CBCT Analysis Of Hyoid Bone Position Before And After Activator Correction In Class Ⅱ Mandibular Retraction Patients

Objective CBCT(Cone-beam Computed Tomography)was used to measure the three-dimensional changes of the position of the hyoid bone before and after treatment with the Activator appliance in the patients with Angle Ⅱ type of mandibular retraction at the peak of puberty.Material and Methods On lateral cephalometric radiographs,ANB angle greater than 4.7 degrees(SNA normal value,SNB less than normal value),cervical vertebral maturity in CVMS Ⅱ~CVMS Ⅲ(peak puberty)were taken as inclusion criteria.Thirty patients with Class Ⅱ mandibular retraction treated by Activator in the Department of Orthodontics,Baotou Iron and Steel Hospital of Inner Mongolia were selected and treated by the same technician before and after treatment.Each patient was scanned to reconstruct the three-dimensional image of craniomaxillofacial region.Twenty-three indicators of measurement software in CBCT were used to analyze the changes of hyoid three-dimensional position before and after correction by paired t-test.The correlation between mandibular forward distance and hyoid measurement items was analyzed.Results 1 After treatment,the linear distance between the supreme point(H1)and the lowest point(H2)of hyoid bone decreased(P < 0.05),while the linear distance from the coronal plane(Z plane)increased(P < 0.05);the linear distance between the supreme point(H1)of hyoid bone and the anterior point(C3)of the third cervical body increased(P < 0.05);the linear distance between the posterior point of hyoid bone and the anterior point of the third cervical body(H1-RGn-C3)decreased(P <0.05).Significant decrease(P < 0.001)and linear distance between left and right bigcorners of hyoid bone(GL,GR)was lower than that of horizontal plane(X plane)before treatment(P < 0.05).2 Compared with before treatment,the linear distance between left and right mandibular corner(GoL,GoR),submental point(Me),anterior chin point(Pog),posterior chin point(RGn)and horizontal plane(X plane)increased(P < 0.05),while the linear distance from coronal plane(Z plane)increased(P < 0.05).3 The angle between hyoid plane and horizontal plane(GR-H1-GL and X plane)increased,while the angle between hyoid plane and coronal plane(GR-H1-GL and Z plane)decreased,but there was no significant difference(P > 0.05).4 The forward distance of mandibular posterior chin point was correlated with the horizontal and vertical position of hyoid bone(P < 0.01).Conclusion Compared with before treatment,the hyoid bone of Class Ⅱ mandibular retraction patients was displaced both anteriorly and inferiorly,and anteriorly and upwardly after treatment with Activator.The distance of mandibular chin forward is positively correlated with the horizontal and vertical changes of hyoid bone.