Experimental evaluation of initial tooth /certified fixed orthodontic courses by Indian dental academy

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

Center of mass
Each body has a point on its mass
, which behaves as if the whole mass is
concentrated at that single point. It is called
the center of mass in a gravity free
environment.
Center of gravity
The same is called the centre of
gravity in an environment when gravity is
present.

Center of mass of a free body is the point through which an applied
force must pass to move it linearly without any rotation. This center of
mass is the free objects “Balance Point”
The center of resistance is the equivalent balance point of a
restrained body.
Center of resistance of a tooth varies depending up on the
- Root length & morphology
- Number of roots
- Level of alveolar bone support

Center of resistance depending upon the level of alveolar bone.

Review
 Mathematical or physical models and finite element
models
 Cadavers have been used to test human teeth
 In vivo measurements are needed to obtain more
reliable results
 Laser-holographic techniques and strain gauge
techniques

Aim
 The purpose of this investigation was to determine
the locations of the centre of resistance and the centre
of rotation of a maxillary central incisor during
retraction in human subjects. The relationship of the
geometric parameters of the teeth and surrounding
alveolar bone to the centre of resistance and centre of
rotation will also be discussed.

Method
Schematic representation of system
for measuring tooth movements.

Lateral view demonstrating geometric relationship
between level of force application and subject’s incisor

Calculation of geometric position of centre of
rotation.

RESULTS
Degree of tooth rotation as function of level of force application.
Positive signs indicate lingual crown tipping and negative signs
represent labial crown tipping. Centre of resistance is located at
level of intersection of y-axis and line representing degree of tooth
rotation

Location of centre of resistance with respect to geometries of root
and alveolar bone for each of 3 subjects (A, subject 1; B, subject
2; C, subject 3).

Location of centre of rotation plotted along long axis of tooth
corresponding to number of level of force application for each of 3
subjects (A, subject 1; B, subject 2; C, subject 3)

DISCUSSION AND CLINICAL IMPLICATIONS
 Burstone and Pryputniewicz (0.6 and 0.67)
 Nikolai (0.45)
 Tanne et al and Vollmer et al (0.76 and 0.58)
 Labio-lingual inclination was not taken into
consideration in those models, and labial and palatal
alveolar bone attachments were thus positioned at the
same level

Therefore considered that the value of X/RP can be an
indicator of the location of the centre of resistance with
respect to the surrounding bony structures when incisors
are retracted.

 There seem to be other factors determining the
location of centre of resistance, such as the amount or
thickness of the alveolar bone and the periodontal
ligament surrounding the tooth.
 Even though only 3 subjects were studied, it is evident
that the location of the centre of resistance depends on
the palatal alveolar bone height and not on the labial
alveolar bone height during anterior teeth retraction.
This finding is of clinical significance for orthodontic
tooth movement.

 The effect of changes in the level of force application
 Class II Division 1 malocclusions - controlled tipping
 Class II Division 2 malocclusions
 In cases of reduced palatal alveolar bone height, as
with subject 1, the centre of resistance is located at a
more apical position; therefore, a greater moment-to-
force ratio is needed for any controlled movement of
maxillary incisors during retraction.

CONCLUSIONS
 The location of the centre of resistance appears to
depend more on the palatal alveolar bone height and
very little on the labial alveolar bone height when
anterior teeth are retracted.
 The location of the centre of resistance of a maxillary
central incisor was found at approximately two-thirds
of the palatal alveolar bone height from the root apex.

 A negative correlation exists between the palatal
alveolar bone height and the moment-to-force ratio
needed for any orthodontically programmed tooth
movement such as controlled tipping, translation, and
root movement during anterior retraction.
 Further in vivo studies are needed to allow more
accurate prediction of tooth movements based on
applied force systems and total anatomical
parameters, such as shapes of the root, surrounding
bony structures, and the periodontal ligament, in
addition to the labial and palatal bone levels.

Experimental evaluation of initial tooth /certified fixed orthodontic courses by Indian dental academy

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Experimental evaluation of initial tooth /certified fixed orthodontic courses by Indian dental academy