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DEVELOPMENT OF OCCLUSION

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					                                 DEVELOPMENT OF OCCLUSION

       Occlusion is the normal relation of the occlusal inclined planes of the teeth when the jaws are closed
-Angle.


Occlusion: Is the changing interrelationship of the opposing surfaces of the maxillary and mandibular teeth,
which occurs during movements of the mandible and the terminal full contact of the maxillary and
mandibular dental arches – Gregory.

         Occlusion of the teeth is not a static condition, as the mandible can assume various positions.
Occlusion may be centric, habitual, mesial, distal, eccentric, labial, lingual, supra, inferior and may other
forms.

Ideal Occlusion:
         Ideal occlusion is a hypothetical formula, which does not and cannot exist in man. Ideal tooth form
and ideal occlusion necessitates an unblemished heredity, an optimum favourable environment and a
developmental history devoid of any accident, diseases or occurrence, which would modify the inherent
growth pattern.

         According to Maxwell an ideal occlusion presupposes and requires.
1) Normally developed coronal contour of properly coordinated mesiodistal and buccolingual dimensions.
2) Normally developed tooth and osseous, muscular other anatomic and emerging structures.
3) A definite geometric and anatomic, individual and collective relationship of denture units.
4) A definite geometric and anatomic relationship of the two dentures, cranium and mandible.




I. FACTORS FOR ACHIEVING NORMAL OCCLUSION:
         The achievement of a normal occlusion is dependent upon a number of factors and may easily be
thwarted by failure of any one of them. To understand these factors and to set them in order logically, the
experience of a developing tooth is traced from its position in the crypt to its final position in the mouth.
These factors are grouped under the following headings:
a) The position, size and relationship of the bone in which the tooth develops.
b) The position and relationships of the tooth with in the bone.
c) The path which the tooth follows to reach the mucous membrane before eruption.




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d) The forces which guide its course after eruption.
e) The forces which start to operate when the tooth contacts with its apparent.

1) BONE RELATION :
       The relationship of the maxilla or mandible to other bones and to each other is probably determined
by a number of factors like hereditary, congenital, hormonal imbalance, traumatic and pathological
condition which interfere with growth. All these conditions will effect upon the nature of the ultimate
occlusion.

a) Hereditary or Racial Influences :
       Most dental and facial characteristics are inherited from the parents, though these may be modified
by environmental factors like prenatal and postnatal influences, pressure habits, nutritional disturbances etc.

       Horawitz S.L. 1958, Heredity is significant in determining the following characteristics and thus
influence the development of normal occlusion.
1) Width and length of the palate
2) Height of the palate
3) Crowding and spacing of the teeth
4) Position and conformation of perioral musculature to tongue size and shape
5) Soft tissue peculiarities – character and texture of the mucosa, frenum size, shape and position etc.

       Certain characteristic traits are seen in a particular race like broad jaw in Negroes, the „Rocker jaw‟
(smoothly cured gonial angle) of the polynesians.

b) Congenital Influences :
       The development and growth of the craniofacial skeleton with associated soft tissues and the primary
teeth, and a few permanent teeth begin prenatally and an interference with this development, either due to
nutritional, metabolic, or other systemic influences, drugs or trauma, may all result in malocclusion.

Example :
Congenital syphilis : Abnormally shaped teeth and malposed teeth.
Birth injury to TMJ : Ankylosis affects mandible growth.
Cerebral palsy (From birth injury)      : Paralysis or lack of muscular co-ordination affects mastication,
deglutition, respiration, and speech and upsets muscle balance which is necessary for establishment and
maintenance of normal occlusion.

c) Tooth Relationship :



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          According to Nanda, there is some evidence that the developmental position of a tooth is also under
strong hereditary control, similar atypical malpositions of individual teeth are seen in twins and sibling.

          The lower permanent molars develop at the root of the coronoid process, oriented with a mesial
inclination. This is corrected subsequently as the tooth erupts forward and upward along a curved path.
Upper permanent molar develops facing backwards in the tuberosity of the maxilla. The upper permanent
molars swing downwards and forwards as they erupt through an arch of a circle whose center would be
somewhere in the region of the apex of the next tooth mesially.

          During intra alveolar eruption, the tooth position is affected by- the presence or absence of adjacent
teeth, resorption of the primary teeth, early loss of primary teeth, localized pathologic conditions and any
factors that alter the growth or conformation of the alveolar process.

3) Tooth Eruption :
          Several theories have been propounded to explain the mechanism of eruption. Each of them fails to
explain all the phenomena that have been observed and eruption is defined as a developmental process that
moves a tooth from its crypt position through the alveolar process into the oral cavity and to occlusion with
its antagonist.

          Permanent teeth do not begin eruptive movements until after the crown is completed. They pass
through the crest of alveolar process at varying stages of root development. It takes 2 to 5 years for the
posterior teeth to reach the alveolar crest following completion of the crowns and from 12 to 20 months to
reach occlusion after reaching the alveolar margin. The roots usually are completed a few months after
occlusion is attained. The moment of emergence into the oral cavity often is spoken of as the „time of
eruption‟.

Various Theories of Eruption :
I. Root Elongation Theory (Hunter, Weber 1778) :
      According to this theory the simplest and most obvious mechanism of eruption would be that the
crowns of the teeth are pushed into the oral cavity by virtue of the growth and elongation of the roots.

Evidence for theory :
          Root of tooth elongates as crown erupts into the oral cavity. This evidence however, is only
circumstantial.
          For this theory to become practical there should be a strong base, which can provide the necessary
rebound mechanism where by the apical root growth forces can be translated into the occlusal eruptive
forces.




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Evidence against this theory :
a) The bone of the socket floor cannot act as a fixed base as it has been understood that as a root grows
   towards the socket floor the bone gets resorbed.
b) Rootless teeth often erupt with out the concomitant elongation of the root. This condition can be seen in
   ;
   1) The erupted crowns of incisors or premolar teeth after the premature extraction of their deciduous
       predecessors.
   2) Submerged teeth often continue the formation of their roots but do not erupt.
   3) Supra eruption of a tooth will occur, when its antagonist is removed by extraction or bite opening,
       long after root formation is completed.

II. Alveolar Bone Remodelling Theory (Brash 1928) :
       Selective resorption and deposition of bone is active around the crypt of a tooth. It was suggested
that continuous bone deposition at the base of the socket causes tooth eruption.

Evidence against this theory :
       Root formation starts after the crown formation is completed. The root grows downward causing
resorption of bone rather than deposition.

       Studies showed that bone resorption takes place at the base of the bony socket, while deposition
occurs only after eruption has commenced therefore it looks more like an effect not a cause.

Evidence for This Theory :
       Bone remodeling of the crypt wall clearly is important to achieve tooth eruption and in experiments
where tooth germ is removed but the follicle is left in position, the eruptive pathway still forms in bone thus
proving the dental follicle and not bone as the major determinant in tooth eruption.


III. Pulpal constriction Theory : (Zucker Kandal et al) :
       According to this theory the growth of the root dentin and subsequent constriction of the pulp may
cause sufficient pressure to move the tooth occlusally.

Evidence for this theory :
       The pulp is progressively constricted by growth of root dentin as the eruption of the tooth occurs.
This evidence is only circumstantial.

Evidence against the theory :




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1) Pulpless teeth often erupt at the same rate as the normal neighborus.
2) Submerged molars and upper cuspids often erupt late i.e. after dentin formation is completed and pulp is
   fully enclosed with obliterated apical foramen.
3) Removal of pulp in the continuously erupting incisors of the rat does not alter the rate of its eruption,
   although dentin formation and pulpal constriction is ceased.

IV. Pressure from muscular action upon the alveolar process :
       Berten suggest that the action of the musculature of the cheeks and lips upon the alveolar process
might serve to squeeze the crown of the tooth into the oral cavity like a pumpkin seed from between the
fingers. This process continues until tooth is in occlusion.

Evidence against theory :
       The teeth erupt which lie lingual to the arch and there are therefore under no muscular action of the
cheeks and lips.


       Mouth breathers with notoriously week action of cheek and lip musculature show a relative increase
or surpa eruption of the clinical crowns of the teeth, while people with strong musculature and powerful
bites show much less erupted clinical crowns of the teeth.

V. Resorption of Alveolar Crest :
       Aichel and Weidenraich suggest that the resorption of the alveolar crest would serve to expose the
crown of the tooth into the oral cavity. This theory is not tenable since histological examination shows that
the alveolar crest is the site of the most rapid and continuous growth of bone.

VI. Vascular Pressure Theory (Massler and Schour 1941) :
       Vascularity and exudation of local fluids around the root apex were believed to raise local pressure,
which would push the tooth along its long axis. It is known that teeth moved in synchrony with the arterial
pulse indicating momentary response to hydrostatic pressure.

Clinical evidence of the relation of vascularity to eruption :
1) Submerged teeth often erupt under the influence of hyperemia induced by mechanical irritants. Thus a
   submerged tooth will erupt after an artificial denture is placed, examination shows that the tissue is
   markedly hypremic.
2) The hyperemia in periodontitis causes a supra eruption of teeth.




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3) Following an intraalveolar fracture the incisal segments continues to erupt while the apical segment
   becomes ankylosed. The site of fracture becomes markedly hyperemic. This hyperemia may be the
   prime factor in causing the migration of incisal segment.

Experimental Evidence :
       Leist and King 1936 sectioned the sympathetic innervation on one side of the jaw in guinea pigs that
produced permanent vasodilatation and marked increase in vascularity on the operated side. They also
observed an increase rate of eruption of teeth on that side.

Periodontal Ligament Traction Theory :
       According to this theory the periodontal membrane plays an important role in the tooth eruption.
They found two causative agents with in the periodontal ligament which can generate eruptive force.
1. Collagen contraction
2. Fibroblast traction

       The periodontal ligament is a fabric of fibers, ground substance and cells. The fibers are composed
of collagen, a glue like protein, while the ground substance is a macro molecular mass of proteins and
polysaccharides. This substance is packed with cells, predominantly fibroblasts.

Collagen Contraction Theory :
       The tractional forces are generated with in the oblique fiber system of the periodontal ligament due
to cross linking and aggregation that occurs during collagen maturation. This contraction of fibers occurs
only during maturation. Therefore to maintain the eruption process there should be continuous turnover of
collagen in the periodontal ligament.

Presence of Intermediate Plexus :
       These comprise of fibers coming from the bone and those coming form the cementum and unite at
the center.    This plexus consist of precollagenus fibers which allows continuous rebuilding and
rearrangement of the periodontal ligament with out change in bone or cementum. This plexus anchors the
tooth firmly and at the same time allows movement.

Fibroblast Contractile Theory :
       Fibroblasts play an important role in eruption because of their contractile properties.
       The fibroblasts are able to contract a collagen gel and bring about movement of root tissue attached
to the gel. Thus they are able to transmit their own contractile forces to the extracellular environment. In
the periodontal ligament 1) Adherence type of contact is present between fibroblasts which permits the




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summation of contractile forces, 2) Fibronexus is present by which the contractile forces are transmitted to
the collagen fibers bundles.

       The fibers are inclined at the correct angle to bring about eruptive movement. This orientation is
established by the developing root. In summary eruptive movement is brought by a combination of events
involving a force initiated by the fibroblast. This force is transmitted to the extra cellular compartment via
fibronexus to the collagen fiber bundles, which are aligned in an appropriate inclination brought about by
root formation, causes tooth movement.

Factors affecting eruption :
       Mechanical disturbance can alter the plan of eruption, cause localized pathosis. Periapical lesions,
pulpits and pulpatomy of a primary molar will hasten the eruption of the successor premolar. If the primary
tooth is extracted prior to the onset of permanent tooth eruptive movement (prior to root formation) the
permanent tooth is very likely to be delayed in its eruption, since the alveolar process may reform atop the
successor tooth, making eruption more difficult and slower. Intrusion or extrusion of the primary incisors
may occur accidentally during early childhood, resulting in disturbance in mineralization of the permanent
successors in the same instance and even occasionally intrusion of permanent incisor.

Sex Differences :
       Except for third molar, in girls the permanent teeth erupt an average of approximately 5 months
earlier than boys.

Sequence of Eruption :
       There is wide variability in the sequence of arrival of teeth in the mouth, same of the variations are
important clinically. In the maxilla, the sequence 6-1-2-4-3-5-7 and 6-1-2-3-4-5-7 account for almost half of
the cases, whereas in the mandible the sequence 6-1-2-3-4-5-7 and 6-1-2-4-3-5-7 include more than 40% of
the cases.


       The rate at which the incisors erupt is much faster than that of the molars at the time of immediate
emergence into the mouth, if one is seeing a child at 6 months intervals, it may look as if the incisor has
came first where as in truth the molar has preceded it but is moving so slowly that the incisor passes it by.
Investigators who have studied eruption sequence at short intervals tend to report the mandibular molars as
erupting first. There seems to be clinical significance attached to either 6-1 or 1-6 sequence.

       Mandibular 6-1-2-3-4-5-7 is favourable for maintaining the length of the arch during the transitional
dentition.




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Factors Determining the tooth position during eruption :
       When the teeth occlude with those of the opposite dental arch (occlusal stage of eruption) a most
complicated system of force determines the position of the tooth. The muscles of mastication exert an
influence through the interdigitation of the cusps. The upward forces of eruption and alveolar growth are
countered by apposition of the apically directed force of occlusion. The periodontal ligament disseminates
these strong forces of chewing into the alveolar bone.

       The axial inclination of the permanent teeth is such that some of the forces of chewing produce a
mesial resultant force through the contact points of the teeth, the “anterior component of force”. The
anterior component of forces often is confused with the mesial drifting tendency, the former is the result of
muscle forces acting through the intercuspation of the occlusal surfaces, whereas the mesial drifting
tendency is an internal disposition of most teeth to drift mesially even before they are in occlusion. The
anterior component of forces is countered by the approximal contacts of the teeth and by the musculature of
the lips and cheeks.




4) Intra Oral Forces :
       All the time that a tooth erupts into the mouth, its roots are separated by a considerable margin from
the wall of its sockets. This permits plenty of latitude for its guidance by other forces to its final position. It
is at this stage that physical forces are most likely to influence the position of the tooth. The forces which are
encountered by the tooth may be divided into buccolingual forces, which are derived from muscles of the
lips, cheeks and tongue and the mesio-distal forces which are exerted through adjacent teeth.
       These forces which are generated by muscles may be either passive or active.

Passive Muscle Forces :
       Certain muscles exert a constant tension (muscle tonus) upon the jaws. At rest a muscle is in a state
of tonus. In this state, a small proportion of fibers contract, the proportion of fibers is constant but they are
not always the same fibers. Successive groups taking over the function of maintaining tension.

       The muscles which have a direct effect on the jaws are those of deglutition, expression and
mastication. The tongue with in the lingual vestibule is applied to the lingual surfaces of the teeth and the
hard and soft palate. The lips and cheeks apply forces to labial and buccal surfaces. At the same time there
is a tension from articularis oris muscle on the upper incisors.




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       When an individual is at rest the mandible is held in such a position that the upper and lower teeth
are normally separated a little. This distance is called “Free way space” or interocclusal clearance. This
position of the mandible is maintained by balance of muscle tonus.

Active Muscle Forces :
       Active muscle forces exert pressure only intermittently, the degree of force however is great than that
of muscle tonus.    One of the most frequently performed activities is deglutition which therefore has
considerable significance to the orthodontist.

II. EFFECTS OF MUSCULATURE, LIPS, TONGUE AND ORAL HABITS :

       The functions of the oro-facial musculature include mastication, deglutition, respiration, speech and
maintenance of head posture. These effects of musculature in relation to occlusion will here be considered
under a series of functional headings.

       Muscle physiology: Studies in muscle physiology indicate that orofacial muscles are a potent force
whether in active function or at rest. The stability of dental relationship is determined by the buccinator
mechanism from outside and by the tongue from inside.           Equilibrium in these forces assumes great
importance in the retention and stability of orthodontically treated dentitions. The effect of muscle function
should be considered both in vertical and horizontal plane.

       Winders (1956) measured the force of perioral and lingual musculature on the dentition and found
that the tongue was capable of exerting more lingual pressure (about 2-3 times much force) than the labial
and buccal musculature. These findings suggested an imbalance in muscle force in the normal condition
(this impression was confirmed by Kydd 1957).

       Briggs 1965 and Lear et al 1965 found that a normal adult in an average swallows 585 times a day
with a range of 233 to 1008 times per day. According to Profitt a typical individual swallows about 800
times per day, while awake, but has only a few swallows per hour while asleep. The total swallows per day
therefore is usually under 1000 times, 1000 second of pressure of course total only a few minutes, not nearly
enough to effect the equilibrium.


       It must be noted that contractile forces exerted by the tongue are dynamic and greater than the
perioral muscle forces, but it lacks steady state condition. Tongue pressure in the maxillary incisor region
has been determined to be 75 +/- 25 g/cm per each swallow and pressure from the sides of palate and molar
teeth is 100 +/- 30 gm/cm (Proffit and Norlon 1970). Lingual pressure against the mandibular incisors and
molars has been recorded as 90 gm/cm. In case of perioral musculature a significant consideration is the



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passive tonic forces. These forces are produced from a constant slight tension. The restrictive influence and
the forces exerted by the perioral musculature can be explained by a study of the buccinator mechanism.
The mechanism is an intermingling of fibers of all preoral muscles to constitute a functioning unit.
Anteriorly the superior and inferior fibers of the articularis oris decussate with the zygomaticus, levator
anguli oris, platysma and laterally with the buccinator.        The buccinator posteriorly inserts into the
pterygomandibular raphae, just behind the dental arches. The fibers of the superior constrictor muscle
decussate at this point and continue posteriorly and medially and attach to the pharyngeal tubercle of the
occipital bone. The buccinator complex of muscle acts like a rubber bondage around the dentoalveolar
region and is important in the maintenance of equilibrium and stability of the dentition.

Posture of The Lips :

       The lip seal is determined by lip length, protrusion of the incisors and the vertical height of the lower
face. The usual lip posture is a closed lip position. In cases with small interlabial gap, the lip contraction
required for a lip seal is minimal, where as in cases of large interlabial gap with small lip length, there may
be significant muscular activity and a contraction of the mentalis muscle may be evident. Due to contraction
of mentalis muscle the chin will be flattened and moves the inferior facial sulcus upward and forward and an
attempt to close the lips increases the posterior component of forces on the incisors. In cases with Class II
division I where protrusion of incisors is marked the patients have a habitual relaxed lip position where as in
Class II division II there is lip redundance and the orbicularis oris and related muscles provide a restraining
effect on the dentition as if teeth were warped in elastic rubber sheath. In cases of Class III malocclusion the
lower lip tends to be stretched against the forward positioned incisors producing a lingual inclination of
these teeth.    Lip function in cases with malocclusion therefore accentuates the existing dental
malrelationships through functional adaptation.

Tongue Position :

       Tongue size, position and function may be direct cause of or an important contributory factor in the
development of malocclusions. A large tongue is usually responsible of wide well developed dental arches,
buccal or labial inclination of teeth with spacing and occasionally leads to posterior open bite and anterior
open bite. If the tongue is too small the dental arches are narrow and the teeth in the buccal segments are
lingually inclined. Some times there is crowding of the teeth, and retro inclination of the incision teeth as
seen in Class II Division 2 malocclusion.

       The position of the tongue will also be affected by the craniofacial morphology. A small gonial
angle and a flat mandibular plane will provide more space for the tongue, as a result the tongue is depressed




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and has a low posture. In other cases when high mandibular plane and a large gonial angle provides less
space for the tongue.

       According to Robert Swinheart normal arch form requires sufficient dimension to accommodate the
teeth. The most important natural forces which can increase mandibular arch dimension are those of the
tongue. The normal expansive forces of the tongue are exerted to their maximum only when it can be
accommodated with in the mandibular arch. The mandibular arch found associated with congenital aglossia
provides proof of the vital importance of normal tongue form and function to normal occlusion.




Swallowing pattern :

1) Mature swallowing pattern
2) Deviant swallowing pattern

1) Mature swallowing pattern
       Swallowing begins with the mandible in a rest position. During swallowing the mandible moves
upward and forward in its path towards closure. In the rest position the tongue is at the level of the
mandibular incisors or slightly lower in cases with normal occlusion. At the initiation of deglutition, the
tongue moves forward and upward touching the lingual surface of the maxillary incisors of the lingual
papillae.

2) Deviant Swallowing Pattern or Tongue Thrust Swallowing :

       In many individuals the infantile swallowing pattern persist and may contribute to the development
of malocclusions.       In this type of swallowing a great variation has been observed in the pattern of
swallowing, tongue tip contact, dental occlusion and the resultant speech defects.

Infantile Swallow :

       In neonates the tongue seems relatively large and in the forward suckling postural position for
nursing. The tip actually inserts through the anterior gumpads and takes part in the anterior lip seal. This
tongue position and the coincident swallowing are termed as infantile or visceral swallow. With the
eruption of incisors at about 6 months of age the tongue positions starts to retract. Over a period of 12 to 18
months as proprioception causes tongue postural and functional changes, there is a transitional period.
Between 2 to 4 years of age the functionally balanced or mature swallow is termed as somatic swallow. This
swallow is seen in normal developmental patterns. The visceral type of swallow can persist well after 4 th



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year of life, however, and is then considered a dysfunction or abnormal because of its association with
certain malocclusion characteristics. The symptoms of a retained visceral swallowing pattern usually
include a forward tongue posture and tongue thrusting during swallowing.

Tongue Thrust :

       It is classified in terms of two categories ;
1) Simple
2) Complex

       In simple tongue thrust the teeth come together during swallowing and there a contraction of the lips,
the mentalis and circumoral muscles to obtain a lip seal. Excessive activity of the circumoral musculature is
abnormal. Swallowing provide a characteristic facial grimace. But during the normal mature swallow
neither the lips nor the muscles of facial expression show any active contraction. Whereas in case of
complex tongue thrust teeth will be apart during swallowing. The condition is accompanied by poor muscle
tone and generalized open bite. During swallowing there is an absence of temporal muscle contraction. The
mandible is dropped immediately before the swallow, and strong contractions of circumoral muscle take
place. This type of tongue thrust is difficult to correct; because inadequacy of neuromuscular co-ordination
is often an attendant condition. Some times even orthodontically corrected case will be unstable.

       Muscular imbalance is frequently observed in individuals characterized by malrelationship of the
jaws and an unpleasant facial appearance. The soft tissue overlying the facial skeleton is stretched and
strained, there by displacing the teeth from their proper position in the dental arch. So therefore normal
performance of functions as respiration i.e. deglutition, tongue and lip action are essential in maintaining
equilibrium between the positions of the teeth and surrounding musculature.




Occlusal Contact :

       The movement of the mandible from its rest position to the position of maximum contact is under
voluntary control. This is modified by a discharge of impulses arising from the proprioceptive nerve
endings situated in the TMJ, tendons, muscles and periodontal membranes of the teeth. The mandible is
guided by the effect of these impulses to a position of full occlusal contact which may not necessarily be the
position of centric occlusion, which is defined as that relationship where the teeth are in contact maximally
and the condyle on each side is resting in the depth of the glenoid fossa. The mechanism is protective in that




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it guides the mandible away from a position in which there may be premature contact of an individual tooth
before maximal dental contact is established. The path taken by the mandibular teeth from the rest position,
to that of maximum occlusal contact is known as “Path of contact”.

       It is against this background that the tooth erupts into its first contact with an apponent. If the
contact is made before maximal contact is reached then it may either cause the mandible to deviate to a new
position on closure or it may be guided by the inclined plane of cuspal contact to a new position. This is
likely to occur in the case of molars because of their cuspal morphology. The large mesiopalatal cusp of the
upper molar will probably lie somewhere with in the central part of the confluence of fissures on the lower
molar. Subsequently the inclined plane of this relationship will guide those teeth into a normal relationship,
provided contact is made frequently. If however the discrepancy of relationship is very great then the
mesiopalatal cusp of the upper molar would not be opposite the central part of the lower molar, but would be
engaged out side it and became guided to a more abnormal relationship. Similarly cuspal guidance may be
responsible for the elimination of small malrelationship of the teeth.




Proximal Contact :

       It is a well known clinical phenomenon that contact between neighbouring teeth in a complete adult
dentition is maintained by a tendency for the posterior teeth to move forward. Certainly at the time of
eruption of these teeth, there seems to be a tendency for slight crowding of the teeth to be emphasized.

       Bash has shown that both upper and lower alveolar borders grow outwards and forward towards each
other, but he has not indicated why this particular direction should be selected. He also maintained that
curvature of the roots of the teeth is evidence of this forward movement, the developing apices are left
behind as tooth moves forward with the alevolus.

       The distal curvature of the root may also be the cause of the migration of the teeth. An examination
of the roots of a complete dentition shows that degree of curvature of the roots increases towards the end of
series. Thus the mesial inclination of the axis of the teeth increases towards the back of the mouth. When
these teeth erupt they erupt along the axis of each tooth, the posterior teeth erupting not only occlusally but
also forwards. This creates pressure which is transmitted mesially from tooth to tooth. The upper incisors
are limited from moving labially by lip pressure and mesially by the teeth of the opposite side. The lower
incisors are also limited from moving labially by the overbite of the upper incisors. The maintenance of
approximal contact is important in a complete dentition. If however tooth is lost, this forward movement




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may prove deterimental, because the posterior teeth may tilt into the space to such an extent that the
occlusion with their opponent is disturbed.

DEVELOPMENT OF DENTITION :

    To understand the development of occlusion one should know the various stages of development of
dentition birth to adult life. These can be divided into four phases.
1) The change occurring from birth to the complete eruption of the deciduous teeth (i.e. birth to 2 ½ years).
2) The changes occurring form the completion of the deciduous dentition to the eruption of the first
    permanent molars (i.e. 2 ½ to 6 years).
3) The mixed dentition period, form the eruption of the first permanent molars to the final shedding of
    deciduous teeth ( 6 to 12 years).
4) The period from the eruption of the second permanent molars at about 12 years onwards.

MOUTH OF NEONATES :
The Gum Pads :
        The alveolar arches at the time of birth are termed as gumpads. They are firm and pink in colour.
They develop in two district parts – a labio-buccal and a lingual portion. The labio-buccal part is
differentiated first and grows more rapidly. It is divided by transverse grooves into ten segments, each
corresponding to a deciduous tooth sac, and is papillomatous at first. The grooves between the canines and
first deciduous molar segment are called the lateral sulci and are the only ones to extend on to the buccal
side.

        The lingual portion, which is differentiated later, remains almost entirely smooth. These portions are
separated by the dental groove, which is the site of origin of the dental lamina. The lingual portion is limited
lingually by the gingival groove. In the upper jaw the gingival groove separates the gum pad from the
palate, and is related to the inner alveolar palate.

        The dental groove of the upper gumpad passes form the incisive papilla laterally and lingually, to
joint the gingival groove in the canine regions, where it continues distally and buccally across that segment
of the gum pad which corresponds to the first deciduous molar crypt. The gingival groove defines the limits
of the palate, both anteriorly and laterally by three straight borders forming part of an oblong.


        The lower gum pad is U-shaped, and the alveolar pad is limited on the lingual aspect by a continuous
groove. Anteriorly the gumpad is slightly everted labially. The gumpad is divided into ten segments by
transverse grooves. But the division is not as clear as that in the upper.

        The groove distal to canine is continued on to the buccal surface and is called labial sulcus.




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       The upper gumpad is wider than the lower and when the two are approximated, there is a complete
overjet all round of the upper over the lower gum pad, with a considerable overjet anteriorly. The lateral
sulcus of the lower gum pad is usually posterior to that of the upper. In the anterior region there is nearly no
contact between the gum pads when they are approximated and the contact is seen only in first molar region.

       A vertical space generally exists between the upper and lower incisor segments of the gumpads even
when they are pressed into occlusion. This space is occupied by the tongue and is not necessarily a
precursor to an anterior openbite.

       At rest the gumpads are separated by the tongue, which protrudes over the lower gumpad to lie
immediately behind the lower lip, and may even protrude a little between the lips.

       The anteroposterior movements of the gumpads are usually small and there is no lateral movement.
       At birth, the gum pads are not sufficiently wide to accommodate the developing incisors, which are
crowded and rotated at their crypts.

       During the first year of life the pads grow rapidly, and the growth is most marked in the lateral
direction. This increase of width permits the incisors to erupt in good alignment and to be spaced.




The primary Dentition :

       The eruption of primary teeth begins at about 6 months after birth and all the primary teeth are
usually erupted by 2 ½ years of age i.e. when the second premolars came to occlusion.

       At this stage the roots of the second molars are usually not yet complete.                  Therefore the
establishment of the primary dentition is usually considered to take place at about 3 years of age when the
root completion takes place and lasts until 6 years of age when first permanent tooth begins to erupt.

       From 3-4 years of age the dental arch is relatively stable and changes very slightly. From 5-6 years
of age the size of dental arch begins to change due to eruptive force of first permanent molars.

       Changes should be carefully observed at this stage which is indicative of what may be the prototype
of future dentition.

Characteristics of Primary Dentitions :




                                                      15
1) Spaced anterior
2) Primate spaces
3) Shallow overbite and overjet
4) Straight terminal plain
5) Almost vertical inclination of the anterior teeth
6) Ovoid arch form

Spacing of Primary Teeth :

       Spacing in the deciduous teeth was first described by Dellabarre in the year 1819. Spacing in
deciduous teeth has been called as physiological spaces by Korkhous and Neumann, developmental spacing
by Fraber in 1962.


       Spaces around canines are called as Simian gap by Baume in 1940, primate spacing by Boyko 1968
and anthropoid spaces by Foster and Hamillon in 1969.

       Spacing of the deciduous teeth is variable. In most normal cases spacing occurs between all the teeth
as they erupt. Occasionally spaces develop between the deciduous incisors subsequently to their eruption.
Failure of incisor spacing to appear before five years of age occurs in about 20% of the cases and indicates
crowding in the permanent dentition.

       Any spaces which exist between the deciduous molars usually close by the time of the eruption of
the first permanent molars. But the spaces between deciduous incisors persist until these teeth are replaced.

       When the deciduous incisors erupt the overbite of the upper incisors are equivalent to the height of
the crowns of a lower incisor, that is the lower incisors are covered by the upper when the teeth are in
occlusion. This deep overbite is reduced progressively by the eruption of deciduous molars and by the more
rapid attrition of the incisors. Later on forward movement of the mandibular arch is associated with a
reduction of overbite and a flattening of all the molar cusps and another factor is the greater increase of
width of the upper arch compared to that of lower arch.

Significance of Primate Spaces :

1) Spaced primary arches generally produce more favorable alignment of the permanent incisors whereas
   40% of the arches with out anterior spacing produce crowded anterior segment.
2) The presence of mandibular primate space is conductive of proper molar occlusion by means of an early
   shift of mandibular molars (primary) into this primate space on eruption.




                                                       16
3) The mean increase in intercanine widths brought about by lateral and frontal alveolar growth is lesser in
       spaced arches than non-spaced arches, so that there is no excessive space for the erupting permanent
       incisors.
4) The width of the permanent incisors is greater than their deciduous counterparts, primate spaces help to
       accommodate these wider teeth.

           It is possible to predict the degree of crowding in the permanent teeth based on the amount of
spacing present in lower deciduous arch.
           According to Lighton B.C.
           Dec.                   Per
Crowding 10 in 10                 10 in 10 dev. Crowding
No spaces                         7 in 10 dev. Crowding
Spaces below 3 mm                 5 in 10 dev. Crowding
3 to 6 mm of spaces               2 in 10 dev. Crowding
Over 6 mm of spaces No crowding

Eruption of Deciduous Teeth:

           Calcification of the deciduous teeth begins about the fourth month of fetal life, near the end of the
month all of the deciduous teeth have begun to develop. Normally no teeth are visible in the mouth at birth.

           At around 6 months of age the deciduous mandibular central incisors appear in the mouth. The age
at which the deciduous teeth erupt are as     follows ;
Maxillary                                       Mandibular
C.I.               : 7 ½ months                 C. I.     :6½
L.I.               : 8 months                   L.I.      :7
Canine : 16-20 months                    Canine : 16-20 (16 months)
                    (18 mos +/- 2)
Ist Molar          : 12-16 (14 +/- 2)           Ist Molar : 12-16 (12 months)
II Molar           : 20-30 (24)                 II molar : 20 – 30 (20 months)
           From Logan W. and Kronfeld, R.J.M. Dent. Ass. 20 : 379, 1933. Slightly modified by McCAll and
Schour.
           The usual order of appearance of the deciduous teeth in the mouth is as follows :
                          ABDCE
                          ABDCE
           The mandibular teeth usually precede the maxillary teeth in the order of appearance




                                                          17
Occlusal Relations :
Development of Molar Relation :
        At birth the mandibular arch is posterior to the maxillary arch. With the eruption of the primary first
molars the (first three dimensional) occlusal relationship is established. The primary posterior teeth occlude
so that the mandibular cusp articulates just a head of its corresponding maxillary cusp. The mesiolingual
cusp of the maxillary molar occludes in the central fossae of the mandibular molars and the incisors are
vertical with minimal overbite and overjet. The mandibular second deciduous molar is wider mesiodistally
than the maxillary giving rise typically to a “flush terminal plane‟ at the end of the primary dentition.

Terminal Plane :

       In most of the deciduous dentition the distal surface of the maxillary and mandibular second primary
molars are in the same vertical plane called the flush terminal plane. This plane is seen most commonly at
the completion of primary dentition. Later on when the maxillary and mandibular first permanent molars
erupt, they are guided into the dental arch by distal surfaces of the second primary molars and acquire a cusp
to cusp relationship, that is the mesial contours of the maxillary and mandibular first permanent molars are
in same vertical plane. This relationship of the molars is normal at this age (early mixed dentition).

       The initial cusp to cusp relationship in the early mixed dentition is maintained until the exfoliation of
the second primary molars, which are followed by the eruption of smaller second premolar. Thus the
change from the initial cusp to cusp molar relationship to the normal adult occlusal relationship is
accounted.

       According to Graber, at the age of 9 to 10 years the combined mesio distal width of the deciduous
cuspid, first molar and second molar is approximately 1.7 mm greater than average combined width of
permanent canine, first premolar and second premolar teeth.

       In the maxilla the combined width difference averages only 0.9 mm. This difference between the
mesio-distal width of CDE and 3,4,5 in either arches is referred to as Leeway space of Nance.

       Greater Leeway space in mandibular arch allows more mesial movements of the mandibular first
permanent molars.

Variations in the Terminal Plane Relationship or Occlusal Changes in the Mixed Dentition :

       As stated earlier the usual flush terminal plane of the primary dentition typically provides an end-to-
end relationship of the first permanent molars. The first permanent molar normally then achieves a Class I
relationship by:




                                                      18
1) Early mesial shift in spaced deciduous dentition by moving the primary molars into the primate space.
2) A late mesial shift after the loss of the second primary molars.
3) Greater forward growth of the mandible than the maxilla.
4) A combination of both, late mesial shift and greater forward growth of mandible.


The anterioposterior relationship of the first permanent molars‟ influenced by the plane formed by the distal
surfaces of the maxillary and mandibular second primary molars.

Distal Step :

       The distal surface of the lower second deciduous molar is more distal to that of the upper. Under this
condition the maxillary and mandibular permanent first molars on eruption will assume similar relationship
to that of the primary morals, and finally land up in Class II pattern. This molar configuration in the mixed
dentition stage is not normal and will not be self correcting into a normal adult molar relationship.

Mesial Step :

       When the mesiobuccal cusp of the maxillary primary molar occludes with the buccal grooves of the
mandibular second primary molars, the distal projection of the maxillary molar produces a mesial step.
Under these conditions the first permanent molars, upon eruption will approximate the Class I adult molar
relationship during the mixed dentition stage. The mesial step or Class I occlusal configuration during the
mixed dentition stage may be maintained in the adult hood as a Class I molar relationship or may develop
into a Class III malocclusion. This development into a Class III malocclusion well depend upon the
magnitude of the mesial step, the magnitude of the Leeway space and the differential growth of maxilla and
mandible. Children with mesial step relationship characterized by prognathic lower jaws or retrognathic
upper jaw will develop Class III malocclusion.

       In some cases a flush terminal plain relationship in the primary dentition may change into
relationships other than the expected end to end and later Class I molar intercuspation. Such a situation may
arise in case with flush terminal plain in the primary dentition, a mild Class II facial skeleton and
insufficient arch perimeter space to permit a late mesial shift of the first permanent molars, the occlusion is
likely to become Class II by the end of the mixed dentition period or an end-to-end molar relationship by the
time of the eruption of the premolars depending on the severity of Class II skeletal growth pattern.

The permanent dentition :




                                                       19
        Except for the cusps of the first permanent molars the permanent dentition is formed after birth. The
first appearance of the permanent dentition is at the age of about 6 years when the first molar erupt.
Eruption is the developmental process that moves a tooth from its crypt position through the alveolar process
into the oral cavity and to occlusion with its antagonist.

        During eruption of succedaneous teeth, many activities occur simultaneously.
1) Primary teeth resorb
2) Root of permanent tooth lengthens
3) The alveolar process increases in height
4) The permanent tooth moves through the bone.

        Teeth do not move occlusally until their crown is completed. They passes throughout the crest of the
alveolar process at varying stage of root development. It takes 2-5 years for posterior teeth to reach alveolar
crest following completion of their crowns and 12-20 months to reach occlusion after reaching alveolar
margin.

        The roots usually are completed a few months after occlusion is attained. The age of eruption of
permanent teeth are as follows.
Maxillary                                   Mandibular
C.I.    : 7-8 years                         C.I.      : 6-7 years
L.I.    : 8-9 years                         L.I.      : 7-8 years
Canine : 11-12 years                        Canine : 9-10 years
Ist P.M. : 10-11 years         1st P.M. : 10-12 years
IInd P.M. : 10-12 years                     IInd P.M. : 11-12 years
Ist M       : 6-7 years        Ist M.              : 6-7 years
IInd M.     : 12-13 years                   IInd M.       :11-13 years
IIIrd M.    : 17-21 years                   IIIrd M       : 17-21 years


Dentition of Various Ages :
6 Years :
                                        6                                         6
                                         E D C B A                    A B C D E
                                        6E D C B A                    A B C D E   6



7 Years
                                        6                                         6
                                         E D C B 1                    1 B C D E
                                        6E D C B 1                    1 B C D E   6




                                                                 20
8 Years
                             6 E D C2 1 12 C D E 6
                            6 E D C2 1 1 2 C D E 6


9 Years :

             6 edc21 12cde6
             6 E D3 2 1 1 23 D E 6


10 Years :
                             6 E4 C 2 1 1 2 C4 E 6
                                 6 E4 3 2 1 1 2 34 E 6




11 Years :
                             7
                                 6 5 43 2 1 1 23 4 5 67
                             7   65 4 3 2 1 1 2 3 45 6 7




                                          21
12 Years :
                                          7
                                              654321 1234567
                                          7   654321 1234567


The changes form Deciduous Dentition to Permanent Dentition :
        The changes from deciduous dentition to permanent dentition can be described under 2 headings.
1) The replacement of primary incisors
2) The replacement of primary canines and primary molars.

1) The Replacement of Primary Incisors (First Transition Period) :

        As the permanent incisors erupt, there is exchange of primary incisors. There is a tendency for the
mandibular permanent incisors to erupt somewhat lingually and to a slightly irregular position, even in
children who have normal dental arches and normal spacing with in the aches because permanent incisor
tooth buds lie lingual and apical to the primary incisors. Maxillary central incisor seem to erupt from labial.

        The permanent incisor teeth are considerably larger than the primary teeth.

        The maxillary arch has first enough space for the accommodation of permanent lateral incisors when
they erupt, after central incisors.
In the mandibular arch however, when the lateral erupt, there is an average 1.6 mm less spaces available for
the 4 mandibular incisors than would required for perfectly aligning them. This difference between the
amount of space needed for the incisors and the amount of space available for them is called the “Incisor
liability”. So during the age of 8 to 9 years there is a period of slight crowding of mandibular permanent
incisors which is normal at the stage of development which is transitory.


The incisal liability can be compensated by;
1) Interdental spacing in the primary incisor region
        The physiologic spaces that exist in the primary dentition are important factors in allowing relatively
large permanent incisors. This is about 4 mm in maxilla and 3 mm in mandible.

2) Increase in intercanine width :

        During the exchange of incisors the obvious change in the dental arch can be observed. That is
intercanine width increases markedly at the time of eruption of mandibular and maxillary lateral incisors.




                                                      22
        According to Moorrees and Chandha (1959) by the time the lateral incisors have completed their
eruption the intercanine width increases by about 3 mm in each arch. Furthermore, in the maxilla the
intercanine width increases by another 1.5 mm when the canines erupt. More width is gained in the boys
then the girls. Therefore girls have greater liability to have incisor crowding, especially mandibular incisor
crowding.

Clinical Implications :

        Clasps on the cuspids attached to the space maintainers must be cut off at this time or should be
designed so as to allow natural increase of intercanine width to access unimpeded.

3) Increase of Anterior Length in the Dental Arch :

        Increase in the length of the dental arch in A-P dimension will also provide space for the larger
permanent incisor to be accommodated. It is necessary for permanent incisors to erupt more labially to
obtain necessary added space.

        Actually the permanent incisors move at about 2-3 mm labially from the location of the primary
incisors.

        The permanent incisors in the mandible are located occasionally on the lingual side of the preceding
primary incisors immediately after their eruption. As the permanent teeth erupt they also tend to move
labially.

        In order not to interfere with the natural pathway of labially oriented eruption of the permanent
incisors, one needs to pay careful attention to any abnormal root resorption of the primary incisors and/or the
use of incorrectly designed space maintainers at regular recall appointments which should occur as early as
possible.

4) Change of Tooth Axis of Incisors :

        One of the characteristic difference between permanent and primary teeth is the tooth axis. Primary
teeth are generally very upright but permanent teeth tend to incline to the labial or buccal surface.

        The interincisal angle between the maxillary and mandibular C.I. is about 115o in the primary
dentition and the angle in permanent dentition is about 130o-131o which indicates that both upper and lower
central incisors are labial.




                                                       23
        This makes the permanent dental arch circumference wider. This is another advantageous condition
for the arrangement of the large permanent incisors. This contributes to 1-2 mm of additional space.

The Ugly Duckling Stage :

        This term was first introduced by Broadbent to describe the midline diastema in the maxillary arch
along with overlapping of the permanent lateral incisors about the age of 8-9 years.
        The mandibular permanent central incisors are almost always in proximal contact from the time they
erupt. However in the maxillary arch, when the centrals erupt, a midline diastema is present which shows
the beginning of their ugly duckling stage. It is found in most of the Bolton research that a space between
these teeth in many cases may persist for a period of 3 to 4 years and may not close until eruption of
permanent cuspids.

        By 7th year the crowns of permanent cuspids have been completed, but they have not yet moved from
their sight of origin. In a relatively short time the incisors cut their way into the mouth with the lateral
incisors crowns flaring out as they came downward and forward to take their positions, in a proximal contact
at the distal of centrals.

        The crowns of the cuspids in the young jaw impinge on the developing roots of the lateral incisors,
driving the roots medially and causing the crown to flare laterally. The roots of the centrals are also forced
towards each other.

Changes During Ugly – Duckling Stage 6-8 Years :

        As the upper lateral incisors develop in a more palatal position than the CI they are overlapped by the
latter. When central incisors erupt the lateral incisors becomes free to move labially but their apices always
remain slightly more palatal than those of the central incisors.

Changes during 8-10 years :

        The crowns of the L.I. have a slight distal inclination and there is a partial closure of midline spaces
as they erupt. The distal inclination of the lateral incisors is due to the developing canines which are high
and closely associated with the roots of the erupting lateral incisors and exert pressure on the apices
resulting in distal tilt of the crowns of the laterals.

Changes During age of 10-12 years :




                                                          24
       About 11years the roots of the L.I. are complete and maxillary canines erupt at about 11 ½ years. As
they erupt the canines move labially and L.I. became more upright, exerting a mesial pressure resulting in
closure of central diastema.




Significance :

1) Ugly duckling stage is a normal developmental stage and should not be mistaken for malocclusion.
2) If the amount of spacing is great, there is less likelihood that the central diastema will close on its own.
   If the diastema is less than 2 mm spontaneous closure occurs and therefore treatment is not indicated. If
   the diastema is greater than 2 mm spontaneous closure is unlikely, therefore treatment is indicated.
3) Thus mid line diastema should be differentiated form other causes like:
   a) Involvement of labial frenum – the L.F. in infancy normally has a low attachment near to the crest of
       the alveolar process in the midline.
   b) In primary dentition the labial frenum can frequently be seen to be attached to the alveolar process
       between the upper C.I. With normal dentoalveolar growth the upper alveolar process grows down
       and the labial frenum attachment becomes progressively higher in the jaw. Occasionally the low
       attachment persists and the frenum apparently causes midline spaces between upper C.I.

It is important to remember that there are other causes for midline diastema; like ;
a) Hypodontia, especially missing upper lateral incisor
b) Proclination of incisors
c) General spacing of dentition
d) The presence of an unerupted conical supernumerary teeth.

Second Molar Eruption :

       After the exchange of the lateral teeth has been completed and the dental arch upto first molar is
established the second permanent molars will begin to erupt. In most cases, just prior to eruption of the
second molar, the dental arch length will be reduced by the eruptive forces immediately mesial and with the
second permanent molar.        With the eruption of second molar in the permanent dentition, the arch
circumference may became shorter than that of the primary dental arch by the utilization of the Lee Way
space with the exchange of the second primary molar to second molar. Proximal caries lesions or the early
extraction of second primary morals will cause further loss of dental arch space. This space decreases
substantially during the eruption period of the second molar and will significantly affect occlusal
relationships in the molar region.


                                                      25
       The mandibular second molar should follow all teeth anterior to it into the arch. The eruption of
maxillary second molar ahead of the mandibular second molar is said to be symptomatic of a developing
Class II malocclusion.

DENTITONAL AND OCCLUSAL DEVELOPMENT IN YOUNG ADULT :
1) Third Molar Development :
       The third molar is unique among human teeth, as it apparently displays no sexual differences in
formation nor is its formation related as closely to somatic growth and sexual maturation as are the other
teeth.
Third molar agenesis occurs in a good number of cases and is seen in about 16% of cases in West American
whites. The role of the third molar in the crowding of mandibular incisors during the late teenage period is a
matter of controversy. A number of simultaneous phenomena‟s such as the arch perimeter shortening, the
increase in incisor crowding the development of third molars and the more forward growth of mandible than
the maxilla during this period all confuses the issue.

       Incisor crowding has been found to correlate better with mandibular increments than with the
eruption of third molars.

       The first molars have been found to be farther forward and incisors more procumbent in individuals
with third molars than those with third molar agenesis. As the difference in first molar position and incisal
procumbency appear before significant development of third molars, the third molars cold not play a
primary role in the position of first molar and incisors.

       But recent studies suggest that the correlation between late incisor crowding and third molars cannot
be accounted.

2) Dimensional Changes :

       The dental arch perimeter decreases a surprising amount during the late adolescent and young adult
periods.

3) Occlusal changes :

       Both overbite and overjet decrease throughout the second decade of life probably due to the
relatively greater forward growth of the mandible.

4) Resorption of Permanent Teeth :




                                                         26
       By the end of the second decade, most persons display idiopathic resorption of one or more teeth.
Nearly 90% of all teeth show same evidence of resorption by the time a person is 19 years of age. In most
of the instances the resorption is mild and confined to apical blunting but 10% show 2 mm to 4 mm of root
resorption.

5) Arrangement of the Teeth in the jaws:
       Dempter et al have reported an exhaustive study of the relationship of the roots to the craniofacial
skeleton. The bicuspid roots are nearly perpendicular to the plane of occlusion. The lower incisor, cuspid
and molar roots are directed obliquely backward. The roots of maxillary teeth, anterior to the second
bicuspid are directed posteriorly and inward whereas roots of maxillary molars are more vertical than the
opposing lower molars.

Mandibular Movements for mastication :

   The forces of mastication exerted upon the teeth are various, but for the purpose of description they can
be divided into those applied in the following directions.
1) Vertical
2) Antero-posterior
3) Transversal

1) Vertical Movements of the Mandible :

       The mandible moves from its position of rest vertically into occlusion and then applies direct vertical
pressure to the upper teeth. In the bucco-lingual direction although the molar axes are not vertical the upper
and lower molars are directly apposed. The axes of incisors are not directly apposed labiolingually; the
upper incisors, being inclined labially, the forward resultant of occlusal pressure is observed partly by the
lips and partly by the palatal curvature of the upper incisor roots. The roots of the lower incisors resist
lingual pressure, because they are flattened mesio distally and are mutually supported as are “bricks of a
Roman arch built without mortar”.

2) Antero Posterior Movements of Mandible :

       Movements of mandible in this direction are not used frequently in mastication because the overbite
of the incisors causes the molars and premolars to be disengaged to any but the smallest excretions are
made. But for some actions, the mandible is protruded sufficiently to bring the incisal edges of the incisors
into occlusion. The movement is at first an incising action and later a shearing action.

3) Transverse Movements :



                                                      27
       In the molar region, labial excretions of mandible at first cause the large mesiopalatal cusps of the
upper molars to glide up the buccal cusp of the lower molars, and if movement is continued, the cusp to cusp
contact of the buccal cusps of the opposite side disengages the molars. Shearing action may be performed
by the premolars and canines and by the action of buccal cusps of the lower premolars engaging those of the
upper premolars.




Lateral and protrusive Functional Occlusion :

       Functional occlusion occurs in the segment of the arch towards which the mandible moves. Lateral
functional occlusion involves the canines and posterior teeth on the side towards which the mandible moves.
Two fundamental patterns of contact may occur or be established in lateral functional (working) occlusion.
1) Canine guided lateral functional occlusion
2) Group lateral functional occlusion

       Canine guided lateral functional occlusion usually occurs in the young and unworn dentition. The
opposing maxillary and mandibular canines provide contact during labial functional excursive movement
with sufficient incline steepness to cause an immediate, complete disclusion of all posterior teeth in
functioning (working) segment. The adjacent lateral and rarely, central incisor may participate in the
guiding contact either actively or passively.

       Contact between teeth other than those in the functioning segment is undesirable. Thus in a canine
guided lateral functional occlusion there is no excentric contact or posterior teeth on either the functioning or
non-functioning side; only the canines actually contact. In an ideal occlusion the functional contact path of
the mandibular canine cusp tracks from the centric contact in the area of the mesial marginal ridge of the
maxillary canine obliquely laterally and posteriorly lowered the cusp tip.

Group Lateral Functional Occlusion :

       Group lateral functional occlusion provides dominant guidance by the canines but involves sharing
of the contact by other posterior teeth in the functional (working) segment. As in canine guided lateral
functional the adjacent lateral and central incisors may share in the contact. There are two potential ranges
of lateral functional contact on the multicuspid posterior teeth.
1) A facial range
2) A lingual range




                                                       28
        The facial range involves the mandibular cusps moving from their areas of centric contact facially
and slightly lingually across the lingual inclines of the maxillary facial cusps. Ideally the lower cusps tips
with pass through the embrasure spaces or grooves, and the actual functional contact will occur on the distal
arms or ridges of these cusps. The facial range contacts may include all the cusps in the segment.

        The lingual range of lateral functional contacts involves the tracking of the maxillary lingual cusp
lips from their areas of centric contact up the facial inclines of the mandibular lingual cusps lingually. These
lingual range contacts usually donot occur and are not desired in the well arranged natural dentition. Thus
lingual range lateral functional contacts in the natural dentition are like non-functional contacts in that they
are undesirable.

        Instead of occlusal contact in the lingual range of lateral function, the lingual cusps of the maxillary
premolars would pass ideally through the occlusal embrasures of the mandibular PMs without contact. The
mesiolingual cusps of the maxillary molars pass through the lingual grooves of the mandibular molars, the
disto lingual cusps of the maxillary molars pass through the lingual occlusal embrasures.

Importance of Overbite and Overjet :
Overbite :
        The vertical labial overlap of the maxillary incisors over the mandibular incisors

        with the teeth in centric occlusion is termed as overbite.

Normal overbite is 2-4 mm and approximately equal to 1/3rd of the height of the crowns of the mandibular
incisors.

Physiological Bite Opening :

        Between 6-7 yrs the first permanent molars erupt in to the mouth. It is at this time that the first of
three assaults on excessive overbite occur. According to Schwartz there are 3 periods of physiological
raising of the bite,
1) With the eruption of first permanent molar at the age of 6.
2) With the eruption of second permanent molar at the age of 12
3) With the eruption of third permanent molar at the age of 18.

        As the upper and lower first molar erupt, the pad of tissue overlying them creates premature contacts,
proprioceptive response conditions the patients against biting on it and acts as natural bite openers and thus
the deciduous teeth anterior to the first permanent molar creep reducing the overbite.




                                                       29
       The magnitude of overbite varies form individual to individual. The different types of over bite
generally seen are ;
a) The lower incisors contact the lingual fossa of the upper incisors (normal overbite relationship)
b) Maxillary and mandibular incisors may meet at this incisal edges (edge-to-edge bite)
c) The lower incisors may contact the cingulum of the upper incisors or may even contact the palatal
   gingiva. (deep bite or closed bite)
d) The upper and lower incisors may not contact, with a gap between this incisal edge (open bite)
e) The lower incisors may be labial to the maxillary incisors (cross bite), reverse overbite).

   Even in a normal overbite relationship there may not be a true contact of the incisal edges of the lower
   incisors with the lingual surface of the upper incisors. Ideally such a contact is necessary to prevent
   supraeruption of lower incisors.
   Flamming H.B. 1961, Angle Ortho. Vol.31, 1961, 53-62.

Observation of Overbite :

a) Visual Examination : May be misleading, when the teeth do not have an actual contact. However gross
   discrepancies may be noticed.
b) Lateral cephalometric radiograph
c) Examination of dental casts from the position aspect.

       Vertical overlapping is not confined to the anterior teeth, it is also in the posterior teeth where the
guiding cusps overlap supporting cusps of the opposing teeth.

       Thus the variations in normal overbite relations in the posterior teeth may result in posterior open
bite, posterior crossbite and these may be unilateral or bilateral.

Factors that Contribute to Overbite :

       All factors that contribute to occlusion, influences overbite relation like- evolutionary, hereditary,
racial, congenital and systemic factors.
       The important factors that influence over bite are ;
a) Equilibrium between forces from cheek and the forces from tongue.
b) Balance of forces from lips and the tongue.
c) Occlusal equilibrium
d) Proper eruption sequence of canine and premolars
e) Proper jaw relations, which in turn is dependent on their growth.




                                                        30
Overjet :

        Overjet is the horizontal overlap between the most labial surface of the mandibular incisor and the
labial surface of the maxillary central incisor edge when the jaws are in centric occlusion. The normal value
is 2-3 mm (Profitt).

        The degree of overjet is related to the axial orientation of the anterior maxillary and mandibular
teeth. Overjet, however is not confined to the anterior segment. It is also seen in the posterior segment
where the guiding cusps overlap the supporting cusps.

        A lot of changes occur in the overjet during the primary and transitional dentition period. In an edge
to edge bite the overjet is zero.

        Excessive increase in overjet is seen in cases where there is proclination of maxillary incisors or
retroclination of lower incisors or both. Decrease in overjet can be seen in cases where there is retroclination
of maxillary incisors and/or proclination of lower incisors.

Curvatures of the Occlusal Plane :

Definition : Occlusal plane is an imaginary surface that is related anatomically to the cranium and that
theoretically touches the incisal edges of the anteriors and the tips of the occluding surfaces of the posterior
teeth, including the canines. Occlusal plane is not straight but curved in all directions.

        Different types of curvatures of the occlusion plane are ;
a) Curve of spee
b) Curve of Wilson
c) Curve of Monson

Curve of Spee :

        It was described by Graf Von Spee in 1980. It refers to the antroposterior curvature of the occlusal
surfaces beginning at the tip of the lower cuspid and following the cusp lips of the bicuspids and molars
continuing as an arc through the condyle. IF this curve is extended, it would form a circle of about 4”
diameter.

        The curve results from variations in the axial alignment of the lower teeth. The long axis of each
lower tooth is aligned nearly parallel to its individual arc of closure around the condylar axis. This requires
a gradual progressive increased mesial tilting of teeth towards molars which creates the curve of spee.



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These are 2 curves of Spee :
1) Maxillary curve of spee : Curve formed by the tip of the canine and the buccal curve of the premolars
   and molars of the maxilla in the downward convex fashion.
2) Mandibular cure of spee. Curve formed by the tip of the canine and the buccal cusps of the premolars
   and molars of the mandible in an upward concave fashion.

Significance of Curve of Spee :

1) Curve of spee helps the achievement of occlusal balance during mastication by encouraging
   simultaneous contact in more than one of the dental arch.
2) If the curve of spee is too great in the mandible more the posterior aspects of the occlusal plane is in the
   line with the condylar path. This results in a less discluding influence on the posterior teeth, causing non
   functional contact of teeth (occlusal interference‟s). Therefore too much curve is not advisable while
   setting artificial teeth for dentures; during orthodontic therapy, a flat curve of spee is preferred.

Curve of Wilson :

       The occlusal curves of Wilson are aligned in the transverse plane. Curves are formed by the cusps of
molars of the right and left sides of the upper and lower jaw. There are 2 curves.
1) Maxillary curve of Wilson – formed by the cusps of molars, right and left of the mandible, in a
   transverse plane, in unworn dentition is in downward convex plane.
2) Mandibular curve of Wilson – formed by cusps of molars, right and left of mandible, in a transverse
   plane, in unworn dentition, is in upward concave plane.


Curve of Wilson are such that mandibular molars are oriented lingually while the maxillary molars are
oriented buccally. Curve of Wilson helps in two ways:
a) Teeth are aligned parallel to the direction of medial pterygoid for optimum resistance to masticatory
   forces.
b) The elevated buccal cusps prevent food from going past the occlusal table.

Curve of Monson :

       The curve of Monson is obtained by extending the curve of spee and curve of Wilson to all cusps and
incisal edges.

Equilibrium Effects on the Dentition :




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       Equilibrium effects on the dentition can be understood best by observing the effects of various types
of pressures.
The duration of force is more important than its magnitude because of the biologic response. This important
point is made clear by examining the response to the forces applied during chewing. For example when
heavy masticatory forces are applied to teeth, the fluid filled periodontal ligament acts as a shock absorber,
stabilizing the tooth for a instant while alveolar bone bends and the tooth is displaced for a short distance
along with the bone.

       If the heavy force is maintained for more than a few seconds increasingly severe pain is felt, and so
the biting force is released quickly. This type of heavy intermittent force has no impact on the long term
position of a tooth.

       Another example for effect of forces on dentition is, pressure from cheeks and tongue. The pressures
are much lighter than those form masticatory function, but are also of much greater duration. Experiments
suggest that even very light forces are successful in moving teeth, if the force is of long duration. The
duration threshold seems to be approximately 6 hours in humans; for example if an injury of the soft tissue
of the lip results in scarring and contract, the incisors in this vicinity will be moved lingually as the lip
lightens against them. On the other hand if its restraining pressure by the lip or cheek is removed, the teeth
move outward in response to unopposed pressure from the tongue.

       These observations make it plain that in contrast to forces from mastication, light sustained pressure
from lips, cheeks and tongue at rest are important determinants of tooth position.

       The periodontal ligament itself can contribute to the forces that make up the dental equilibrium. It
seems clear now that the eruptive force is generated with in the periodontal ligament though it seems likely
that some metabolic activity can and does produce forces that serve as part of “active stabilization for teeth”;
directly contributing to the equilibrium.

SAFETY VALUE MECHANISM :

       By the end of the 9th year in girls and 10th year in boys, the mandibular intercanine arch width
dimensions is essentially completed. In the maxilla there is little anterior maxillary arch width increase in
girls after 12 years of age. The difference in increase of maxillary dimension is due to the fact that the
pubertal growth spurt in girl is from 10 to 12 years of age. While in boys it is from 12 to 18 years of age.
This clinical implications are quite obvious. The final horizontal growth increments in the mandible,
particularly in the males, causes a forward movement of the mandibular base with its teeth. The basal
change eliminates any flush terminal plane tendencies that have persisted beyond the mixed dentition. But



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the bodily mandibular thrust forward is unmatched by comparable maxillary horizontal growth changes.
Hence the maxillary intercanine dimension serves as a safely valve for this basal discrepancy.


CONCLUSION:

Problems related to development and eruption leads to a good percentage of malocclusions and hence it is
mandatory for the orthodontist to know the normal developmental process. This seminar was an attempt to
give a broad outline of the development of occlusion.




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