Holograph / orthodontic courses by Indian dental academy

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INDIAN DENTAL ACADEMY

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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 RECORDINGExperimental 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 studiesWictorin 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 RESULTSHolographic 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.

n d = ------- 2Where d = Displacement in mm.

n = no. of fringes.

= He- Ne laser wavelength = 632.810¯ 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.


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Holograph / orthodontic courses by Indian dental academy

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