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About 13 months into treatment, the crowding had been relieved, but the Class II malocclusion and overjet remained. An .016" x .022" stainless steel archwire with 13º additional labial crown torque in the anterior region and an accentuated curve of Spee was placed in the maxillary arch. Hooks were welded to the archwire on both sides between the canines and first premolars. Two miniscrew implants^* (1.8mm in diameter, 7mm long) were placed in the alveolar bone, about 1mm distal to vertical lines from the contact points between the first and second molars, to anchor distal movement of the entire maxillary dentition (Fig. 2). The hooks were kept short to generate a clockwise moment and thus avoid extrusion of the maxillary posterior teeth (Fig. 3). Power chains^** were attached between the hooks and miniscrews, with a retraction force of about 200g on each side.

Discussion

In a case such as this, each miniscrew would ideally be implanted in the attached gingiva, adjacent to or apical to the level of furcation of the first molar, so that the vertical line of force would pass through the center of resistance of the entire maxillary dentition.⁵ The more apically the miniscrew is placed, the greater the vertical force vector that will be applied to the posterior teeth. In practice, however, it is difficult to locate the precise position of the center of resistance. Therefore, to avoid producing a counterclockwise moment, the line of force of the power chain should pass slightly below the estimated center of resistance (Fig. 3).

The distance between the molar roots will determine the amount that the maxillary arch can be moved backward or forward.^5-7 More interproximal alveolar bone is available between the maxillary second premolar and first molar roots and between the maxillary first molar and second molar roots than in other locations.⁶ Additional space for distal movement can be obtained by angulating the miniscrews 30-40º superiorly to the perpendicular of a plane tangent to the buccal cortical bone.⁷

In our clinical experience, it is better to place the archwire hooks between the canines and first premolars than between the lateral incisors and canines. Not only does this reduce soft-tissue impingement, but it also prevents transverse forces between the hooks and miniscrews from causing unwanted expansion of the posterior dentition. In premolar extraction cases treated with sliding mechanics, the posterior arch is naturally constricted as a result of posterior anchorage loss. In nonextraction cases, where the arch length is longer, the posterior teeth resist constriction during retraction of the entire dentition. A palatal expander may still be needed to compensate for the relative narrowness of the maxillary arch as it is moved backward. No lingual molar tipping, such as that reported by Park,² was observed in the present case, which might be due to the initial buccal tipping of the posterior teeth caused by the palatal expander.

After treatment, the maxillary incisors were slightly proclined because of the angulation in the bracket prescription and the 13º labial crown torque in the archwire. A bodily distal incisor movement of about 3mm was evident in the cephalometric superimpositions, however, resulting in a slight retrusion of the upper lip compared to the Ricketts E-line (Table 1). The mandibular plane angle was well maintained.

Minor extrusion of the upper second molars, which is commonly seen after distalization of the posterior teeth,^3,4 occurred in this case despite the vertical force vector and clockwise moment applied to the archwire (Fig. 5A, Fig. 5B). The upper second molars were also tipped back, creating a marginal ridge discrepancy between the first and second molars. To prevent this, the upper second molar tubes should have been bonded more occlusally.