Abstract: BACKGROUND: The Twin-block orthodontic appliance is commonly used for the correction of Class II malocclusion. Its mechanism of action in stimulating mandibular growth has been confirmed in many studies, but its impact on maxillary growth is not very clear.
OBJECTIVE: By establishing a finite element model to analyze the stress distribution of the maxillary complex, surrounding bone sutures, and maxillary dentition in patients with Class II malocclusion wearing Twin-block orthodontic appliances.
METHODS: One patient with Class II malocclusion who underwent orthodontic treatment at Qingdao Hospital/Qingdao Municipal Hospital of Shandong Rehabilitation University was selected. The bite force data of the patient when wearing the Twin-block orthodontic appliance was measured, and CBCT data were collected. A finite element model was established, including the maxillary complex, peripheral sutures, Twin-block orthodontic appliance, and maxillary dentition. ABAQUS software was used to simulate the stress distribution in the maxilla and maxillary dentition when the patient was wearing the Twin-block appliance.
RESULTS AND CONCLUSION: The equivalent stress on the maxillary anterior teeth was significantly smaller than that on the posterior teeth, and the maximum equivalent stress on both sides of the teeth were 4.797 5 MPa and 8.716 1 MPa, respectively, which were located at the first premolar. The maximum displacements were presented at the maxillary incisors on both sides of the teeth, which were 0.080 5 mm and 0.081 0 mm, respectively. The maximum equivalent stress on the bone suture was 1.284 MPa, which was mainly concentrated in the pterygopalatine suture and the frontal-maxillary suture on both sides, and there was almost no difference in the force of the rest of bone sutures; the maximum displacement of the bone suture was 0.07 mm, with the pterygopalatine suture having the largest displacement, followed by the frontal-maxillary suture. The maximal equivalent stress on the maxillary complex was 27.18 MPa, which was mainly concentrated on both sides of the anterior pyriform foramen of the maxilla, around the nasofrontal suture and around the pterygopalatine suture at the posterior part of the jaws. The maximal displacement of the maxilla was 0.07 mm, which was mainly concentrated on the maxillary alveolar bone. All these findings show that the occlusal force acts on the maxillary complex through the Twin-block appliance, resulting in clockwise rotation of the maxilla and steepening of the dentition plane. Measures should be taken to compensate for this tendency, for example, by considering maxillary molar elongation and intrusion in the process of occlusion, which are not only able to flatten the occlusal plane, but facilitate the mandibular protraction, thereby further improving Class II malocclusion orthodontic treatment. 

Key words: malocclusion, tissue construction, bone tissue engineering, Twin-block appliance, functional correction, finite element analysis