A Retrospective Study on the Cephalometric Markers That Predict the Treatment Outcomes of Twin Block Functional Appliance

Despite the widespread use of functional appliances, broad variations are reported on the treatment response. This retrospective study aims to investigate the pre-treatment cephalometric predictors on the treatment outcome of Twin block in growing patients with class II malocclusion Methods After screening, a total of 90 patients treated by Twin block were included in the study. The treatment outcome was the advancement of the chin position, assessed by the distance of skeletal pogonion to the vertical reference plane perpendicular to Frankfurt plane ( △ Pog-VRP). Moreover, △ Pog-VRP is adjusted by facial growth evaluated by △ N-Ba to minimize the growth discrepancy among individuals (adj △ Pog-VRP). The pre-treatment cephalometric measurements were compared with independent t-test. Stepwise multivariate regression analysis was performed to determine the pre-treatment cephalometric predictors for the treatment outcome.


Conclusion
With the limitation of the study, we provide evidence that patients with a smaller Na-Go-Me angle are more likely to experience a greater chin advancement following Twin block treatment.

Background
Class II malocclusion is one of the most prevalent orthodontic conditions worldwide, affecting around one third of the populations. Based on the newly combined data from 37 studies enrolling around 11,000 subjects, the prevalence of Class II malocclusion is estimated to be 9.91% (95% CI: 7.41%-13.79%) in Chinese school children, calling for proactive and effective interventions [1]. Indeed, several types of functional appliances that aim for the correction of class II division 1 malocclusion in actively growing patients have been applied for decades, including Twin Block, activator, herbst, bionator, etc [2]. Yet, the nature of the treatment effect induced by functional appliances is still controversial. Some studies revealed marked skeletal alterations following functional appliance treatment, either as an increase in mandibular length [3,4] or as a favorable growth of condyle and chin advancement position [5,6]. While others claimed that the successful effects of functional appliances were mainly attributed to dentoalveolar components, rather than the skeletal effects [7,8]. Despite the disputed arguments, functional appliances are still widely used as they bring a harmonious facial pro le in growing individuals.
The Twin block, rstly introduced by Clark, consists of an upper and lower block. It positions the mandibular forward by interlocking the occlusal bite block [9]. Due to the small size and no visible extraoral components, it is generally better accepted by teenagers [3], and is therefore one of the mostly used functional appliances in clinic. Many studies have revealed the bene cial effects of Twin block, including an increased growth and forward movement of the mandible, proclination of the lower incisors, retroclination of the upper incisors, distal movement of the upper molars, and mesial movement of the lower molar. [10] Moreover, enlargement in the oropharynx and hypopharynx airway dimensions was also observed following Twin block treatment [11].
The treatment effects of functional appliances have been well validated, however, the skeletal and dental response to the treatment vary from patient to patient, possibly due to the discrepancy in the dentalskeletal patterns among individuals [12]. Therefore, studies have been carried out to determine whether the pretreatment cephalometric markers could predict good response following functional appliance treatment. It is well demonstrated that a decreased gonial angle may serve as good predictors for greater increment in mandibular growth [13] and chin point advancement [14,15]. Moreover, overbite, inclination of the lower incisor and lip, mandibular length, ramus height, anterior and posterior face height are suggested to be signi cantly associated with the treatment response, depending on the functional appliance used and outcome measurement [16][17][18]. On the contrary, some argued that no relationship exists between mandibular morphology, vertical skeletal pattern and favorable dentoalveolar/skeletal responses to Twin block therapy [19].
The majority of the studies that determine the cephalometric predictors were carried out in white Caucasians. Whether such association can be generalized to other races remain to be further explored.
Indeed, markedly different skeletal pro les exist between Chinese and Caucasians with Class II Division 1 malocclusion. For example, Chinese presented with a signi cantly more prognathic maxilla and less retrusive mandible in reference to Caucasian, as indicated by larger S-N-A and S-N-B angles respectively [20]. As such, this present study aims to investigate the association of pretreatment cephalometric markers with Twin block treatment outcomes in Chinese subjects at pubertal stage.

The inclusion criteria
This study was carried out on a total of 200 growing patients who consecutively completed Twin block treatment at the Department of Orthodontics, School of Stomatology, Wenzhou Medical University from Jan 2015 to June 2020. The inclusion criteria were as follows: 1) Overjet > 7mm with angle class II division I malocclusion; 2) ANB > 4°; 3) Cervical stage (CS) 3-4 at the beginning of functional treatment (T1) and CS 4-5 at the end of therapy (T2); 4) Lateral cephalometric graphs with high quality were available at T1 and T2; 5) No adjunct orthodontic treatment either prior to or during the Twin block therapy. 6) Patients with good compliance, indicated by self-reported full-time wear of Twin block appliance. A total of 90 subjects were included in the study after screening.

Treatment protocol
All the patients were treated by two specialists (HZ & QS). The basic design of the Twin-block is as previously described with brief modi cation [21]. In particular, the upper anterior teeth were engaged in the device with maxillary labial bow. Bite registration was recorded with the incisors in an edge to edge position. All the patients were instructed to wear Twin Block for 24h except during sports and tooth brushing. The patients were reassessed every 4-6 week till the completion of the treatment. The acrylic of the posterior bite block was kept intact to allow vertical control. The treatment duration is within 9-12 months till class I molar relationship is achieved.

Cephalometric analysis
The lateral Cephalometric images were taken prior to and 3 months after the completion of functional appliance treatment. The reference points and lines are displayed in Fig All the tracing on the lateral cephalometric graphs were done by a single examiner (JD) with the use of cephalometric software Uceph (Sichuan University, China). One week after the rst measurements, 20 cephalometric graphs were randomly retraced and reassessed for all the variables included in the study. It was found that the intraclass correlation coe cient (ICC) value was more than 0.90 (0.90-0.99) for all the variables measured, indicating a minimal measurement error of the tracings. The assessment of the CS was performed by JD and veri ed by HZ. Disagreement were discussed till satisfaction were reached by both.

Data curation and statistical analysis
The descriptive numerical data were presented as mean (standard deviation, SD), and categorical data were shown as N (%). The outcome was the alteration in Pog-VRP following treatment. Those who had △Pog-VRP value above the median were categorized in to good response group (GRG), while the rest were divided into poor response group (PRG). In an attempt to minimize the growth and development differences among individuals, △Pog-VRP was further divided by the alteration in the cranial base length (N-Ba). Accordingly, subjects with adj△Pog-VRP (△Pog-VRP/△N-Ba)) above the median were grouped as adjGRG, while the rest were adjPRG.
Independent students' t test was used to compare the numerical results between the groups (GRG v.s PRG; adjGRG v.s adjPRG), while chi-square test was performed for the categorical results. Spearman correlation was performed to assess the association of Na-Go-Me angle with continuous variable △Pog-VRP and adj△Pog-VRP. As for the multivariate regression analysis, the predictive variables to determine the treatment outcome included age, gender, CS and baseline cephalometric measurements with stepwise method. All statistical analysis were performed with SPSS19 (IBM, USA). A two tailed P < 0.05 was set as statistically signi cant.

Results
Based on the advancement of Pog, 45 subjects whose △Pog-VRP was above the median (> 2.3 mm), were categorized into GRG, while the rest half were PRG (≤ 2.3 mm). The demographic information is indicated in Table. 1. In general, there is no signi cant difference between PGR and GRG in age (PRG,    Next, those variables with P < 0.10 in Tables 2 & 3, together with age, gender and CS were included in the regression analysis to determine the association with the treatment outcome. As shown in Table 4, age (β = -0.21,95%CI, -0.19 to -0.003, P = 0.04), and Na-Go-Me (β = -0.26, 95%CI, -0.06 to -0.01, P = 0.01) were found to be the independent predictors of Pog advancement (Model 1). In addition, when the Pog advancement was adjusted by △N-Ba, we found that only Na-Go-Me (β = -0.29 95%CI -0.06, -0.01, P < 0.01) was included in the nal model (Model 2). Additionally, the signi cant linear correlation of Na-Go-Me with △Pog-VRP (R 2 = 0.09, P < 0.01) and △adjPog-VRP (R 2 = 0.09, P < 0.01) was depicted in the scatter plot (Fig. 2) height, anterior and posterior lower face height were predictors for great ANB reduction [18]. Moreover, it has been suggested that growing patients with Co-Go-Me angle smaller than 125.5° are expected to experience a greater increment in mandibular length indicated by Co-Gn [13]. Similar results were obtained in other studies, in which Co-Go-Me degrees were signi cantly associated with chin position advancement [14,15]. The outcome of Twin block was assessed by the advancement of skeletal Pog point in the present study since chin position is more directly related to the balance and harmony of the facial pro le. Whereas an increase in the size of mandibular does not fully re ect the advancement of the mandibular as it can be camou aged by the growth in the vertical dimensions [22]. Moreover, the skeletal Pog was chosen over soft tissue Pog because of the relatively large individual variation and low accuracy of soft tissue measurements [23].
The present study showed that the favorable group had a smaller ∠SN-MP, ∠Ar-Go-Me, ∠Na-Go-Me, ∠MP-OP and sum of angles, and an increased PFH/AFH. Interestingly, such differences in the above variables markedly increased when the cohort was regrouped in adjGRG and adjPRG. These ndings suggest that a low divergent skeletal pattern is likely to have greater chin advancement following Twin block treatment. Notably, lower gonial angle (∠Na-Go-Me) was found to be signi cantly associated with the treatment outcome after adjusting the confounders. In the Bjork-Jarabak analysis, the gonial angle, which could be divided into upper and lower gonial angle, provides information on the he relationship between the ramus and corpus of the mandible. A decreased gonial angle indicates a more horizontal growth pattern, which is accompanied by counterclock rotation of the mandibular and reduced anterior face height [24]. Speci cally, the upper gonial angle indicates how oblique the ramus is, and lower gonial angle suggests the slants of mandibular body [25]. The gonial angel (∠Co-Go-Me) was demonstrated to predict the treatment outcome in previous studies [14,15,26]. However, the information on the upper and lower gonial angle was not speci ed in their analysis. Here, we provide further evidence that it was the lower gonial angle that mainly contributes to the decrease in the gonial angle, which lead to the favorable treatment response.
Ideally, a reference group from the same population with class II skeletal relationship at growing stage are highly recommended to discriminate the "true" outcome produced by functional appliance. However, the ethical concerns make it di cult to delay the treatment for patients with such conditions [27]. As such, the present study further divided △Pog-VRP by △N-Ba in an attempt to minimize any growth difference among the individuals. The reason why △N-Ba was chosen is based on previous evidence that the distance from Nasion to basion is not in uenced by the functional treatment [28]. Moreover, the growth of cranial base is closely associated with that of the mandibular [29]. Therefore, the adjusted △Pog-VRP could be interpreted as the twin block induced effects.
Several limitations have to be addressed for the current study. The retrospective nature of the data collection could increase the possibility of bias. Moreover, long-term design study is highly warranted to investigate the stability of functional appliance treatment in subjects with different skeletal pattern. Finally, the predictive parameters found in the present study should be validated in future prospective studies.

Conclusions
Within the limitation of the study, we provide evidence that patients with a low divergent skeletal pattern are likely to experience more improvement in the advancement of chin position following Twin block treatment. The Na-Go-Me angle is found to be the statistically signi cant predictor for favorable treatment outcome, regardless of the of the potential in facial growth.