To examine whether the modulation of the mandibular stretch reflex sensitivity depends on the food resistance, we applied an additional external force on the mandible, counteracting closing of the jaw each cycle. Two experimental sessions were performed in random order, i.e., without force and with an additional force of 20 N. We observed pronounced reflexes at the onset of jaw closing, during the closing phase, and at occlusion. No or only weak jaw-jerk reflexes were present during jaw opening. The reflex amplitudes at occlusion were larger when an external force was present. This increase in reflex amplitude may be the result of an adjusted gamma motoneuron activity, from pre-motor inhibition, or from both. The reflex amplitudes elicited during jaw closing were not correlated with the phase of the movement.

An epidemiological study of maxillofacial fractures requiring surgical treatment at a tertiary trauma centre between 2005 and 2010

The epidemiology of maxillofacial fractures shows considerable regional variation as a result of local demographic and socioeconomic factors. We have assessed the epidemiological characteristics of such fractures at our centre in The Netherlands. The medical records of 394 patients who were treated surgically for maxillofacial fractures between 1 January 2005 and 31 December 2010 were analysed retrospectively. The male:female ratio was 3:1. There was a peak incidence in the second and third decades of life among men. The number of injured patients/year remained stable during the selected period. The incidence was highest in the spring and at weekends. Fractures of the mandible and zygoma were the most common. Road traffic crashes were the most common cause of injury (42%) and mainly involved bicycles. A total of 165 (15%) of the patients were intoxicated, and 142 patients (36%) had other serious injuries. Most patients (n=248, 63%) were treated within a day of presentation. Two hundred and thirty-two patients (59%) spent 4 days or fewer in hospital. The presence of other injuries was associated with a prolonged stay in hospital. Groups at particular risk of maxillofacial fractures are young men and cyclists. The use of helmets by cyclists could achieve a large reduction in injuries to the brain and upper face.

Speed-dependent Control of Cyclic Open-Close Movements of the Human Jaw with an External Force Counteracting Closing

Previous work with open-close movements of the jaw in which food resistance was simulated by an external force has shown that additional activity of the jaw-closing muscles to overcome the force is mainly of sensory origin. When the force was expected, a small anticipatory response was also observed, starting before the onset of the force. The movement rates in these experiments corresponded to natural chewing rates of about 60 cycles per minute. Our aim was to investigate how anticipatory and peripherally induced muscle activity change with movement speed. Peripheral feedback to the muscles may increase at higher movement speed, possibly resulting in stronger reflex activity. On the other hand, when the force is expected, more preprogrammed muscle activity may be generated with faster movements, in anticipation of the force. Three movement rates were studied: 30 cpm (slow), 60 cpm (normal speed), and 120 cpm (fast). The results show that muscle activity to move the jaw increases sharply with movement speed. Extra muscle activity needed to overcome the force also increases with movement speed. However, the contribution by peripherally triggered muscle activity does not increase. In contrast, preprogrammed extra muscle activity in anticipation of the force increases sharply with movement speed. It is concluded that the control strategy for these movements is speed-dependent, with a shift to relatively more anticipatory muscle activity at higher movement speeds, making the movement more ballistic.

Digital image processing versus visual assessment of chewed two‐colour wax in mixing ability tests

Two-colour chewing gum and wax have been widely used as test foods to evaluate the ability to mix and knead a food bolus. The mixing of the colours has been assessed by computer analysis or by visual inspection. Reports contradict each other about whether computer analysis and visual assessment could equally well discriminate between the masticatory performances of groups of participants with different dental status. This study compares the results of computer analysis of digital images of chewed two-colour wax with the results of visual assessment of these images. Sixty healthy subjects participated and chewed on red-blue wax for 5, 10, 15 and 20 chewing strokes. The subjects were divided into three groups of 20, matched for age and gender, according to their dental status: natural dentition, full dentures and maxillary denture plus implant-supported mandibular overdenture. Mixing of the chewed wax was determined by computer analysis of images of the wax and by visual assessment of the images by five examiners. Both the computer method and the observers were able to distinguish the mixing abilities of the dentate subjects from the two denture wearer groups. Computer analysis could also discriminate the mixing abilities of the two denture groups. However, observers were not able to distinguish the mixing abilities of the two denture groups after 5, 10 and 15 chewing strokes. Only after 20 chewing strokes, they could detect a significant difference in mixing ability.