Skeletal and dentoalveolar effects of Twin-block and bionator appliances in the treatment of Class...

ORIGINAL ARTICLE

Skeletal and dentoalveolar effects of Twin-block and bionator appliances in the treatmentof Class II malocclusion: A comparative studyAshok Kumar Jena,a Ritu Duggal,b and Hari Parkashc

New Delhi, India

Introduction: The purpose of this study was to evaluate the skeletal and dentoalveolar effects of theTwin-block and bionator appliances in the treatment of Class II Division 1 malocclusions. Methods: Fifty-fivegirls from North India with Class II Division 1 malocclusion and the same physical growth maturation statuswere selected for the study. The subjects were divided among a Twin-block group (n � 25), a bionator group(n � 20), and a control group (n � 10). Pretreatment and posttreatment lateral cephalometric radiographs ofthe treatment group subjects, and prefollow-up and postfollow-up radiographs of the control group subjects,were traced manually and subjected to the pitchfork analysis. Results: Statistical software was used for1-way analysis of variance and multiple comparisons (post-hoc test, Bonferroni). A P value of .05 wasconsidered statistically significant. Neither the Twin-block nor the bionator appliance significantly restrictedforward growth of the maxilla (P � .476). Mandibular growth in the Twin-block subjects was significantlygreater than in controls (P � .005). Mandibular growth was comparable in the control and the bionatorsubjects. Molar correction, overjet reduction, and proclination of the mandibular incisors were significantlygreater (P � .000) in the treated subjects compared with the controls. Conclusions: Both the Twin-block andbionator appliances were effective in correcting molar relationships and reducing overjets in Class II Division1 malocclusion subjects. However, the Twin-block was more efficient than the bionator in the treatment of
Class II Division 1 malocclusion. (Am J Orthod Dentofacial Orthop 2006;130:594-602)
Class II malocclusions can manifest in variousskeletal and dental configurations.1-5 MostClass II patients have a deficiency in the

anteroposterior position of the mandible.6 Several treat-ment options are available for managing Class IIproblems, and functional appliances have been used formany years in the treatment of Class II Division 1malocclusions.7-12 Several varieties of functional appli-ances are currently in use that aim to improve skeletalimbalances. Alteration of maxillary growth, possibleimprovement in mandibular growth and position, andchange in dental and muscular relationships are theexpected effects of these functional appliances. It hasbeen claimed that the forward growth of the maxillacan be inhibited,13-15 redirected,16 or unaffected17-19

by functional appliances. The effect of functional ap-

From the Division of Orthodontics, Department of Dental Surgery, All IndiaInstitute of Medical Sciences, Ansari Nagar, New Delhi, India.aSenior resident.bAssociate professor.cProfessor and head.Reprint requests to: Ritu Duggal, Department of Dental Surgery, All IndiaInstitute of Medical Sciences, New Delhi, India; e-mail, [email protected], August 2004; revised and accepted, February 2005.0889-5406/$32.00Copyright © 2006 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2005.02.025
594

pliances on mandibular growth is controversial. Someauthors suggested that mandibular growth can be in-creased with functional appliance treatment,20-24 butothers believe the appliances have no real effect onmandibular length.25,26 However, most researchers agreethat the appliances produce retroclination of the max-illary incisors10,27,28 and proclination of the mandibularincisors.29,30 There is no consensus on how the molarcorrection occurs.

Two of the more popular functional appliances usedtoday are the Balters’ bionator8,31 and Clark’s Twin-block.32 Few studies have compared the effects of theseappliances. Both are tooth-borne, but the Twin-block isdesigned for full-time wear to take advantage of allfunctional forces applied to the dentition, including theforces of mastication. The purpose of the study was toevaluate the skeletal and dentoalveolar effects of theTwin-block and bionator appliances in the correction ofClass II Division 1 malocclusions.

MATERIAL AND METHODS

The subjects for this study were selected from theOrthodontic Clinic, Division of Orthodontics, Depart-ment of Dental Surgery, All India Institute of MedicalSciences, New Delhi; 55 girls from North India having
the same cervical vertebrae maturation index were

American Journal of Orthodontics and Dentofacial OrthopedicsVolume 130, Number 5

Jena, Duggal, and Parkash 595

chosen.33 Each met the following criteria: (1) Class IIDivision 1 malocclusion with normal maxilla andretrognathic mandible, (2) stage 3 cervical vertebramaturation index (transition stage), (3) full-cusp AngleClass II molar relationship on either side, (4) mandib-ular plane angle less than or equal to 25°, (5) little or nocrowding or spacing in either arch, and (5) overjet of 6to 10 mm.

Girls with a history of orthodontic treatment, ananterior open bite, a severe proclination of the maxil-lary and mandibular teeth, or a systemic disease affect-ing growth were not considered for this study.

Ten subjects constituted the control group; theyreceived no treatment but were followed until the endof the study. The remaining 45, contstituting the treat-ment group, were divided into a Twin-block (n � 25)and a bionator (n � 20) group.

The subjects in the treatment group were treatedwith standard Twin-block or bionator appliances. Sin-gle-step mandibular advancement was carried out dur-ing wax bite registration. An edge-to-edge incisalrelationship with 2 to 3 mm bite opening between thecentral incisors was maintained for all subjects. TheTwin-block and bionator appliances were all made bythe same operator (A.K.J.). The Twin-block patientswere instructed to wear the appliance 24 hours per day,especially while eating; they could be removed fortoothbrushing. The patients in the bionator group wereinstructed to wear the appliance at least 15 hours perday. All subjects in the treatment group were checkedevery 4 weeks until the end of active functionalappliance therapy. Interocclusal acrylic trimming wasperformed in all patients to allow unhindered verticaldevelopment of the mandibular buccal segments. Acti-vation of the labial bow was avoided during treatment.Appliance use was discontinued when overjet andoverbite were reduced to 1 to 2 mm or when the patienteither was deemed to have finished active appliancetherapy or went on to further appliance therapy. Wear-ing times varied greatly, depending on the level ofpatient cooperation and the rate at which the deciduousteeth exfoliated.

Lateral cephalometric radiographs with the teeth inocclusion were obtained for all subjects before the startof treatment and at the end of active functional appli-ance therapy, or at the beginning and end of theobservation period. All cephalometric films were takenwith the same machine with the same settings.

The pitchfork analysis was used to evaluate skeletaland dentoalveolar changes that contributed to the cor-rection of Class II malocclusions.34 This analysis usescephalometric superimposition to measure physical
movement of the maxillary and mandibular molars and
incisors relative to their dental bases, as well as thedisplacement of the maxilla and mandible relative tothe cranial base. Measurements are defined as positiveif they contribute to Class II correction and negative ifthey aggravate the Class II relationship. The magnitudeof changes during treatment and the source of thechanges—eg, skeletal or dental—were also deter-mined. The algebraic sum of the various components isequal to the change in molar relationship and overjet.The pitchfork diagram summarizes the various compo-nents of change (Fig 1). Pretreatment and posttreatmentcephalograms were traced for each patient at the sametime, as suggested by Johnston.34 All measurementswere made 3 times, manually, with an electronic digitalcaliper, and the means were used for statistical analysis.

Statistical method

A master file was created and the data statisticallyanalyzed on a computer with software (SPSS, Chicago,Ill). A data file was created under dBase and convertedinto microstat file. The data were subjected to descrip-tive analysis for mean, range, and standard deviation ofall variables. One-way analysis of variance and post-hoc test (Bonferroni) for multiple comparisons wereused. Probability (P value) of .05 was considered

Fig 1. Pitchfork diagram: Cranial Base, base of cra-nium; Maxilla, maxillary change in relation to cranialbase; Mandible, mandibular change in relation to cranialbase; ABCH, anteroposterior change in relationshipbetween maxilla and mandible; Total U6, total maxillarymolar movement; Total L6, total mandibular molarmovement; Total Molar, ABCH � total U6 � total L6,change in molar relationship; Total U1, total maxillaryincisor movement; Total L1, total mandibular incisormovement; Overjet, ABCH � total U1 � total L1,change in incisor relationship.

statistically significant.

American Journal of Orthodontics and Dentofacial OrthopedicsNovember 2006

596 Jena, Duggal, and Parkash

RESULTS

The mean age of the subjects at the beginning of thestudy and the duration of the study are shown in Table I.The results of all measurements in the pitchfork anal-ysis are shown in Table II and Figures 2 to 4. Positivevalues are those contributing to the correction of theClass II malocclusion, and negative values are thosethat aggravated the Class II relationship.

Skeletal changes are shown in Table II and Figure 5.Mean movements of the maxilla were �1.98,

�1.33, and �1.56 mm in the control, Twin-block, andbionator groups, respectively. Both appliances had arestraining effect, but it was greater with the Twin-block than the bionator. However, comparisons ofmaxillary growth between the subjects in the 3 groupsshowed no statistically significant differences (P �.476).

The mean changes in mandibular position were5.02 mm in the Twin-block group, 4.42 mm in thebionator group, and 3.37 mm in the control group. Thedifference between the control and Twin-block groupswas large and statistically significant (P � .004). Thedifference between the control and the bionator groupswas minimal and not statistically significant (P � .386).

Table I. Mean ages and duration of study among contr

Control group (n � 10)Mean � SD

Age at start of treatment (y) 10.97 � 0.46Duration of study (mo) 16.37 � 0.94

Table II. Treatment changes for all measurements

Parameters

Control group (n � 10) Twin-block gr

Mean � SD95% CI for

mean Mean � SD

Maxilla �1.98 � 0.60 �2.41-1.55 �1.33 � 1.77ABCH 1.39 � 0.43 1.08-1.69 3.69 � 1.53Mandible 3.37 � 0.57 3.02-3.83 5.02 � 1.78U6 tip �0.32 � 0.22 �0.48-�0.16 0.02 � 0.48U6 bodily �0.86 � 0.36 �1.12-�0.60 0.05 � 0.75Total U6 �1.18 � 0.53 �1.56-�0.80 0.07 � 1.21L6 tip 0.04 � 0.34 �0.20-0.27 0.63 � 0.74L6 bodily 0.23 � 0.73 �0.29-0.75 0.82 � 0.77Total L6 0.27 � 1.06 �0.48-1.02 1.45 � 1.26Molar correction 0.48 � 0.80 �0.09-1.04 5.11 � 1.78Total U1 �0.55 � 0.51 �0.92-�0.18 1.45 � 1.33Total L1 �0.60 � 0.24 �0.77-�0.43 1.27 � 0.96Overjet 0.24 � 0.72 �0.28-0.74 6.31 � 1.71

C, Control group; TB, Twin-block group; B, bionator group; NS, No*P �.05; †P �.01; ‡P �.001.

The difference between the Twin-block and bionator

groups was also small and not statistically significant(P � .110).

The anteroposterior change in the relationship be-tween the maxillary and mandibular base made a meanpositive contribution in all 3 groups. The greatestchange in apical base (ABCH) occurred in the Twin-block group (3.69 mm), followed by the bionator group(2.86 mm) and the control group (1.39 mm). TheABCH between the control and Twin-block groups wasstatistically significant (P �.001), compared with thechange between the control and bionator groups(P �.05). However, the difference in ABCH betweenthe Twin-block and bionator groups was not statisti-cally significant (P � .107).

Dental changes are shown in Table II and Figure 6.In the control group, the mean total movement of

the maxillary first molar (U6) was �1.18 mm (�0.32mm tipping and �0.86 mm bodily movement). In theTwin-block and bionator groups, the mean total move-ment of U6 was 0.07 mm (0.02 mm tipping and 0.05mm bodily movement) and �0.31 mm (�0.18 mmtipping and �0.13 mm bodily movement), respectively.In the Twin-block group, U6 was moved distally.Forward movement of U6 was less in the bionator

in-block, and bionator groups

Twin-block group (n � 25) Bionator group (n � 20)Mean � SD Mean � SD

11.40 � 0.90 11.00 � 1.3012.78 � 4.00 16.18 � 2.52

� 25) Bionator group (n � 20)

Pvalue

Intergroupcomparison

C-TB C-B TB-BCI forean Mean � SD

95% CI forMean

-�0.60 �1.56 � 1.12 �2.09-�1.04 .476 NS NS NS-4.32 2.86 � 1.18 2.30-3.42 .000 ‡ * NS-6.14 4.42 � 1.66 3.36-5.16 .004 † NS NS-0.21 �0.18 � 0.71 �0.51-0.15 .215 NS NS NS-0.40 �0.13 � 0.74 �0.48-0.21 .003 † * NS-�0.57 �0.31 � 1.36 �0.95-0.32 .025 * NS NS-0.94 0.56 � 0.51 0.28-0.76 .039 * NS NS-1.14 0.79 � 0.56 0.53-1.05 .068 * NS NS-1.98 1.35 � 0.82 0.93-1.70 .016 * * NS-6.10 3.90 � 1.45 3.18-4.54 .000 ‡ ‡ †

-2.00 0.59 � 1.67 �0.18-1.37 .000 † NS NS-1.67 1.50 � 0.76 1.14-1.86 .000 ‡ ‡ NS-7.15 4.95 � 2.00 4.04-5.92 .000 ‡ ‡ *

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group than in control group. The movement of U6 in



the Twin-block group was significantly different fromthe control group (P �.05). However, there was nostatistically significant difference between the controland bionator groups (P � .198) or between the 2treatment groups (P � .840) for total movement of U6.

The mean total movement of the mandibular firstmolar (L6) in the control group was 0.27 mm (0.04 mmtipping and 0.23 mm bodily movement). In the Twin-block group, total mesial movement of L6 was 1.45mm, significantly (P �.05) greater than in the controls.Such movement included 0.63 mm tipping and 0.82mm bodily movement. Thus, significant forward move-ment of L6 was a factor contributing to molar correc-tion in the Twin-block group. In the bionator group,

Fig 2. Pitchfork diagram of changes in control group.

Fig 3. Pitchfork diagram of changes in Twin-blockgroup.

total movement of L6 was 1.35 mm. The net change in

molar position was due to tipping (0.56 mm) and bodilymovement (0.79 mm). The mesial movement of L6 inthe bionator group also differed significantly (P �.05)from the control group. However, mesial movement ofL6 in the treatment groups was comparable, and thedifference was not statistically significant (P � 1.000).

Molar correction is the algebraic sum of ABCH �total U6 � total L6. Molar correction was significantlygreater (P �.001) in the treatment group than in thecontrol group (Twin-block, 5.11 mm; Bionator, 3.90mm; control, 0.48 mm). Although mesial movement ofL6 contributed to molar correction in the treated sub-jects, molar correction was mostly due to ABCH. Theamount of molar correction in treated subjects wasdirectly related to the amount of ABCH.

The change in the maxillary incisors (U1) in thecontrol group was �0.55 mm. This was small com-pared with the maxillary skeletal change (�1.98 mm)in the control group, and it is a good example ofdentoalveolar compensation. In the Twin-block andbionator groups, U1 retroclined 1.45 and 0.59 mm,respectively, indicating the appliances had restrainingeffects. The difference in incisor change between thecontrol and Twin-block groups was statistically signif-icant (P �.01). However, comparison of U1 changebetween the treatment groups showed no significantdifference (P � .122).

In the control group, the mandibular incisors (L1)retroclined 0.60 mm; this was unfavorable for Class IIcorrection. In the treatment group, L1 proclined 1.27mm in the Twin-block group and 1.50 mm in thebionator group. Such proclination helped the overjetcorrection. The difference in L1 change between the

Fig 4. Pitchfork diagram of changes in bionator group.

control and treatment groups was statistically signifi-



cant (P �.001). No significant difference in L1 changewas found between the Twin-block and bionator groups(P � 1.000).

The change in overjet is the total change in incisorrelationship; it is the algebraic sum of ABCH � totalU1 � total L1. Overjet corrections were 0.24 mm in thecontrol group, 6.31 mm in the Twin-block group, and4.95 mm in the bionator group. In the treatment group,more than half of the overjet correction was contributedby ABCH. Intergroup comparison showed a statisti-cally significant (P �.001) difference in overjet correc-tion between the control and treatment groups. In theTwin-block group, overjet correction was also signifi-cantly (P �.05) greater than in the bionator group.

DISCUSSION

Our results showed that forward growth of themaxilla was slightly less in the treated patients than inthe controls, but the difference was not statisticallysignificant. When the mandible was postured forwardby the functional appliances, a reciprocal force acteddistally on the maxilla, redirecting growth.35 Neitherappliance effectively restricted forward growth of themaxilla. This agrees with some studies20,26,36-42 butcontradicts others.12,32,43,44 Thus, the design of theappliance and the duration of appliance wear were notmajor factors in the headgear effect of functionalappliance therapy.

The effect of functional appliance therapy on man-dibular growth is a major controversy. Many researchershave claimed that extra mandibular growth occurs withthe Twin-block36-39,43,45 and bionator appliances.43,46

In this study, we showed a statistically significantdifference in mandibular growth between the Twin-block and control subjects. We observed 1.65 and 1.05

-3

-2

-1

0

1

2

3

4

5

6

Control group

Twin-block group

Bionator group

elbidnaMHCBA allixaM

Fig 5. Comparison of skeletal changes among control,Twin-block, and bionator groups.

mm extra mandibular growth in the Twin-block and

bionator groups, respectively, compared with the con-trols. Toth and McNamara36 found 3.0 mm additionalincrease in condylion to gnathion length during astandardized 16-month period of Twin-block therapy,Lund and Sandler47 found 2.4 mm extra mandibulargrowth in a 12-month period, and Mills and McCul-loch38 found 4.2 mm more growth. Illing et al43

reported a 3.9 mm increase in mandibular growth withthe bionator appliance. These observations agree withthe results of investigations with other functional ap-pliances.48-51

On the other hand, some authors claim that themandible does not experience additional growth withfunctional appliance therapy.14,18 In our study, mandib-ular change was greater with the Twin-block appliancethan with the bionator. Duration and timing (duringfunction) of appliance wear could be responsible for thedifference. A randomized controlled trial by Tullochet al52 reported small mandibular changes with thebionator. Keeling et al53 made similar conclusionsabout growth modification with the bionator. However,O’Brien et al40 reported more mandibular changes withthe bionator than with the Twin-block.

The ABCH value represents the maxillomandibulardifferential, or the movement of the mandible relativeto the maxilla. A positive value indicates that themandible outgrew the maxilla, and a negative valuemeans that the maxilla outgrew the mandible. ABCHwas 1.39 mm in the control group, indicating 1.39 mmgreater anteroposterior movement of the mandible thanthe maxilla. Rushforth et al54 found 1.9 mm ABCH in17.3 months in Class II Division 1 control subjects. Inour study, the ABCH in the Twin-block group was 3.69mm in 12.78 months. The outgrowth of the mandiblewas significantly greater in the Twin-block than in the

-2

-1

0

1

2

3

4

5

6

7

Control groupTwin-block groupBionator group

Total U6 Total L6 Total Molar Total U1 Total L1 Overjet

Fig 6. Comparison of dental changes among control,Twin-block, and bionator groups.

untreated controls. ABCH was also greater in the



bionator group than in the control group. However,differences in ABCH between the 2 appliances werenot significant. The greater anteroposterior movementof the mandible in the Twin-block group contributed togreater Class II molar correction. Thus, we showed thatmandibular growth is more efficient with the Twin-block appliance than with the bionator or other func-tional appliances.54

Mesial movement of the mandibular molars anddistal movement of the maxillary molars or restraint ofthe maxillary molars as the maxilla moves forward areideal conditions for the correction of a Class II molarrelationship. Dentoalveolar changes with tooth-bornefunctional appliances have been widely discussed. Inour study, the maxillary first molars moved 1.18 mmforward in the control group; this was considerednormal. In the Twin-block subjects, the maxillary firstmolars moved slightly to the distal side (0.07 mm)compared with the forward movement of the maxilla(�1.33 mm). Restraint of the molars by the Twin-blockappliance could be responsible for this effect. Tumerand Gultan37 made a similar observation. However, Tothand McNamara36 found 1.5 mm distal molar movementduring Twin-block appliance treatment, and Lund andSandler47 noted 1.6 mm movement. Clark12 also founddistalization of the maxillary molars with the Twin-block appliance, and Mills and McCulloch45 concludedthat the headgear effect caused relative distalization ofthe maxillary molars during Twin-block treatment.Forward movement of the maxillary molars was 0.31mm in the bionator group. Compared with movementof the maxilla (�1.56 mm), it appeared that the forwardmovement of U1 was restricted by the bionator. How-ever, we showed that the Twin-block is more efficientthan the bionator in preventing forward movement ofthe maxillary molars.

The mean forward movement of the mandibularfirst molars was 0.27 mm in the control group. Lundand Sandler47 noted only 0.1 mm mesial movement ofthe mandibular first molars in their Twin-block controlsubjects, and Toth and McNamara36 found 0.5 mmmesial movement during a 16-month study. In ourstudy, the forward movement of the mandibular molarsin the Twin-block subjects was 1.45 mm, significantlygreater than in the control group. More forward move-ment of the mandibular molars in the Twin-blocksubjects was a factor contributing to the Class II molarcorrection. In the Twin-block subjects, Mills and Mc-Culloch45 reported more mesial eruption of the man-dibular molars, and Lund and Sandler47 noted substan-tial (2.4 mm) forward movement compared with thecontrols (0.1 mm). However, in contrast with our study,
Toth and McNamara36 found equal forward movement
of the mandibular molars in the Twin-block and controlgroups. We found 1.35 mm mesial movement of themandibular first molars in the bionator subjects; thiswas relatively less than in the Twin-block group. DeAlmeida et al46 found 1.4 mm mesial movement of themandibular molars in a 13-month period of bionatortherapy; this agrees with the results of our study.

The total molar movement (molar correction) is thesum of the movements of the maxillary and mandibularmolars with ABCH. The means that 5.11 and 3.90 mmof molar correction in the Twin-block and bionatorgroups, respectively, are largely due to the mandibleoutgrowing the maxilla, rather than to significant max-illary and mandibular molar movement. Molar correc-tion in the control group was only 0.48 mm. Thus, inuntreated subjects, although mandibular growth wasgreater than maxillary growth on an average, dentoal-veolar compensation appeared to have kept the buccalsegment relationship fairly static. In this study, 72.2%of the skeletal changes contributed to molar correctionin the Twin-block group, whereas 73.3% of the skeletalchanges contributed to molar correction in bionatorgroup. In contrast to this study, O’Brien et al40 foundonly a 41% skeletal contribution to molar correctionwith the Twin-block appliance. Their finding was alsosimilar to that of Tulloch et al.52 In our study, treatmentwas started during the peak pubertal growth spurt, andthis could have caused more skeletal contribution tomolar correction by the Twin-block and bionator ap-pliances. Thus, we showed that molar correction by theTwin-block appliance is not only greater but alsooccurs in a shorter time compared with the bionator.However, dentoalveolar changes contributed to rapidand greater molar correction with the Twin-blockappliance.

A widely accepted consensus is that the Twin-blockand bionator appliances cause retroclination of themaxillary incisors and proclination of the mandibularincisors.36-38,40,43,46 In our study, maxillary incisormovement was �0.55 mm in control group. However,the amount of incisor movement was less comparedwith movement of the maxilla (�1.98 mm), indicatinggood dentoalveolar compensation. In the Twin-blockand bionator groups, the maxillary incisors retroclinedby 1.45 and 0.59 mm, respectively. This could be dueto the so-called headgear effect of the labial bowappliance. However, this effect was disproved by manyauthors.40,52,53 Toth and McNamara36 concluded thatlingual tipping of the maxillary incisors is due to thecontact of the lip musculature during Twin-block treat-ment. This lingual tipping can also be due to the labialwire in both appliances, which might come into contact
with the incisors during sleeping, causing them to



retract.55 Toth and McNamara36 found less lingualtipping of the incisors in subjects wearing Twin-blockappliances without a labial bow. Trenouth39 found14.37° lingual tipping of the maxillary incisors with theTwin-block appliance. Lund and Sandler47 achievedsignificant maxillary incisor retraction using a maxil-lary labial bow, in contrast to Mills and McCulloch,45

who did not use a labial bow and found little change inmaxillary incisor position. Illing et al43 found greaterreduction in the proclination of the maxillary incisorswith the Twin-block appliance (9.1°) than with thebionator appliance (7.7°). Our results also support theresults of other authors.12,32,42,44,46

The most prominent dentoalveolar effect in thetreated subjects was proclination of the mandibularincisors, which was significantly greater than in thecontrols, and was probably a result of the mesial forceon the mandibular incisors induced by protrusion of themandible.36,43,44,56 In our study, the Twin-block andbionator appliances caused 1.27 and 1.50 mm ofmandibular incisor proclination. The slightly greaterproclination in the bionator group could be because theappliance was worn longer. Illing et al43 also foundmore mandibular incisor proclination with the bionator(4.0°) than the Twin-block (2.0°). Toth and Mc-Namara36 found 2.8° of forward tipping and 0.7 mm offorward movement of the mandibular incisors duringTwin-block treatment. Lund and Sandler47 reported7.9° proclination, and Mills and McCulloch45 found3.8° proclination with the Twin-block. We found anoverall �0.60 mm mean movement of the mandibularincisors in the control group. Such uprighting of themandibular incisors could be due to the restrainingeffect of the lower lip.

The change in overjet is the total change in incisorrelationship and is the algebraic sum of the ABCH �total U1 � total L1. As a result of treatment, overjetdecreased significantly in both appliance groups. Thegreatest reduction was in the Twin-block group (6.31mm), followed by the bionator group (4.95 mm) and thecontrol group (0.24 mm). In the treatment group,ABCH was the major factor contributing to overjetcorrection; other factors were restriction of forwardmaxillary growth, retroclination of the maxillary inci-sors, and proclination of the mandibula incisors. In thisstudy, the Twin-block appliance produced more skele-tal and dentoalveolar changes than the bionator, thusaccounting for more overjet correction. Mills and Mc-Culloch45 and Baccetti et al57 reported that 50% ofoverjet correction was due to skeletal changes withTwin-block appliance. Recently, in a multicenter, ran-domized controlled trial, O’Brien et al40 reported only
27% skeletal change in overjet correction. However, we
showed 58.47% and 57.77% skeletal contribution foroverjet correction with Twin-block and bionator appli-ance therapy, respectively.

Thus, timing of the appliance therapy—at the peakof the pubertal growth spurt—played a crucial role,contributing more skeletal effect for molar and overjetcorrection in the treatment of Class II Division 1malocclusions.

CONCLUSIONS

Early orthodontic treatment with the Twin-blockand bionator functional appliances appeared to beeffective in correcting molar relationships and reducingoverjets in children with Class II Division 1 malocclu-sions. The following conclusions can be drawn fromthis study.

1. Neither appliance was efficient in restricting for-ward growth of the maxilla.

2. Both appliances increased mandibular growth, butthe Twin-block induced more mandibular growththan the bionator.

3. Both appliances were significantly effective inrestricting forward movement of the maxillarymolars.

4. Both appliances resulted in mesial movement ofthe mandibular molars, with the Twin-block pro-ducing slightly more movement than the bionator.

5. Both appliances helped dramatically in molar cor-rection, and the Twin-block corrected the molarrelationship more efficiently than the bionator.

6. Forward movement of the maxillary incisors wasrestricted by the appliances.

7. The Twin-block and bionator appliances causedsignificant forward movement of the mandibularincisors.

8. Both appliances were effective for overjet re-duction in Class II Division 1 malocclusionpatients, but the Twin-block appliance was bet-ter than the bionator.

REFERENCES

1. Craig CE. The skeletal patterns characteristics of Class I andClass II Division 1 malocclusions in norma lateralis. AngleOrthod 1951;21:44-56.

2. Gilmore WA. Morphology of the adult mandible in Class IIDivision 1 malocclusion and in excellent occlusion. AngleOrthod 1950;20:137-46.

3. Henry RG. A classification of Class II Division 1 malocclusion.Angle Orthod 1957;27:83-92.

4. Renfroe EW. A study of the facial patterns associated with ClassI, Class II, Division 1, and Class II, Division 2 malocclusions.Angle Orthod 1948;18:12-5.

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