Mastication / orthodontic course by indian dental academy


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Mastication / orthodontic course by indian dental academy

  1. 1. MASTICATION Contents 1) Introduction 2) Definition 3) Importance of mastication 4) Masticatory apparatus 5) Muscles of mastication  Masseter  Temporalis  Medial pterygoid  Lateral pterygoid 6) T.M.J. 7) Tongue 8) Neural masticatory receptors 9) Major functions of masticatory system 10) Parafunctional movements  Bruxism  Clenching  Nail biting  Pencil chewing etc 11) Clinical implications  T.M.J. referred pain  Orofacial pain  Muscles trismus  High points in restorations 12) References 13) Conclusion
  2. 2. INTRODUCTION: Feeding or ingestion is the process of transferring food into the gut for digestion. In many animals the mouth is merely the anterior opening of the gut. Where food is either swallowed as whole, or in large chunks with little or no mechanical processing. However in terrestrial mammals the situation is generally different. During mammalian evolution, changes occurred in the morphology of skull, teeth, jaws, and associated orofacial structures that permitted an addition stage of mechanical processing of food in the mouth, prior to swallowing. This process of mechanical breakdown of food in the mouth is mastication or chewing. Mastication can therefore be regarded as an interruption in the process of transporting food through oral cavity en route to the gut. DEFINITION: L.M. Harrison : defined mastication as a process of chewing food. IMPORTANCE OF MASTICATION: 1) Increases the surface area of food so that digestive enzymes can act on a greater area. 2) In case of most fruits and raw vegetables where the surface coating of the food is made up of indigestible cellulose or hemi cellulose, mastication causes exposure of inner digestive material. 3) It helps in the flow of saliva. 4) It helps in subsequent deglutition. Grinding of food to a very find particulate consistency prevents excoriation of the G.I.T. and increases the ease with which food is emptied from the stomach into the small intestine and then into all segments of the gut.
  3. 3. MASTICATORY APPARATUS: These involve the organs and structures primarily functioning is mastication viz. 1) Teeth 2) Muscles of mastication 3) T.M.J. 4) Tongue 5) Accessory organs of mastication Teeth: Teeth are inarguably the principle organ of mastication and are generally adopted for the functional requirement of the diet. Mammalian dentition is heterodont i.e. the teeth in different parts of the mouth differ in anatomical form and function. The anterior teeth have sharp edges for grasping, incising or tearing food while posterior teeth are specialized for cutting flesh of grinding fibrous plant materials. The human dentition is made up of 32 teeth. Each tooth can be divided into two basic parts. 1) Crown and the Root: The root is attached to the alveolar bone by means of specialized connective tissue fibers called PDL. PERIODONTAL LIGAMENT: The PDL attaches the tooth firmly to its bony socket and also helps to dissipate the forces supplied to the bone during mastication and acts as a natural shock absorber. The 32 teeth are distributed equally in the alveolar bone of the maxillary and mandibular arches of which 16 teeth are aligned in alveolar process of maxilla and 16 in alveolar process of mandible.
  4. 4. The maxillary arch is slightly larger than mandibular arch and tooth sizes are also greater than mandibular teeth. The permanent teeth can be grouped into 4 classification as follows (according to morphology of crowns) : 1) Incisors 2) Canines 3) Premolars 4) Molars Incisors: teeth located in the anterior most region are incisors and are shovel shaped with an incisal edge. 4 maxillary incisors are larger than mandibular incisors. Main function is used for incising or tearing food during mastication. Canines: Distal to incisors are canines located at the corners of the arches and are generally the longest teeth with a single cusp and root. Two maxillary and 2 mandibular canines are present. In humans, canines usually function and incisors and are used for ripping or tearing of food. Premolars: 4 maxillary and 4 mandibular premolars are present. - Since they have 2 cusps they are called bicuspids. - The presence of these two cusps greatly increases the biting surfaces of these teeth. Their main function is to begin the effective breakdown of food substances into smaller particles. Molars: The last class of teeth are the molars. There are 6 maxillary and 6 mandibular molars. - The crown of each molar has 4-5 cusps. - This provides for large, broad surface upon which breaking and grinding of food can occur.
  5. 5. Thus each tooth is highly specialized according to its function. The exact interarch and intrarch relationships of the teeth are extremely important and greatly influence the health and function of the masticatory system. MUSCLES OF MASTICATION : Muscles that power the jaw movement during mastication are known as muscles of mastication. Other muscles like tongue, muscles in lips and cheeks also aid in mastication. Muscles of mastication can be classified and anatomically into 2 categories: 1) Those between the cranium and mandible viz Masseter, Temporalis and Pterygoids. 2) Those between mandible and hyoid bone viz. Anterior Digastric, Geniohyoid and Mylohyoid. Functionally masticatory muscles can be classified as, i) Jaw elevators: Masseter, temporalis and medial pterygoid. ii) Jaw depressors: Anterior Digastric, Geniohyoid, Mylohyoid and Lateral pterygoid. The muscles Masseter, Temporalis, Medial and Lateral pterygoid are considered the principal muscles of mastication. MASSETER: It is a quadrilateral muscle consisting of 3 layers which blend anteriorly. i) The superficial layer ii) Middle layer iii) Deep layer
  6. 6. Muscle Origin Insertion i) Superficial layer It arises by a thick aponeurosis from the maxillary process of zygomatic bone from anterior 2/3rd of the inferior border of zygomatic arch. Its fibers pass downwards and backwards to insert into the angle and lower posterior half of lateral surface of mandibular ramus. ii) Middle layer Arises from the medial aspect of anterior 2/3rd of zygomatic arch and from the lower border of post 1/3rd of zygomatic arch Inserts into the central part of ramus of mandible iii) Deep layer Arises from deep surface of zygomatic arch It inserts into the upper part of the mandibular ramus and into the coronoid process. Relations: Superficial: Skin, Platysma, Risorius, Zygomaticus major and Parotid gland. Deep: Temporalis and ramus of mandible Posterior: Margin is overlapped by parotid gland. Nerve supply: Anterior branch of mandibular nerve. Actions: Elevates the mandible to occlude the teeth in mastication. TEMPORALIS :
  7. 7. Origin Insertion Arises from whole of temporal fossa (except the part formed by the zygomatic bone) and from deep surface of temporal fascia. Its fibres converge and descend into a tendon which passes through the gap between zygomatic arch and side of skull, and attaches to medial surface, apex, anterior and posterior borders of coronoid process and anterior border of mandibular ramus of mandible almost to the last molar tooth. Relations: Superficial: Skin, Auriculars anterior and superior, temporal fascial, superficial temporal vessels, Auriculotemporal nerves, temporal branches of facial nerve, zygomatic temporal nerve, epicranial aponeurosis, zygomatic arch and Masseter. Deep: Are femoral fossa, lateral, lateral pterygoid, the superficial head of medial pterygoid, a small part of buccinator, the maxillary artery, deep temporal nerves and buccal nerve and vessels. Nerve supply: Temporalis is supplied by the Deep temporal branches of anterior trunk of the mandibular nerve Actions: 1) Elevation: temporalis elevates the mandible and also closes the mouth and approximates the teeth. 2) Posterior fibres retract the protruded mandible. 3) Also contributes to side-to-side gliding movements. LATERAL PTERYGOID: It is a short, thick muscle with two parts or heads.
  8. 8. - Upper head - Lower head Muscle Origin Insertion Upper head It arises from the infratemporal surface and infratemporal crest of greater wing of sphenoid bone. Pterygoid fovea Lower head It arises from the lateral surface of lateral pterygoid plate Anterior margin of articulating disc and capsule of TMJ. Relations : Superficial: are ramus of mandible, the maxillary artery, tendon of temporalis, and Masseter. Deep: are part of medial pterygoid, the sphenomandibular ligament, middle meningeal artery and mandibular nerve. Nerve supply: supplied by branch from anterior trunk of mandibular nerve. Actions: Upper head: Elevates the mandible and medial movement from laterally displaced position (Aids mainly in chewing). Lower head: Depresses the mandible, protrusion of mandible and side-side movements. Medial pterygoid: This is a quadrilateral muscle. It has a small superficial head and a large deep head and forms the major part of the muscle. Muscle Origin Insertion Small head From the tuberosity of the maxilla and adjoining bone. The fibers run downwards and backwards and laterally insert into the roughened area on medial
  9. 9. surface of the angle and adjoining part of ramus of mandible below and behind the mandibular foramen and Mylohyoid groove. Deep head From the medial surface of lateral pterygoid plate and adjoining part of palatine bone. Nerve supply: Nerve to medial pterygoid i.e. a branch of the main trunk of mandibular nerve. Actions: 1) Elevates the mandible 2) Helps to protrude the mandible 3) Side-to-side movements i.e. chewing movements Temporomandibular joint: 1) The area where craniomandibular articulation occurs is called the T.M.J. 2) It provides for hinging movement in one plane, hence can be considered a ginglymoid joint. At the same time it also provides for gliding movements, which classifies it as an arthroidal joint. Thus it is technically considered as a ginglymoarthroidal joint. 3) The T.M.J. is formed by the mandibular condyle fitting into the mandibular fossa of the temporal bone. separating these two bones from direct articulation is the articular disc. Functionally, the articular disc serves as a nonossified bone that permits the complex movements of the joint. The T.M.J. can be discussed under the following headings. 1) Articular surface 2) Articular disc 3) Ligaments Articular surface :
  10. 10. 1) The upper articular surface is formed by the following parts of the temporal bone. i) Articular eminence ii) Anterior part of the mandibular fossa. 2) The inferior articular surface is formed by the head of the mandible. 3) The articular surfaces are covered with fibrocartilage. 4) The joint cavity is divided into upper and lower parts by an intrarticular discs. Articular disc: 1) The articular disc is an oval fibrous plate that divides the joint into an upper and a lower compartment. 2) The upper compartment permits gliding movements, and the lower, rotatory as well as gliding movements. 3) The disc has a concavo convex superior surface and a concave inferior surface. 4) The periphery of the disc is attached to the fibrous capsule. Ligaments: these are i) Fibrous capsule ii) The lateral ligament iii) The sphenomandibular ligament iv) Stylomandibular ligament i) Fibrous capsule: is attached above to the articular tubercle, the circumference of the mandibular fossa and the squamotympanic fissure and below to the neck of the mandible. The capsule is loose above the intra-articular disc, and tight below it. The synovial membrane lines the fibrous capsule and the neck of the mandible. ii) The lateral (temporomandibular) ligament: it reinforces and strengthens the lateral part of the capsular ligament. Its fibres are directed downwards and backwards. It is attached above to the auricular
  11. 11. tubercle, and below to the posterolateral aspect of the neck of the mandible. iii) The sphenomandibular ligament: it is an accessory ligament, which lies on a deep plane away from the fibrous capsule. It is attached superiorly to the spine of the sphenoid, and inferiorly to the lingula of the mandibular foramen. It is a ruminant of the dorsal part of Meckel’s cartilage. The ligament is related laterally to: - The Lateral pterygoid - The Auriculotemporal nerve - Maxillary artery - Inferior alveolar nerve and vessels Medially there are - Medial pterygoid - The chorda tympani nerve and - The wall of the pharynx Near its lower end it is pierced by the Mylohyoid nerve and vessels. iv) The Stylomandibular ligament: is another accessory ligament of the joint. It represents a thickened part of the deep cervical fascia which separates the parotid and sub mandibular salivary glands. It is attached above to the lateral surface of the styloid process, and below to the angle and posterior border of the ramus of the mandible. Relations of T.M.J.: Lateral : Shin of fascial, parotid gland and temporal branches of the facial nerve. Medial: The tympanic plate separates the joint from the internal carotid artery. Spine of sphenoid, with the upper end of the sphenomandibular
  12. 12. ligament attached to it. The Auriculotemporal and chorda tympani nerves, Middle meningeal artery. Anterior: lateral pterygoid, massetric nerve to vessels. Posterior: the parotid gland separates the joint from the external auditory meatus. Superficial temporal vessels and Auriculotemporal nerve. Superior: Middle cranial fossa Middle meningeal vessels Inferior: Maxillary artery and vein Blood supply: Branches from superficial temporal and maxillary arteries Nerve supply: - Auriculotemporal nerve - Massetric nerve Biomechanics of T.M.J.: The T.M.J is a compound joint. Its structure and function can be divided into 2 distinct systems. 1) One joint system is the tissues that surround the inferior synovial cavity (i.e. the condyle and the articular disc). Since the disc is tightly bound to the condyle by the lateral and medial discal ligaments, the only physiologic movement that can occur between these surfaces is rotation of the disc on the articular surface of the condyle. The disc and its attachment to the condyle are called the condyle disc complex. This joint system is responsible for rotational movement in the T.M.J. 2) The second system is made up of the condyle discomplex functioning against the surface of the mandibular fossa. Since the disc is not tightly attached to the articular fossa free-sliding movement is possible between these surfaces in the superior cavity. This
  13. 13. movement occurs when the mandible is moved forward (referred to as translation). Translation occurs in the superior joint cavity between the superior surface of the articular disc and the mandibular fossa. Thus the articular disc as a nonossified bone contributing to both joint systems. Normal functional movement of the condyle and disc during the full range of opening and closing. The disc is rotated posteriorly on the condyle as the condyle is translated out of the fossa. The closing movement is the exact opposite of opening. Tongue: 1) The tongue is a highly muscular organ of deglutition, taste and speech. It plays several key roles in food ingestion and subsequent intraoral processing. 2) It is partly oral and partly pharyngeal in position, and it is attached by its muscles to the hyoid bone, mandible, styloid processes, soft palate and the pharyngeal wall. 3) It has a root, an apex, a curved dorsum and an inferior surface. Movement of the tongue involves mainly an antero-posteriorly directed cyclic pattern which is linked with vertical movements of the jaws. Tongue retraction occurs mainly when the teeth are apart, while tongue is protruded when the teeth are closer together in the occlusal phase and early opening phase of chewing cycle. Tongue may also act as an organ of mastication. Soft foods may be squashed / mushed by the tongue against the hard palate. It is divided by the U-shaped sulcus terminalis into an anterior, oral or presulcal part facing upwards and a posterior, pharyngeal or post sulcal part facing posteriorly. The anterior part forms about two-thirds of the tongue’s length.
  14. 14. Oral (Presulcal) part : is located in the floor of the oral cavity, this has an apex touching the incisor teeth, a margin in contact with the gums and teeth and a superior surface (dorsum) related to the hard and soft palates. Its general sensory nerve is the lingual branch of the mandibular, the chorda tympani branch of facial nerve. Pharyngeal (postsulcal / part) forms the base of the tongue; it lies posterior to the palatoglossal arches within the oropharynx, forming its anterior wall. Devoid of papillae, it is no elevations due to lymphoid nodules embedded in the submucosa collectively termed the lingual tonsil. Muscles of tongue: A middle fibrous septum divides the tongue into right and left halves. Each half contains 4 intrinsic and 4 extrinsic muscles. Intrinsic muscles Extrinsic muscles 1) Superior longitudinal 2) Inferior longitudinal 3) Transverse and 4) Vertical 1) Genioglossus 2) Hyoglossus 3) Styloglossus 4) Palatoglossus Blood supply: Lingual artery, a branch of external carotid artery. The root is supplied by the tonsillar and ascending pharyngeal arteries. Nerve supply: The Lingual nerve is the nerve of general sensation and Chorda tympani is the nerve for taste in the ant 2/3rd except for vallate papilla. - The Glossopharyngeal nerve is the nerve for both general sensation and taste for posterior 1/3rd of the tongue including circumvallate papilla. - The posterior most part of the tongue is supplied by the Vagus nerve through the Internal laryngeal branch.
  15. 15. Accessory organs of mastication: 1) These play essentially supportive role. 2) Lips can aid in the ingestion of food and provide an anterior oval seal to prevent spillage of food from mouth. 3) Tongue and cheek combine to direct the bolus on the occlusal surface of posterior teeth. 4) Salivary glands provide the intra oral lubrication for these activities. Neural masticatory receptors: 1) The various coordinated masticatory activities of the mandible are reflected by the approximate muscle function. 2) Each muscle is innervated by α (alpha) -efferent motor neurons that supply the extrafusal muscle film. 3) Where as γ-efferent supply the intrafusal fibres of the muscle spindle. 4) Each muscle comprises fibres that exhibit rapid twitch contraction or slow twitch contraction. There are also muscle fibres with intermediate properties. 5) Contraction of individual muscle fibres is a function of muscle unit. Muscle unit comprises a single α (alpha) motor neurons, its α (alpha) – efferent nerve fibre and number of muscle fibres. 1) Muscle spindle: 1) They comprise stretch sensitive, slowly adopting specialized intrafusal muscle fibre that are 2) contained with in a capsule laying parallel to the extrafusal muscle fibres. Spindle generally has a double afferent innervation.
  16. 16. i) Large – group Ia myelinated afferent fibres terminate in the central region of each intrafusal fibre called as primary or annual spiral ending. ii) Smaller group II myelinated afferent fibre ending on either side of the central region as spray or secondary endings. There is a concept that the muscle spindle may be involved in correcting small errors between the intended and actual mandibular movements and maintaining a constant posture against the effect of gravity. 2) Golgi tendon organs: These are the receptors primarily located at muscle tendon junctions or TMJ capsule. They are innervated by Ib myelinated afferent fibres. There is no evidence of such units within the masticatory muscles. 3) Periodontal mechanoreceptors: The periodontal ligament mechanoreceptors respond to forces applied to the teeth. These mechanoreceptors have a wide range of properties. i) Some are excited by just often microns of tooth displacement. ii) Some are less sensitive and respond only to much larger forces. iii) Some exhibit directional sensitivity, with nerve fibres responding maximally to forces in one particular direction. iv) Some are slowly adopting and produce continuous discharge when constant stimulus is applied. v) Some adopt more rapidly, producing only a few impulses immediately when stimulated. vi) Some are very rapidly adopting units and do not respond unless a very rapid stimulus is applied.
  17. 17. vii) Some are very slowly adopting units and provide a constant discharge that can be increased / decreased by applying forces in specific direction. Most single fibres respond to mechanical stimulation of just one teeth, but some also respond to stimulation of upto 3 adjacent teeth. Cell bodies of these fibres are located in trigeminal ganglion, with some others in the trigeminal mesencephalic nucleus. 4) Mucous membrane receptors: There are some cells in the mesencephalic nucleus, main sensory and spinal trigeminal nuclei that respond to pressure in the palate, particularly in the region just distal to central incisors. 5) Joint receptors: Free nerve fibres composite the predominant receptors in TMJ capsule. The lateral aspect of joint capsule and lateral ligament also contains Ruffin, Pacinian and Golgi receptors and are supplied by a branch of Auriculotemporal nerve. Control of mastication: Though mastication is a ‘voluntary’ process, little conscious effort is involved, actually chewing occurs anatomically in much the some way as walking or breathing. A number of theories have been put forward to explain how mastication is controlled. Most of these theories include a contribution from reflex actions. Jaw reflexes: Reflex can be defined as an automatic / involuntary activity brought about by relatively simple circuits without consciousness being necessary involved. Jaw reflexes effort the vertical relationship between upper and lower jaw as well as horizontal relationship which involve lateral and anteroposterior movement of mandible with respect to the maxilla.
  18. 18. Thus jaw reflexes can be discussed under 2 headings viz. - Vertical jaw reflexes and - Horizontal jaw reflexes Vertical jaw reflexes: Vertical jaw reflexes can be considered under 2 broad categories. 1) Those evoked by stimulation of receptors with in the muscles themselves. i) Jaw jerk reflex ii) Jaw unloading reflex 2) Those which are responses to stimuli of external origin (eg. food) i) Jaw opening reflex ii) Reflexes which involve activation of the jaw elevator muscles. i) Jaw jerk reflex : Jaw jerk is the simplest of the jaw reflexes in that if the only one mediated by a monosynaptic pathway. It is analogous to the knee jerk and is a stretch reflex whereby stretching the jaw elevator muscles usually by applying a downward tap on the chin-produces a reflex contraction of these muscles. The significance of this reflex lies not in it happening as such during normal function but in that it demonstrates the existence of feedback between the jaw elevator muscles and their own motor neurons. This feedback mechanism helps in the fine control of jaw movements during normal functions to take account of varying external circumstance. E.g. change in the consistency of food as it is broken up during mastication. The reflex arc for the jaw jerk is known to start within the jaw elevator muscles at the muscle spindle primary ending which via their primary afferent nerve make direct monosynaptic connections with the motor neurons in the trigeminal motor nucleus. ii) Jaw unloading reflex :
  19. 19. This reflex involves some jaw opening but most be distinguished from those reflexes known as jaw opening reflex. Since its trigger is very different. Jaw unloading reflex is evoked when a hard object which is being bit breaks suddenly thus ‘unloading’ the jaw elevator muscle together with an activation of jaw depressor muscles. The result is that the opposing teeth do not come strongly into contact with one another after breaking trough the hard object and that is this way, potential damage to the masticatory apparatus is avoided. This reflex is heavily dependent on receptors in the jaw and muscles. When one is biting on an object which one knows or suspects to be brittle, one sends not only powerful excitatory signals to the jaw elevator motor neurons but also, as a precaution, weaker excitatory signals to the jaw depressor motor neurons. Jaw elevator motor neurons receive positive feedback from their own muscle spindle via jaw jerk pathway. Signals from jaw elevator muscle spindles produce an inhibitory effect on the antagonist, jaw depressor motor neurons – this is known to occur in spinal cord. Thus while biting on the object there will be 2 excitatory drives to jaw elevator motor neurons while the depressor motor neurons will be receiving a mixture of excitatory and inhibitory drives. When the object breaks, the sudden shortening of elevator muscles will result in decrease in spindle activity and hence in overall excitatory drive to the jaw elevator motor neurons and the inhibitory drive to the jaw depressor motor neurons. In turn this causes the decreased activity in the jaw elevator muscles and increased activity in the depressor. iii) Jaw opening reflex :
  20. 20. The term jaw opening reflex can be misleading since there are several reflexes which can in one or other way, cause jaw opening – including jaw unloading reflex. Simplest of there is the disynaptic reflex activation of motor neurons to the Anterior Digastric muscle in response to the mechanical or noxious stimulation in or around the mouth. The 1st synapse is believed to be in the trigeminal sensory nuclear complex mast probably in nucleus oralis or nucleus interpolasis and 2nd synapse located in the trigeminal motor nucleus. Horizontal jaw reflexes: These reflexes involve lateral, protrusive and passively retrusive movements of the jaw in response to stimulation of intraoral mechanoreceptors. These reflexes are for less well understood than the vertical once mainly because they are more difficult to investigate. There is a possibility that those reflexes are triggered by horizontal loading of the teeth and that consequently they might play a role in adjusting the final closure of the jaws from the moment of 1st tooth contact until the intercuspal position is reached. Masticatory mandibular movements: The range of masticatory mandibular movements were first described by Ulrich and Bernet at the turn of 20th century. They showed that there was no fixed axis of mandibular rotation. Mandibular movements occurs as a complex series of interrelated three dimensional movements. They can be broken down into 2 basic components. Two types of movements occur in the TMJ. 1) Rotational: when the body is turning about axis. 2) Translational: when all the points within a body have identical motion.
  21. 21. Every possible 3 dimensional movement can be described in terms of these 2 components. It is easier to understand mandibular movement when the components are described as projections in 3 perpendicular planes. 1) Sagittal 2) Horizontal and 3) Frontal (vertical) planes Reference planes: Sagittal plane: In the Sagittal plane, the mandible is capable of a purely rotational movement as well as translation. Rotation occurs around the terminal hinge axis, which is an imaginary horizontal line through the rotational centers of the left and right condylar processes. The rotational movement is limited to about 12mm of incisor separation before the T.M. ligaments and structures anterior to the mastoid process force the mandible to translate the initial rotation or hinging motion is between the condyle and the articular disc. During translation, the lateral pterygoid muscle contracts and moves the condyle disk assembly forward along the posterior incline of the tubercle. Condylar movement is similar during protrusive mandibular movement. Horizontal plane: In their plane, the mandible is capable of rotation around several vertical axes e.g. lateral movement consists of rotation around on axis situated in the working (laterotrusive) Condylar process, with relatively little concurrent translation. This slight lateral translation is known as Bennett movement, mandibular side shift, or laterotrusion. This is frequently present. This may
  22. 22. be slightly forward called lateroprotrusion or slightly backward called lateroretrusion. The orbiting (or nonworking) condyle travels forward and medially as limited by the medial aspect of the mandibular fossa and the temporomandibular ligament. Finally, the mandible can make a straight protrusive movement. Frontal plane: When a lateral movement occurs in the frontal plane, the mediotrusive (non-working) condyle moves down and medially, while the laterotrusive (or working) condyle rotates around the Sagittal axis perpendicular to this plane. Due to the anatomy of the medial wall of the mandibular fossa on the mediotrusive side, transtrusion may be observed. Due to the anatomy of the mandibular fossa on the laterotrusive side, this may be lateral and upward or lateral and downward (laterotrusion) and laterodetrusion. A straight protrusive movement occur in the frontal plane, with both condylar processes moving downward as they slide along the tubercular eminences. Border movements: Mandibular movements are limited by the T.M.J and ligaments, the neuromuscular system and the teeth. Posselt first who described the extremes of the mandibular movements, which he called as the border movements. Posselt used a 3-dimensional representation of the extreme movements the mandible is capable of All possible mandibular movements occur with its boundaries. Starting at the intercuspal positions in the protrusive pathway, the lower incisors are initially guided by the lingual concavity of the maxillary anterior teeth. This leads to gradual loss of posterior tooth contact as the
  23. 23. incisors reach the edge-to-edge position. This is represented in the Posselt’s diagram by the initial downward slope. As the mandible moves further protrusively, the incisors slide over a horizontal trajectory representing the edge-to-edge position (the flat portion in the diagram), after which the lower incisors move upward until new posterior tooth contact occurs. Further protrusive movement of the mandible typically takes place without significant tooth contact. The border farthest to the right of Posselt’s solid represents the most protruded opening and closing stroke. The maximal open position of the mandible is represented by the lowest point in the diagram. The left border of the diagram represents the most retruded closing stroke. This movement occurs in 2 phases. The lower portion consists of a combined rotation and translation, until the condylar processes return to the fossae. The record portion of the most retruded closing stroke is represented by the top portion of the border that is farthest to the left in Posselt’s diagram. It is strictly rotational. Posterior and anterior detriments: The characteristics of mandibular movement are established posteriorly by the morphology of the T.M.J’s and anteriorly by the relationship of the anterior teeth. The posterior determinants are shape of the articular eminences, anatomy of the medial walls of the mandibular fossae configuration of the mandibular condylar processes. Impact of skeletal variables on occlusal form of restorations. Posterior determinants Variants Impaction restoration Inclination of articular eminence Steeper Posterior cusps may be taller. Flatter Posterior cusps must be shorter Medial wall of glenoid fossa Allows more lateral translation. Posterior cusps must be shorter.
  24. 24. Allows minimal lateral translation. Posterior cusps may be taller. Intercondylar distance Greater Smaller angle between laterotrusive and mediotrusive movement. Lesser Increased angle between laterotrusive and mediotrusive movement. The anterior determinants: are the vertical and horizontal overlaps and the maxillary lingual concavities of the anterior teeth. These can be altered by restorative and orthodontic treatment. A greater vertical overlap causes the direction of mandibular opening to be more vertical during the early phase of protrusive movement and creates a more vertical pathway at the end of the chewing stroke. Increased horizontal overlap allows a more horizontal jaw movement. Envelope of motion: By combining mandibular movements in the three planes (i.e. sagittal, horizontal, frontal) or 3-dimensional envelope of motion can be produced, that represents the maximum range of movement of the mandible. The superior surface of the envelope is determined by tooth contacts, whereas the other borders are primarily determined by ligaments and joint anatomy that restrict or limit movement. Three dimensional movements: To demonstrate the complexity of mandibular movement, a seemingly simple right lateral excursion will be used. As the musculature begins to contract and move the mandible to the right, the left condyle is propelled out of its centric relation position. As the left condyle is orbiting anteriorly around the frontal axis of the right condyle, it encounters the posterior slope of the articular eminence, which causes an inferior movement of the condyle around the sagittal axis with resultant tilting of the frontal axis.
  25. 25. Additionally, contact of the anterior teeth produces a slightly greater inferior movement in the anterior part of the mandible than in the posterior part, which results in an opening movement around the horizontal axis. Because the left condyle is moving anteriorly and inferiorly, the horizontal axis is shifting anteriorly and inferiorly. This example illustrates that during a simple lateral movement, motion occurs around each axis (i.e. sagittal, horizontal, vertical) and simultaneously each axis tilter to accommodate to the movement occurring around the other axes. All this happens within the envelope o0f motion and is intricately controlled by the neuromuscular system to avoid injury to any of the oral structures. MAJOR FUNCTIONS OF MASTICATORY SYSTEM: Functional movements: The three major functions of the masticatory systems are 1) Mastication 2) Swallowing 3) Speech Most functional movements of the mandible take place inside the physiologic limits established by the teeth, the T.M.J’s and the muscles and the ligaments of mastication. Hence these movements are rarely coincident with border movements. Mastication: Mastication is defined as the act of chewing foods. It represents the initial stage of digestion, when the load is broken down into small particle sizes for case of swallowing. It is a complex function that uses the muscles, teeth and periodontal supportive structures, as well as the lips, cheeks, tongue, palate and salivary glands.
  26. 26. Chewing stroke: Mastication is made up of rhythmic and well controlled separation and closure of the maxillary and mandibular teeth. This activity is under the control of the Central Pattern Generator (C.P.G) located in the brain stem. The complete chewing stroke has been described as a tear shaped movement pattern. It can be divided into an opening movement and a closing movement. They closing movement has been further subdivided into the crushing phase and the grinding phase. During mastication similar chewing strokes are repeated over and over as the food is broken down. Tooth contacts during mastication: 1) When food is initially introduced into the mouth, few contacts occur. As the bolus is broken down, the frequency of tooth contact increases. 2) In the final stages of mastication, just before swallowing, contacts occur during every stroke. 3) Two types of contacts have been identified : i) Gliding contact, which occurs as the cuspal inclines pass by each other during the opening and grinding phases of mastication. ii) Single contact, which occurs in the maximum intercuspal position. 4) The mean percentage of gliding contacts that occur during chewing has been found to the 60% during the grinding phase and 56% during the opening phase. 5) The overage length of time for tooth contact during mastication is 194 msec. 6) It has been demonstrated that the occlusal condition can influence the entire chewing stroke.
  27. 27. 7) During mastication the quality and quantity of tooth contacts constantly relay sensory information lack to the CNS regarding the character of the chewing stroke. 8) This feed back mechanism allows for alteration in the chewing stroke according to the particular food being chewed. 9) Generally, take cusps and deep fossae promote predominantly vertical chewing stroke, whereas flattened or warn teeth encourage a broader chewing stroke. When the posterior teeth contact in undesirable lateral movement, the malocclusion produces on irregular and less repeatable chewing stroke. 1) Normal persons with good occlusion masticate with chewing strokes that are well rounded, with definite borders and less repeated. 2) The chewing strokes of persons with TMJ pain show a repeat pattern. The strokes are much shorter and slower and have an irregular pathway. The mouth than opens slightly, the tongue pushes the food onto the occlusal table, and after moving sideways, the mandible classes into the food until the guiding teeth contact. This cycle is completed as the mandible returns to its starting position. This pattern repeats itself until the food bolus has been reduced to particles that are small enough to be swallowed, at which point the process can start over. The directed of the mandibular path of closure is influenced by the inclination of the occlusal plane with the teeth apart and by the occlusal guidance as the jaw approaches. Intercuspal position: The chewing pattern observed in children differs from that found in adults. Until about age 10, children begin the chewing stroke with a lateral
  28. 28. movement. After age 10, they start to chew increasingly the adults, with a more vertical stroke. Speech occurs when a volume of air is forced from the longs by the diaphragm through the larynx and oral cavity. Controlled contraction and relaxation of the vocal cards (we bonds of the larynx) create a sound with the desired pitch. Once pitch is produced the precise from assumed by the mouth determines the resonance exact articulation of the sound. Speech occurs during expiration. Speaking: The teeth, tongue, lips, floor of the mouth and soft palate form the resonance chamber that affects pronunciation. During speech the teeth are generally not in contact, although the anterior teeth may come very close together during ‘C’ ‘CH’, ‘S’ and ‘Z’ sounds, forming the speaking space. When pronouncing ‘F’ the inner vermilion border of the lower lip traps air against the incisal edges of the maxillary incisors. Parafunctional Movements: These can be described as sustained activities that occur beyond the normal functions of mastication, swallowing and speech. The various parafunctional activities are: 1) Bruxism. 2) Clenching. 3) Nail biting. 4) Pencil chewing etc. Parafunction is manifested by long periods of increased muscle contraction and hyperactivity. Excessive occlusal pressure and prolonged tooth contact occur, which is inconsistent with the normal chewing cycle. Over a protracted period this can result in excessive wear, widening of P.D.L and mobility, migration or
  29. 29. fracture of teeth. Muscle dysfunction such as myospasms, myositis, myalgia and referred pain (borderers) may also occur. Bruxism: Sustained grinding, rubbing together, or gnashing of teeth with greater than normal chewing farce is known as Bruxism. This activity may be diurnal, nocturnal or tooth. The etiology of bruxism is often unclear. Some theories relate bruxism to malocclusion, neuromuscular disturbances, responses to emotional distress, or a combination of these factors. Clenching: Is defined as forceful clamping together of the jaws in static relationship. The pressure thus created can be maintained over a considerable time with short periods of relaxation in between. The etiology can be associated with stress, anger, physical exertion, or intense concentration on a given task, rather than on occlusal disorder. Effects: Abfractions i.e. cervical defects at the CEJ may result from sustained clenching. Also the increased load may result in damage to the periodontium, temporomandibular joints and muscles of mastication. The elevator muscles may become over developed. A progression of muscle splinting, myospasms, and myositis may occur. Biting force: (Forces of mastication) The maximum biting force that can be applied to the teeth varies from individual to individual. Generally males can bite with more force than females can. In females maximum biting load ranges from 79-99 pounds (35.8 – 44.9 kg). A male’s biting load varies from 118-142 pounds (53.6 – 64.4 kg). The greatest maximum biting force reported is 975 pounds (443 kg).
  30. 30. The biting force also varies from tooth -tooth. The maximum amount of force applied to a molar is usually several times that which can be applied to the on incisor. The range of maximum force applied to the 1st molar is 91-198 pounds (41.3 – 89.8 kg). The maximum force applied to control incisors is 29 – 5’ pounds (13.2 – 23.1 kg). The maximum biting force appears to increase with age up to adolescence. The factors influencing biting force are: 1) Particular tooth. 2) Dietary consistency. 3) Degree of chronic periodontal disease. 4) Jaw separation. 5) Natural / artificial teeth. 6) Biting practice and parafunctional overuse. 7) Craniofacial morphology.
  31. 31. Conclusion References: • Gray’s Anatomy- 38th Edition • Textbook of Medical Physiology, 9th Edition: Guyton and Hall • Essentials Of Oral Physiology: Bradley • Scientific Basis Of Eating: R.W.A Linden • Human Anatomy Vol 3: B.D Chaurasia • Medical physiology 5th Edition: Sujit K.Chaudhuri