Development Of Dentition & Occlusion

DEVELOPMENT OF DENTITION & OCCLUSION JinishnathFinal year part 1Dept of orthodontics

INTRODUCTION The development of dentition is an important part of craniofacial growth as the formation, eruption, exfoliation and exchange of teeth take place during this period. This is an assimilation of facts, predictions, studies, in both static and dynamic situations; the factors influencing them and their clinical implications.

PRE-NATAL DEVELOPMENTOF TEETHThe embryonic oral cavity is lined by stratified squamous epithelium known as the oral ectodermAround the 6th week of intra uterine life, the infero-lateral border of maxillary arch and supero- lateral border of mandibular arch shows localised proliferation of oral ectoderm resulting in the formation of a horse-shoe shaped band of tissue called dental lamina Dental lamina plays a important role in the development of dentition

Early Dental LaminaTongue Dental lamina Vestibular laminaThe deciduous teeth are formed by the direct proliferation of the dental laminaThe permanent molars develops from the distal proliferation of the dental laminaAnd permanent teeth which replace the deciduous teeth develop from the lingual extension of the dental lamina

In certain areas the dental lamina proliferate and forms knob like structure that grows into underlying mesenchyma.Each knob represents a future deciduous tooth and is called enamel organEnamel organ passes through a number of stages ultimately forming the teethBased on shape of the enamel organ develops can be divided into,Bud stageCap stage Bell stage

BUD STAGE Differentiation of dental lamina leads to formation of round, ovoid swelling at 10 different points corresponding to future position of deciduous teeth. These are the primordia of enamel organ. Enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells. Dental papilla : It is the area of ectomesenchymal condensation subjacent to enamel organ. Cells of dental papilla will form tooth pulp & dentine. Dental sac: It is area of ectomesenchymal condensation surrounding the tooth bud & dental papilla. Cells of dental sac will form cementum & periodontal ligament.

CAP STAGE :Characterized by a shallow invagination of deep surface of a bud.Outer & inner enamel epithelium.Cuboidal cells cover the convexity of the cap.Columnar cells cover the concavity of the cap.Stellate Reticulum: Polygonal cells begin to separate as more intercellular fluid is produced and forms cellular network called stellate reticulum.Enamel Knot: Cells in center of the enamel organ are densely packed. This knot projects towards underlying dental papilla.Vertical extension of enamel knot forms enamel cord. Both the structures disappear before enamel formation begins.Dental papilla & dental sac :Dental papilla is for mature organ of dentine & shows active budding of capillaries.Dental sac are important for formations of cementum & periodontal ligament.

BELL STAGE :Stage of Histodifferentiation and Morph differentiation.During histodifferentiation some cells of dental organ diffentiates into specific form and shape. This is seen in early Bell stage. During morph differentiation Dental organ assumes characteristic shape of the tooth. This is seen in late Bell stage.The invagination of the epithelium deepens and its margins continue to grow and enamel organ assumes a bell shape.During histodifferentiation cells acquire their functional assignment. Odontoblasts are differentiated from mesenchymal cells with formation of dentin the cells of inner dental epithelium transform into ameloblasts and enamel matrix is lead down opposite the dentin. Presence of dentin is absolutely essential for laying down of enamel.Differentiation of epithelial cells are essential for differentiation of epithelial Odontoblasts and initiation of dentin formation. Future dentino enamel junction is outlined and the form of crown is established.Tooth germ shows the following structures

Dental organ :a) Outer dental epithelium: A single rows of cuboidal cells is thrown into folds and contain blood vessels at late bell stage.b) Stellate reticulum: There is increase in intercellular fluid and layer expands. The cells assume star shape with long processes that anastomose with adjacent cells.c) Stratum intermedium: Several layers of squamsus cells appear between stellate reticulum and inner dental epithelium and are called stratum intermedium. This layer is essential for enamel formation. It helps in calcification of enamel and is a reserve source for new ameloblasts.d) Inner dental epithelium: This consists of single layer of cells that differentiates into tall columnar cells, the ameloblasts. They have a hexagonal shape on cross section and are 4u in diameter and 40u in height. These cells influence the underlying mesenchymal cells, which differentiates into Odontoblasts.

II. Dental Papilla : It is the mesenchyme enclosed portion of the Dental organ. The peripheral cells under the influence of inner dental epithelium assume an cuboidal shape first & columnar later and are called Odon oblast, which produce dentin. The basement membrane separating the epithelial dental organ and dental papilla is called membranaperformativa which forms future dentino enamel junction.III. Dental sac:Before formation of dental tissues begins, the dental sac shows a circular arrangement of its fibres and resembles a capsular structure. With the development of the root the fibers of dental sac differentiates into the periodontal fibres that become embedded in the developing cementum and alveolar bone.

Apposition The tooth germ forms calcified tissues of the tooth, the enamel, the dentin and the cementum. There is a layer like deposition of an extra cellular matrix resulting in additive growth. There is regular and rhythmic deposition, which is incapable of further growth.

FORMATION OF ROOTRoot start forming after dentin formation has reached future cementoenamel junction. Both dental organ and dental papilla play part in formation of root.Hertwig's epithelial root sheath :* The outer and inner dental epithelium meets one another at future cervical area and is called cervical loop.* This cervical loop forms epithelial sheath of Hertwig, which moulds the shape of the root and initiates dentin formation.

The root sheath consists of only outer and inner dental epithelium.

The inner layer of cells remains short and do not produce enamel. These cells induce the differentiation of cell of dental papilla into Odontoblasts, which lay a layer of dentin. At the same time the continuity of Hertwig's sheath is destroyed due to infiltration of connective tissue and the root sheath breaks up into small strands of epithelium called epithelial rests of Molassez.

While the coronal part of the sheath degenerates, the apical part continues to grow in length and aid in lengthening of root.

The cells of dental sac differentiate into cementoblasts, which lay cementum over the outer surface of the dentin in root portion. At the same time precollagenous fibers appear between cementoblasts and become continuous with outer surface of dentin. They become collagenous and are transformed into cementoid tissue, which calcifies to form cementum. The sheath is folded first at future cemento enamel junction into a horizontal plane. This is called epithelial diaphragm.

Differential growth of the epithelial diaphragm in multirooted teeth causes the division of the root trunk into two or three roots. During the general growth of the enamel organ the expansion of its cervical opening occurs in such a way that long tongue like extensions of the horizontal diaphragm develop.

Before division of the root trunk occurs the free ends of these horizontal epithelial flaps grow toward each other and fuse. The single cervical opening of the coronal enamel organ is then divided into two or three openings.Enamel pearls: If the cells of epithelial root sheath remain adherent to the dentin surface, they may differentiates into fully functioning ameloblasts and produce enamel. Such droplets of enamel called enamel pearls.

THEORIES OF TOOTH ERUPTION Bone remodelingRoot growth Vascular pressure Periodontal ligament traction

POST-NATAL DEVELOPMENT OF DENTITION BIRTH TO COMPLETE PRIMARY DENTITION1ST INTERTRANSITIONAL PERIOD1ST TRANSITIONAL PERIOD2ND INTERTRANSITIONAL PERIOD2ND TRANSITIONAL PERIOD ADULT DENTITION

BIRTH TO COMPLETE PRIMARY DENTITION (Birth- 3yrs) The tooth buds of all primary teeth are present and in various stages of development at the time of birth. About 7 to 8 months after birth, all the teeth except the 7’s & 8’s are present in some stage of development. By the 1st year, a normal & desirable dentition will usually exhibit spacing

The 1st primary tooth to erupt is the lower central incisor between 6&8 months of age, followed by the U.Centrals, U.Laterals & L.Laterals. The 1st primary molar erupts by about the 14th month. The primary cuspids & 2nd primary molars erupt by about 21/2 yrs of age.

FIRST INTERTRANSITIONAL PERIOD( 3-6 yrs ) This is the period between the completion of eruption of Primary dentition & the emergence of the Permanent teeth. According to Baume(1950) Primate Space(Type I Spacing) Spacing invariably is seen mesial to the max canine &distal to the mand canines these physiological space are called primate spaces or simian spaces or anthropoid spaces as they are seen commonly in primates These spaces help in placement of canine cusps of the opposing arch

The tooth buds of the 4’s & 5’s begin to form.

FIRST TRANSITIONAL PERIOD ( 6-8 yrs ) Eruption of the 1st permanent molars:The Terminal plane is very important in determining the interocclusalrelationship of the 1st permanent molars.Vertical plane Mesialstep Distal step

When deciduous 2nd molar in flush terminal plane , the 1st molar erupts into a cusp to cusp relationship,Which later transform into a class 1 molar relation using primate space and leeway spaceIf deciduous molar in distal step, the permanent molar erupt into a class 11 relation, this molar configuration is not self correctingIf primary 2nd molar in mesial step relationship, leads to class 1 molar relation in mixed dentition this may remain or progress to a half or full cusp class 111 with continued mandibular growth

Exchange of incisors : The primary incisors begin to exchange with the permanent incisors before & after the eruption of the 6’s.The total sum of the M-D width of the 4 permanent incisors is > that of the primary incisors by about 7mm in the maxilla & 5mm in the mandible.Incisor liability(Mayne 1968 ) -Interdental spacing in primary incisors should exist.-Intercaninearchwidth growth should occur. - Intercaninearchlength should increase through anterior positioning of the permanent incisors. Favourablesize ratio between the primary & permanent teeth.

SECOND INTERTRANSITIONAL PERIOD( 8-10 yrs ) :This period is referred to as the “Mixed Dentition period”/ “Ugly Duckling stage” ( Broadbent 1937 )Around the age of 8 yrs a midline diatema is commonly seen in the upper archWhich is usually misinterpreted by the parent as malocclusion

Crown of the canine impinge on developing lateral incisor roots, thus driving the roots medially and causing the crown to flare laterallyThe roots of central incisor are also forced together thus causing midline diastemaThis period from the eruption of lateral incisor to canine is termed as the ugly duckling stageWith the eruption of canine, the impingement from the root shifts incisally thus driving the incisor crown medially, resulting in closure of the diastema

The “GABLE EFFECT” : The mand. cuspids & bicuspids are in the shape of a ‘V’, in sequence, in relation to the occlusal plane. The sequence of eruption being 3,4,5 in the mandible, 3 makes its way much ahead of 4 & 5.

SECOND TRANSITIONAL PERIOD( 10- 12 yrs ) Here, exchange of teeth occurs between c,d,e & 3,4, 5. Emergence of 7’s takes place by virtue of increase in arch-circumference, after the Dental arch upto 6, is established .(mixed to permanent )During the exchange & emergence, smooth utilization of “Leeway space” . The sequence of eruption of the teeth in:Maxilla - 4, 3, 5 ( 3,4,5 /4,5,3 are exceptions ). Mandible - 3,4, 5.

The Leeway Spaces , the sum of the mesiodistal width of the primary canine and the primary first and second molars is larger than the sum of their succedaneous teeth, namely, the permanent canine and first and second premolars. This differenceis called the leeway space and is present in both the maxillary and mandibular arches . Themost favorable dental arch pattern is when leeway space is excessive The leeway space is larger in the mandibular arch than in the maxillary arch.

On the average, the 1.8 mm in the lower arch In the upper arch, the leeway space averages only 0.9 mm per side. combined sizes of the unerupted teeth are larger than the space available. This condition is called a leeway space deficiency, and dental arch crowding often results. The leeway space differential between the two arches allows the first permanent molars to move mesially relatively more in the mandibular arch than in the maxillary arch.

ADULT DENTITIONThe permanent dentition stage of dental development starts after the shedding of the last primary tooth and the eruption of all the permanent teeth excluding third molars. This is considered to be between 18 & 25 yrsNose & chin become more prominent to enhance profile.Increase in jaw growth continues ( mandible ) to accommodate the 8’s.

FACTORS AFFECTING OCCLUSAL DEVELOPMENT GENERAL FACTORS1.Skeletal Factors2.Muscle Factors 3.Dental factors LOCAL FACTORS

Skeletal factorsConditions that affect jaw growth are ;1. Any pathological condition2. Inherited & acquired congenital malformation 3. Trauma or infection during the growing years

Muscle factors : Final tooth position is largely governed by muscle action, particularly muscles of the lips, cheeks & tongue.Lip form Lip activityTongue size resting position & functionAdaptive resting posture or adaptive swallowingEndogenous tongue thrustThumb & finger suckingNeutral zone

Dental factorsThe third major factor affecting occlusal development is the relationship between the size of the dentition & the size of the jaws. But it is more realistic to consider dentition size in relation to the dental arch size, than to jaw size. Effects of excessive dentition size-Overlapping & displacement of teeth -Impaction of teeth-Space closure after extractions

Effects of early loss of primary teeth - Function & oral health - Over-eruption of opposing teeth - Psychological effects on child & parent -Position of permanent teethEffects of asymmetric loss of primary teeth -Mid line shift Space maintenance

Local FactorsAberrant developmental position of individual teeth -Trauma -Malposed crown -Dilacerated root -Unknown etiology( perm. Max. canines )Presence of supernumerary teeth -Supplemental (teeth of normal form ) -Conical (the mesiodens ) -Tuberculate (usually palatal to the upper centrals, delaying their eruption )

Development Of Dentition & Occlusion

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