PURPOSE: Full-time wearing of an orthopedic brace has demonstrated effectiveness in limiting curve progression in adolescents with idiopathic scoliosis. However, treatment adherence is challenging, with an average wearing time of 13 h/day. Despite this issue, barriers to brace adherence have rarely been studied. The aim of this study was to develop and validate a new instrument tool to evaluate factors influencing brace adherence. METHODS: Our study followed the COnsensus-based Standards for the selection of health Measurement INnstruments criteria (COSMIN). A conceptual framework was initially defined, and experts elaborated, reviewed, and selected candidate items. We also investigated the MOBI's factorial structure and its psychometric properties. RESULTS: The MOBI initial version included 32 items related to four conceptual barriers to adherence, namely social/emotional, treatment, patient, and health system/professional. The factorial analysis led to an 18-item inventory with an internal consistency of 0.85 with four better-defined barriers (treatment social/emotional support structure, patient's self-image and perception, treatment adverse effects, and treatment acceptability. The MOBI-18f correlates with the SRS-22 domain treatment satisfaction and pain and the SF-12 mental health. Patients with poor brace wear time and more severe scoliosis will score higher on the MOBI-18f questionnaire. CONCLUSION: The MOBI-18f is a reliable and valid measure of patients' adherence to brace treatment. This questionnaire can be used to develop interprofessional adherence support intervention in AIS patients undergoing brace treatment.