Occlusal outcome after orthodontic treatment with preadjusted straight-wire and standard edgewise appliances

Reliable assessment of patient records after treatment with a reliable tool is important in measuring treatment success in an objective way. Multiple indices have been introduced that make case evaluation easier [1‐5] and although the Peer Assessment Rating tool [4] is one of the most widely used, it does mostly measure malocclusion improvement and not precisely measure tooth positions within the occlusion. The Index of Complexity Outcome and Need (ICON) [5] was ambitiously developed to easily evaluate case complexity, treatment need, and malocclusion improvement in an objective way, but its esthetical component might receive disproportionate weight for assessments of treatment outcome.

A more efficient method to assess outcome of orthodontic treatment might be the Objective Grading System (OGS) from the American Board of Orthodontics (ABO) [6] that uses cast models and orthopantomograms after debonding. The ABO-OGS tool gauges finishing quality of the final occlusion based on eight criteria reflecting ideal intercuspation and function. Ideal arch alignment and intercuspation are scored with 0 points, deviations from ideal for each criterion are scored with 1–2 penalty points per tooth and a case can ultimately be categorized as “success” or “failure”—a system that shows high accordance in both inter- and intraexaminer level. The ABO-OGS has been extensively used in recent years— to compare several orthodontic treatment models and techniques [7‐16] with enhanced reliability, validity, and precision when evaluating the progress or final outcome of fixed appliance treatment [17, 18]. Furthermore, clinical data indicated that treatment outcomes fulfilling the standards of the ABO-OGS led to a balanced anterior temporalis activation and improved chewing function as reported by patients [19]. Finally, ABO-OGS is able to detect fine changes that occur in the occlusion postdebonding [20, 21] and evidence supports that better finishing quality with ABO-OGS leads to better long-term outcomes [22].

Fixed appliance treatment has become an integral part of modern orthodontics and has been a major focus point of orthodontic research. Since their initial introduction by Edward H. Angle, considerable development has been seen, with appliances becoming more and more sophisticated. A major historical point was the development of the preadjusted appliance by L. Andrews [2], which was followed by several variations in the values for tip and torque prescribed for each tooth. In recent years, focus has been added to try to make orthodontic treatment as efficient as possible, so as to minimize adverse effects and maximize patient satisfaction [23]. A recent systematic review on various orthodontic fixed appliances [24] concluded that at the present there is very little evidence from controlled trials to form clinical recommendations about their comparative efficacy. Comparisons between preprogrammed and nonprogrammed appliances are limited and, in their majority, suffer from methodological limitations of uncontrolled studies including confounding and assessor bias.

The aim of this study is therefore to compare the results of orthodontic treatment with preadjusted straight-wire (SWIRE) and nonprogrammed standard edgewise (SEDGE) appliances. The primary research question is whether SWIRE appliances result in better occlusal outcome measured by ABO-OGS after class II extraction treatment compared to SEDGE appliances.

A total of 56 patients were included in this study, who were treated either with SWIRE (n = 28) or with SEDGE appliances (n = 28). No significant differences existed for most characteristics (Table 1). Among the included patients 25 (45%) were male, the mean age was 13.5 years (SD = 2.4 years), the mean overjet 5.0 mm (SD = 2.2 mm), the mean overbite 3.1 mm (SD = 2.4 mm), the mean ANB 3.9° (SD = 2.2°), and the mean SN-ML 36.2° (SD = 5.6°). A total of 40 patients (71%) were treated with 4 premolar extractions and 43 patients (77%) received a transpalatal arch. The only statistically significant difference between groups was the use of headgear, where more SWIRE patients received a headgear than SEDGE patients (89% versus 64%, respectively; P = 0.03).

Analysis with the DI (Table 2) showed that the two groups were on average similar in terms of baseline malocclusion severity (P = 0.26). The only significant differences between the two groups pertained to the DI criteria of anterior open bite and crowding. On average, patients in the SWIRE groups had significantly more anterior open bite (medians of 1.5 and 0, respectively; P < 0.001) and more crowding than SEDGE patients (medians of 4 and 2, respectively; P = 0.04).

The total ABO-OGS score was similar in the SWIRE and SEDGE groups (means of 31.3 and 34.0 points, respectively) with no statistically significant difference (P = 0.26; Table 3, Fig. 1a). Similar scores were also seen for most separate ABO-OGS criteria (P > 0.05) with the sole exception of overjet, where SWIRE patients had significantly better alignment than SEDGE patients (means of 3.9 and 5.4 points, respectively; P = 0.03). The treatment duration was significantly lower in the SWIRE group compared to the SEDGE group (means of 25.7 and 32.5 months, respectively; P < 0.001; Fig. 1b). This translated to an average difference of −6.8 months (95% CI = −9.6 to −4.0 months) between groups. The number of visits was similarly lower in the SWIRE group compared to the SEDGE group (means of 24.5 and 31.5 visits, respectively; P < 0.001) with an average difference of −7.2 visits (95% CI = −10.3 to −4.2 visits). This was not due to different interval between the visits (means of 4.8 and 4.5 weeks, respectively; P = 0.11; compared post hoc). Finally, analyses adjusted for the influence of any potential confounders (Appendix 2) found no considerable differences from the main analysis.

Intraexaminer agreement and repeatability was almost perfect with a concordance correlation coefficient of 0.98 (95% CI = 0.96–0.99) and a Bland–Altman average difference of −0.52 (95% limits of agreement = −4.08 to 3.05). Interexaminer agreement and repeatability was somewhat worse with a concordance correlation coefficient of 0.87 (95% CI = 0.79–0.92) and a Bland–Altman average difference of −0.82 (95% limits of agreement = −9.52 to 7.87).

On the other hand, treatment with SWIRE appliances was found to be more efficient than with SEDGE appliances in terms of shorter duration and fewer visits. This agrees with a previous retrospective study on extraction treatment that reported shorter treatment durations with SWIRE appliances [33]. A possible explanation for this might be that during space closure in the SWIRE group the already attained built-in prescription of the appliance for each tooth is retained to some degree. On the contrary, torque lost during space closure in the SEDGE group might need to be reapplied at the finishing stage, which might result in prolonged treatment times.

The problematic control of tooth inclination/torque during treatment is supported by another retrospective study of nonextraction treatment by Soltani et al. [34], where SWIRE treatment was associated with significantly improved buccolingual inclination of teeth compared to SEDGE treatment. However, a much smaller gain of 2 months in total was seen for the SWIRE group [34], but this might be due to nonextraction treatment and the lack of space closure. This difference with respect to the control of tooth inclination is corroborated by another retrospective study of extraction treatment, where SWIRE patients scored significantly better for the angulation and inclination of the maxillary posterior teeth compared to SEDGE patients [33]. Another retrospective study found significant differences in anchorage loss of the lower incisors between treatment with SWIRE and SEDGE appliances [35], which might lead to longer treatment times needed to upright proclined incisors. Finally, another study found that patients treated with SEDGE appliances did not exhibit ideal functional occlusal relationships, whereas most individuals after SWIRE appliance therapy had an ideal (i.e., mutually protected) occlusion [36]. On the other hand, an unpublished retrospective study of extraction/nonextraction cases found no statistically significant difference in either treatment duration or PAR scores between SWIRE and SEDGE appliances [37].

Although root resorption was not measured in this study, a previous retrospective study [38] reported that extraction treatment with SEDGE was associated with significantly greater resorption of the upper incisors compared to the SWIRE group. This might be attributed to more efficient force control in the SWIRE group, where tooth inclination and torque is gradually applied on the teeth already through the aligning superelastic archwires of increasing size and stiffness, whereas these are applied abruptly during the finishing stage in the SEDGE group. Finally, a previous randomized trial did not find a significant difference in either root resorption of the upper incisors or treatment duration between fully programmed (Roth) and partly programmed fixed appliances [39]. However, no pure nonprogrammed SEDGE appliance had been used in this trial, while both extraction and nonextraction cases were pooled together and this might have influenced the results.

The strengths of this retrospective observational study are its a priori registered protocol [40] and its open data provision that increases its transparency [41]. The performed sample size calculation means that the study was adequately powered to identify clinically relevant differences between appliances, while blinded outcome assessment reduces the potential for bias. However, the present study also has certain limitations that need to be considered. First, it was a nonrandomized retrospective study, which means it is more prone to bias than both prospective nonrandomized and randomized studies [42]. Furthermore, as most clinicians pick one appliance prescription and stick to it, it was not feasible to have both SWIRE and SEDGE appliances being administered in the same clinic. Thus, two different centers were compared. Pretreatment case severity was measured with the ABO DI in order to assess and control for baseline differences, but this is no panacea. Finally, important adverse effects like root resorption and gingival recessions were not assessed in this study, even though these are often taken into consideration during clinical decision-making regarding implemented appliances and techniques.

The results of the present retrospective study within its limitations indicate that on average similar occlusal outcomes according to the ABO-OGS are possible after extraction-based orthodontic treatment with the use of either straight-wire or standard edgewise appliances. However, treatment with standard edgewise appliances might take longer on average than treatment with straight-wire appliances. These findings should be confirmed by future prospective clinical studies comparing these two appliances, preferably with a randomized trial design.