Mini-implants and the efficiency of Herbst treatment: a preliminary study

Results

Two miniscrews in two different patients of the EG lost stability during treatment. One was immediately substituted with a new one inserted in an adjacent site, while the second one was tightened deeper until stability was achieved again. All patients achieved both skeletal improvement and dental correction to class I relationships (Figures 4 and5). The mean increase in lower incisor inclination at the end of treatment was 1° (range 0° to 2°) in the EG and 7° (range 4° to 10°) in the CG. The mean decrease in the A point, nasion, B point (ANB) angle at the end of treatment was 3.6° (range 2° to 5°) in the EG and 2° (range 1° to 3°) in the CG (Table 1). As the sample size was very small, no statistical analysis was performed.

Discussion

This preliminary study describes the association of a modified Herbst appliance and TADs comparing the effects of this new protocol to traditional Herbst treatment. To date, with a history of over 100 years, the Herbst appliance is still considered a state-of-the-art solution and the most used noncompliance device for class II treatment[23–26]. All fixed bite-jumping devices generate both skeletal and dental effects, resulting as a main side effect in a proclination of the lower incisors, contributing to overjet reduction[14–16]. This undesired effect can minimize the skeletal effects and impede the desired forward displacement of the mandible, thereby preventing the establishment of a solid class I final dental relationship and a tight intercuspation. Although many attempts have been made to avoid lower incisor proclination, e.g., with the use of cast mandibular appliances, archwires with torque bends, or brackets with selective torques, absolute anchorage control has not yet been achieved and loss of mandibular anchorage can be anticipated[27, 28]. This is especially undesirable in patients where the lower incisors already exhibit a compensatory proclination at the start of treatment, for which reason, further proclination should be avoided during the correction of the sagittal discrepancy. This was taken into consideration when the patients were assigned to the experimental or the control group. A randomized assignment to groups was considered unethical, and the five cases with the greatest proclination of the lower incisors measured to the mandibular plane were therefore assigned to the EG. To reduce bias, the same bracket prescription was used in all the cases, and patients either requiring extractions or with lower arch crowding greater than 4 mm were excluded from the sample. Therefore, the changes in lower incisor position that resulted from leveling and alignment or the space closure phases were excluded. Most cephalometric errors are either due to experience of the operator or to landmark identification[29]. All landmark identification and measurements were carried out twice by the same experienced operator. In case of discrepancy between the first and second measurements, a third measurement was then performed.

Although TADs have become a daily clinical tool in orthodontics, their use, in combination with the Herbst appliance, has not yet been standardized into a treatment protocol or reported in clinical trials. This preliminary study describes how a combination of miniscrews and Herbst appliance can control the proclination of the lower incisors during treatment. The mandibular alveolar process has optimal cortical bone thickness and adequate inter-root distance for insertion of miniscrews with standard characteristics (1.5-mm diameter and 6-mm thread length)[30, 31]. Following the insertion and in the presence of primary stability, a stiff connection to the Herbst appliance was required. This was achieved by means of a 0.12-mm stainless steel ligature wire. In this way, the Herbst was directly connected to the lower dentition and indirectly to the lower basal bone with the aim of enhancing the skeletal effect and reducing the dentoalveolar compensations in the mandibular arch. Direct orthopedic load on miniscrews should be avoided as it most likely would increase the risk of failure of these devices designed to withstand standard orthodontic forces[32]. In all cases, the Herbst appliance was kept in place for a minimum of 9 months (mean duration of Herbst phase 9.6 months) to induce adequate skeletal and neuromuscular adaptation[33]. The stability of the Herbst in both groups and the stability of the TADs and tightness of the ligature wires in the EG were routinely checked monthly. The two miniscrews that lost stability were either positioned too close to a dental root[34] or loosened due to adverse local conditions (poor bone quality, inflammation, etc.). It is well known that mini-implants experience failure, and various success rates are reported without significant differences between immediately loaded or delayed loaded samples[35]. The conditions of the load (immediate and indirect) could not be blamed for the loss of stability. The success rates of the insertion sites, between the mandibular premolars and the second premolar and first molar, have not shown to differ from other mandibular or maxillary sites in growing patients[36]. Patient gender and side of insertion can also be considered irrelevant in relationship to success rates. Although the mandibular inter-radicular space between the first and second premolars is, on average, greater than the one between the second premolar and first molar[37], individual variations should be considered and periapical radiographs should always be used for implant site determination.

At the end of treatment, all cases displayed class I canine and molar relationships and reduction of the ANB angle. The results thus confirm the efficiency of the Herbst appliance in the correction of class II malocclusions[38]. This corroborates the findings of Schaefer et al.[39] who reported normalization of the dentoskeletal parameters following overall treatment in their stainless steel crown Herbst sample. However, there was a clear difference of the change in lower incisor inclination between the two groups studied. The side effects were reduced in the EG (Table 1). While the mean increase in the inclination of the lower incisors from T1 to T2 in the CG is comparable to the one reported in other similar studies with greater sample size[14, 39], as a result of the absolute anchorage delivered by the TADs, this proclination was smaller in the EG. In addition, the mean reduction in ANB angle was greater in the EG (Table 1), confirming the fact that reducing the dentoalveolar compensations during Herbst treatment can help to achieve a greater skeletal effect. Although the EG showed reduced dentoalveolar compensations compared to the CG, the results should be interpreted with caution due to the small sample size.