For patients who need to reduce tooth show from vertical excess problems, adequate intrusion mechanics should be chosen according to the needs respectively. The methods included using a conventional orthodontic appliance, such as intrusive arch6 or headgear and J-hook2,3
to apply intrusive force for upper anterior teeth. The intrusive arch could intrude the anterior teeth, and extrude, tip-back of posterior teeth at the same time.6
To prevent unwanted molars extrusion, we could lace the molars to posterior miniscrews or insert the intrusive arch directly to posterior miniscrews. However, the intrusive arch is a “shape-driven mechanics”. The force system changes according to the V bend position and angulation; the insertion site of the intrusive arch at the molar auxiliary tube or miniscrews also increased friction in sliding mechanics.
In this case, we combined used the bilateral miniscrews of the infrazygomatic crest and anterior sub-apical miniscrew to retract, intrude and control the torque of maxillary incisors. Compared with the intrusive arch, the anterior sub-apical miniscrew can provide pure intrusive force to anterior teeth (force-driven mechanics). These mechanics do not have the side effect of extrusion and tip-back of posterior teeth which could make the chin profile worsened; neither does it cause any additional friction in the force system. During the intrusion of anterior teeth, we provided light retraction force to prevent unwanted root resorption and anterior flaring.
While using miniscrews to sustain the intrusive force, the appropriate biomechanical design is required for efficient incisor intrusion and less risk of root resorption. According to Lee et al., a good periodontal condition should be maintained during the intrusion.7
The force magnitude should be low and constant. The recommended force for the intrusion of the upper four incisors should less than 100 g.6
And periodical periapical X-ray check of root condition in every 4-6 months is recommended. If retraction is also undertaken at the same time, the retraction force could not be too heavy to prevent loss of anterior torque control. In this case, we use light force provided by an elastic thread from the archwire to the subapical screw. The force exerted through this approach has been shown to effectively provide constant force level below 100g.8,9
We found some root blunting in the upper anterior region in the post-treatment panoramic radiograph, which is acceptable considering the amount of intrusion performed.
Concerning the amount of intrusion of posterior teeth and its relapse rate, Yao et al. found the mean intrusive movement of the maxillary first molars was 3-4 mm.10
Baek et al. found the maxillary first molars were intruded by 2.39 mm during treatment and at the 3-year follow-up, and the relapse rate was 22.88% in open bite cases.11
Eighty percent of the total relapse of the intruded maxillary first molars occurred during the first year of retention. Sugawara et al. also found the average relapse rate was about 30% at the 1-year follow-up in open bite cases.12
However, in Class II hyperdivergent cases who need total maxillary intrusion to improve facial esthetics, the treatment result showed good stability after the retention period from 12-21 months.13-15
This might be owing to the etiological basis of different malocclusions, and their ability for neuromuscular adaptations. In our case, the upper molars were intruded 1.6 mm after the treatment. However, at the finishing stage, the patient showed good stability of the occlusion at each visit, under the cancellation of intrusive force for the upper dentition. Yet concerning the unpredictability of active retention protocol, we did not design any active retainer for the patient, further follow up for the patient’s stability is required (Figure 8)
In this case, U6 was intruded 1.6 mm after treatment, while the lower molars slightly extruded during uprighting after treatment. If we could maintain or even intrude the lower molars, the counterclockwise rotation of the mandible would be further maximized and expressed thoroughly. However, if we take the patient’s facial proportions at frontal view into consideration, the treatment effect already turned her slight dolichofacial pattern into a mesofacial pattern, and her lower facial height proportions (subnasale-stomion/ stomion-menton) is already less than 1/2; it could be a worsening of facial esthetics if we increase the amount of intrusion for lower dentition regardless of facial proportions. Thus, more vigorous intrusion and counterclockwise rotation of mandible is not a treatment alternative for this patient.
According to Proffit's “envelope of discrepancy”, maxillary and mandibular incisors could be retracted 7 mm and 3 mm maximally by orthodontic tooth movement alone. If tooth movement aided by miniscrews, a larger amount could be accomplished.16
In our case, the U1 and L1 both retracted 6.4 mm. Sarikaya et al. found that lingual movements of the maxillary and mandibular incisors reduced the lingual bone width in both arches.17
And some of the patients demonstrated bone dehiscence. Pan et al.18
found during anterior retraction, there is a risk for the U1 root to contact the incisive canal and might cause external apical root resorption.18
Wainwright found that once the cortical plate had been penetrated, the buccal root surface became devoid of cortical bone.19
Although some osteogenesis took place during the 4-month retention period, it was insufficient to cover the root completely.
If we look at the anterior alveolar ridge width and their relationship to mandibular symphysis and facial patterns, skeletal Class II hyperdivergent patients or the ones with thin symphysis are the patients who own thinnest anterior alveolus width in maxilla and mandible, according to literatures.20-22
In these hyperdivergent cases, too much uprighted incisor inclination will result in fenestration at root apex, while buccal proclination will cause labial marginal bone dehiscence.23,24
From our post-treatment cephalometric radiograph, upper incisor position was still confined in the maxillary alveolus, while lower incisor was in contact with the lower lingual cortex. Besides, the total alveolar bone width harboring mandibular incisor roots is clearly very narrow, both in buccal and lingual directions, which indicates thinning of the alveolus in both arches during treatment. Although there are no signs of attachment loss/ gingival recession from the latest follow-up records, we should keep careful monitoring the periodontal condition over lower incisors to prevent iatrogenic consequences in the long run. Furthermore, cone-beam computed tomography (CBCT) can aid in accuracy in evaluation of bone fenestration and boundary condition rather than cephalometry in the alveolar bone.24
Thus, if there’s no recovery of periodontal bone after the recall CBCT check, periodontal bone grafting will be indicated in reestablishing healthy periodontium in this case.