Application Of The Modified Twin-block Appliance For Treating The Children With Class â…¡ Division 1 Mandible Retrusion

Class II division 1 malocclusion is common in clinic, in which patients with descrepency often appear mandible retrusion. It has an adverse effect on growth, beautiful outlook and psychological develepment. Functional appliance is considered the most effective device to treat mandible retrusion in nonage. By appling orthopedic force, it stimulates mandible growth and improves bimaxillary relationship. To guide clinical application, we use the modified Twin-block functional appliance to treat growing children with ClassⅡdivision 1 mandible and discuss its clinical effects and mechanism in this study.Objective: 1. To evaluate the craniofacial tissue changes in patients treated with the modified Twin-block functional appliance. 2. To discuss changes of condyle process position and modification of temporomandibular joint after the device treatment. Method: Nineteen patients in growth with ClassⅡdivision 1 malocclusion were randomly chosen and treated with the modified Twin-block appliance. The course of treatment ranged from six months to ten months (average eight months). Through the cephalometric roentenograms, TMJ lateral tomograms and Helkimo index, we observed the modification of hard and soft tissues and evaluate the clinical effects of early treatment.Results:1. Mandible changed significantly and growed in favorable direction. SNB degree increased 3.05°and ANB degree decreased 2.67°. Mandibular length (Co-Pg) and mandibular body length (Go-Pg) increased 6.36mm and 3.49mm respectively. The distance from Pg to Olp plane increased 3.09mm. All results were significant in clinic(P<0.01). The appliance did not inhibited maxillary growth, which resulted in no changes of SNA degree. The increase of anterior facial height (N-Me), lower anterior facial height (ANS-Me) and lower posterior facial height (Ar-Go) showed statistical significant difference(P<0.01), increasing 3.72mm,2.84mm,2.49mm respectively.It illustrated that the modified Twin-block appliance stimulated the growth of lower facial and corrected discrepancy between sagital and vertical skeletal relationships.2. The results did not show an adverse rotation of jaw bones, and there was no change in the mandibular plane angle, SN-PP angle and the occlusal plane angle with treatment.3. After therapy, upper incisors and molar had 2.33 mm and 1.82mm backward movement, and the mandibular molar and incisor moved forward 4.28 and 4.21 mm, respectively. Class II molar relationship changed 6.03mm in average, of which 45.77% was due to skeletal and 54.23% due to dental changes. Overjet corrected on average 6.61mm, of which 41.75% was due to skeletal and 58.25% due to dental changes.4. The modified Twin-block appliance therapy brought about increasing the facial angle of soft tissue, thickening mental soft tissue. There were statistically significant difference in both H angle and mentalabial sulcus depth(P<0.05). Li-H distance, Pn-Sn, Sn-H distance and Ai-Sn were observed no changes. The results were explained that the modified Twin-block appliance stimulated growth of lower facial tissue and was beneficial to improve the profile.5. The therapy did not bring about TMD symptom.6. The position of the condyle seemed no change throughout therapy. Before therapy, the condyle was in the middle of glenoid fossa. After patients worn the modified Twin-block appliance for four months, condyle moved forward. The distances of upper and posterior glenoid spaces were increased, and that of anterior glenoid fossa space was decreased. So the ratio of anterior and posterior spaces was reduced. After 6-10 months'active treatment, condyle returned to its former position.Conlusions:The modified Twin-block appliance was used in clinic to correct growing patients with Class II division 1 malocclusion and mandible retrusion. By appling the appliance, it stimulated the sagital and vertical growth of mandible, and improved profile. It had no adverse effect on TMJ. The appliance activated condyle step by step and bring about persistent noumenon stimulation, affecting growth rate of supporting bone tissue and result of bone trabecula, and all changes were consistent with TMJ adaptive remodeling. This study showed that the modified Twin-block appliance is effective in treating patients with Class II division 1 malocclusions and has excellent curative effect. However, further study is necessary to understand the long-term effects of the modified Twin-block appliance in clinic.