Vertical Skeletal and Dental Changes in Early Treatment of Class II Malocclusion Calogero Dolce, Lisa K. Babb, Susan P. McGorray, Marie G. Taylor, Gregory J. King, and Timothy T. Wheeler

Increases in the vertical dimension can be detrimental in certain facial types or unstable in others. By using cephalograms, this study examined the skeletal and dental changes as well as their permanence in a group of children participating in a randomized clinical trial on the timing of Class II orthodontic treatment. Children (mean age, 9.6 years) were assigned to either a headgear/biteplane (n = 94), bionator (n = 86), or observation (n = 82) group. Cephalometric data were obtained from tracings at baseline, after Class I molar was obtained or 2 years after treatment had elapsed, and after 1 year of retention or observation. In general, the skeletal changes, such as total anterior and posterior face height and mandibular plane angle, ob- served during the active phase of early treatment persisted until the end of phase I. Phase I treatment also produced dental changes, such as molar eruption and tip. With the exception of the eruption of the mandibular molars in the bionator group, these changes could not be maintained until the end of phase I. Whether the subjects were in retention or not had little impact on any of the studied parameters. Mandibular plane angle, pretreat- ment, and retention had an affect on the mandibular molar tip over the course of the study. (Semin Orthod 2002;8:141-148.) Copyright2002, Elsevier Science (USA). All rights reserved.

C ontrolling the vertical dimension is critical to functional appliance treatment because

clockwise rotation can have deleterious effects in patients who have a high mandibular angle; like- wise counter clock rotation in those patients, who exhibit a low mandibular plane angle may be unstable. Numerous studies have reported on the skeletal and dental vertical changes pro- duced by functional appliance treatment.

Jakobsson a was among the first to point out that activator treatment altered the vertical di- mension, which was represented by an apparent clockwise rotation of the palatal plane, an in-

From the Department of Orthodontics, Colle W of Denti~tu, UnL ve~ity of Florida, Gainesville, FL.

Supported by ~TH/NIC1)R DE08715 Address corTe~pondence to Calogero Dol~, DI)S, PhD, Depart-

ment oJ Orthodontics, Box 100444, J[tMHC, Gaine~ville, I~L 32610-0444.

(,'~'rigl~t 2002, ELsevier Science (USA). All rights reserved. 1073-8746/02/0803-0005535.00/0 doi: l O. 1053/sodo.2002.125433

creased lower anterior lace height, a downward positioning of A-point, and an increase in the total mandibular length for both the headgear and activator groups relative to the observation group. There was also a significant increase in the mandibular plane angle associated with ac- tivator treatment. Interestingly, this study did "not support the hypothesis that activator treat- ment can bring the mandible forward and affect condylar growth. ''~

Overall, the available literature is often times the result of retrospective efforts that are com- monly plagued by selection biases and insuffi- ciently matched controls. It is also common to find relatively small experimental groups and a narrow, if not shortsighted, selection of experi- mental parameters. In light of these and other shortcomings, some findings seem to be fairly constant among the reviewed investigations. Of the studies that explored vertical ramus height a n d / o r vertical position of the lower molar (rel- ative to the mandible), there was a predominant

8eminars in Ot~hodontics, Vol 8, No 3 (September), 2002: pp 141-148 141

142 Dolce et al

agreement that both of these parameters in- crease with early t rea tment involving ei ther headgear or activatorY -s With regard to anter ior and posterior facial height, about half of the time the total anter ior face height was studied with no concur ren t study of poster ior facial height. ~,5,6 When the propor t ion poster ior lower facial height to anter ior lower facial height was investigated, the ratio was greater than one if a pos t t rea tment observation period was in- cluded. 4,9,~° During t rea tment itself, a ratio less than one was seen. This means that posttreat- ment changes in growth are occurring relative to the observation group in the vertical dimension.

This study evaluated the vertical t rea tment effects of the headgea r /b i t ep lane (HG/BP) and bionator during the early t rea tment of the Class II malocclusion. More specifically, the aim of this work evaluated changes in (1) the poster ior and anter ior facial height, (9) the alveolar height of the maxillary and mandibular molars, and (3) the orientat ion of the palatal and man- dibular plane as well as the molars that may occur within the periods studied for all the groups.

Materials and Methods

The details of this randomized clinical trial have been previously described by Keeling et al. 11 Briefly, this was a prospective randomized clini- cal trial designed to examine the timing of Class II or thodont ic t reatment. The inclusion and ex- clusion criteria have already been described, u A stratified block randomizat ion was used to assign a subject into one of the following groups: bionator, H G / B P , or observation. Subjects were stratified according to the following criteria: (1) severity of molar Class II malocclusion (mild, bilateral ½ cusp; modera te , one side -9'/4 cusp; and severe, one side full cusp), (2) mandibular plane angle (low, <30°; normal , 30 ° - 40°; and high, >40°) , (3) race, (4) gender, and (5) re- tention. Each subject had complete or thodont ic records taken at every data collection point as they progressed through the study. The end of phase I t reaunent occurred when two project orthodontists independent ly agreed that a bilat- eral Class i molar relationship was achieved or 2 years had elapsed f rom the start of t reatment. Treated subjects were then randomized to reten- tion or no retention. The retent ion scheme was

to cont inue to wear the bionator or H G / B P every other night for 6 months, followed by ob- servation for 6 months. The nonre tent ion scheme lasted 1 year and began immediately after treatment. All subjects had the following data collection points (DC): (1) DC1, initial records; (2) DC3, end of early Class II t reatment or observation; and (3) DC5, end of retention per iod and observation period.

Cephalometric Analysis, Calibration, and Digitization

All lateral cephalograms were taken on the same machine. The superimposit ions of lateral ceph- alometric tracings f rom DC 1 to DC5 were used to detect vertical changes in the sample popula- tion. All skeletal measurements were pe r fo rmed relative to an xy-axis super imposed on the struc- ture of Sella (S) with the x c o m p o n e n t of the grid drawn parallel to the Frankfort horizontal line. Additional templates were then con- structed for the maxilla, mandible, uppe r and lower molars, condyle, and gonial angle. The details on the template construction have previ- ously been described. 12 All points were regis- tered with crosses on both the templates and the final tracing for each film. Figure 1 depicts the angular measurements , and Figure 2 depicts the linear measurements that were taken on the tracings.

Before tracing, four crosshatches were estab- lished on each film, acetate pape r was taped to the left-hand margin, and the crosshatches were transferred to the acetate. The dental and skel- etal templates were or iented independent ly on each radiograph by best-fit visual approxima- tion, and the registration crosses of each struc- ture were transferred to the final tracing for each film. In addition, points represent ing the anter ior and posterior extent of the functional occlusal plane (FOP) were marked on the ace- tate. The FOP was identified as the line of max- imum intercuspation of the first p e r m a n e n t mo- lars, p e r m a n e n t first bicuspids, a n d / o r primary first molars.

The following measurements were executed: (1) mandibular plane (MP) angle, the angle fo rmed by the mandibular plane and the x-axis; (2) palatal plane (PP) angle, the angle fo rmed by the PP and the x-axis; (3) occlusal plane (OP) angle, the angle fo rmed by the FOP and the

Class I I Malocclusion 143

PP

OP

MP

Figure 1. Diagram of the angular measurements ex- ecuted in the stud}, relative to the x-axis. U6tip, max- illmy molar tip relative to the palatal plane; L6tip, mandibular molar tip relative to the mandibular plane.

x-axis; (4) tip of the uppe r molar, the angle fo rmed by the tangent to the anter ior bo rde r of the uppe r molar and the PP; (5) tip of the lower molar, the angle fo rmed by the tangent to the anter ior borde r of the lower molar and the man- dibular plane; (6) total poster ior face height (TPFH), the distance between the x-axis and gonion as projected onto the y-axis; (7) uppe r posterior face height (UPFH), the distance be- tween the x-axis and condylion as projected onto the y-axis; (8) lower poster ior face height (LPFH), the distance between condylion and gonion as projected onto the y-axis; (9) total anter ior face height (TAFH), the distance be- tween the x-axis and men ton as projected onto the y-axis; (10) uppe r anter ior face height (UAFH), the distance between the x-axis and the anter ior nasal spine (ANS) as projected onto the y-axis; (11) lower anter ior face height (LAFH), the distance between ANS and m e n t o n as pro- jec ted onto the y-axis; (12) uppe r molar erup- tion (U6PP), the perpendicular distance be- tween the mesial contact point of the uppe r molar and the PP; and (13) lower molar erup- tion (L6MP), the perpendicular distance be- tween the mesial contact poin t of the lower mo- lar and the mandibular plane.

R e l i a b i l i t y a n d S t a t i s t i c a l Analysis

Reliability was evaluated by estimation of the intraclass correlat ion coefficient of reliability for each vertical cephalometr ic variable. This coei L ficient can be in terpre ted as the propor t ion of the variance that is caused by subject-to-subject variability. The estimate can range f rom 0.00 to 1.00, with higher values indicating more reliable measurements .

In t raexaminer reliability was assessed for 20 cephalometr ic radiographs, which were traced twice. The results of the in t raexaminer reliability for the 14 measurements in this study range f rom 0.933 for uppe r molar tip to 0.998 for total poster ior facial height.

Descriptive statistics were used to examine differences in the cephalometr ic variables be- tween the t rea tment groups. These differences were identified analysis of variance. Chi-square tests were used to detect differences between t rea tment groups between categoric variables such as gender and race. Linear regression mod- els were developed to examine the jo in t impact of race, gender, mandibula r plane angle, age, initial molar class severity, pre t rea tment , reten- tion, and t reaunent group on change of the

UPFH I , \

LPFH

L \t IoAF,

t LAFH Figure 2. Diagram of the linear measurements in- volved in the study. Presented here are the UAFH, LAFH, UPFH, and the lower posterior facial height LPFH. Also depicted are the perpendicular heights of the maxillary molar as measured from the PP (U6PP) and mandibular molars as measured (L6MP) molars as measured from the MP.

144 Dolce et a l

cephalometric variables between DC1 to DC5. A P value of less than .05 was considered statisti- cally significant.

To standardize the data, the results presented in Tables 1 through 4 are annualized, that is the difference between time point 1 and time point 2 is divided by the time between measurements to obtain a rate of change. This method was chosen so as to correct for the time needed to achieve a given correction. In this manner, a given amount of change is weighed more heavily if it occurred over a shorter period of time.

Results

Baseline characteristics tbr the 262 subjects are presented in Table 5. With the exception of the alveolar height of the mandibular molar, all other measured variables were not significantly different at the beginning of treatment. The mean active treatment (DC1-DC3) time, in years, for the bionator, HG/BP, and observation group was 2,2 + 0.7, 2.1 -+ 0.8, and 2.2 -+ 0.4, respectively. Although the total phase I (DC1- DC5) treatment time, in years, for the bionator, HG/BP, and observation group was 3.4 _+ 0.6, 3.2 -+ 0.7, and 3.2 -+ 0.4, respectively

The annualized skeletal and dental effects of treatment (DC1-DC3) are shown in Tables 1 and 2. Overall, the HG/BP group experienced sev- eral statistically significant changes in the mea- sured parameters that were not observed by in the bionator group when compared with the obsmwation gronp. The dental effects included the distal tipping of the maxillary molars (U6 tip) in the HG/BP group. Interestingly, the mandibular molar (L6 tip) tipped distally in all the three groups; however, the greatest amount of change was seen in the H G / B P group (Table 1). Eruption of the maxillary (U6PP) molars occurred in all three groups, the changes in the I tG /BP group were statistically significant from the other groups. The final observed dental

change was the eruption of the mandibular (L6MP) molars. Similar eruption of the mandib- ular molars occurred in both the HG/BP and bionator group when compared with the obser- vation group (Table 2). The observed skeletal changes were mainly in the HG/BP group and included the increase in mandibular plane angle and lengthening of the UAFH. Skeletal changes, which were observed in both the HG/BP and bionator group, were increases in LAFH and TAFH. Again the H G / B G group showed the greatest amount of change. TPFH increased equally in the HG/BP and bionator groups when compared with the observation group (Ta- ble 2).

Tables 3 and 4 depicts the changes for the total phase I time period (DC1-DC5). When the treatment phase and the 6 months of re tent ion/ nonretent ion and 6 months of obse~wation are combined, many of the dental effects of treat- ment for the headgear group have relapsed. For example, the change in the maxillary and man- dibular molar tip have disappeared. However, all of the skeletal effects seen during treatment per- sisted to the end of phase i for the HG/BP group with the exception of the anterior upper face height. All treatment effects for the biona- tar group persisted to the end of the phase I time period.

Because other factors besides treatment group may play a role in cephalometric changes, linear regression models were used to examine the joint impact of treatment and gender, race, mandibular plane angle, age, molar severity, pretreatment, and retention during the DC1 to DC5 time period. Accounting for the additional variables did affect the treatment group differ- ences by analysis of variance in three cases. Change in mandibular plane angle did not dif- fer significantly between the H G / B P group and the other two groups after including the addi- tional variables. Tip back of the lower molar was affected by the following variables: retention

Table 1. Annualized Angular (°) Changes From the Treaunent Phase (DCI-DC3)

Bionator H G / B P Control Mean ± SD Mean ± $1) Mean ± S1) P Value D~Jkrerlce

Palatal plane -0.02 -+ 0.6 0.03 ± 0.9 0.12 ± 0.7 MP -0.23 -+ 0.8 0.50 + 1.0 -0.39 ± 0.5 Occlusal plane -0.31 -+ 1.3 0.01 -+ 1.6 -0.51 -+ 1.2 U6 tip 0.35 ± 1.9 -1.22 + 2.9 0.84 -+ 1.9 L6 tip -0.42 ± 2.3 -1.66 -+ 2.3 0.24 + 1.4

.39

.0001

.06

.0001

.0001

H/BC

H/BC H/BC

Class H Malocc lus ion 145

Table 2. Annualized Linear (mm) Changes From the Treatment Phase (DC1-DC3)

Bionator H G / B P Control Mean ± 57) Mean ± S1) Mean ± 5D P Value Diffl, rence

UPFH 0.66 ± 0.8 0.42 ± 0.9 0.39 _+ 0.7 .06 LPFH 1.56 ± 0.9 1.73 + 1.1 1.37 ± 0.8 .07 TPFH 2.22 + 0.9 2.15 ± 0.9 1.76 ± 0.7 .0024 BH/C UAFH 1.25 _+ 0.7 1.56 ± 0.7 1.13 ± 0.7 .0001 H/BC LAFH 1.35 ± 0.9 1.66 + 0.9 0.92 ± 0.6 .0001 H/B/C TAFH 2.61 ± 1.1 3.23 ± 1.2 2.05 ± 0.8 .0001 H/B/C U6PP 0.65 + 0.4 0.92 + 0.5 0.69 ÷ 0.4 .0001 H/BC 1,6MP 0.72 m 0.6 0.76 ± 0.5 0.49 ± 0.3 .0011 HB/C

(P = .04), m a n d i b u l a r p l ane angle (P < .0001), p r e t r e a t m e n t (P = .0413), a n d t r e a t m e n t g roup with H G / B P differ ing f rom controls (P = .0177). UPFH change was affected by age at start of DC1 (P = .0245), a n d when add i t iona l vari- ables were inc luded , t r e a t m e n t g roup achieved only bo rde r l i ne s ignif icance (P = .0545). For all o the r variables, the inc lus ion of add i t iona l vari- ables (covariates) did no t change the t r e a t m e n t g roup differences we observed with univar ia te analysis.

Regression models were deve loped to exam- ine the impac t of t r e a t m e n t g roup an d o the r variables ( length of t r ea tment , gender , race, m a n d i b u l a r p l ane angle , age, mo la r severity,

p r e t r ea tmen t , a n d r e t en t ion ) on the a m o u n t of change . The ini t ial m a n d i b u l a r p l ane angle had an impac t on the o u t c o m e for the change in the

OP with those who initially had an average or h igh angle e x p e r i e n c i n g more change in the OP angle. The a m o u n t of t ime that the subject was in t r e a tmen t had a positive effect on the erup- t ion of the molars. Those who had a l onge r t ime span be tween DC1 and DC5 e x p e r i e n c e d m o r e u p p e r a n d lower mola r e rup t ion . Those subjects whose base l ine lower mola r he igh t was larger expe r i enced more e rup t ion , and those who had r e t e n t i o n expe r i enced less m a n d i b u l a r mo l a r e rup t ion . Both the base l ine value a n d ini t ial m a n d i b u l a r p lane angle had a positive effect on the a m o u n t of mo la r tip seen. For bo th the

Table 3. Ammalized Angular (°) Changes From Phase I

u p p e r a n d lower molar , the larger the base l ine mo l a r tip (the less it was t ipped back), the more it t ipped back d u r i n g phase l. The init ial man- d ibu la r p l ane angle had a similar effect with the subjects with a m o d e r a t e (be tween 30 ° a n d 40 °) m a n d i b u l a r p lane angle a n d those with a high angle (greater than 40 ° ) e x p e r i e n c i n g m o r e tip- p ing back of bo th the u p p e r a n d lower mola r d u r i n g the DC1 to DC5 t ime per iod. The larger the base l ine value for the UPFH, the smal ler the change was seen. In addi t ion , male pa t ients a n d those who had a l onge r span of t ime be tween DC1 to DC5 e xpe r i e nc e d m o r e of an increase in the TPFH. G e n d e r had an effect on the u p p e r an te r io r face height , with males e x p e r i e n c i n g m o r e change . Also, the greater the a m o u n t of t ime that e lapsed be tween DC1 a n d DC5, the grea ter the a m o u n t of change seen for the an-

ter ior upper , lower, a nd total face heights for all subjects

D i s c u s s i o n

The f indings of this study show that H G / B P or b i o n a t o r t r e a t m e n t p r o d u c e vertical changes in skeletal a n d den ta l s tructures, which are signifi- cantly d i f fe rent f rom the observat ion group. Al- t h o u g h the resu l tan t skeletal changes are stable

1 year after t r e a t m e n t irrespective of r e ten t ion , the den ta l changes have a p ropens i ty to relapse.

Treatment (DC1-DC5)

Bionator H G / B P Cont~vl Mean ± SD Mean +~ SD Mean ± SD P Value Diff,:fence

Palatal plane 0.02 -+ 0.5 0.01 ± 0.5 MP -0.31 ± 0.7 -0.05 ± 0.6 Occlusal plane -0.33 -+ 0.9 -0.62 ± 1.0 U6 tip 0.93 ± 1.1 0.66 ± 1.4 L6 tip 0.37 + 1.4 0.62 ± 1.3

0.10 ± 0.5 .25 -0.33 ± 0.6 .005 -0.53 ± 0.8 .15

0.82 ± 1.4 .28 -0.18 ± 1.1 .14

H/BC

146 Dolce et al

Table 4. Annualized Linear (mm) Changes From Phase I Treatment (DC1-DC5)

Bionator H G / B P Control Mean ± SD Mean ++- SD Mean ÷- SD P Value Difference

UPFH 0.48 _+ 0.6 0.46 ± 0.5 0.29 ± 0.5 .0349 BH/C LPFH 1,67 ± 0.7 1.60 -+ 0.7 1.48 ± 0.6 .22 TPFH 2.15 ± 0.8 2,06 -+ 0.7 1.78 + 0.6 .0024 HB/C UAFH 1.30 +- 0.5 1,41 ± 0.5 1.23 + 0.6 .09 LAFH 1.19 ± 0.7 1.18 ± 0.5 0.89 -+ 0.5 .0008 BH/C TAFH 2.49 -+ 0.9 2.59 ± 0.7 2.12 + 0.7 .0003 BH/C U6PP 0.76 _+ 0.4 0.85 +_ 0.3 0.72 _+ 0.3 .0520 L6MP 0.66 ± 0.4 0.57 ± 0.3 0.50 -+ 0.3 .0177 B/C

Posterior Face Height

In this study, TPFH was depic ted as the vertical

distance f rom the x-axis to condyl ion (UPDF) plus

the distance fi-om condylion to gon ion (LPFH).

Al though statistical significance was not me t for

U P F H and LPFH, the TPFH significantly in-

creased dur ing active t rea tment with e i ther

H G / B P or bionator. This skeletal change was last-

ing because it was present at the end of the reten-

t ion phase. We speculate that the condyle de-

scended relative to the x-axis to a greater ex ten t

than the ramus increase in length for the following

reasons. First, the b iona tor g roup nearly r eached

significance for U P F H for both t rea tment (P =

.06) and overall differences that were de tec ted

th rough the univariate analysis (P = .54) when

account ing for o the r variables. Second, the LPFH

changes did no t compa re with the UPFH. These

results contradic t o the r studies that have attrib-

u ted increases in TPFH to an increase in ramal

length.'~.4.~5a6 However, they suppor t Jakobsson 's 1

results showing the vertical descent o f the condyle

dur ing early t r ea tment with the headgear or biona-

tor. Similar, animal studies have shown that con-

tinuous, heavy distal forces to the upper molars

can alter resorpt ion at craniofacial sutures in grow-

ing monkeys, la,14 Specifically, the zygomaticomax-

illary, zygomaticotemporal , and the j unc t i on be-

tween the maxillary tuberosity, pterygoid plates o f

the sphenoid, and the pyramidal process experi-

ence m o r e resorpt ion with cont inual heavy head-

gear force c o m p a r e d with an un t rea ted observa-

tion group. Given these observations and the fact

tha tJakobsson I found a si~fif icant increase in the

U P F H for his headgea r group, it is possible that

the g lenoid fossa itself is affected by headgear

t r ea tment in the human. Because the fossa itself is

part of the tempora l bone, it may be physically

moved or rotated by forces transmitted through

the zygomat icotempora l suture, thereby displacing

the condyle f rom the g lenoid fossa e n o u g h to

cause an adaptive response. Alternatively, the tran-

sitory bite open ing that resulted f rom the molar

t ipping could tr igger an adaptive response in the

TMJ. Whatever the case, an increase in the TPFH

appears to be a lasting effect 1 year after headgear

t r ea tment was discontinued.

W h e t h e r this increase in the T P F H and pos-

Table 5. Group Comparison of the Studied Variables at Baseline

Bionator H G/BP Control Mean +- S1) Mean ± SD Mean + SD P Value

Angular measures (°) Palatal plane 3.10 ÷ 2.4 2.83 +- 2.0 3.18 + 2.3 .54 Mandibular plane 25.20 ÷ 5.9 25.98 +- 5.1 26.06 -+ 5.0 .51 Occlusal plane 12.99 ± 4.3 13.17 -+ 4.2 13.45 ± 4.4 .79 U6 tip 78.10 ± 4.8 77.97 -+ 5.1 79.10 + 4.9 .26 L6 tip 88.46 ± 5.6 89.33 + 5.1 88.62 -+ 4.3 .46

Linear measures (ram) UPFH 17.75 ± 2.8 17.97 -+ 2.6 17.64 -+ 2.8 .72 LPFH 51.43 ÷ 4.0 50.54 ± 3.1 51.46 ± 3.0 .12 TPFH 69.18 -+ 5.3 68.51 ± 4.0 69.10 + 4.0 .54 UAFH 40.77 ± 4.9 41.39 ± 4,8 41.46 ± 4.4 .67 LAFH 59.93 ± 4.6 59.72 +- 4.1 60.09 ± 4.4 .85 TAFH 100.70 +- 6.6 101.11 -+ 5.8 101.25 ± 5.8 .83 U6PP 16.99 + 2.0 17.26 ± 1.9 17.55 ± 1.9 .17 L6MP 27.38 -+ 2.1 26.28 -+ 2.3 27.60 ± 2.1 .01

Class H Malocclusion 147

sible descent of the condyle in the b ionator group reflects primarily a hard- or soft-tissue adaptat ion within the TMJ is debatable. Ani- mal studies involving an anter ior posit ioning of the mandible with a fixed functional appli- ance have found both types of effects. The dep- osition of new bone at the poster ior and supe- rior aspects of the glenoid fossa and similar adaptive changes in the condyle involving the intermediate zone of condylar cartilage at the posterior aspect have been noted. 17-21 Further- more, when the mandible is p ro t ruded forward and forced to function in this position full-time, the fibrous retrodiscal tissue has been found to proliferate posteriorly. 2°

Vertical Control With High Pull Headgear

Controlling the vertical dimension has always been of utmost importance during orthodontic treat- ment. Although much has been written on con- trolling the vertical dimension during headgear treatment, 2,2z27 most of these articles are either anecdotal ~5-27 or fail to follow-up with what hap- pens after the headgear is removed2 ,2e-24 It is spec- ulated that headgear treatment may result in an increase in the anterior face height and mandibu- lar plane angle with a correspondingly negative esthetic impact on the higher mandibular plane angle Class II patients. In the present study, during the DC1-DC3 treatment phase the H G / B P group experienced an increase of 0.50 ° in mandibular plane angle, whereas the bionator and observation group saw a decrease of 0.23 ° and 0.39 °, respec- tively. This decrease in mandibular plane angle was maintained during the 1-year retention/obser- vation period. Interestingly, during this re tent ion/ observation period the changes in the mandibular plane angle reversed in the H G / B P group. Con- sequently, phase I treatment (DC1-DC5) essen- tially resulted in no annualized change in mandib- ular plane angle.

All of the high mandibular plane angle (>40 °) subjects were treated with a high-pull headgear with no corresponding increase in the annualized change of the mandibular plane an- gle at the end of phase I. Because the initial mandibular plane angle did not have an impact on the increases in the total and lower anter ior face height for phase I, we can conclude that when high mandibular plane angle cases are treated with a high-pull headgear, there is no

greater increase of the anter ior face height over the observation group. Jakobsson ~ in his study failed to show a significant increase in the man- dibular plane angle.

Molar Eruption and Facial Height

As the mandibular molar erupts, it typically mi- grates mesially. 2-~) This and the fact that that Class II correct ion is facilitated if there is counter- clockwise rotat ion of the OP are often viewed as desirable events dur ing Class II or thodont ic t reatment. In this study, lower molar erupt ion was encouraged by the selective removal of acrylic occlusal to the lower molar in the biona- to t g roup and by the use of an anter ior bi teplane in the headgear group. Our data show that al- though both treated groups exper ienced lower molar erupt ion compared with the observation group dur ing the t rea tment phase (DC1-3), only the bionator group retained this change until the end of phase I (DCI-5), with no correspond- ing change in the angle of the OP. Conse- quently, a l though it is possible that erupt ion of the lower molar is contr ibut ing to the molar correct ion in some way, it does not necessarily have an effect on the inclination of the OP. The maxillary molars e rup ted dur ing DC1-DC3 in the H G / B P group with the changes f rom DC1- DC5 being nearly significant (P = .052).

Both the bionator and H G / B P group experi- enced significant mean increases in both the total and lower anterior face height during tlle treat- ment phase, which persisted through the end of phase I. It is possible that mandibular molar erup- tion in the bionator group contributed to the in- creases in the lower anterior face height, whereas in the H G / B P group, the previously mentioned changes have been attributed to a combined effect from both upper and lower molars.

Headgear's Effect on Lower Molar Tip

Many of the dental t rea tment effects for the H G / B P group were transient and relapsed by the end of phase I of the study. Among these was the distal t ipping of the molars. As expected, the use of a headgear p roduced distal t ipping of the maxillary molars. What was surprising was the distal t ipping of the mandibula r molars. Because no appliances were used in the lower arch dur- ing t rea tment with the HG/BP , the tipping ef- fect on the lower molar may be because of an

1 4 8 Dolce et al

altered occlusal vector from the upper molar. Alternatively, because the subjects who were in the headgear group were instructed to wear the biteplane full time, it may be that the biteplane itself has an effect on the lower molar tip. There is no evidence in the literature that indicates that the biteplane wear can result in a tip back of the lower molar. The headgear's effect of tip- ping back the molars may be responsible for the increase of the LAFH, increase in the TAFH, and increase of the mandibular plane angle dur- ing the active treatment phase (DC1-DC3).

Conclusions

Considerable variability is present in the vertical response to Class II orthodontic treatment with the bionator and HG/BP. The HG/BP groups experienced more changes in vertical parame- ters than the bionator or observation group. Also, the observed skeletal changes remained stable until the end on phase I, whereas the dental changes seen at the end of active treat- ment were lost by the end of phase I.

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