Download Mechanical Reaction of Facial Skeleton to Rapid Palatal Expansion

10.5005/jp-journals-10021-1200
RESEARCH ARTICLE
Revathi Peddu et al
Mechanical Reaction of Facial Skeleton to
Rapid Palatal Expansion Devices using Laser
Holography: An in vitro Study
1
Revathi Peddu, 2Uma Maheswari, 3Kalyani Sreedhar Mallavarapu
4
Shyam Kumar Bandaru, 5Sai Prakash Adusumilli, 6SRK Reddy
ABSTRACT
Introduction: Rapid palatal expansion of the midpalatal suture is an effective method for the correction of transverse malocclusions. This study
was conducted to compare the reaction of circum-maxillary sutures and center of resistance of molar to the Hyrax appliance with different
degrees of activations and the Spring jet appliance using laser holography on a freshly macerated skull.
Materials and methods: A freshly macerated skull with well-aligned teeth and intact dental arches was used as the experimental model. The
Hyrax appliance and Spring jet appliances were fabricated and analyzed qualitatively and quantitatively by using double exposure hologram.
Hyrax was activated two ¼ turns, four ¼ turns and eight ¼ turns. The Spring jet is activated and double exposure hologram was recorded.
Results: Results were obtained qualitatively by observing the fringes, and quantitatively by counting the number of fringes. The Spring jet showed
fringes only in the first molar area, whereas the Hyrax showed fringes along the circum-maxillary sutures and the first molar area; and the number
of fringes increased with the number of turns.
Conclusion: Hyrax appliance activation produced mechanical reactions on the teeth, alveolar bone, maxilla and the circum-maxillary bones and
sutures. The displacement and fringes increased progressively with two, four and eight turns activation of hyrax. The pattern of the fringes was
more circular around the nasomaxillary complex and zygomaticomaxillary sutures, suggesting rotational displacement of the maxilla. The
number and pattern of fringes produced by the Spring jet appliances suggest that it produces only dentoalveolar changes and minimal orthopedic
affects.
Keywords: Rapid palatal expansion, Hyrax appliance, Spring jet appliance, Laser holography.
How to cite this article: Peddu R, Maheswari U, Mallavarapu KS, Bandaru SK, Adusumilli SP, Reddy SRK. Mechanical Reaction of Facial
Skeleton to Rapid Palatal Expansion Devices using Laser Holography: An in vitro Study. J Indian Orthod Soc 2013;47(4):426-432.
INTRODUCTION
Rapid palatal expansion of the midpalatal suture is an important
part of the clinician’s armamentarium in the correction of
malocclusions. Rapid maxillary expansion by opening of
the midpalatal suture is extremely advantageous in the
treatment of:
1. Both surgical and nonsurgical Class III cases especially
the nonsurgical ones.
2. Cases of real and relative maxillary deficiency.
3. Cases of inadequate nasal capacity exhibiting chronic nasal
respiratory problems.
4. The mature cleft palate patient.
1,2,4-6
Professor, 3Senior Lecturer
Department of Orthodontics, Meenakshi Ammal Dental College
Chennai, Tamil Nadu, India
2
1,3-6
Department of Orthodontics, Sibar Institute of Dental Sciences
Guntur, Andhra Pradesh, India
Corresponding Author: Revathi Peddu, Professor, Department of
Orthodontics, Sibar Institute of Dental Sciences, Guntur, Andhra Pradesh
India, Phone: 09848885654, e-mail: [email protected]
Received on: 29/4/13
Accepted after Revision: 25/6/13
426
5. Selected arch length problems to avoid the profile
disturbances so frequently associated with removal of
teeth (Andrew).1
It has been reported that the transverse forces applied by
the Hyrax appliance is of sufficient magnitude to overcome
the bioelastic strength of the sutural elements to bring about
the orthopedic separation of the maxillary segment. Newer
appliances like the Spring jet introduced by Aldocarano2 which
incorporates the NiTi open coil spring in a telescopic device
has been shown to produce rapid maxillary expansion.
The mechanical reactions of hard bony and dental tissues
are a very important aspect in the overall dynamics of
orthodontic appliance activity. An acquaintance with these
processes allows the orthodontist to influence the structures
of the maxillary complex remote from the dentoalveolar
region.
The amount of rapid maxillary separation was displayed
nearly three-quarters of a century ago3 by invasive procedures
like reflecting a flap of palatal mucosa put forward by
Federspiel4 while Price5 employed simple probing between
the separated palatal process. Isaascon6,7 measured the forces
using deformation gauge on a metal piece of the expander.
The mechanical reactions to the rapid maxillary expansion has
been analyzed by various methods like Teleradiographic head
JIOS
Mechanical Reaction of Facial Skeleton to Rapid Palatal Expansion Devices using Laser Holography: An in vitro Study
Films (Krebs,8,9 Hershey10), occlusal radiographs by Wertz.11
Experimental animals studies were done by West (1964),12
Starnbach, Bayne, Cleall Subtelny (1966)13 and Gardner and
Kronman (1971).14 Laser holography has been shown to be a
valuable method in which the initial microstresses (or) fringe
patterns resulting from various applied forces to the human
skull can be seen and recorded.
Holography is a technique of recording three-dimensional
images of objects. This method has become vital in
measurement techniques where influence of very small
deformation of objects under study becomes significant. Leith
and Upatniks applied laser to holography in 1960 and found
series of application in dentistry such as study of elastic
deformations of soldered gold joints,15 qualitative studies of
various dental structures, measurement of elastic deformation
of prosthodontic appliance,11 determination of centers of
rotation of teeth.
Hence, this study sought to compare the reaction of
frontonasal, nasomaxillary, intermaxillary, zygomaticomaxillary, zygomaticotemporal sutures and center of
resistance of molar to the Hyrax appliance with different
degrees of activations and the recently introduced Spring jet
appliance using laser holography on a freshly macerated skull.
Fig. 1: Freshly macerated skull
AIMS AND OBJECTIVES
1. To study the mechanical reactions of frontonasal,
internasal, nasomaxillary, intermaxillary, zygomaticomaxillary, zygomaticotemporal sutures and an area
corresponding to the center of resistance of the first molar
using Hyrax appliance with the different degrees of
activations (two ¼ turns, four ¼ turns, eight ¼ turns).
2. To study the mechanical forces produced on the sutures
with the Spring jet appliance.
3. To compare the mechanical forces between the Hyrax
appliance and Spring jet appliance.
Fig. 2: Skull coated with magnesium oxide
MATERIALS AND METHODS
Preparation of Experimental Models
A freshly macerated skull (without mandible) with well-aligned
teeth and intact dental arches (Fig. 1) obtained from
Government Medical College, Chennai, was used as the
experimental model. The skull was stored in Ringer’s lactate
solution to minimize the changes. The skull was coated with
magnesium oxide (Fig. 2) just prior to recording of the
hologram.
Preformed bands were adapted on the first premolar and
molars of the maxillary dentition of the skull, impressions
swere made using alginate impression material and the bands
were then transferred. Stone cast were poured with the bands
in place. Hyrax appliance was adapted (11 mm expansion) and
soldered to the bands, and the appliance was fabricated (Fig. 3).
Spring jet appliance was fabricated in the other model (Fig. 4).
Fig. 3: Hyrax placed on the model
The hyrax appliance consists of a midpalatal jackscrew
(11 mm Dentarum) assembly which as four rigid 0.060 inch
stainless steel shafts radiating outward from the screw. Each
shaft was adapted to the premolar and molar bands to which
they were soldered.
The Journal of Indian Orthodontic Society, October-December 2013;47(4):426-432
427
Revathi Peddu et al
Spring Jet (American Orthodontics)
Fig. 4: Spring jet appliance placed on the model
It consisted of a director unit which was formed into a ‘U’
shape keeping the legs parallel to each other at 5 mm distance.
They were cut to proper length and the ends were smoothened.
The second direction unit was formed as a mirror image of
the former one. The two units slide into one another and were
checked for friction free movement. The second unit is then
cut to size and the units were again rechecked. Double ‘U’
was positioned on the working model and the anteriorposterior tube positions were noted relative to the first molars
and fixed in place. The connecting framework 0.645 mm was
formed on each side and they were soldered, cleaned and
polished. According to force deflection chart (Chart 1), two
7 mm springs segments were assembled on double ‘U’ sections
with locks.
The expansions screws and the frameworks were placed
close to the roof of the palate, without impinging on the palatal
soft tissue.
Activation of the Spring jet: The activation lock was
released using Allen key (Fig. 7). The appliance was thus
activated and double exposure hologram was recorded.
Stabilization of The Experimental Model (Fig. 8)
Fig. 5: Two one-fourth turns activation of hyrax–frontal and lateral view
The skull with the fabricated screw set up was then fastened to
a heavy metal framework. The metal frame consists of two
vertical bars attached to the base. Vertical bars were used to
fix the skull in the occipital region with the help of the screws.
Iron struts were welded to the base so as to hold the skull
rigidly to prevent any minor movements during the experiment.
The skull was fixed such that the occlusal plane was kept
parallel to the base of the support.
Holographic Setup (Figs 9 and 10)
The holographic unit consisted of a source of He-Ne laser
with wavelength of 0.6238 microns, beam splitter, mirrors,
beam expanders, object photographic plate and plate holder.
The skull was used as the object.
RESULTS
Fig. 6: Four one-fourth turns activation of hyrax–frontal and
lateral view
Activation of the Appliance
Hyrax: This appliance was activated two ¼ turns (Fig. 5) by
using the key provided. Then the double exposure hologram
was recorded. Similar procedures was repeated for four ¼
turns (Fig. 6) and eight ¼ turns.
428
These holographic plates were developed and the images were
recorded with a camera in two-dimensional photographs. The
holograms were interpreted from the photographs by following
method put forward by Ryszard J Pryputriewicz (1969). A grid
of three horizontal and three vertical lines which represent
the projections of the planes in which the deformations and
displacement of the sutures were measured, were drawn on
the photographs of the interferograms (Fig. 11). The fringes
Chart 1: Force-deflection chart from the Spring jet appliance
400 gm Spring 0.018 × 0.055
Compression (mm) 1
Maximum force
39
Minimum force
30
2
112
90
3
185
155
4
260
232
5
327
300
6
404
377
7
473
442
JIOS
Mechanical Reaction of Facial Skeleton to Rapid Palatal Expansion Devices using Laser Holography: An in vitro Study
Fig. 10: Recording of hologram
Fig. 7: Activation of Spring jet appliance–frontal and lateral view
Fig. 11: Interpretation of results
Fig. 8: Stabilization of the skull
Interpretation of Results
Holographic interpretation can be done in two ways:
1. Qualitative analysis: By direct observation of the fringes
(Graphs 1 and 2).
2. Quantitative analysis: By making measurements either
by directly counting the number of fringes (or) fringe
spacing on the hologram (or) by making measurements on
photographs (Graphs 3 and 4).
The sutures were represented by the following grids:
Nasomaxillary suture
Zygomaticomaxillary
Fig. 9: Experimental apparatus–holographic unit
were counted for each suture on the x and y axis. The x-axis
lines were represented as a b c c1 and y-axis as ABC Al on the
lateral view and d e f and D E F on the frontal view respectively.
Nasomaxillary suture, zygomaticomaxillary, zygomaticotemporal and center of resistance of molar were evaluated in
the lateral view and frontonasal, intermaxillary, zygomaticomaxillary sutures were evaluated in the frontal view.
Zygomaticotemporal
Center of resistance of molar
Frontonasal
Intermaxillary
aA
Right/Left
bBcDcF
aC
c1 A1
dE
fE
The displacements were calculated from the number of
fringes using the following formula:
n
d = ———
2
The Journal of Indian Orthodontic Society, October-December 2013;47(4):426-432
429
Revathi Peddu et al
where, d = displacement in nm
n = no. of fringes
632.8 × 109 nm
 = He—Ne laser wavelength = ———————
2
Analysis of the interferograms showed that transverse
maxillary expansion by the Hyrax appliance induced initial
mechanical reactions not only of the alveolar bone and teeth
but also of the entire maxilla, all the circum-maxillary sutures
and the surrounding bones. These changes were evident even
when the Hyrax was activated initially by two turns. The
direction of interference fringes on the bony surface was also
identical for all loading degrees; whereas their number increase
corresponded to the increased amount of widening. This
indicates that the mechanical reactions in these experiments
were qualitatively, the same at the lowest as well as at the
highest loading degree.
Quantitatively the number of fringes increased with two,
four, eight ¼ turns. There was minimal fringe pattern with two
¼ turns activation of Hyrax with the same number of fringes
in the frontonasal, intermaxillary, zygomaticomaxillary,
zygomaticotemporal and center of resistance of molar. Similar
trend could be observed at the four-turn activation. As the
activation force was increased to eight turns, the number of
fringes increased significantly at all sutures, and the fringes
were seen in a more circular direction around the nasomaxillary
complex and zygomaticomaxillary suture (Tables 1 and 2).
Interpretation of the fringe pattern of the Spring jet
appliance indicated moderate changes at the intermaxillary
suture comparable to four ¼ turns activation of Hyrax. There
was very minimal change at the other frontonasal,
zygomaticomaxillary, zygomaticotemporal sutures, and the
most of the fringes were concentrated around the center of
resistance of the molars (see Tables 1 and 2).
DISCUSSION
To understand the actual function of an orthodontic appliance
the modern orthodontist must continually expand his
knowledge of orthodontic forces induced at the point of
contact of the appliance with soft and hard orofacial tissues,
the distribution of these forces and their transformation while
passing through the tissue, the biologic and mechanical tissue
reactions to the applied forces. One of the foremost
proponents of the concepts of rapid palatal expansion is the
Graph 1: Holographic interpretation of firnge numbers frontal view
Graph 2: Holographic interpretation of fringe numbers lateral view
Graph 3: Holographic interpretaion of lateral view—amount of
displacement in nm
Graph 4: Holographic interpretation of lateral—amount of
displacement in nm
430
JIOS
Mechanical Reaction of Facial Skeleton to Rapid Palatal Expansion Devices using Laser Holography: An in vitro Study
Table 1: Holographic interpretation of frontal view—fringe numbers
Hyrax
Frontonasal
suture
Number of fringes
Intermaxillary
Zygomaticomaxillary
suture
suture
2
4
8
Spring jet
1
2
6
0
1
2
4
3
Right
Left
1
2
7
1
1
2
7
1
Table 2: Holographic interpretation of frontal view—amount of
displacement (in nm)
Hyrax
2
4
8
Spring jet
Frontonasal
suture (nm)
000316.4
000632.8
0001898.4
0
Intermaxillary
suture (nm)
000316.4
000632.8
0001265.6
000949.2
Zygomaticomaxillary
suture
Right
Left
000316.4
000632.8
0002214.8
000632.8
000320.2
000598.4
0002208.2
000636.6
Haas Appliance in 1961, and since then rapid palatal expansion
has been the mainstay in management of skeletal transverse
discrepancies. In this study, holographic interferometry was
applied to evaluate the skeletal displacements of sutures with
two different expansion appliances.
Andrej Zentner et al (1996),16 derived a method that
combined the advantage of classical optical interferometry
and those of holography and allows the measurement of
displacement of relatively large surface at the level of light
wavelength. In several earlier investigation, researchers have
proved the practicability, validity, and advantages of
holographic interferometry,17 when applied in orthodontic
research. Skeletal displacement was evaluated in the present
work using a zero-order fringe interpretation technique. It was
chosen because it has several important advantages including
precision, the ability to discriminate between various types
of displacement, simplicity of the optical set-up and
convenience of using photographs for evaluation.
Holographic study was performed on a freshly macerated
skull as the modulus of elasticity of bone is several orders of
magnitude higher than that of the remaining connective
tissues,18 and skeletal reaction therefore probably represents
the major component of the displacement in vivo.19 The use
of a macerated skull in the present study seemed justified as
an approximation of the situation in vivo. Furthermore, a
macerated skull is to be used because a wet specimen would
require substantially longer exposure times during holographic
procedures and could change in shape through evaporation.20
The use of a single skull served the purpose of the present
study, i.e. to compare the effects of Spring jet and Hyrax
expansion appliances applied in different degrees of activation
under otherwise identical experimental conditions. The use
of several skulls was rejected in order to avoid the additional
variable caused by individual skull, holography which might
obscure subtle qualitative and quantitative differences in
displacement. Furthermore, a single skull was used due to the
difficulty of obtaining a freshly macerated young skull with
full complement of teeth and the cost of holographic plates.
Satisfactory precision accuracy and reproducibility of the
experimental arrangement employed in the present
investigation were confirmed in the precursor trials.
The results of the present investigations showed that the
transverse orthopedic expansion produced mechanical
reactions on the teeth and the entire maxilla, all the circummaxillary sutures and the surrounding bones. The activation
of the Hyrax:
Two ¼ turns produced minimal displacement (000316.4
nm) at the frontonasal, intermaxillary, zygomaticomaxillary
and zygomaticotemporal sutures, while moderate
displacement was noted at the center of resistance of molar
(000949.2). There was a progressive increase with four ¼ turns
at the sutures, and center of resistance of the molar. With an
activation of eight ¼ turns, the fringe effect was maximum at
the sutures and the center of resistance of first molar
(0002214.8) (0003480.4). In most rapid palatal expansion
appliances according to Hass ¼ revolution of the screw equals
0.2 mm of lateral displacement. Thus two ¼ turns produces
0.4 mm and four ½ turns produces 0.8 mm, eight ¼ turns
produces–1.6 mm. Hass also reported the forces produced by
this appliances as ranging from 3 to 10 pounds. 21 The
displacement in the holographic study is recorded in
nanometers. Hence, it cannot be quantitatively compared to
the forces produced clinically. Increase in the fringes
associated with increase in the appliance activations coincides
with study by Dubravko Pavlin and Dalibor Vukicevic (1984).22
This suggested that increase in arch width with every screw
activation occurs partially because of the deformation within
the alveolar bone and partially because of the changed position
of the maxilla in relation to the skull bones.
The qualitative holographic interpretation demonstrated
convergence of the fringes at discrete areas approximating
the frontonasal suture. The fringe pattern on the zygoma was
oblique in direction indicating the displacement of the bone
by rotation and lateral bending. The fringes were clearly visible
on the surrounding maxillary bones, which indicated that the
expansion forces were transmitted to these bones. Similar
results were obtained by Stanley Braun, Alexandre Bottrel
(Sept 2000).23 Lee et al (1997)24 who studied the holographic
interpretation with protraction and rapid palatal expansion
found similar results.
The Spring jet, introduced by Aldocarano (1999)2 is a
relatively new appliance incorporating a NiTi open coil spring
which produces 473 gm of force on activation. The present
study included the Spring jet appliance to study its effect and
compare the reactions with that of Hyrax. The results indicate
that the Spring jet appliance produces changes mainly at the
intermaxillary and the center of resistance of molar. Thus, it
can be inferred that changes are mainly concentrated on the
dentoalveolar region only. When comparing the number of
fringes produced with the activation of Spring jet appliance in
the intermaxillary area, a total number of three fringes were
recorded. This could be compared with the Hyrax activation
of four turns, where the number of fringes was only two. While
The Journal of Indian Orthodontic Society, October-December 2013;47(4):426-432
431
Revathi Peddu et al
no fringes were noted in the sutures remote from the
dentoalveolar region viz–zygomaticotemporal, zygomaticomaxillary and frontonasal with this Spring jet appliance. Thus,
it can be inferred that the mechanical reaction of the Spring
jet were more in the intermaxillary area and an area close to
the center of resistance of the first molar. The mechanical
reaction of Spring jet on the sutures cannot be compared due
to lack of the studies on the appliance.
Clinically; thus it can be concluded on the basis of these
findings that the remodeling with in the maxillary sutures are
responsible for the jaw expansion and downward and forward
movement of the maxilla associated with the rapid palatal
expansion. Results of the Spring jet appliance suggest that it
produced mostly dentoalveolar and very minimal orthopedic
changes.
This holographic study was made on a dry human skull and
could not be directly correlated with the biomechanical
reactions of the live structures due to the change of mechanical
properties of bone on loss of humidity and soft tissues. The
decreased humidity causes the gradual increase in friction and
decreases displacement progressively.25
Nevertheless, the knowledge of the mechanical reaction
allows orthodontist for a better understanding about the final
therapeutic effects of appliance and the correct application
of orthopedic appliance on sutures and bones of craniofacial
system.
CONCLUSION
The results were interpreted using zero order fringe
interpretation technique, on the photographs, on selected
vertical and horizontal planes. The results can be concluded
as follows:
1. Activation of Hyrax appliance produced mechanical
reactions on the teeth, alveolar bone, maxilla and the
circum-maxillary bones and sutures.
2. As the displacement increased, the number of fringes
increased progressively with two, four and eight turn
activation of Hyrax.
3. The pattern of the fringes was more circular around the
nasomaxillary complex and zygomaticomaxillary sutures
suggesting rotational displacement of the maxilla.
4. The number and pattern of fringes produced by the Spring
jet appliances suggest that it produces only dentoalveolar
changes and minimal orthopedic effects.
Thus, increase in dental arch width with Hyrax
appliance can be attributed deformation of alveolar
process, lateral dental tipping and rotational movement of
maxilla within the sutures. Hence, the Hyrax appliance is
indicated in cases requiring maximum orthopedic changes
and Spring jet can be advocated in dentoalveolar transverse
discrepancies.
REFERENCES
1. Haas AJ. Palatal expansion: just the beginning of dentofacial
orthopedics. Am J Orthod 1970;57:219-255.
2. Carano A. The Spring jet for slow palatal expansion. J Clin Orthod
1999;33(9):527-531.
432
3. Timms DJ. A study of basal movement with rapid maxillary
expansion. Am J Orthod 1980;77:500-507.
4. Federspiel MN. Discussion on paper by Dewey M: development
of the maxillae with reference to opening the median suture. Items
of Interest 1913;35:271-282.
5. Price WA. Some contributions to dental and medical science.
Dent Summary 1914;34:253-290.
6. Isaacson RJ, Wood JL, Ingram AH. Forces produced by rapid
maxillary expansion part-I design of the force measuring system.
Angle Orthod 1964;34:256-260.
7. Isaacson RJ, Ingram AH. Forces produced by rapid maxillary
part-II forces presented during treatment. Angle Orthod
1964;3:261-269.
8. Krebs A. Expansion of the mid-palatal suture studied by means
of metallic implants. Trans Eur Orthod Society 1959;17:491-501.
9. Krebs A. Midpalatal suture expansion studies by the implant
method over a seven-year period. Rep Congr Eur Orthod Soc
1964;40:131-142.
10. Hersley HG, Stewant BL, Warren DW. Changes in nasal airway
resistance associated with rapid maxillary expansion. Am J Orthod
1976;69:274-284.
11. Wertz RA. Changes in nasal airflow incident to rapid maxillary
expansion. Angle Orthod 1968;38:1-9.
12. West IM. Histologic study of sutural tissue changes accompanying
palatal splitting in the monkey. Master’s Thesis University of
Illinois; 1964.
13. Starbanch H, Bayne D, Cleall J, Subtelty JD. Facioskeletal and
dental changes resulting from rapid maxillary expansion. Angle
Orthod 1966;36:152-164.
14. Gardner GE, Kronman JH. Cranioskeletal displacements caused
by rapid palatal expansion in the rhesus monkey. Am J Orthod
1971;59:146-155.
15. Wictorin L, Bjelkhagen H, Abrumson N. Holographic investigation
of the elastic deformation of defective gold joints. Acta Odontol
Scand 1972;30:659-670.
16. Zentner A, Sergl HG, Filippidis G. A holographic study of variations
in bone deformations resulting from different headgear forces in
a macerated human skull. Angle Orthod 1996;6:463-472.
17. Pryputniewicz RJ, Bowley WW. Techniques of holographic
displacement measurement: an experimental comparison. Appl
Opt 1978;17:1748-1756.
18. Yamada H, Evens FG. Strength of biological materials. Baltimore:
Williams & Wilkins; 1970.
19. Kragt G, Ten Bosch JJ, Borsboom PC. Measurement of bone
displacement in a macerated human skull induced by orthodontic
forces a holographic study. J Biomech 1979;12(12):905-910.
20. Fuchs P, Schott D. Holografische inferometry zur darstellung von
verformungendesmenschlichen gesichtsschadels. Schweiz
Monstsschur Zohnmed 1973;83:1468-1482.
21. Ladner PT, Muhl ZF. Changes concurrent with orthodontic
treatment with maxillary expansion is a primary goal. Am J Orthod
Dentofacial Orthop 1995;108:184-193.
22. Pavlin D, Vukicevic D. Mechanical reactions of facial skeleton to
maxillary expansion determined by laser holography. Am J Orthod
1984 Jun;85:498-507.
23. Braun S, Bottrel JA, Lee KG, Lunazzi JJ, Legan HL. The
biomechanics of rapid maxillary sutural expansion. Am J Orthod
2000;118:257-261.
24. Lee KG, Ryu YK, Park YC, Rudolph DJ. A study of holographic
interferometry on the initial reaction of maxillofacial complex during
protraction. Am J Orthod 1997;111:623-632.
25. Vest CM.- Holographic interferometry. New York: Wiley; 1979.
465 p.