Unexpected Tooth Movement Under Bonded Retainers

Diagnosis and Treatment Planning

A 24-year-old female presented with the chief complaint of a progressive buccal inclination of the lower left canine (Fig. 1).

She had previously been treated in our department for a Class II, division 1 malocclusion, beginning at age 9 (Fig. 2).

At that time, she displayed habitual mouthbreathing, along with a low resting position of the tongue. The lateral cephalogram showed adenoid hypertrophy and strained lip closure. Cephalometric analysis indicated a skeletal Class II relationship and a hyper-divergent jaw relationship (Table 1).

Treatment Progress

Orthodontic treatment had been performed with a removable upper appliance for maxillary expansion, followed by an Andreasen activator and then full fixed Tip Edge^* preadjusted appliances. The proclined upper incisors were retruded, while the lower incisors showed a slight proclination during treatment. The hyperdivergent jaw relation-ship persisted (Fig. 3).

An .024" stainless steel retainer wire was bonded only to the two lower canines.

Treatment Results

After 26 months of supervised retention, the patient was dismissed from regular appointments (Fig. 4).

Cephalometric analysis showed that the skeletal Class I and the hyperdivergent jaw relationships, as well as the incisor inclination, remained stable during the retention period (Table 1).

Nine years later, the patient reported that after debonding of her initial stiff-wire retainer some years earlier, another dentist had bonded a new lower retainer to all six anterior teeth. Clinical examination found a flexible spiral-wire retainer with five out of six intact bonding sites, from the lower left canine to the right lateral incisor. The patient could not recall a particular time when the unwanted tooth movement had begun, but we removed the retainer to prevent any further progression. New diagnostic records were taken.

During retreatment planning, all previous plaster casts and photographs were carefully analyzed. A comparison of intraoral photographs from the initial debonding appointment and the end of the supervised retention period indicated not only a minor proclination of the central incisors, but a slight tendency of the lower left canine toward buccal inclination (Fig. 4). The plaster casts from these time points were then digitized using a desk-top scanner^** to create standard tessellation lan-guage (STL) files. A 3D superimposition was per-formed with Viewbox 4^*** software, using a protocol previously reported for the superimposition of maxillary dental casts.^16,17 In our case, because of the lack of stable anatomical landmarks in the mandible, the crowns of the premolars and first molars were used as reference points (Fig. 5A).^15,18 After superimposition, a color-coded distance map was created to indicate deviations of the superimposed models from 1.5mm to –1.5mm, highlighting the movement tendency of the lower left canine (Fig. 5B).

The plaster casts from the retreatment records were digitized and super-imposed on the postretention casts in the same manner. That superimposition and color map emphasized the worsening inclination of the left lower canine, as well as the resulting retrusion of the central incisors (Fig. 6).

Discussion

At first glance, this case seemed to be a typical incidence of unintended tooth movement under a fixed flexible spiral-wire retainer. Careful analysis of all available records, however—with the added advantage of 3D superimposition—revealed the movement tendency of the lower left canine during retention with the initial stiff retainer wire. The flexible spiral-wire replacement retainer seemed to intensify, rather than limit, this inclination tendency.

Although the etiology of such complications is not yet known, some potential factors have been discussed. Iatrogenic activation during bonding has been assumed by some authors and cannot be completely ruled out in the present case.^7-9,13,14 The patient reported, however, that her new retainer had been bonded some years earlier; any undesirable effects of an active retainer should occur within a few weeks of bonding. Wire fatigue or deformation from masticatory forces,^8,9,11,14 as described by several in vitro studies,^19,20 could also have been involved. Our patient’s flexible spiral- wire retainer did show some signs of wire fatigue—at the time she presented in our clinic, it was bonded to only five teeth, with an assumed wire fracture between the lower right lateral incisor and canine.

To date, only three systematic retrospective studies have examined unwanted tooth movements despite fixed retention with multistranded retainer wires. Kucera and Marek, in an analysis of 3,500 retention patients, found 1.1% of the sample to be affected by a twist effect, an “X” effect, or other unprescribed tooth movements.⁹ This group was associated with a lower average age at debonding, a proclined position of the lower incisor relative to APo, and a greater mandibular-plane angle. Wolf and colleagues, in a study of 30 consecutively treated patients using 3D superimposition of dental casts, reported that 30% were moderately affected and 13.32% severely affected by the twist effect, noting an association with intercanine expansion or excessive overjet correction during orthodontic treatment.⁷ Klaus and colleagues found 14.1% of the lower fixed retainers in 163 patients to be affected by unwanted tooth movements.¹⁵ Compared to a control group, the affected patients were more likely to present with oral dysfunctions or habits prior to orthodontic treatment. These patients were also more likely to display an incisal overlap without interincisal contact—attributable to an abnormal resting position of the tongue or a tongue-thrust swallowing habit—resulting in orovestibular forces on the retained segments.¹⁵ Although Shaughnessy and colleagues have suggested that the magnitude and duration of tongue pressure are less than the force needed to bend a wire,⁸ the unwanted tooth movement may have been caused not by the wire itself, but by the altered biomechanical properties of the retained teeth. Because of the fixed wire, the retained segment works like a cantilever arm, transmitting the forces exerted on single teeth to the entire segment.

Several of these pretreatment- or treatment-related factors were present in our case: although the patient initially had normally positioned lower incisors (L1-APo = +1mm), she exhibited an excessive mandibular-plane angle (ML-NSL = 38°) and habitual mouthbreathing, along with a low resting position of the tongue. Additionally, her overjet was reduced by 8mm during orthodontic treatment.