The forces of occlusion that maintain the relationship between teeth include growth, muscle tonus, approximal contact, and mastication. Growth and development must be proportional to ensure proper positioning of teeth. Muscle tonus provides constant light force on teeth while mastication applies intermittent heavy forces from vertical, anterior-posterior, and transverse movements. Approximal contact and mesial drift from continuous growth at the alveolar bone and root surfaces helps maintain contacts between teeth. Imbalances in these forces can affect the normal relationship between teeth and arches.

1. Dr. Ahmed Maher Fouda
Assistant professor of
Orthodontics
Faculty of Dentistry- Mansoura
University
by

2. The position of each tooth within the jaw
is maintained by certain factors known as
the forces of occlusion . Normally there is
a balance of these forces which produce a
harmonious relationship of the teeth to
each other.
Any imbalance of these forces must affect
the normal relation of the teeth and / or
arches.

3. The forces of occlusion are:
 1- Growth
 2- Muscle tonus
 3- Approximal contact
 4- Mastication

4. I- Growth;
Normal downward and forward growth of the face, by
harmonizing the size and form of title jaw with the
teeth may profoundly affect the position of the teeth.
Not only should the size of each jaw be adequate but
their relation should be correct.

7.  To achieve this , it is necessary that the growth of all those
bones that contribute to the formation of the face should
be proportionate. Growth of the soft tissues, particularly
muscles, should also be harmonious .

9. II - Muscle tonus:
is the continuous and passive partial contraction of the
muscles
Forces generated by the muscles may be either:
1-passive due to muscle tonus which is continuous but
very light.
2-active associated with muscle activity which is always
greater and intermittent.

10. 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 of tonus a small proportion of fibers are contracted,
the proportion of fibers is constant but they are not always
the same fibers .
Successive groups taking over the function of maintaining
tension. It is known that the number of fibers contracted is
proportional to the amount of stretch put upon the muscle.

12.  Muscles that elevate the mandible:
The masseter : elevation and retraction of
mandible.
The temporalis : elevation and retraction of
mandible.
The medial pterygoid : elevates mandible, closes
jaw, helps lateral pterygoids in moving the jaw
from side to side.
 Muscle that depress the mandible:
The lateral pterygoid : depresses mandible

13.  Temporalis muscle

14.  Medial pterygoid muscle

15.  Masseter muscle

17.  The muscles which have a direct effect on the jaws or
the dento-alveolar structures are those of deglutition,
expression and mastication.
 MUSCLES OF DEGLUTATION:
 muscle of the mouth
 Muscle of soft palate
 Muscles of pharynx
 MUSCLES OF EXPRESSION:
 Buccinator , orbicularis oris and mentalis

18.  Muscles of mastication:
 The masseter
 The temporalis
 The lateral Pterygoid
 The medial pterygoid

22.  The tongue within the vestibule is applied
to the lingual surfaces of the teeth and
the hard and soft palates. The lips and
cheeks are applied to the labial and
buccal surfaces of the teeth.

23. The upper incisors are prevented from moving labially by
the lip pressure and the lower incisors by the overbite
of the upper incisors.
There is a tension from the orbicularis oris muscle on the
upper incisors and it should be remembered that
orbicularis oris muscle derives some of its fibers from
the buccinator muscle which blends with it at the angle
of the mouth.

25. The buccinator passes
backwards and inwards
around the maxillary
tuberosity to be inserted
into the pterygomandibular
raphae. From the posterior
aspect of this ligament arise
some of the fibres of the
superior constrictor muscle
which other fibres are
inserted into prevertebral
fascia and base of the
cranium. There is thus a
continuous chain of muscle
encircling the pharynx and
dental arches .

26.  The buccinator muscle, often limits
buccal movement of upper third molar,
causing the upper dental arch to assume a
horse-shoe shape .

29. Three of the muscles of mastcation (temporalis , masseter and
medial pterygoid ) support the mandible against gravity and the
action of the submandibular muscles . The former exert an upward
pull and the latter downward pull.

31. Masseter
Temporalis
Medial PterygoidThe masseter attaches superiorly to the medial and inferior aspect of the zygomatic arch and inferiorly to the lower border
on the lateral surface of the ramus of the mandible.
The temporalis attaches superiorly to the floor of the temporal fossa to the fascia that overlies it. Inferiorly it attaches to the
coronoid process and to the anterior surface of the ramus.
The medial pterygoid runs from the medial surface of the lateral pterygoid plate and the palatine bone to the lower edge of
the medial surface of the ramus of the mandible.
The lateral pterygoid attaches to the lateral surface of the lateral pterygoid plate and the greater wing of the sphenoid bone.
It runs to the pterygoid fovea at the neck of the mandible and the capsule of the temperomandibular joint.
Lateral
Pterygoid

33. 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 about 2-8
mm in the premolar region . This
distance is called the freeway space
or interocclusal clearance.
This mandibular position is maintained
by the balance of muscle tonus . This
balance affects not only the vertical
but also the antero-posterior relation
of the teeth at rest.

34. active muscle forces :
 This kind of force exert pressure
intermittently. The degree of force is larger
than that of muscle tonus. Also the
frequency varies.
 One of the activities is the deglutition that
concern the orthodontist very much, also
speech , mastication and expression should
not be ignored.

36. III - APPROXiMAL CONTACT :
There is a continuous growth
of bone at the fundus of
the sockets of teeth and at
the crests of the alveolar
processes .
This growth is rapid in
young persons , slows
down in the thirties but in
the healthy dentition it
never ceases entirely.

38.  There is also
constant growth of
bone on the distal
wall of each socket,
whereas the mesial
wall shows
resorption of bone
alternating with
localized reparative
apposition .

39. The roots of the teeth also
enlarge and this enlargement
does not cease when the root
is fully formed. By continuous
apposition of cementum the
roots grow slightly in its
transverse diameters and more
rapidly in length.

40.  Elongation of the
root and growth of
bone at the
fundus, of the
socket are
correlated with a
continued vertical
eruption of the
tooth during its
functional period.

43. The changes on the mesial and distal alveolar walls are
correlated to a movement of all teeth toward the
midline, i.e., mesial drift .

45. The continued occluso-mesial
movement is necessary to
compensate for the loss of tooth
substance at the occlusal and
contact surface .
it is well known clinically that the
contact between neighboring
teeth in a complete adult
dentition is maintained by a
tendency for the posterior teeth
to move forward.

46. The deciduous teeth are more or less vertical in the
alveolus, so that when occlude, there is little
dislodging force ..

49.  When the roots of a complete permanent
dentition are examined, they show that
the degree of root curvature increase
towards the back of the mouth, i.e.,
distal inclination of the teeth axes
increases towards the end of the series

52.  These teeth, therefore, erupt slightly
mesially inclined, and when they come into
occlusion, they exert upon each other an
equal and opposite force.

54. As a result of their axial inclinations, this force will be divided
into vertical and horizontal compenents of force. The vertical
components will cancel each other and leaving just a mesial force
on all the teeth .This mesial force is called the anterior
component of force which transmits to the teeth by approximal
contact.

55. IV : Mastication:
The forces of mastication exerted on the teeth
can be divide into those which applied in the
following direction:
 1-vertically
 2-antero-posteriorly
 3-transversely

56. The mandible moves from its position of rest vertically into
occlusion and then applies direct vertical pressure to the upper
teeth. In the mesiodistai direction,there is a forward resultant,
most marked in the third molar region. In the bucco-lingual
direction, the upper and lower molars are directly opposed
although their axes are not vertical.

57. The axes of the incisors,
however, are not directly
opposed labio-lingually; the
upper incisors being inclined
labially. The forward resultant
of occlusal pressure is
absorbed partly by the lips and
partly by the palatal curvature
of the upper incisor roots.

58.  The roots of the lower incisors resist
lingual pressure because they are
flattened mesio-distally.

59. Movements of the mandible in this direction are not used
frequently in mastication because the overbite of the incisors
causes the molars and premolars to be disengaged if the smallest
excursions are made. However, for some action the mandible is
protruded sufficiently to bring the incisors into occlusion.

60. The movement is at first an incising the lateral margin of the orbit
to the junction of the temporal ridge and the supra-orbital ridge.

61.  The pterygoid plates
which lie behind the
tuberosity of the
maxilla and pass
upwards and backwards
to the inferior surface
of the great wing of the
sphenoid bone.

62. In the molar region, lateral excursions of the mandible at first cause
the large mesio-palatal cusps of the upper molars to glide up the
buccal cusps 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 actions may be performed by the
premolars and canines, the buccal cusps of the lower premolars
engaging those of the upper premolars.

63. Bone, unlike other connective tissue responds to mild
degree of pressure and tension by changes in form.
Those changes are accomplished by resorption of
existing bone and deposition of new bone. In this
respect bone is more plastic than any other connective
tissue.

64. During mastication, certain stresses and strains are exerted not
only upon the alveolar bone but also upon the whole face and
anterior part of the base of the cranium. In order to withstand
these, the bones which are made up of thin plates are thickened
into well defined ridges. Most of these ridges pass upwards from
the alveolar bone towards the base of the cranium. Some, however,
lie transversely or antero-posteriorly to withstand forces in these
directions .

67. 1 - The canine eminence which is a thickened ridge arises
from the region
of the upper canine . It extends upwarde on the facial
surface of the maxilla to the medial end of the infraorbital
margin and continues up the lateral wall of the nose to the
mesial end of the supraorbital ridge .

68. 2 - The key ridge or the root of the Zygomatic process which arises
on the lateral wall of the maxilla over the roots of the first
permanent molars and becomes the zygomatic process . It
continues upwards to approximal contact .The anterior coinpenent
of force on one side of the arch cancels out that of the other side
in the midline provided that there is an intact dental arch. If the
interproximal contacts are destroyed by loss of teeth, those behind
the gap tend to drift forward, whilst those in front will lack
forward development

70. The supraorbital ridges and infraorbital margins .

71.  The arched vault of the palate.

72.  The zygomatic arch which lies
horizontally and it is the continuation of
the zygomatic process . It is continued
posteriorly with the temporal ridge.

73. The mylohyoid ridge and its continuation upwards and backwards to
the condyle .

75.  The external oblique ridge winch is a
thickening of bone passing backwards and
upwards on the external surface to the
root of the coronoid process and up its
anterior border.
external oblique ridge

76.  The inferior border of the mandible is
thickened and the gonial angle is slightly
everted and thickened where the
masseter muscle is inserted .