Leveling the curve of Spee with a continuous archwire technique: A ...

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368 Bernstein, Preston, and Lampasso After treatment, the incisors retroclined a mean distance of –0.10 mm relative to the A-Po line, resulting in an overall (T3) mean proclination of 1.41 mm (SD, 1.57 mm). A negative value for the measurement represents lingual movement of the tip of the tooth. There was a statistically significant difference between the T1 and T2 measurements P ( .0001) (Table IV). American Journal of Orthodontics and Dentofacial Orthopedics March 2007 leveled completely, whereas 32% had slight residual COS at T2. For the latter patients, the average amount of COS at T2 was 0.19 mm; this is probably clinically insignificant. The T2 models all showed Class I molar and canine relationships with properly finished buccal DISCUSSION It is generally accepted that, with a few notable exceptions, leveling a deep COS makes an important contribution to the success of orthodontic treatment. 18-20,34,39-43 In a study that compared sectional and continuous archwire treatments of adolescent patients with Class II, deep-bite, low-angle malocclusions, it was found that both techniques corrected deep bites. 4,5 Although these studies compared the effective - ness of overbite correction as measured on cephalometric x-rays, they did not measure the COS, or the effectiveness and long-term stability of leveling the COS. Our study was prompted by a belief that there was a need for a long-term, comprehensive, radiographic, and study model analysis of the effectiveness and stability of leveling the COS. Before we analyzed the data from the cephalometric investigation, the radiographic assessment of the COS performed in this investigation required validation. Integral to this process were the COS measurements 21 recorded by Carcara et al in their study model analysis of the COS in the same sample group. The 2 sets of measurements were compared and analyzed to determine whether the method of recording the COS from lateral cephalographic x-rays produced the same results as those recorded from the study models. There were no statistically significant differences (P .01) between the radiographic and the study model COS measurements for any of the 31 patients at T1, T2, and T3. 21 occlusions and normal overjets and overbites. Results of the paired t test comparing the COS at T2 with that at T3 indicated a statistically significant change (P .001) in this metrical character. The COS increased from a mean of 0.19 mm at T2 to a mean of 0.69 mm at T3. Thus, the COS relapsed on average 0.5 0mm over a mean period of 7 years 5 months after the fixed lingual canine-to-canine mandibular retainers were removed (an average of 11 years 5 months postdebond). Although the relapse in the COS might be statistically significant, it is a relatively small change, which, in turn, probably represents a normal physiologic process. The correlation percentage between the 2 meth - ods was approximately 97%. Only 1 patient who was measured to be level after treatment in the study model analysis was not considered level in the cephalometric analysis. Taking into account all factors that could have influenced the recordings of the COS by the 2 methods and the closeness of the results, we believe that the proposed radiographic assessment of the COS is valid. The cephalometric radiographs were taken on a Quint Sectograph that was set to focus on the cranial landmarks on the left side of the face. From this study, it seems that the continuous archwire technique is an effective orthodontic approach for leveling a COS in Class II Division 1 nonextraction deep-bite patients whose initial COS was 2 to 4 mm. Sixty-eight percent of the patients studied here were 19-21,27 A previous study, although con - firming that the leveling of the COS is a stable treatment outcome, could not show that the amount of leveling was correlated with the amount of relapse of this parameter. 44 Unfortunately, those authors did not specify the treatment technique used to treat their patients, who had various malocclusion types. In our study, the COS tended to relapse more in subjects with the deepest COS at T1. The Pearson correlation coefficient (initial COS vs posttreatment changes) was 0.380 (P .0349); this was statistically significant with set at 0.05 but not with set at 0.01. At least 1 other study confirmed our finding that the more the COS is leveled during treatment, the more it relapses after treatment. 45 That study had a relatively short mean posttreatment time of 2 years 8 months in their patients, who were treated with various appliances, and who had different malocclusion types. Although dental heights were recorded in the study of mixed malocclusions, no attempt was made to compensate for the affects of growth on these dimensions. The overall long-term (T3-T1) effect of orthodontic treatment with the continuous arch technique was an average of 72.97% reduction in the pretreatment COS. Ten of the 31 patients remained 100% level over a time span of 5 to 25 years after orthodontic treatment. Only 5 patients had residual COS of over 1 mm, and none was deeper than 2 mm. This study indicates that relapse in the COS occurred slowly and over an extended period of time. Although there are speculations in the literature about the contributions of various occlusal elements involved in leveling the COS, these reports do not quantify the contributions. 3,19,43-47 At the outset of this study, there was an attempt to use a mandibular superimposition method, 48 patterned after Björk’s 21
American Journal of Orthodontics and Dentofacial Orthopedics Volume 131, Number 3 Bernstein, Preston, and Lampasso 369 structural method, 49 to measure the treatment-induced changes in dental heights. Whereas this technique was suitable for measuring changes in the mandibular incisor region (L1-MP), it produced method errors that were unacceptably large for the other 2 dimensions studied (L4-MP and L6-MP). Our results indicate that most of the leveling of the COS was accomplished by relative extrusion of the premolars (mean, 2.81 mm) as measured by the L4-MP perpendicular distance (Table III). It is, however, likely that some of the mean increase in this dimension could be attributed to bone apposition at the inferior border of the mandible. Without control growth data for Class II subjects, we decided to use the growth charts derived from normal white American youths. In support of the use of normal growth data, our selection criteria favored subjects with normal to shorter-than-average lower anterior facial heights as measured by the MP to SN angle (32°). The appositional growth in the premolar area was taken to be the average of the available mean growth increments of the molar and incisor regions. 36 In this study, the mean contribution of growth to the L4-MP perpendicular distance was calculated as 1.55 mm. 36 If this estimated growth is taken into account, then the mean net extrusion of the premolar was 1.26 mm. Likewise, taking appositional 36 growth (1.7 mm) into account, it can be said that during treatment the mean change in the L6-MP dis- 36 tance was 0.61 mm. Allowing for growth (1.4 mm) from T1 to T2, the perpendicular distance L1-MP decreased by a mean of –0.87 mm. The premolar and incisor changes were statistically significant (P .01) before and after the possible effects of growth were taken into account. These findings agree with previous studies suggesting that straight-wire techniques level the COS by a combination of premolar extrusion and incisor intrusion. 4,5,19,46 Those studies did not, how - ever, provide data to quantify the suggested tooth movements. The relapse of the COS (T3-T2), although small, was nonetheless statistically significant (P .01). The mean observed relapse in the COS could be attributed to changes in the relative vertical heights of 2 of the dental elements that were used to define the curve. Taking posttreatment growth changes into consideration, the mandibular incisor (L1-MP) erupted a mean distance of 0.66 mm, whereas the molar erupted a mean distance of 0.23 mm after treatment. The premolar, however, had no additional mean vertical dental height change after treatment and thus had no relapse. These results confirm previous findings of a slight return of the COS after treatment. 19-21,27 archwire mechanics. Several authors noted the effects of continuous 3-9 These effects include flaring of the mandibular incisors, extrusion of the mandibular molars, and clockwise opening rotation of the OP. Some features of the continuous arch technique, including the –5° of torque in the mandibular incisors and the –6° of distal tip in the mandibular molars, are somewhat unique to this technique. These features, along with heat-treated stainless steel archwires with reverse COS in the mandibular arch and omega stops tied back to the molar tubes, might play a role in preventing the side effects reported with other straight-wire techniques. 3-9 All subjects in this study had mild to moderate mandibular incisor crowding that, with a straight wire and a nonextraction approach to treatment, would have been aggravated by the leveling of the COS. 39 Despite the incisor crowding in our sample of patients, the effects of the orthodontic treatment on the position of their mandibular incisors were minimal. The angles (L1-NB, L1-MP) were not statistically significantly changed as a result of treatment (P .01). The mean treatment change of the significantly (P .01) altered variable (L1 to A-Po) was 1.51 1.57 mm. Because the T1 average position for the L1 to A-Po was –1.18 mm, a mean proclination of 1.51 mm placed the tip of the L1 ahead of the A-Po line by a mean of 0.33 mm. It is thus fair to say that, in this sample, orthodontic treatment did not result in excessive flaring of the mandibular incisors. Since there was very little alteration in the measures of the mean positions of the mandibular incisors during treatment, it was expected that little posttreatment change would occur. This was indeed the case; the posttreatment changes in the 3 measurements of mandibular incisor position studied here were small and not statistically significant (P .01). The L1 to A-Po distance relapsed an average of –0.10 mm to leave the tip of L1 just 0.23 mm ahead of the A-Po line. Both the T2 and the T3 recordings for the L1 to A-Po distance were close to the stated ideal position of this parameter. 46 In this group of patients, it can be - con cluded that the treatment-induced average advancement of the mandibular incisor was clinically acceptable and stable in the long term. From the data, it appears that the mean treatment change (T2-T1) for the mandibular molar inclination to the mandibular plane was –6.42° (SD, 3.04°). This finding indicates that the full 6° of tip back built into the molar attachments was expressed in most patients. The additional 0.42° might have been due to the use of reverse curved archwires that would tend to cause the posterior teeth to tip back even farther than the tip incorporated in the prescription of the appliance. This
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