Frictionless Segmented Mechanics for Controlled Space Closure

Incisor protrusion is a fairly common condition that often results in lip protrusion and increased facial convexity.¹ A prevalent treatment strategy for this circumstance involves the extraction of the first premolars to immediately create the necessary space for incisor retraction.² However, the space closure phase can be challenging for clinicians, because in cases where the mesialization of posterior teeth is undesirable it needs to be avoided or at least minimized during the retraction of anterior teeth.

Alveolar or extra-alveolar temporary anchorage devices (TADs) and extraoral forces have played important roles in cases that require maximum anchorage,¹ but patients may not be receptive to further surgical treatment in addition to the extraction itself and may be reluctant to use external devices. Therefore, clinicians should be cognizant of alternative clinical strategies for anchorage control. Attention should also be especially given to incisor torque control since uncontrolled tipping of the incisor crowns may result in sagittal and vertical problems.³

The following case report describes a relatively simple, controlled segmented mechanical approach that utilizes differential moments without friction to minimize anchorage loss and control incisor torque. This treatment option enabled definable and predictable force systems to be applied so that the intended treatment outcome could be predictably achieved.

The patient was a 31-year-old woman whose chief complaint was that her maxillary and mandibular front teeth were "forwardly placed" and her smile was "unesthetic." Facial photographs taken by the clinician demonstrated a convex facial profile and a slightly asymmetric face (Figure 1 through Figure 3). She also presented with lip incompetence and was unable to close her lips without mentalis strain.

She was diagnosed with an Angle class I malocclusion,⁴ mild crowding in the mandibular arch, bimaxillary protrusion, and lack of canine guidance (Figure 4 through Figure 6). A lateral cephalometric analysis showed a steep mandibular plane (SN-GoGn 37°) with severe labioversion of the anterior teeth (1.SN 109°, IMPA 100°, 1.1 109°) (Table 1). Her lips were protrusive, with the upper lip to Ricketts "E" line at 2 mm and the lower lip to "E" line at 8 mm (Figure 7 and Table 1).⁵ A panoramic radiograph showed no abnormalities in the bone or the periodontal and periapical aspects. She had no notable medical history.

The treatment objectives were fivefold: improve the lateral profile and lip line, obtain ideal inclination of the anterior teeth, maintain the class I molar and canine relationships, achieve ideal overbite and overjet, and attain a mutually protected functional occlusion.⁶

Four treatment plan options were identified and presented. The first was to extract the four first premolars and retract the six anterior teeth in each arch simultaneously with TADs. The second treatment option was to extract the mandibular first molars, each of which had a large amalgam filling, and the maxillary first premolars, and to fully retract the entire maxillary and mandibular dentitions using TADs. The third alternative was to perform an anterior segmental osteotomy combined with genioplasty. The fourth option was to retract the anterior teeth with a two-step space closure without using TADs, instead using frictionless controlled segmented mechanics that utilize differential moments with power arms for anchorage control. The patient declined the surgical treatment plan and the use of TADs and did not want her mandibular first molars extracted. Therefore, the fourth treatment plan was selected.

A transpalatal bar and lower lingual arch were placed to avoid molar rotations and undesirable transverse changes. Standard edgewise brackets and tubes were passively bonded to the teeth to allow the prompt distalization of the canines after the extractions. The distalization of the canines was initiated 1 week after extraction of the premolars using a "T-loop" spring made with a 0.016-in x 0.016-in chromium-cobalt wire, to achieve closure of up to two-thirds of the spaces (Figure 8).

After retraction of the canines, the next step was to retract the incisors using two power arms on each quadrant connected with a power chain, which delivered a force of 100 g on each side (Figure 9). The posterior power arms on the molars were made with 0.017-in x 0.025-in stainless steel wires and were distally placed in the auxiliary tubes of the first molars. The anterior ones were part of a 0.019-in x 0.025-in stainless steel wire passing through the slots of the incisors, which created an incisor segment. The anterior and posterior power arms were positioned as close as possible to the center of rotation of the molar and incisor segments.

To address the treatment objectives for space closure, the power-arm heights were adjusted to be 3 mm to 5 mm apical to the bracket position (shortening of the anterior power arms provided controlled but not excessive lingual/palatal torque).⁷ Segmented mechanics were used to slightly intrude the mandibular incisors during the retraction of the maxillary incisors (Figure 10).

Once the retraction of the incisors was completed, a panoramic x-ray and maxillary and mandibular impressions were taken to evaluate root parallelism and possible marginal ridge discrepancy. The patient was then debonded and rebonded for final alignment and leveling. When both arches had been leveled and aligned, continuous 0.019-in x 0.025-in stainless steel arch wires were inserted for torquing control (Figure 11). The archwires were sectioned distal to the canines and vertical elastics were utilized in the posterior teeth for 1 week to hone and settle the occlusion.

The appliances were removed after a 24-month treatment period, at which point a maxillary wraparound and a mandibular bonded premolar-to-premolar fixed retainer were installed. A slight space was left distal to the maxillary lateral incisors for composite build-ups because of a tooth size discrepancy.

Facial photographs showed the improvement of the patient's profile after lip retraction, a balanced face with proportional vertical thirds, and a more appealing smile (Figure 12 through Figure 14). The protrusive incisors were retracted as needed. Intraoral photographs showed an Angle class I occlusion with normal overjet and overbite (Figure 15 through Figure 17). The mutually protected functional occlusion was achieved with stable and simultaneous occlusal contacts of all teeth in centric relation and eccentric contacts guided by the anterior teeth.

The post-treatment cephalometric (Figure 18) and panoramic radiograph demonstrated the positive changes achieved with the treatment. The cephalometric numbers confirmed that the maxillary and mandibular incisors were considerably uprighted (Table 1). The superimpositions revealed no extrusion of the maxillary and mandibular molars (Figure 19). As anticipated, the preferred molar positions were maintained almost unchanged, with minimal anchorage loss.

At the 2-year follow-up, the final result had remained stable.

Bimaxillary protrusion is characterized by severe buccal tipping of the anterior teeth; it results in lip protrusion and increased facial convexity.¹ Conventional treatment includes the extraction of the first premolars to modify the facial profile by retracting the anterior teeth and keeping the canines and first molars in class I relationship.² The retraction stage of the anterior teeth is a highly critical phase of the orthodontic therapy and requires precise mechanics to avoid unwanted movements and anchorage loss during treatment.

Two methods have been reported in the literature for this purpose.⁸ One is the en-masse retraction of incisors and canines; the other is a two-step retraction that begins with the distalization of the canines, which is followed by incisor retraction. Both methods may involve complex techniques,⁹ complicated spring (force system) designs,¹⁰ excessive friction,¹¹ and the use of TADs¹² and may create an indeterminate system. Controlling the force system is essential to precise tooth movement. Force direction, magnitude, and constancy, in addition to moment-to-force ratio, are crucial variables determined by the orthodontist during treatment.¹¹ The force systems should move teeth at an ideal rate (0.8 mm to 1.2 mm per month)¹³ in an extended range of activation while producing a relatively constant force system. This reduces tissue injury and the number of appointments and, at the same time, results in tooth movement with a nearly constant center of rotation.¹⁰

This case report describes a mechanical system that provides definable and predictable orthodontic forces. Moreover, constant force levels can be maintained, and the moment-to-force ratio at the centers of resistance can easily be regulated to produce the desired tooth movement. The estimated position of the center of resistance for the incisor segment was located within the mid-sagittal plane, approximately 6 mm apical and 4 mm posterior to a line perpendicular to the occlusal plane extending from the labial alveolar crest of the central incisor.¹⁴ The resulting force system can easily be modified by altering the magnitude and direction of force in relation to the center of resistance of the anterior segment; this can be done by changing the height of the anterior or posterior power arms.

Excessive retroinclination of the incisors can be moderated by the shorter distance from the force line of action to the center of resistance of the four incisors and by the buccal crown torque created by the binary of forces generated by the rectangular wire in the bracket slot.¹⁰ The power arms inserted in the auxiliary tubes of the maxillary first molars play an important role in anchorage for two reasons. First, this allows the placement of the vector of force close to the center of resistance of the teeth (in the trifurcation of the roots),¹⁵ which minimizes the mesial crown tipping created by the counter-clockwise moment. Second, the applied force causes a clockwise couple of force in the molar tube due to the gap between the wire and the inner walls of the tube, which may create distal crown tipping (Figure 20).

The use of a more determinate system and differential moments for space closure may also allow the orthodontist to minimize side effects, such as anchorage loss, without the use of TADs and extraoral devices. TADs have shown to be a stable source of anchorage for retraction of maxillary and mandibular anterior teeth.^10,11 However, as seen in the case presented here, patients sometimes decline the use of invasive methods or extraoral devices that would provide anchorage control. With this in mind, the use of frictionless segmented mechanics with differential moments can be a beneficial clinical alternative to produce a predictable force system between the posterior and anterior segments, enabling the magnitude of the moments and forces delivered to be well controlled in the three planes of space and yielding minimum anchorage loss.¹

Anchorage preservation is a critical factor in treating patients with alveolar dental protrusion. As observed in the superimpositions presented in this case, anchorage loss was minimal but the result was comparable with traditional methods of achieving maximum anchorage.¹¹ Moreover, as stated in the literature, when first premolars are extracted, the posterior teeth can be expected to move forward approximately one-third of the space, leaving the other two-thirds for crowding relief and incisor retraction.¹⁶ Therefore, absolute anchorage of the posterior teeth may not be essential to retract the anterior teeth as long as the orthodontist maintains approximately two-thirds of the extraction space, which can be achieved easily with this segmented technique.

Another advantage of the method presented here is the lack of friction during retraction of the incisors. It is estimated that 50% of an applied orthodontic force is dissipated due to friction.⁴ Therefore, the total force applied in an orthodontic treatment should be twice the force necessary to produce an effective force in the absence of friction. Excessive force, however, may increase bracket friction and escalate the potential loss of posterior anchorage. Additionally, positive correlations likely exist between increased force levels and root resorption.¹⁷ In this segmented method, the anterior and posterior segments are connected only by power chains or nickel-titanium coil springs, which eliminate friction.

An excellent final result was achieved with frictionless segmented mechanics, differential moments, no anchorage devices, and strategic planning of the relationship between the force line of action and the centers of resistance of the anterior and posterior segments. Taken all together, a controlled retraction of the incisors into the extraction space was achieved with minimum anchorage loss in a relatively short period of time.

This frictionless segmented mechanics method with differential moments and strategic planning of the relationship between the force line of action and the centers of resistance of the anterior and posterior segments can be used for patients who require anchorage control, such as for bialveolar protrusion cases. This approach may permit the application of definable and predictable force systems to enable clinicians to predictably and confidently achieve the desired treatment outcome.

The authors acknowledge Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES), which contributed a scholarship to Dr. Silveira's PhD residency.

Ildeu Andrade, Jr., DDS, MS, PhD
Associate Professor, Department of Orthodontics, Pontifícia Universidade Católica de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil

Fernando Amaral Moreira Guimaraes, DDS
Private Practice, Itaúna, Minas Gerais, Brazil

Giordani Santos Silveira, DDS, MS
PhD Student, Department of Orthodontics, Pontifícia Universidade Católica de Minas Gerais, Belo Horizonte, Minas Gerais, Brazil