Relevance. The research is relevant because of a high prevalence of anterior and posterior crowding in children with mixed and permanent dentition (from 42 to 86 %), for whom the correction of the dentition form and the size is recommended by fixed expanding appliances with a screw. After the teenage growth spurt, the growth and development of jaws end, steady pathological signs form, there are psychological problems in a face and smile selfperception, and the question of the orthodontic treatment duration arises.Purpose. To study the effectiveness of treatment of patients with maxillary premolar area constriction during growth spurt using two modifications of fixed Hyrax expander.Material and methods. We examined 22 male patients aged 10.6 to 13 y.o. (average age 11.2 ± 1.6 years old) with permanent dentition, constriction of the posterior teeth and crowding of incisors, treated with a fixed expander (Hyrax appliance) in the upper jaw during the growth spurt. The patients formed two groups. Group 1 (n = 11) had the Hyrax screw at the level of the upper first premolars, and Group 2 (n = 11) had the Hyrax lock located at the first permanent molars. The patients activated the expander independently by ¼ turn every other day for six months. Then the appliance remained in the mouth for three months. We studied the maxillary expansion results on plaster models before and after six months of the expander fixation using the Tonn, Pont, Korkhaus, Bolton method.Results. The study has established that orthodontic correction of transversal abnormalities with a fixed Hyrax expander affects the maxillary posterior teeth expansion, changes their parameters and improves their movement. At the same time, in Group 1, the width between the first premolars and the first molars reached the norm calculated from the sum of the mesiodistal dimensions of the upper incisors, and in Group 2, the width between the first permanent molars exceeded the normal, and inter-first-premolar distance did not reach the normal values for three months of activation.Conclusion. Fixed Hyrax expander showed the best results if the screw was at the level of first premolars during the teenage growth spurt.
