Mandibular changes during functional appliance treatment

Mandibular changes appliance treatment

during functional

Colin Nelson, Michael Harkness, and Peter Herbison* Dunedin, New Zealand

The purpose of this prospective trial was to determine the changes in position and size of the mandible in children treated with either the Fr~mkel function regulator or Harv~)ld activator. Forty-two 10- to 13-year-old children with Class II, Division 1 malocclusions were matched in triads according to age and sex and randomly assigned to either control, Fr&nkel function regulator, or Harvold activator groups. There were no statistically significant differences between the groups at the beginning of the study. After 18 months, significant increases in gonial angle and articulare-pogonion length in the Harvoid group were attributed to a change in the location of articulare because the condyles were positioned downward and forward at the end of treatment. The main effects of both appliances were to allow vertical development of the mandibular molars and increase the height of the face. The Harvold appliance also proclined the lower incisors and increased mandibular arch length. We could find no evidence to support the view that either appliance was capable of altering the size of the mandible. (AM J ORTHOD DENTOFAC ORTHOD 1993;104:153-61.)

T h e notion that functional appliances are capable of stimulating mandibular growth is widely held, and it could be argued that it is supported by reports of successfully treated cases and the results of many retrospective studies in which mandibular size or position appear to have changed favorably. 1-t3 Such an interpretation may be premature, however, because in some studies, treatment groups have been selected on the basis of cooperation 7 or results ~,6,8.12.13 in which favorable mandibular growth may have occurred. In other studies either the samples, the methods of selection and/or the pretreatment comparisons are not fully r e p o r t e d 1"3'6"9'1~ o r the control and treatment groups dif- fered at the outset m'4'~'tt't3 making it difficult to determine the extent of any treatment effect.

Undoubtedly, the best method for ensuring that differences in outcome between control and treatment groups can be attributed to the treatment method is to undertake a prospective study and randomly assign subjects to all groups at the outset. ,4 In the only randomized control trial of functional appliances Jakobsson I~ randomly assigned each subject to either an Andresen activator, an extraoral traction, or a control group from triads of children at the same stage of dental development and with Class II, Division 1 malocclusions. It is not clear, however, whether the initial selection of subjects was based on their likely favorable response to

From the Department of Orthodontics, School of Dentistry, University of Otago. This research was supported by the Medical Research Council of New Zealand. *Department of Preventive and Social Medicine, University of Otago. Copyright �9 1993 by the American A~sociation of Orthodontists. 0889-5406193151.00 + 0.10" 8/1/34462

treatment with either an activator or extraoral traction. Jakobsson reported that activator treatment had no effect on either condylar growth or the anteroposterior position o f the mandible which conflicts with the results of studies mentioned previously.

In this prospective investigation we have sought to determine the changes in position and size of the mandible in subjects with Class II, Division 1 malocclusions treated with either a Harvold activator or a Friinkel function regulator. The Harvold activator was chosen because it is a lingual appliance derived from the tra- ditional activator and the Friinkel function regulator because it is a vestibular appliance.

MATERIALS AND METHODS

Materials. The subjects for this investigation were 42 school children, between the ages of 10.0 years and 12.9 years, mean age 11.6 years, referred to the Department of Orthodontics. Consecutive subjects with Class II, Division 1 malocclusions '6 were recruited.t0 the study after its purpose had been explained to each child and his or her parent(s), and informed consent obtained. Sex and age-matched (to within 12 months) subjects were then grouped in triads, and one subject in each triad was randomly assigned to either the control, the Friinkel function regulator (FFR), or the Harvold

"activator (HA) group (Fig. 1). The mean difference between theyoungest and the oldest subjects in the same triad was 0.44 year, and the range was 0.17 to 0.85 year. Details of the groups are given in Table I. One of three orthodontists, with experience in functional appliance therapy, was randomly assigned to either treat or supervise each subject. All subjects were treated without extractions or any other form of orthodontic treatment and followed for 18 months.

Methods. The Frtinkel function regulators (FR-2) used in

.153

1 5 4 Nelson, Harkness, and Herbison American Journal of Orthodontics and Dentofacial Orthopedics Auez~st 1993

~ onsecu'ive 10-13 year old children I wil.'t Class 11 divLsio'a 1 I

malocchisiom ]

I " 1

I I

I t-test males v's females I at the start of ~e study " :

T m

at the start oft/'m study and the I , .d over 18 monti'Ls in HA group using differences over 1S months I ~ [ increase in stature as the covariate

lr Duncan's new multiple renge test I to determine sign.il'ieent groups I

t m

Correlation coefficients to [ determine associations bet~,oen

molars and face height

Fig. 1. Experimental design.

this study were constructed according to the description given by McNamara and Huge ~7 but with the addition of double hanger wires to reduce downward rotation of the lingual shield)* On average, the Fr~inkel function regulators opened the bite 6.4 mm from the intercuspal position and advanced the mandible4.8 mm. The activators used were constructed according to the description given by Harvold t9 and, on average, opened the bite 10.5 mm from the intercuspal position and advanced the mandible 7.0 mm. The children were insiructcd to wear the appliances 4 hours a day in the first week, 8 houri a d~y in the second week, 12 hours a day in the third week, and a minimum of 14. hours a day thereafter.

Lateral cephalometric radiographs, intraoral and,facial photographs, study models, and height and weight measurements were taken at the commencement of the study and at 6, 12, and 18 months later. The lateral cephalometrie radiographs were taken while each subject was standing with his or her head held by a Wehmer cephalostat and with teeth occluded in the intcrcuspal position. The anode to target distance was 166 cm, and the image magnification was X1.0866. The data have not'been been corrected for magnification.

The outlines of Bjork's stable mandibular structures 2~ (the external and internal symphyseal outlines, bony trabec- ulae present within the symphyses of the initial and 18-month radiographs, the third molar tooth germs before root formation and the mandibular canal) and the reference points given in Table II and Fig. 2 were traced by C.N. from the initial radiograph onto matte acetate film. According to the method outlined by Johnston, 9 the initial and 18-month films, and if necessary the 6- and 12-month films, were viewed together to assist location of the stable structures and reference points. Each tracing was then orientated over the stable structures on the 18-month film of the same subject, and the reference points marked in again. Sella was located on the 18-month radiograph with the aid of a separate tracing of the anterior cranial base taken from the initial radiograph. This template was orientated to obtain the best fit o f the anterior cranial base, the anterior contour of the pituitary fossa, 22 and the intersection of the averaged greater wings and planum of the sphenoid bone : For each subject a symphyseal trabecula present on both radiographs was designated fiducial point 1 (Fide), and a point on either the inferior border of the developing third molar or the superior surface of the mandibular canal was designated fiducial point 2 (Fid2). The mandibular reference plane was established by drawing a line between Fid~ and Fid2.

The coordinates of each point on the tracings were re- corded with a reflex metrograph 2~ and converted to linear and angular measurements by a microcomputer linked to the metrograph. The reference points and measurements used are given in Fig. 2, Tables II and lIl. The measurements used to determine the horizontal and vertical changes in articulate, gonion, menton, and pogonion are shown in Fig. 3. Differ- ences were obtained by subtracting the initial measurements from the 18-month measurements.

The initial and 18-mont h radiographs of 10 randomly selected subjects Were retraced and remeasurcd more than 8 weeks later by C.N. The errors in identification of the reference points, superimposition of the tracings, and recording of the points were estimated using Dahlberg's formula. :4 Al- though the errors for the cofidylar angle (3.16~ Ar angle (3.07~ and the lower incisor axes LIAo (1.08 ~ and LIAr8 (1.18") were relatively large and may have been reduced if there had been a shorter period between the tracings and if condylar templates had been used, the remainder of the errors did not exceed 0.65 ~ (gonial angle~s) or 0.68 mm (Co-Gojs). Apart from the condylar and Ar angles, the errors are comparable to those reported by others who used both structural methods ~--'5 and conventional cephalometrie analyses. '-'~

Statistical analysis. The study was large enough to have about an 80% power of detecting a one standard deviation difference wiih p < 0.05. The matching of subjects was dis- carded for the analysis. Student's t test for unpaired data was used to compare the ages, stature, and cephalometric measurements of the boys and the girls. An analysis of variance was used to determine whether statistically significant differences existed between the groups at the beginning of the study and in the magnitude of the changes over the 18 months. Duncan's new multiple range test was then used to identify

American Journal of Orthodontics and Dentofacial Orthopedics Nelson, t tarkness, and Herbison 155 Volume 104, No. 2

T a b l e I. T h e n u m b e r o f s u b j e c t s in e a c h g r o u p

Male I

Number of subjects l 1 recruited

Moved from the area

Failure to attend Failure to wear ap-

pliance Number of subjects 6

in complete triads

Number of subjects 5 in incomplete triads

Number of subjects 11 left in the study at 18 months

Control FFR HA

Female Male Female Male [ Female Total I 6 10 6 I0 7 50

1 1 2

i 1 5

4 4 6 4 30

2 1 2 1 1 12

6 7 6 7 5 42

FFR; Frankel functional regulator. HA; Harvold activator.

Table I I . R e f e r e n c e p o i n t s

N Nasion S Sella Co Condylion

Ar Articulare

Go Gonion

Me Menton

Pg Pogonlon

B B point

Lie Lower incisor edge Lia Lower hwisor apex Ldt Lower molar distal cusp tip Ldc Lower molar distal contact point Fid~ Fiducial poinh

Fid2 Fid,wial point2

The most anterior point of the frontonasal suture. The center of sella turcica, determined by inspection. The intersection of the long axis and articular surface of the mandibular

condyle. Determined with the aid of tangents to the anterior and posterior borders of the condylar process and the articular surface.

The intersection of the outline of the pharyngeal surface of the cranial base and the posterior surface of the mandibular condyle.

The midpoint of the angle of the mandible. Determined by bisecting the angle formed by the mandibular line and the tangent to the posterior outline of the angle from articulare.

The intersection of the symphyseal outline and the lower border of the mandible.

The most anterior point on the outline of the bony chin. Determined by the tangent to the bony chin from nasion.

The most posterior point on the outer contour of the mandibular alveo- lar process. It should lie opposite the apical third of the incisor roots. When there is no curvature in this region and determination of B point is not possible by the method already described, it is chosen with the aid of either preceeding or succeeding films.

The tip of the crown of the most prominent, mandibular central incisor. The apex of the most prominent mandibular central incisor. The tip of the distal cusp of the mandibular first molar. The posterior contact point of_the mandibular first molar. A point on a natural structure within the mandibular symphysis, e.g.,

a trabecula present on the initial and 18-month radiographs. A point 9 n either the inferior border of the developing mandibular third

molar or the superior surface of the mandibular canal present on the initial and 18-month radiographs.

groups significantly different. To determine whether the rate o f growth during the study contr ibuted to the significant differences be tween the Harvold activator and control groups, the increase in stature was used as a covariate in an analysis

o f covariance. Finally, Pearson product -moment correlation coefficients were used to determine the associations between increases in the heights o f the lower molars and the SN-GoMe

angle and the N-Me.

156 Nelson, Harkness, and Herbison American Journal of Orthodontics and Dentofacial Orthopedics August 1993

L

M

Fig. 2. Reference points.

RESULTS

The results are given in Tables IV, V, VI, and VII.

Pretreatment comparisons

At the start of the study mandibular arch length was, on average, 2.03 mm longer in the boys than in the girls because the majority of the boys (22 of 25), but less than half of the girls (7 of 17), were in the mixed dentition (Table IV). The data from the boys and the girls in each group were combined, and the three groups (control, Frfinkel function regulator, and Harvold activator) were compared with the analysis of variance. There were no statistically significant differences between the three groups at the beginning of the study (Table V).

Treatment comp~irisons

There were 16 statistically significant differences between the groups at the conclusion of the study (Table' vi).

During the study period the subjects in the Harvold activator group grew, on average, 1.33 cm more in stature than the subjects in the control group. But nei- ther this difference nor the smaller differences between the Frfinkel function regulator and control groups were statistically significant.

Ar

4

Fid2

I

Fig. 3. Horizontal and vertical distances from articulare, gonion, menton, and pogonion to mandibular reference plane and fiducial points (1, Ar horizontal; 2, Ar vertical; 3, Go horizontal; 4, Go vertical; 5, Me horizontal; 6, Me vertical; 7, Pg horizontal; 8, Pg vertical).

Position. There were no significant differences between the groups in either the total rotation of the mandible, the intramatrix rotation of the mandible, or the posterior face height (S-Go). The S-N mandibular plane angle (SN-GoMe) increased significantly in the Harvold activator group compared with both the control and the Frfinkel function regulator groups. Anterior face height (N-Me) increased 3.29 mm more in the Harvold activator group than the control group, and 1.81 mm more in the Friinkel function regulator group than the control group. Both differences were statistically significant.

Size. Mandibular body length (Go-Pg) in the Harvold activator group increased significantly compared with the control group. This statistically significant difference was lost when the increase in stature was used as a covariate. The distance from articulare to pogonion (Ar-Pg) increased significantly in both the Fr~inkel function regulator and Harvold activator groups, as compared with the control group. On the other hand, there were no significant differences between any of the groups in mandibular length, measured from condylion to pogonion (Co-Pg). The gonial angle opened 0.61 ~ in the Harvold activator group and closed 1.03 ~ in the control group and 0.45 ~ in the Fr~inkel funct!on regulator group. The mean difference of 1.64 ~ between the Harvold activator and the control groups was statistically significant.

In the Harvold activator group articulare (Ar horizontal) and gonion (Go horizontal) moved more posteriorly than in the control group. This latter finding was also lost when the increase in stature was used as a covariate. Only articulare moved posteriorly in the Fr~nkel group. There were no other significant differences in either the vertical or horizontal movement of articulare, gonion, menton, or pogonion.

Dentoah'eolar. During the study the mandibular in-

American Journal of Orthodontics and Dentofacial Orthopedics Nelson, Harkness, and Herbison 1 5"/ Volume 104. No. 2

Tab le III. Measurements

Total rotation lntramatrix rotation SN-GoMe Gonial angle SNB S-Go N-Me Ar-Go Co-Go Go-Pg Ar-Pg Co-Pg Condylar growth Condylar angle

Ar change Ar angle

LIA Arch length -6 height "6 AID change

Ar horizontal

Ar vertical Go horizontal

Go vertical Ale horizontal

Ale vertical Po horizontal

Po vertical Ar-Fid 2-Fid 1

The angle between S-N at the start of the study and 18 months later. The angle between Go-Me at the start of the study and 18 months later. The angle between SN and Go-Me. The internal angle between Ar, Go and Me. q'he angle between S, N and B. The distance between S and Go. The distance between N and Me. The distance between Ar and Go. The distance between Co and Go. The distance between Go and Pg. The distance between Ar and Pg. The distance between Co and pg. The distance between Co at the start of the study and Co at 18 mouths. The angle between Co-Go at the start of the study and the line joining

Co at the start of the study and Co at 18 months. Angles posterior to Co-Go are positive, angles anterior to Co-Go are negative.

The distance between Ar at the start of the study and Ar at 18 months. The angle between Ar-Go at the start of the study and the line joining

Ar at the start of the study and Ar at 18 months. Angles posterior to Ar-Go are positive, angle anterior tO At-Go are negative.

The angle between Lie-Lia and Go-Me. The distance between Ldc and Lie. The perpendicular distance from Ldt to Go-Me. The difference between Ldc at the start and Ldc at 18 months. Mea-

sured parallel to Go-Me from Ldc to the perpendicular to Go-Me through Pg.

The perpendicular distance from Ar to the perpendicular to the mandibular reference plane through Fid 2.

The perpendicular distance from Ar to the mandibular reference plane. The perpendicular distance from Go to the perpendicular to the mandib-

ular reference plane through Fid 2. The perpendicular distance from Go to the mandibular reference plane. The perpendicular distance from Me to the perpendicular to the man-

dibular reference plane through Fid 1. The perpendicular distance from Me to the mandibular reference plane. The perpendicular distance from Po to the perpendicular to the mandib-

ular reference plane through Fid 1. The perpendicular distance from Po to the mandibular reference plane. The internal angle formed by Ar, Fid 2 and Fid 1.

cisors procl ined significantly in the Harvold activator group compared with the Fr/inkel funct ion regulator and the control groups. Mandibular arch length increased

significantly in the Harvold activator group compared

with" the control group. The height o f the mandibular first molar above the

mandibu la r plane (Go-Me), increased significantly in both the Fr~inkel funct ion regulator and the Harvold activator groups compared with the control group. In

- both t reatment groups increases in lower molar height were significantly correlated with increases in N-Me but not increases in the SN-Go Me angle (Table VII).

DISCUSSION

We could find no evidence to support the c la im that ei ther the Fr~inkel function regulator or the Harvold activator increase the overall length of the mandib le .

W h e n Rober tson 26 and Hami l ton et alfl 7 reported s imilar

negat ive f indings, they were accused o f not us ing func- t ion regulators correctly, zs'z9 We argue that our use of the appl iances is above such cri t icism because the changes we found are comparable with those reported

by others. ~'3"u There is a special p roblem when articulare is used,

instead of condyl ion , to define mandibular length. Be- .cause it is the intersection of the outl ines of the pharyngeal surface of the cranial base and the posterior surfaces of the condylar processes, its location will depend, to a certain extent, on the posit ion of the mandible. Thus , in cases where the condyles are forward, articulare may occur on a more posterosuperior part o f the condylar process and the Ar-Pg will be longer than if the condyles are in their fossae. Several investigators have reported that after t reatment with Frfinkel funct ion

1 58 Nelson, Harkness, and Herbison American Journal of Orthodontics and Dentofacial Orthopedics August 1993

Table IV. Comparison of the boys and girls at the beginning of the study

Boys (n = 25)

Variables Mean [ SD I

Girls (n = 17)

Mean ] SD

Age (~'r) 11.39 0.85 11.99 0.68 0.021 Stature (cm) 146.09 5.26 148.66 7.24 0.189 SN-GoMe (degrees) 34.78 5.38 33.88 6.14 - 0.617 Gonial angle (degrees) 130.54 5.63 127.30 5.52 0.072 SNB (degrees) 75.26 2.23 74.84 3.76 0.650 S-Go (mm) 73.35 3.63 71.70 3.84 0.166 N-Me (mm) 114.57 5.14 111.55 5.94 0.087 Ar-Go (mm) 42.70 3.51 42.02 3.66 0.549 Co-Go (mrn) 54.76 3.57 53.73 3.08 0.340 Go-Pg (mm) 71.97 4.02 72.49 4.10 0.687 Ar-Pg (mm) 102.04 3.83 100.72 4.46 0.311 Co-Pg (nm~) 109.85 4.20 108.30 4.56 0.261 LIA (degrees) 93.51 7.40 93.27 5.94 0.909 Arch length (mm) 40.88 2.44 38.85 3.05 0.022

height (mm) 27.19 2.56 27.08 2.04 0.888

Significant values are in bold type; unpaired t test.

Table V. Comparison of the control group, the Frankel function regulator group and the Harvold activator group at the beginning of the'study

Control (n = 17) FFR (n = 13) HA (n =12)

Variables Mean l SD Mean l SD Mean l SD

Age (yr) 11.53 0.93 11.70 0.68 11.70 0.89 0.250 Stature (cm) 148.06 6.80 147.67 5.74 145.23 5.82 0.457 SN-GoMe (degrees) 35.16 6.43 33.82 5.52 34.00 4.88 0.786 Gonial angle (degrees) 128.81 7.30 129.98 3.99 129.01 5.22 0.858 SNB (degrees) 74.57 3.04 74.88 3.15 76.05 2.41 0.392 S-Go (ram) 72.54 4.14 72.21 3.77 73.39 3.38 0.730 N-Me (mm) 114.53 6.09 I 11.85 5.24 113.30 5.36 0.442 Ar-Go (ram) 41.53 4. ! 7 43. I I 3.81 42.94 1.88 0.410 Co-Go (mm) 53.59 4.26 54.29 2.57 55.47 2.58 0.343 Go-Pg (mm) 73.16 3.71 71.38 4.36 71.67 4.10 0.435 Ar-Pg (mm) 101.30 4.55 101.93 4.56 101.34 3.06 0.910 Co-Pg (mrn) 108.95 4.41 108.95 4.05 109.90 4.89 0.822 LIA (degrees) 92.65 7.99 93.81 6.14 94.07 5.94 0.835 Arch length (turn) 39.68 3.09 40.19 2.88 40.44 2.64 " 0.771

height (ram) 27.21 2.18 26.21 2.19 28.06 2.48 0.139

ANOVA P

regulators 3~ and Andresen activators 3~ that condyles were positioned inferiorly and, somewhat, forward relative to the original condyle-fossa positions. We argue that similar changrs in condylar position occurred in the present study because in both treatment groups articulare moved more posteriorly and the Ar-Pg was significantly longer, without a corresponding increase in the Co-Pg, than in the control group. Jakobsson ~s reported similar findings in his study and also argued that the increase in the Ar-Pg was due to a change in condylar position. From this evidence we conclude that measurements with articulare should not be used to

describe mandibular length in groups treated with functional appliances.

In view of the association between the rate of mandibular growth and the rate of statural growth 3-'.33 and the relatively large increase in stature in the Harvold group (although it was not statistically significant), we used the increase in stature as a covariate to identify the significant findings because of growth. As a result of this adjustment, the significant increases in mandibular body length (Go-Pg) and posterior movement of gonion (Go horizontal) have been attributed to growth (Table VI). The latter finding is of interest because it

American Journal of Orthodontics and Dentofacial Orthopedics Nelson, Harkness, and Herbison 159 Vohone 104, No. 2

Table VI. C o m p a r i s o n o f the g roups by u s i n g the d i f fe rences b e t w e e n m e a s u r e m e n t s at the start o f the

s tudy and 18 m o n t h s la ter

Variables

Control (n = 17)

Mean ] SD

FFR (n = 13)

Stature (cm) 8.94 2.91

Position Total rotation (degrees) - i .98 ! .65 - 2.19 i .50 - !. 13 lntramatrix rotation (degrees) 1.22 1.62 1.76 0.76 1.28 SN-GoMe (degrees) - 0.85 I. 16 - 0.45 1.24 0.73

1' t t t SNB (degrees) 0.44 0.70 0.75 0.81 0.66 S-Go (mm) 3.49 1.15 4.38 1.78 4.37 N-Me (mm) 3.32 1.66 5.13 1.59 6.61

Size t ~ t Ar-Go (mm) 2.95 1.58 3.22 2.01 2.88 Co-Go (mm) 4.18 i .64 4.08 2.44 3.07 Go-Pg (mm) 1.54 1.94 2.48 1.23 3.32

l i Gonial angle (degrees) - 1.03 1.98 - 0.45 l. 14 O.61

Ar-Pg (mm) 3.32 1.41 4.71 ! .27 5.70 t t t t

Co-Pg (mm) 4.38 " 1.90 5.07 2.28 5.70 Condylar growth (ram) 6.47 2.70 7.77 2.33 7.22 Condylar angle (degrees) 3.67 24.50 9.80 24.78 - 4.64 Ar change (mm) 5.27 2.11 6.62 1.87 6.85 Ar angle (degrees) -0 .39 26.62 4.32 15.85 - ! 1.32 Ar horizontal (mm) 2.23 1.84 3.84 I. 17 5.15

t t t t Ar vertical (mm) 3.97 2.91 5.19 2.08 3.83 Go horizontal (mm) i .53 2.19 2.79 !. 15 3.65

T 1̀ Go vertical (mm) -0 .88 2.30 - 1.67 0.74 - 1.I0 Me horizontal (mm) O. 13 0.28 0.56 0.84 O. 18 Me vertical (mm) 0.14 0.23 0.09 0.22 0.00 Pg horizontal (mm) 0.02 0.14 0.01 0.14 0.03 Pg vertical (mm) O.0l O. 15 0.03 0.15 0.08 Ar-Fid2-Fidl (degrees) -2 .88 3.25 -3 .72 1.97 -2 .03

Dentoalveolar

HA (n = 12)

I Mean. ] SD Mean SD

9.87 1.59 10.27 4.51

A.NOVA P

0.509

2.26 0.306 1.47 0.529 1.64 0.011

1.28 0.636 2. ! 5 0.257 2.70 0.000

1.99 0.887 2.17 0.325 2.16 0.043t

1.52 0.037

2.64 0.004

2.63 0.292 3.69 0.482

27.71 0.377 3.53 O.195

24.50 0.237 2.44 0.001

3.57 0.424 2.47 O.026tt

| .67 0.473 0.48 0.106 0.34 0.418 0.17 0.962 0.17 0.359 3.24 0.362

LIA (degrees) 0.18 3.13 0.80 2.96 3.86 4.08. 0.017 1 t ~ t

Arch length (mm) - I. 19 1.44 - 0.46 1.36 0.25 1.61 0.043

height (mm) 1.03 0.80 2.09 1.08 2.61 1.48 0.002 t t t T

MD change (mm) -0 .91 1.08 -0 .52 1.10 -0 .17 1.21 0.224

significant values are in bold type. The bars indicate groups found by, Duncan's new multiple range test to be different at the 5% level of significance. Analysis of covariance, "I'P = 0.085, "~tP = 0.059.

-suggests tha t the smal l , bu t s ta t i s t ica l ly s igni f icant , in-

c rease in gonia l ang l e ( A r - G o z M e ) in the H a r v o l d g roup

is due to a r t icu lare sh i f t ing pos te r io r ly r a the r than

c h a n g e s at e i the r m e n t o n or gon i on .

T h e S N - m a n d i b u l a r p l ane ang le ( S N - G o M e ) in-

c r eased s l ight ly in the H a r v o l d ac t iva to r g roup and de-

c reased in bo th the control" and Fr~inkel func t ion reg-

u la to r g roups . O the r s ~5"3~'35 h a v e repor ted s imi la r smal l ,

bu t s ta t i s t ica l ly s igni f icant , i nc reases in the S N - m a n -

d ibu la r p l ane ang le du r ing t r ea tmen t wi th ac t iva tors and

l i t t le or no c h a n g e du r ing t r ea tmen t wi th func t ion reg-

u la tors , 7"36"37 p r e s u m a b l y b e c a u s e o f the lesser b i te -

o p e n i n g used w i th the la t ter app l iance . T h e as soc ia t ion

b e t w e e n the inc rease in m o l a r he igh t and the increase

160 Nelson, Harkness , and Herbison American Journal of Orthodontics and Dentofacial Orthopedics August 1993

Table VII. Correlation coefficients relating the increase in lower molar height to increases in SN-GoMe

and N-Me

Variables

Lower molar height (ram)

Control(n= 17) I FFR (n = 13) I HA (n = 12)

SN-GoMe (degrees) 0.11 - 0.3 i 0.23 N-Me (mm) 0.38 0.64* 0.68*

*P < 0.01.

in the SN-GoMe angle was not significant, which suggests that other factors, such as downward displacement of the upper facial skeleton during treatment 3s may contribute to the increase in the SN-GoMe angle. Our investigation of the changes at gonion and menton (Table VI) do not support the claim by Luder 3a that opening

of the SN-GoMe angle is due to altered remodeling of the lower border of the mandible.

In both appliance groups, anterior face height (N- Me) increased significantly. Similar changes during functional appliance treatment have been reported by Chang et al. ~~ The increase in N-Me was also significantly correlated with the increase in molar height in

both appliance groups (Table VII). The stability of menton, relative to the reference plane, indicates that de-

position at menton did not contribute to the increase in N-Me (Table VI).

Others 1~ have also reported activators either

procline or advance the mandibular incisors. Although the incisors were capped with acrylic, this was ob- viously ineffective in preventing the incisors from pro- clining and arch length from increasing. In response to the bite opening employed by both appliances the height of the first molars also increased significantly. Similar changes in molar height during treatment with functional appliances have also been reported by others.6.7.10.4o

CONCLUSIONS

In this prospective trial we could find no evidence to support the view that either the Fr~inkel function regulator or the Harvold activator are capable of altering the size of the mandible. In the Harvold group, significant increasesqn the gonial angle and Ar-Pg were attributed to a change in the location of articulare because the condyles were positioned downward and forward at the end of treatment.

The main effects of both appliances were to allow vertical development of the mandibular molars and increase the height of the face. The Harvold activator also proclined the mandibular incisors and increased mandibular arch length.

We are indebted to Drs. John C. Muir and Heli Vinkka- Puhakka who participated in the planning and conduct of this trial.

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Mandibular changes during functional appliance treatment