a pilot study of the effect of masticatory muscle training ... · for the chewing test, the child...

C 19*7 European Orthodontic Society

A pilot study of the effect of masticatory muscletraining on facial growth in long-face children

Bengt Ingervall and Elias BitsanisOrthodontic Clinic, University of Bern, Switzerland

SUMMARY Daily chewing of a tough chewing material consisting of resin from a pine tree (Mastixfrom the island of Chios, Greece) was instituted in 13 children (aged 7-12 years) with long-facemorphology. The chewing exercise therapy was maintained for one year and aimed at revealingthe possibility of strengthening the masticatory muscles and influencing facial growth.

Masticatory muscle strength was monitored by measurement of bite force and electromyo-graphic recording of the activity of the anterior temporal and masseter muscles during biting andchewing. The facial morphology was recorded with profile cephalograms and dental casts.

During the one-year experimental period, there was a significant increase in bite force andmuscle activity during maximal bite. The change was already evident at the first control recording3 months after the start of the chewing exercise therapy. The muscle activity during chewing ofapple and peanuts did not change significantly.

The facial growth was characterized by anterior mandibular rotation in 9 of 12 cases while aposterior rotation occurred in 2 cases. The anterior rotation was, on average, 2.5 degrees and thusconsiderably greater than would be expected during normal growth. There were no signs ofreduced vertical growth of the maxilla, a reduced rate of molar eruption or increased growth of themandible.

Introduction

It is well-known that individuals with a so-calledlong-face morphology have weak facial muscles.The long-face morphology (dolichofacial type)is characterized by a large anterior face height(particularly lower face height), a small posteriorface height, a steep mandibular plane and a largegonial angle. This type of facial morphology pre-disposes to skeletal open bite. In the short-face(brachyfacial type) with the opposite facialcharacteristics (small anterior face height, flatmandibular plane, small gonial angle) the masti-catory muscles are strong.

The variation of masticatory muscle strengthwith facial type is known from measurements ofbite force in adults by, among other workers,Ringqvist (1973), Helkiroo and Ingervall (1978)and, recently, Profit et aJ. (1983). A tendency tolow bite force values in children with the long-face morphology was found by Proffit and Fields(1983). A corresponding variation of the activityof the masticatory musculature with facial typewas demonstrated electromyographically in

adults by Moller (1966) and in children byAhlgren (1966), Ingervall and Thilander (1974)and Ingervall (1976).

It is not known whether the facial morphologydetermines the strength of the masticatorymuscles or whether a strong musculature in-fluences the form of the face. Studies of youngadults with small and large bite forces revealed asmaller variation in the facial morphology in theindividuals with large bite force (Ingervall andHelkimo, 1978). This argues for the primaryimportance of the masticatory muscles in form-ing the face. Strong muscles form the face to aspecific type while the facial form in individualswith weak muscles varies more because themuscles exert less influence.

If the facial form is partly determined by thestrength of the masticatory muscles, this couldopen up therapeutic possibilities. Exercises aim-ing at strengthening the muscles of masticationcould have a beneficial influence on a growingface of the long-face type. Such an attempt wasreported by Spyropoulos (1985), who demon-strated a decrease of frontal open bite in children

16 BENGT INGERVALL AND ELIAS BITSANIS

after a prolonged period of chewing of a specialtough chewing material in order to strengthen themasticatory muscles. The present study was in-spired by the results of Spyropoulos and aims atfurther elucidating the possibilities of influencingthe form of the face in children with a tendencyto a long-face, by exercise with tough chewingmaterial. At the same time, the influence of thechewing exercises on the musculature was studiedby measurement of bite force and electromyo-graphic muscle activity.

Subjects and methods

Thirteen children (8 boys and 5 girls) were in-cluded in the study. Their ages at the start of theinvestigation varied between 7 years, 6 monthsand 12 years, 7 months (median age 9 years,4 months). They were selected on the grounds ofa facial morphology of the 'high-angle' type, i.e.the inclination of the mandible was large (a largeangle NSL/ML). The anterior face height waslarge and the gonial angle (angle RL/ML) wasalso large. They had a tendency to skeletalfrontal open bite and were in a period of activegrowth.

Morphological recordingsThe facial morphology of the sample as seenin profile cephalometric radiographs is given inTable 1. The reference points and lines used in

the cephalometric analysis are given in Figures 1and 2. The profile radiographs were taken withthe mandible in the intercuspal position. Table 1also gives the mean and standard deviation of thevariables measured in an unselected referencesample of 9-year-old children (73 boys and 77girls) with the same ethnic background as thepresent subjects (Feller, 1986).

Figure 1 Reference points used in the cephalometricanalysis.

Table 1 Mean and range of profile cephalometricvariables of the subjects studied and mean and stand-ard deviation of the same variables in 150 unselected9-year-old children (reference sample).

Variable

n-$-ar°s-n-ss°s-n-sm0

ss-n-srn0

NSL/NL0

NSL/OL0

NSL/ML0

NL/ML0

s-tgo/n-me x 100n-sp'/sp'-me x 100RL/ML0

n-s-gn°is-io (nun)ii-io (mm)

Present subjects

Mean

126.280.374.8

5.56.7

21.542.435.859.073.8

131.872.84.6

- 0 . 5

Range

114-13876-8670-800-90-11

17-2734-5032^454-6465-86

125-14266-790-10

- 5 - 3

ReferencesampleMean

124.281.076.64.46.8

18.733.726.965.581.4

124.967.54.8Z7

s.d.

4.93.43.22.23.23.84.74.44.45.95.43.52.11.9 Figure 2 Reference lines used in the cephalometric

analysis.

MASTICATORY MUSCLE TRAINING AND FACIAL GROWTH 17

The reference line OL was denned as a linefrom mo Dissecting the distance is-ii (Feller,1986). The point io was constructed as the per-pendicular projection of the point ii on to the linemo-is (upper occlusal line). The distances is-ioand ii-io give the overjet and overbite respectivelyand were in cases of double projections measuredfrom the central incisor in the most extreme posi-tion. The quotients s-tgo/n-mc x 100 and n-sp'/sp'-mexlOO express the relation between theposterior and anterior face heights and the upperand lower anterior face heights, respectively.

The widths of the upper and lower dentalarches between the first permanent molars weremeasured on dental casts with sliding calipersto 1/10 mm. The measuring points were thosedescribed earlier (Thuer et al., 1985).

The total rotation of the mandible from thestart to the end of the period of observation wasanalysed by superimposition of profile cephalo-graphs using natural reference structures of themandible as described by Bjork and Skieller(1983).

Measurement of bite forceThe bite force in the region of the right and theleft first permanent molars was measured with abite force recorder as described by Floystrandet al. (1982). Thin acrylic splints covering theocclusal surfaces of the teeth in the two dentalarches were used in order to allow biting on thesensor loading it at right angles (Fig. 3). Threemeasurements on each side of the dental archwere made in random order. The highest of thethree recorded values was used as the maximumbite force at that location.

Figure 3 Measurement of bite force.

Electromyographic recordingThe activity of the anterior portion of the tem-poral muscle and of the masseter muscle wasrecorded bilaterally with bipolar hook electrodes(Ahlgren, 1967). The placement of the electrodeswas as described earlier (Ingervall and Thilander,1974; Ingervall and Egermark-Eriksson, 1979).

Recordings were made with a Disa electro-myograph and a Gould electrostatic writer (fordetails see Ingervall and Bitsanis, 1986) in therest position of the mandible, during maximalbite in the intercuspal position and during chew-ing of apple and peanuts. For the recordings, thechild was placed in a dental chair, in an uprightposition, without a head-rest. For the chewingtest, the child was given a standard sized piece ofapple or 3 peanuts and asked to chew in thehabitual way. The order of the recordings was asfollows: in the rest position of the mandible;during chewing of apple, and of peanuts; duringmaximal bite in the intercuspal position, and anew recording in the rest position of the mandible.

The mean voltage amplitude in the rest posi-tion was measured when the muscle activity wasminimal during at least five seconds (mean ofrecordings 1 and 5). The mean voltage amplitudeof the characteristic activity during maximalbite was measured. The maximal mean voltageamplitude during the closing phase of the chew-ing cycle was determined as the mean of six ran-domly selected cycles during an act of chewing.

Muscle training with chewing gumThe training was started after initial morpho-logical recordings with x-ray cephalometry anddental casts, and electromyographic and biteforce recordings. A special type of chewing-gumwas used. The material, available under the nameof Mastix, is a natural product (resin from a pinetree growing on the Greek island of Chios). Theproduct is well known and popular as a chewingmaterial in Greece. It is much tougher to chewthan ordinary chewing-gum and retains its toughconsistency during long periods of chewing. Thechildren were requested to chew the resin for atleast two hours per day and to make daily recordsof the duration of the chewing.

Experimental period and control recordingsThe children were followed up for one year.During this time, they chewed the resin asdescribed above. Appointments were arranged


Table 2 Mean bite force (in N) and mean voltage amplitude (in /*V) in the rest position ofthe mandible and during maximal bite and chewing at recordings 1-4. Mean of right andleft sides.

Bite forceAmplitude in rest positionAnterior temporal muscleMasseter muscleAmplitude during maximal biteAnterior temporal muscleMasseter muscleAmplitude during chewing of appleAnterior temporal muscleMasseter muscleAmplitude during chewing of peanutsAnterior temporal muscleMasseter muscle

Recording

1

334

2.31.4

336170

295151

331169

2

372

2.41.4

483258

315164

364193

3

395

2.90.9

507299

336180

413231

4

436

1.41.3

513283

320175

390208

Significant difference

l - 2 « , l - 3 \ 1-4*. 2^**, 3-4*

N.S.N.S.

1-2", 1-3**, 1-4**1-2*, 1-3*. 1-4**

N.S.N.S.

N.S.1-3*

x = 0.01 < P < 0.05, xx = 0.001 < P < 0.01

every fourth week for the measurement of biteforce. Electromyographic recordings were madeat the start and after 3, 6 and 12 months ofchewing-gum therapy. After 12 months, newcephalometric recordings and new dental castswere made.

Results

Bite force and muscle activityThe bite force at the start of the experiment(recording 1) and after 3 (recording 2), 6 (record-ing 3) and 12 (recording 4) months of muscletraining is given in Table 2 and Figure 4. TheTable also shows the electromyographicallyrecorded muscle activity in the rest position andduring biting and chewing. The values for biteforce and muscle activity were calculated as themean of the right and left sides.

On average, there was a gradual increase inbite force during the one year experimental periodand a statistically significant increase was foundon comparison between each of the recordings,except between recordings 2 and 3. Wilcoxon'smatched pair signed ranks test was used for allstatistical analyses. The muscle activity in therest position of the mandible was unchangedduring the period of observation.

The amplitude during maximal bite in theintercuspal position was significantly greater atrecording 2 and at the following two recordings

4OO •

3 O O -

2 0 0 ••

100x

MasseterMuscle

-4-Start 3 months** 6 months 12 months

Figure 4 Average bite force (in Ncwtons) and amplitude ofthe anterior temporal and masseter muscles during maximalbite (in fiV) on Tour occasions during the experimental period.

than at recording 1. There was also a numericalincrease between recordings 2 and 3. Duringchewing, -the maximal amplitude increasednumerically from recording 1 to recording 2 andto recording 3. This increase was statistically


Table 3 Maximal mean voltage amplitude duringchewing in per cent of the .mean voltage amplitudeduring maximal bite.

Recording

1

Amplitude during chewing of appleAnterior temporal muscleMasseter muscle

88 65 66 6289 64 60 62

Amplitude during chewing of peanutsAnterior temporal muscle 99 75 81 76Masseter muscle 99 75 77 74

significant only for the masseter muscle whenchewing peanuts (Table 2). The amplitude duringchewing in per cent of the amplitude duringmaximal bite is given in Table 3.

Due to the marked rise in amplitude betweenrecordings 1 and 2 during maximal bite and theless marked change in amplitude during chew-ing, the proportions between the amplitude usedduring chewing and during maximal bite changeddrastically (Table 3). After 3 months of chewing-gum therapy, a considerably smaller proportion

of the maximal muscle activity was used duringchewing than before the period of muscle train-ing. After the change brought about during thefirst 3 months of muscle training, the relationsremained unchanged during the rest of the periodof observation.

Changes in facial and bite morphology

The changes in facial morphology from the startof the period of muscle training to the recordingsafter 12 months are shown in Table 4.

During the one-year period of observation,there was a significant increase in mandibularprognathism (angle s-n-sm) and a change insagittal jaw relation (angle ss-n-sm), and a sig-nificant decrease in inclination of the occlusalline (angle NSL/OL) and of the mandible (angleNSL/ML). The vertical jaw relation (angle NL/ML) decreased significantly. There was a sig-nificant change in the relation between theposterior and the anterior face heights (indexs-tgo/n-me). On average, the posterior faceheight increased 2.5 mm and the anterior faceheight 2.0 mm. There was a significant increase inoverbite (distance ii-io). The widths of the dental

Tabk 4 Changes in cephalometric variables (mean and range) betweenrecordings 1 and 4. A minus sign denotes a smaller value at recording 4.

Variable

n-s-ar°s-n-ss°s-n-sm°ss-n-sm0

NSL/NL0

NSL/OL0

NSL/ML0

NL/ML0

s-tgo/n-me x 100n-sp'/sp'-me x 100RL/ML0

n-s-gn°is-io (mm)ii-io (mm)n-me (mm)s-go (mm)

Difference between recordings 1 and 4

Mean

0.0- 0 . 2+ 0.8- 1 . 0

0.0- 1 . 3- 1 . 3- 1 . 2+ 0.9+ 0.2-0.9-0.7-0.4+ 1.0+ 2.0+ 2,4

Range

+ 1.8—2.4+ 2.0--1.6+ Z9—1.2+ 0.6—2.3+ 1.1 — 0.8+ 0.7—3.8

0.0—3.5+ 0.5—2.5+ 3.0—1.0+ 3.6—6.4+ 2.5—2.5+ 0.5—2.1+ 4.0—4.0+ 3.0—2.0+ 4.5- + 0.5+4.5-+ 1.5

Significance

N.S.N.S.

0.01<P<0.05P < 0.01

N.S.P<0.01P<0.01P<0.0l0.01 <P<0.05

N.S.N.S.N.S.N.S.

0.01 < P < 0.05P < 0.01P<6.01

Riolo elal. (1974)

+ 0.1-0.1+ 0.2-0.2+ 0.4+ 0.2+ 0.3

+ 0.7-0.1

+ 2.3f+ 1.7f

reduced to the same degree of magnification as in the present study


arches were practieally unchanged (averageincrease in both arches 0.3 mm).

Table 4 also gives the annual changes duringnormal growth in the untreated longitudinalsample of Riolo et al. (1974). For comparisonwith the present material, the values from Rioloet al. were matched with those of the presentsubjects with respect to sex and age.

Superimposition of the profile radiogramsfrom recordings 1 and 4 showed an average totalanterior rotation of the mandible of 2 degrees(P < 0.01). It was technically possible to evaluatethe degree of rotation in 12 of the 13 subjects.The mandible rotated posteriorly in two subjects(1.0 and 1.5 degrees, respectively), did not rotatein one subject, and rotated anteriorly in ninesubjects (1.0 to 3.5 degrees). The amount ofanterior rotation was 3.0 degrees in two subjectsand 3.5 degrees in three subjects. Superimposedtracings of four children with great anteriorrotation are shown in Fig. 5.

Discussion

The mean bite force of the present group ofchildren at the start of the study was somewhatsmaller than that in an unselected group of thesame age examined with the same methods (Dineand Kober, 1985) but rose during the experi-mental period to a value comparable to thatfound in 11 -year-old children by the aboveauthors. The gradual increase in bite force of thepresent subjects is probably an effect of themuscle training but may also partly be ascribedto a normal increase with age (average bite forcein 9-year-old children 357 N and in 11-year-oldchildren 424 N, Dine and Kober, 1985). Theaverage rise in bite force in the present childrenduring one year was, however, greater than thatnormally observed during two years.

The same electromyographic method as in thepresent investigation has been used in a study ofchildren with Angle Class II, division 1 mal-occlusion (Ingervall and Bitsanis, 1986). In thatstudy (median age of the children. 10 years, 10months) the mean amplitude of the anteriortemporal muscle during maximal bite was 416 ftVand that of the masseter muscle 229 /*V. The pre-sent children had 20-25 per cent lower amplitudeduring maximal bite at the start (recording 1)than the children with distal occlusion but at theend of the period of muscle training they had23 per cent higher amplitude than the other

children. After only 3 months of muscle trainingthe amplitude was 13-16 per cent higher thanthat of the children with distal occlusion. Duringisometric muscle contraction (as during maximalbite) there is a linear relation between muscleforce and the electromyographic amplitude(Ralston, 1961; Ahlgren and Oewall, 1970;Ahlgren et al., 1985). The increase in amplitudeduring maximal bite thus reflects an increase inmuscle strength and is obviously an effect of themuscle training.

All observations of the changes in the cephalo-metric variables point in the same direction, i.e.a decrease in the inclination of the occlusal andmandibular planes through anterior rotation ofthe mandible. This development is accompaniedby an increase in mandibular prognathism andoverbite.

The children studied during the year tended tonormalize with respect to facial morphology oncomparison with the 9-year-old reference groupdescribed in Table 1. The facial morphology ofthe reference group conforms very closely to thatof the sample of Riolo et al. (1974) which wasstudied longitudinally. The children studied inour region have on average the same facialmorphology as the sample studied by Riolo et al.and that of a Scandinavian series (Feller, 1986).The one-year changes of the present experimentalgroup were therefore compared with the annualchanges in the sample of Riolo et al.lt is obviousfrom Table 4 that the significant annual changesof the present subjects were much greater andopposite to those of the children studied by Rioloet al. The therapeutic muscle exercises carriedout by our subjects have thus probably had aneffect on the facial morphology.

The impression of a therapeutic effect of themuscle training is further substantiated by theanalysis of the total mandibular rotation bythe method of Bjork. The average anteriormandibular rotation in the 12 children analysedwas 2 degrees, with an anterior rotation of3.0-3.5 degrees in five cases. In eight anteriorlyrotating untreated boys followed longitudinallyby Bjork and Skieller (1983), the rotation peryear at an age comparable to that of the presentchildren was around 1 degree. The anterior rota-tion in most of our children is thus considerablygreater than would be expected from the findingsin untreated children.

The superimposition on natural mandibularreference structures revealed that the anterior


8y. 5 m.m.

/ 9i1y.5m.

Figure 5 Superimposed profile tracings of four children with marked anterior growth rotation of the mandible during the yearof muscle training.


mandibular rotation was accompanied by resorp-tion at the gonial angle in six cases. The resorp-tion at the mandibular border in the gonial regionvaried between 0.5 and 2.5 mm (mean 1.3 mm).This resorption explains why the angle NSL/MLdecreased less than the anterior rotation (averageanterior rotation in nine cases 2.5 degrees, withan average decrease of the angle NSL/ML of1.6 degrees).

The considerable anterior mandibular rota-tion found in many of the present children mightpossibly be explained by reduced midfacial ver-tical growth due to increased masticatory musclestrength. If the vertical growth of the mandibularcondyles exceeds that of the midface and of thealveolar processes, the mandible will rotateanteriorly, as pointed out by, for example Isaac-son et al. (1981). In none of the present childrenwas the vertical growth of the maxilla halted,however. The distance between the cephalometricreference point pm and the NSL increased be-tween 0.5 and 1.5 mm (average 1.1 mm). Thedistance between pm and the ethmoid registra-tion point in the cranial base (used by Riolo etal.) increased on average by 1.2 mm. This is veryclose to the annual increase found by Riolo et al.(1974). Bjork and Skieller (1976) also found anaverage lowering of the palate of 1 mm per yearduring the growth period.

The lowering of the upper first molar from thenasal line was on average 1.1 mm in the presentchildren. This conforms to annual changes foundby Riolo et al. (1974) and Bjork and Skieller(1976).

The increased muscle strength thus seems notto bring about a reduction of the vertical growthof the maxilla or to retard the eruption of themaxillary teeth. The eruption of the lower firstmolar measured in relation to the mandibularline (ML) was identical (mean value 0.6 mm)with that found by Riolo et al. (1974). The lowermolar eruption was, however, larger (mean value1 mm) when measured in relation to the referenceline based on the natural reference structures.The difference is due to the instability of themandibular line because of the resorption in thegonial area.

There is evidence, as emphasized, of anincreased anterior mandibular rotation duringthe year of muscle training. There is no evidenceof a decreased maxillary vertical growth or adecreased molar eruption during the periodof muscle training. The anterior mandibular

rotation may, however, be brought about by anincreased mandibular growth which could bethe result of the functional stimulation due to theincreased muscle strength. The growth of themandibular condyles was estimated by measur-ing the distance ar-gn. This distance increased onaverage by 2.5 mm during the year of training,which is identical with the annual increase inthe sample of Riolo et al. The great anteriormandibular rotation can thus not be explainedby increased mandibular growth either.

The facial morphology of the present childrenwas compared with that of children with normalocclusion or children randomly selected withrespect to facial morphology. The objectionmight be made that the long-face children followanother pattern during their facial growth.Recent longitudinal studies of the growth ofchildren with different facial types have, how-ever, failed to reveal any such differences. Thegrowth pattern was parallel and the annual incre-ments the same in long-face children as in childrenwith average facial morphology (Bishara andJakobsen, 1985).

Acknowledgement

This study was supported by SchweizerischerNationalfonds zur Forderung der wissenschaft-lichen Forschung (Grant Nr. 3.832-0.83) and bythe Research Fund of the Swiss Dental Associa-tion (Project Nr. 132).

Address for correspondence

Professor B. IngervallKlinik fur KieferorthopadieFreiburgstrasse 7CH-3010 BernSwitzerland

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a pilot study of the effect of masticatory muscle training ... · for the chewing test, the child...

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