DEVELOPMENT OF DENTITION AND OCCLUSION……. BY NIDA SUMRA. INTERN.
DEVELOPMENT OF TEETH...
The embryonic oral cavity is lined by stratified squamous epithelium known as the oral ectoderm.
Around 6th week the infero lateral border of the maxillary arch and the supero lateral border of the mandibular arch show localized proliferation of the oral ectoderm resulting in the formation of the horse shoe shaped band of tissue called the dental lamina.
The deciduous teeth are directly formed by the proliferation of the lamina.
The permanent molars develop as a result of its distal proliferation.
Succedanous teeth develop from a lingual extension of the lamina.
The ectoderm in certain areas of the dental lamina proliferates and forms knob like structures that grow into the underlying mesenchyme.
Each of these knobs represent a deciduous tooth and is called the enamel organ.
Based on the shape of the Enamel Organ the development of teeth is divided into 3 stages:- 1. Bud stage. 2. Cap stage. 3. Bell stage.
This is the initial stage of tooth development where the enamel organ resembles a small bud.
The enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells.
The tooth bud continues to proliferate resulting in a cap shaped enamel organ.
The outer cells of the cap covering the convexity are cuboidal – the outer enamel epithelium.
The cells lining the concavity of the cap→ tall columnar – the inner enamel epithelium.
The polygonal cells between the outer and the inner epithelium forms a cellular network – the stellate reticulum.
The ectomesenchymal condensation ie:-dental papilla and dental sac are pronounced during this stage.
Due to the continued uneven growth of the enamel organ it acquires a bell shape
A few layers of flat squamous cells between the inner enamel epithelium and the stellate reticulum – stratum intermedium.
As the enamel formation starts the stratum intermedium collapses to a narrow zone reducing the distance between the outer and the inner epithelium.
Dental sac → circular arrangement of fibers → periodontal ligament.
The junction between inner enamel epithelium and odontoblast → dentinoenamel junction.
ENAMEL and DENTIN…
ERUPTION OF PRIMARY TEETH…
NATAL and NEONATAL TEETH...
Very rarely teeth are present at birth called as natal teeth.
If they erupt during the 1st 30 days then they are called as neo natal teeth.
Mostly located in the mandibular incisor region.
ERUPTION OF PERMANENT TEETH…
Chronology of Human Permanent Dentition
PRE EMERGENT ERUPTION...
Eruptive movements begin soon after the root begins to form.
Two processes are necessary for pre emergent eruption:- 1. There must be resorption of bone and primary tooth roots overlying the crown of the erupting tooth. 2. The eruptive mechanism itself then must move the tooth in the direction where the path has been cleared.
POST EMERGENT ERUPTION.
Once the tooth erupts into the mouth it approaches the occlusal level and is subjected to the forces of mastication. • The amount of tooth eruption after the teeth have come into occlusion equals the vertical growth of ramus in a patient who is growing normally.
TERMS… The stage of relatively rapid eruption from the time a tooth first penetrates the gingiva to the occlusal level is called the post emergent spurt. This is followed by the phase of very slow eruption termed the juvenile occlusal equilibrium. When the pubertal growth ends a final phase in tooth eruption called the adult occlusal equilibrium is achieved.
If the antagonist is lost at any age a tooth can erupt more rapidly demonstrating that the eruption mechanism remains active and capable of producing significant tooth movement even late in life.
In children with cleidocranial dysplasia not only the resorption of primary teeth and bone deficient but heavy fibrous gingiva and multiple supernumerary teeth also impede normal eruption.
Retained primary teeth…
Congenitally missing teeth…
DEVELOPMENT OF OCCLUSION AND DENTAL ARCHES… The human dentition is in a dynamic state constantly changing throughout life. A knowledge of these changes assists the clinician in determining whether or not a specific occlusion will be: sustained , worsen ,or self correct over time.
THE DEVELOPMENT OF OCCLUSION OCCURS OVER 3 DISTINCT STAGES. 1. The deciduous dentition begins soon after birth and is completed with the eruption of all deciduous teeth. 2. The eruption of the first permanent molar ushers in the mixed dentition . 3. Loss of the last deciduous tooth denotes the transition to the permanent dentition.
PERIODS OF OCCLUSAL DEVELOPMENT. 1. Pre-dental period. 2. The deciduous dentition. 3. The mixed period. 4. The permanent dentition period.
PRE - DENTAL PERIOD...
GUM PADS… The alveolar processes at the time of birth.
Dental groove:-separates the labiobuccal and the lingual portion.
Transverse groove:-divides the gum pad into ten segments representing each deciduous tooth.
Gingival groove:-separates the gumpad from the palate and the floor of the mouth.
Lateral sulci:-present between the canine and the 1st molar.
INFANTILE OPEN BITE…
When the upper and the lower gum pad are approximated there is a complete overjet all around.
This infantile open bite is considered to be normal .
It helps in sucking.
THE DECIDUOUS DENTITION…
ERUPTION AGE and SEQUENCE…
The mandibular central incisors are the first to erupt around 6-8 months of age.
A variation of 3 months from the mean age is accepted to be normal.
The sequence of eruption is :- A – B – D – C – E.
The primary dentition is usually established y the age of 3 years.
CHARACTERISTICS OF DECIDUOUS DENTITION…
Spacing usually exists between the deciduous teeth for the normal development of permanent teeth.
These spaces are called as developmental or physiological spaces.
Spacing invariably is seen mesial to the maxillary canine and distal to the mandibular canine.
These spaces are called as primate, simian or anthropoid spaces.
The deep bite is accentuated by the fact that the deciduous incisors are more upright then their permanent counterparts.
This is later reduced due to:- 1. Eruption of deciduous molars. 2. Attrition of incisors. . 3. Forward movement of mandible due to growth.
FLUSH TERMINAL PLANE…
THE MIXED DENTITION …
ERUPTION AGE and SEQUENCE..
The mixed dentition period begins at around 6 yrs of age with the eruption of the 1st permanent molar.
This period can be divided into the following 3 phases:- 1. 1st transitional period. 2. Inter transitional period. 3. 2nd transitional period.
FIRST TRANSITIONAL PERIOD…
EMERGENCE of 1st PERMANENT MOLAR
The mandibular 1st molar is the first permanent tooth to erupt at around 6yrs of age.
The location and relation of the 1st permanent molar depends on the distal relationship between the upper and lower 2nd deciduous molars.
Flush terminal plane → class Ι (early and late mesial shift.)
Mesial step → class Ι.
Differential growth of mandible in forward direction persists → class ΙΙΙ.
Distal step → class ΙΙ.
EARLY and LATE MESIAL SHIFT…
Early shift occurs during the early mixed dentition period . →
The eruptive forces of the 1st permanent molar is sufficient to push the deciduous 1st and 2nd molars forward to close the primate spaces and establish classΙ molar relationship.
In the cases when the primate spaces are absent the permanent 1st molar drift mesially utilizing the leeway space.
This occurs in the late mixed dentition period and is called the late shift. →
During the 1st transitional period the deciduous incisors are replaced by the permanent ones.
The permanent incisors are considerably larger then the deciduous teeth they replace.
This difference between the amount of space needed and the amount of space available is called as incisal liability.
The incisor liability is about 7mm in the maxillary arch and 5mm in the mandibular arch.
This is overcome by 3 factors :- 1. Inter-dental spaces. 2. Inter-canine width. 3. Incisor inclination.
The physiologic spaces seen in the primary dentition are utilized to partly account for the incisal liability.
The permanent incisors are much more easily accommodated in normal alignment in cases exhibiting adequate inter-dental spaces.
During the transition from the primary incisors permanent incisors an increase in inter-canine width of both maxillary and mandibular arch is observed.
The primary incisors are more upright than their permanent counterparts.
Since the permanent incisors are more labially inclined they tend to increase the dental arch perimeter.
INTER TRANSITIONAL PERIOD..
In this period between the permanent incisors and the 1st permanent molars are the deciduous molars and canines. This phase is relatively stable and no change occurs.
SECOND TRANSITIONAL PERIOD…
LEEWAY SPACE of NANCE…
The second transitional period is characterized by the replacement of deciduous molars and canines by the permanent premolars and cuspids respectively.
The combined mesio distal width of the permanent canines and premolars is less than that of the deciduous canines and molars.
This excess space is called leeway space of Nance.
1.8mm – maxillary arch.
3.4mm – mandibular arch.
UGLY DUCKILNG STAGE…
A transient or self correcting malocclusion is seen in the maxillary incisor region between 8-10 yrs at the time of eruption of the permanent canines.
This situation is described by Broadbent as ugly duckling stage as children tend to look ugly during this phase of development.
As the developing permanent canines erupt they displace the roots of the lateral incisors mesially.
This results in transmitting the force on roots of the centrals which also get displaced mesially.
Hence a distal divergence of the two centrals causes midline spacing.
After the eruption of canines the pressure is transferred from the root to the coronal area of the incisors and the malocclusion is corrected. →
THE PERMANENT DENTITION PERIOD…
The permanent dentition forms within the jaws soon after birth except for the formation of the cusps of the 1st permanent molar which form before birth.
The permanent incisors develop lingual or palatal to the deciduous incisors and move labially as they erupt.
The premolars develop below the diverging roots of deciduous molars.
ERUPTION SEQUENCE… In maxillary arch:-
6-1-2-3-4-5-7. In mandibular arch:-
ANGLE defined occlusion as the normal relation of the occlusal inclined plane of the teeth when the jaws are closed.
The term occlusion has both static and dynamic aspects.
STATIC OCCLUSION… Static refers to the form , alignment of teeth within and between the arches and the relationship of teeth to their supporting structures.
Dynamic refers to the function of the stomatognathic system as a whole comprising teeth supporting structures, temperomandibular joint , neuromuscular and nutritive system.
It is a pre conceived theoretical concept of occlusal structures and relationships that include idealized principles and characteristics that an occlusion should have.
An occlusion in which balanced and equal contacts are maintained throughout the entire arch during all excursions of the mandible.
Traumatic occlusion is an abnormal occlusal stress that is capable of producing or has produced injury to the periodontium.
Anterior cross bite which produces a traumatic bite resulting in gingival recession of the involved tooth. →
TYPES OF CUSP…
Centric holding cusps:- (stamp cusps) The facial cusp of mandibular and palatal cusps of maxillary posterior teeth. They occlude in the central fossae and marginal ridges of opposing teeth.
Non supporting cusps:- (shearing or guiding cusps.) The maxillary buccal and the mandibular lingual cusps . They contact and guide the mandible during excursions.
ARRANGEMENT OF TEETH…
Human dentition exhibits 2 types of teeth arrangement when the upper and the lower teeth occlude:-
Cusp fossa occlusion:- The stamp cusp of one tooth occludes in a single fossa of a single opponent. Tooth to tooth occlusion.
Cusp embrasure occlusion:- Each tooth occludes with two opposing teeth. Tooth to two teeth occlusion.
IMAGINARY OCCLUSAL PLANES AND CURVES…
CURVE OF SPEE.. CURVE OF WILSON..
Curve of Spee:- It refers to the antero posterior curvature of the occlusal surfaces beginning at the tip of the lower cuspid and following the cusp tips of the bicuspids and molars continuing as an arc through the condyle.
Curve of Wilson:- This is a curve that contacts the buccal and lingual cusp tips of the mandibular teeth. The curve of Wilson is medio lateral on each side of the arch.
CURVE OF MONSON.
It is the curve of occlusion in which each cusp and incisal edges touch or conform to a segment of a sphere in diameter with its centre in the region of glabella.
CENTRIC RELATION and OCCLUSION
Centric relation is the relation of the mandible to the maxilla when the mandibular condyles are in the most superior and retruded position in their glenoid fossa with the articular disk properly interposed.
Centric occlusion is that position of the mandibular condyle when the teeth are in maximum intercuspation.
They are areas of teeth that contact the opposing teeth.
Posterior centric contacts
Anterior centric contacts.
Eccentric occlusion refers to contact of teeth that occurs during movement of mandible.
Eccentric occlusion can be of 2 types:- 1 - Functional occlusion.
Protrusive functional occlusion. 2 - Non functional occlusion.
Also called as the Working side occlusion.
It refers to the tooth contact that occur in the segment of the arch towards which the mandible moves.
LATERAL FUNCTION OCCLUSION
It includes contacts that occur on canines and posterior teeth on the side towards which the mandible moves.
It is divided into two parts:- 1. Canine guided occlusion. 2. Grouped lateral occlusion.
CANINE GUIDED and GROUPED LATERAL OCCLUSION…
CANINE GUIDED OCCLUSION. During lateral mandibular movements the opposing upper and lower canines of the working side contact thereby causing disclusion of all posterior teeth on the working and balancing side.
GROUPED LATERAL OCCLUSION. In addition to canine guidance when certain other posterior teeth on the working side also contact during lateral movement it is called as Grouped lateral occlusion.
It includes eccentric contacts that occur when the mandible moves forward.
The mandibular anterior teeth contact along the lingual inclines of the maxillary anterior teeth.
NON FUNCTIONAL OCCLUSION.. The tooth contacts that occur in the segment away from which the mandible moves. For eg:-if the mandible is moved to the left side contacts occur on the right side.
ANDREWS 6 KEYS TO NORMAL OCCLUSION….
KEY 1 MOLAR INTERARCH RELATIONSHIP…
The mesiobuccal cusp of the upper 1st molar should occlude in the groove between the mesial and medial buccal cusp of the lower 1st molar.
KEY 2 MESIO DISTAL CROWN ANGULATION...
A line that passes along the long axis of the crown through the most prominent part of the center of the labial or the buccal surface is called the long axis of the clinical crown.
The gingival part of the long axis must be distal to the occlusal part of the axis.
KEY 3 LABIO LINGUAL CROWN INCLINATION.
If the gingival area of the crown is more lingually placed than the occlusal area:-positive crown inclination.
If the gingival area is more labially or buccally placed:- negative crown inclination.
Maxillary incisors:- +ve inclination.
Mandibular incisors:- -ve inclination.
Posteriors:- -ve inclination.
KEY 4 ABSENCE OF ANY ROTATION…
Normal occlusion is characterized by absence of any rotation. →
Rotated posterior teeth occupy more space while rotated incisors occupy less space. Incisor rotation
KEY 5 TIGHT CONTACTS…
To consider an occlusion normal there should be tight contacts between adjacent teeth. Spaced dentition. Tight contacts
KEY 6 CURVE OF SPEE…
A normal occlusion plane according to Andrews should be flat wit the curve of Spee not exceeding 1.5mm
CHANGES IN ALIGNMENT AND OCCLUSION..
In modern population there is a strong tendency for crowding of the mandibular incisors to develop in the late teens or early twenties.
3 major theories to explain this are:- 1. Lack of normal attrition in modern diet. Mandibular crowding. 2. Pressure from the 3rd molars. 3. Late mandibular growth.
LACK of NORMAL ATTRITION in the DIET.
Shortening of arc length and mesial migration are a natural phenomenon.
Hence crowding will not develop if the amount of tooth structure is lost during the final stages of growth.
Therefore as compared to the primitive population who had a coarse diet the modern diet is soft.
Hence less attrition and more chances of crowding.
PRESSURE FROM THE 3rd MOLARS.
In most of the cases 3rd molars are impacted because the jaw length does not increase enough to accommodate them via backward remodeling of the ramus.
LATE MANDIBULAR GROWTH…
Late incisor crowding develops as the entire mandibular dentition moves distally relative to the mandible in late mandibular growth.
The mandibular incisors tend to be displaced lingually.