INTRODUCTION

Since its introduction by Clark,¹ the Twin Block (TB) functional appliance has gained increasingly wide popularity among clinicians worldwide for the treatment of Class II malocclusion. TB was voted as the preferred functional appliance by 75% of British orthodontists.² By engaging upper and lower acrylic plates with interlocking bite-blocks, TB induces a favorable forward mandibular displacement upon closure.¹ Significant proclination of lower incisors had been consistently reported by TB studies evaluated by a recent systematic review and meta-analysis.³ This complication, generally acknowledged for functional appliances, is attributed to a protrusive effect on the lower incisors exerted by the lingual appliance components while the mandible attempts to rebound to normal resting posture.⁴

Given the fundamental role of interarch relationships of anterior teeth in treatment decision making, a number of studies have tested the efficiency of modified TB appliances in controlling the inclination of upper and/or lower incisors. Inclusion of Southend clasps in the lower anterior region significantly limited incisor proclination according to Trenouth and Desmond.⁵ On the contrary, when a labial bow or torquing springs had been incorporated in the upper plate, no statistically significant differences in the angular position of incisors were observed between the two alternative designs.^6,7

Acrylic capping was previously recommended to prevent labial tipping of the lower incisors in functional appliance treatment.⁸ Such TB modification proved to minimize lower incisor proclination compared with untreated matched historical control subjects.⁹ To our knowledge, no available studies compare the influence of TB acrylic extension on lower incisor proclination vs the conventional appliance design. Therefore, the primary aim of this investigation was to examine the effect of acrylic capping on incisor proclination in orthodontic patients immediately after TB treatment. Second, we aimed to identify factors that might influence post-TB inclination of lower incisors.

MATERIALS AND METHODS

Sample Collection

This retrospective study was carried out on 56 patients consecutively treated with TB functional appliances between August 2013 and June 2015 at the staff clinic of the Department of Orthodontics, University Medical Center Groningen. In this study, 29 participants (7 boys, 22 girls; mean age at the start of treatment, T1, 12.6; standard deviation, 0.8) were treated with TB appliance with acrylic capping on the lower incisors (TB-AC) and 27 (14 boys, 13 girls; mean age at T1, 12.4, standard deviation, 0.7) were treated with TB with ball-ended clasps in the lower anterior region (TB-NAC) (Table 1).

Table 1. Descriptive Statistics of the Study (TB-AC) and Control (TB-NAC) Groups

View Fullscreen

The inclusion criteria were as follows: white patients, Class II molar relationship of at least 1/2 premolar width at T1, nonextraction treatment plan, Class I canine and molar relationship at appliance removal (T2), and available T1 and T2 cephalograms of adequate quality for accurate landmark identification.

At an alpha level of 5% and power of 95%, it was calculated that 26 patients were required in each group to detect a significant difference of 5° in lower incisor inclination angles.

Cephalometric Analysis

Digitization of all cephalograms was performed by means of cephalometric analysis software (Viewbox 3.0; dHAL Software, Kifissia, Greece) by the first author (MCvdP), who was blinded to the type of appliance. A number of skeletal and dental points were selected to determine lower incisor inclination and craniofacial pattern (Figure 1A). Inclination of the long axes of lower incisors was estimated in relation to 3 different reference mandibular planes commonly used in cephalometrics (Figure 1B): the line drawn from Me to Go,¹¹ the line drawn from Gn to Go,^12,13 and the tangent to the lower border of the mandible.^14–16 Thirty randomly selected tracings were repeated by the same investigator 2 weeks after the first series of tracings to calculate intraobserver agreement.

View Figure

• Download as Powerpoint
• Download Figure

RESULTS

There was excellent intraobserver reliability, and ICC values ranged between 0.91 and 1.00.

Skeletal and dental measurements are summarized for both groups in Table 2. No statistically significant differences (P > .05) were observed between groups at T1 and T2 with respect to ANB and lower incisor inclination.

Table 2. Skeletal and Dental Measurements in the Study (TB-AC) and Control (TB-NAC) Groups Before and after Active Twin Block Treatment

View Fullscreen

Regression analysis showed that appliance design (with or without acrylic capping) did not have a significant effect on lower incisor inclination regardless of definition used (P < .05) (Table 3).

Table 3. Coefficients, P Values, and 95% Confidence Intervals from the Regression Analysis for Lower Incisor Inclination; L1-GoGn (T1), L1-GoMe (T1), L1-MP (T1), Pretreatment Lower Incisor Inclination Values

View Fullscreen

Pretreatment lower incisor inclination was the only statistically significant predictor for final tooth inclination after TB treatment (L1-GoGn: β = 0.57, 95% CI = 0.30, 0.84, P < .001; L1-GoMe: β = 0.56, 95% CI = 0.28, 0.84, P < .001; L1-MP: β = 0.46, 95% CI = 0.17, 0.75, P = .003).

Treatment duration with the exception of mandibular plane related inclination (β = –0.93; 95% CI = –1.85, –0.02; P = .045) and overjet demonstrated weak evidence of association with lower incisor inclination (Table 3).

DISCUSSION

Proper positioning of lower incisors in the alveolar bone by use of orthodontics is essential to reach favorable anterior occlusal contacts. Increased incisor proclination and tooth movement out of the osseous envelope of the alveolar process may be associated with higher tendency for developing gingival recessions.¹⁷ Moreover, reduced thickness of the free gingival margin, a narrow mandibular symphysis, inadequate plaque control, and aggressive brushing technique were identified as factors that may lead to gingival recession after orthodontic tipping and/or translation movement.¹⁸ As a consequence, insufficient control of the labiolingual inclination of lower incisors in patients undergoing functional appliance therapy will profoundly affect the course of treatment.

Unlike previous findings,⁹ TB design with acrylic capping of the lower anterior teeth in the present study could not restrain significantly incisor proclination during TB treatment phase. Treatment effect in the aforementioned study was estimated by subtracting the natural growth occurring in the control subjects from the treatment changes observed in the study group. Nonetheless, from a methodologic point of view, the selection of untreated control individuals from longitudinal growth study archives^19–22 may be treated with skepticism. Differences in dentofacial characteristics between populations and time periods²³ and absence of another TB alternative in the study of Sidlauskas⁹ hinder hypothesis testing regarding the effect of acrylic extension over the incisal edges. Baysal and Uysal²⁴ attributed the lack of differences in lower incisor position between TB and untreated groups to the acrylic capping of the lower incisors. The authors referred to appliance design details in their own previous publication,²⁵ which surprisingly claimed to reproduce Clark's original description with interdental clasps rather than acrylic capping in the incisor region.¹

Inclination of the lower incisors before TB insertion had a positive relationship to the posttreatment inclination angles, that is, the larger the incisor inclination before treatment, the larger the proclination after appliance removal. Given the cumulative effects of TB treatment,³ leveling of the curve of Spee,²⁶ and arch expansion during the fixed appliance stage on the inclination of lower incisors, space gaining procedures may be required to reposition flared lower incisors in the dentoalveolar envelope and prevent or address soft tissue sequelae.

Our analysis provided weak evidence that pretreatment overjet and length of TB treatment may be significantly associated with increased post-TB inclination of lower incisors. Hypothetically, a larger protrusive movement of the mandible to address the existing sagittal discrepancy may exaggerate mesial force application on the lower incisors, and subsequently, tooth proclination. Furthermore, longer appliance wear may lead to more stable results and prevent the mandible from shifting to the pretreatment position.

This investigation presents certain limitations. First, as in all retrospective studies, patient information on exposure was retrieved from previously collected records, and therefore conclusions should be interpreted with caution. Randomized clinical trials implementing random assignment of individuals to alternative TB designs with proper allocation concealment may minimize selection bias reduction, control unobserved confounders, and improve internal validity.²⁷ Second, the accuracy of assessing the lower border of the mandible on two-dimensional cephalometric images may be questioned. By averaging the lower border when two outlines were visible, observer bias might have been introduced. Additionally, identification of landmarks such as Go and lower incisor apex may be dubious.^28,29 Nevertheless, use of the specific mandibular planes has proven valid for measuring lower incisor inclination, and these measures are in excellent agreement with each other for growing orthodontic patients aged 12–16 years.³⁰ Finally, the actual appliance wear time per day, which may have influenced the active TB treatment duration and, potentially, the magnitude of treatment effects, remained unknown. However, a TB appliance is assumed to be worn daily longer than any other device.² Patient compliance in our study was clinically evaluated by the achieved correction of overjet and molar relationship, the appliance retention and appearance, and patient's speech while wearing the TB appliance. Patients who had failed to show substantial progress and eventually discontinued TB treatment were excluded. Electronic wear time documentation using incorporated microsensors would have aided quantification of patient adherence to prescribed wear time,³¹ though flaws in identifying specific temperature profiles have been described elsewhere.³²

When interpreting the results of TB studies that looked into incisor inclination changes, attention should be focused on methodologic issues like small sample size;³³ early treatment timing;¹⁹ varying appliance wear protocols;^20,34 prolonged treatment, including retention phase;^19,25 and cephalometric evaluation after fixed appliances stage.^21,22 On the other hand, our study, the first one to report on the effect of acrylic capping on the lower incisor inclination in patients treated with two TB modifications, appears methodologically advantageous. Similarity of groups with respect to age, origin, and treatment duration; involvement of one clinician in treatment procedures; and sample-size calculation strengthen the validity of our conclusions. Large-scale randomized controlled clinical trials will shed more light on the contribution of TB appliance components on lower incisor inclination.

CONCLUSIONS

• The findings of this study suggest that the increase in lower incisor inclination may not be significantly restrained by adding acrylic extension in the anterior region of the lower plate of TB appliance.

• Pretreatment lower incisor inclination may be significantly associated with tooth inclination after active TB treatment and should be taken into account into treatment decision making.