Correction of an Asymmetrical Class II Malocclusion Using Predictable Force Systems
Asymmetrical malocclusions can be caused by a number of skeletal, dental, and soft-tissue factors.1-3 If the etiology is mainly dental, the asymmetry may have developed from abnormal dental eruption, premature loss of deciduous teeth, or loss of permanent teeth. If the etiology is primarily skeletal, the patient may have a developmental or acquired asymmetry in either or both arches.4
It is extremely important to differentiate between dental and skeletal asymmetry before establishing treatment alternatives and objectives. Computed tomography, anteroposterior and 45º lateral oblique cephalograms, and submental-vertex radiographs can aid in the diagnosis and treatment planning of these cases. The most important tool, however, remains the clinical examination of the patient.3 An asymmetrical patient usually presents with a Class I occlusion on one side and a Class II or III occlusion on the other, so that the upper and lower midlines do not coincide with each other or the facial midline.5
The following case shows how a dental asymmetry can be corrected using predictable force systems.
Diagnosis and Treatment Plan
A 14-year-old patient in the permanent dentition presented with the chief complaint of crowded and overlapping anterior teeth. She had a Class II subdivision left malocclusion (Fig. 1). The maxillary midline had shifted 1mm to the right and the mandibular midline 2mm to the left from the facial midline. Cephalometric analysis indicated a retrognathic mandible with lower incisor flaring and a lower arch that was skewed to the left (Table 1). The overjet was 4.5mm, and the overbite was 50%. The patient reported premature loss of the mandibular deciduous teeth on the left side, which was diagnosed as the major etiologic factor in the dental asymmetry.
Following a comprehensive clinical and data-base analysis, we devised a treatment plan involving extraction of the upper and lower first premolars to achieve a symmetrical buccal occlusion, midline correspondence, appropriate overjet, and adequate retraction of the flared lower incisors.
Full-arch .022" appliances were bonded, and leveling and alignment were carried out with continuous .014" nickel titanium archwires. This caused the mandibular midline deviation to become even more evident. Maxillary space closure was begun with separate canine retraction, using an .016" x .022" stainless steel continuous base archwire and an overlaid .017" x .025" Beta III CNA intrusion arch, engaged from the first molar auxiliary tubes and tied to the four anterior teeth (Fig. 2). The intrusion arch was designed to provide two mechanical advantages: molar tipback moments for intraoral anchorage, and an intrusive force on the incisors to prevent any deepening of the bite from archwire deflection as the canines were retracted6 (Fig. 3).
To correct the mandibular midline, an .017" x .025" stainless steel archwire was split into buccal segments between the first molars and second premolars and an anterior segment from canine to canine, with a passive loop extending apically toward the center of resistance of the anterior teeth. An .017" x .025" Beta III CNA cantilever from the right first molar auxiliary tube was bent buccally and connected to the loop with elastomeric chain (Fig. 4). The cantilever was activated to achieve an efficient midline correction through pure translation of the anterior segment5 (Fig. 5).
The lower midline was corrected in eight weeks without any reactivation of the cantilever or tipping of the lower anterior teeth (Fig. 6). Maxillary canine retraction was completed in 12 weeks with no apparent anchorage loss or bite deepening (Fig. 7).
The maxillary incisors were then retracted with Beta III CNA mushroom loops, allowing the use of differential moments during space closure6,7 (Fig. 8). Activation of the M-loop increases the applied moment and thus the moment-to-force ratio, providing better anterior root control and posterior anchorage (Fig. 9). In the lower arch, an .017" x .025" stainless steel archwire was placed with Class II elastics to achieve simultaneous protraction of the mandibular molars.
After 16 more weeks of treatment, the upper incisor retraction was finished, but a small space remained on the lower left side. An .016" x .022" stainless steel archwire was placed, with an off-center V-bend distal to the lower left canine for space closure using differential moments8 (Fig. 10, Fig. 11). The space was closed in eight weeks, after which the roots were uprighted and the case finished. Total treatment time was 23 months (Fig. 12).