Soft and Hard Tissue Changes Following Treatment of Class Ⅱ Division 1 Malocclusion with Twin-Block and Myofunctional Appliance:A Pilot Study

Ling XIE ,Ping WANG ,Jianhua WU

ABSTRACT Background Many cases of Class Ⅱ deformities have been reported to be treated with prefabricated appliances.The aim of this study was to distinguish the clinical effect of traditional custom-made appliances and prefabricated appliances in the treatment of Class Ⅱ division 1 malocclusion.Therefore,soft and hard tissue changes following treatment of Class Ⅱ division 1 malocclusion using the twin-block (TB) appliance was compared to that using the Myofunctional Research Company (MRC) appliance (K1+K2) combined with oral myofunctional treatment (OMT) (MRC+OMT).Methods The study included 22 children (6 boys and 16 girls aged 9-11 years) with Class Ⅱ division 1 malocclusion along with mandibular retrognathism with a 5-12 mm overjet,basic normal maxillary status,and stage 2 or 3 cervical vertebral maturation (CVM).Participants were randomly assigned into two groups,the TB group and the MRC+OMT group for 12 months.Standardized lateral cephalograms were used to assess skeletal,dental,and soft tissue changes from pre-to post-treatment.Independent t-tests were used to compare the initial and final cephalometric status and tissue changes between the groups.Results The TB and MRC+OMT groups resulted in different degrees of lateral changes;however,improvements of skeletal and soft tissue indices were significantly greater in the TB group than in the MRC+OMT group.Conclusion TB was more effective than MRC+OMT in treating children aged 9-11 years with Class Ⅱ division 1 malocclusion.However,further research using custom-made appliances with OMT is recommended,and further investigations are needed to confirm these findings.

KEY WORDS Class Ⅱ division 1 treatment;Twin-block;Myofunctional Research Company;Oral myofunctional treatment

INTRODUCTION

Class Ⅱ malocclusion is one of the most prevalent orthodontic conditions in adolescents,and Class Ⅱ division 1 malocclusion represents a significant percentage (approximately 30%) of the orthodontic conditions treated in clinical practice[1].According to Moyers analysis,50% of these patients have mandibular retrognathism[2].Class Ⅱ division 1 malocclusion is more severe than Class Ⅰ malocclusion according to the cervical spine and mandibular length (Co-Cn) classification in youth,with a 2-mm peak,which is smaller than the 2.9-mm peak typically observed throughout adolescence[3].Class Ⅱ malocclusion results in problems associated with mandibular growth,such as jaw development deficiencies.Additionally,patients with Class Ⅱ malocclusion have larger ANB angles due to retrusive mandibles rather than protrusive maxilla[4].Many cephalometric studies have shown a strong link between craniofacial morphologic features and pharyngeal dimensions,which has prompted many researchers to develop a series of methods and devices to treat Class Ⅱ skeletal malocclusion.

Various functional appliances,such as the Mono-block,Activator,Bionator,and Function Regulator,which were developed by Robin,Andresen,Balters,and Frankel,respectively,have been shown to be useful for stimulating the growth of the mandible for the treatment of Class Ⅱ skeletal malocclusion[5-8].

The TB orthodontic device,an improved version of the Activator,was invented by Clark in 1982[9].It is composed of bite guides and guide planes for the upper and lower jaws.The occlusal contact of the upper and lower guide plates is locked at a 70 ° angle,and the mandibular bone is moved forward by an oblique guiding force during the occlusal process to improve the sagittal malalignment of the upper and lower jaw.

Muscle dysfunction is also a common cause of malocclusion.The incidence of malocclusion in children with bad oral habits has been shown to be significantly higher than that in children without bad oral habits.As Kondo[10]has indicated,myofunction therapy,when used to treat malocclusion,can effectively align and restore perioral muscle function,thereby significantly improving the orthodontic effect and long-term stability,which is in accordance with the“muscle function hypothesis”proposed by Moyers[11].

It is believed that the stability of the position of the teeth and the shape of the teeth and jaws depend on the balance of the strength of the perioral muscles.Based on this theory,various prefabricated appliances have been developed,such as the Eruption Guidance Appliance and the MRC appliance.It is recommended that the use of these prefabricated appliances be accompanied by myofunctional exercises[12].

Several previous studies have compared the TB to other functional appliances,such as the Bionator,Forsus,and modified TB[13-15].These studies have shown that TB is more effective for the functional treatment of Class Ⅱ skeletal malocclusion caused by the retrusion of the mandible,resulting in positive changes,especially to the lower dental arch and the maxillary incisors.In addition,studies comparing soft and hard tissue changes pre-and post-treatment with customized and prefabricated appliances have been conducted[16].However,the participants of these studies were all aged >11 years.Interestingly,a recent randomized controlled trial has shown significant improvement in Class Ⅱ skeletal and dentofacial characteristics using the Activator compared to the T4K appliance in 10-year-old children[16].In that study,soft and hard tissue changes pre-and posttreatment using a customized type of appliance combined with muscle function training and the MRC appliance were not clear.The aim of this study,therefore,was to compare soft and hard tissue changes in children with Class Ⅱ division 1 malocclusion treated with either TB (TB group) or a combination of MRC (K1+K2) and OMT (MRC+OMT group).

METHODS

Participants

This study comprised 22 children (6 boys and 16 girls aged 9-11 years) with Class Ⅱ division 1 malocclusion,mandibular retrognathism with a 5-12 mm overjet,basic normal maxillary status,and stage 2 or 3 CVM.Participants were randomly assigned into two groups,the TB and MRC+OMT groups.This study was conducted at the Department of Pediatrics,Hefei Stomatological Hospital,from January 2017 until January 2019.

Only children whose parents consented were invited to participate in this study.All participants’ parents were provided with information sheets and signed informed consent.The recruitment of patients and follow-up flow diagram is shown in Fig.1.

The inclusion criteria were as follows:1.Stage 2 or 3 CVM;2.Mixed dentition without any report of changes in number,shape,or size of the teeth;3.Good health without oral respiration,rhinitis,nasal septum curvature,or adenoid tonsils three months after surgery;4.Good compliance during the entire treatment course;5.Overjet between 5 and 12 mm.The exclusion criteria were as follows:1.Stage 5 or 6 CVM;2.A history of orthodontic treatment;3.Maxillary prognathism;4.Poor compliance;5.Poor oral hygiene.

Randomization

Patients were randomly divided into the two groups.The final sample consisted of 22 patients (4 patients did not have their follow-up appointment on time) at a ratio of 1:1,with 11 participants in each group.Six males and 16 females aged 9-11 years were included in this randomized study.There were 3 males and 8 females with an average age of 9.9 ± 1.1 years in the TB group and 3 males and 8 females with an average age of 9.7 ± 1.3 years in the MRC+OMT group.Baseline sample characteristics for each group are shown in Table 1.

Fig.1 Flow chart of patient allocation

Patients in these groups underwent TB or MRC+OMT treatment.Cephalogram radiographs were taken before the initiation of treatment and after the 12-month treatment.The total duration of treatment was 1.22 ± 0.26 years.Class Ⅱ division 1 malocclusion was diagnosed at baseline when all the following signs were present:bilateral full-or half-cusp Class Ⅱ molar relationship,large overjet (>4 mm),and sagittal skeletal Class Ⅱ relationship (ANB angle >4 °) associated with the retrognathic mandible (SNB angle <78 °).

Cephalometric Analysis

Standardized lateral cephalogram radiographs were taken for each patient in both groups at T0 and T1.Children’s X-ray cephalometry software (Version 2.0,BJ Appliance Health Science &Technology Co.Ltd.,China) was used to digitize the landmarks and calculate the linear measurements and angles.All marks used for measurements were drawn three times by a doctor with 10 years of experience using standardized magnification,and the average measurement for each variable was used for cephalometric analysis.Linear and angular measurements were used to evaluate the soft and dentoskeletal tissue changes.

The initial oral diagnosis model was used,and the occlusal position was recorded.The lower jaw of thepatients was extended forward to the point where the upper teeth were incised end to end,and the jaw was opened 2-4 mm.Additionally,the position of the tongue and the magnitude of contraction of the buccal and labial muscles were estimated.

Table 1 Baseline sample characteristics

For the subjects in the TB group,the TB appliance was adjusted once a month,and the fitting pad was adjusted by approximately 1-2 mm,allowing the probe to be inserted.Individuals in the MRC+OMT group wore the MRC for 2 hours while they were awake and throughout the night.The K1 and K2 were worn for the first and second half of the year,respectively.Lip,cheek,and tongue muscle training and nasal breathing training were also provided.The OMT exercises were modified to provide strengthening,and evaluations were conducted after one year of monthly visits.

Treatment

Twin block

The TB treatment consists of maxillary and mandibular removable appliances.In this study,a maxillary labial bow was also used to aid the proclination of the anterior retention and control the incisors in cases where they were proclined.The occlusal plane was interlocked at approximately 70 °.Additionally,an expansion screw was turned once a week to expand the maxillary arch.All patients wore the appliance for 24 hours a day,including while eating,only removing it during contact sports,swimming,and for cleaning.At the 1-year follow-up,the occlusal plane was trimmed for polishing,and the stage was found to have improved from Class Ⅱ to Class Ⅰ.

MRC trainer

The MRC is a prefabricated appliance (Myofunctional Research Co.,Australia),and the type of MRC is chosen based on the length of the anterior teeth and age of the patient.Each MRC consists of tooth channels,labial bows,a tongue tag,a tongue guard,and lip bumpers.In this study,the patients wore their appliance for 2 hours while they were awake and throughout the night.The patients were required to put their tongues on the tongue tag and close their lips.All the teeth,including the last molars,were required to be positioned in the tooth channels.The patients then underwent myofunctional treatment according to the level of muscle dysfunction.

During the daytime,the patients were asked to perform myofunctional treatments,including training the buccal,labial,and tongue muscles,each for 5 minutes,twice a day.All treatments aimed to build nasal respiration to correct the tongue position and to improve abnormal swallowing.The K1 was used for the first 6 months;K2 was subsequently used for the following 6 months.

OMT included the following:(1) Abdominal breathing exercises:The patients closed their upper and lower lips and breathed through their nose;(2) Tongue:The tip of the tongue was placed against the front of the palate,and the tongue was slid backward;(3) Facial:The patients performed the following sequence:pucker lips-hold-smile-hold;(4) Suction movements were performed by contracting only the buccinators.These exercises were performed with repetitions and holding positions.The angular and linear measurements illustrated in Fig.2 and Fig.3 were used to evaluate soft and hard tissue changes (see Supplementary Table 1 for the reference values for the variables used in the study).We overlapped the patients’ head shadows pre-and post-intervention for both groups.The TB treatment is illustrated in Fig.4A.The MRC treatment is illustrated in Fig.4B.

Fig.2 Cephalometric landmarks,planes,and measurements

Statistical Analysis

Continuous variables are expressed as mean ± standard deviation (SD) when appropriate,and the variables that did not have gaussian distributions are expressed as median and quartiles.The normality of data was tested using the Shapiro-Wilks test.The cephalometric values were compared between the TB and MRC+OMT treatment groups.Differences between time points were calculated,and the mean differences were compared between the treatment groups using independent samplet-tests.For the variables that were not normally distributed,the Mann-Whitney U-test was used to determine differences.Pearson correlation coefficients for skeletal and airway measurements were also calculated;P<0.05 indicated statistical significance.All statistical analyses were performed using SPSS software version 22.0 (SPSS Inc.,Chicago,IL,USA).

Fig.3 Soft tissue measurements used for cephalometric analysis

RESULTS

Preoperative and postoperative soft and hard tissue measurements were assessed over 1 year using lateral cephalometric radiographs.There was no significant difference in the quantitative CVM between the two groups.All patients in both the TB and MRC+OMT groups completed the treatments,and no adverse events were recorded during the entire 12-month follow-up period.The measurements for the 17 skeletal variables for both groups are shown in Table 2.Inter-group comparisons revealed more meaningful improvement with TB than with MRC+OMT from T0 to T1.This was shown by the significantly greater increase in the SNB angle in the TB group (x=2.30 ± 1.79) than in the MRC+OMT (x=0.40 ± 1.82) group (P=0.03) and the significantly larger decrease in the ANB angle in the TB group (x=-2.10 ± 1.92) than in the MRC+OMT group (x=-0.3 ± 1.34) (P=0.03).Moreover,a remarkably larger increase in both L1-MP (x=-9.90 ± 1.19) (P=0.01) and L1-NB (x=11.10 ± 3.49) (P=0.03) was observed in the MRC+OMT group than in the TB group.No other significant differences were detected for the remaining dentoalveolar variables.

Soft tissue changes are detailed in Table 3.Both groups had different degrees of lateral changes,and the experimental group showed more balanced structures than the control group (P<0.05).The NLA,FCA,UFH,ULL,LLP,UL-EP,and LL-EP showed significant changes in the TB group (P<0.05).In contrast,only ULL and LL-EP showed significant changes in the MRC+OMT group.Additionally,the FCA decreased significantly more in the TB group (x=-0.90 ± 2.43) than in the MRC+OMT group (x=1.80 ± 1.03) (P=0.01),and the UL-EP decreased more in the TB group (x=-1.70 ± 1.11) than in the MRC+OMT group (x=0.10 ± 3.48) (P=0.049).Moreover,thexincreased significantly more with TB (x=0.50 ± 2.19) than with MRC+OMT (x=0.10 ± 1.09) (P=0.01).

According to the patients’ pre-and post-treatment head shadows (Fig.4A-B),forward growth of the mandible was observed in the TB group,and the inclination of the lip toward the lower anterior teeth was not clearly evident.The mandibular growth appeared more downward in the MRC+OMT group than in the TB group,and the tilt of the lip toward the lower anterior teeth was more evident.

Fig.4 Schematic diagram profiles of skeletal and soft tissue changes derived from LCR between pre-treatment (black) and post-treatment (green)

DISCUSSION

In the present study,the efficacy of the TB appliance was compared to that of the MRC (K1+K2) appliance (+OMT) for mandibular advancement in growing patients with Class Ⅱ division 1 malocclusion by assessing the soft and hard tissue changes between the two groups using lateral cephalograms.The outcomes after 12 months of active treatment showed that while both groups had significant skeletal changes,TB was significantly more effective,as it resulted in a larger increase in the SNB angle and a greater reduction in the ANB angle than in MRC+OMT.These improvements in the distal placement of the maxilla and maxillomandibular relationship were similar to the findings of several previous studies[17].

When the TB appliance was used to treat Class Ⅱ division 1 malocclusion,significantly greater improvements in skeletal positioning were found,especially concerning the dimensions of the mandible and the maxillomandibularrelationship,which is consistent with the results reported in some previous studies.However,the resulting increase in the length and height of the mandible and the significantly larger increase in the length of the mandible were not consistent with the findings of most studies,which have found only an increase in the height of the mandible[18-20].Burhan[17]reported significant increases in the length and height of the mandible using the bitejumping and TB appliance.In that study,improvement in the maxillomandibular relationship mainly depended on the amount of increase in the mandible alone,with no significant effects on the maxilla[21].Meanwhile,this pilot study showed greater improvements in the SNB,ANB,LI-MP,and LI-NB measurements in the TB group than in the MRC+OMT group.This result might be explained as follows.First,the TB is a custom-made appliance that allows for the anterior mandible to be precisely repositioned by the construction wax bite,whereas the MRC is a prefabricated appliance without personalized accurate alignment.Second,TB is designed to expand the maxillary arch so that its width is aligned with the midline of the mandible and to move the mandible forward smoothly.The expansion of the palatal cleft and maxilla is conducive to widening the gap[22],thus greatly reducing the possibility of tooth extraction.Third,TB is made of acrylic material,which is harder than the trainer material,especially the starting appliance (i.e.,the soft blue trainer).The high elasticity of K1 can also be easily chewed on by children,making it difficult to maintain the mandible in a stable forward position.

Table 2 Comparison of skeletal variable changes between the two groups

Recent research has shown that myofunctional appliances can also be used to treat Class Ⅱ division 1 malocclusions with retrusive mandibles.OMT balances the functionof the perioral muscles and,when combined with prefabricated functional appliances,has been shown to improve the oral environment through re-education of the musculature and respiratory patterns,thus effectively treating malocclusions[23].However,the incidence of malocclusion in children is remarkable due to factors such as the regional economy,oral hygiene practices,lifestyle,and individual preventive measures.Bad oral habits are important factors that cannot be ignored in the context of acquired factors since the proportion of children with bad oral habits is significantly higher than that of children without bad oral habits[24-25].Accordingly,soft tissue and dental profiles can be improved significantly using both MRC+OMT and TB treatments.

Table 3 Comparison of dentoalveolar variable changes between the two groups

After approximately 12 months of treatment in this study,the mandible was guided into a new position.Shen[26]reported the formation of new condylar bone after guiding mandibular bone to the normal occlusal state after 7 months based on computed tomography,cone beam computed tomography,and magnetic resonance imaging.Additionally,the SNA angle was shown to have decreased,and the lower incisor proclination had increased[27].However,we observed the lower incisors were slightly upright with TB but tilted labially with MRC+OMT in this study.

Dental changes can be assessed to help determine the skeletalodental effects of functional appliances[19].Similarly,as many children’s mouths and facial features are not harmonious,we provided orofacial myofunctional training twice a day,with each group of muscles exercised for 5 minutes,and the MRC K1 and K2 were worn at night for the first and second half of the year,respectively.The patients’ dental arch morphology was more stable after only weeks of perioral muscle function training and MRC wear,and the U1-SN was smaller.With an increase in L1-MP,dental compensation is more highly expressed,and the nasolabial angle is also increased,which is due to the simple use of the MRC,with no registration of alignment or accurate mandibular repositioning.Additionally,K1 is an elastic device,which makes it difficult for children to maintain accurate mandibular and maxillary positioning.In this study,the upper incisor lip tilt improved in both groups.However,given that the TB appliance allowed for better control of the labial arch,it was superior to MRC in controlling the inclination of the anterior teeth and guiding the growth of the mandible.During the 12-month observation period,treatment with TB was more conducive to the integrity of the dental arch shape and the coordination of facial soft tissue.Therefore,this prefabricated appliance was shown to be more effective in reducing overjet,overbite,and mandibular crowding and establishing the Class Ⅰ canine relationship than no treatment[28].Moreover,by eliminating the pressure of the tongue,mouth,and lips and establishing muscle balance,good facial harmony and occlusal stability were achieved for many cases.The MRC comprises a stabilizer and a muscle function trainer that can promote and maintain balanced perioral muscle function.Most patients could achieve good occlusal relationships and facial shapes.However,some patients with individual malocclusion need additional fixed correction,perhaps not require more complex treatment during adolescence[29].

Our study has several potential limitations.First,the follow-up period of 12 months is relatively short.Second,the small number of participants is an inherent limitation.However,the significant difference in results between the two groups could be ascribed to the different treatment methods,achieving the aim of the current study.Future investigations are necessary to confirm our results.

CONCLUSION

TB treatment is significantly more effective than MRC+OMT in normalizing soft tissue appearance,particularly the facial convexity angle.

ACKNOWLEDGEMENTS

I would like to express my gratitude to all those who helped me during the writing of this thesis.I gratefully acknowledge the help of my co-worker,Mr.Jianhua Wu,who offered me valuable suggestions during my academic studies.I also owe a special debt of gratitude to all the professors at the Hefei Stomatological Hospital,whose devoted clinical teaching helped me to prepare this article and from whom I have benefited a great deal.

Finally,I would like to express my gratitude to my coworkers at the Department of Pediatric Dentistry who continually helped and supported me without a word of complaint.

ETHICS DECLARATIONS

Ethics Approval and Consent to Participate

This study received approval from the Ethics Committee of the Hefei Stomatological Hospital.All parents of the participants provided written informed consent before study enrollment.

Consent for Publication

All the authors have consented to the publication of this article.

Competing Interests

The authors declare that they have no competing interests.The authors state that the views expressed in the article are their own and not the official position of the institution or funder.

AUTHORS’ CONTRIBUTIONS

LX conceived of the idea,analyzed the data,and led the writing;PW collected the data;JW provided the case photos and drew the overlaps.All authors have read and approved the final manuscript.

SUPPLEMENTARY MATERIALS

Supplementary Table 1 Reference values of the variables