Clinical Set Up

All patients received treatment with the 0.022-inch preadjusted edgewise appliance system, Roth prescription (GAC, International, Bohemia, NY). Once the initial leveling and aligning were complete, 0.017 × 0.025-inch stainless steel arch wire, with crimpable hooks (TP Orthodontics, LaPorte, Ind) placed distal to lateral incisors, was inserted in the upper arch. To ensure that the wire was passive, it was left in place for at least 4 weeks before initiating retraction. Titanium mini-implants (1.3 mm in diameter and 8 mm in length) were inserted between the roots of the first molar and the second premolar in both upper quadrants. The surgical procedure for implant placement involved incision of the overlying mucosa, preparation of the insertion site with a pilot drill under constant irrigation with a coolant, and placement of the mini-implants with a screw driver. The mini-implants were checked for stability and were immediately loaded with precalibrated nickel-titanium closed coil from the implant head to the crimpable hooks. A force of 150 g was applied bilaterally for en masse retraction of the upper anterior teeth. Direction of the applied forces was upward and backward (Figure 1). Conventional mechanics were used for the lower arch.

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Data Collection

The data were obtained by the analysis of paired lateral cephalograms for the 23 patients. In order to minimize the effects of any residual growth and to estimate treatment changes primarily due to retraction of anterior teeth, radiographs were taken just before initiation of retraction (T1) and immediately after complete space closure (T2). The length of time between two cephalograms was not more than 14 months for any of the subjects.

Data Interpretation

The tracings were done by one investigator with verification of anatomic outlines and landmarks by others. The suspicious structures and landmarks were retraced to their mutual satisfaction. Landmarks, cephalometric planes, and linear and angular parameters used in this study have been elaborated in Figures 2 through 5. Horizontal and vertical changes of certain landmarks were measured in relation to a Cartesian coordinate system. A Frankfort horizontal plane was constructed by subtracting 7° from the sella-nasion line. This served as the x-axis, and a line perpendicular to it through sella served as the y-axis (Figure 2). Linear and angular measurements were performed to the nearest 0.5 mm and 0.5°, respectively.

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Study Model Analysis

Study casts were obtained at T1 and T2. Intermolar and intercanine width changes were measured to quantify the anterior and posterior arch width changes for the maxillary arch before and after retraction with mini-implants.

Statistical Analysis

All statistical analyses were performed using the SPSS software package (SPSS for windows 98, version 10.0, SPSS Inc, Chicago Ill). The interpretation of the treatment changes between T1 and T2 was obtained by applying the Wilcoxon signed rank test.

The method of error was calculated using Dahlberg's method error formula, ME = (Σ d²)/2n, with d being the difference between the two determinations of the same variable, and n the number of double measurements (Table 1). The error varied between 0.346 and 0.934 and was within acceptable limits. Correlation analysis applied to the same measurement showed that the highest r value was 0.975 for Pog-Sv (chin position) with the lowest value 0.865 for the SNA angle.

Table 1.  Error of the Method

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Dentoskeletal Effects

In the present study, a surgical approach to treatment was not desired by the patients or their families, and although the underlying sagittal jaw discrepancies in some cases were severe, the selective extraction of permanent teeth was considered acceptable. The mini-implants maintained absolute anchorage and enabled simultaneous retraction of all six anterior teeth, into the extraction spaces to correct overjet ≥7 mm (range: 6–13.4 mm). Previously, with traditional mechanics, 1.6 mm to 4 mm of mesial movement of the molars has been noted.910 With the use of adjuncts for anchor preservation, up to 2.4 mm of anchor loss has been observed.1112 In sharp contrast, our study showed a net distal and intrusive movement of the maxillary molar, although not significant (P > .05; Table 3). After space closure, the contact between the canine and second premolar was established. At this point any further continuation of the retraction force resulted in its transmission to the posterior segments through the interdental contacts.

Table 3.  Descriptive Statistics for the Dental and Soft Tissue Changes (T2–T1^a)

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The coil springs in the majority of cases were left in place for at least a couple of months after space closure to obtain a tight overjet or close some residual spaces. This might have caused some distal movement and intrusion of the molars as observed in the cephalometric radiograph. Also, the vertical vector of the total force can cause some binding (or increase the friction) of the archwire to the brackets or tubes, preventing sliding and causing the total force to be transmitted through the archwire to the entire dentition. Similar results have also been reported in recently published case reports.67

Controlling the vertical plane during orthodontic treatment is often a difficult and unpredictable task. Although in our sample the UFH/LFH ratio decreased while PFH/AFH increased (P < .05; Table 2), indicative of a counter-clockwise rotation of the mandible, no significant change in the mandibular plane angle (SN-Go-Gn, PP-MP) or chin projection (Pog-Sv) was noted. This can perhaps be explained by the compensating eruption of the mandibular molars (0.52 ± 0.75 mm) in the lower arch. By focusing Class II correction on methods that control eruption and intrude teeth in both arches, greater amounts of true mandibular rotation and greater improvements in the chin projection might be expected, especially in high angle patients. It has been shown that posterior bite blocks and vertical chin cups, with or without headgear, can redirect condylar growth, increase posterior facial height, and reduce mandibular plane angles.1314 However, with such techniques patient compliance is always an issue.

A small, but statistically significant level of intrusion was observed for the maxillary incisors (−1.32 ± 1.08 mm). The force (F) exerted by the nickel-titanium coil springs (bilaterally) had two distinct components to it: a larger and predominantly retractive force (r) and a smaller intrusive force (i), causing en masse retraction and some intrusion of the anterior teeth (Figure 6). Despite extensive retraction of maxillary anterior teeth, there was no change in the SNA angle (−0.18 ± 1.23°). Insufficient torque control over the incisors during retraction might have been the cause. As the retraction force was below the estimated center of resistance15 (Cres), a clockwise moment (M) on the anterior segment was produced. Besides, a 0.017 × 0.025-inch SS archwire was used for retraction, which shows about 15° of play16 in a 0.022-inch slot, making torque control over the retracting incisors difficult. We could have supplemented the retraction unit with a torquing auxiliary for augmenting translatory mechanics, but that would have created additional forces on the maxillary arch, thereby compromising the true effects of skeletal anchorage.

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In the transverse dimension, the maxillary arch did not show any significant changes when measured at the intercanine width. However, a statistically significant decrease in the intermolar width was noted (Table 5). This can be attributed to the deformation that took place in the rectangular archwire because of the distal pull exerted by the coil springs (Figure 7). As the force was buccal to the Cres of the posterior segments, it caused the posterior teeth to move inward, resulting in a decrease in the overall intermolar width. This effect can be counterbalanced by using a transpalatal arch or by applying a buccal crown torque on the molar.

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Soft Tissue Effects

Significant changes were observed in all measured soft tissue parameters. A mean increase of 11.5° was observed for the nasolabial angle, while the upper and lower lips were retracted by 2.41 mm and 2.73 mm, respectively (Table 3). In a study by Luecke and Johnston,3 the upper and lower lips were retracted by 2.2 mm and 1.4 mm, respectively, relative to the E plane in upper premolar extraction cases. Paquette et al17 found only 0.7 mm retraction of the upper lip relative to the E plane. Demir et al18 reported mean retraction of 1.31 mm and 0.52 mm for the upper and lower lips, respectively. The increased retraction of the lips in the current sample was primarily due to the strict retraction requirements of the patient sample and larger mean maxillary incisor retraction.

It has been suggested that the way anchorage is managed, not the mere extraction of teeth, determines the magnitude of anterior dental retraction and the resulting change in lip position.19 Additionally, the upper incisors tipped lingually more than they were bodily retracted, causing increased lip retraction. It would be interesting to mention that despite extensive retraction, the mean scores for the upper and lower lips were well within Ricketts' esthetic ideal.20 Taking into account the flexible and mobile lip texture, a rather large variability in lip position can be expected on the lateral cephalogram, even when patients are instructed to keep their lips relaxed and their teeth in occlusion.21 An inability to quantify this variable remains a drawback of this study. Also, lip response as a proportion of incisor retraction decreases as the amount of incisor retraction increases, indicating that lips have some inherent support.22 The more regional effect of incisor retraction should be expected because even with orthognathic surgery, soft tissue changes decrease as the distance from the surgical site increases.

On a broader perspective, the final outcome of this study further widens the application of mini-implant anchorage, especially in patients showing moderate to severe Class II malocclusions. The possibility of using mini-implants as an alternative to orthognathic surgery in borderline cases should be further explored in the near future.