Alain Woda

Adaptation of healthy mastication to factors pertaining to the individual or to the food

Mastication is a physiological process controlled by the central nervous system and modulated by inputs from the mouth. Both the intrinsic characteristics of the subject and the extrinsic characteristics of the chewed food are responsible for variations of the masticatory function. Age, gender and dental state constitute the most studied intrinsic factors whereas hardness, rheological characteristics such as plasticity or elasticity, and food size are the better known extrinsic factors. These factors cause physiological adaptations which can occur during individual cycles or the whole sequence of mastication. Electromyographic and jaw movements (kinematic) recordings are commonly used to study mastication, from which, several variables can be measured. Vertical and lateral amplitudes and, velocities of jaw movements, are only given by kinematic recordings. Bioelectrical activities per cycle or per sequence are closely linked to masticatory forces and are measured from electromyographic recordings. Number of cycles, sequence duration and masticatory frequency can be measured from both types of recordings. The objective of this review is to provide an overview of the variations of the measured masticatory variables that occur when mastication adapts to changes in characteristics of the individual or the food.

Evidence for a peripheral origin of the tonic nociceptive response to subcutaneous formalin

&NA; The orofacial formalin test in the rat is a valid and reliable model of nociception and is sensitive to various classes of analgesic drugs. The noxious stimulus consists in an injection of diluted formalin (2.5% in saline) into the upper lip. The behavioural nociceptive response is measured in terms of the amount of time the animal spends rubbing the injected area. Two distinct periods of intense rubbing activity can be identified, a first phase occurring in the first 3 min and a second phase lasting from 12 to 39 min after formalin injection. The present study verified the peripheral origin of the first phase of the formalin response and examined whether the second phase is produced by peripheral activation of afferent fibres and/or by a phenomenon of central facilitation induced by the neural activity that occurs during the first phase. This was determined by assessing the effect of a local anaesthetic agent (lidocaine) administered into the formalin injection site, before or after the first phase of the formalin response. Local injection of 50 &mgr;l of lidocaine prior to formalin completely abolished the first phase of the formalin response but this blockade did not significantly influence the appearance and development of the second phase. Thus, the primary afferent activity that normally occurs during the first phase of the formalin response is not a prerequisite for the expression of the second phase. A higher dose of lidocaine (150 &mgr;l) induced, in addition, inhibition of the first part of the second phase. Fifty or 150 &mgr;l of lidocaine injected after the first phase produced a blockade of the second phase, with a dose‐dependent duration (6 min and 18 min, respectively). Our results demonstrated that both the first and second phases of the nociceptive response to formalin depend on primary afferents activation and, consequently, that the second phase cannot be mediated by central sensitisation alone.

The orofacial formalin test in rats : effects of different formalin concentrations

&NA; In this study of the orofacial formalin test in rats, the effects of different formalin concentrations (0.2%, 0.5%, 1.5%, 2.5%, 5% and 10%) on the behavioural nociceptive response (face rubbing) was investigated. The histological responses of the skin were also evaluated. Increasing the concentration of formalin caused a parallel aggravation of histological signs of tissue inflammation and injury. All concentrations provoked an early phase of nociceptive response, but its intensity was not concentration‐dependent. The 2nd phase of response to formalin only occurred for concentrations of 1.5% and higher. A positive relationship between the formalin concentration and the amplitude of the rubbing activity measured between 12 and 45 min after injection could be observed until 2.5% but with the highest concentrations (5 and 10%), the amplitude of the response decreased. Our findings indicate that the orofacial formalin test should be carried out using concentration between 0.5 and 2.5%. This is essential to assess increase as well as decrease in pain intensity. Moreover, this will have the effect of minimizing the suffering of the experimental animal.

Nonfunctional and functional occlusal contacts: A review of the literature

A review of the literature on occlusal contacts emphasizes the following points. 1. Contacts in centric occlusion do not correspond to any ideal occlusal diagram. 2. Canine protection and group function appear to correspond to two successive states of the evolving dentition under the effect of abrasion. In most lateral occlusions, two maxillary teeth, of which one is the canine, are involved. 3. During mastication, tooth contacts exist. They occur most often during a sliding movement in which the direction and the origin are variable. This justifies the concept of an occlusal field of mastication. 4. During unilateral mastication, the chewing of the food is performed by working as well as nonworking contact. This imposes the distinction between the chewing and nonchewing sides (functional) and the working and nonworking sides (kinematic). 5. Centric occlusion is the occlusion most often used during mastication. It is also the occlusion for which the masticatory forces are the greatest. 6. The bibliographic data concerning occlusal contracts during swallowing are largely contradictory. It nevertheless seems that the occlusal contacts obey laws nearly the same as those governing the occlusal contact during mastication (sliding on an occlusal field of deglutition, and the importance of centric occlusion).

The Challenge of Mastication: Preparing a Bolus Suitable for Deglutition

The main function of mastication is to transform a solid food into a bolus that can be swallowed safely. The bolus characteristics such as particles size or cohesiveness, are continuously sensed during mastication and they are important in initiating deglutition. This study examined the following question: What is the condition of the bolus just before swallowing? Ten subjects with normal dentition aged 37.5 ± 3.7 years were asked to chew without swallowing six different foods (three nuts and three vegetables) while the number of cycles and the duration of the sequence were recorded. The particle size distribution shown by the expectorated food bolus just before swallowing was examined by image analysis. The results showed that, for a given food, the sizes of the bolus particles just before swallowing were comparable in all subjects. However, the number of cycles and duration of the sequence varied between subjects. Taken together these data strongly suggest that the granularity of the bolus before swallowing has to reach a predetermined state which is obtained by using an individual chewing strategy. This suggests that the bolus structure reflects a key factor for homeostasis and explains the large interindividual variability of the mastication physiologic parameters.

The rostral part of the trigeminal sensory complex is involved in orofacial nociception.

Abstract Single units responsive to noxious mechanical stimulation of orofacial receptive fields were recorded within the ventrobasal complex of the rat thalamus. The induced activities were compared before and after deafferentation of the subnucleus caudalis by a trigeminal tractotomy performed at the obex level. The receptive fields activated by noxious stimulation were classified as ‘oral’ when included in the oral, perioral or paranasal areas, and as ‘facial’ when included in facial regions distant from the oral cavity. After tractotomy, the unit responses to noxious stimulation of an oral field remained unchanged in 8 cases, decreased in 3 cases, and were suppressed in 4 cases. For units responding to noxious stimulation of a facial field, the responses were suppressed in 8 cases, decreased in two cases and remained unchanged in two other cases. So it appears that the rostral part of the trigeminal sensory complex (1) receives nociceptive afferents mainly from the oral and perioral areas and (2) is a relay in ascending pathways which convey painful sensations.

Role of Physical Bolus Properties as Sensory Inputs in the Trigger of Swallowing

Background Swallowing is triggered when a food bolus being prepared by mastication has reached a defined state. However, although this view is consensual and well supported, the physical properties of the swallowable bolus have been under-researched. We tested the hypothesis that measuring bolus physical changes during the masticatory sequence to deglutition would reveal the bolus properties potentially involved in swallowing initiation. Methods Twenty normo-dentate young adults were instructed to chew portions of cereal and spit out the boluses at different times in the masticatory sequence. The mechanical properties of the collected boluses were measured by a texture profile analysis test currently used in food science. The median particle size of the boluses was evaluated by sieving. In a simultaneous sensory study, twenty-five other subjects expressed their perception of bolus texture dominating at any mastication time. Findings Several physical changes appeared in the food bolus as it was formed during mastication: (1) in rheological terms, bolus hardness rapidly decreased as the masticatory sequence progressed, (2) by contrast, adhesiveness, springiness and cohesiveness regularly increased until the time of swallowing, (3) median particle size, indicating the bolus particle size distribution, decreased mostly during the first third of the masticatory sequence, (4) except for hardness, the rheological changes still appeared in the boluses collected just before swallowing, and (5) physical changes occurred, with sensory stickiness being described by the subjects as a dominant perception of the bolus at the end of mastication. Conclusions Although these physical and sensory changes progressed in the course of mastication, those observed just before swallowing seem to be involved in swallowing initiation. They can be considered as strong candidates for sensory inputs from the bolus that are probably crucially involved in the triggering of swallowing, since they appeared in boluses prepared in various mastication strategies by different subjects.

Is the goal of mastication reached in young dentates, aged dentates and aged denture wearers?

The objective of the present study was to assess the impact of age and dentition status on masticatory function. A three-arm case-control study was performed. Group 1 (n 14) was composed of young fully dentate subjects (age 35.6 +/- 10.6 years), group 2 (n 14) of aged fully dentate subjects (age 68.8 +/- 7.0 years) and group 3 (n 14) of aged full denture wearers (age 68.1 +/- 7.2 years). Mastication adaptation was assessed in the course of chewing groundnuts and carrots to swallowing threshold. Particle size distribution of the chewed food, electromyographic (EMG) activity of the masseter and temporalis muscles during chewing, and resting and stimulated whole saliva rates were measured. Aged dentate subjects used significantly more chewing strokes to reach swallowing threshold than younger dentate subjects (P < 0.05), with increased particle size reduction, longer chewing sequence duration (P < 0.05) and greater total EMG activity (P < 0.05) for both groundnuts and carrots. In addition, aged denture wearers made significantly more chewing strokes than aged dentate subjects (P < 0.001) to reach swallowing threshold for groundnuts. Particle size reduction at time of swallowing was significantly poorer for denture wearers than for their aged dentate counterparts, despite an increase in chewing strokes, sequence duration and EMG activity per sequence. Masticatory function was thus adapted to ageing, but was impaired in denture wearers, who failed to adapt fully to their deficient masticatory apparatus.

Effect of lingual nerve block on burning mouth syndrome (stomatodynia): A randomized crossover trial

&NA; Burning mouth syndrome (stomatodynia) is associated with changes of a neuropathic nature the main location of which, peripheral or central, remains unknown. A randomised, double‐blind crossover design was used to investigate the effects of lingual nerve block on spontaneous burning pain and a possible correlation with the effects of topical clonazepam, the patients response to a psychological questionnaire, and the taste and heat thresholds. The spontaneous burning was measured with a visual analogue scale (VAS) just before and 15 min after injection. The decreases in VAS score after lidocaine or saline injection were not significantly different (2.7 ± 3.9 and 2.0 ± 2.6, respectively; n = 20). However, two groups of patients could be identified: in a “peripheral group” (n = 10) the VAS decrease due to lingual nerve injection was 4.3 ± 3.1 cm after lidocaine and 0.9 ± 0.3 cm after saline (p = 0.02). In a “central group” (n = 7), there were an increase in pain intensity score (−0.8 ± 2.6 cm) after lidocaine and a decrease (1.5 ± 3.0 cm) after saline (p = 0.15). An increase in the hospital anxiety and depression (HAD) score and a decreased taste sensitivity and heat pain threshold of painful oral area were seen in patients compared with age‐and‐sex‐matched controls (p < 0.05). Topical clonazepam treatment tended to be more effective (p = 0.07) and HAD score lower (p < 0.03) in the peripheral than in the central group. These results suggest that the neuropathic disorder associated with stomatodynia may be predominantly peripheral, central or mixed depending on the individual. Topical application of clonazepam and HAD may serve as indicators of which mechanism is dominating.

Effects of tractotomy on nociceptive reactions induced by tooth pulp stimulation in the rat

The effects of a trigeminal tractotomy on nociceptive reactions induced by electrical stimulation of an inferior incisor were studied in freely moving rats. Behavioral tests were based on the observation of three reactions: jaw opening reflex, face scratching with forepaws, and head rotation toward the stimulated side. These reactions appear successively when the stimulation intensity increases. The results were evaluated by comparing a test group with a sham-operated group. Both groups were prepared surgically in the same way but only the former had undergone tractotomy. The results were as follows: The threshold of the jaw opening reflex triggered by stimulation of the tooth pulp ipsilateral to the tractotomy was not modified; however, the threshold of the face-scratching and the head rotation reactions rose (P less than or equal to 0.05) after tractotomy. These data suggest that the rostral part of the trigeminal sensory complex is involved in oral nociception. However, the modification of the face-scratching and head rotation thresholds reveals that the subnucleus caudalis also participates in this function when a certain level of pain is reached. The thresholds of the three nociceptive reactions evoked by stimulation of the contralateral tooth pulp were significantly lower in the test group than those in the sham-operated group (P less than or equal to 0.001), which is interpreted as being indicative of an modulating effect of the subnucleus caudalis on the contralateral trigeminal sensory complex. This effect was probably revealed by the use of a sham-operated group as a control.