(1)
Faculty of Medicine of Montpellier, Montpellier, France
Abstract
The masticatory function is realised by the temporomandibular joint (TMJ) activated by masticatory muscles: mandibular strap and temporal range for the closure, and lateral pterygoid for the opening. The biomechanical organisation of the somato-gnathic complex depends on the type of feeding explaining its different types in animals. Human beings are intended to use all kinds of food and their mouth is also designed for articulated language. This justifies the discordant TMJ with a meniscus and the particular protraction movement of the condyle during the aperture of the mouth given by the lateral pterygoid muscle which is fixed on both the mandible and the meniscus. It is a tendency to functionally link mandibular and upright postures, and as a hypothesis, it can be suggested to find in the thalamic organisation some similarities in the management of these two bilateral stabilisations very often asymmetrical (dominance and laterality).
7.1 Introduction
The somato-gnathic complex comprises a certain number of organs around the oral cavity whose main function is chewing (or manducation). This concerns not only solid foods but also words (phonation) with the major role of the lingual modulator of sounds and cleaner of mouth toward the pharynx.
The mandible is the mobile bone of the complex. It is articulated to the skull base by the temporomandibular joint (TMJ). It is set in motion by the masticatory muscles responsible for: mouth opening by the lateral pterygoid muscle combined synergistically to the digastric one and mouth closure by the mandibular strap formed by the masseter and the medial pterygoid and by the temporal range providing selective clamping of the dental arches (Fig. 7.1).
Fig. 7.1
Left temporal and masseter muscles. (a) Dissection of temporal: 1. Vertical anterior part; 2. Frontal bone; 3. Oblique middle part; 4. Parietal bone; 5. Horizontal inferior part (antagonist of the lateral pterygoid); 6. Occipital bone; 7. Temporal bone; 8. Zygomatic arch. (b) Dissection of masseter: 1. Superficial part almost perpendicular to the occlusal line; 2. Styloid process; 3. Deep part; 4. Meatus acusticus externus; 5. Mastoid process. (c) Dissection of an hemiface: 1. Temporal with its tendon; 2. Zygomatic arch; 3. Masseter; 4. Posterior belly of digastric
On the embryological plan, the whole face with the three floors (orbital, maxillary and mandibular) derives from the first visceral arch colonised by the trigeminal nerve (V). The second visceral arch gives the hyoid bone – supporting the tongue and the larynx – which is linked to the mandible: (i) by the muscles of the floor of mouth (mylohyoid and anterior belly of the digastric belonging to the first arch); (ii) by the posterior belly of the digastric; and (iii) by the stylohyoid attached to the styloid process (territory of the second arch colonised by the facial nerve, VII). This explains the multiple functional synergies between the mandible and larynx.
The comparative anatomy has shown clearly the morphological and functional specialisation of stomato-gnathic complex depending on the type of feeding in different species. Therefore, the masticatory apparatus and teeth are preset accordingly.
Ruminants feed on plants, and their entire digestive tract from the mouth to the anus is built based on this type of food. They have cylindrical transverse condyle, allowing a lateral displacement of the jaw adapted for grinding and finishing plants from merycic rumen, the first gastric pouch.
Carnivores chase their mobile provender made mainly of meat. They need a strong and stable clamp jaw with a strong and retentive closing system on the prey that is realised by the canines offset hook, which is possible with a 1-DF TMJ hinge and very powerful temporal muscles. A lagomorph, like a rabbit, performs rapid jaw shear movements through a condyle lying in anteroposterior axis with pterygoid and masseter muscles responsible for the powerful alternative mandibular movement. The hen, known by its absence of teeth, compensates for this disadvantage by the use of a gizzard mill effective for grinding grain.
Finally, man is undoubtedly an omnivore and the structure of his digestive tract and glands proves it. Its TMJ allows all movements of specialised types briefly described before: opening, closing, antetraction, retrotraction and diduction.
It seems interesting to analyse in a comprehensive vision the original solution provided to the technical problem of eating [1].
7.2 Movements of the TMJ
They are numerous and conditioned by the configuration of the TMJ and the muscular actions.
7.2.1 Joint
This is probably the one throughout the body that is the most sought, in chewing food and in phonation. It must withstand very important occlusive forces and allow multidirectional movements, involving rotation and translation. Therefore, all joint surfaces are covered with fibrocartilage and contact zones of hyaline cartilage [2]. The following two original technical features can be described.
7.2.1.1 The Articular Contact Surfaces
On the mandible, there is the condylar process with its head or articular condyle placed above the neck and, on the temporal bone, the articular tubercle above extending forwardly the mandibular or glenoid fossa. It is therefore a question to have two convex cartilage surfaces by nature discordant in contact.
Both mandibular condylar heads form an angle of their transverse portion of 140° which is open toward the front with individual variations. In addition, the mandibular neck is slightly inclined backward and the head is flexed forward to be opposed to the tubercle of the temporal. Moreover, when the mandible is in place, and not placed on a table in the position which it used to be when described by ancient anatomists, the mandibular ramus or rising part is vertical and the body of the mandible goes obliquely downward (which is a test of anatomical quality of the drawing of the mandible in place) (Fig. 7.2).
Fig. 7.2
Temporo-mandibular joint (TMJ). (a) Sagittal section: 1. Fissura tympanosquamosa; 2. Tympanal bone; 3. Retrocondylar fibro-venous cushion (bilaminar zone); 4. Posterior thick part of meniscus within the mandibular fossa of the temporal bone; 5. Mandibular condyle; 6. Inferior articular cavity of the condylo-meniscal joint; 7. Articular tubercle of temporal bone; 8. Superior articular temporo-meniscal cavity; 9. Lateral pterygoid muscle (LPM). (b) Dissection of the disc and lateral pterygoid muscle: 1. Collar; 2. Posterior part of the disc covering the mandibular condyle; 3. Anterior part of the disc and superior articular cavity; 4. Meniscal superior head of LPM; 5. Inferior part of LPM fixed on fovea pterygoidea below the condyle. (c) Sagittal section of TMJ: 1. Posterior SCC; 2. Lateral SCC; 3. Incudomallear joint; 4. Tympano-squamous fissure; 5. Vaginal crest of tympanic bone; 6. Retrocondylar fibro-venous cushion; 7. Meniscus; 8. Mandibular condyle; 9. LPM; 10. Parotid gland
7.2.1.2 The Meniscus
All species with a mandible have a biconcave articular disc (meniscus) with a thick posterior rim which fits exactly into the mandibular fossa in intercuspidation position. This disc creates the connective damping of compression forces during dental clamping. A thinner anterior rim receives the meniscal attachment of the lateral pterygoid muscle. A middle area interposes exactly between the condyle and the articular tubercle.
The articular disc continues back through the capsule that attaches to the neck and with the mandibular periosteum as well. There are thus two separate articular cavities. The first superior one has an S shape extending from the upper portion of the posterior rim of the disc to beyond the articular tubercle on the superior face of the lateral pterygoid meniscal part. The second inferior one extends from the posterior part of the condylar head to its cartilage edge, at the base of the condylar insertion of the lateral pterygoid.
A significant anatomical detail is, behind the disc, the existence of a fibro-venous retrocondylar space, also called posterior connective vascular complex or less precisely bilaminar area (Fig. 7.3). It is made of a fibrous connective tissue in which lies a very rich venous plexus. It occupies the space between the disc overlying the condylar head and the anterior part of the tympanic bone joined to the temporal bone by the petrotympanic fissure of Glaser. This structure plays the role of a rear stop for the mandibular condyle.
Fig. 7.3
MRI of the right TMJ in coronal sections. (a) Mouth closed: 1. Medial pterygoid muscle; 2. Collar; 3. Mandibular condyle; 4. Zygomatic arch. (b) Mouth open: 1. Articular tubercle of temporal; 2. The retrocondylar fibro-venous cushion like an open accordion with very visible thin veins; 3. Mandible
This is the appropriate technical solution to the problem of opening the mouth which is done by pulling forward the mandibular condyle. That imperatively requires an extensible and flexible structure filled with venous blood which changes the posterior empty fibrous tissue and make possible the protraction of the condyle. This original feature can be seen on an MRI coronal section and can be compared with an accordion full of blood instead of air.
7.2.2 Joint Mobilisation
It is done by a series of muscles responsible for the different mandibular movements.
7.2.2.1 Closing the Mouth and Dental Clamping Intercuspation
This is the active part of the food chewing, which is carried out by two systems.
The Mandibular Strap
It is formed by two symmetrical components, outside the masseter muscle [3, 4] and inside the medial pterygoid. One should note the oblique orientation of the two muscles pulling on the mandibular body perpendicular to the occlusal line [5]. This solves the problem of the need for cusp compression forces correctly oriented to avoid lateral stresses that may cause dental malposition. It is obvious that the mechanical perfection is impossible to imagine, which justifies and maintains the orthodontist’s work.
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
