European Journal of Onhodotuta 14(1992) 162-171 © 1992 European Orthodonuc Society

No posterior mandibular displacement in Angle Class II,

division 2 malocclusion as revealed with electromyography

and sirognathography

Urs Thiier, Bengt Ingervall, Walter Biirgin, and Arthur DemischOrthodontic Clinic, University of Bern, Switzerland

SUMMARY The activity of the anterior and posterior temporal, and of the masseter muscles wasstudied by electromyography and the position of the mandible by sirognathography. Therecordings were made in 22 children, aged 8-13 years, with Angle Class II, division 2 malocclusionbefore and during treatment of their malocclusion. The treatment comprised two phases:proclination of the upper incisors and bite raising with a removable plate, and the subsequentcorrection of the distal occlusion with an activator. The aim of the study was to reveal signs ofanterior mandibular positioning during the treatment.

The electromyographic recordings were made in the rest position of the mandible, and duringmaximal biting, chewing, and swallowing. The sirognathographic recordings comprised thepositions of the mandible at rest, at intercuspation, and during tooth contact during chewingand maximal mandibular movements.

The muscle activity at rest was unchanged during the period of observation. The activityduring maximal biting, chewing, and swallowing decreased during the phase of proclination,which was interpreted as a result of occlusal instability. The positions of the mandible at rest, atintercuspation, and during chewing were stable during the treatment. Neither the electromyo-graphic recordings nor the recordings of mandibular positions revealed any signs of anteriormandibular positioning during the treatment of the Class II, division 2 malocclusion.

Introduction

The question of a possible posterior mandibulardisplacement in Angle Class II, division 2 mal-occlusion is controversial. The morphologicalcharacter of this type of malocclusion (retroclin-ation of the upper incisors in combination witha deep bite) invites such an assumption. Swann(1954) stated that one-third of cases with thismalocclusion have a posteriorly displacedmandible in the intercuspal position. Based oncephalometric findings during orthodontic treat-ment, Erickson and Hunter (1985) concludedthat in about one-quarter of their sample themandible may have repositioned anteriorly dur-ing the treatment. On the other hand, Gianellyet al. (1989) found no difference in the positionof the mandibular condyles betweer Oiss IImalocclusion cases with steep incisors and adeep bite, and Class II cases without thesecharacteristics. There was, therefore, no evid-ence that the steep upper incisors, in combina-

tion with the deep bite, had forced the mandiblebackwards on closure into the intercuspalposition.

Subjects with Angle Class II, division 2 mal-occlusion have been found to have a largerdistance between the retruded and intercuspalmandibular positions than individuals with nor-mal occlusion (Ingervall, 1968). This is at vari-ance with the theory of a posterior mandibulardisplacement. If the mandible was posteriorlydisplaced in the intercuspal position, a furtherlarge retrusion would be unlikely.

In a previous nvestigation (Demisch et al.,1992), we made longitudinal recordings of therelationship between the retruded and intercus-pal mandibular positions in children with ClassII, division 2 malocclusion during their ortho-dontic treatment. The results corroborated theearlier findings by Ingervall (1968) of a largedistance between the two mandibular positionsbefore the start of the treatment. The develop-

M A N D I B U L A R DISPLACEMENT IN ANGLE II, DIVISION 2 163

merit of the distance between the two mandib-ular positions during the treatment presentedno evidence of an anterior mandibular reposi-tioning when the upper incisors were proclinedor during the subsequent treatment of the distalocclusion with an activator.

In the sample studied, the activity of thetemporal and masseter muscles was alsorecorded with electromyography, and positionsand movements of the mandible with siro-gnathography. Such recordings have previouslybeen made during the treatment of childrenwith Angle Class II, division 1 malocclusionwith an activator in order to reveal a possibleforward mandibular positioning induced by theactivator treatment (Ingervall and Bitsanis,1986; Thuer ex ai, 1989; Ingervall and Thuer,1991). No signs of an anterior mandibular posi-tioning during the treatment of Class II, division1 malocclusion were found.

The present study reports the results of theapplication of these methods of investigation inthe treatment of children with Class II, division2 malocclusion. A change in the muscle activityin the rest position of the mandible or duringbiting, chewing, and swallowing during thetreatment could be indicative of mandibularrepositioning. The same would be true if sirog-nathographic recordings revealed a change inthe rest position of the mandible or in theposition of tooth contact during chewing, aswell as of mandibular border movements duringthe course of the treatment.

Subjects and methods

The study included the same 22 children (11boys and 11 girls) that took part in the previousinvestigation (Demisch et ai, 1992). Their agevaried between 8 years, 2 months and 12 years,8 months (median age 10 years, 2 months). Thechildren were selected from among those enrol-ling for treatment at the Orthodontic Clinic,University of Bern. They had an Angle ClassII, division 2 malocclusion and fulfilled thefollowing criteria: a bilateral distal occlusionwith retroclined upper incisors and a largeoverbite, and no symptoms or signs of mandib-ular dysfunction.

The treatment protocol as well as the resultsof the treatment are described in detail in theprevious report (Demisch et at, 1992). Briefly,the treatment comprised three phases. In the

first phase, the upper incisors were proclinedand the deep bite was corrected with an upperremovable plate. In the second phase, the distalocclusion was corrected with an activatoraccording to Herren (Herren, 1980; Demisch,1984). The third phase was a phase of retention,which was accomplished with a retention activ-ator. All 22 children completed the first phaseof the treatment. The treatment was then discon-tinued in one child because of lack of co-operation.

Electromyography (EMG)

The activity of the right anterior and posteriortemporal muscles, the right masseter muscle,and the left posterior temporal muscle wasrecorded with a DISA electromyograph withdirect (15C04) and mean voltage (31C17) chan-nels. The lower and upper frequency filters wereset at 20 and 1000 Hz, respectively. The EMGsignals were displayed on an electrostatic writer(Gould ES 1000). Bipolar hook electrodes(Ahlgren, 1967) were used. The placement ofthe electrodes was as described earlier (Ingervalland Thilander, 1974; Ingervall and Egermark-Eriksson, 1979).

Recordings were made in the rest position ofthe mandible (calibration 50 /iV/division, paperspeed 5 cm/sec), during maximal bite in theintercuspal position (calibration 200 /zV/divi-sion, paper speed 5 cm/sec), and during chewingand swallowing of apple and peanuts (calib-ration 200 /iV/division paper speed 10 cm/sec).

The sequence of the recordings was as follows:

(1) in the rest position of the mandible;(2) during chewing and swallowing of apple;(3) during chewing and swallowing of peanuts;(4) during maximal bite in the intercuspal

position;(5) a new recording in the rest position.

For the chewing tests, the child was given astandard piece of apple or three peanuts, andasked to chew and swallow in the habitual way.

Electromyographic recordings were made ontwo occasions (median time 27 and 22 days)before the start of the treatment. The nextelectromyographic recording was made at theend of phase one of the treatment, i.e. afterproclination of the upper incisors and correctionof the deep bite (median time 207 days afterthe start of the treatment). A fourth and fifth

164 U. THUER ET AL.

electromyographic recording was made 3 and12 months, respectively, after the start of theactivator treatment, which was initiated imme-diately on completion of phase one of thetreatment.

Analysis of the EMG recordingsThe muscle activity in the rest position wasdetermined by measuring the mean voltageamplitude when the muscle activity was minimalduring at least 5 seconds. For determination ofthe muscle activity during maximal bite, themean voltage amplitude of the characteristicactivity was measured.

The following measurements were made ofthe muscle activity during chewing (mean of sixrandomly selected chewing cycles during the actof chewing):

(1) maximal mean voltage amplitude and dura-tion of the activity during the closing phaseof the chewing cycle;

(2) characteristic mean voltage amplitude dur-ing the opening phase of the chewing cycleand duration of the opening phase.

The muscle activity during swallowing was ana-lysed by measurement of the maximal meanvoltage amplitude and duration of activity. Inthe further analyses, the activity in the rightand left posterior temporal muscles, as well asthe two recordings of the rest position activitywere averaged.

Recording of mandibular positions andmovementsThe positions and movements of the mandiblewere recorded with a Sirognathograph (SiemensAG, Bensheim, Germany), which registered thelocation of a magnet fastened at the labial foldinferior to the lower central incisors. The mag-net was incorporated in a custom-made acrylicholder glued to the labial surfaces of the teeth(Fig. 1). The position of the holder was individu-alized so as not to interfere with the occlusionor disturb mandibular movements. For eachsubject, the same holder was used throughoutthe period of investigation.

With the Sirognathograph, changes in posi-tion of the magnetic field are recorded by meansof an antenna system attached to the head witha face-bow (Fig. 2). The antennae convert thechanging magnetic field into electrical signalsthat are fed into a computing unit. The signals

Figure 1 Magnet on lower incisors for sirognathographicrecording.

Figure 2 Subject with antennae for sirognathographicrecording.

are processed into analog voltages representingmovement in three planes as the axes of a three-dimensional co-ordinate system.

The positions and movements of the magnetwere simultaneously recorded in the sagittal andfrontal planes with the aid of two x/y plotters.Simultaneously, the analog signal from theSirognathograph was digitized and stored in acomputer for subsequent analyses. During therecordings, the subject sat upright with his orher head in natural balance without the use ofa headrest. To help keep the head at rest duringthe recordings, a plumb-line bob was hung fromthe ceiling in front of the subject. The weighthelped an observer to check the head position.

During the period of observation, majorchanges in the position of the teeth and in theintermaxillary relationship were accomplishedby the treatment. Two different reference sys-tems were therefore used. Before the start ofthe treatment and during the phase of proclin-

M A N D I B U L A R DISPLACEMENT IN ANGLE II, DIVISION 2 165

ation of the upper incisors, the positions andmovements of the magnet at the lower incisorswere referred to a magnet placed in the deepestpart of the palate (procedure I). This magnetwas incorporated in an acrylic plate that fittedthe upper jaw and was unchanged during thefirst phase of the treatment. At the start of eachrecording session, the acrylic plate was placedin the mouth and the signals from the magneton the plate were recorded in the computingunit as the zero point for the subsequent record-ings from the magnet in the lower jaw. Afterthis calibration, the magnet in the upper jawwas removed and replaced by the magnet in thelower jaw. The locations of the magnet in thelower jaw thus always refer to the location ofthe magnet in the upper jaw, which was station-ary during the period of observation.

During the phase of activator treatment theedge-to-edge position of the upper and lowercentral incisors was used as a reference positionfor the determination of the location of themandible in the other positions and movements(procedure II). Each recording session whenprocedure II was used (phase of activator treat-ment) commenced with the recording of theposition of the magnet at the lower incisorswhen the mandible was in the edge-to-edgeposition. This procedure was identical with apreviously described method (Thuer et al.,1989).

Recordings and measurements with procedure I

In procedure I, after the establishment of thereference point, the following recordings weremade in the order given:

(1) mandibular position in intercuspation;(2) mandibular position at rest;(3) movements and positions of the mandible

during chewing of apple;(4) new recordings of the mandibular position

at rest.

The recordings displayed on the x/y plotterswere used as visual control while the actualmeasurements were made with a cursor on thegraphic display of the computer. The positionsof the mandible in intercuspation and duringtooth contact while chewing were measured asx and y co-ordinates in relation to the zeropoint of the reference system (Fig. 3). Eachchewing cycle during the act of chewing (untilswallowing) was analysed. Those with a closing

distance within 0.1 mm of the minimal value(indicating tooth contact) were chosen, meas-ured, and averaged. The position of the mand-ible in the rest position (x and y co-ordinates)was measured in relation to the mandibularposition in intercuspation.

Recordings were made in 20 children on twooccasions (median time 27 and 22 days) beforethe start of the treatment, and thereafter 3months after the start of the treatment and atthe end of the phase of proclination of theincisors. The deep bite and the position of theupper incisors made the placement of the mag-net at the lower incisors impossible in two cases.The results of the two recordings made beforetreatment as well as of the two recordings madein the rest position at each recording sessionwere averaged.

Recordings and measurements with procedure II

The recordings and measurements with proced-ure II were the same as in an earlier study(Thuer et al., 1989), where the method has beendescribed in detail.

The following recordings were made at thestart of the treatment with an activator andafter 3 and 12 months of activator treatment:

(1) mandibular position in incisor edge-to-edgeprotrusion and the movement from this intothe intercuspal position;

(2) maximal movements of the mandible (open-ing, laterotrusion, protrusion);

(3) mandibular position at rest and the move-ment from the rest position into the intercus-pal position;

(4) movements and positions of the mandibleduring chewing of apple;

(5) new recording of the mandibular positionat rest.

All measurements in the sagittal and frontalplanes were made on the graphic display of thecomputer. In principle, the measurementsincluded the determination of the overjet andthe overbite, the size of the maximal movementsof the mandible, the free-way space, and theanteroposterior position of the mandible in therest position, the point of tooth contact in theanteroposterior direction during chewing, anytransverse deviation of the mandible during max-imal opening or in the rest position. The samerules as for procedure I for the measurement of

166 U. THUER ET AL.

(a)

(c)

(b)

(d)

Figure 3 (a) x and >>-co-ordinates for determination of the mandibular position at intercuspation with procedure I. (b)and (d) Magnitude of freeway space (y) and distance between intercuspal and anteropostenor position of the mandible inthe rest position (x) as measured with procedures I and II, respectively, (c) Distance between edge-to-edge and intercuspalmandibular positions in horizontal (.t) and vertical (y) planes recorded with procedure II.

the chewing cycles and the averaging (chewingand rest position recordings) were applied.

Recordings during activator treatment weremade in 15 children at the start of this phaseof treatment and, thereafter, at 3 and 12 months.

Statistical methods

Accidental errors of the method (si) were calcu-lated with the formula

d2si= ,

V Inwhere d is the difference between repeatedobservations.

Differences between recordings on consec-utive occasions were tested with Wilcoxon'smatched-pairs signed-ranks test.

ResultsElectromyography

The two electromyographic recordings madebefore treatment were tested for systematicdifferences with Wilcoxon's matched-pairs sign-ed-ranks test. Only two variables (the amplitudeof the masseter muscle in the opening phaseduring the chewing of apple and the amplitudeof the anterior temporal muscle during the chew-ing of peanuts) differed significantly between thetwo recordings. No other significant differenceswere found. The results of the two recordingsmade before the start of the treatment were,therefore, averaged for the further analyses.

The results of a subsequent electromyo-graphic recording were tested for significantdifferences from the results of the previous

MANDIBULAR DISPLACEMENT IN ANGLE II, DIVISION 2 167

recording (consecutive recordings) withWilcoxon's matched-pairs signed-ranks test. Nosignificant differences were found for the muscleactivity in the rest position of the mandible orin the amplitude during the opening phase ofthe chewing cycle.

Significant differences between subsequentrecordings during the first phase of the treat-ment were found for the amplitude during max-imal bite and during the closing phase of thechewing cycle as well as during' swallowing(Table ' ) . In addition the activity in the poster-ior temporal muscle during maximal biteincreased (median difference 66 /̂ V) from3 to 12 months after the start of treatmentphase 2.

There were no significant differences between

consecutive recordings in the duration of theclosing or opening phases during chewing ofapple and only one during chewing of peanuts.Thus, the duration of the closing phase in theposterior temporal muscle increased from amedian value of 355 msec at 3 months to375 msec at 12 months after the start of theactivator treatment.

There were no significant changes with timein the duration of the activity during swallowingof peanuts, but during the swallowing of applethe duration increased in the anterior temporalmuscle from a median value of 530 msec to695 msec, and in the masseter muscle from440 msec to 545 msec from 3 to 12 months afterthe start of the activator treatment (phase twoof treatment).

Table 1 Median and amplitude (in /JV) and range for the muscle activity in the rest position during maximalbite and during the closing phase of the chewing cycle as well as during swallowing.

Rest positionAnt.temporal m.Post.temporal m.Masseter m.

Maximal biteAnt.temporal m.Post.temporal m.Masseter m.

Chewing of appleAnt.temporal m.Post.temporal m.Masseter m.

Chewing of peanutsAnt.temporal m. 'Post.temporal m.Masseter m.

Swallowing of appleAnt.temporal m.Post.temporal m.Masseter m.

Swallowing of peanutsAnt.temporal m.Post.temporal m.Masseter m.

•0.01 < P < 0.05;

Before t

Median(

2.4110.3

427360181

322278183

377330229

24421399

244263149

••0.001 <

reatment

Rangen = 22)

0-7.14.1-21.6

0-5.7

61-742144-66733-486

170-684121-54349-530

206-751162-64490-548

97-438104-47422-533 •

77-877111-524

0-399

P < 0.01, ***

After proclination

Median

26.80.7

188***2 0 2 " *

88

270214*147

307*2 7 0 " *182.

138**174*55

170"166**66

P < 0.00

Rangei = 22)

0-10.81.4-32.4

0-8.1

88-57744-449

9-525

122-435105^450"51-370

98-624153-44748-411

33-42238-43822-288

22-52261-38822-330

After 3 monthsof activatortreatment

Median Range

261.4

186234

88

235 •239108

292242124

16617744

14919430

1; indicates significant

(" = 21)

0-8.81.4-14.9

0-8.1

27-66641-44711-433

114-750124-517 •

8-375

61-1027156-666

19-668

11-24433-2940-411

44-65566-4220-466

difference from

After 12 monthsof activatorstreatment

Median

('

2.77.11.4

212235*144

202273130

230291172

12720861

15526644

previous t

Range, - 2 1 )

0-8.11.4-39.2

0-5.4

88-585111-588

5-570

98-826113-59741-470

133-933149-50244-788

44-84477-4330-433

44-82266-3880-411

•ecording.

168 U. THfJER ET AL.

Positions and movements of the mandible

Only one variable (the vertical position of thepoint of tooth contact during chewing) recordedwith sirognathographic procedure I differed sig-nificantly between the two recordings madebefore the start of the treatment. The accidentalerrors of the method in the determination ofthe anteroposterior and vertical positions of themandible at intercuspation were 1.38 and1.40 mm, respectively. The corresponding valuesfor the position of tooth contact during chewingwere 1.55 and 1.12 mm, and for the rest position0.77 and 1.35 mm.

No significant differences were found whenconsecutive recordings made before the start ofthe treatment and during the phase of proclin-ation of the upper incisors were tested withWilcoxon's matched-pairs signed-ranks test(Table 2). Thus, the intercuspal position of themandible, like the position of tooth contactduring chewing and the rest position remainedstationary during the phase of proclination ofthe incisors. There was a numerical, though notsignificant, increase in the vertical position atintercuspation during the period of treatment.This is an effect of the bite-raising that wasachieved with the orthodontic appliance duringthis phase of the treatment. The free-way spaceremained constant, however. On average, themandible in the rest position was positioned

posterior of the intercuspal position. In onlyone individual was the mandible at rest situatedanterior to the intercuspal position before thestart of the proclination. After 3 and 12 monthsof proclination, four and seven subjects, respect-ively, had a mandibular rest position anteriorof the intercuspal position.

The size of the border movements of themandible (maximal opening, protrusion, andlateral movements) did not change significantlyduring the- phase of activator treatment asrevealed by the sirognathographic recordings ofprocedure II. The same was true for the positionof the mandible in the frontal plane duringmaximal opening and at rest. The distancebetween the edge-to-edge position of the mand-ible and the intercuspal position, i.e. the overjet,decreased from the start of the activator treat-ment to the second observation and from thesecond to the third recording session (Table 3).This is the improvement of the overjet broughtabout by the activator treatment. The overbite,like the position of the mandible at rest, and attooth contact during chewing, remained con-stant during the period of observation (Table 3).At the start of the activator treatment sevensubjects had a rest position anterior to theintercuspal position. This proportion remainedconstant, with eight and seven subjects, respect-ively, at the recordings at 3 and 12 months afterthe start of the activator treatment.

Table 2 Median position and range (in mm) of the mandible at intercuspation and during tooth contactduring chewing. The table also shows the position of the mandible at rest relative to the intercuspal position.

Position at intercuspationrelative to the zero pointof the co-ordinate system

AnteroposteriorVertical

Difference between the toothcontact during chewing andthe intercuspal position

AnteroposteriorVertical

Difference between intercuspaland rest positions

AnteroposteriorVertical (freeway space)

Before treatment

Median

-14.512.9

0.1-0.4

1.74.5

Rangen = 20)

-19.9--5.36.9-18.7

-1.5-1.4-2.9-1.4

-0.3-4.22.4-6.7

After 3months of

Median

-14.614.0

0.6-0.6

1.34.1

proclination

Range= 20)

-22.0--8.15.6- 18.8

-2.7-7.1-5.0-1.8

-2.1-4.42.2-6.4

After proclination

Median Range(n = 20)

-15.014.0

0.6-0.5

1.24.4

-22.0--10.09.4-21.4

-5.2-3.3-3.9-2.0

-6.1-5.81.6-7.3

MANDIBULAR DISPLACEMENT IN ANGLE II, DIVISION 2 169

Table 3 Median position (in mm) and range, of the mandible at intercuspation and the position of themandible at rest and at tooth contact during chewing in relation to the intercuspal position.

Position at intercuspationrelative to the zero pointof the co-ordinate system

AnteroposteriorVertical

Difference between intercuspaland rest position

AnteroposteriorVertical

Difference between intercuspalposition and position of toothcontact on chewing

AnteroposteriorVertical (freeway space)

Start of activator treatmenl

Median( i

5.53.7

1.04.5

0.40.2

Range= 15)

2.3-8.11.6-5.8

-3.7-5.11.8-10.1

-0.6-2.0-2.3-1.3

After 3 months of activator[ treatment

Median('

2.6"3.2

-0.34.9

0.10.0

Rangei=15)

1.3-4.91.6-7.0

-3.3-3.42.7-6.6

-4.9-1.5-1.4-1.2

After 12 months of activatortreatment

Median(i=15)

1.6"3.3

1.04.4

0.20.2

Range

0.7-3.31.1-5.0

-1.2-3.32.0-6.1

-1.0-2.2-0.5-1.9

' 0.001 < P < 0.01; indicates significant difference from previous recording.

Relationship between the retruded andintercuspal mandibular positions, andelectromyographic and sirognathographicvariables

In the subjects studied, the relationship betweenthe retruded (RCP) and intercuspal (ICP) man-dibular positions was recorded with a gnatho-thesiometer. The results of these recordings havebeen reported previously (Demisch et al., 1992).The anteroposterior difference between the twopositions was found to decrease from the startof the treatment until the end of the activatortreatment and to increase again somewhat dur-ing the phase of retention after the treatment.

In order to evaluate whether the anteropos-terior difference between RCP and ICP, and thechange in this distance during the period ofobservation were related to results of the electro-myographic and sirognathographic recordings,rank correlation coefficients of Spearman werecalculated. The variables examined were theanteroposterior distance between RCP and ICP,the activity of the temporal and massetermuscles in the rest position, and the anteropos-terior position of the mandible at intercusp-ation, in the rest position and during toothcontact while chewing. Coefficients of correla-tion were calculated between the variables men-tioned as recorded before treatment, after

proclination of the upper incisors, at the startof the activator treatment and after 12 monthsof activator treatment as well as between thechanges in these variables between the occasionsmentioned.

There were no significant correlationsbetween the anteroposterior RCP-ICP distanceand the other variables before or after theproclination of the upper incisors. Nor werethere any significant coefficients of correlationfor the changes of these variables during phaseI of the treatment. There was also no significantcorrelation between the muscle activity and themandibular positions recorded with sirogna-thography.

After 12 months of activator treatment, therewas a correlation (rho = 0.59, 0.01

170 U. THUER ET AL.

0.05) anterior positioning of the mandible atintercuspation in six subjects. Three of thesechildren also chewed in a more anterior positionat the end of the phase of proclination, butthree still used their old, more posterior mandib-ular position. It may be assumed that the threechildren who still used their original mandibularposition for chewing occupied a slightly pro-truded convenience position at closure to inter-cuspation from the rest position. In fact, oneof these children had the greatest anterior repo-sitioning of the mandible at rest.

Of the three children who both positionedtheir mandible more anteriorly at intercuspationand also chewed in a more anterior positionone had a very large (> 4 mm) distance betweenRCP and ICP. It may be assumed, therefore,that the anterior positioning of the mandible atintercuspation and chewing in this child alsorepresents an anterior convenience position. Theremaining two children had small RCP-ICPdistances both before and at the end of thephase of proclination. In these two children,therefore, the more anterior position of themandible at intercuspation and chewing at theend of phase I of the treatment is unlikely tobe a convenience protrusion, and may indicatea true mandibular repositioning after proclin-ation of the upper incisors.

Discussion

During the period of observation no significantdifference was found in the muscle activity in therest position of the mandible for any of themuscles studied. The results are, therefore, inline with previous findings during the recordingof the muscle activity during the treatment ofAngle Class II, division 1 malocclusion with anactivator (Ingervall and Thuer, 1991). This resultsuggests that the rest position of the mandibleremained stable both during the phase of proclin-ation of the upper incisors and during the follow-ing phase of correction of the distal occlusionwith an activator. The recordings of the muscleactivity in the rest position, therefore, gave noevidence of an anterior repositioning of themandible during any of the phases of treatment.

During the phase of proclination of the upperincisors, there was a simultaneous bite raisingaccomplished by the removable acrylic plate withits anterior bite plane. During this phase of thetreatment, the activity of the anterior and poster-

ior temporal muscles decreased during maximalbite, during chewing and swallowing. This isprobably a result of occlusal instability inducedby the bite raising and wearing of the appliance.A stable occlusion has been shown to be aprerequisite for maximal muscle activity duringbiting (Ingervall and Egermark-Eriksson, 1979;Ingervall et ai, 1979; Bakke and Moller, 1980)and an improvement in the occlusion has beenshown to result in an increase of the maximalmean voltage amplitude during chewing (Ingerv-all and Egermark-Eriksson, 1979; Ingervall etai, 1979; Bakke et ai, 1982; Ingervall and Carls-son, 1982; Moller et ai, 1984). The oppositeeffect was shown to result from the introductionof an experimental interfering occlusal contact(Riise and Sheikholeslam, 1984).

The duration of the muscle activity duringswallowing of apple increased during the activ-ator treatment. This is probably also a con-sequence of occlusal instability. Similar effectson the amplitude during biting, chewing andswallowing were found in the previous study ofthe muscle activity during treatment of AngleClass II, division 1 malocclusion (Ingervall andBitsanis, 1986). The activity during maximalbiting was also studied in children with ClassII, division 1 malocclusion being treated withan activator or with headgear as well as duringtreatment of children with Class I malocclusion(Ingervall and Thuer, 1991). A decrease in theactivity during the course of the treatment wasalso found in these groups. In both studiesmentioned the decrease was also taken to be aneffect of occlusal instability.

There was, on average, no significant changein the position of the mandible during the phaseof proclination of the upper incisors either atintercuspation or in the rest position or duringchewing. The sirognathographic recording,therefore, did not reveal any signs of anteriorrepositioning of the mandible during this phaseof the treatment for the group. The analysis ofindividual cases showed, however, that a reposi-tioning of the mandible may sometimes havetaken place as exceptions from the general trend.

The results of the sirognathographic record-ings during the phase of activator treatmentlargely agreed with those of the previous studyof children with Class II, division 1 malocclu-sion (Thuer et ai, 1989), i.e. a successive reduc-tion of the overjet with an unchanged overbite,as well as a stable anteroposterior position at

MANDIBULAR DISPLACEMENT IN ANGLE II, DIVISION 2 171

rest and unchanged position during chewing inrelation to the intercuspal position. In neither ofthe two investigations was there a decrease inthe range of maximal protrusion of the mandibleduring the activator treatment. It has beenrecommended that the range of maximal protru-sion be measured during the treatment and hasbeen claimed that a reduction is indicative of ananterior mandibular repositioning (Petit andChateau, 1984). In summary, neither the sirogna-thographic recordings during the phase of pro-clination nor those during activator treatmentrevealed any signs of anterior mandibular posi-tioning except possibly in isolated individuals.

The correlation analysis of the present studydid not reveal any relation between the RCP-ICP distance and the muscle activity at rest orwith the sirognathographic variables duringphase I of the treatment. The results of thefunctional recordings, therefore, do not favourthe interpretation of a large RCP-ICP distanceas a sign of persistent anterior mandibularpositioning at intercuspation. It is more likelythat the large RCP-ICP distance recorded in afew subjects at the end of the phase of proclin-ation and bite raising (Demisch et ai, 1992) isexplicable as a convenience protrusion of themandible at intercuspal wax bite recording inthese individuals.

A relationship between the anteroposteriorRCP-ICP distance and the overjet was foundat the recording 12 months after the start ofthe activator treatment. This is probably aneffect of less advanced progress of the treatmentwith remaining excessive overjet in some chil-dren (12 children still in the active phase ofactivator treatment).

Address for correspondence

Dr Urs ThuerKlinik fur KieferorthopadieFreiburgstrasse 7CH-3010 BernSwitzerland

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