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Embryology is the study of formation and development of the embryo. The development of the human embryo from the time of fertilization through the birth week is an important period for human appearance . Every individual spends the first nine months of its life within the womb. During this period it develops from one-celled structure to an organism having billions of cells. An embryo has no face as such Webster delimits the face as the front part of the head comprising the nose,cheeks,jaws,mouth,forehead and the eyes. www.indiandentalacademy.com
The key primordial begin to gather,and form slight swelling,depression and thickening that are rapid which undergo a series of mergers, and enlargements that will transfer them,forming a cluster of separate masses into a Face. Prenatal life may be arbitrarily divided into three periods: The period of the ovum (from fertilization to the end of the fourteenth day) The period of the embryo (from the fourteenth day to about the fifty-sixth day) The period of the fetus (from about the fifty-sixth day until the two hundred and seventieth day-birth) www.indiandentalacademy.com
Understanding the development of the structures of the face requires knowledge of the pharyngeal or branchial arches. These arches form on either side of the foregut and correspond to the primitive branchial arches. The pharyngeal arch consists of a core of mesenchyme covered externally by ectoderm and covered internally by endoderm. Pharyngeal(branchial ) apparatus The pharyngeal (brachial)arches begin to develop during the fourth week in utero. www.indiandentalacademy.com
The pharyngeal apparatus consists of a series of bilaterally paired arches,pouches(clefts)grooves and membranes This apparatus gives rise to a significant number of structure of the head and neck. The structure of the pharyngeal apparatus are numbered sequentially. The first four paired arches make up the lateral wall of the primordial pharynx,which develop from the foregut.
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The pharyngeal arch apparatus Cranial Endoderm
Pharyngeal pouch Mesenchyme
Pharyngeal membrane
Pharyngeal cleft Nerve Artery Ectoderm
Caudal
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Weeks 2 through 8 are especially important because the tissues and organ system are developing from the original three germ layers. Formation of Germ layers At a very early stage in development,the embryo proper acquires the form of a three-layered disc.,called the embryonic disc. The three layers that constitute this embryonic disc are Endoderm(endo=inside) Ectoderm(ecto=outside) Mesoderm(meso=in the middle) www.indiandentalacademy.com
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Ectoderm The ectoderm contributes to the formation of the face is well around the stomodeum by the fourth week of embryonic life. It is evident that the ectodermal structures bounding the stomodeum participate not only in the formation of the face, but also in the formation of the nasal and oral cavities. Mesoderm The mesenchyme that fills the pharyngeal arches is derived from three sources.. The Paraxial
Lateral plate
musculature
Neural crest skeletal
(this musculature is innervated by one nerve that innervates all the muscles of that particular arch)
www.indiandentalacademy.com Finally each arch has its own artery.
Folding of the Embryo Beginning in the fourth week of development the flat trilaminar embryonic disc folds in two planes to form a more typical-appearing,cylindric,C-shaped embryo. Folding in the cranial 窶田audal plane is mainly a result of rapid longitudinal growth of the central nervous system. Growth of the somites account for much of the lateral folding.
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Folding of the Embryo‌‌ Folding brings the endodermal-lined yolk sac into the embryo and creates the primordial gut;the foregut,midgut,and the hindgut. At the early stages of embryonic development, the vertebrate face has a common plan. A series of small buds of tissue called the facial primordial forms around the stomodeum, which forms the primitive mouth. The facial primordial are made up mainly of neural crest cells that have migrated from the cranial crest and settled. www.indiandentalacademy.com
A summary of the derivatives of the first and second pharyngeal (ie, branchial)arches is as follows: ·
Pharyngeal arch I
Arch I-Meckels cartilage (Mandibular arch) This pair of arch has distinct maxillary and mandibular prominence Its major contribution is in the development of face o Cranial nerve - Maxillary and Mandibular division of trigeminal nerve (cranial nerve V) o Artery - Maxillary (terminal branch) www.indiandentalacademy.com
o Muscles - Muscles of mastication (ie, temporalis, masseter, pterygoids), mylohyoid, anterior belly of digastric, tensor tympani, tensor veli palatini o Skeleton - Maxillary cartilage (incus, alisphenoid), mandibular or Meckel cartilage (malleus), arch dermal mesenchyme (maxilla, zygomatic, squamous portion of temporal bone, mandible) www.indiandentalacademy.com
Pharyngeal arch II (hyoid arch) Reichert’s cartilage Its major contribution is the development of hyoid bone,the lesser horn and the superior portion of the body
o Cranial nerve -Facial nerve VII o Artery - Stapedial o Muscles - Muscles of facial expression (ie, orbicularis oculi, orbicularis oris, risorius, buccinator, platysma, auricularis, frontalis), stapedius muscle, posterior belly of digastric, stylohyoid muscle o Skeleton - Stapes, styloid process, stylohyoid ligament, lesser cornu of hyoid, upper part of the body of the hyoid bone www.indiandentalacademy.com
Derivatives of pharyngeal pouches,Grooves & membranes
Pouch 1-Tympanic cavity,auditory tube,mastoid antrum Pouch 2-Crypt lining of palatine tonsils Pouch 3-Inferior parathyroid glands,thymus
Pouch 4-Superior parathyroid glands,Ultimobranchial body Groove1-External auditory meatus Groove2 & 4-Cervical sinus Membrane 1-Tympanic membrane
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Development of the face Facial development result mainly from enlargement and movement of the frontonasal prominence and four prominences from pharyngeal arch1,the paired maxillary prominences ,and the mandibular prominence.These structures surround the stomodeum. The upper jaw develops from 5 main buds of tissue: a single median frontonasal mass ,the 2 lateral nasal prominences on both sides,, the 2 maxillary prominences The lower jaw develops from the paired mandibular primordia (mandibular prominences). www.indiandentalacademy.com
Paired maxillary and mandibular prominences are derivatives of the first pair of branchial or pharyngeal arches. All these prominences are produced by the proliferation of the neural crest cells that migrated into the arches from the neural crest during the fourth week. The neural crest cells give rise to the connective tissue components Frontal prominence Nasal placode Maxillaryprominence
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Mandibular prominence Arch I
·
Fourth week of development
o Primordia of the face appear at the cephalic end of the embryo. o Two nasal placodes cap the bulbous frontal prominence. o The optic discs appear posterolateral to the frontal prominence. O Three paired branchial arches have formed. o The first arches split into maxillary and mandibular prominences. The hyoid arches are the second pair. www.indiandentalacademy.com
o Between the first arches and frontal prominence, the buccopharyngeal membrane becomes fenestrated. Fifth week of development · o Nasal pits develop in the nasal placodes, and the rims of the placodes differentiate into medial and lateral nasal prominences. o The lens vesicles invaginate and close within the optic discs. www.indiandentalacademy.com
o The mesenchyme of the mandibular arch fills in across the midline. o The caudal end of the medial nasal prominences begins to fuse with the maxillary prominences. · At the beginning of the sixth week of development o The nasals have shifted to a more ventral, central position. o Growing and shifting subectodermal mesenchyme smooths out the furrows between prominences and arches, and the second arch becomes more massive. www.indiandentalacademy.com
o Six auricular hillocks, which will become the pinna of the ears, form on the mandibular and hyoid arches. By the end of the sixth week of · development o Medial and lateral nasal prominences fuse. o Maxillary prominences begin the formation of the upper jaw. o The midline approximation of the medial nasal prominences forms the nasal septum. www.indiandentalacademy.com
· At the beginning of the seventh week of development o The tip of the nose is elevated between the medial nasal prominences and is visible in profile. o Eyelids become prominent. o The pinna of the ear takes shape. · End of the seventh week of development O The pattern of facial features has taken on a human appearance. However, facial proportions develop during the fetal period. www.indiandentalacademy.com
· The fusion of the medial nasal prominences is complete, forming the central axis of the nose and the philtrum of the lip. From the beginning of the eighth week of development to birth, final development of the face occurs slowly and results mainly from changes in the proportion and relative positions of the facial components. At the end of the 8 week the face has taken on a human appearance.
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Developmental Defects 1. Cleft (hare) lip and cleft jaw: the medial nasal processes do not fuse with the maxillary processes. 2. Cleft palate and/or uvula: the palatine shelves fail to fuse with each other and/or with the primary palate. www.indiandentalacademy.com
3.Median cleft of upper lip: The 2 medial nasal processes fail to fuse. This is often accompanied by mental retardation. 4.Median cleft of lower lip: Failure of fusion of 2 mandibular arches.
5. Oblique facial cleft: The
nasolacrimal duct remains exposed due to the lateral nasal process failing to fuse with the maxillary process It can occur along with a cleft lip. www.indiandentalacademy.com
Development of Dentition The development of dentition is an integral part of craniofacial growth and an Unique structure of body Tooth is derived from ectoderm & mesoderm The first indication of tooth formation is seen at about sixth week of intrauterine life Life cycle of the tooth Histo differentiation
Initiation Proliferation
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Apposition
Morphodifferentiation
Initiation The epithelium overlying the convex border of the alveolar process becomes thickened and projects into the underlying mesoderm. This epithelial thickening is called the dental lamina. Specific cells within the dental lamina have the potential to form the enamel organ of certain teeth by responding to those factors that initiate or induce tooth development. A lack of initiation result in absence of teeth(anodontia)most frequently the permanent upper laterals,third molars,and lower second premolars. Abnormal initiation result in development of supernumerary teeth
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Proliferation Enhanced proliferation activity ensures at the point of initiation and results successively in the Bud,Cap,and Bell stage of the odontogenic organ. Proliferative growth causes regular changes in size & proportion of the growing tooth germ As the enamel organ grows downwards into the mesenchyme (of the alveolar process)its lower end assure a cup-shaped appearance.The cup comes to be occupied by a mass of mesenchyme called the dental papilla. The enamel organ and the dental papilla together constitute the tooth germ.At this stage the developing tooth looks like like a cap:it is ,therefore ,described as the cap stage of tooth development.
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Histo differentiation It succeeds the proliferative stage which undergoes definite morphologic and functionally changes. The cells of the enamel organ that line the papilla become columnar.These are called ameloblasts. Mesodermal cells of the papilla that are adjacent to the ameloblasts arrange themselves as a continuous epitheliumlike layer.The cells of this layer are called odontoblasts Ameloblast lay down enamel on the superficial(outer)surface of the basement membrane.The odontoblasts lay down dentine on its deeper surface.As layer after layer of enamel and dentine are laid down,the layer of ameloblasts and the layer of odontoblasts move away from each other.
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The ameloblasts and odontoblasts are separated by a basement membrane.The remaining cells of the papilla form the pulp of the tooth.The developing tooth now looks like a bell (Bell stage of tooth development )
Morphodifferentiation & Apposition Morphodifferentiation is impossible without proliferation. The basic form and relative size of the tooth is established by morphodifferentiation.
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In advanced bell stage ,the boundary between inner enamel epithelium and odontoblast outline the future dentinoenamel junction.In addition,the cervical portion of the enamel organ gives rise to the epithelial root sheath of Hertwig. Apposition is the deposition of the matrix of the hard dental structures. Appositional growth is characterized by regular and rhythmic deposition of the extracellular matrix,which is of itself incapable of further growth.
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Development of Permanent tooth
The Permanent tooth bud appears in the 4th & 5th intrauterine months, at about the same age at which mineralization of the deciduous teeth commences. The dental lamina gives off series of buds, one of which lies on the medial side of each developing milk tooth. These buds from enamel organ give rise to the permanent teeth The permanent molars are formed from bud that arise from the dental lamina posterior to the region of the last milk tooth At birth the germs of all the deciduous teeth,and of the permanent incisors,canine and first molars show considerable development. The germs of premolar and of the permanent second molars are .rudimentary. The germ of third molar iswww.indiandentalacademy.com formed after birth.
The stage of dentitions Gum pads
Primary dentition
Mixed dentition
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Permanent dentition
Gum pad stage:It extends from birth until the eruption of the first primary tooth in the oral cavity. (The pre-dentition period.) -This is from birth to six months. -At this stage, there are no teeth. Clinically, the infant is edentulous -Both jaws undergo rapid growth; the growth is in three planes of space: downward, forward, and laterally (to the side). Forward growth for the mandible is greater. Greater the anteroposterior dimension of the gumpads,greater the possibility of the child developing malocclusion --occasionally, there is a neonatal tooth present at birth.
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Primary dentition stage: (first transitional period /Deciduous dentition period. ) It extend from the time of eruption of the primary teeth until the eruption of the first permanent tooth around 6yerars of age. – The dentition of this period manifests several important characteristics. 1.There is either spacing between the teeth or there is not. 2.There is no sagittal and transversal dimensions alteration,only vertical changes are seen. 3.The distal surface of the maxillary & mandibular second primary molars maintain vertical relationship.
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-The tooth buds anticipate the ultimate occlusal pattern. -Mandibular teeth tend to erupt first. --Eruption times can be variable. Spaced dentition is supposed to be good as space in between the teeth is utilized for adjustment of permanent successors Primate spaces occur in about 50% of children. They appear in the deciduous dentition. The spaces appear between the upper lateral incisor and the upper canine. They also appear between the lower canine and the deciduous first molar.
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Flush terminal plane This dentition is completed after the eruption of the second primary molars,indicating the location for permanent teeth.Therefore it’s the key factor that influences the future occlusion of the permanent dentition. The second primary molar relation is described in terms of relationship between terminal planes. In flush terminal plane :The distal surface of upper and lower teeth are in straight line.It’s a favourable relationship to guide the permanent molars.
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Mesial step:In mesial step the distal surface of the lower molar is more mesial to that of the upper.Invariably it is favorable to guide the permanent molar into a class I relationship. Distal step:The distal surface of the lower molar is more distal to that of upper.Its unfavorable as it guides the permanent molars into distal occlusion. flush terminal plane
Distal step
Mesial step ClassIII ClassI(with late mesial shift) End-end ClassIII(normally) ClassI(desirable)
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ClassII
 Incisor liability: Mayne has coined the term incisor liability Permanent incisors are larger than primary incisors,this difference in size is termed incisal liability. Eruption of permanent incisors make the permanent arch circumference wider,normally the interincisal angle is 150 degree in primary dentition and 120 degree in permanent dentition The period of transition determine the successful alignment of the permanent teeth toward an ideal arch form
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Anterior teeth relation Over bite:The average over bite in primary dentition is 2mm Overjet :The average over jet is 1-2mm with a normal range of 2-6mm. With the excessive wear of the primary canines and molars,the whole lower dental arch may move anteriorly,the incisors may assume an edge-to-edge interrelationship by the age of six
Sequence of eruption Deciduous
Eruption Time
Central Incisor
6-12 months
Lateral Incisor
9-13 months
Canine
16-23 months
First Molar
13-19 months
Second Molar
23-33 months
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Exfoliation of Primary teeth The replacement is a multifactorial process. When the permanent tooth begins to erupt,it is apparently guided by the gubernaculum.. The pathway for the erupting anterior teeth is essentially lingual to their predecessors resulting in an oblique lingual pattern of resorption. Resorption is not a continuous process,but is rather one of alternating period of destruction and apposition
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Mixed dentition stage: (second transitional period) Starts with the eruption of the first permanent tooth,usually the mandibular central incisor,and is normally completed at the time the last primary tooth is shed. During this period ,the vertical dimension of the face is increased,allowing for a heightening of the alveolar ridges to accommodate continual root growth of the cuspids and premolars.
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-If the deciduous dentition is spaced dentition with flush -terminal plane of second deciduous molars,the eruptive force -cause a closing of of the existing space causing a decrease in - arch length-(Early mesial shift) -
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When no space exist,the erupting first permanent molars is not able to close spaces.In these case when the primary molars exfoliates the permanent molar migrates to use up the “leeway space�-(Late mesial shift) www.indiandentalacademy.com
The leeway space. -The deciduous anteriors--incisors and canines are narrower than their permanent successors mesiodistally. -The deciduous molars are wider that their permanent successors mesiodistally. -This size difference has clinical significance. The difference is called the leeway space. -The leeway space in the lower arch is approximately 3.4 mm. -The leeway space in the upper arch is approximately 1.8 mm. In normal development, the leeway space is taken up by the mesial migration of the first permanent molars.
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Since there are no premolars in the primary dentition, the primary molars are replaced by permanent premolars. If any primary teeth are lost before permanent teeth are ready to replace them, some posterior teeth may drift forward and cause space to be lost,] This may cause crowding and/or misplacement once the permanent teeth erupt, which is usually referred to as maloclussion.
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'Ugly duckling stage.: ' On occasion, the permanent incisors 'spread out' due to spacing. In the older literature, is called by the 'ugly duckling stage.' With the eruption of the permanent canines, the spaces often will close.
This is the norm between ages 7 through 12 years of age, and usually is not connected with a permanent space between the teeth. This stage of development needs special comment because its often mistaken as anwww.indiandentalacademy.com orthodontic problem.
Permanent dentition Maxillary / mandibular occlusal relationships are established when the last of the deciduous teeth are lost. The adult relationship of the first permanent molars is established at this time. The growth of the jaw continues during this period,developing room for the third molars
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Sequence of eruption Permanent Teeth
Eruption Time
Central Incisor
6-8 years
Lateral Incisor
6.5-9 years
Canine
9-12 years
First Premolar
8-12 years
Second Premolar
8.5-13 years
First Molar
6-7 years
Second Molar
11-14 years
Third Molar (Wisdom Tooth)
17-25 years
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Physiology of the Stomatognathic system • Stomatognathic- of or relating to the jaws and the mouth :webstar’s medical dictionary • As a dentist and especially as an orthodontist,it is essential that we have a sound knowledge of and understand the importance of the stomatognathic system • Until we know what is normal we will not be able to recognize aberrancy
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Functional Osteology
• Bone is one of the hardest tissues of the human body and it’s most adaptive and responsive to functional forces • Meyer -1864:proposed the trajectorial theory of bone formation • 1870:Julius Wolff said that the trabecular arrangement was due to functional forces and any change in direction or intensity of the forces would change the internal architecture & external form of the bone • It was expressed by a mechanical mathematical law= law of orthogonality www.indiandentalacademy.com
• Tajectories need not pass at right angles to each other—even their course may be wavy • Increased function increases bone density while decreased function decreases . • Muscles have a great influence over bones and may even change their shape • Thus the timed manipulation of the musculature may be beneficial to the orthodontist • It is established that cartilaginous and membranous bones react differently to forces www.indiandentalacademy.com
Beninghoff’s trajectories He Concluded saying that trajectories can be demonstrated in the maxillo-facial area • Lines pass through both the spongy and compact bone • They obey no individual bone margins •
Maxilla Transfers the whole stress to the cranium which has as thin cortices that are interconnected by trabeculae Stress trajectory can be considered as the craniofacial unit www.indiandentalacademy.com
3 main pillars of trajectories: • All arise from the alveolar bones and end in the base of the skull The canine pillar The zygomatic pillar The pterygoid pillar • Curve around the sinuses,nasal and orbital opening • The supra and infra orbital ridges and the zygomatic buttress reinforce these pillars
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Mandible • Absorbs all the stresses
Thick cortices and more radially arranged trabecuae Stress trajectories • From beneath the teeth-join together in comman pillar—ends at the condyle
•Mandibular canal and nerve are protected-unloading of nerve •Thick cortical layer at the base provides resistance to bending forces •Trajectories from the symphysis, gonial angle and coronoid process join this main pillar www.indiandentalacademy.com
Myology
Property of muscles (that are important for help in kinetic activity are) Elasticity= the ability to return to original length and shape after contraction or extension Normally relaxed muscle withstand only a certain amount of elongation Contractility=the ability to contract under innervational impulse. Strength of contraction depends on Frequency of stimuli Number of fibers involved
• The greatest of contraction is seen when the muscle approximates its resting position
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Other principles of muscle physiology • All or none law Intensity of contraction is independent of the strength of the action potential as long as the threshold is crossed Seen only when the muscle is not fatigued • Muscle tonus State of slight constant tension and form the basis of reflex posture • Resting length Constant and Permits maintenance of postural relationship • Reciprocal innervations and inhibition Inhibition of contraction may be brought about by excitation of the antagonist.It is through this that movements are coordinated www.indiandentalacademy.com
Buccinator mechanism There is strong inter relationship between bone and muscle Muscles are a potent force-at rest or when active Teeth and supporting tissues are influenced by the contiguous musculature The balance between the tongue and the peri-oral musculature influences the morphology of the dental arches and teeth The shape and size of the tooth root and the periodontal spaces also helps in the final adaptation
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• During swallowing and mastication- tongue exerts 2-3 times more force than the peri oral muscles • Counter acted by the tonal contraction,atmospheric pressure,peripheral recruitment from the buccal musculature • Even though doubts still exist about such a balance of force-we must admit that aberration of muscles can and do lead to marked malocclusion • The continuous band of muscle that encircle the dentition and attach to the pharyngeal tubercle comprises the buccinator mechanism
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Tongue • Opposing the buccinator mechanism is a very powerful structure- the tongue • It has multiple functions • Most developed in a new born • In infancy extrinsic suspensory muscles attach the tongue to the various osseous structures and are responsible for gross movement in the horizontal direction(suckle-swallow)
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• The tongue has amazingly versatile functional possibilities by virtue of the fact it is anchored at only one end. • This freedom permits the tongue to deform the dental arch. •
It may Leads to proclination of the maxillary incisors,constriction of buccal segment and may create an open bite.
• Compensating action of mentalis causes retraction of the lower incisors • Abnormal position and size of the tongue is often seen in the presence of enlarged tonsils and adenoids.
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Muscles of the Tongue
INTRINSIC
Superior longitudinal-Shortens the tongue & makes the dorsum concave Inferior longitudinal-Shortens the tongue & makes the dorsum convex Transverse-Makes the tongue narrow & elongated Vertical-Makes the tongue broad & flattened
EXTRINSIC Genioglossus-
Protrudes the tongue out of the mouth by pulling the posterior part forwards
Hyoglossus-depresses the tongue Styloglossus -pulls it upward & backward Palatoglossus-brings the palatoglossal arches together, thus shutting the oral cavity from the oropharynx www.indiandentalacademy.com
Temperomandibular Joint
• The TMJ is the ball and socket joint that allows the lower jaw to swing open and close. It’s a Compound –movable joint It articulation with the mandibular condyle and the inferior surface of the sqamous portion of the temporal bone •The condyle: This is the "ball" in the joint. It is a part of the mandible (lower jaw), and is covered in a layer of cartilage which allowswww.indiandentalacademy.com for smooth motion within the joint assembly
•The glenoid fossa: The fossa is the "socket", or depression in which the condyle sits. It is located in the temporal bone of the skull.
The front of the fossa is a more gentle slope of bone called the articular eminence. The eminence is also covered with cartilage. The articular disc: The articular disc is also called the meniscus. It is made of hyaline cartilage. The ligament behind the meniscus is called the retrodiscal pad in deference to its function as a shock absorber www.indiandentalacademy.com
•The joint capsule is the covering of the TM joint .It isolates the contents of the joint and allows free movement of the condyle and articular disk within a small "swimming pool" of synovial fluid. The capsule has lots of blood vessels and nerves as well as connective tissue. • It is lined by synovial membrane.
Head of the condyle is tubular or ellipsoidal Unique feature of the joint is that there are actually 2 joint spaces- separated by the disc Thus the joint is capable of dual function Lower cavity—rotary/hinge movement Upper cavity—gliding/translatory movement www.indiandentalacademy.com
• The rotary movement occurs by itself when opening from occlusion to physiologic resting after this even gliding motion takes place • When opened in his manner the articular disc glides over the articular eminence • The condyle rotates against the inferior surface of the disc. • Lateral pterygoid helps to move the disc anteriorly
In the lateral shift/bennet movement the condyle on the working side rotates and moves laterally. • The condyle on the other side moves forwards and mesially in an arc •
• Posterior limit formed by the squamo-tympanic fissure— medially by the petro-tympanic fissure www.indiandentalacademy.com
• Due to over closures-the disc is moved forward and the condyle rides over the posterior border of the disc. Impinges on the retrodiscal soft tissues-clicking an discomfort to the patientmisdiagnosed as arthritic changes ,pain may be due to impingement or due to pterygoid muscle spasm(mpd) • The main reason for the mpd is neurogenic or psychological. The TMJis beautifully engineered to handle multiple tasks endowed to it • To maintain it-proper occlusal management an maintenance of vertical dimensions is essential
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Functional movements of the mandibl •The mandible is the only movable bone in the head and face. •It has 13 muscles attached to it •Due to limitations of morphology and structure of the TMJ it can be moved only in certain directions •It has postural stability along with primary movements
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Forced retrusion
Posterior fibres of the temporalis,masseter,suprahyoid muscles
Opening
lateral pterygoid-main. Supra,infra hyoidgenio,mylo hyoids,digasric>stabilising Temporalis,masseter,medial pter.>controlled relaxation
Closure
Temporalis, masseter >main action medial ptery. >part action Lateral ptery >cntrolled relaxation
Protrusion
Lateral and medial pter.
Lateral movement
Temporalis + lat. pter www.indiandentalacademy.com
COMPENSATORY MUSCLE FUNCTIONS • While mastication may call for most potent effort from the associated muscles, the most frequent demands are made by deglutition, respiration, speech and posture. • At times two or more of these functions are carried on simultaneously. • where there is malocclusion or abnormal morphologic relationship certain compensatory or adaptive muscle functions may arise, either to restrain the dental malocclusion or to actually increase the discrepancy. www.indiandentalacademy.com
Positions of the mandible • • • • • • • •
Basic positions are Postural resting position (Physiologic rest) Centric relation Initial contact Centric occlusion Most retruded position (terminal hinge position) Most protruded position Habitual resting position
• Habitual occlusion position
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POSTURAL RESTING POSITION It is one of the earliest posture positions to be developed. Mandible is suspended from the cranial base by the cradling musculature. Here the jaws are separated by a constant distance. Factors influencing the postural position are the following: • Body and head posture • Sleep • Psychic factors influencing muscle tonus • Age • Proprioception from the dentition and muscles • Occlusal changes such as attrition • Pain • Muscle disease and muscle spasm and • Temporomandibular joint disease. www.indiandentalacademy.com
CETRIC RELATION • As far as muscle physiology is concerned, centric relation may be defined as unstrained, neutral position of the mandible in which the anterio superior surfaces of mandibular condyles are in contact with the concavities of the articular disks as they approximate the postero inferior third of their respective articular eminent. •
This means that the mandible is deviating neither to the right nor to the left and is neither protruded no retruded.
•
Such a relation can be the same as the postural resting position, the point of initial occlusal contact and centric occlusion. www.indiandentalacademy.com
INITIAL CONTACT • As the mandible moves from physiologic rest or the postural resting position toward occlusion of teeth, if all is normal it maintains a centric relation position as far as the articular fossae are concerned. •
If there is normal occlusion the point of initial contact produces no change in the function of temporomandibular joint and all inclined planes are brought together simultaneously in the maxillary and mandibular teeth.
•
Premature contacts are seen quite frequently, they can initiate deflections in the mandibular path of closure.
•
This causes traumatic forces to be exerted on the teeth and severe cases will produce temporomandibular joint problems www.indiandentalacademy.com
CENTRIC OCCLUSION • Centric occlusion is a static position • It is harmonious with the centric relation • Teeth brought into contact with unstrained relation of the condyles • Few patients can show centric occlusion • Premature contact,loss of teeth,Overeruption of teeth,overextension of artificial restorations,malpositions of individual teeth-all these mitigate against the establishment of a centric occlusion
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MOST RETRUDED POSITION (Terminal Hinge Position) • It’s a Reproducible retruded mandibular position with the teeth in occlusion • It has become a common starting point in occlusal analysis and rehabilitation • Some patients can easily retrude a few mm. while others find it difficult • Mandible should not be guided or forced beyond the unstrained position of the mandible-unbiologicalwould compress the tissues.
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MOST PROTRUDED POSITION • This position is variable from individual to individual than the retruded position,however,it is reproducible • Shows the range of movement of the mandible • Flaccidity of capsular ligament allows condyle to over ride the anterior margin of the eminence,leading to dislocation.
HABITUAL RESTING POSITION
There are certain types of malocclusions that prevent the patient from achieving a physiologic resting position.
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• Eg.,in class II div 2 malocclusion with maxillary incisors inclined lingually there is a tendency to force the condyles posteriorly and superiorly in the articular fossae • The physiologic resting positions can be changed due to mental disturbances, enlarged adenoids, temporomandibular joint pathology, psychic trauma, selective paralysis by poliomyelitis and confirmed mouth breathing etc.
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HABITUAL OCCLUSAL POSITION • In normal occlusion the centric occlusion and habitual occlusion should be the same. But the occlusal relationship can be changed when there is an environmental imbalance induced by improper restoration, tooth loss etc. • It is vitally important that the dentist make sure that the habitual occlusal position and the centric occlusal position are the same and are in harmony with centric relation and the postural resting position of the
mandible.
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Functions of the system • • • •
Deglutition Mastication Respiration Speech
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DEGLUTITION
Infantile swallow ( visceral swallow)
Mature swallow ( somatic swallow)
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CHARACTERISTICS OF INFANTILE SWALLOW
The jaws are apart, with the tongue between the gum pads The mandible is stabilized by contraction of the muscles of the 7th cranial nerve & the interposed tongue. The swallow is guided, & to a great extent controlled by sensory interchange between the lips & the tongue. www.indiandentalacademy.com
TRANSITION PERIOD
At about the 5 to 6th month of age, as the incisors begin to erupt, certain proprioceptive impulses come into play & the peripheral portions of the tongue starts to spread laterally. An average infant would show a dominant & exclusive thrusting swallow for the first 6 months of life,a transitional thrusting & lateral spread of tongue during the next year & a dominant somatic swallow thereafter.
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CHARACTERISTICS OF MATURE SWALLOW
The teeth are together
The mandible is stabilized by contraction of the mandibular elevators,which are primarily 5th cranial nerve muscles The tongue tip is held against the palate, above & behind the incisors There are minimal contractions of the lips during the mature swallow. www.indiandentalacademy.com
DEGLUTITION CYCLE FLETCHER gave 4 phases 1.Preparatory swallow 2.Oral phase of swallowing 3.Pharyngeal phase of swallowing 4.Esophageal phase of swallowing
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PREPARATORY SWALLOW • Starts as soon as liquids are taken in, or after the bolus has been masticated • The liquid or bolus is then in a swallowpreparatory position on the dorsum of the tongue. • The oral cavity is sealed by lip & tongue. www.indiandentalacademy.com
Oral phase of swallowing • Soft palate moves
upward & the tongue drops downward & backward • Larynx & hyoid bone move upward • Smooth path for the bolus as it is pushed from the oral cavity by the wave like rippling of the tongue • Oral cavity is stabilized by the muscles of mastication, & www.indiandentalacademy.com maintains the anterior
Pharyngeal phase of swallowing • Begins as the bolus passes through the fauces • The pharyngeal tube is raised upward en masse • Nasopharynx is sealed off by closure of the soft palate against the posterior pharyngeal wall • Hyoid bone & base of tongue move forward as the pharynx & tongue continue their peristaltic – like movement of the bolus of the food www.indiandentalacademy.com
Esophageal phase of swallowing • Commences as the food passes the cricopharyngeal sphincter • While peristaltic movement carries the food through the esophagus, the hyoid bone , palate & tongue return to their original positions www.indiandentalacademy.com
Mastication • In the infant food is taken by suckling-unlearned or automatic reflex • Rhythmic caving in of the cheeks, bobbing of the hyoid bone,snake like movement of the tongue, anterior movement of the tongue and nodding movement of the head • Mandible is protruded • During deglutition rhythmic contraction of the tongue and facial muscles aids in stabilizing the mandible
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6 phases outlined by Murphy • 1)preparatory phase=> food in mouth-tongue positions it-mandible moves to working side • 2)food contact=> characterized by a pauseprobably to determine food consistency • 3)crushing phase=> starts with high velocity and slows down as food is crushed • 4)tooth contact=> takes place with slight change in direction-muscles are already ready for this • 5)grinding phase=> transgression of the mandible across the maxillary teeth-most patients chew unilaterally-cyclic event • 6)centric occlusion=> mandible back to terminal position-next cycle begins www.indiandentalacademy.com
Respiration Inherent reflex activity Demand on muscles are more subtle and more difficult to appreciate Posture has a significant effect on respiration Oral musculature is responsible for vital positional relationships-maintains the airway All the learned jaw functions are built around and accommodated to the tongue and mandibular positions which make clear airway possible Maintenance of the respiratory spaces and airway is significant in facial growth Respiration in the child helps keeping the pharynx patent as well in the development of speech . www.indiandentalacademy.com
Speech Learned activity –depends on maturation of organism Makes use of muscles that have other functions WEST lists the “other than speech functions� as follows 1. innate,automatic,vegetative-swallowing,breathing,gagging 2. learned,automatic,vegetative-biting,chewing,sucking 3. learned,automatic,emotional-grimaces,mannerisms 4. innate,automatic,emotional-laugh,smile,sobbing 5. learned,non automatic,discriminatory,voluntary-movement of with tongue,spreading lips,kissing,blowing 6. learned,automatic practical reactions-whistling,playing instrument,humming www.indiandentalacademy.com
Muscles involved Walls of the torso,the respiratory tract, the pharynx, the soft palate,the tongue,the lips and face and nasal passageways are all concerned in the production of speech. Simultaneous breathing is necessary to provide vibrations for sound Tongue,lips and velopharyngeal closure modify outgoing stream to produce sound . Normal structure is necessary for production of sound-in cleft palate cases its not possible even with muscular compensation Speech mechanism acts on the breath streams in a number of ways,controlling the air mechanism direction,flow,release,pressure,general and lingual air path www.indiandentalacademy.com
With respect to tongue • The lips and the tongue undergo maturational changes • Degree of lip protrusions varies the length of the vocal tract • With the reduction of suckle swallow activity,more delicate peripheral lip movements are noted. • In infancy-suckle reflex is active and extrinsic muscles are well developed-only later the intrinsic muscles capable of speech develops
Velopharyngeal valve(closure) Very important for the dentist-in conditions of cleft palate-inadequate valving is seen-even in rehabilitated cases • Upward and backward movement to contact the post pharyngeal wall is important for certain sound production www.indiandentalacademy.com
Speech difficulties related to Malocclusion
Speech sound
Problem
S/z(sibilants) T/d
(linguoalveolar stops)
lips
(labiodental fricatives)
Th/sh/ch
gap between incisors,Anterior open bite
difficulty in production
F/v
Related malocclusion
Irregular incisors,lingual position of maxillary incisors
distortion distortion
(linguodental fricatives)
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Skeletal classIII Anterior open bite
Conclusion… • The study of the intra oral and peri oral structurestheir normal morphology and activity is very important for us. • Unless we know what is normal it is difficult to recognize and correct abnormalities • Also, the structures are all inter related ::follow scientific principles and order in their development. • It becomes important to follow the functions of the structures which ultimately leads to changes in the morphology.. www.indiandentalacademy.com
Bibliography • Orthodontics:Principles and practices-T.M Graber • Orthodontics:Graber and Vanarsdal • Contemporary Orthodontics: William.R.Proffit • Handbook Of Facial Growth :Donald.H.Enlow
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