Mandibular movements / fixed orthodontic courses


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Mandibular movements / fixed orthodontic courses

  1. 1. MANDIBULAR MOVEMENTS INDIAN DENTAL ACADEMY Leader in continuing dental education 1
  2. 2. Once it is accepted as it must be that the movement of the jaw are complex and variable ,then it became imperative to learn as much as possible about jaw movements in order to reproduce those aspects of its movements considered necessary for proper functioning of the occlusion, either natural or artificial. 2
  3. 3. There are broad agreements concerning jaw movements like the mandible performs habitual movements and border movements ,opening movements and closing movements, protrusive and lateral movements . 3
  4. 4. • The main influences on normal jaw movements are the teeth ,the joints and the surrounding muscles and ligaments • Normally the mandible moves in a habitual manner to accomplish speech, mastication, deglutition, respiration , sucking ,whistling etc. • Abnormally the jaw moves in a habitual and often pernicious manner as in bruxism • In the edentulous patient the influence of the teeth is lessened by their movable relation of the mandible and maxilla. 4
  5. 5. 5
  6. 6. The mandible, or lower jaw, is the largest and the strongest bone of the face. It has a horse shoe shaped body which lodges the teeth, and a pair of projections or rami. The ramus on either side extends vertically and slightly laterally from the posterosuperior aspect of the body. The upper part of the body is continuous as the alveolar process. It generally surrounds and supports the teeth, but when they are lost, it becomes the bony 6 base for dentures.
  7. 7. The ramus terminates superiorly in two processes. Of these, the coronoid process is anterior to the condyloid process, which is capped by the condyle. The constricted area just inferior to the condyle is called the neck of the condyle. Between coronoid process and condyloid process is the mandibular notch, which is concave superiorly. 7
  8. 8. The mandibular foramen, through which the inferior alveolar nerves and vessels enter, lies on the medial aspect of the ramus, approximately midway between the lowest point of the notch and the inferior surface of the mandible. The anterior border of the ramus presents two ridges. The lateral ridge continues onto the body as the external oblique line. The medial ridge is called the temporal crest and is almost continuous with the mylohyoid ridge of 8 the body of the mandible.
  9. 9. The mental foramen is located in the vicinity of the apex of the premolar teeth. When the teeth are lost and resorption occurs, it may progress downward to involve the mental foramen. On the lingual surface of the midline, the genial tubercle may exhibit prominences on both sides of the midline. 9
  10. 10. 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. 10
  11. 11. 11
  12. 12. 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. 12
  13. 13. 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. 13
  14. 14. 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. 14
  15. 15. 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. 15
  16. 16. 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. 16
  17. 17. 17
  18. 18. 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. 18
  19. 19. 19
  20. 20. The Glossary of Prosthodontic Terms, 7 th edition, the Academy of Prosthodontics, 1999 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. 20
  21. 21. 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. 21
  22. 22. 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. 22
  23. 23. 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 23
  24. 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). 24
  25. 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. 25
  26. 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 26
  27. 27. 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. 27
  28. 28. 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. 28
  29. 29. 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. 29
  30. 30. 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. 30
  31. 31. 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. 31
  32. 32. 32
  33. 33. 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. 33
  34. 34. 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. 34
  35. 35. 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. 35
  36. 36. 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 36 contraction of lateral pterygoids.
  37. 37. 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 37 etc.
  38. 38. 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. 38
  39. 39. The combined amount of Bennett movement (ISS+PSS) is the Bennett angle of the orbiting condyle (non-working condyle). In other words, B.A. Is the angle formed 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). 39
  40. 40. 40
  41. 41. Origin: Lateral surface of the skull Insertion: Coronoid process and anterior border of the ramus Function : Elevates and retracts jaw Assists in rotation Active in clenching 41
  42. 42. 42
  43. 43. Origin : Zygomatic Arch Insertion : Angle of mandible Function : Elevates and protracts jaw Assists in lateral movements Active in clenching 43
  44. 44. Origin : Pterygoid fossa and mesial surface of lateral pterygoid plate Insertion: Medial surface of angle of Insertion mandible Function: Elevates jaw, causes lateral movement and protrusion 44
  45. 45. SUPERIOR LATERAL PTERYGOID Origin : Infra temporal surface of greater wing of sphenoid Insertion: Articular capsule and disc ,neck of the condyle Function: Position disc in closing 45
  46. 46. 46
  47. 47. Origin : Lateral surface of lateral pterygoid plate Insertion : Neck of the condyle Function: Protrudes and depresses jaw causes lateral movements 47
  48. 48. Origin : Inner surface of the mandible Insertion: Hyoid and mylohyoid raphe Function : Elevates and stabilizes hyoid 48
  49. 49. GENIOHYOID Origin : Genial tubercle Insertion: Hyoid Function : Elevates and draws hyoid forward 49
  50. 50. Origin : Tendon linked to hyoid Insertion: Digastric fossa Function: Elevates hyoid, depresses jaw 50
  51. 51. MOVEMENT MUSCLES Elevation of chin (closing) Masseter Medial pterygoid Anterior part of temporalis Depression of chin (opening) Lateral pterygoid Digastric Geniohyoid and mylohyoid with infra hyoid muscles 51
  52. 52. MOVEMENT MUSCLES Protraction Lateral pterygoid Medial pterygoid Masseter Retraction Temporalis Digastric Chewing Medial and llateral pterygoid Masseter Temporalis 52
  53. 53. 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. 53
  54. 54. 54
  55. 55. Every possible three-dimensional movement can be described in terms of these two components. It is easier to understand mandibular movement when the components are described as projections in three perpendicular planes: sagittal, horizontal, and frontal reference planes and three axis of rotation. 55
  56. 56. 56
  57. 57. 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. 57
  58. 58. 58
  59. 59. 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. 59
  60. 60. 60
  61. 61. 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. 61
  62. 62. 62
  63. 63. 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. 63
  64. 64. 64
  65. 65. 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. 65
  66. 66. 66
  67. 67. 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. 67
  68. 68. 68
  69. 69. 69
  70. 70. Mandibular motion consists of curved, and more often, elliptical motion. The related axes of rotation in the three planes of space are associated with this three dimensional motion. Although mandibular motion is controlled by the neuromuscular complex, physiologic axes of rotation exist as an integral part of motion itself. 70
  71. 71. Transverse Hinge Axis The transverse hinge axis which passes through both condyles is associated with rotation of the mandible in the vertical (sagittal) plane. Motion is always perpendicular to its axis of rotation by definition. 71
  72. 72. 72
  73. 73. Vertical Axis The physiologic vertical axis of rotation is associated with rotation in the horizontal (transverse) plane and is located in the working condyle. 73
  74. 74. 74
  75. 75. SAGITTAL AXIS The physiologic sagittal axis of rotation is associated with rotation in the frontal plane. The balancing condyle rotates about the sagittal axis which is located through the working condyle 75
  76. 76. 76
  77. 77. 77
  78. 78. 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. 78
  79. 79. 79
  80. 80. 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. 80
  81. 81. 81
  82. 82. 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. 82
  83. 83. 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. 83
  84. 84. 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. 84
  85. 85. 85
  86. 86. This movement occurs in two phases: The lower portion consists 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. 86
  87. 87. 87
  88. 88. 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. 88
  89. 89. 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 89 concavities of the maxillary anterior teeth.
  90. 90. 90 Comparison of border & chewing movements of soft food
  91. 91. 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. 91
  92. 92. 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. 92
  93. 93. 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 press receptors play an important role in the development of functional chewing cycles." 93
  94. 94. 94
  95. 95. 95
  96. 96. 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. 96
  97. 97. 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”. 97
  98. 98. 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. 98
  99. 99. 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. 99
  100. 100. 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. 100
  101. 101. 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. 101
  102. 102. 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. 102
  103. 103. 103
  104. 104. 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. 104
  105. 105. 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. 105
  106. 106. 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. 106
  107. 107. 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. 107
  108. 108. 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. 108
  109. 109. 109
  110. 110. The muscles that hold move or stabilize the mandible do so because they receive impulses from the central nervous system. Mandibular motion at conscious level results in voluntary movement where as at subconscious level due to stimulation of oral or muscle receptors cause involuntary movement. 110
  111. 111. Receptors in the oral mucous membrane are stimulated by touch pain thermal changes or pain and pressure where as other receptors are principally located in the periodontal ligaments, mandibular muscles and ligaments provide information as to the location of mandible in space and thus called PROPRIOCEPTORS 111
  112. 112. • Impulses form oral receptors Trigeminal nuclei • From proprioceptors Mesencephalic nuclei of the brain From these 2 receptors Cerebral cortex 112
  113. 113. • From the cerebral cortex It comes though three ways Via the thalamus to the sensoriomotor cortex (conscious level) to produce voluntary change in the position of the mandible By way of a reflex arc to the motor nuclei of the Trigeminal nerve to cause involuntary movement By combination of the these two ways through the subcortical areas as the hypothalamus, basal ganglion. 113
  114. 114. In edentulous patients the periodontal ligament are lost thus the source of control in the positioning of the mandible are lost thus to compensate this centric occlusion must be in harmony with the centric relation and meet evenly in the normal range of functional activity and these impulses can be generated by voluntary thought which are transmitted through the motor nuclei and from there to the muscle of mastication so the mandible performs the desired activity 114
  115. 115. Mastication is a programmed event residing in a chewing centre located in the brain stem (in the reticular formation of the pons ) The cyclic nature of mastication (jaw opening and closure ,tongue protrusion and retrusion) is a result of of action of this central pattern generation. 115
  116. 116. The alteration of the chewing pattern or character (rate, force, duration)are related to the consistency of the bolus of the food. The relatively continuous flow of impulses through the specific pathway form the receptors to the CNS and back to the musculature establishes a memory pattern for mandibular movements. 116
  117. 117. Thus when natural teeth are present a individual sub consciously develops these memory patterns But these patterns are disturbed when the teeth are lost or a new restoration is placed with an occlusion which is not in harmony with mandibular movement leads to pain ,pathosis and mental stress 117
  118. 118. 118
  119. 119. 119
  120. 120. 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. 120
  121. 121. 121
  122. 122. Fig shows an envelope of motion (maximum border movements) in the sagittal plane as described by a 122 dentate subject.]
  123. 123. 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. 123
  124. 124. 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 124 restorations.
  125. 125. 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). 125
  126. 126. 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. 126
  127. 127. 127
  128. 128. 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. 128
  129. 129. 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. 129
  130. 130. 130
  131. 131. 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. 131
  132. 132. 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. 132
  133. 133. 133
  134. 134. 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. 134
  135. 135. 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. 135
  136. 136. 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. 136
  137. 137. 137
  138. 138. 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. 138
  139. 139. 139
  140. 140. 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. 140
  141. 141. 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. 141
  142. 142. 142
  143. 143. 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. 143
  144. 144. 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 144 Rest.
  145. 145. 145
  146. 146. 146
  147. 147. BIBLIOGRAPHY • • • • • • GPT-7thedition(1999) BOUCHERS ROSENSTIEL HEARTWELL SHARRY WEINBERG articles 147
  148. 148. Thank you Leader in continuing dental education 148