Patient records were selected from 578 patients with Class II treated with myofunctional appliances between 1981 and 2021 at the Academic Centre for Dentistry Amsterdam and several orthodontic practices in the Netherlands. In all clinics, the same activator was used regardless of patient characteristics; some practices used only Ton Kooiman appliances (T-appliances), whereas others used only headgear activators (HACs) ( Table I ).

Cephalograms were taken pretreatment (T0) and posttreatment (T1). Treatment consisted of activator therapy followed by fixed appliances to refine occlusion and tooth position.

Inclusion criteria were pretreatment skeletal Class II malocclusion (bilateral sagittal end-to-end or more severe Class II molar relationship), completion of the first mixed dentition phase at the start of treatment (9-14 years), initial overjet >4.5 mm, activator treatment with T-appliance or HAC activator lasting between 6-15 months, correction of overjet and molar occlusion to Class I without extractions, and availability of T0 and T1 cephalograms ( Table II ).

The T-appliance is a removable activator supported by both the maxilla and the mandible. The sagittal relation between the upper and lower parts of the T-appliance can be altered by activating 2 U-loops, enabling the sagittal activation of the appliance ( Fig 1 , A ). The HAC consists of a monobloc activator with 2 arms for high-pull traction ( Fig 1 , B ). ^,

A , T-appliance B, HAC.

The HAC bite registration was taken approximately 7 mm mesial to the Class II occlusion, with a bite raise exceeding the vertical rest position—typically approximately 7-8 mm in the molar region. The force generated by the high-pull headgear is directed such that it produces an intrusive force on the anterior portion of the dental arch in hypodivergent and normal patients, whereas it targets the posterior portion of the dental arch in hyperdivergent patients. Patients were instructed to wear the HAC for 12 hours per day to achieve optimal results. For the HAC, the headgear arms were adjusted to be approximately parallel to the occlusal plane at insertion, with an inclination of ∼10°-15° downward relative to the Frankfurt horizontal plane. Activation was achieved by stepwise advancement of the arms by 1-2 mm at follow-up visits, depending on treatment progress and patient tolerance. The high-pull headgear generated an extraoral force of approximately 250-350 g per side. The direction of this force varies with the vertical skeletal pattern: In hypodivergent and normodivergent patients, it produces an intrusive effect primarily on the anterior portion of the dental arch, whereas in hyperdivergent patients, the force is directed more posteriorly, targeting the posterior portion of the dental arch. Patient compliance and appliance adjustments were monitored at each appointment.

The T-appliance facilitates growth modification through forward positioning of the mandible, compensating for disto-occlusion by 3-4 mm and achieving a bite raise of 2-4 mm between the incisal ridges. This force is continually maintained throughout the treatment by activating the adjusting springs at the back of the appliance. The T-appliance is worn for 14-18 hours per day, ensuring consistent force application for mandibular advancement.

Exclusion criteria were poor patient cooperation, treatment with extractions or missing permanent teeth, craniofacial anomalies (cleft palate, hemifacial microsomia, Pierre Robin syndrome, etc.), facial trauma, previous orthodontic treatment, a medical history or systemic disease interfering with growth and craniofacial development, and poor-quality cephalograms.

Before the study, sample size calculation demonstrated that 327 patients were necessary to perform a study with a power of 0.80 and α = 0.05 (G∗Power, version 3.1.9.6; Heinrich Heine Universität, Düsseldorf, Germany). However, based on the inclusion and exclusion criteria described above, 188 patients were eligible for inclusion, meaning that the power was lowered to 0.50. These patients were stratified based on the pretreatment mandibular plane–palatal plane angle: Hypodivergent (≤19°), normodivergent (20°-29°), and hyperdivergent (≥30°). As only 12 patients were classified as hypodivergent, this subgroup was excluded. Chi-square tests showed no significance between the initial population and the population after exclusion regarding gender, divergence, and appliance.

4 normodivergent and 4 hyperdivergent patients were excluded from soft tissue assessment as the soft tissue was not discernible. This omission, however, had no effect on the statistical evaluation.

The selected cephalograms were calibrated in such a way that 10 mm corresponded to 70 bits in the measuring program ImageJ (version 1.49; U.S. National Institutes of Health, Bethesda, Md) for Windows. The measurements were performed independently by 2 examiners. To test the intra and interexaminer reliability in locating landmarks and measuring dimensions, the observers performed all measurements on 30 randomly selected T1 cephalograms twice, at a 2-week interval. The intraclass coefficient and interclass coefficient tests showed values between 0.986 and 0.998 and 0.987 and 1.000, respectively, which are considered excellent.

Landmarks were mostly midsagittal; non-midsagittal were averaged. All landmarks used in this study are described in Table III , ^,,, and the measurements on the cephalograms are categorized into 5 sets.

Measurements parallel to the functional occlusal plane were expressed as Se–Landmark (∥FOP) and correspond to the POL reference line.

Measurements parallel to the Frankfort horizontal plane were expressed as Se–Landmark (∥FFH) and correspond to the PFFH reference line.

The first set consisted of horizontal measurements of distance from dentoskeletal planes to sella parallel to FOP ( Fig 2 ) and parallel to FFH. In addition to the 6 landmarks measured in the study by Pancherz, other landmarks were added to assess the sagittal skeletal development. A detailed image of all the landmarks in this set is provided in the Supplementary Figure .

Horizontal dentoskeletal planes, parallel to FOP.

The second set followed the study of Hack et al. The dentoskeletal planes depicting the distance of the landmarks to Se parallel to FFH (first set) and to FOP (second set) are as follows: PTM, Zyg, Art, A Point, Subspinale A, Prst, Is/Ii, Infr, B Point, Supramentale B, Pog, Gon, and the most mesial point of first maxillary/mandibular molar (Ms/Mi) ( Figs 2 and 3 ).

Vertical dentoskeletal planes, perpendicular to FFH.

The third set involved horizontal planes from soft tissue landmarks to Se, parallel to FFH, as shown in Figure 4 ( blue uninterrupted lines): Nas, NP, Subnas, LabrS/LabrI, PlMent, and STPog. The Nas-Lab angle was the angle between Columella-Sella-Labrale Inferior ( Fig 4 , black uninterrupted lines).

Horizontal dentoskeletal ( blue ) planes and soft tissue ( black ) landmarks, parallel to FOP.

The fourth set contained vertical planes constructed through dentoskeletal landmarks, perpendicular to the palatal plane (PP) and the mandibular plane (MP), according to the analysis by Pancherz. ( Fig 3 ). For the following skeletal landmarks, the distance perpendicular to FFH was assessed: Gon, Zyg, Me, Infr, and Prst. Vertical position of two maxillary landmarks was assessed by measuring the perpendicular distance between the landmark and the spina plane (SP): Is and M1s. Vertical position of two mandibular landmarks was assessed by measuring the perpendicular distance between the landmark and the mandibular plane (MP): Ii and M1i. The lower face height (LFH) was measured directly between Me and SNA. The angle between the PP and MP quantifies the intermaxillary divergence.

The final set ( Fig 5 ) displayed overall skeletal and alveolar planes to display the direction of development of the lower face : Se-Prst, Se-Infr, Cd-Gn, Art-Gn, Art-Gon, and Gon-Gn. Moreover, the following angles were calculated: Art-Gon-Gn, Se-Gn/FFH, Se-Pr/FFH, Se-Infr/FFH, Cd-Gn/FFH, Art-Gn/FFH, Art-Gon/FFH, and Gon-Gn/FFH.

Planes that are used to evaluate the skeletal and alveolar dimensional development of the lower face.

The data were statistically analysed with SPSS (version 27.0; IBM Corp, Armonk, NY) for Macintosh. Intra and interexaminer reliability was assessed with the intraclass and interclass coefficient tests. Statistics were carried out based on the measurements of all 5 sets. One-way analysis of variance (ANOVA) test was performed initially to identify if the normodivergent and hyperdivergent groups differ statistically significantly regarding the changes (T0-T1) in the distances Se-Infr, Se-B, and Se-Pog measured both parallel to FFH and FOP.

Two-way ANOVA tests were performed on all points to identify whether there were statistically significant interactions between the normodivergent and the hyperdivergent group, treated with either of the 2 appliances.

When the 2-Way ANOVA test showed a significant interaction between appliance and divergence, independent samples t tests were carried out as post-hoc analysis. For all analyses, an α level ≤0.050 was considered significant.

The characteristics of the study population with regard to distribution of the appliances, the age at start and end of activator treatment, and total activator treatment duration are presented in Tables I and II . Eventually, 82 males (aged 148 ± 14 months) and 106 females (aged 145 ± 11 months) were included in this study.

Table IV shows the mean scores of the sagittal development of primary landmarks: Infr, B, and Pog. The normodivergent and hyperdivergent groups have an Se-Infra/FOP development of 5.19 ± 3.89 mm and 4.77 ± 3.79 mm, respectively. As for the Sella-Infr/FFH development, the normodivergent and hyperdivergent groups are 4.45 ± 3.90 mm and 3.64 ± 3.15 mm, respectively. Consequently, Infradentale shows a decrease in forward development as the divergence increases.

Se-B/FOP and Se-Pog/FOP show the same decreasing trend, indicating less effectiveness of myofunctional therapy as the angle palatal plane/mandibular plane increases. However, no statistically significant differences for all 6 sagittal dimensions were found between the normodivergent and hyperdivergent groups. Measurements parallel to FOP suggest more effectiveness compared with measurements parallel to FFH.

Table V describes the interactions tested by 2-way ANOVA between divergence and appliance in all 5 sets (FOP, FFH, soft tissue, vertical, and overall dimensions). The following changes in dimensions and 1 angle show statistically significant interactions: Se-Zyg/FOP, Se-A/FOP, Se-Pog/FOP, Se-PTM/FFH, Se-A/FFH, Se-Nas/FFH, Se-NP/FFH, Se-LabrI/FFH, LFH, Se-Gn/FFH, Se-Inf/FFH, Gon-Gn/FFH, and Gon angle.

Table V

Means and SDs of skeletal sagittal, soft tissue, vertical, and dimensional landmark developments and angles for normodivergent and hyperdivergent groups treated with a T-appliance or HAC (2-way ANOVA)

	Outcome	Normo HACmean (SD)	Normo Tmean (SD)	Hyper HAC mean (SD)	Hyper Tmean (SD)	P (appliance × divergence)
Skeletal sagittal (parallel to FOP)	Se–PTM (∥FOP)	0.91 (1.81)	0.81 (2.24)	0.87 (1.97)	0.69 (1.30)	0.543
	Se–Zyg (∥FOP)	2.35 (3.08)	1.06 (3.22)	0.89 (2.41)	1.33 (1.88)	0.041 ∗
	Se–Art (∥FOP)	0.78 (2.34)	0.48 (2.97)	0.22 (1.65)	−0.64 (1.44)	0.394
	Se–A (∥FOP)	2.59 (3.15)	1.45 (3.06)	0.95 (3.00)	1.69 (1.97)	0.036 ∗
	Se–Prst (∥FOP)	1.98 (3.59)	0.94 (4.23)	0.73 (2.63)	0.97 (3.10)	0.217
	Se–Is (∥FOP)	1.54 (4.11)	1.23 (4.80)	−0.49 (3.40)	−0.25 (2.95)	0.682
	Se–Ii (∥FOP)	5.41 (3.22)	4.35 (4.28)	4.62 (3.18)	5.08 (2.89)	0.148
	Se–Ms (∥FOP)	1.65 (3.48)	0.52 (4.46)	1.53 (2.71)	1.44 (2.85)	0.314
	Se–Mi (∥FOP)	5.61 (3.34)	5.71 (4.68)	5.25 (2.83)	5.89 (2.26)	0.601
	Se–Infr (∥FOP)	5.61 (4.50)	4.19 (4.48)	4.44 (3.26)	5.03 (3.04)	0.097
	Se–B (∥FOP)	4.37 (9.20)	3.35 (4.61)	3.85 (3.50)	4.78 (3.24)	0.299
	Se–Pog (∥FOP)	5.72 (4.35)	3.61 (5.23)	4.27 (3.80)	5.56 (3.47)	0.010 ∗
	FOP–FH angle	0.24 (2.76)	−0.10 (2.90)	0.35 (2.33)	1.00 (2.19)	0.214
Skeletal sagittal (parallel to FFH)	Se–PTM (∥FFH)	0.78 (1.48)	0.35 (0.89)	0.23 (1.80)	0.68 (0.88)	0.027 ∗
	Se–Zyg (∥FFH)	1.53 (2.52)	0.78 (2.09)	1.09 (2.83)	−0.08 (1.66)	0.545
	Se–Art (∥FFH)	1.09 (1.69)	0.60 (1.51)	0.61 (2.14)	0.25 (1.02)	0.796
	Se–A (∥FFH)	2.02 (3.51)	0.56 (2.31)	1.13 (3.02)	1.44 (2.48)	0.050 ∗
	Se–Prst (∥FFH)	1.07 (3.64)	−0.33 (2.63)	0.97 (3.29)	−0.14 (2.04)	0.748
	Se–Is (∥FFH)	0.63 (4.16)	−1.04 (3.42)	0.84 (3.96)	−1.86 (3.11)	0.366
	Se–Ii (∥FFH)	4.76 (3.96)	3.05 (3.30)	4.23 (3.45)	3.00 (3.58)	0.669
	Se–Ms (∥FFH)	0.69 (3.03)	0.43 (2.11)	0.56 (3.95)	0.44 (2.34)	0.873
	Se–Mi (∥FFH)	4.86 (3.48)	4.41 (2.46)	4.45 (3.82)	4.17 (2.07)	0.865
	Se–Infr (∥FFH)	5.65 (4.32)	3.33 (3.19)	4.16 (3.54)	3.19 (2.86)	0.225
	Se–B (∥FFH)	4.61 (3.49)	2.36 (3.53)	3.52 (3.34)	2.86 (2.42)	0.121
	Se–Pog (∥FFH)	4.80 (3.94)	3.81 (3.28)	3.05 (4.08)	3.14 (2.63)	0.355
Soft tissue (parallel to FFH)	Se–Nas (∥FFH)	3.81 (3.06)	2.31 (2.62)	1.88 (1.76)	2.72 (2.08)	0.023 ∗
	Se–NP (∥FFH)	6.49 (4.63)	4.10 (6.69)	2.52 (7.46)	3.82 (3.55)	0.050 ∗
	Se–Subnas (∥FFH)	3.87 (3.99)	3.69 (5.96)	1.71 (2.35)	1.26 (5.60)	0.848
	Se–LabrS (∥FFH)	2.91 (4.06)	2.45 (4.10)	1.12 (6.84)	0.82 (2.50)	0.916
	Se–LabrI (∥FFH)	5.53 (4.92)	3.76 (5.63)	1.40 (3.73)	2.47 (3.45)	0.048 ∗
	Se–PlMent (∥FFH)	4.89 (11.28)	5.28 (4.70)	3.17 (2.87)	4.47 (3.31)	0.685
	Se–Chin (∥FFH)	4.06 (11.07)	5.83 (5.82)	3.42 (3.78)	4.18 (3.30)	0.657
	Nas-Lab angle	3.43 (9.00)	−0.59 (9.73)	2.52 (7.95)	2.38 (8.18)	0.162
Skeletal vertical	V-Ment	7.11 (4.13)	5.07 (3.28)	5.87 (4.75)	5.44 (7.64)	0.296
	V-Infr	5.17 (3.51)	3.73 (2.17)	3.26 (2.65)	3.03 (2.24)	0.158
	V-Gon	5.57 (4.40)	4.29 (3.14)	4.19 (2.36)	3.03 (2.92)	0.913
	V-Zyg	1.33 (2.11)	1.00 (1.47)	0.97 (1.96)	0.53 (1.11)	0.842
	V-Prost	2.35 (2.95)	1.60 (1.72)	1.65 (2.42)	1.19 (2.49)	0.701
	V-LFH	5.35 (3.03)	3.73 (2.38)	4.10 (2.13)	4.14 (2.43)	0.039 ∗
	V-Ms	1.65 (1.49)	1.09 (1.49)	1.94 (1.39)	0.89 (1.04)	0.261
	V-Is	1.59 (2.44)	0.91 (1.40)	0.71 (2.12)	1.11 (2.20)	0.091
	V-Mi	2.39 (1.75)	2.07 (1.44)	2.19 (1.83)	1.97 (1.32)	0.848
	V-Ii	−0.19 (1.61)	−0.10 (2.71)	0.45 (1.70)	−0.28 (1.50)	0.125
	Div angle	−0.47 (2.06)	−0.04 (1.95)	−1.03 (2.21)	−0.50 (1.59)	0.876
Skeletal dimensional	Cond-Gn	9.37 (8.17)	5.51 (3.85)	5.57 (3.30)	4.86 (2.44)	0.062
	Gon angle	−0.28 (3.15)	0.50 (2.83)	0.49 (2.99)	−0.09 (2.74)	0.046 ∗
	Se-Gn	8.93 (4.45)	6.40 (3.71)	5.38 (3.75)	5.19 (2.63)	0.049 ∗
	Se-Pr	3.16 (4.18)	1.97 (3.95)	1.11 (2.73)	0.83 (2.66)	0.396
	Se-Inf	7.70 (3.49)	5.47 (3.06)	4.98 (3.35)	4.69 (2.20)	0.049 ∗
	Art-Gn	7.95 (4.36)	6.00 (3.14)	5.73 (4.20)	4.83 (3.44)	0.393
	Art-Gon	5.02 (2.85)	3.60 (2.59)	3.27 (3.11)	2.83 (2.18)	0.259
	Gon-Gn	4.54 (4.00)	2.77 (2.61)	2.47 (2.64)	2.78 (2.21)	0.031 ∗
	Se-Gn-FFH	−0.19 (1.61)	−0.10 (2.71)	0.45 (1.70)	−0.28 (1.50)	0.076
	Se-Pr-FFH	0.63 (1.71)	1.20 (1.81)	1.07 (1.32)	0.81 (1.24)	0.080
	Se-Inf-FFH	1.73 (1.73)	1.65 (1.65)	1.57 (1.57)	2.87 (2.87)	0.154
	Cd-Gn-FFH	0.26 (1.91)	0.33 (1.69)	0.47 (1.83)	0.06 (1.74)	0.741
	Art-Gn-FFH	0.54 (1.95)	0.40 (2.24)	0.51 (1.87)	0.47 (1.76)	0.860
	Art-Gon-FFH	0.11 (3.10)	−0.77 (2.6)	−0.24 (2.78)	0.31 (2.65)	0.111
	Gon-Gn-FFH	0.05 (2.27)	−0.30 (2.39)	−0.13 (2.13)	−0.53 (2.08)	0.945

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