Holographic interferometry

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INTRODUCTION
Holography is the name given to the
process of wave front reconstruction and
comes from the Greek words ‘holos’,
meaning whole and ‘gramma’ meaning
message. So it gives whole message.
In recent years, holography has been used
increasingly in biomechanical research in
dentistry, especially in orthodontics.

 The experiments on the dynamics of
human teeth during the masticatory
function, and the investigations of the
mobility and movement of teeth.
 The center of rotation produced by
orthodontic forces was also determined.
 The mechanical reactions of skull
bones and sutures induced by orthopedic
extra oral forces and the strain distribution
on the maxillary complex and
displacement of the teeth caused by
maxillary expansion forces were analyzed.

So far, the experimental methods for
measuring the movements and
deformations of hard tissue of the
dentofacial complex were based mainly on
either the histologic analysis of the
apposition or resorption of bone and
sutural tissue or on cephalometric
measurements.

Histologic methods for determining the
mechanical bony reactions are indirect and
give only a rough estimation of these
processes.
Furthermore, because of the limited
sensitivity of these methods, linear
and angular changes in the deeper
bony structures can be determined
only when high-magnitude force is
used for rapid expansion.

These methods do not provide useful results
for the measurement of such changes when
low-magnitude forces for slow expansion are
applied.
A noninvasive and highly accurate
holographic interferometric method allows
the precise detection of very subtle initial
mechanical reactions of hard tissue to forces
and direct measurement on the specimens.

METHOD OF RECORDING
Experimental apparatus consists of a flat optical
table with air suspension. The illumination for
recording and reconstruction of holograms was
provided by a 15 mW He-Ne laser.
The laser, the optical components for steering and
shaping of object and reference beams, the
experimental model, and the photosensitive
material used for recording of holograms were
rigidly mounted on top of the optical table by
means of magnetic bases.

Holographic set-up

The highly coherent and
monochromatic light from the laser
source is split into two beams by
means of a beam splitter .
One of the beams is directed by
mirrors, expanded by means of a
spatial filter (microscope objective
and a pinhole assembly), and is used
to illuminate the experimental model
to be recorded.

This beam, referred to as the object beam, is
modulated by a reflection from the
experimental model and carries all the
information about the instantaneous condition
of the it’s surface.
The second beam is known as the reference
beam and is not modulated by any intervening
object.

Holographic set-up
a: Laser.
b: Beam splitter.
c: Beam expander.
d: Holographic plate.
e: Spatial filter.
ob: Object beam.
rb: Reference beam

If both of these beams are allowed to impinge
on some kind of a surface, they will produce a
set of fringes, on that surface, as a result of
their mutual interference.
The spacing of the fringes is entirely
dependent on the angle between the two
beams and the wavelength of the light used;
the fringe opacity is related to the intensities
of the interfering beams.

The fringe pattern resulting from the
Superposition of two beams can be
recorded in the photographic emulsion
which, upon photographic processing,
becomes a hologram. The holographic
plate consists of a highly sensitized
emulsion that coated on a glass plate.
The hologram bears no resemblance to
the original object, to interpret a hologram
we need to reconstruct the image.

The hologram can be reconstructed with
the original system setup used in
recording, but now it is illuminated with
the reference beam alone.
This reconstruction wave must be
identical to the reference wave used
during the formation of the hologram.This
process is called reconstruction.

Double exposure holographic
interferometry
First the normal state of the object was
recorded.On the same record plate before
processing, another exposure
corresponding to the deformed portion of
the object
(for example: skull with an activated
appliance in palate)was recorded.

This double exposed plate was processed after
reconstruction, one corresponding to the original
position and another corresponding to the
deformed position of the experimental model.
These two reconstructed image waves interfered
giving rise to an image with number of bright and
dark fringes on the image.
From the fringe pattern the magnitude and
direction of displacement can be calculated.

Bright & dark fringes on the image

Quotes of some studies
Wictorin et al (1972) were some of the first to use
holographic interferometry in dentistry.
Wedenhal P.R(1974) evaluated the dynamics of human
teeth & found that mobility of teeth & related structures
can be measured with a range of 0.5-3.0 microns.
Dermaut et al (1981) studied the bone deformation by
class II elastic forces on double exposure holograms.
Gerrit kragt and Herman S Duterloo(1982) evaluated
the initial effects of orthopedic forces on a macerated
human skull using laser holography.

Kragt et al (1982) evaluated the reaction of
cervical headgear to the
maxilla.Holography used to compute .
Palvin et al (1984) studied mechanical
reactions of facial skeleton to maxillary
expansion determined by holographic
interferometry.
Duterloo et al (1985) studied macerated
human child skulls subjected to high-pull
headgear traction.Displacements were
measured using holography.

Dermaut et al (1986) evaluated intrusion
mechanics on a macerated human skull
using holography.
Harradine et al (1990) carried out a pilot
study using holograms as substitute for
study casts.
Legan et al (2000) evaluated displacements
in the craniofacial complex during mid
palatal suture opening using holography.
Batista et al (2002) evaluated stress area
induced by rapid expansion appliance using
double exposure holography.

Holograms as substitutes for
study casts
A hologram not only provides two-
dimensional information about an object,
as found in photographs, but also depth
information. This transforms the image
of the object into three dimensions.

Plaster casts are mainstay of clinical
orthodontic records since many years.
DISADVANTGES
 Fragile & prone for fracture
 Bulky & expensive to store and transport.
 Need to keep for long periods.

LEGAL REQUIREMENTS
The minimum legal requirement in British
hospitals is to
 retain study casts for 8 years after
completion of treatment,
 or until the patient reaches the age of 25
for treatment carried out on minors
 or until the patient attains age of 25
years ,for medico legal reasons.
In U.S, the patient records should be kept until
10 years after orthodontist retires from practice.

Advantages of holograms
Schwaninger et al (1977) proposed potential
advantages of holograms as a substitute for study
casts.
They are
 Holograms are very resistant to damage.
 Better suited to transport by post.
 Can be stored with patient’s clinical records.
 Accurate measurements such as intercanine
width can be measured.

HOLOGRAMS
The
chairside
viewer
holographic
camera
HOLOGRAM

In a study by Harradine et al ( 1990 Aug
AJO) with four clinicians and 56 patients,
three of the clinicians found the
holograms to be acceptable alternatives
to study casts in routine clinical
orthodontic practice.
Current holographic cameras enable
those who are not experts to produce
holograms very simply, but careful clinical
and laboratory techniques are required to
ensure that these holograms correctly
record the occlusion.

disadvantages
 significant consideration is the
consequence of incorrect occlusion of
the models when the holograms are
being made.
 Once made, a hologram cannot be
adjusted as a set of study casts .

A STUDY DONE IN OUR DEPARTMENT
USING HOLOGRAPHY
An in-vitro study was carried out in our
department regarding mechanical
reaction of facial skeleton to rapid palatal
expansion devices using laser
holography,which was done by using the
facility available in ANNA UNIVERSITY.

A freshly macerated human skull taken
& then fastened to heavy metal
framework.With the help of preformed
bands hyrax appliance secured and
holography was performed.Then spring
jet appliance secured and holography
done,up-on activations.

hyrax appliance secured

spring jet appliance

Holography being recorded on
macerated human skull

Upon recording holography,the plates were
developed & the images were recorded with a
camera in 2-Dimensional photographs.
The interpretation of photos can be done by
following a method put forward by RYSZARD J
PRYPUTRIEWICZ (1969).
A grid of 3 horizontal & 3 vertical lines which
represent the projections if the planes in which
the deformations & displacement of the sutures
were measured,were drawn on the photo of
interferograms.

INTERPRETATION OF RESULTS
Holographic interpretations can be done in two
ways.
1. qualitative analysis:By direct observation of
the fringes.
2. Quantitative analysis:By making
measurements either by directly counting the
no. of fringes or fringe spacing on the hologram
or by making measurements on the
photographs

The displacements were calculated from the
no. of fringes using the following formula.
nλ
d = -------
2
Where d = Displacement in mm.
n = no. of fringes.
λ = He- Ne laser wavelength = 632.8×10¯ 9
nm
----------
2

INTERPRETATION OF PHOTOS

The results of our study can be concluded as
follows:
1. Activations of hyrax produced mechanical
reactions on teeth,alveolar bone,maxilla &
the circum-maxillary bones and sutures.As
the activations increased, the no of fringes
increased positively.
2. The pattern of the fringes were more
circular around the naso- maxillary complex
and zygomatico- maxillary sutures
suggesting rotational displacement of the
maxilla.

3. The number & pattern of fringes produced
by the spring jet suggest that it produces only
dento alveolar change with minimal
orthopedic effect.
Conclusion of our study : hyrax appliance is
indicated in cases requiring maximum
orthopedic changes & the spring jet can be
advocated in dento-alveolar transverse
discrepancies.

Conclusion
As mentioned and highlighted
holography is a remarkable tool
with extensive research potential
for clinical application. The further
enlightenment with this tool is a
question, which has to yield
positive results.

LET US REMEMBER THE PERSON WHO SHOWED US THE WAY
TO FREEDOM

Holographic interferometry

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