Mechanisms of Class II correction induced by the crown Herbst appliance as a single-phase Class II therapy: 1 year follow-up

Cephalometric analysis

Cephalograms were taken with the Frankfort horizontal parallel to the floor with teeth in occlusion and lips relaxed. All cephalograms were hand-traced by the same examiner (G.J.) on acetate paper. The tracings were digitized with Dentofacial Planner Plus software (Dentofacial Software, Toronto, Canada) on a computer with a digitizing screen (Numonics Cooperation, Montgomeryville, USA). All cephalograms were adjusted to 0% enlargement.

Two cephalometric landmarks, frontomaxillary nasal suture (FMN) and T-point (the most superior point of the anterior wall of sella turcica, at the junction with tuberculum sellae) have been used by Franchi et al.[5] as the reference points for studies of growing individuals. Skeletal and dentoalveolar changes were recorded using a grid system: the x-axis was drawn through T and FMN points, while the y-axis was constructed perpendicular to x-axis through T-point.

The following landmarks were digitized, and the changes were assessed in the constructed grid system: condylion (Co), gonion (Go), point A, point B, pogonion (Pg), menton (Me), upper first molar mesiobuccal cusp tip (ms), lower first molar mesiobuccal cusp tip (mi), upper incisor tip (is), and lower incisor tip (ii).

The following linear and angular measurements were performed:

Class II correction was calculated as described earlier by Pancherz[4], but the y-axis served as the vertical reference line.

Statistical analysis

Statistical analysis was performed with SPSS for Windows software (SPSS, Chicago, IL, USA). The data were tested for normality of distribution (Shapiro-Wilk test) and equality of variances (Levene's test). Independent t tests were used to detect the differences in the skeletal and dentoalveolar characteristics between the treated and control groups at the start of treatment as well as in the changes of the measurements during the observation intervals (T1-T2, T2-T3, and T1-T3).

The power of the study was calculated considering the ANB angle as a sensitive variable for the assessment of the orthopedic effects of Class II treatment. With a clinically significant change of 2.0 degrees in the ANB angle, a standard deviation (SD) for this angle of 1.3° (as derived from a previous study on the effects of the stainless steel crown Herbst)[15] and alpha of 0.05, the calculated power for the independent sample t test exceeded 0.90.

Method error

Twenty cephalograms selected randomly from the treated sample were retraced and digitized by the same investigator (G.J.) on two separate occasions at least 2 weeks apart to calculate the method error with Dahlberg's formula[16] and to assess the intraclass correlation coefficient (ICC). The method error for linear measurement ranged from 0.3 mm (lower incisor to T-FMN line) to 0.7 mm (lower mandibular cusp to T-FMN line) and for angular measurements, −0.2 degrees for ANB angle to 1.2 degrees for lower incisor inclination. ICC for linear measurements varied from 0.949 for overjet to 0.998 for point B to the y-axis. For angular measurements, the values ranged from 0.962 (for maxillary plane to T-FMN line) to 0.988 (for SNA and SNB angles).

Written informed consent was obtained from the patients for the publication of this report. The approval of conducting the present research was received from Kaunas Regional Committee of Ethics for Biomedical Research (Kauno Regioninis Biomedicininu Tyrimu Etikas Komitetas), Nr. BE-2-34.

Skeletal changes

The Herbst appliance had no effect on the sagittal position of the maxilla. No significant change was found in the horizontal and vertical positions of the maxillary and mandibular landmarks with respect to the stable basicranial reference system. During active treatment, the total mandibular length (Co-Gn) showed a statistically greater increase (1.3 mm, Table 3) with respect to the controls, an increase that was maintained in the posttreatment period (T1-T3 change, 1.5 mm), though not at a statistically significant level (Table 5). This alteration in the amount of mandibular growth during active treatment was associated with a significantly greater increase in the SNB angle (0.7°) and a significantly greater decrease in the ANB angle (−1.1°) that remained stable in the follow-up (T1-T3 change, 0.9°, and −0.6°, respectively).

As for the skeletal vertical parameters, the palatal plane angle (pp to T-FMN) showed a significant clockwise rotation of 1.2° in the treated vs. the control group; this effect was maintained throughout the posttreatment period. No significant change was recorded in the inclination of the mandibular plane to FMN or in the Co-Go-Me angle.

Dental changes

The treatment group showed a significant reduction of overjet (−2.7 mm) and correction of overbite (−2.3 mm) with respect to the controls. Although during the posttreatment period, a slight relapse was recorded for both overjet and overbite, statistically significant corrections were still present when considering the T1-T3 period (−1.4 and −1.0 mm, respectively).

The upper first molars moved backward at a statistically significantly amount (2.7 mm) and tipped distally (7.1°) in comparison to the untreated group. Although a major portion of the tipping rebounded during the follow-up (to 2.7°), the Class I relationship of the buccal segments was preserved, and the upper molars remained significantly tipped back by 4.6° in the treated sample with respect to the controls. During treatment, the upper incisors showed a significant uprighting (−2.6°) that was maintained when considering the T1-T3 follow-up period (−2.3°).

During active treatment (T1-T2), the lower first molars moved forward significantly by 2.5 mm and extruded significantly by 1.0 mm with respect to the controls. When considering the overall observation period (T1-T3), the lower molars still showed a significant forward movement of 2.1 mm while the extrusion of the lower molars was not statistically significant in comparison to control values. The lower incisors proclined significantly (4.6°) in the treated sample as opposed to the slight uprighting (−0.8°) in the controls during T1-T2. During the T2-T3 period, the lower incisors uprighted (−3.2°) as did the controls (−0.6°); when considering the overall observation period (T1-T3), the proclination of the lower incisors was still significantly greater in the treated group with respect to the controls by 2.8°. Although the appliance exerted a significant vertical force to the upper molars, no significant change in their vertical position was recorded. Vertical changes of the lower incisors in the study sample could be attributed to their proclination.

Class II correction mechanisms

The components of overjet and molar relationship correction during treatment and posttreatment changes are summarized in Figures 1 and2.

In the correction of molar relationship during treatment, an important parameter was the distal movement of the upper molars, which rebounded almost completely during the follow-up period. Thus, at the end of the observation period, the molar relationship was maintained by further forward growth of the mandible and forward movement of the lower molars.

The appliance produced an overjet reduction by changing the dental and skeletal parameters equally. During the follow-up period, the changes primarily were dentoalveolar in nature.

Skeletal changes

The increase in mandibular length induced by cHerbst therapy was found to be similar to that reported in some other studies[15, 18–20]. During the follow-up period, the mandibular length increased with the same amount in both the treatment and the control groups. Thus, at the end of the 1-year observation period, the net increase of the mandible in the study group was 1.5 mm more compared to the controls; however, this difference was not significant statistically. This finding is consonant with the study by Pancherz and Fackel[21] in which the band Herbst appliance was used as the only appliance. When comparing the skeletal and dentoalveolar changes 31 months before and after treatment, they concluded that the Herbst appliance had only a temporary impact on the existing craniofacial growth pattern. Nevertheless, the occlusal relationships were improved, and the Class I relationship was maintained 1 year after treatment.

In the majority of the studies in which the first phase treatment with the Herbst appliance was followed by fixed appliances, a statistically significant net increase in mandibular length at the end of treatment was recorded[5, 6, 10, 15]. In most of those studies, an acrylic Herbst was used along with stepwise activation of the appliance[5, 6, 10]. The design and construction of the acrylic Herbst could suggest better dentoalveolar anchorage as well as the inhibition of vertical development because of the interocclusal coverage of the splint. In a direct comparison of the acrylic splint and stainless steel crown Herbst appliance, however, the investigation of Burkhardt and co-workers[20] indicated that the two appliance designs produced similar changes in horizontal and vertical skeletal position.

The moderate effect on mandibular length also can be explained at least partially by the difference in treatment strategies. Because previous studies on the factors influencing relapse after Herbst treatment[22, 23] emphasized the importance of attaining a stable occlusion at the end of Herbst appliance therapy, the treatment objective of the present sample was to establish Class I relationship during the active phase of treatment rather than to ‘overtreat’ the occlusion in to a ‘super Class I’ relationship. At the start of treatment, the appliance was activated to an edge-to-edge relationship, and during treatment, the buccal segments were controlled for settling, as mentioned earlier. As the lower incisors moved forward and the upper incisors retroclined, no further activation was possible in order to avoid the creation of a negative overjet.

Several previous studies have suggested that the following treatment strategy should be employed to induce increase in condylar growth: (1) stepwise advancement, (2) a 6-month duration for each instance of advancement, and (3) initial advancement of at least 5 mm[24–26]. In the present sample, not all of these requirements could be realized due to the predominant dentoalveolar effects of appliance. Although the cHerbst appliance had no significant effect in restraining sagittal growth of the maxilla, a significant restriction in vertical growth of the posterior maxilla was observed, an observation that also been reported in other studies of the Herbst appliance[4, 5, 10, 11].

Dentoalveolar changes

Similar to other studies[9, 22], significant dental relapse/rebound was recorded during the follow-up period, even though a Class I relationship was maintained. Interestingly, in overjet correction, skeletal changes contributed slightly more, while molar correction was achieved mainly by dental movements. Other studies have reported comparatively equal contribution of both components[4, 5, 11].

The Herbst appliance was shown to have a headgear effect, and the upper molars were distalized and tipped backward significantly. This finding was in agreement with other studies[4, 5, 10, 11, 18]. During the follow-up period, the upper molars tended to rebound to a more mesial position by 1.3 mm. However, a Class I relationship of the buccal segments was maintained. A similar observation was noted by Burkhardt et al.[20], who explained this phenomenon to be a result of a favorable growth pattern and dentoalveolar compensation.

The appliance produced a moderate lingual tipping of the upper incisors, while the lower incisors were proclined significantly. Similar findings also have been demonstrated in some other samples[4, 5, 10, 11]. Apparently, such factors as the method of Herbst appliance anchorage in the mandible[27] or the amount of initial activation[22] does not affect the proclination of the lower incisors.

One year after treatment, the lower incisors rebounded by 2.6°, resulting in overjet and overbite correction loss. Pancherz and Hansen[27] found that 80% of the lower incisors proclination recovered within 12 months; however, they noted that rebound of the incisors was not associated with significant crowding. An increase in overjet 3 years after successfully treated Class II malocclusion also was reported for the twin block appliance[28]. The appliance design facilitated forward movement of the molars within the mandible that was slightly greater than that reported in other samples[4, 5, 11]. Forward movement of the lower molars assisted in molar relationship correction because the overall mandibular forward movement was slightly smaller than that reported in previous studies[4, 5, 11].