1986 Vanden Bulcke Imprtant

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The center of resistance of anterior teeth duringintrusion using the laser reflection technique andholographic interferometry

Marc M. Vanden Bulcke,* Luc R. Dermaut,** Rohit C. L. Sachdeva,*** andCharles J. Burstone****Ghent, Belgium, and Farmington, Corm.The aim of this investigation was to define the location of the center of resistance of variousconsolidated units of the maxilla ry anterior dentition using a dry human sku ll when subject to intrusiveforces. The laser reflection technique and the holographic interferometric technique were employedto measure the displacement of the dentition to the applied forces. The units studied consistedof a) two central incisors, b) four incisors, and c) six anterior teeth. The incorporation of the lateralincisors into the upper incisor segment resulted in a small distal shift of the center of resistance22 mm). However, when the canines were included into this unit, the center of resistance shifteddista lly by a significantly larger amount 2 7 mm). Increasing the magnitude of intrusive forcesapplied to the various un its appeared to have no effect on the location of the center of resistance.(AM J ORTHOO DENTOFAC ORTHOP 90: 21 l -220, 1986.)Key words: Intrusion, center of resistance, holography, laser reflection, segment

T e ability to define the location of the cen-ter s) of resistance of biologic structures is of impor-tance in the control of all types of orthodontic move-ment-for example, the retraction of molars by a head-gear or the intrusion of incisors. The term center ofresistance is sometimes confused with the terms cen-ter of gravity , center of mass, and center of ro-tation. r2-7

In a free body, movement is determined by thedirection and point of application of the acting forcewith respect to the center of mass of the object. A forcethat has its point of application at the center of massor a series of forces of which the resultant passesthrough the center of mass wil l lead to translation ofthe object. If an object is restrained-for instance, atooth or a group of teeth in bony sockets-movementis determined by the direction and point of applicationof a force with respect to the center of resistance ofthese bodies. When the applied force or the resultant ofa number of applied forces passes through the centerThis stody was funded by U.S. Public Health Service Grant DEKl6817, andF.K.O. 33.0083.83, Belgium.*Fkmer Assistant, O rthodontic Departm ent, University of Connecticut; As-sistant, orthodontic Departm ent, University of Gbent.**Chairman, Orthodontic Department, University of Ghent.***Assistant Professor, Orthodontic Departm ent, University of Connecticut.****Chairman, Orthodontic Department, University of Connecticut.

of resistance, the object will then undergo translation.The resistance to the applied forces generated by

the surrounding structures of a restrained group of teethdiffers from that of a free body; hence, the point ofapplication of the force or resultant of multiple forces)that brings about translation of a group of teeth maydiffer from the center of mass.In the orthodontic literature, the centers of resis-tance are most often defined on a hypothetical basis orestimated through clinical experience.5 Some confusionarises as distinct ions are made between dental and skel-etal centers of resistance21 because, relative to the cra-nial base, both the tooth and bone may displace. Thecenter of rotation is the point around which an objectappears to rotate from the initia l to final stages of themovement. For translation, the center of rotation is atinfini ty. Various model systems have been developedto study the location of the center of resistance of theupper anterior teeth by means of laser holography. Ina previous study, ribbon-arch sectional wires wereused as intrusive mechanisms and tied into Begg brack-ets. Unfortunately, this system allowed for independentmovement of the teeth about their centers of rotationsto some extent, making it impossible to define the centerof resistance of the whole anterior segment. As a resultof this investigation, attempts were made to rigidlyconsolidate the anterior teeth by use of edgewise brack-

211


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212 Vanden Bulcke et al . Am . J. Orthod. Dentofac. Orthop.September 1986

Fig. 1. Occlusal view of the spl ints incorporating two upperincisors (a), four upper inciso rs (b), and six upper anteriorteeth (c) .

ets with the slots filled with a rectangular wire. Un-fortunately, such a system also allowed some indepen-dent motion of the teeth.

In the present investigation, metal splints were usedto rigidly hold the anterior teeth to minimize the indi-vidual tooth movements. The number of teeth, the pointof force application, and the magnitudes of forces werethe controlled variables. To detect the initial movementsof the teeth to the applied forces, two laser techniqueswere used: laser reflection and holography. Forces wereapplied directly to the splint to simulate the applicationof an intrusive force system as suggested in the seg-mented-arch technique.. The intrusive arch proposedby Burstone and Nanda- provides an efficient mech-anism to intrude teeth. The point contact establishedanteriorly enables the clinician to place intrusive forceson the anterior segment without errat ic force systemscoming into play.

The objective of this study was to determine thelocation of the center of resistance for different rigid

Flg. 2. Sagittal A) and frontal (6) views of the splint incorpo-rating four upper incisors. Note the distal extension arm of thespl ints and the hooks th at were the p oints of appl ication for theintrusive forces.

units of the anterior maxillary dentition when intrusiveforces were applied to them.

The direction of pure intrusion was defined as avertical displacement perpendicular to the occlusalplane. The center of resistance for this study wasdefined as the point or the zone through which an in-trusive force when applied causes pure intrusion trans-lation) of the selected group of anterior teeth as ob-served from the lateral sagittal) view.MATERIALS AND METHODS

The experiments were performed on a dry humanskull presenting with a full complement of dentitionand a symmetric dental arch. The upper central inc isorswere inclined at 112 to the palatal plane, and 105 tothe sella-nasion plane. To ensure maximum skull sta-bilization, the cranium was embedded in hard stonewithin 5 mm of the maxillozygomatic sutures; this as-sembly was then fixed to a solid support. Araldit 208,displaying elastic properties somewhat similar to theperiodontium Dijkman, 196913), was used to fix theteeth into the sockets. A catalyst/acrylic proportion ofl/IO was used to accelerate setting time. Siliconeimpressions of the test skull dentition were taken, work-ing models made, and wax models for the anterior


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V o l u m e 9 03umber Center of resistance of anter ior teeth dur ing intrusion 213

Table I. Displacement values for the two-, four-, and six-teeth segments applying intrusive forces of 50 g,100 g, and 200 g among points a to f using the laser reflection technique

Loading position50 g l@g 2008

Displacement* SD Displacement* SD Displacement* SDTwo-teeth segment displacements

a + 14.0b + 10.3C t4.2d -1.4e - 15.0f - 27.2

Four-teeth segment displaceme ntsa +8.3b +5.4C +3.6d -e -2.9f -5.1

Six-teeth segment displacementsa +4.2b +2.6C + 1.3d -e -f -0.9

2.69 +31.1 1.16 +49..5 2.070.40 +21.7 0.84 +38.6 0.911.11 + 10.6 2.53 + 16.5 0.66

0.78 -6.2 0.62 -11.5 0.831.57 -34.3 1.15 -56.4 1.23

0.74 -57.7 0.62 - 10.1 2.29

0.62 + 17.0 1.41 +27.8 1.250.80 + 12.0 1.71 +20.5 1.070.61 +6.8 0.92 + 14.8 0.48

- - - +1 0.841.83 -6.5 0.70 - 10.9 0.740.35 - 10.8 1.51 - 18.8 1.27

1.12 +8.9 0.95 + 17.6 1.110.53 +5.3 1.84 + 14.7 0.840.42 +3.8 0.29 + 10.4 1.27

- + 1.4 0.62 +3.4 1.17- - - - -

1.25 - 1.5 0.79 -2.6 0.70*Displacement of speckle measured in tenths of millimeters with the traveling microscope.

sjplints fabricated. The splints weie cast in chromium-cobalt alloy. Three splints were fabricated to fit thedifferent consolidated units under study two, four, andsix anterior teeth, Fig. 1). The splints were fabricatedso that their distal ends extended to the second pre-molars. The splints were rigidly cemented to the teethwith zinc oxyphosphate. A small aluminum plate1.5 x 3 cm) was fixed at their midpoints, perpendic-

ular to the occlusal plane. These small plates were usedto measure the initia l displacements of the splints fol-lowing loading. V-shaped hooks positioned along thesplints served as points of attachment for force appli-cation Fig. 2, A and B). Two forces, equivalent inmagnitude and perpendicular to the occlusal plane, weiegenerated by dead weights and applied at points sym-metric to the midline. At the midpoint of the segmentthat is, between the two central inc isors), only a singleforce was applied.

The splints embracing two, four, and six teeth werenumbered 1, 2, and 3, respectively. The loading po-sitions were indicated with the letters a, b, c , d, e, andf. For loading position a, the force was applied be-tween the central incisors; for position b, betweenthe central and lateral incisors; for position c, be-tween the lateral incisors and canines; for position d,between the canines and first premolars; for position

e, between the first and second premolars; and forposition f, distal tq the second premolar Fig. 2, B).Two laser measuring techniques were used: a holo-graphic interferometric technique and a laser reflectiontechnique. An argon laser, Spectra Physics model 165 ,*with 265 power supply wavelength 6,328 A) was used.The skul l and the optical components were positionedon a vibration-free table to allow both registration tech-niques to be used with minimal rearrangement of thetechnical equipment.

For the holographic registration, the double ex-posure technique as described in the literature2.4-9was used. The pictures were registered on holotestplates by Agfa-Gevaert 4 X 5 X 0.060 inch HDBE56).t For practical reasons, the skull was loadedduring the first half of the exposure time and not loadedduring the second half. Frontal and occlusal hologramswere recorded for each of the splints and successiveloads of 50 g, 100 g, and 200 g were applied. Thetechnique provides an overall impression of the indi-vidual displacements of the craniofacial componentsincluding the dentoalveolar complex) to applied forces.

The number of fringes on the exposed plate is a measure*Spectra-Physics, Inc.. San Jose, Calif .tAgfa-Geva elt Rex, Inc., Secaucus, N.J.


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Figs. 3 through 5. Graphic representations of the resul ts obtained using the laser reflection technique.Hori tonial axis-displacement of speckle measured in tenths of mi l l imeters. Vert ical axis-points offorce appl ication. Fig. 3 (shown above). Two-teeth segment with appl ied intrusive loads of 50 g,100 g, and 200 g.

of the magnitude of the displacement. Also, rigid-bodymotion of the skull and its stabilizing unit may be de-tected using this method. This is important because anydisplacement of the skull to the applied forces mayinfluence the interpretation of the results. The holo-graphic technique was used to substantiate the resultsobtained with the laser reflection technique for the var-ious experimental investigat ions performed. It is im-portant to note that the holograms do not give any in-dication of the direction of displacement-that is, ifthe motion is away or toward the observer. However,the laser reflection technique is effective in providingsuch information.

For the experimental investigations involving theuse of the laser reflection technique,** the laser beamwas direcied onto the middle of the aluminum platefixed perpendicular to the metal splint). The reflected

beam was intercepted by a microscope magnificationx 30). When looking through the objective, a well rec-ognizable speckle was brought to the center of the hair-line by adjusting the objective of the microscope. Thesplint was then loaded, and the direction and amountof displacement of the reflected speckle were noted.

An upward movement of the speckle downwardwhen observed through the objective of the microscope)indicated a counterclockwise rotation of the splint. Adownward movement of the speckle upward when ob-served through the objective of the microscope) indi-cated a clockwise rotation of the anterior segment. Noinotion of the speckle on applying the intrusive forceindicated pure intrusion. It was therefore inferred

from this that the intrusive force acted through the cen-ter of resistance of the unit. Measures for all centers ofresistance were determined from a plane parallel to themidsagittal plane.ANALYSIS OF THE MEASURING RESULTSOBTAINED WITH THE LASER REFLECTIONTECHNIQUE

Intrusive loads were applied to the three splints andsix values for displacement of the speckle were recordedfor a given load. Means and standard deviations of themeasures of speckle displacement appear in Table I.

For various loading conditions of the splint, thespeckle was observed to move upward or downward.These displacements were assigned a positive or neg-ative sign that corresponded to counterclockwise orclockwise rotation of the splint, respectively. In TableI, the displacement values are recorded for the two-,four-, and six-teeth segments for intrusive forces of50 g, 100 g, and 200 g loaded at points between aand f. The results, are also presented graphically inFigs. 3 through 5. On the vertical axes are plotted thepoints of application of the intrusive force and on thehorizontal axes the displacement measures of the splint.A positive abscissa represents a counterclockwise ro-tation and a negative abscissa a clockwise rotation.Two-central-incisors segment: Splint 1 Fig. 3)

When 50 g of an intrusive load was placed on thetwo-central -incisors segment through positions a to f, displacement occurred. Counterclockwise rotation


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Fig. 4. Four-teeth segment with appl ied intrusive loads of 50 g, 100 g, and 200 9. (For complete legend,see Fig. 3.)

was produced at loading positions a, b, and c,with c lockw ise rotation at the loading positions d, e, and f. The point of intersection of the graphwith the vertica l axis corresponded to the point of forceapplication that induced no rotation; by definition, theintrusive forces go through the center of resistance. Thisloading position lies between cl and d; nearer tothe latter than the former. It may be concluded that ifthe rigid ity of the segment is maintained, an intrusiveforce placed distal to the canine eminence will resultin pure intrusion of the central incisors with respectto the cranium. When the force levels applied wereraised to 100 g and 200 g, the respective graphs inter-sected the vertica l axes at the same point as that of the50 g graph. This indicates that the center of resistancedoes not change with increasing force magnitude in thisparticular model system.

Forces applied at positions mesial to the center ofresistance created rotations of much greater magnitudethan those applied distal to the center of resistance.Four -incisors segment: Splint 2 (Fig. 4)

When the four incisors were segmented and an in-trusive force of 100 g was applied, counterc lockwiserotation was observed for the loading positions a,b, and c, with clockwise rotation for the loadingpositions e and f. For the loading position d,pure intrusion was observed. An intrusive force appliedbetween the canine and the first premolars induced pureintrusion of all four incisors. Varying the amount of theforce 50 to 200 g) did not change the location of thecenter of resistance.

No obvious differences were observed between the

Fig. 5. Six- teeth se gment with applied intrusive loads of 50 g,100 g, and 200 g. (For complete legend, see Fig. 3.)

magnitudes of the rotations created by intrusive loadsapplied mesial or distal to the loading position d.Six-anterior-teeth segment: Splint 3 (Fig. 5)

When an intrusive force of 200 g was placed on thesix anterior teeth at positions a through f, rota-tions occurred. Counterclockwise rotation was ob-served at the loading positions a, b, c, andd, with clockwise rotation at the loading positionf. At position e, the applied force passed throughthe center of resistance of the six-teeth segment sinceno rotation was observed.

A load of 100 g did not change the location of the


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Fig. 6. Frontal holograms with two-teeth segmen t, appl ied intrusive loads of 50 g (A), 100 g (B), and200 g (C).

Table II. Comparison between the number offringe patterns observed on the hologram anddisplacement values obtained by the laserreflection technique for similar loadingcondition for the two-incisor segment at loadingposition a

Intrusiveforce

50 g1mg2mg

Hologram Laser rejlection(Fig. 6, A through C) value*

7 fringes 14.016 fringes 31.134 fringes 49.5

*Displacement of speckle measured in tenths of millimeters with thetraveling microscope.

center of resistance. For an intrusive force of 50 gapplied at a position between d and f, neitherpositive nor negative rotation occurred. The force wasobviously too light.ANALYSIS OF THE HOLOGRAPHIC RECORDS

Frontal and palatal views were taken of each of thethree splints loaded at the six loading positions dis-cussed previously. The forces applied were those com-monly suggested for use in clinical practice-50 g forsplint 1, 100 g for splint 2, and 200 g for splint 3. Forthe purpose of this study, only the centers of resistancelocated from the frontal views of the holograms wereof significance. The findings on the palatal view holo-grams were in agreement with the findings of the fron-

tal views and the results obtained by the laser reflec-tion method; therefore, the palatal views wil l not bediscussed.

The interpretation of the holograms was based onthe relationships of the various fringe patterns observedon the aluminum plates. When the distances betweenthe fringes on the plates were the same, this indicatedpure rotational movement. When the fringes were par-allel to the incisal edge of the tooth, the displacementmay be interpreted as clockwise or counterclockwiserotation. However, the distinction between the posterioror anterior rotations cannot be read from the hologram.The rigid body motion of the skul l was tested withsplint 1 and intrusive forces of 50 g, 100 g, and 200 gapplied between the central incisors Fig. 6, A throughC). Increasing the amount of force caused no changein the direction of the fringes. However, the number offringes observed increased linearly with the applicationof increased levels of force. This finding related wellwith the measures obtained by the laser reflection tech-nique Table II).

Slight bone deformation was visib le with loads of50 g one fringe). This deformation increased as theapplied load changed from 100 g two fr inges) to200 g 4 fringes). However, the zygomatic structuresappeared to remain unaffected.Two-upper-jncisors segment: Splin t 1 Fig. 7)

An intrusive force of 50 g was applied and placedsymmetrically for all possible positions on the splint


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Fig. 7. Frontal holograms with two-teeth segmen t, appl ied intrusive load of 50 g. A through F, Loadingpositions a through f.

that is, through positions a to f ). It was observedthat the density of the fringe pattern decreased graduallyfrom the loading positions a through c and in-creased again from loading positions d through f.The fringe patterns in the regions co and d dis-played similar densities; however, the loading directionswere distinctly different. The holograms in Fig. 7, Cand D did not give any indication as to the direction inwhich the rotation took place. The displacement valuesobtained by means of the laser reflection technique anddisplayed in Table I demonstrate that the decreasingfringe patterns corresponded to the decreasing coun-

terclockwise rotations of the splint and that the increas-ing fringe patterns corresponded to the increasing clock-wise rotations, thus definitely suggesting that the splintobserved on the hologram in Fig. 7, C displaced in acounterclockwise direction and on the hologram in Fig.7, D in a clockwise direction.Four- incisors segment: Splin t 2

The sp lint connected the four incisors and theamount of intrusive force applied was 100 g. Intrusiveloads were applied at positions a between the cen-tral incisors) through f distal to the second pre-


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Fig. 8. Graphic representation of the resul ts obtained using the laser reflection technique for the two-teeth, four- teeth, and six-teeth segmen ts with respectively appl ied loads of 50 g, 100 g, and 200 g.

molars). It could be observed that the density of thefringe patterns decreased from the loading positionsa through c. At position d, no fringes wereobserved on the aluminum plate; this implies that theintrusive force passed through the center of resistance.The number of fringes increased from the loading po-sitions e through f. These findings concurred withthe findings obtained using the laser reflection techniquelisted in Table I and displayed graphically in Fig. 4.Six-anterior-teeth segment: Splint 3

The splint connected the six upper anterior teethand the magnitude of the applied force ~8s 200 g.Intrusive forces were applied at all positions from athrough f. The density of the fringe pattern decreasedfrom loading positions a to d. At position e,no fringes were seen. A fringe pattern was visib le atloading position f. This corresponded to the findingsof the laser reflection study. Only with intrusive forcesapplied distal to the second premolars could pure ver-tical displacement of the splint be achieved.DISCUSSION

The dentition and bone comprise a functional unitresponsive to orthodontic forces. Bone deformation cancertainly influence the location of the center of resis-tance relative to the cranium. Therefore, it is importantto recognize that the center of resistance of the dentitioncannot be thought of in isolation of its anatomic rela-tionships in any study of this nature.

The laser reflection and holographic interferometric

techniques are very useful tools to measure the initialdisplacements of a body subjected to an applied load.The resul ts obtained with the laser reflection methodclearly demonstrate that varying the levels of the intru-sive forces applied to the anterior teeth causes little, ifany, shift of the center of resistance for a rigidly con-nected segment irrespective of the number of sym-metric units of the upper anterior dentition consoli-dated). The application of greater magnitudes of intru-sive forces at the same point of application should, asexpected, result in similar displacement characteristicsof the dentoskeletal unit; however, in practice the great-est limitation in applying high-force levels to the den-tition is imposed by the biologic response to such forces,which may comprise tissue destruction, root resorption,and possible loss of anchorage.

The holographic study, independent of the laser re-flection technique, demonstrated the same location forthe centers of resistance for the various units studied.In addition, the posterior movement of the centers ofresistance as a result of the incorporation of more teethinto the splint was seen using both techniques. Thissuggests that either of the methods described here maybe used reliably to study tooth displacement.

Fig. 8 demonstrates the results obtained using thelaser reflection technique for the two-teeth, four-teeth,and six-teeth segments with applied loads of 50 g,100 g, and 200 g, respectively. Displacements of spec-kles are plotted on the horizontal axis; points of forceapplication are plotted on the vertical axis. A positivefigure on the horizontal axis refers to a counterclock-


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Center of resistance o f anter ior teeth dur ing intrusion 219

wise rotation of the anterior unit and a negative figureto a clockwise rotation. The intersection points of thegraph with the vert ical axis refer to the points of ap-plication of forces that result in a vertical translation ofthe unit.The center of resistance displaces posteriorly asmore teeth are incorporated into the segment. The sh iftbetween the two-teeth unit and the four-teeth unit is22 mm. However, substantial posterior shift of thecenter of resistance is seen when the canines are addedto the four-teeth unit 7 mm = a premolar width). Thismay be caused by the bending of the bony structuressurrounding these teeth.

It is realized that the center of resistance of a bodycan vary as a function of the various anatomic rela-tionships of the dentoskeletal unit and biologic factorssuch as the physiologic reaction of living tissue toforces, the degree of humidity of bone, changes in axialinclination of the anterior teeth, and varying tooth sizes.In all probability, these factors may have some effecton the position of the center of resistance and wereexcluded from this experiment.

However, the importance of this type of researchlies in the clues it may provide in establishing the natureof the response of the dentition to applied forces whenloading conditions and anatomy are controlled. Theapplication of the optimal force system in ortho-dontics may then become a reali ty if the results ofin vitro studies are correlated with in vivo animal andclinical studies.SUMMARY AND CONCLUSIONS

The goal of this study was to define the location ofthe centers of resistance of various units of the anteriorsegments to applied intrusive loads by means of boththe laser reflection and holographic interferometry tech-niques. Tooth displacements were measured on a dryhuman skull.

One must be judicious in directly applying the re-sults obtained from investigat ions using dry humanskulls to study the displacement characteristics of thedentoskeletal unit in the clin ical situation. However,such models have the advantage of simulat ing thein vivo system; for instance, the relationship of thevarious anatomic structures are maintained intact and,more important, force variables can be controlled, keep-ing the anatomic geometries the same.1. For an anterior segment comprising two centralincisors, the center of resistance was located on a pro-jection line parallel to the midsagittal plane on a pointsituated at the distal half of the canines.

2. For an anterior segment that included the four

incisors, the center of resistance was situated on a pro-jection line perpendicular to the occlusal plane betweenthe canines and first premolars.

3. For a rigid anterior segment that included the sixanterior teeth, the center of resistance was situated ona projection line perpendicular to the occlusal planedistal to the first premolar.

4. The centers of resistance of the anterior segmentsincorporating two or four anterior teeth were within If: 2mm of each other. However, inclusion of the caninesin the anterior segment resulted in the center of res is-tance moving dista lly by approximately one premolarwidth 7 mm). This effect may have been the result ofthe resistance of bony structures at the level of thecanines and some bending of the maxi llary complex aswas observed on the holograms.

5. No appreciable shift in the location of the centersof resistance of the various segments studied was de-tected as varying magnitudes of intrusive force wereapplied.

The authors would like to thank ProfessorP. Boone andJ. Morton for their valuable suggestionsduring the courseofthis study, and Barbra Rich for her secretarialassistance.REFERENCES1. DuterlooHS: Extra-oraleraktie.University f Groningen, T he

Nether lands, 1981.2. Hocevar R: Understanding, planning, and managing tooth move-ment: Orthodontic force sys tem theory. AM J ORTHOD 80: 457-477, 1981.

3. Chr ist iansen RL, Burstone CJ: Centers of rotation within theper iodontal space. AM J ORTHOD 55: 353-368, 1969.4. Nikolai RJ: Analytical mechanics and analysis of or thodontictooth movements . AM J ORTHOD 82: 764-766, 1982.

5. Burstone C J: Application of bioengineering to clinical ortho-dontics. In Graber TM , Swain BF (edi tors): Orthodontics, cur-rent principles and techniques. S t. Louis, 1985, The C. V. Mos byCompany, pp 193-229.6. Burstone CJ, Pryputniewicz RJ: Holographic determination ofcenters of rotation produced by orthodontic forces. AM J ORTHOD77: 396-409, 1980.7. Smith R J, Burstone CJ: Mechan ics of tooth movem ent. AM JORTHOD 85: 294-307, 1984.

8. Liu SV, Herschleb CW: Control led movement of maxi l lary in-cisors in the Begg technique. AM J ORTHOD 80: 300-315, 1981.9. Burstone CJ : The rationale of the segmented arch. AM J ORTHOD11: 805-822, 1962.

10. Burstone CJ: The mec hanics of the segmen ted arch technique.Angle Orthod 36: 99-120, 1966.11. Nanda R: The di fferential diagnosis and treatment of excessiveoverbi te. Dent Cl in North Am 25: 69-84, 1981.12. Dermaut LR, Vanden Bulcke MM: Evaluation of intrusive me-chanics of the type segmental arch on a macerated humanskull using the laser reflection technique and holographic inter-ferometry. AM J ORTHOD 89: 251-263, 1986.13. Di jkman J FP: Distr ibution of forces in orthodontic treatment.Thesis, Universi ty of Ni jmegen, 1969.


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Dermaut LR, Beerden L: The effects of Class II elastic force ona dry skul l measured by holographic interferometry. AM J OR-T H OD79: 296-304, 1981.Pavl in D, V ukicevic D : Mechanical reactions of facial skeletonto maxi l lary expansion determined by laser holography. AM JORTH OD 3: 498-507, 1984.Pryputniewicz RJ, Burstone C J: The effect of t ime and forcemagnitude on orthodontic tooth mo vemen t. J Dent Res 58: 1754-1764, 1979.Kragt G: Initial dentofacial orthopedic reactions. A holographicstudy. Thesis, Universi ty of Groningen, 198 1.Wedendal PR , Bjelckhagen HI: Dental holographic interferome-try in vivo, uti liz ing a ruby laser syste m. Acta Odontol Stand32: 345-356, 1974.Burstone CJ, Pryputniewicz RJ, Bowley WW : Holographic mea-surement of tooth mobi l i ty in three dimensions. J Per iont Res13: 283-294, 1978.

20. Ryden H: The laser reflection method: Measurements of toothmovem ents. Thesis, Universi ty of Stockholm, 1980.

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Retwint requests to:

Stockl i P, Teuscher U: Combined activator headgear orthope-dics. In Graber TM, Swain BF (edi tors) : Orthodontics, currentpr inciples and techniques. St. Louis, 1985, The C. V. MosbyCompany.

Am. J. Orthod. Dentofac. Orthop.September 1986

Marc Vanden BulckeKl iniek voor Tand-, Mond- en KaakziektenAfdeling OrthodontieAkademisch ZiekenhuisDe Pintelaan 135B-9000 GhentBelgiumRohit SachdevaDepartment of OrthodonticsUniversi ty of Connecticut

Health CenterFarmington, CT 06032

1986 Vanden Bulcke Imprtant

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