Material and methods

The sample size was determined using G∗Power for paired t test. To determine the effect size, we did a preliminary study using measurements of 5 patients on a primary outcome (ANB). A minimum of 14 patients for 1 treatment group was required to achieve a statistical power of 0.95 with α of 0.05.

After obtaining ethical approval from the University of British Columbia Clinical Research Ethics Board (no. H20-01192) and approval from 2 authors (S.K.T. and L.J.A.) for using orthodontic records from their office, the records of 140 patients with a Class II relationship treated with MA appliance were obtained between 2017 and 2021. The records of 444 patients treated with the Herbst appliance between 2008 and 2019 were screened by one of the investigators (L.J.A.), and 69 patients with complete records were available for the study. The inclusion criteria consisted of consecutively treated patients with a skeletal Class II malocclusion with an ANB angle of ≥4°; mandibular plane angle of <40°; and lateral cephalometric radiographs taken before treatment (T1) immediately after the advancement phase (T2); and after completion of second phase treatment (T3). Exclusion criteria included patients with Class II Division 2 malocclusion, those past the peak puberty growth spurt or above cervical vertebral stage 4, missing radiographs, diagnosed with medical conditions or syndromes, congenitally missing teeth, or not completing treatment. Using the eligibility criteria, 20 MA patients and 20 Herbst patients matched in skeletal maturation and craniofacial morphology were included in the final sample. Each group had an equal number of males (n = 9) and females (n = 11). Both groups were evenly distributed in the cervical vertebral maturation (CVM) staging, with 3 at cervical vertebral stage (CS) 2, 4 at CS3, and 13 at CS4. The average age for the 2 groups was 12.7 ± 1.8 years for the Herbst group and 13.1 ± 1.5 years for the MA group. Age was not used for matching the 2 samples because of gender differences.

The lateral cephalograms of 10 randomly selected patients from each group were traced and superimposed twice, 2 weeks apart, to determine the intrarater reliability of the method for landmarks and tracings. Intraexaminer reliability of the skeletal maturation of the patients was assessed using the CVM index. The findings of the CVM stage were scored on all patients and repeated after a 2-week interval for intrarater reliability.

Patients in this group were treated with the Herbst appliance (Specialty Appliances, Cumming, Ga) ( Fig 1 ). Before the insertion of the Herbst appliance, brackets were placed on the maxillary incisors, and a sectional wire was placed to align the anterior teeth. The Herbst appliance was modified by removing the transpalatal arch for improved patient comfort, and buccal archwire tubes with a telescoping mechanism were used instead of rods and tubes to overcome their drawback of impinging on the ascending ramus. When the maxillary arch expansion was required for arch development, an expansion screw was added and activated at 0.25 mm/d until the required expansion was achieved. An initial advancement of 3 mm was usually prescribed, then gradual increments of 1-2 mm advancements every 2 months until a negative overjet of −1 to −2 mm was achieved.

Design of the Herbst appliance.

In the subsequent phase of comprehensive treatment, once the Herbst appliance was removed, GAC mini twin 0.022-in brackets were bonded to all teeth. The archwire sequence in the maxillary arch was 0.014-in nickel-titanium (NiTi), 0.016-in stainless steel (SS), 0.019 × 0.025-in NiTi, 0.018 × 0.025-in SS, and in the mandibular arch was 0.014 NiTi, 0.016 × 0.022 NiTi, and finished with 0.020 SS, 0.018 × 0.025-in SS wire. Treatment was completed once the buccal relationship had been corrected to a Class I occlusion with maxillary and mandibular dental arches well aligned and overbite and overjet within normal limits.

Patients in the MA group were treated with Invisalign with MA (Align Technology, Tempe, Ariz) ( Fig 2 ) until the molar was corrected to a Class I relationship on at least 1 buccal segment, or the overjet was edge-to-edge in centric relation position. Before prescription of an initial advancement, a deep overbite of ≥8 mm would require some leveling of the curve of Spee; an overjet of <2 mm in Class II Division 2 would require proclination of maxillary incisors; in the presence of a posterior crossbite, the maxillary arch would be expanded with aligners to correct the crossbite; and if the maxillary molars were severely rotated, this would be corrected before advancement of the mandible. During the advancement phase, the mandible was advanced sequentially 2 mm every 8 weeks until the incisors were edge-to-edge. Alignment of the anterior teeth from the first premolar to the first premolar in maxillary and mandibular dental arches was prescribed. The curve of Spee was also leveled by the intrusion of the mandibular anterior teeth. No interproximal reduction was performed during the advancement phase, so expansion and proclination were the primary mechanisms for alignment. At the end of the advancement, the patient’s dentition was re-scanned for the second phase of treatment. All patients were treated with additional aligners without the precision wings to complete orthodontic treatment. All patients had precision cuts for Class II elastics to be worn either full-time or part-time, depending on if further improvement to the molar relationship was required. The less severe side would be corrected to Class I in asymmetrical malocclusions. In the postadvancement phase of treatment, any remaining Class II relationship would be corrected through maxillary molar sequential distalization or a Class II elastic simulation jump. If the mandibular incisors had been proclined during alignment, a minor interproximal reduction of 0.2 mm would have been prescribed in the anterior mandibular arch to retrocline the mandibular incisors. Treatment was completed once the buccal relationship had been corrected to a Class I occlusion with maxillary and mandibular dental arches well aligned and overbite and overjet within normal limits.

Design of Invisalign with MA.

The radiographs were de-identified by way of a random number being assigned to each study subject and entered mock patient files created in Dolphin Imaging software (version 11.7; Dolphin Digital Imaging and Management Solutions, Chatsworth, Calif) with the assigned number and a random initial as first name and last name. For each patient, 3 time point tabs were created, and each radiograph was added to that specific time point; these were T1, T2, and T3. All radiographs were traced using a custom analysis in the Dolphin software.

Using the Dolphin Imaging software, a custom analysis was created using variables from Pancherz, Bjork, VanLaecken et al, and Wigal et al ( Figs 3-5 ). The measurement for each variable was performed twice. Linear and vertical measurements were recorded to the nearest 0.1 mm, and angular measurements to the nearest 0.1°. Analysis of the sagittal skeletal and dental changes was recorded along the occlusal plane and to the occlusal plane perpendicular (OLp) from the first cephalogram, which formed the reference grid. The grid was transferred to subsequent cephalograms by superimposing the tracings on the midsagittal cranial structure using the custom analysis in the Dolphin software.

Cephalometric landmarks and reference lines for sagittal measurements.

Cephalometric landmarks and reference lines for vertical measurements.

Cephalometric landmarks and reference lines for angular measurements.

To determine the amount of skeletal and dental contribution to treatment with the Herbst and MA appliance, changes in overjet and molar relationship were calculated with the formula described in Table I .

Twenty radiographs were randomly selected from both treatment groups, and radiographs were retraced at least 2 weeks apart after the initial tracing, and the intraclass correlation coefficient (ICC) was used to evaluate the intrarater reliability of the measurements ( Table II ). The ICC was conducted using a 2-way mixed model, focusing on absolute agreement between single measures. The ICC values for most error measurements of landmarks, tracings, and superimpositions were >0.80, indicating good agreement between the 2 measurements.

Results

The final sample consisted of 20 patients treated with the Herbst appliance, matched with 20 patients treated with the MA appliance. No significant differences were found in skeletal maturation or CVM stage between the Herbst (3.50 ± 0.76) and the MA group (3.50 ± 0.76).

Table III shows the Herbst (T1) and MA group (t1) craniofacial morphology at the start of treatment. The 2-sample t tests showed significant differences in 2 out of 9 variables in craniofacial morphology between the Herbst appliance (T1) and MA (t1) groups. The mandible was more retrusive in the MA group (75.4° vs 78.4°), and mandibular incisors were more proclined in the MA group (101.8° vs 94.6°).

The average treatment duration with the MA appliance was 14.4 months, followed by 17.9 months of treatment with clear aligners, giving an overall treatment duration of 32.3 months. The average treatment duration for the Herbst appliance was 18.5 months, followed by 18.3 months of treatment with a fixed appliance of 18.3 months, giving an overall treatment duration of 36.8 months.

Table IV shows the treatment changes with the Herbst appliance at T2 − T1 and T3 − T1. The matched paired t test results showed significant changes in 19 out of 25 variables between T1 and T2 measurements. For sagittal changes with the Herbst appliance, overjet reduction with the Herbst appliance was −4.8 mm with the over corrections. It was a combination of 0.6 mm forward movement of the maxilla, 4.8 mm forward movement of the mandible, −0.2 mm retraction of the maxillary incisors, and 0.4 mm proclination of the mandibular incisors. The skeletal contribution was −4.2 mm (88%), and the dental contribution was −0.6 mm (12%). Molar correction with the Herbst appliance was −6.4 mm. It was a combination of 0.6 mm forward movement of the maxilla, 4.8 mm forward movement of the mandible, −0.7 mm backward movement of the maxillary molars, and 1.5 mm forward movement of the mandibular molars. The skeletal contribution was −4.2 mm (52%), and the dental contribution was −2.2 mm (48%). Figure 6 shows the overjet correction components and the molar relationship change between T1 and T2.

Table IV

Changes in the Herbst appliance group for the periods T2 − T1 and T3 − T1

Variable	T2 − T1	P value	T3 − T1	P value
Olp-Co	0.39 ± 0.71	0.03 ∗	0.90 ± 1.15	0.003 ∗∗
Olp-A-pt	0.67 ± 1.28	0.03 ∗	1.45 ± 1.62	0.0008 ∗∗∗
Olp-Ms	−0.12 ± 0.88	0.57	2.51 ± 2.20	<0.0001 ∗∗∗
Olp-Mi	7.67 ± 2.46	<0.0001 ∗∗∗	7.60 ± 1.98	<0.0001 ∗∗∗
Olp-ls	0.46 ± 2.18	0.36	2.14 ± 2.50	0.001 ∗∗
Olp-Ii	5.23 ± 2.91	<0.0001 ∗∗∗	6.22 ± 6.83	<0.0001 ∗∗∗
Olp-B-pt	5.08 ± 2.78	<0.0001 ∗∗∗	5.25 ± 2.74	<0.0001 ∗∗∗
Olp-Pg	4.84 ± 3.30	<0.0001 ∗∗∗	5.52 ± 3.48	<0.0001 ∗∗∗
Olp-Gn	6.21 ± 3.56	<0.0001 ∗∗∗	6.61 ± 3.45	<0.0001 ∗∗∗
Wits	−3.42 ± 1.93	<0.0001 ∗∗∗	−2.82 ± 1.42	<0.0001 ∗∗∗
Ols-A-pt	1.40 ± 1.25	<0.0001 ∗∗∗	0.96 ± 1.58	0.01 ∗
ANS-Me	2.09 ± 2.67	0.002 ∗∗	3.19 ± 2.73	<0.0001 ∗∗∗
ls-NL	−0.09 ± 1.68	0.82	0.14 ± 1.37	0.66
li-ML	−0.93 ± 1.36	0.007 ∗∗	0.04 ± 1.89	0.93
Overbite	−3.29 ± 1.84	<0.0001 ∗∗∗	−2.35 ± 1.70	<0.0001 ∗∗∗
Msc-NL	−0.40 ± 1.33	0.20	0.42 ± 1.48	0.22
Mic-ML	1.18 ± 0.92	<0.0001 ∗∗∗	2.78 ± 1.52	<0.0001 ∗∗∗
SNA	−0.90 ± 1.54	0.02 ∗	−1.19 ± 1.69	0.005 ∗∗
SNB	0.93 ± 1.70	0.02 ∗	0.50 ± 1.84	0.24
ANB	−1.83 ± 1.39	<0.0001 ∗∗∗	−1.69 ± 1.03	<0.0001 ∗∗∗
SNL-NL	0.70 ± 2.01	0.14	0.82 ± 1.51	0.03 ∗
SNL-ML	0.08 ± 1.73	0.85	−0.52 ± 2.08	0.28
SNL-OLs	2.95 ± 2.94	0.0003 ∗∗∗	2.85 ± 3.58	0.002 ∗∗
ls/NL	−6.77 ± 8.73	0.003 ∗∗	−3.50 ± 8.07	0.07
li/ML	7.39 ± 6.91	<0.0001 ∗∗∗	3.85 ± 8.49	0.04 ∗

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