Age-related changes in mastication.

The paper reviews human mastication, focusing on its age-related changes. The first part describes mastication adaptation in young healthy individuals. Adaptation to obtain a food bolus ready to be swallowed relies on variations in number of cycles, muscle strength and volume of emitted saliva. As a result, the food bolus displays granulometric and rheological properties, the values of which are maintained within the adaptive range of deglutition. The second part concerns healthy ageing. Some mastication parameters are slightly modified by age, but ageing itself does not impair mastication, as the adaptation possibilities remain operant. The third part reports on very aged subjects, who display frequent systemic or local diseases. Local and/or general diseases such as tooth loss, salivary defect, or motor impairment are then indistinguishably superimposed on the effects of very old age. The resulting impaired function increases the risk of aspiration and choking. Lastly, the consequences for eating behaviour and nutrition are evoked.

Tongue-mandible coupling movements during saliva swallowing.

The purpose of this study was to measure the tongue and mandible positions and displacements in relation to the maxilla in the midsagittal plane to characterize the different saliva swallowing patterns by recording their kinematics. A 2D electromagnetic articulograph using four transducer coils, three attached to the upper surface of the tongue midline plus one attached to the chin anterior part allowed continuous evaluation of tongue and chin movements in twelve young adults in good general health. During 170 s sequences recorded at a frequency of 100 Hz, subjects were at rest, silently reading a text they had chosen. The subjects were free to swallow during the sequence. Deglutition of accumulated saliva was analysed after averaging all values obtained during successive 250 ms periods. We identified three elementary swallowing patterns. Mean duration of tongue-mandible movements were 1·51 ± 0·17 s, 1·63 ± 0·14 s and 2·00 ± 0·08 s for the first, second and third patterns respectively. In the light of other studies based on intra-oral pressure recordings, our results help to understand the tongue-mandible coupling behaviours involved in managing an in-mouth saliva bolus during the three elementary swallowing patterns identified.

Mood-induced variations of mandible and tongue postures.

Twelve young adults in a good general health were observed during habitual posture of tongue and jaw in different emotional conditions induced by watching three video sequences. The position of the mandible was tracked by the displacements of an electromagnetic sensor glued to the chin. The tongue-to-palate distance was obtained by 2-D location of three electromagnetic sensors placed on the tongue upper midline surface. Head displacements were evaluated with a sensor fixed to an upper central incisor and were subtracted from corresponding displacements of tongue and chin sensors to obtain the real tongue and mandible positions during continuous recording sequences. Emotional conditioning by a fear movie influenced the vertical position of the mandible: the mean interarch distances during the fear movie (2·34 ± 0·24 mm) were significantly different from those measured during the tender (3·13 ± 0·35) and neutral (3·42 ± 0·80) movies, respectively (anova repeated measure, SNK; P < 0·05). anova repeated measure indicated that the tongue-to-palate distance differed significantly when the subjects were watching the conditioning movies (P = 0·003), the tip of the tongue taking a lower position during the fear movie than during the tender and neutral movies.

Is the tongue position influenced by the palatal vault dimensions?

The influence of the palatal vault dimensions on tongue position is here studied through evaluation of the in-mouth air cavity (IMAC) volume when the mandible is in maximal intercuspal position. A sample of 35 women (mean age 21.2 +/- 1.0) and 15 men (mean age 22.1 +/- 0.9) was selected. The sagittal cross-section area of the IMAC, which is modulated by the tongue position, was measured on lateral cephalograms. Dental casts were used to measure the palatal vault volume, which was defined by the occlusal plane, the hard palate and the posterior face of the second molars. Palatal vault volume allowed deduction of the IMAC volume through a rule of three procedure relating volume to area ratios. No IMAC could be calculated from cephalograms of 10 subjects who had the tongue stuck to the palate. For the 40 other subjects, the IMAC volume was 8.9 +/- 4.8 mL. It was 2 mL larger in men (n = 14) than in women (n = 26) and was the largest in skeletal Class III and the smallest in skeletal Class II (P > 0.05). IMAC volume was strongly correlated with palatal vault height but neither with palatal width nor length. It was thus assumed that the height of the palatal vault could influence the most observed position of the tongue but this does not exclude a possible growth influence of the tongue on its surrounding skeletal structures.

Age and gender-related differences in premolar and molar functional areas.

The aim of the present study was to underline the difference in premolar-molar functional areas according to both age and gender in a human adult population. Premolar and molar series were marked manually on opposed casts with articulating paper and functional areas were evaluated by quantification of these markings. It was noticed that on average larger functional areas were displayed by the older group (19 individuals, mean age 71.2 years, minimum 68 years, maximum 73 years) in comparison with the younger one (24 individuals, mean age 27.4 years, minimum 22 years; maximum 33 years). It was also observed that the average total functional area (4 hemiarches) was larger in the male subgroup (n: 23; mean age 45; young adults, 14; old adults, 9) compared with the female one (n: 20; mean age 49; young adults, 10; old adults, 10). Nevertheless, this gender difference in premolar-molar functional areas was related to the individual occlusal areas. As a matter of fact, when the individual occlusal area was taken into account for each series in the form of a ratio (functional area/occlusal area), the values obtained were not different between both gender subgroups studied. When comparing premolar and molar functional ratios (functional area/occlusal area) between age groups, it appears that the values obtained were slightly higher for the premolar series (ratios for young and older adults, respectively: Premolar series: 23.6+/-7.5%, 30+/-6.5%; Molar series: 23.1+/-6.8%, 27.3+/-6.7%). This difference was interpreted in relation to the reference (occlusal area) chosen.

Effect of age on salivary flow obtained under feeding and non-feeding conditions.

Age, health status and disease treatments are thought to influence salivary flow. In this study, age effect on salivation was compared in non-feeding (at rest and during parafilm chewing) and feeding (during meat chewing) conditions for two groups of healthy subjects, 25 young subjects (mean age 27.4 years) and 20 old subjects (mean age 71.2 years). In non-feeding conditions, parotid flow was assessed at rest (3 min) and during parafilm chewing (1 min) from the absorptive capacity of a cotton roll placed in front of the upper duct apertures. Remaining saliva emanating mainly from the submandibular/sublingual glands was determined at rest by a sublingual cotton roll. In order not to impede in the chewing process during parafilm chewing, no cotton roll was placed in the lower part of the mouth and the remaining saliva was simply spit out for evaluation. Assessments were made under feeding conditions during the mastication of meat of different textures. The saliva content of the bolus was evaluated at different stages of the chewing process by weighing the mouth contents after spitting. No direct age effect was found on the different salivary flow rates measured during different conditions of stimulation. However, a significant correlation was found between the salivary flow rates at rest and those obtained during meat chewing in the elderly group but not in the young group. In elderly adults, rest salivary flow rate appears as a good predictor of salivary flow during the consumption of food. Within each group, significant correlations were found between salivation elicited by meat and by parafilm chewing. These results confirm the lack of direct global age effect on salivary flow rate by chewing in the 3 min after the stimulation, although adaptations to the measurement conditions are different between both groups of subjects.

Influence of age and dental status on chewing behaviour studied by EMG recordings during consumption of various food samples.

OBJECTIVE: To evaluate the effects of age and dental status on chewing performance in humans. DESIGN: Electromyography recordings (EMG) were made during chewing of six foods (rice, beef, cheese, crispy bread, apple, and peanuts) to compare the masticatory patterns of four subject groups with different ages and dental status. SUBJECTS: Nineteen elders (mean age 67.2 years) classified into three categories according to their number of opposing post-canine teeth pairs (i.e. functional units) and a control group of 10 young adults (mean age 26.5 years) with a high number of functional units. MAIN OUTCOME MEASURES: Number of chewing cycles, chewing time, total muscle activity and muscle activity per chew, burst and inter-burst durations, maximum and mean voltages from EMG recordings. RESULTS: Time-related EMG parameters increased from young subjects to elderly subjects with high, middle and low dental status. Parameters related to EMG voltages per chew decreased in the same order among the different groups of subjects. These tendencies were observed for all the studied products. Subjects with weak muscle contraction may compensate for their poor chewing performance by lengthening both chewing cycle and sequence duration. Additional alterations in the chewing patterns were observed when age effect was associated with a dental status degradation in terms of number of functional units. CONCLUSION: Impairment in mastication for the elderly is due to both ageing and decreasing number of functional pairs of post-canine teeth.

Correlations between functional and occlusal tooth-surface areas and food texture during natural chewing sequences in humans.

The dental-arch surfaces preferentially used in mastication were studied by measuring functional and occlusal surface areas and comparing these to the number of chews required to swallow foods of different texture properties. The functional surface of the teeth was defined as the total area of visible wear facets on post-incisal teeth, adding to it the contacting areas of restored teeth where no facets were visible. Occlusal surface area was taken as the total area of the occluding parts of post-incisal teeth. Both surfaces were measured with computer image processing on dental-stone casts of the teeth of 31 young adults. Functional surface areas (mean 168 mm(2), four quadrants) were positively correlated with occlusal surface areas (mean 739 mm(2), four quadrants). The left:right area ratios were more variable for functional than for occlusal surfaces. Functional surface-area ratios markedly different from 1.0 might reflect functional side-preponderance of masticatory activity. Correlations between tooth surface area and the number of cycles were examined with five different food samples of known texture during side-imposed mastication. Depending on the elastic moduli of the foods, significant negative correlations were found between the left:right ratios of functional or occlusal surface areas and the left:right ratios of cycle numbers. The rheological properties of the food particles chewed were assumed to be the key factor in the correlations with either the functional or anatomical occlusal surfaces.

Variations in human masseter and temporalis muscle activity related to food texture during free and side-imposed mastication.

Adjustments of mastication to food texture have been examined in various studies, but the notion of food texture is often ill defined and usually assessed in terms of hardness. The goal of this study was to examine the pattern of activity in masseter and temporalis muscles during mastication of different food samples with known textural properties and to determine the interindividual variability. Electromyograms were recorded from the right and left masseter and temporalis muscles in 36 young adults during 'free-style' and side-imposed mastication. Five different types of food with known rheological properties were used. Both temporalis and masseter activity increased with increased stress at maximum strain of the chewed samples. A power function optimally described the relation between muscle work per chew and the mechanical measurements of food; this confirmed that the masticatory process is adjusted to accommodate to food texture. Temporalis muscle activity was more influenced by food texture than was masseter muscle activity. Less muscle work was needed to prepare the food bolus for swallowing during free-style mastication. However, 25% of the participants showed no differences between unilateral side-imposed mastication and 'free-style', suggesting that they might have greater chewing efficiency on one side.

Masseteric facilitation induced by electrical stimulation of rat orofacial tissues.

The effects of a conditioning electrical shock applied to the periodontium of the lower incisor or the glabrous area of the lower lip on the jaw-closing reflex in the anesthetized, non-paralysed rat were studied. The masseteric reflex was triggered by stimulation of the mesencephalic nucleus as a test shock and was recorded from the masseter muscle. There was facilitation of the jaw-jerk reflex, which culminated at an interval of 10-15 ms between the conditioning and the test shocks. This facilitation was not suppressed by digastric excision or by blocking a possible rebound closing reflex evoked by jaw opening. No inhibitory influence was observed. This facilitatory effects relies on an A alpha input and on cell bodies making up the mesencephalic nucleus. The direct excitatory electrical events observed in the masseter muscle after periodontal or labial stimulation proved to be due to the diffusion of the bioelectrical activity generated in the neighbouring jaw-opening muscles.

Responses of trigeminal subnucleus oralis nociceptive neurones to subcutaneous formalin in the rat.

Extracellular recordings of 33 single nociceptive neurones of the trigeminal subnucleus oralis (SNO) were made in rats under halothane nitrous oxide anaesthesia. These neurones were tested for their responses to a s.c. injection of formalin in their receptive field. Such a chemical noxious stimulation is known to induce a biphasic response of nociceptive dorsal horn neurones, the second period of which would be due to inflammation. Twenty-three neurones were characterized as nociceptive non-specific (NnS) and 10 as nociceptive specific neurones (NS). Formalin activated both SNO NS and NnS neurones, but, when they responded, NS neurones (n = 5) showed only the first phase of activity while NnS neurones showed either one (n = 13) or two phases (n = 6). Biphasic responses were most often observed for NnS neurones with A delta- and C-fibre inputs. These results indicate that the time-course similarity between the behavioural and the neuronal responses to formalin exists only for some SNO convergent neurones and that therefore the SNO does not seem to be very involved in the inflammatory component of the pain caused by formalin.