Comparative Evaluation of Twin Block Appliance and Fixed Orthodontic Appliance in Early Class II Malocclusion Treatment: A Randomized Controlled Trial

INTRODUCTION

Class II malocclusion is one of the most common problems in orthodontic practice, accounting for approximately one-third of the patients seeking orthodontic treatment in some populations.¹ Class II had a global distribution of 19.56% in permanent dentition. Europe has the greatest incidence of class II (33.51%). African population showed the lowest occurrence of class II malocclusions (7.5%). Class II division 1 malocclusion is the most frequent orthodontic issue. It accounts for approximately 12 to 49% of all orthodontic difficulties.² Class II malocclusion, like other types of malocclusions, causes esthetic, functional, and psychological problems, the severity of which depends on the amount of anteroposterior discrepancy and its interaction with the surrounding soft tissue structures.³

Various factors contribute to the development of class II malocclusion, but mandibular retrognathism is the most common.³ Different functional appliances are used to correct a class II malocclusion by utilizing, eliminating, or directing muscle function forces, tooth eruption, and growth modification.⁴

The debate over the mode of action of functional appliances is about the efficacy of the appliances in increasing mandibular growth, which leads to a long-standing improvement in the skeletal pattern.^5–10 Although research results showed that functional appliances could increase mandibular unit length (Co-Gn),⁵ other researchers concluded that functional appliances did not affect mandibular growth.^6,7 The mandibular changes need to be evaluated carefully. According to Johnston, what might be interpreted as growth is in fact mandibular displacement or a combination of the two types of change.^8,9 This could be seen in measurements that use point articulare (Ar), because the measurements are unable to discriminate between growth and displacement.¹⁰ Distalization of the upper buccal segment and retroclination of the upper anterior teeth, along with mesial movement of the lower buccal segments and proclination of the lower labial segments helped in the establishment of class I relationship.¹⁰

Treatment timing is a critical factor for the successful treatment of any skeletal malocclusion. There is controversy about the best time for correction of class II malocclusion. Some researchers preferred orthodontic therapy in the mixed dentition period,¹¹ while others contended that early treatment is sometimes a waste of time and resources.¹² Early treatment of class II malocclusion is suggested to result in more stable results because the skeletal growth is adjusted and enhanced.¹³ Baccetti et al.¹⁴ reported that the best time to treat class II malocclusion is in circumpubertal cervical vertebral maturation (CVM) stages 2 and 3.

The Twin block appliance has become the most popular removable functional appliance during the last few decades.¹⁵ Additionally, systematic reviews found it to be the most efficient in producing skeletal changes.¹⁶ It has been proven to be comfortable, efficient, and esthetic,^17,18 and can be used in both permanent and mixed dentition.¹⁹ There are several studies that evaluated the dental and skeletal effects associated with the Twin block appliance and compared them with a control group or with other appliances such as the Herbest or the Activator appliances.²⁰ However, there are few studies that compared the Twin block to the fixed orthodontic appliance in the treatment of class II malocclusion.

The target of the Twin block appliance is to enhance mandibular growth. The main functioning mechanism in the normal dentition is the occlusal inclined plane. This plane is crucial in determining the final occlusal relationship. If the mandible occludes in distal relation to the maxilla in normal function, the forces of occlusion acting on the lower teeth have a distal component of force. Additionally, the mandible is locked in a distally occluding functional position by the inclined planes created by the cusps of the upper and lower teeth.¹⁵

The aim of this study was to compare skeletal, dentoalveolar, and soft tissue changes between Twin block and early fixed orthodontic appliance for class II division 1 malocclusion treatment.

MATERIALS AND METHODS

RESULTS

The patients in both groups were well matched according to gender, weight, height, body mass index, and finally CVM. Except for overjet, no statistically significant difference was found between the control and Twin block group (Table 1). For cephalometric measurements, intra-class correlation coefficient ranged from 0.85 to 1.0, while the inter-class correlation coefficient ranged from 0.87 to 0.99. Except for U1-NF and Ar angle, the initial values of all cephalometric measurements of patients in both groups showed similar values with no statistically significant differences (p ≥ 0.05).

Twin block group showed statistically significant differences nearly in all skeletal measurements (Table 2, Fig. 1). Regarding skeletal maxillary, mandibular, and maxillomandibular relationship, all variables (Co-A, SNB, SND, and ANB) showed significant change after treatment or an observational period between two groups. SNB increased remarkably by 4° in the Twin block group, but only by 0.68 in the control group. Twin block group showed also an increase in Co-A by 5.48 mm comparable to 2.38 mm in control group. As a consequence of the forward advancement of the mandible in Twin block group, facial convexity (N-A-Pog) decreased by 2.68 mm. However, the decrease in facial convexity in control group was insignificant (p ≥ 0.05) (Fig. 2). Mandibular plane angle showed a significant decrease by 1.34 mm in Twin block appliances after treatment. All facial heights anterior facial height, posterior face height, upper anterior face height, and lower anterior face height (AFH, PFH, UAFH, LAFH) showed a significant increase in both groups after treatment or observational periods.

Additionally, Twin block group showed statistically significant difference either increase or decrease in all dental parameters except L6-MP as shown in (Table 3, Fig. 3). Control group showed significant decrease in U1-NA (°), U1-NA (mm), U1-NF (°), Z angle after observational periods by 3.24°, 1.44 mm, 3.28°, and 2.99°, respectively (Fig. 4).

There was a significant decrease in vertical dimensions (SN-GoGn) in the Twin block group compared to the control group (p = 0.002). The Frankfort-mandibular plane angle (FMA) showed a significant change between both groups (p = 0.001). However, face height (PFH, UAFH, and LAFH) increased significantly after Twin block treatment. These changes were significant and remarkable in Twin block group over control group (2.53 mm, 3.36 mm, and 2.92 mm, respectively). Among cranial base variables, N-S-Ar and S-Ar-Go showed significant change between the groups (p = 0.030 and p = 0.006), (Table 4, Fig. 5).

Among dentoalveolar variables, U1-NF and L6-MP showed significant changes (p = 0.033 and p = 0.036) between groups either by a decrease or increase in their values respectively. In comparison to the control group, overjet decreased significantly by 7.09 mm, while overbite showed a significant decrease by 2.19 mm. Among soft tissue variables, the nasolabial angle and Z angle showed significant change (p ≤ 0.001*) between groups. The nasolabial angle and Z angle showed increase in Twin block group by 1.39° and 2.75°, respectively. However, they showed a decrease in control group by 8.15° and 1.28°, respectively (Table 5, Fig. 5).

Finally, there was a significant and remarkable enhancement of anteroposterior relationship in Twin block group in comparison to control group. Additionally, there was a significant decrease in vertical dimensions (SN-GoGn) in Twin block group compared to control group (p = 0.002). Also, significant enhancement in the facial profile of the patients in Twin block group was observed.

DISCUSSION

The results of the current trial confirmed the findings of Lund and Sandler,²⁴ Mills and McCulloch,²⁵ and Trenouth²⁶ that the Twin block appliance produced an orthopedic effect in both the anteroposterior and vertical directions, resulting in an improvement in the facial profile. In the current trial, a significant increase in mandibular length (Co-Gn) was observed in Twin block group as reported in other studies.²⁷ Illing et al.²⁸ found an increase in mandibular unit length measured from point condylion (Co) and Ar to gnathion (Gn). Toth and McNamara²⁹ found an increase of 5.7 mm in mandibular unit length (Co-Gn) during a 16-month period when compared with controls which increased by 2.7 mm. This increase in mandibular length was a net result of an increase in both ramal (Co-Go) and body length (Go-Gn). Many studies asked whether the increase of mandibular length was due to actual growth of the mandible or forward posturing of the mandible which affects cephalometric points as point (Ar) and the (B) point.³⁰ In the current trial, there were no significant differences regarding the mandibular length change between the two groups; therefore, the increase in mandibular length is supposed to be due to the actual natural growth of the mandible. Early intervention with the Twin block appliance increased the SNB angle by 4.0° which is greater than the increase reported in previous studies.^31,32 Illing et al.²⁸ reported 0.8°, while Mills and McCulloch²⁵ reported 1.9° increase in Twin block group. The larger change of SNB in the current trial could be attributed to the longer duration of wearing the Twin block or to a difference in skeletal age between the studies. Twin block postures the mandible forward, which explains the increase in the SNB angle; in the current study, the SNB angle increased by 4.0° in the Twin block group while it increased by 0.5° in the control group.

In the present trial, besides the stimulating effects of Twin block on the mandible, there was an insignificant inhibitory effect of Twin block on the maxilla as reported by previous studies.^28,31–33 This conclusion is based on the similar change in the effective maxillary length in the Twin block and the control groups. Thus, the Twin block may have no effect on the anteroposterior maxillary position (point A). The labial bow in Twin block restricted proclination of upper anterior teeth and this may result in an inhibitory effect on point “A”. Sagittal jaw discrepancy, therefore, significantly improved in the Twin block group. The increase in SNB and decrease in SNA lead to a significant decrease in ANB by 4.17° in Twin block group in comparison to control group which was 0.52°. Likewise, Alhammadi et al.³² found a statistically significant reduction in ANB angle by 2.49° in Twin block group, while control group increased by 0.29°.

The overjet improvement was 7.09 mm in the Twin block group and 0.5 mm in the fixed appliance group, and there was a statistically significant difference between the change in the two groups (p ≤ 0.001*). Mills and McCulloch²⁵ noted a reduction of 5.6 mm in patients treated with a Twin block appliance for 14 months. The obvious difference in the change occurred in the overjet between the two groups suggest a clinical significance too. Patients in the Twin group enjoyed a large decrease in the overjet during the 12-month follow-up which is suggested to be due to the forward positioning of the mandible. The dentoalveolar changes included lingual tipping and displacement of upper anterior teeth, labial tipping, and displacement of lower anterior teeth in the Twin block group and in the fixed appliance group as well. These results are in agreement with the results of previous studies.^25,28,34 Dentoalveolar changes in both groups contributed, in part, to the overjet improvement. Also, there was a significant reduction of overbite in Twin block group by 2.19 mm. This was in accordance with findings by Illing et al.²⁸ who reported a decrease in overbite by 1.8 mm. However, the decrease in overbite in the fixed group was only 0.88 mm.

According to Mills and McCulloch,²⁵ the improvement of anteroposterior relationship improved facial profile as indicated by N-A-Pog, which showed a significant reduction in Twin block group. This, in turn, reduced the convexity of the facial profile. The facial profile was also improved by a reduction in the vertical growth pattern as SN-GoGn and FMA angles significantly decreased in Twin block group and the change was significant between the two groups. This change in SN-GoGn is in partial agreement with those stated by Elfeky et al.,³¹ and Mills and McCulloch²⁵ who reported an insignificant decrease in SN-MP angle. The attributed cause for this reduction in vertical dimension is the maxillary plane rotation as reported in a previous study.²⁸ The insignificant increase in SN-GoGn and FMA angles in the fixed group could be attributed to the slight extrusion of molars by the action of a fixed appliance and might be clinically insignificant. In the current study, AFH, UAFH, LAFH, and PFH significantly increased after wearing Twin block appliances, there is a common agreement in previous studies that Twin block leads to an increase in LAFH.^28,34 Elfeky et al.³¹ reported a significant increase in LAFH, while Illing et al.²⁸ reported an insignificant increase. Elfeky et al.³¹ and Mills and McCulloch²⁵ reported a significant increase in the PFH in the Twin block group over the control group. The patients who received the fixed appliance also showed a significant increase in AFH, UAFH, LAFH, and PFH. Comparing the changes that happened in the AFH, UAFH, LAFH, and PFH between the two groups showed a significant difference in the UAFH, LAFH, and PFH and an insignificant difference in AFH. A significant increase in the PFH that coincided with the increase in the AFH, especially in the fixed group could explain the decreased clockwise rotation of the mandible. The ramal length increased by 5.16 mm in the fixed appliance group which was significantly larger than the increase in the Twin block group (3.23 mm). The effect of the molar intrusion in the Twin block group and the molar extrusion in the fixed appliance should also be considered. Therefore, the change in the AFH was insignificantly different between the two groups. Additionally, the mean pretreatment value of FMA was 23.89 ± 5.58 and 24.47 ± 4.84 in the Twin block and control groups, respectively indicating horizontally growing patients.

The smaller the cranial base angle, the more forward the mandibular position, which increases the tendency of a class III jaw relationship, and the larger the cranial base angle, the more backward the position of the mandible, which increases the tendency of a class II jaw relationship. Also, in contrast to the maxilla, the mandible is affected more by changes in the cranial base angle.³⁵ Anderson and Popovich³⁶ found more class II occlusions in large cranial base angle subjects. Kerr and Adams³⁷ concluded that the size and shape of the cranial base influence mandibular position by determining the anteroposterior position of the condyles relative to the facial profile. In the current trial, the anterior cranial base and posterior cranial base increased significantly in Twin block group, but the change was insignificant in comparison to control group. Mills and McCulloch²⁵ reported an insignificant increase in the posterior cranial base and a significant increase in the anterior cranial base comparing them with control group. Almedia et al.³⁸ reported that individuals with class II malocclusion had greater anterior and posterior lengths of the cranial base and more obtuse-angular measurements regarding the skull base in comparison to class I.

Saddle angle showed a significant decrease and gonial angle showed a significant increase in Twin block group as reported in a previous study.²⁵ However, the Ar angle significantly decreased in the current trial, as reported in previous studies,³⁹ but there is a controversy with Mills and McCulloch.²⁵ Bhattacharya et al.³⁵ reported that cranial base angles affect the rotation of the mandible, saddle angle was correlated with Y-axis and mandibular plane angle (SN-GoGn). Correlation suggested that increase in the cranial base flexure can cause a clockwise rotation of the mandible. In the current trial saddle angle decreased after wearing Twin block appliance and this lead to a decrease in SN-GoGn. Abdelkarim et al.⁴⁰ reported that patients with mandibular retrognathism had the highest values of saddle angle (123.3°), followed by patients with normal mandibular posture (122.7°). Thus, after Twin block appliance, the mandible moved forward attaining less value of saddle angle. The gonial angle is the representation of the form of the mandible. This angle has an important role in predicting growth and it also has specific effects initially on growth, profile changes, and the condition of the anterior teeth of the lower jaw.⁴¹ Sharma et al.⁴² also reported an increase in the gonial angle following class II Twinblock treatment and discussed that this phenomenon may be the result of changing muscle functions or due to sagittally directing condylar growth, leading to greater increments in total mandibular length.

Regarding soft tissue changes, significant positive improvements in the facial profile of the patients were observed. The soft tissue facial angle (Gl’SnPog’) decreased by 3.73 in Twin block group. Shahamfar et al.⁴³ reported a significant decrease in the Gl’SnPog’ after Twin block therapy. Facial convexity was also evaluated by FH to N’-Pog’ and it increased by 2.5° after wearing the Twin block appliance, which is in accordance with Sumitra and Tandur⁴⁴ who reported a significant change by 2.2 ± 1.3 in FH-N’-Pog. However, in the fixed appliance group, facial convexity did not increase as much as the Gl’SnPog’ increased by 1.56°. According to Shahamfar et al.,⁴³ the advancement of the mandible increased the Z angle by 3.26° in the Twin block group. However, in the fixed group, the Z angle decreased by 2.99°. The decrease in SNA and palatal tipping in upper anterior teeth lead to an increase in Z angle in the Twin block group.

Finally, Twin block appliance is preferable to fixed appliance in severe class II malocclusion with increased overjet because it shows a significant and remarkable advancement of the mandible, whereas fixed appliance is preferable in mild cases, especially if crowding exists.

LIMITATIONS

Being a short-term study is a limitation to draw conclusive results about the difference in the orthopedic effect between two-phase orthodontic treatment that include a first phase of Twin block use in comparison to treatment with orthodontic fixed appliance. Therefore, a long-term follow-up of the patients who participated in the current study is planned.

CONCLUSIONS

The Twin block postured the mandible forward, improving the maxillomandibular relationship and the facial profile. Mandibular length increased similarly in both groups, as did the effect on the maxilla. Dentoalveolar changes contributed to the significant overjet improvement in the Twin block group. A significant increase in the PFH parallel to the increase in the AFH, especially in the fixed group, controlled the deleterious effect on the profile. Finally, the clinical application of these results may indicate the importance of functional appliance in treatment of cases with class II malocclusion due to mandibular retrusion during growth spurt period.

CLINICAL SIGNIFICANCE

Because of its favorable skeletal effect, early treatment of class II due to mandibular retrusion with Twin block functional appliance is recommended. Early treatment with fixed appliance affects mainly the dentoalveolar component. Long-term follow-up is needed for further insights.

AVAILABILITY OF DATA AND MATERIALS

The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

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