Leader in Dental Education

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com


From birth to adulthood the dental occlusion undergoes
significant changes.

The

term

OCCLUSION

refers to the manner in which
the maxillary and mandibular
teeth articulate


Easier said than done, the whole process is infact a complex
series involving morphology and angulation of teeth, roles of
muscles of mastication, tempromandibular joint, the skeletal
components and functional jaw movements.

In an earlier disscussion, the growth and development of dental
arches and occlusion has already been emphasized, this
presentation aims at giving an insight at the various factors that
play an active role in this formation.

In a nutshell, the whole process can be
summed as follows….
Development phases of dentition from birth to adult life……
i.

Birth to complete eruption of deciduous teeth Birth – 2 ½

ii. Completion of deciduous dentition to eruption of first
permanent molar

2 ½ - 6 years

iii. Mixed dentition period

6 – 12 years

iv. Permanent dentition period

> 12years


BONE
RELATION

TOOTH
RELATION

FUNCTIONAL
ERUPTION

DEVELOPMENT

INTRAORAL
FORCES

OCCLUSAL
FORCES

BONE RELATION….
The position, size and relationship by which the maxilla and
mandible relate to one another and to other bones is governed by a
variety of factors :
i.

Heredity

ii. Congenital factors which may
not be hereditary
iii. Hormonal imbalance
iv. Traumatic and pathological
conditions

TOOTH RELATION….
•

Some evidence suggests that tooth

development is strongly linked to hereditary
control
•

The permanent incisors develop lingual to

the roots of their predecessors
•

Maxillary canines develop high in the

maxilla close to the floor of the orbit
•

Mandibular canines develop near to the

inferior border of the mandible
•

Premolar crowns develop below and

between the roots of the deciduous molars

• The lower permanent molars develop at
the root of the coronoid process, oriented
with a mesial inclination which in the
course of eruption takes a curved path in
a upward and forward direction
•

The upper permanent molars swing

downwards and forwards as they erupt
through an arc of a circle whose center is
in the region of the apex of the next tooth
mesially

•

Sometimes a defect in the development process maybe observed by the

presence of an ectopic tooth or an impacted tooth.
•

One tooth shown to have a high degree for impaction to occur are the

maxillary canines. One of the reasons postulated for this occurrence is the
remoteness in the eruption pattern of this tooth.
• Mandibular premolars are known for their ectopic eruption pattern, although
this is a very rare occurrence. Etiology for this behavior is thought to be
because of the deep position of the tooth buds between the root apices of
deciduous molar that give it the freedom to ‘wander’.


ERUPTION….
The HOWs and WHYs about tooth eruption are
satisfactorily explained by theories put forth by various authors.
There is however no single theory that gives a clear definition on
how tooth eruption occurs.
BLOOD PRESSURE THEORY – Soft tissues at the developing
end of the tooth are very vascular and increase in vascularity has
been observed in cases of rapid tooth eruption
ROOT GROWTH THEORY – This theory postulates that tooth
eruption is accompanied by growth of root. But variations of this
occur such as, a tooth may fail erupt even though root formation is
over. Again, cases of tooth eruption are known to occur at birth
with very little root present.

EPITHELIAL COIL THEORY – suggested by Warwick
James. It states that epithelial remnants of tooth band and organ
cause a progressive channel for tooth eruption. These epithelial
coils found above the tooth determine the site of eruption.
HAMMOCK LIGAMENT THEORY – suggested by Sicher. He
says that the tooth in the crypt is surrounded by a a band of fibrous
tissue, which is present at the open apex of the root and is attached
to the bone halfway up the wall of the crypt . At this point are
present many fluid droplets which act as cushion to protect the
developing tooth germ.

INTRA – ORAL FORCES….
At the time of tooth eruption the
roots are separated a considerable distance
from the socket wall. This gives chance for
a lot of play in the tooth.
Due to this various forces may act during eruption of the tooth and
cause it to be deviated in its eruption. These forces maybe from,
musculature of the lips, cheeks and tongue and may also be from
mesio-distal forces exerted from adjacent teeth.

It is to be understood that,
muscles are never at a state
of rest but in a state of
TONICITY. This means
that a small proportion of
fibres even at rest are always
in a state of contraction.

Forces generated by the muscles
maybe active or passive. Active
muscle forces exert forces that are of
greater force and frequency and the
pressure exerted is only intermittent.
One example of such a force system is
that of deglutition. The others include
speech, mastication, and expression.
Passive muscle forces are continuous
but light, and mainly due to muscle
tonus. Examples of such muscle
systems include the Buccinator
mechanism,
tension
from
the
Orbicularis Oris.

Lip and cheek pressures lost due
to tropical disease. The teeth
have moved out buccally because
of pressure from the tongue and
no

balancing

force

from

buccinator and orbicularis oris.


OCCLUSAL FORCES….
It is a well know fact that in adult dentition the teeth are held in
close approximation with each other because of the tendency of
the posterior teeth to move forward.
As a result, at the time of
eruption there is a chance
for crowding in the
dentition to occur.
Upper and lower alveolar borders grow in a direction that is
outwards and forwards as pointed out by Brash. He also suggested
that evidence for this was the curvature of the roots.

This forward movement of the dentition may however
prove detrimental in cases where a tooth becomes lost because of
some pathology. During this time, the tooth just distal has a
tendency to tilt into that space. This can result in loss of contact
with the opponent result in malocclusion.
Thus, a pressure is transmitted
mesially from tooth to tooth. The upper
incisors are thus prevented from moving
mesially by lip pressure and the lower
incisors are prevented by overbite of the
upper incisors

Trajectorial theory of forces
(Wolff’s Law)


Bone is the hardest material in the body and yet the most
plastic. It is this resilient feature of the bone that is taken
advantage of in meeting the various demands of functional
forces.
From a historical perspective, the first studies on
function of the bone were done on the femur.
1867 - von Meyer and Culmann occurrence on stress
lines on long bones like femur
1870 - Julius Wolff proposition of the trajectorial
theory of forces (Wolff’s law)
1881 - Wilhelm Roux biological basis for bone
formation

1925 - Benninghoff beam hypothesis

The story unfolded … …
In 1867, anatomist von Meyer in Zurich, while drawing the
pattern of trabecular architecture in long bone of femur, realized
with the help of Culmann who was an engineer visiting Zurich,
that the trabeculae followed certain patterns similar to internal
stress patterns seen in the Fairbairn crane that he was designing.


However it was only 2 years later
that, anatomist and orthopedic surgeon von
Meyer while doing his dissertation on the
normal and abnormal bone formation in
animals brought to light that the patterns
which Meyer and Culmann saw were infact
stress patterns produced as a result of bone
adaptation to functional forces.
He brought out a law based on these trajectories of stress patterns
he observed which came to be known as ‘Wolff’s law’. The law
states as follows:
“ Every change in the form and the function of a bone or of their
function alone is followed by certain definite changes in their
internal architecture, and equally definite secondary alterations in
their external confirmation, in accordance with mathematical laws
”

In truth, Wolff's law laid the mathematical formulae for
the development of such internal stresses and it was around the
same time that Roux in 1881 propounded that bone formation
occurred as a result of stresses of tension or pressure on the
bone. He hypothesized that apposition and resorption by cells
determines change in bone structure and that this change is
brought about by action of local forces. He infact gave an insight
to the biological phenomenon behind this occurrence.

In 1925, Benninghoff, suggested the beam hypothesis
after studying lines of stresses in skull. He postulated that
these lines were trajectories corresponding to functional
stresses.
The trajectories in the maxilla can be broadly classified
as horizontal and vertical trajectories.
Horizontal trajectories :
a.

Hard palate

b.

Orbital ridges

c.

Zygomatic arches

d.

Palatal bones

e. Lesser wings of sphenoid

Vertical trajectories include:
a) Fronto-nasal buttress
b) Malar-zygomatic buttress
c) Ptearygoid buttress


One of the deficiencies of the beam hypothesis is that
only mechanical stresses resulting from bite forces on the
dentition are considered. In many instances, mechanical
stresses resulting from muscle contraction produce large
bending moments and therefore large bone strain adjacent to
several

cranial

and

zygomaticotemporal

facial

and

sutures

reaction

temporomandibular joint.


such

forces

as

the

from

the

CONCLUSION:
This presentation has aimed at reavealing information on the
functional factors that affect the developing dentition.
“An orthodontist can establish a perfect occlusal
relationship, but unless he takes into consideration the
effects of the use of these teeth, unless he makes
allowances for the manifold environmental functional
influences, the delicately responsive bony structures are apt
to change, and the tooth positions will change with them.”
GRABER

Leader in Dental Education

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