The aim of the present study was to evaluate Class II correction mechanisms with the cHerbst appliance by comparing the changes in the treated group with the growth changes of untreated Class II individuals. In the present study, only the subjects with stable Class I relationship 1 year after the single active treatment phase with cHerbst were included[13]. It also would have been of interest to analyze the unsuccessful cases. However, from the subjects who returned for follow-up visits, only eight growing patients experienced relapse, a number that was too small to evaluate statistically. Thus, this study evaluated the treatment effects of the cHerbst appliance alone because no additional orthodontic appliances (including retainers) were used during treatment stage as well as during the 12-month follow-up period.
A considerable advantage of the study was that the treatment of the consecutive subjects was conducted by a single operator who followed a strict protocol. The operator was not involved in the analysis of the data, thus reducing bias of the study. The treated and control groups were matched on the basis of skeletal maturity. Although historical control groups may present with limitations[17], in the current study, the use of historical controls was necessitated by the lack of ethical reasons to leave Class II patients untreated at the circumpubertal growth period, a stage of development that is known to represent the optimal time for orthopedic modifications[6].
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].