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Mandibular movements / /certified fixed orthodontic courses by Indian dental academy


The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and offering a wide range of dental certified courses in different formats.

Indian dental academy provides dental crown & Bridge,rotary endodontics,fixed orthodontics,
Dental implants courses.for details pls visit ,or call

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  • 1. MANDIBULAR MOVEMENTS INDIAN DENTAL ACADEMY Leader in continuing dental education
  • 2. There are four determinants of mandibular movements. Two posterior, one anterior and a neuromuscular determinant.
  • 3. POSTERIOR DETERMINANT The TMJ and its suspensory ligaments, centres of rotation, axes of rotation, translation of these centres. ANTERIOR DETERMINANT - Visible component The contacting areas of upper and lower teeth, inclines of cusps and nature of occlusion in centric relation and eccentric movements.
  • 4. NEUROMUSCULAR DETERMINANT The role of muscle spindles, proprioceptive engrain and neuromuscular response to occlusal conditions. The two posterior determinants are fixed. The third determinant, namely occlusion can be modified by the dentist to certain limits. The fourth neuromuscular determinant can be reflexly modified by the dentist indirectly as he alters-the third determinant, namely, viz. occlusion.
  • 5. If we modify occlusion (viz. by restorative procedures, occlusal equilibration, orthodontic therapy and extractions.) the fourth determinant, namely , the neuromuscular determinant will show a favourable response by release of inhibited movement or cessation of bruxism The ability of dentist to modify the occlusal contact pattern of teeth to alter proprioceptive stimuli and muscle function is known as occlusal programming .
  • 6. Example: A case of TMJ dysfunction with a known interceptive premature contact or a slide in centric. Now, if the dentist corrects the occlusal discrepancy, then he is able to alter the proprioceptive signals received from the teeth, which resulted in TMJ dysfunction.
  • 7. In other words, the dentist was able to indirectly modify the neuromuscular response (the fourth determinant) by modifying occlusion (third determinant) to alter proprioceptive stimulus. These altered proprioceptive stimuli are now able to release the inhibited movements or spasm of the musculature. This ability of the newly created occlusion (occlusal rehabilitation, occlusal equilibration, splint, etc) to Programme muscle function is referred to as occlusal programming
  • 8. MANDIBULAR MOVEMENT As for any other movement in space, complex three-dimensional mandibular movement can be broken down into two basic components: translation, when all points within a body have identical motion, and rotation, when the body is turning about an axis.
  • 9.
  • 10. Every possible three-dimensional movement can be described in terms of these two components. In addition, it is easier to understand mandibular movement when the components are described as projections in three perpendicular planes: sagittal, horizontal, and frontal reference planes
  • 11.
  • 12. 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, an imaginary horizontal line through the rotational centers of the left and right condylar processes.
  • 13.
  • 14. The rotational movement is limited to about 12 mm of incisor separation before the temporomandibular ligaments and structures anterior to the mastoid process force the mandible to translate. 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.
  • 15.
  • 16. Horizontal Plane In the horizontal plane, the mandible is capable of rotation around several vertical axes. For example, lateral movement consists of rotation around an axis situated in the working (laterotrusive) condylar process with relatively little concurrent translation. A slight lateral translation-known as Bennett movement, mandibular sideshift, or laterotrusion is frequently present.
  • 17.
  • 18. This may be slightly forward or slightly backward (lateroprotrusion or lateroretrusion). The orbiting (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.
  • 19.
  • 20. Frontal Plane. When observing a lateral movement in the frontal plane, the mediotrusive (or nonworking) condyle moves down and medially while the laterotrusive (or working) condyle rotates around the sagittal axis perpendicular to this plane.
  • 21.
  • 22. Again, as determined by the anatomy of the medial wall of the mandibular fossa on the mediotrusive side, transtrusion may be observed as determined by the anatomy of the mandibular fossa on the laterotrusive side, this may be lateral and upward or lateral and downward (laterosurtrusion and laterodetrusion). A straight protrusive movement observed in the frontal plane, with both condylar processes moving downward as they slide along the tubercular eminences.
  • 23.
  • 24. CONDYLAR MOVEMENTS During lateral movements of the jaw, nonworking condyle is drawn inward from centric position by the lateral pterygoid and as a result it translates in a forward, downward and anterior direction. The opposite working of condyle rotates and moves outward (latero protrusion - Bennett's movement). I.O.W External pterygoid moves the orbiting condyle medially and the rotating condyle moves out. The bodily shift during laterotrusion of working condyle is known as Bennett's shift.
  • 25. Condylar guidance The Glossary of Prosthodontic Terms, 7 th edition, the Academy of Prosthodontics 1999: I Condylar guidance : Mandibular guidance generated by the condyle and articular disc transversing the contours of the glenoid fossae. 2. Condylar guidance : The mechanical form located in the upper posterior region of an articulator that controls movement of its mobile member. Condylar path: That path traveled by the path mandibular condyle in the temparomandibular joint during various mandibular movements.
  • 26. Protrusive condyle path: The path path the condyle travels when the mandible is moved forward from its initial position. Lateral condylar path: The path of path movement of the condyle disc assembly in the joint cavity when a lateral mandibular movement is made. Condylar inclination : The direction of the lateral condyle path
  • 27.
  • 28. Working side Rotating side (rotating condyle) Ipsilateral side (ipsilateral condyle) Laterotrusive side (laterotrusive condyle) Pivoting side (pivoting condyle)
  • 29. Non working/ balancing side / Idling side Orbiting side (orbiting condyle) Contra lateral side (contra lateral condyle) Mediotrusive side (mediotrusive condyle) Advancing condyle / translating condyle
  • 30.
  • 31. Protrusive movement Sagittal protrusive condylar path - forward and downward translation of mandibular condyle.
  • 32. Lateral movement i Sagittal lateral condylar path Medial and downward movement of the nonworking condyle. This path is longer and steeper than sagittal protrusive condylar path. The angle between them is the Fischer angle (5 degrees).
  • 33.
  • 34. Horizontal lateral condylar path Consists of immediate and progressive mandibular lateral translation (ISS & PSS). ISS occurs when nonworking condyle moves from centric relation straight medially (1.0 mm). PSS occurs during the translitory forward movement of nonworking condyle. It is directly proportional to the forward movement of nonworking condyle.
  • 35. Bennett (1908) studied working condylar path and called it Bennett movement, now referred as laterotrusion. Bennett showed that the working condyle moved outwards during sideward movement of mandible in frontal plane, whereas the non-working condyle moved inward. Bennett described this bodily shift of mandible without having any knowledge of Balkwill's description in 1866 of the same side shift.
  • 36. The orbiting condylar path (horizontal lateral condylar path) consists of two components namely; an immediate and progressive mandibular lateral translation. Immediate lateral translation - Immediate side shift ISS, occurs when non working condyle moves from centric relation straight inward or medially. Progressive lateral translation progressive side shift PSS, is the translatory portion of lateral movement.
  • 37.
  • 38. Although Bennett has described about this movement which became popularly known as Bennett movement, the original discovery of this movement should go to BALKWILL. As early as 1870 Balkwill observed that the mandible opened and closed on an axis that runs through the condyles, that the condyles move downwards and forwards in protrusion and also the mandible moves bodily from side to side.
  • 39. His observation was forgotten and remained in the archives of London library. Without being aware of Balkwills work, Bennett demonstrated that the TMJ permitted three kinds of movement.
  • 40.
  • 41. Bennett's movement refers to the condylar movements on the working side and Bennett's shift is the bodily side shift of the mandible on the working side generally in horizontal direction.
  • 42. The Glossary of Occlusal Terms, International Academy of Gnathology, 1979:
  • 43. Bennett's movement (transtrusion, side shift) - The bodily side thrust or shift of the mandible regulated by the anatomical configurations of the glenoid fossa or the capsular ligaments.
  • 44. The Glossary of Prosthodontic Terms, 6 th edition, the Academy of Prosthodontics, 1994 Bennett movement ( Sir Norman Godfrey Bennett, British dental surgeon, 1870 1947) :. • Laterotrusion n: condylar movement on the working side in the horizontal plane. This term may be used in combination with terms describing condylar movement in other planes, for example, laterodetrusion, lateroprotrusion, lateroretrusion and laterosurtrusion.
  • 45. Laterodetrusion n : lateral and downward movement of the condyle on the working side. Lateroprotrusion n : a protrusive movement of the mandibular condyle in which there is a lateral component. Lateroretrusion n : lateral and backward movement of the condyle on the working side. Laterosurtrusion n : lateral and upward movement of the condyle on the working side. Bennett's movement is composed of two phases an immediate side shift and a progressive side shift
  • 46.
  • 47. The Glossary of Prosthodontic Terms, 7th edition, the Academy of Prosthodontics 1999: Mandibular translation: The translatory (medio-lateral) movement of the mandible when viewed in the frontal plane. While this has not been demonstrated to occur as an immediate horizontal movement when viewed in the frontal plane, it could theoretically occur in an essentially pure translatory form in the early part of the motion or in combination with rotation in the lateral part of the motion or both.
  • 48. Bennett angle : The angle formed between the sagittal plane and the average path of the advancing condyle as viewed in the horizontal plane during lateral mandibular movements.
  • 49. Early mandibular translation: The translatory portion of lateral movement in which greatest portion occurs early in the forward movement of the nonworking condyle as it leaves centric relation. Immediate mandibular translation: The translatory portion of lateral movement in which the non-working condyle moves essentially straight and medially as it leaves the centric relation position.
  • 50. Progressive mandibular translation (Guichet) 1: The translatory portion of mandibular movement when viewed in a specified body plane. 2 : The translatory portion of mandibular movement as viewed in a specified body plane that occurs at the rate or amount that is directly proportional to the forward movement of the non-working condyle.
  • 51. Timing of Bennett's movement: Amount of immediate side shift and progressive side shift. The rate or amount of descent of contra lateral condyle and the rotation and lateral shift of Ipsilateral condyle Immediate Side Shift (ISS) Progressive Side Shift (PSS). It is the bodily shift of mandible in horizontal direction. This is regulated by the shape of glenoid fossa, looseness of capsular ligament and the contraction of lateral pterygoids.
  • 52. ISS is the first movement the mandible makes when initiating lateral excursion. ISS occurs when the non-working condyle moves medially from its centric position in the fossa during lateral movement. It takes place at the beginning of lateral movement. This is not an exact 90' or a right angled medial movement in horizontal plane. This horizontal movement varies according to the shape of glenoid fossa etc.
  • 53. ISS ranges from 0.2 mm to 2 mm in width, with a mean 1.0 mm (Lundeen, Wirth). Using an electronic recording device Hobo found it to be 0 to 2.6 mm with a mean value of 0.42 mm. Beyond this (ISS), the condyle moves forward, downward and inward or medially. Guichet referred this movement component as Progressive Side Shift (PSS). Lundeen and Wirth found that ISS varies with individuals, whereas PSS showed a value of 7.5 mm among different subjects.
  • 54. The combined amount of Bennett movement (ISS+PSS) is the Bennett angle of the orbiting condyle (non-working condyle). In otherwords, B.A. Is the angle fon-ned by the orbital condylar path (horizontal lateral condylar path) and sagittal plane. It varies 2- 44 degrees; with a mean value of 16 degrees (Hobo, Mochizuki).
  • 55. Hanau (1922), recommended a formula for Bennett angle L = h/8+12 Adjustment in articulator from the sagittal lateral condylar path obtained by lateral check bites. Hobo in his studies using electronic mandibular recording device showed no significant correlation between BA and sagittal lateral condylar path.
  • 56. Therefore the use of Hanau's formula for obtaining BA adjustment in sermadjustable articulators like (Hanau and Dentatus) is questionable. New generation of articulators such as Hanau radial shift, Denar Mark II, Pandent, Panahoby have ISS and PSS adjustments.
  • 57.
  • 58. TEMPOROMANDIBULAR JOINTS The major components of the temporomandibular joints are the cranial base, the mandible, and the muscles of mastication with their innervation and vascular supply. Each joint can be described as ginglymoarthrodial, meaning that it is capable of both a hinging and a gliding articulation.
  • 59.
  • 60. An articular disk separates the mandibular fossa and articular tubercle of the temporal bone from the condylar process of the mandible. The articulating surfaces of the condylar processes and fossae are covered with avascular fibrous tissue (in contrast to most other joints, which have hyaline cartilage). The articular disk consists of dense connective tissue; it also is avascular and devoid of nerves in the area where articulation normally occurs.
  • 61. Posteriorly it is attached to loose vascularized connective tissue, the retrodiscal pad or bilaminar zone( Called bilaminar because it consists of two layers: an elastic superior layer and a collagenous inelastic inferior layer), which connects to the posterior wall of the articular capsule surrounding the joint.
  • 62. Medially and laterally the disk is attached firmly to the poles of the condylar process. Anteriorly it fuses with the capsule and with the superior lateral pterygoid muscle. Superior and inferior to the articular disk are two spaces, the superior and inferior synovial cavities.
  • 63. These are bordered peripherally by the capsule and the synovial membranes and are filled with synovial fluid. Because of its firm attachment to the poles of each condylar process, the disk follows condylar movement during both hinging and translation, which is made possible by the loose attachment of the posterior connective tissues.
  • 64. LIGAMENTS The body of the mandible is attached to the base of the skull by muscles and also by three paired ligaments: the temporomandibular (also called the lateral), the sphenomandibular, and the stylomandibular. Ligaments cannot be stretched significantly, so they limit the movement of joints.
  • 65.
  • 66. The temporomandibular ligaments limit the amount of rotation of the mandible and protect the structures of the joint, limiting border movements. The spheno-mandibular and stylomandibular ligaments limit separation between the condylar process and the disk; the stylomandibular ligaments also limit protrusive movement of the mandible.
  • 67. MUSCULATURE Several muscles are responsible for mandibular movements. These can be grouped into the muscles of mastication and the suprahyoid muscles.
  • 68. Muscles of mastication
  • 69. The former include the temporal, the masseter, and the medial and lateral pterygoids; the latter are the geniohyoid, the mylohyoid, and the digastrics.
  • 70. Lateral Pterygoid It is a muscle which runs in a horizontal direction. This location make it the chief muscle for the protraction of mandible. As it relaxes, the posterior fibres of temporalis muscle pull the condyle back to its centric position. When it contracts it draws forward the condyle along with the disc.
  • 71. This muscle is responsible for the initial opening of hinge movement. If the external pterygoids on one side contracts and the other remains relaxed, then the mandible will be moved laterally to the other side.
  • 72. External ptergyoid is not a muscle used for chewing. It only places the mandible to open into any position forward so that incision of food can be made with anterior teeth by the contraction of masseter and temporalis. It can also place the mandible into lateral position, so that the same muscles can permit chewing at the molar and bicuspid region. It guides the mandible into lateral position and keeps it steady when chewing take place in lateral position.
  • 73. Functions of superior and inferior heads of lateral pterygoid. A. Harmonious contraction of both the heads of muscle There is synchronization of superior and inferior head during protraction thus permitting the condyle and disc to move forward in unison. Simultaneous relaxation of these two heads of the muscle permit the condyle disc assembly to go back to centric position.
  • 74. B. Independent functions of the two heads of muscle The superior and inferior heads of the muscle function as two different muscles. The superior head is active only on closing. It braces the disc ,against the posterior slope of the eminentia . The inferior head is active on mouth opening.
  • 75.
  • 76. Medial Pterygoid – Helps in lateral positioning of mandible. The external pterygoid moves the condyle forwards while the internal pterygoid on one side moves the body of the mandible laterally to the opposite side. It thus contributes to Bennett movement. Acting together it elevates the mandible. Acting alone it draws the mandible laterally.
  • 77. Masseter The superficial portion of masseter elevates the mandible. Deep fibers of masseter run more horizontal in direction and they assist in retraction of mandible.
  • 78.
  • 79. Temporalis Since the posterior fibres are directed forwards and towards the ascending ramus when they contract, they retrude the jaw. The middle fibers run almost vertical and their contraction elevates the mandible. The anterior fibers run backward and their contraction protrudes the mandible. When all the - fibres of temporalis contract simultaneously they close the mandible.
  • 80. Temporalis and masseter muscles are closing muscles of the mandible. They also retrude the mandible and are partners in action. It is interesting to observe that the temporalis is attached to the upper part of the ascending ramus, while masseter is inserted down below in the ramus.
  • 81. Further, the temporalis is inserted on the medial surface, while the masseter is inserted on the outer surface of the ramus of the mandible. As a result of this pattern of insertion, the simultaneous contraction of these muscles helps to position the mandible without unstabilising it during function.
  • 82. Temporalis and lateral pterygoid are antagonistic in their function. It should be noticed that there is no muscle to oppose the action of lateral pterygold (protraction) to retract the mandible from behind. There is no muscle inserted into the posterior aspect of the condyle to retract the condyle or the articular disc. This function of retrusion is performed by the temporalis muscle attached to the coronoid process. The simultaneous contraction of middle and posterior fibres of temporalis assisted by deep fibres of masseter and posterior belly of digastric retrude the mandible.
  • 83. MANDIBULAR RETRUSION PROTOGONIST (mover muscle) Ternporalis + Digastric + Deep fibres ANTAGONIST MUSCLE Lateral Pterygoid of masseter
  • 84. MANDIBULAR PROTRUSION PROTOGONIST Lateral Pterygoid ANTAGONIST Temporalis + Digastric + Deep fibres of masseter
  • 85. HINGE CLOSURE Opening on retrusive arc-Digastric, geniohyoid Closure on retrusive arc --Post fibres of temporalls + Deep fibres of masseter exerting a backward pull Depression Lateral --Elevation
  • 86. BENNETT SHIFT Masseter on one side with the contraction of pterygoids of opposite side.
  • 87.
  • 88. Mandibular movements are limited by the temporomandibular joints and ligaments, the neuromuscular system, and the teeth. Posselt was the first to describe the extremes of mandibular movement, which he called border movements.
  • 89.
  • 90. Posselt used a three-dimensional representation of the extreme movements the mandible is capable of. All possible mandibular movements occur within its boundaries. At the top of both illustrations, a horizontal tracing represents the protrusive movement of the incisal edge of the mandibular incisors.
  • 91.
  • 92. 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 incisors reach the edge-to-edge position. This is represented in Posselt's diagram by the initial downward slope.
  • 93. As the mandible moves farther 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.
  • 94. The border farthest to the right of Posselt's solid represents the most protruded opening and closing stroke. The maximal open position the mandible is represented by the lowest point in the diagram. The left border of the diagram represents the most retruded closing stroke.
  • 95.
  • 96. This movement occurs in two phases: The lower portion co-sists of a combined rotation and translation, until the condylar processes return to the fossae. The second portion of the most retruded closing stroke is represented by the top portion of the border that is farther to the left in Posselt's diagram. It is strictly rotational.
  • 97.
  • 98. Most functional movement of the mandible (as occurs during mastication and speech) takes place inside the physiologic limits established by the teeth, the temperomandibular joints, and the muscles and ligaments of mastication; therefore, these movements are rarely coincident with border movements.
  • 99. Chewing When incising food, adults open their mouth a comfortable distance and move the mandible forward until they incise, with the anterior teeth meeting approximately edge to edge. The food bolus is then transported to the center of the mouth as the mandible returns to its starting position, with the incisal edges of the mandibular anterior teeth tracking along the lingual concavities of the maxillary anterior teeth.
  • 100. Comparison of border & chewing movements of soft food
  • 101. The mouth then opens slightly, the tongue pushes the food onto the occlusal table, and after moving sideways, the mandible closes into the food until the guiding teeth (typically the canines) contact.' The 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 direction 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.
  • 102. Chewing pattern observed in children differs from that found in adults. Until about age 10, children begin the chewing stroke with a lateral movement. After the age of 10, they start to chew increasingly like adults, with a more vertical stroke. Stimuli from the pressoreceptors play an important role in the development of functional chewing cycles."
  • 103.
  • 104.
  • 105. Mastication is a learned process. At birth no occlusal plane exists, and only after the first teeth have erupted far enough to contact each other is a message sent from the receptors to the cerebral cortex, which controls the stimulai to the masticatory musculature. Stimulai from the tongue and cheeks, and perhaps from the musculature itself and from the periodontium, may influence this feedback pattern.
  • 106. 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”.
  • 107. When pronouncing the fricative "F," the inner vermilion border of the lower lip traps air against the incisal edges of the maxillary incisors. Phonetics is a useful diagnostic guide for correcting vertical dimension and tooth position during fixed and removable prosthodontic treatment.
  • 108. PARAFUNCTIONAL MOVEMENTS Parafunctional movements of the mandible may be described as sustained activities that occur beyond the normal functions of mastication, swallowing, and speech. There are many forms of parafunctional activities, including bruxism, clenching, nail biting and pencil chewing. Typically, parafunction is manifested by long periods of increased muscle contraction and hyperactivity.
  • 109. Concurrently 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 the periodontal ligament (PDL), and mobility, migration, or fracture of the teeth.
  • 110. Muscle dysfunction such as myospasms, myositis, myalgia and referred pain (headaches) from trigger point tenderness may also occur. The two most common forms of parafunctional activities are bruxism and clenching. Increased radiographic bone density is often seen in patients with a history of sustained parafunctional activity.
  • 111. BRUXISM. Sustained grinding, rubbing together, or gnashing of the teeth with greater-than-normal chewing force is known as bruxism. This activity may be diurnal, nocturnal, or both. Although bruxism is initiated on a subconscious level, nocturnal bruxism is potentially more harmful because the patient is not aware of it while sleeping.
  • 112.
  • 113. Therefore, it can be difficult to detect, but it should be suspected in any patient exhibiting abnormal tooth wear or pain. The prevalence of bruxism is about 10% and is less common with age." 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."
  • 114. A study on cohort twins has demonstrated substantial genetic effects, the condition has been related to sleep disturbance and the symptoms of bruxism are three times more common in smokers.
  • 115. Altered mastication has been observed in subjects who Brux and may be due to an attempt to avoid premature occlusal contacts. ( occlusal interferences). There may also be a neuromuscular attempt to "rub out" an interfering cusp. The fulcrum effect of rubbing on posterior interferences will create a protrusive or laterotrusive movement that can cause overloading of the anterior teeth, with resultant excessive anterior wear.
  • 116. It is common for wear on anterior teeth to progress from initial faceting on the canines to the central and lateral incisors. Once vertical overlap diminishes as the result of wear, posterior wear facets are commonly observed. However, the chewing patterns of normal subjects can be quite varied, and the relationship, if any, between altered mastication and occlusal dysfunction is not clear.
  • 117. The causes of bruxism are difficult to determine. One theory states that bruxism is performed on a subconscious reflexcontrolled level and is related to emotional responses and occlusal interferences. In certain malocclusions, the neuromuscular system exerts fine control during chewing to avoid particular occlusal interferences.
  • 118. As the degree of muscle activity necessary to avoid the interferences becomes greater, an increase in muscle tone may result, with subsequent pain in the hyperactive musculature, which in turn can lead to restricted movement. The relationship, if any, between bruxism and temporomandibular disorders is still unclear."
  • 119. Patients who brux can exert considerable forces on their teeth, and much of this may have a lateral component. Posterior teeth do not tolerate lateral forces as well as vertical forces in their long axes. Buccolingual forces, in particular, appear to cause rapid widening of the periodontal ligament space and increased mobility
  • 120. CLENCHING Clenching is defined as forceful clamping together of the jaws in a 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 an occlusal disorder.
  • 121. As opposed to bruxism, clenching does not necessarily result in damage to the teeth because the concentration of pressure is directed more or less through the long axes of the posterior teeth without the involvement of detrimental lateral forces.
  • 122. Abfractions- 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. Typically, the elevators will become overdeveloped.
  • 123. A progression of muscle splintir myospasm, and myositis may occur, causing the patient to seek treatment. As with bruxism., clenching can be difficult to diagnose and difficult if not impossible for the patient to voluntarily control.
  • 124.
  • 125. Parallelogram of forces. From the standpoint of the prosthodontist, the skull presents some interesting facts that need to be taken into consideration. The factor of muscle pull in relation to the direction and strength of each muscle used in positioning the mandible after the loss of teeth is an important consideration. The parallelogram of forces can be studied only in relation to the entire skull.
  • 126. The direction of these forces has much to do with the seating or unseating of dentures. The occlusal vertical dimension affects this direction of forces, a fact that makes positioning of the mandible after the loss of teeth so important
  • 127.
  • 128. In an explanation of the clinical implications of mandibular movements, it is helpful to define the limits of possible motion and certain mandibular reference positions. Recent tests indicate that edentulous patients can make reproducible lateral border movements when stabilized baseplates are used to support the pantograph.
  • 129.
  • 130. Fig shows an envelope of motion (maximum border movements) in the sagittal plane as described by a dentate subject.]
  • 131. The tracing was made from motion picture film when the pathway of a bead attached to a lower cen-tral incisor was plotted. The tracing starts at P,which represents the most protruded position of the mandible with the teeth in contact. As the mandible is moved posteriorly while tooth contact is maintained, a dip in the top line of the tracing occurs as the incisal edges of the upper and lower anterior teeth pass across one another.
  • 132. CO (centric occlusion) is reached when the opposing posterior teeth are maximally intercuspated. When the mandible is further retruded, as most people with natural teeth can do, the most posterior relation of the mandible to the maxillae is depicted by CR (centric relation). Centric relation and the mandibular position where centric occlusion occurs are two reference positions that are of extreme importance in constructing dental restorations.
  • 133. Single restorations are generally constructed to be in harmony with centric occlusion (that is, with the mandible positioned at CO). Multiple restorations, and certainly complete dentures, are so constructed that their occlusion will be in harmony with centric relation (i.e., with the mandible positioned at CR).
  • 134. As the teeth separate, the mandible moves to its most retruded position from CR and the patient can continue to open in this retruded position, with no apparent condylar translation, to approximately MHO (maximum hingeopening position). Any opening beyond MHO will force the condyles to move forward and downward from their most posterior position. CR-MHO represents the posterior terminal hinge movement.
  • 135.
  • 136. This movement is used clinically to locate the transverse hinge axis for mounting casts on the articulator. The posterior terminal hinge movement and centric relation at the vertical level of tooth contact coincide at CR. This terminal hinge movement can be made only by a conscious effort.
  • 137. At approximately MHO the patient can no longer retain the mandible in the most retruded position; and as further opening occurs the mandible begins to move forward with translation of the condyles in a forward direction. Obviously, different muscles and impulses come into play. At MO (maximum opening) the jaws are separated as far as possible and the condyles are in or near their most anterior position relative to the mandibular fossae.
  • 138.
  • 139. The most forward line on the tracing, running from MO to P, represents the pathway of the mandible as it is moved from its most open position upward to its most protruded position until the teeth contact at P, which was the starting point for tracing the envelope of motion. Any mandibular movement observed from the side will fall within this envelope of motion since it represents all extreme positions into which the mandible can be moved. However, few normal mandibular movements follow the border tracings; normal mandibular movements occur somewhere in front of the terminal hinge movement line, CR-MHO.
  • 140. The dotted line beginning with the teeth in centric occlusion (at CO) and extending downward and then upward anterior to the path of the posterior terminal hinge movement line (CR-MHO) is a tracing of the masticatory cycle viewed in the sagittal plane and superimposed on the envelope of motion. The arrows pointing downward indicate the pathway of the bead attached to the lower central incisor during the opening part of the chewing cycle, and the arrows pointing upward indicate the pathway during the closing part of the cycle.
  • 141.
  • 142. Note that the pathways occur anterior to the line representing the terminal hinge movement. This holds true for most persons with natural teeth. However, if restorations are so constructed that centric occlusion and centric relation coincide at CR, many of the chewing cycles will terminate at CR. This applies also to people whose occlusions have been equilibrated for centric relation. The important point to remember is that for edentulous patients the teeth should contact evenly throughout the normal range of function.
  • 143. When the patient is relaxed and the jaw is in the resting Position, obviously the teeth are not in contact. Mandibular rest position normally occurs somewhere downward and slightly forward of CR, as indicated by Rest, This is defined as the habitual postural Position of the mandible when the patient is at ease and upright.
  • 144. The only muscle activity required is the minimal tonic contraction necessary to support the mandible against the force of gravity. The rest Position is an important reference in prosthodontics, particularly for complete denture patients, since it is a guide to reestablishing the proper vertical dimension of occlusion.
  • 145.
  • 146. The envelope of motion as seen in the frontal plane roughly resembles a shield. Such an envelope whose tracing was made from a motion picture film when the pathway of a bead attached to the lower central incisor was plotted. The tracing begins with the teeth in centric occlusion (at CO). As the mandible is moved to the right with the opposing teeth maintaining contact, a dip in the upper line of the tracing is created as the upper and lower canines pass edge to edge.
  • 147.
  • 148. The mandibular movement is continued as far to the right as possible. Then the opening movement is started and continued with the mandible in the extreme right lateral position until maximum opening occurs (at MO). From MO (the position of maximum opening) the mandible is moved in an extreme left lateral excursion as it is closed until the opposing teeth make contact.
  • 149. Then, with the opposing teeth maintaining contact, the mandible is moved from the extreme left lateral position back to where the opposing teeth again contact in centric occlusion, CO. The dip in the left side of the superior border movement is made when the upper and lower left canines pass edge to edge.
  • 150.
  • 151. The dotted line beginning at approximately the middle of the tracing and extending upward (indicated by the upwardpointing arrows) represents the upward component of the masticatory cycle as the subject chews a bolus of food on the left side.
  • 152. Note that the dotted line contacts the superior border of the envelope at CO, indicating that the opposing teeth have penetrated the bolus and come into contact with one another. The masticatory cycle moves to the right when the subject opens from centric occlusion as indicated by the downward dotted line (downwardpointing arrows). In the frontal view the rest position is located slightly downward and to the left for this individual, as indicated by Rest.
  • 153.
  • 154.
  • 156. Thank you Leader in continuing dental education