Chewing

Chewing

1.Introduction

Chewing is the first step in digestion. It combines, in the same sequence, several motor activities, such as grasping, incising food, transporting it intraorally and breaking it up. It thus achieves the mechanical preparation and

Insalivation of the food bolus in order to make it suitable for swallowing.

 This rhythmic act, very complex, is carried out thanks to the coordinated activity of the masticatory muscles but also of the facial, lingual and hyoid muscles.

 Unilateral dominant chewing may lead to asymmetric maxillofacial development or, in adults, joint dysfunction and periodontal lesions. Masticatory efficiency may be reduced by nervous, muscular or dental pathologies.

2. Maturation of chewing

This function appears with the establishment of the temporary dentition, and in particular with the occlusion of the temporary molars which coincides with the transition to an increasingly solid and resistant diet. The periodontal receptors are stimulated, the trigeminal motor function (V) develops and will gradually replace

That of the fascia (VII). The wedging of the mandible is carried out by isotonic contraction

Then complete isometric of the elevator muscles; the lips-cheeks and tongue couple dissociate, the partners acquire functional independence. 

This maturation is rapid and the chewing type is stable and well coordinated around 4 or 5 years or when the first molars come into occlusion.

3. Role and effectiveness of chewing

The purpose of chewing is to facilitate insalivation and thereby allow chemical digestion to begin and create a lubricated food bolus that can be swallowed.

This masticatory efficiency is directly linked to the number and surface area of ​​occlusal contacts in maximum intercuspidation.

4. Physiology of chewing

Mastication is accomplished through rhythmic mandibular movements in three spatial dimensions that allow the fragmentation and crushing of food between the dental arches associated with coordinated movements of the tongue, cheeks and lips that ensure the transport, formation and control of the food bolus.

Each time the mandibular interincisal point returns to its initial position, the mandible has completed a masticatory cycle.

 There are three types of chewing:

• unilateral alternating chewing  : the most frequent and most physiological.

• strict or dominant unilateral chewing : the working side is almost always the same;

• bilateral chewing : the food is crushed simultaneously on both sides.

Many subjects have a preferred side of chewing. Thus, for many authors, the preferred side would correspond to the one ensuring the maximum contacts during occlusal guidance.

5. The chewing sequence :

• A chewing sequence corresponds to all the movements from ingesting food to its complete swallowing.
• It is the set of cycles carried out between taking a bite of food and swallowing it which constitutes a chewing sequence.
• Chewing is accomplished in an uninterrupted succession of these cycles, 15 on average in a sequence. If the side used for chewing changes during a sequence, it is called alternating bilateral chewing. 80% of the patients examined had this type of chewing, while 12% chewed on only one side (unilateral chewing). Only 8% of these patients had bilateral chewing in which food was crushed on both the right and left sides.
• Divided into three phases:

1st phase: preparation series

Food is gathered and broken into pieces of a size compatible with chewing and then moved by the tongue to the back and molar occlusal surfaces.

2nd phase: reduction series 

It ensures the majority of the breakdown of food through mandibular movements corresponding to the masticatory cycles. 

3rd phase: pre-swallowing series

 The posterior part of the tongue lowers, moves forward creating a posterior space and its anterior part rises allowing the food to slide posteriorly.

6. Typical chewing cycles :

• The reduction of the bolus by the teeth is ensured by rhythmic opening-closing movements combined with propulsion-retropulsion and diduction movements of the mandible. Each time the mandible returns to its starting position in occlusion, after having completed an opening-closing trajectory, it is said to have completed a chewing cycle.
• The trajectory of the incisal point during a single cycle is represented by an ellipse.
• During a typical chewing cycle, the incisal point moves down slightly away from the midline toward the non-chewing side. Then it rejoins the midline and continues down toward the chewing side. The mandible begins to move up and the teeth on the chewing side come into contact with the bolus, while the incisal point moves upward remaining away from the midline; at this point the mandible is in the working position. Then the teeth come into contact through the working cuspal slopes. Following an interdental slide, the incisal point abruptly rejoins the midline where it returns to the position corresponding to the maximum intercuspation occlusion (MIO) from which the next cycle will begin again. 

7. Range of masticatory movements

The average amplitude of the displacement of the mandibular interincisal point:

• in the vertical direction, varies from 16 mm to 22 mm.

• in the transverse direction, varies from a few mm to 1 cm.

• in the sagittal direction, is approximately 6 mm.

8. Characteristics of masticatory cycles

A cycle lasts less than a second. The speed is not constant, which allows four phases to be individualized. The speed is fast during opening, and remains so during the beginning of the elevation phase (rapid elevation phase) and only decreases after the teeth have started to crush the food bolus (slow elevation phase).

During the fourth and final phase, the OIM break, movement speed drops to O 

 This pause lasts about 20% of the total time of the chewing cycle.

More generally, the masticatory cycle changes characteristics following occlusal modifications.

9. Variability of chewing cycles

The shape of the trajectories, the duration, the speed of the different phases of the chewing cycles vary according to several parameters, in the same individual and from one species to another.

– Depending on age: In children with temporary dentition, the masticatory cycle is characterized by a large lateral deflection towards the working side at opening and a lesser displacement at closing. The amplitude of the lateral movement at opening tends to decrease with age. The number of masticatory cycles required increases with age.

• Depending on the situation of the cycle in the sequence: During the time preparatory to swallowing, the cycles are more irregular both in frequency and in amplitude or shape.
• The variability existing between cycles within a chewing sequence. In the same individual are also influenced by the nature of the food chewed.
• Each individual uses a certain type of cycle more frequently, so it is possible to speak of interindividual variability. 

Usually the masticatory cycle changes characteristics following pathologies or occlusal modifications, Gibbs and Lundeen report several examples of mandibular prognathisms presenting totally vertical cycles. 

10. Envelope of chewing movements recorded at the level of the condyle:

The condyle moves obliquely from back to front and from top to bottom along trajectories that can be up to 8 mm long on the chewing side and more than 1 cm on the chewing side.

Contralateral. 

11. Muscle activities during chewing

Masticatory cycles are the result of the successive and coordinated activity of the muscles of mastication: Elevators – depressors – propulsors and retropulsors

If mandibular movements are important, it is the reciprocal activity of the tongue and the muscles of the cheek and lips which ensures the position of the food bolus.

             At the opening 

• Contraction of: – mylohyoid – geniohyoid – Digastric – external pterygoid.
• Passive stretching of the levators. 

             At closing:

• Contraction of: internal pterygoids – Masseter – Temporal.
• Passive stretching of the depressors

         Lateral Movements:

• Ipsilateral contraction of posterior and medial temporal fibers.
• Contralateral contraction of the internal and external pterygoids and the anterior temporal fibers

              To the propulsion

             Bilateral contraction of the external pterygoids.

12. Neurophysiological approach to chewing

Mandibular movements during chewing are essentially conditioned by two factors:

• An anatomical factor: guidance is provided by the occlusal structures. 
• A neurophysiological factor which, through programming from the central nervous system, brings the muscular and joint complex into harmony with this

Guidance for optimal functional efficiency. It seems that there is a central generator that holds engrams and is responsible for the rhythm of functional movements: the mastication center.

Although the study of the nervous mechanisms of mastication is complex, a schematic representation of operation can be proposed;

Diagram of the functioning of the central and peripheral nervous system during chewing .

13. Functional angles of mastication of Planas (AFMP):

It is the inclination relative to the occlusal plane of the path of the mandibular interincisal point during lateral movements to the right and left, maintaining maximum dental contact. A difference between the right and left AFMP indicates dominant unilateral chewing on the side of the weakest angle. The value of these angles also reflects dental wear due to chewing. It must be analyzed according to the patient’s age and the stage of dentition. 

14. Unilateral dominant chewing syndrome

In this syndrome the patient chews exclusively or preferentially on one side. 

The growth stimuli generated during chewing by the sliding of the TMJ on the swinging side and the powerful occlusal friction on the working side remain unilateral and lead, in children, to asymmetric maxillofacial development. 

We observe on the chewing side:

• a shorter and distally positioned hemimandible due to reduced growth stimuli at the ATM level;

• a higher mandibular body, especially at the molar level, in relation to the strong functional demands;

• a larger condyle and a more pronounced condylar slope;

• a marked development of the maxilla towards the outside and the front; This development induces a deviation of the median.

On the opposite side we see:

• excessive lengthening of the hemimandible by stimulation of condylar growth during sagittal movements of the non-working condyle;

• a more elongated condyle and a lesser condylar slope;

There is a frontal tilt of the occlusal plane upward on the chewing side and downward on the contralateral side. In adults, unilateral dominant chewing may

Induce or aggravate periodontal or joint damage.

 15. Conclusion

Chewing is a very important function because it is what organizes the occlusion (According to Planas, correct chewing is of the alternating unilateral type in order to promote harmonious and symmetrical growth of the maxillofacial sphere. This is why, when chewing is correctly accomplished, all temporary teeth wear out enormously between the ages of three and six. 

Chewing

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Chewing