Author contribution: Zyabkin IV, Grachev NS, Frolov SV — study concept and design; Ataeva DM, Galkina TA — collection and processing of material; Magomedova AM — statistical data processing; Frolov SV, Magomedova AM — text authoring; Polev GA — editing; Frolov SV, Grachev NS — surgical treatment.

Compliance with ethical standards: the study was approved by the Ethics Committee of the Federal Research and Clinical Center for Children and Adolescents of the Federal Medical Biological Agency of Russia (Minutes #1 of January 18, 2021), conducted in accordance with the principles of biomedical ethics formulated in the 1964 Declaration of Helsinki and its subsequent updates. All study participants signed informed voluntary consent.